Good Old Solar Cells

Here's something completely different...A collection of solar cells, starting with selenium sensors (remember the "electric eye" on automatic doors?), from around 1933.

https://solarmuseum.org/cells/

This collection from the Museum of Solar Energy is far from complete, but shows a nice bit of history, and has great high-resolution photography. 

And so you don't have to look it up, Vanguard, the first US satellite, and the first satellite to have a solar array -- six cells from Bell Labs, was launched in 1957. It had two low-power radio transmitters for tracking and for gathering data on the ionosphere. The battery-powered transmitter ran for about 2 weeks; the solar-powered transmitter ran for more than six year -- until the radiation in the Van Allen belt killed the electronics, including the solar cells...

In a previous life I made solar cells and modules, and built factories -- in the US -- to make them. Here's one of the cells, from around 1982 when we were developing Solarex's cast polycrystalline silicon wafers.

Chesapeake Bay Ice

It was -9F in Duluth, MN this morning, with Lake Superior "smoke on the water" www.duluthharborcam.com/. So winter is coming...It doesn't ice over every year, but here's the upper Chesapeake back in February, 2006.

From Crystal Beach, west toward Turkey Point light.

 From Betterton Beach. Grove Point and then Turkey Point in the background. Then Havre de Grace waaay back.

Mouth of the Sassafras River looking northwest toward Aberdeen Proving Ground.

Rock Hall "beach" (Ferry Park) with the Bay Bridge to Annapolis on the horizon.

Rock Hall harbor...some good advice.

Our Hunter 25.5 "The Spoon" (don't ask me, I didn't name her...), in hibernation mode on the Sassafras.

Old Boat Graveyards, and the Jersey Boys

Is that an Elco PT boat? Or is it from Higgins, or a Huckins?

on Worton Creek, upper Chesapeake Bay, February, 2009

Elco was located in Bayonne, NJ; the Higgins yard was in New Orleans, LA; and Huckins is in Jacksonville, FL.

from the Huckins Yacht photo archive (www.huckinsyacht.com/gallery-detail.cfm?y=12)

Based on the Huckins photo, it looks like the Worton Creek PT boat is not one of theirs...

    .

    .

    .

Thank you, Google! Mystery solved. Turns out it's PTF-19 (www.ptfnasty.com/ptf19.html), a Viet Nam-era "fast" attack boat built by Trumpy, the iconic wooden yacht builder whose yard was located on Spa Creek in Eastport, MD until 1973. John Patnovic, who owns the Worton Creek Marina, apparently is restoring the boat for historical display (or was planning to...). Recently he and his crew were highlighted in Professional Boatbuilder (No. 178, April/May 2019) for rebuilding a 74-foot Spencer sportfisherman that was run aground shortly after being commissioned. They've got more wrecks on their rebuild agenda, but PTF-19 is not listed as one of them. I found a reference to the Trumpy being sold at an old boat boneyard in Suffolk, VA in 2004, and then moved up the Bay by barge. As of 2019 it was still sitting there, and it looks a lot more weathered.

Here is some more interesting boat history to add to this "mystery." It looks like Post Yachts (www.postyacht.com), founded by Russell Post in 1957, is still producing boats in this location. During WWII, Post worked for Ventnor Boat Works in Atlantic City, which built rescue and supply boats for the government. After the war, he and Charley Leeks co-founded one of the best-known sportfisherman companies, Egg Harbor Yachts. In 1956 Post sold the company, "retired" for a year, and then built his own yacht factory in Mays Landing. He ran Post Yachts for another 20 years, then sold the business and the brand. In 2011, after the Great Recession, the Post facility declared bankruptcy, and apparently the molds and boat building operation moved to Worton Creek, where the mystery boat was sitting. Check it out on Google Earth.

Here's some more history of Russell Post and other boat builders of his post-war generation...

captkarlanderson.com/uncategorized/the-boat-builders-of-south-jersey/

And finally, the connection to Phil Bolger, whose first published design was a 32-foot sportfisherman in 1952 and designed for Egg Harbor Yachts...

www.yachtingmagazine.com/jersey-boys/

More details on Phil Bolger, his boat design apprenticeships, and his connection to the Jersey Boys is here, www.duckworksmagazine.com/03/r/vintage/nquarterly/phil/bolger.htm.

More on Flotation, and Masts

In a previous article I mentioned a couple of options for replacing or adding flotation to your boat. I pointed out that dry bags and wakeboat ballast bags were more economical than purpose-built flotation bladders. But since small boat folk are usually pretty good DIYers, how about making your own bladders? If you're interested, here's a link to Mik Storer's article on making them, www.storerboatplans.com/boat/sailing-boat/cruising-dinghy/diy-pvc-buoyancy-bags-and-dry-bags-for-dinghies-and-canoes/ .

Materials that you need:

• Bostik Unigrip 999 or 1669 adhesive
• 18 oz Hypalon PVC fabric
• Air valve

A good source for materials for making and repairing dry bags and bladders is NRS (www.nrs.com), a top dry bag and raft manufacturer, as well as supplier for river running gear. They sell fabric, adhesive, and valves. And they have a bunch of useful how-to pages...

• www.nrs.com/learn/clifton-urethane-adhesive-instructions
• www.nrs.com/learn/how-to-replace-a-raft-valve
• www.nrs.com/learn/install-d-rings-and-footcups
• www.nrs.com/learn/inflatable-repair-adhesives-guide

Sailrite (www.sailrite.com) is another good source of PVC-coated materials, typically used for biminis and tarps (lighter weight and considerably cheaper than NRS):

• HH-66 adhesive
• 18 oz Shelter-Rite fabric
• 13 oz Weblon Regatta

Since I'm pointing to him, I should also mention that Mik Storer is probably best known for his iconic Goat Island Skiff design. And if you want to know anything about how to build and rig a lug sail, and get the most performance out of it, his website is where you should start. I noticed that he also has a couple of articles on windsurfer masts used for spars -- www.storerboatplans.com/plan/gis/windsurf-mast-lug-yard-goat-island-skiff/ . He's one very frugal sailor...

Electric Boats

Jim Campbell heading out to the race course in his eCanoe in 2016. 

Check out this video of “Wye Island Electric Boat Marathon 2017” from Andy Rebele, 

https://vimeo.com/237483010

The race starts at the Miles River Yacht Club in St. Michaels, circles Wye Island with a mandatory stop at the Wye Landing, and then back to yacht club. Height restriction enforced by a fixed bridge that you've got to get under. I've heard that some support crews use that to "drop ship" coffee and snacks to their driver and he roars underneath, with mixed results...The race is run on the Friday of the annual Mid-Atlantic Small Craft Festival, the first weekend in October. For more details on the race and resources, see the electric boat website, www.electricboats.org/wye-island-challenge

Old Boat Graveyards

On the upper Sassafras River, MD...Yes, it looks like a steel version of a Jim Brown Searunner...a rather unique boat.

Old Boat Graveyards

British Twin-keeler Sunray 21 near the C&D Canal a few years ago. It's strange to see a twin-keeler so far from home, but both the Chesapeake and Delaware Bays are rather shallow (and the boat was registered in Delaware).

There is only one Sunray 21 listed in the Sailing Texas photo gallery, confirming that this is a pretty rare boat in the US. As usual, Bill's Log (bills-log.blogspot.com/2015/08/sunray-21.html) is a good source of more information on the boat and provides a key link to what looks like still active owners' group, leisureowners.org.uk/

Small Boat Flotation and Howard Hughes' Flying Boat

Some boats, like Boston Whaler skiffs and MacGregor powersailers, are built with flotation foam between the inner and outer laminations. Roger MacGregor was quite willing to flood any of his boats just to prove that they would not sink and would keep you safe, even though they were water-ballasted. And if you check out his old sales videos on YouTube, he even did it a few times. 

When the boat is not built with integral flotation, you might have to really look for it. For example, the “built-in” flotation on a stock Potter 15 consists of three parts. The easiest-to-see, assuming it's still there, will be two large blocks of styrofoam, one located beneath each of the cockpit seats. They keep the aft end of the boat afloat in the event of cockpit and cabin flooding. Each block provides about 3 cubic feet (1 foot x 1 foot x 3 feet long) or 192 pounds (3 cu-ft x 64 pounds of seawater per cubic foot) of flotation. Since the empty weight of a Potter 15 is about 500 pounds, the foam blocks in the stern are not enough to keep the boat afloat if it floods. 

The rest of the flotation is much harder to find -- because it is underneath the cabin floor. On the forward end of the boat there is a bulkhead that crosses the hull under the mast support. If you open up the cabin floor panels and look forward, you will see it. Foam fills the space ahead of this bulkhead to support the cabin floor and to keep the forward end of the boat from sinking. Unfortunately, without removing the floor or cutting an inspection port, there is no easy way to inspect the forward flotation. Fortunately, Potter's builder did provide a way for any water that gets into the boat to drain back to the cabin, where you can get at it. I estimate the volume of under-floor foam is 6 to 8 cubic feet, or 300 to 400 pounds of flotation. 

Add the three parts together and you come up with 700 to 800 pounds of built-in foam flotation. This is enough to keep even a loaded Potter from sinking, and there are old sales photos showing a young woman standing in a scuttled Potter full of holes in the hull. If you find that photo, note that there are no waves, it is nice calm water. Sufficient flotation is necessary, but not sufficient to keep you safe. Placement of the flotation -- where it is located -- is the other aspect that needs to be addressed. How many runabouts have we seen where the occupants end up clinging to a capsized hull? Because most of the flotation is under the floor, a big enough wave could roll a flooded hull and it will "turtle" -- the flotation in the floor now up high. So, the best flotation is up high -- near the gunnels -- to reduce the tendency to roll over. Since much of the flotation of a Potter (and many other small sailboats) is also low in the hull, they have the same predilection to turtle when flooded, especially if the centerboard is not locked down or is lost, and the mast fills with water. Steve Potter described capsizing his Potter 15, and explains why it happened (“Capsize at Clipper Cove,” Small Craft Advisor, No. 56, March/April 2009, page 22). His experience is applicable to a lot of production and home-brew small craft.

If you find that the styrofoam blocks in your Potter (or other boat) are still under the cockpit seats, be aware that they can work themselves loose, which is not a good situation. And if they got loose before you bought the boat, they might even be gone. Some Potters have been "customized" with hatches cut into the seats, and the foam was removed to increase storage for anchors or fenders. Yikes! Hope the hatch covers stay secured. So it is important to make sure that enough flotation is still in the boat, and that it is secure and not rattling around. If the blocks are loose, then you can try to re-secure them using a good polyurethane construction adhesive such as PL Premium (test to make sure that it doesn't dissolve the foam). If necessary, fabricate a suitable “wedge” (using polystyrene foam sheet) to force the foam block up against the bottom of the cockpit seat. 

If the styrofoam blocks are saturated with water (it's an "open cell" foam) or if they are missing, you might want to replace them. Extruded polystyrene foam sheet, which is closed-cell and won't absorb water, is available up to 2 inches thick. The 2-foot wide sheets are especially useful and each piece, 2.8 cubic feet of foam, is good for about 170 pounds of buoyancy. Build up a “block” and secure it using PL Premium adhesive. It is also possible to add even more flotation by sticking polystyrene foam to the bottom of the underside deck surfaces. And if you are planning some cool weather sailing, more foam can be used to line the inside of the cabin walls. In the Potter, it is possible to insert additional sheet foam between the cockpit seat back and the hull -- the best place for more flotation.

An option to sheet foam is to add flotation or buoyancy bags, which have become common on small boats, kayaks, and canoes (e.g., www.nrs.com/category/3049/kayak-touring/float-bags). In addition to adding buoyancy, these devices also reduce the amount of water that might fill the cabin or compartment. Buoyancy bags are not cheap, and the selection seems fairly limited -- there's just not that much demand. In contrast, dry bags, used for rafting and camping, are readily available from many sources and come in volumes of up to around 90 liters (seatosummitusa.com/collections/outdoor-gear-dry-bags/products/ultra-sil-pack-liner). And the prices are very reasonable. To replace a Potter 15's styrofoam block (displacing 3 cubic feet) you would need a flotation bag with a volume of at least 85 liters (1 cubic foot is equal to 28.3 liters), and a quality dry bag would run about $50. You can fill them with inflated play balls or beach balls -- they are very cheap (find them on sale in the spring), tough, and can be inflated using any pump with a needle. If you are old enough to remember Howard Hughes, you might have heard of his gigantic flying boat, the "Spruce Goose," so called because it was built toward the end of WWII using laminated plywood -- steel and aluminum was in short supply due to the war effort. 

After it's single test flight in 1947 -- for about a mile at a maximum altitude that was under a 100 feet -- the plane was stored in an air-conditioned hangar Long Beach, CA until 1993. It's now at the Evergreen Museum (www.evergreenmuseum.org/the-spruce-goose). Hughes had already survived several airplane crashes during test flights of his planes. So he filled the plane's cargo compartments with beach balls secured with nets to provide removable positive flotation -- just in case.

Unlike the "Spruce Goose," the most effective position for these flotation bags in small boats is probably at the level where the flooded boat will settle, around the gunnels. And they should also be secured so that they'll stay put in a capsize (unlike in the above photo).

How about attaching flotation to the outer gunnels of a trailer-sailer, like on the hard-bottom RIBs? A string of fenders can add a lot of flotation, but they are expensive. A 22-inch fender gives about 1/2 cubic foot of buoyancy and costs around $30, although you can frequently find them on sale. And if you can figure out how to lash it to your gunnels, Duckworks sells a 9 x 60-inch beach roller ($70) that can double up by providing about 2 cubic-feet of additional flotation (duckworks.com/beach-rollers/). Wakeboats use "ballast bags" that are rated by pounds of water. A 375-pound bag has dimensions of 14 x 65 inches, or 6 cubic-feet, for $100 (www.evo.com/fat-sacks/straight-line-big-bag-375-ballast). That's a lot of flotation if it is pumped up with air instead of water. Two 350-pound ballast bags (5 cubic-feet per bag, $135, at www.wakemakers.com/launch-pad-350-twins.html) would make ideal replacements for the styrofoam blocks in the Potter's stern.

For lots of reasons it's important to load the boat so that the heaviest items, such as water bottles, anchors, and chain rodes, are secured as low as possible, ideally between the cabin floor and the hull bottom. In addition, it is useful to store lighter items, such as clothes and sleeping bags, in sealed “dry bags” that are secured as far forward and as close to the deck as possible. This puts flotation up high and, together with a couple of flotation bags up high under the cockpit seats, should help keep the boat from turtling if it is flooded. If heavy weather is expected, right-side-up buoyancy can be further enhanced by securing dock bumpers to the gunwales using the deck cleats and railings.

And it’s worth repeating the obvious: the Potter -- and any small cabin boat -- has it's maximum amount of flotation in its cabin. If the cabin hatch boards are in place and the sliding hatch is closed during a knockdown, much of the green water will be kept out of the cabin, the boat will stay afloat, and it cannot turtle. Water inside the cockpit should drain out (make sure the plug is out, or is easy to pull out). I sail my Potter with the hatch boards in but the sliding hatch open, for ventilation and so that I can get to stuff in the cabin, and the odds are still excellent that a knockdown will not cause cabin flooding. And whenever the wind pipes up, the best insurance is to "batten down the hatches" and I close up the cabin.

The Potter 15, like many small boats, has a hollow aluminum mast and boom, which can help resist the tendency of the boat to capsize if it fills with water if the ends of the mast (and boom) are plugged. The spars don’t have to be “watertight,” even a wad of soft foam is better than nothing. If the mast is plugged it may slowly fill with water from all the little holes at the fittings, but until it does you have about a half cubic foot of buoyancy (30 pounds, or more than an typical inshore life vest), which should help keep the mast up off the bottom. If you sail where “turtling” into mud is a distinct possibility, then adding a masthead float (a la Hobie cat) might actually be the best solution, at the cost of windage and weight at the top of the mast.

In Roger MacGregor's sales video for the powersailer, was the ballast tank flooded, or empty Answer: flooded.

What I miss...

 Janes Island State Park, Crisfield, MD. April 2015

What I miss...

 

Oxford Model Boat Show, 2013

Chesapeake skiffs by Ed Thieler, Easton, Md...

Eddie Somers, Crisfield, MD...

What I miss...

 

Mid-Atlantic Small Craft Festival, St. Michaels, MD, October 2013

More Design Information and Tools

Competition Composites Inc (CCI) in Ontario, Canada, builds foils for what looks like hundreds of one-design racing and cruising boats, from A Scows to Y Flyers. But alas, nothing for Potters or MacGregors...They also have a bunch of useful design information on foil design and performance on their website, if you're trying to figure out why you're not be pointing quite as high as you think you should. What's the right NACA profile? Techy, but I think it's approachable and explained well.

Here's the link to CCI's design page: http://cci.one/site/marine/design-tips-fabrication-overview/?doing_wp_cron=1607906222.2815411090850830078125

Mast and Sail Stress Analysis -- Part 2

Legal Disclaimer: I am not a structural or marine engineer. And I've seen my fair share of home-brew failures. Use this information only as a guide to help understand the factors that figure into mast stress. Don't change the rig from what the designer provided unless you've thought about what can go wrong. And if you're not sure about your scantlings, you should consult a professional engineer to verify that that they will provide the necessary margin of safety. Finally, if you think you might want to do some boat designing, I recommend that you read "Gougeon Brothers on Boat Building" (on-line at the West Systems website) and buy a copy of Dave Gerr's "Elements of Boat Strength" (International Marine, 2000). 

Finally, REALITY!

In Part 1 we figured out how much a mast will theoretically bend (and break?) under load. But does it?

First, a quick summary to highlight some of the conclusions if you skipped the math...

[1] The pressure on the sail is equal to the wind speed, in knots, times itself, times 0.005. At a wind speed of 10 knots, for example, you can figure on (10 x 10 x 0.005 =) 0.5 pound of pressure for every square foot of sail area.

[2] Doubling wind speed increases the pressure on the sail by a factor of four; tripling the wind speed increases the pressure by a factor of nine. Therefore, if that 10 knot breeze gusts up to 20 knots, the pressure on the sail would increase from 0.5 to 2 pounds of pressure per square foot of sail area. Of course, this assumes that you don't luff the sail.

[3] To keep a small boat level, the torque (force times distance) generated by sail pressure must equal the torque generated by the keel, or by hiking out. A 100-SF sail can apply a force of up to 200 pounds in a 20 knot gust. If the sail's center of effort is 5 feet above the waterline, that sounds like 1,000 foot-pounds of torque that could possibly cause a small boat with a centerboard to capsize.

[4] The amount of torque that it takes to bend and ultimately break a mast is dependent on the dimensions and the material used to build the mast. Aluminum -- or carbon fiber -- is not necessarily the best material. It all depends...

Sail maker Stuart Hopkins, of Dabbler Sails, measures mast bend to design and tweak a sail. The following procedure to measure spar bend is based on his "Determining Spar Bending," (www.dabblersails.com/blog/blog.pl?type=show&id=43&pic_id=4)...He doesn't talk about breaking a spar, but his methodology might let you evaluate a mast for "ultimate strength before you get out on the water.

First, support the mast at the tip and where it passes through the partners. Support gaffs and lug, gunter, or lateen yards and booms (the the sail is not loose-footed) at their ends. As shown above, the force of the wind, in pounds per square foot, is given by 0.005 times the wind speed squared in knots. For example, a wind speed of 10 knots is equal to a pressure of (10 x 10 x 0.005 =) 0.5 pounds per square foot.

Next, apply a weight (such as water, which weighs 8.3 pounds per gallon) equal to this pressure to the spar, assuming that the sail's center-of-effort is approximately in middle of the spar. Hang the weight from the midpoint of the spar (for masts and gaffs) or from the halyard attachment point (for lug, gunter, and lateen yards).

According to Hopkins, to get the correct bend the proper load to the spar is the product of the wind pressure times the area of the sail, using the following rule of thumb based on the type of rig. The factors he uses are: 

• 1/2 of the sail area for masts carrying Bermudian, gaff, or sprit sails;
• 1/2 of the sail area for the gaff and boom on a lateen sail; and
• 1/3 of the sail area for gaffs and lug or gunter yards

Hopkins gives an example: measure the bend of an unstayed mast carrying a 100-SF Bermudian sail in 10 knots of wind, using water as the load. 

Since the sail is Bermudian, the sail area (100 SF) used for the load calculation is reduced by half, to 50 SF. The pressure at 10 knots of wind is 0.5 psf, so the load to be applied is (50 SF x 0.5 psf) 25 pounds. Water weighs about 8 pounds per gallon, so we need (25 pounds/8 pounds per gallon) 3 gallons of water. Therefore, the bend of a 100-SF Bermudan sail in 10 knots of wind on an unstayed mast requires 3 gallons of water attached to the midpoint between the mast partner and the tip. 

Once the spar is weighted down, measure the deflection using a string line fixed between the partner and mast tip. If the spar is rectangular, the wider, stiffer section (fore-aft section) should be in vertical axis. Imagine the sail pressure bending the mast top back.

To evaluate the spar deflection at higher wind speeds, adjust the load's weight, or the volume of the water. For example, to evaluate the mast bend with the same sail at 20 knots, increase the load by a factor of four. For the 100-SF Bermudian sail example, increase the volume of water from 3 to 12 gallons, or approximately 100 pounds of load. When I run these tests, I use old weightlifting weights instead of water (12 gallons would take a mighty big bucket, but you can hang successive weights on the spar -- just watch your toes). 

Want to test your home-made unstayed mast? Clamp the end up to the mast partner to something stable (a boat trailer or truck tailgate) and start hanging weights on the mast at the sail's center-of-effort height. I would use the full pressure weight for this test, instead of derating it for measuring the mast bend. For example, use 200 pounds of load for the 100-SF Bermudian sail in a 20-knot puff (and about 450 pounds if you think you'll see a 30-knot gust). The problem will be finding enough weights -- how about using some cinder blocks? If you can careen your boat, I think you could also test the mast partner (or a tabernacle) and the mast using the hanging weight technique. Next time I build a small boat and rig, I'm going to try it and see.

Sail and Mast Stress Analysis -- Part 1

Warning: Extremely Technical Content

(if this is too techy for you, stay tuned for Part 2, which is more practical than theoretical)

Question: How much will a solid, constant diameter, unstayed wood mast deflect at different wind speeds? And will it break?

I mess around with small boats and sail on the upper Chesapeake Bay where the wind speeds are moderate as long as there’s not a thunderstorm. I like to experiment with different sails and rigs. I'm also a retired tech-builder (aka, old-school engineer), and this note addresses several questions I had concerning the stress on an unstayed mast on a small boat. There is a link to a "stress calculator" for determining beam deflection and stress, and it allows you to choose different materials like wood and aluminum. It seems to work pretty good, and I have been using it to look at hollow spars as well as the solid wood mast that I looked at in this note. It can also handle square and rectangular spars ("beams" in the calculator). Much of what I'm presenting here is based on Jim Michalak's excellent discussion of mast stresses in his newsletters, and hopefully you can still get them as a primary source. Thank you, Jim and Stuart Hopkins of Dabbler Sails, for educating me, and hopefully this will be useful to someone else who is asking the same questions (before they break that mast!).

Legal Disclaimer: I am not a structural or marine engineer. Use this information as a guide to help understand the factors that figure into mast stress. You should always consult a professional engineer to verify that your scantlings will provide the necessary margin of safety. And if you are really serious about this stuff, take a look at Dave Gerr's "Elements of Boat Strength" (International Marine, 2000). 

1. Relate wind speed to pressure on the sail:

Pressure on sail, P, in pounds per square foot is given by

        P = 0.005 x (V)**2, or 0.005 x V x V

where V is wind speed in knots

• At 5 knots, P = 0.125 psf
• At 10 knots, P = 0.50 psf
• At 15 knots, P = 1.12 psf
• At 20 knots, P = 2.0 psf

>> Conclusion: Doubling wind speed (e.g., gusts) increases sail pressure by a factor of four; tripling the wind speed increases the pressure by a factor of nine.

>> Rule of Thumb: You can figure on 1/2-pound of pressure per square foot of sail area at 10 knots of wind. And 2 pounds of pressure per square foot at 20 knots of wind. Would your sail and rig support you if you lie on top of it?

2. Calculate the maximum force on the mast due to wind -- pressure x area

Assume boat is level and stationary with respect to wind. Assume wind is directly into sail. And for this example, assume the area of sail is 100 SF

• At 5 knots, max force is (0.125 x 100) 12.5 pounds
• At 10 knots, max force is 50 pounds
• At 15 knots, max force is 112 pounds
• At 20 knots, mas force is 200 pounds

The force scales with area. If the area of the sail doubles to 200 SF, the force doubles. At 20 knots of wind, the force on the rig with a 200 SF sail will therefore be 400 pounds.

--------------

>> Rule of Thumb: A 100-SF sail in 10 knots of wind applies a 50 pound load to the mast. And a 20-knot gust increases the max force or load by a factor of four, to 200 pounds!

>> The capsize torque from that 20-knot gust is 200 pounds of sail load times the distance from the boat's center-of-gravity (CG) to the sail's center-of-effort (CE). For example, the capsize torque is 2,000 foot-pounds if the CG-to-CE distance is 10 feet (200 pounds x 10 feet).

To keep the boat level, you need to generate an equivalent torque in the opposite direction. For the above example, if you sit on the gunwale which is 3 feet from the CG (beam is 6 feet), you need 2,000 ft-lbs / 3 ft = 666 pounds of ballast on the windward rail to compensate for a 20-knot puff. Or, you can luff the sail. Conclusion: Never cleat the sheet in gusty winds.

3. Calculate bending of mast about the partner due to sail's wind load

>> Deflection, D in inches, of a cantilever beam fixed on one end is

        D = P x (L**3) / (3 x E x I)

where

P = applied force, the sail pressure, in pound per square inch
L = distance between the sail's center-of-effort and the mast partner, in inches
E = Young's modulus of mast material in psi
I = moment of inertia of the mast in inches**4

Notice that the mast deflection is proportional to the applied load, P -- if you double the sail pressure, then the mast deflection doubles. Recall that sail pressure increases by the square of wind speed; if the wind speed doubles the sail pressure increases by a factor of four. Therefore the mast deflection (and the stress, until the mast begins to break) at 20 knots will be 4 times the deflection at 10 knots.

Also note that the deflection, D, scales with the cube of L (that is, L x L x L), where L is the height of the center-of-effort above the mast partner. If you can change just one variable, this is the one that makes the biggest difference. Double the sail CE height, and the deflection increases 8X. In the other direction, a small change in CE by lowering the sail CE a bit will result in a huge reduction in D.

If the mast is twice as stiff ("E" times "I" doubles), the mast deflection decreases by half. The moment of inertia ("I") will increase if the mast cross section increases. "Less bendy" wood will have a higher Young's modulus and increase the "E x I" (oak is more flexible than Douglas fir and has a low Young's modulus; pine is lower than oak. The Gougeon book, in Appendix B, has a list of Young's moduli for other woods).

To determine the mast deflection, we first need to determine how high the sail's center-of-effort is above the mast partner. For example, assume the mast is 15-16 feet long and the partner is two feet above the bottom end of mast. For a typical 100 SF lug or gaff sail, the center-of-effort would be about 5 feet above the foot of the sail. If we place the boom gooseneck two feet above the partner, then d, the distance between the mast partner and the sail's center-of-effort, is 5 + 2 = is 7 feet or 84 inches. Two feet below the partner into the step; top the sail is about 14 feet above the step. The top of the mast is one to two feet above the gaff at full sail lift.

We need to know what kind of wood the mast is made from; this allows us to determine the Young's modulus (or modulus of elasticity) of the wood:

• white pine = 0.99-1.24 x 10*6 psi (Gougeon, Appendix B)
• Sitka spruce = 1.23-1.57 x 10*6 psi (Gougeon)
• oak = 1.6 x 10*6 psi
• Douglas fir = 1.56-1.95 x 10*6 psi (Gougeon)

Finally, we need to know the diameter of the mast. An an example, determine the the deflection of a solid (not hollow) mast made with a constant diameter of 3 inches. Let us assume that the mast is made from well-dried Douglas fir so that the Young's modulus is at the high limit, 1.95 x 10*6 psi.

The moment of inertia, I (with dimension of inches to the 4th power), for a round beam is given by

        I = 3.14 x (d**4) / 64

where d is the diameter (in inches) of the mast. For our example 3-inch mast,

        I = 3.14 x (3 x 3 x 3 x 3) / 64

or moment of inertia (I) for the for 3 inch mast = 3.97 inch**4. From above, the deflection, D, of the mast (in inches) with an applied load (in pounds) from the wind in the sail is

        D = P x (L**3) / 3 x E x I

For example, for the 3-inch well-dried Douglas fir mast that's 15 feet long with the CE at 7 feet,

        D (inches) = P x (84 x 84 x 84) / (3 x 1950000 x 3.97), or
        D (inches) = 0.0255 x P (pounds)

At a wind speed of 5 knots, the max pressure of a 100 SF sail is 12.5 pounds and the 3-inch fir mast will deflect 0.32 inches. At a wind speed of 10 knots, the pressure will increase by a factor of 4 to 50 pounds, and the mast deflection will increase to 1.28 inches. And at a wind speed of 20 knots, the mast deflection -- if the sail is not reefed or luffed -- will increase to 5.10 inches. Whoa! Look at that! Is my mast going to break?!?

Note 1: If you want to look at other loads or mast dimensions (e.g., diameter, or height of CE above the partner) use the link to "Beam Deflection Calculator -- Solid Round Beam", http://www.engineering.com/calculators/beams.htm. Be aware that the Young's modulus they are using for "wood" is lower than the value for Douglas fir.

Note 2: If you want to look at a solid square mast section, the moment of inertia is

        I = (b**4)/12 where b = the dimension of the side in inches

Note 3: For a solid rectangular cross section, there are two moments for the narrow (x) and long (y) dimensions

        Ix = b * (h**3)/12 and Iy = h * (b**3)/12

where "b" = short width and "h" = long width (height) of the rectangular beam

Note 4: For these or for hollow sections, use the Beam Deflection Calculator link above.

4. Calculate the stress in the mast at the partner -- Will it break?

The stress, S (in pounds per square inch), at the mast partner is given by

        S = M x c / I

where "M" is the torque moment (in inch-pounds) due to the sail pressure on the mast; "c" is the distance from "neutral axis", or center of the mast, to the "extreme fiber", which is the outside edge of the mast. And "I" is the moment of inertia, the same that we calculated above.

The torque moment is simply the sail load, P (in pounds), times the distance (in inches) from the sail's CE to the mast partner, L; or M = P x L. So that the stress in the mast at the partner due to the sail pressure is given by

        S = (P x L x c) / I

Note that the stress is proportional to the sail pressure; when sail pressure doubles, and stress doubles. Stress is also proportional to the distance above the mast partner; lowering the center of the sail will reduce the stress at the partner. The moment of inertia has the dimension of the mast "c" in it, so reducing the diameter reduces the moment of inertia faster, and has the effect of significantly increasing S.

For a wind speed of 10 knots, the pressure on mast from the 100-SF sail is 50 pounds. We assumed the center-to-partner distance of L = 84 inches. And for a 3-inch mast "c" = 1.5 inches. The moment of inertia, I, of a 3-inch solid round mast is 3.97 inch**4; and therefore the stress in the mast at the partner is

        S (at 10 knots) = (50 x 84 x 1.5) / 3.97 = 1,587 pounds per square inch.

From "Gougeon Brothers on Boat Construction" (Appendix B), the breaking strength of Douglas fir is listed at 5,700-10,200 pounds per square inch. So we can conclude that at a wind speed of 10 knots, the stress on the 3-inch mast at the partner (for a 100 SF sail) is well below the breaking strength of fir.

However, with a gust of 20 knots the sail pressure increases to 200 pounds. In this case the stress on the partner is significantly bigger, S = 6,350 pounds per square inch, which exceeds the lower bound of breaking strength of fir -- and the mast may fail. Maybe well-dried, knot-free fir, with breaking strength at the upper limit, will survive the 20 knot gust. Of course, in the real world, there are a number of ways that the actual pressure on the mast could end up being less (e.g., a lug sail that will twist off in a puff, or we luff the sail, or bear up).

At this wind speed, luffing, reefing, using a tapered mast (to "twist off" wind pressure), adding shrouds and/or backstays or another similar strategy to insure mast and rig survival will be required.

5. Bonus -- What happens if the mast is made of aluminum tubing?

For a windspeed of 10 knots, the pressure on mast from the 100-SF sail is 50 pounds. We assumed the sail has a center-to-partner distance of L = 84 inches. We will stick with the 3-inch diameter mast and choose aluminum tubing with a wall thickness of 0.125 inches. Using the stress calculator (link shown above), the stress in the hollow aluminum mast at the mast partner will be

        S (at 10 knots) = 5,390 pounds per square inch

and the mast deflection (at 10 knots) is 0.80 inch -- that is, barely noticeable.

Since the ultimate strength of 6061-T6 aluminum is listed as 35,000 pounds per square inch, which is about seven times the maximum stress on the mast at 10 knots of breeze, there should be little concern about mast failure. For a gust of 20 knots, the stress will increase to 21,500 pounds per square inch, which is still in the safe region for aluminum. However, with a deflection is 3.2 inches, the rig will begin to "twist off" -- or at the very least it will look like it is going to fail -- which will hopefully be recognized by the crew.

References:

Jim Michalak, Boat Designs Newsletter, www.jimsboats.com/webarchives/date.htm

• "Sail Rig Spars", 9/1/1998 
• "Hollow Spars", 2/1/2000
• "Ballast Calculations 3", 1/1/2006 -- wind speed to sail pressure

Meade Gougeon, "Gougeon Brothers on Boat Building," 5th Edition (McKay Press, 2005)

Small Catboat Rigs

Duckworks BBS has a new lightweight 10-foot camp-cruise kit dinghy called "Scout" that's now in development. They have designed a sporty 70-SF square-headed, full-batten, loose-footed cat rig for it, produced by Neil Pryde (duckworks-boat-builders-supply.mybigcommerce.com/scout-sailing-package/). It is sleeved to fit onto a two-part windsurfer mast (chinooksailing.com). The price for the rig is listed at $1,050. I'm guessing that just the sail will sell for around $500, without the battens. 

I saw the boat in the photo below a few years ago at CBMM's Labor Day auction. The mast is aluminum. And instead of the usual aluminum tube boom, the boat has a wishbone boom, which looks like it's directly off a windsurfer. 

It also had a very interesting method of attaching the the wishbone boom to the mast. The wishbone uses some jib T-track and a pinstop shackle instead of a the usual sprit rig's snotter. Presumably it carries a sleeved sail with a cutout for the sail track. Fascinating...

The clew end of the wishbone was supported with a light line to a block at the top of the mast. The multi-part sheet suggests a large-area sail, but it looks like overkill. The foot of the sail must cleat off at the foot of the mast -- a la Hobie Wave -- where the boom lift was cleated. 

Today, in spite of searching the internet, I can't determine the maker or the model of this boat. Is is a shrunken "Naiad 18," which was designed by Mark Ellis of the iconic Nonsuch yachts (nonsuch.org), who was a strong proponent of wishbone booms? Maybe it's a Puffin (www.puffinboatcompany.com)? Maybe it's something off the board of St. Michaels' most famous designer, Jay Benford, who penned Annie Hill's world-cruiser, "Badger" (anniehill.blogspot.com). Maybe it's a one-off. At the time I saw it I was only interested in the rig, and there were too many people crowded around to investigate more closely or get a full-hull shot. I took these photos, and then it was "sold!" and gone. 

A crisp sail. Beautiful foils. Harken hardware. Shaw & Tenney oars. The whole shebang probably went for a few hundred dollars...somebody went home with a great deal.

I'm going to duplicate this wishbone rig on a small boat someday just to see how it works. And if you know more about this particular boat and design, please contact me.

[photos in 2014/15 archive, CBMM auction 2015]

Mini-Trawler Conversion -- Part 6

Hardtop Dodger

Shade and shelter are now required features on all my boats, and the mini-trawler was designed with a hard dodger to extend the cabin space. In the original build plan each of the cabin side panels was cut from a sheet of plywood, which would keep things simple, reduce the number of parts, and eliminate several epoxy/cleat steps. The practical problem that I needed to address once I started was how to build inside the shop, and then get the boat out -- the garage door opening provided only seven feet of vertical clearance. The build plan changed almost immediately and the dodger became an add-on.

In retrospect, that was a good decision. Keeping the boat on the trailer, rather than building on a floor frame, made it very convenient to dolly the boat around as the work progressed. And I preferred to keep the boat inside the shop for protection from the weather until I could get the transom finished and all the new plywood painted and protected. I briefly considered several variations of removable tops, and then started researching and sketching versions of “pop tops,” common on some of the smaller sailboats (like the one on our old Hunter 25.5). 

I ended up with an unusual variation on a pop-top: mine has only one set of folding supports, and there is a second set of fixed supports that double as the dodger sides. The advantage of a pop-top design, in addition to being able to get the boat in and out of a normal garage door, is that the windage with the boat on the trailer would be significantly reduced. And if the wind out on anchor turns up, it might be nice to be able to literally “batten down the dodger.” Furthermore, dropping the top would also make tarping the boat over the winter months (or during any extended storage) much simpler and easier. 

My pop-up dodger was designed in "3-D", not on a computer or on paper, as shown in the photo below. It uses two oak struts to support the front of the hardtop, and two vertical panels from ¼-inch plywood to support the back once the top is up. The hardtop would be supported by the wedge-shaped crosspieces, which would be secured to the rear vertical stiffener panels once the top is up. When the stiffeners are removed, for transport mode, the top scissors back and is supported by the top edges of cabin/cockpit sides. In the photo I was still figuring out how things would fit: the top wedge-shaped part in is actually on backwards, front to rear, in this photo. And the horizontal batten attached to the wedge represents the length of the dodger top.

Here’s another view once the design was a bit more developed. The top and the wedge are both still on backwards (sometimes the right design takes a while to come together...) but I was getting close to getting the basic design locked down (the panel attached to the cabin side is a mock-up for a cabin window).

And here is the final version, with the top wedges with the correct orientation.
The hard dodger top itself was a repurposed panel from an earlier project boat (the electric catamaran that didn’t make it in prime time and was dismantled). The front supports are bolted to the top and to cabin sides. The wide aft panels are removable. The grey rear supports are hinged at the aft crosspiece on the top, and they can swing up when the top is stowed. With the top up, the bottom ends are attached to the cabin sides, and they can fully support the top without the wide panels. But the wide panels add a lot more stiffness and strength to the structure, and give some protection from wind and spray. They give a great feeling of security when standing in the cockpit because you are surrounded by structure on three sides.

The repurposed dodger top was big enough to cover the cockpit, and it was nicely cambered to stiffen it and to shed water. As an experiment to save epoxy (and exposure to epoxy for large-area coating jobs), I had tried painting the 1/4-inch marine plywood panel, top and bottom, with a couple of coats of Titebond III before it was primed and painted. But after only a few years of exposure to sun and snow, the marine plywood had begun to check. My conclusion: Titebond is not an adequate substitute for epoxy as a sealer, at least not on my boats.

After some heavy sanding to remove the worst of the checking on the dodger top, I used polyester landscape fabric and epoxy to reseal the weather side -- another experiment to reduce costs by using it instead of glass fabric. The polyester fabric was cheap, but it was too light and rucked up instead of lying flat. I also stiffened up the fore/aft strength by epoxying cleats around the perimeter.

The outboard is one of my good old 1965 Evinrude 6-hp “Fisherman” outboards. The burned-up red gelcoat on the Cutter's topsides was sanded and primed and then roller-painted (no tipping) "work boat grey" -- one can of Rustoleum grey mixed with one can of white. The "window" is another mock-up.

Even with the beefing up and epoxy-fabric coating, the dodger top is still light, easy to lift, and the two oak struts are strong enough to support the dodger until the side panels are attached. With the struts constraining the motion to fore-and-aft, the top is always in control until the side panels are locked on. And because of the built-in camber, it’s also stiff and strong enough to support a couple of small solar panels that I will use for battery charging. 

At the launch ramp it only takes a few minutes to raise or lower the hardtop. There are four screws that secure it. And when it is down, it covers the cockpit and cabin hatchway. It's high enough that I don't have to stoop when I'm standing in the cockpit, and there's enough clearance that stepping into the boat from the dock has not been a problem. However, it will be nice to have some handholds either on the top or the side.

So, you can see that I got the mini-trawler built and painted and down to the Bay by the end of October -- the leaves are still on the trees on the Bush River. No bad, not bad at all. And it didn't leak...