Little flattieboat with twin skin sprit sail

[Dieter51]

Hi, I would like to do a little diskt about a flat bottom boat made of Paulownia wood with a Twin Skin Ram Air Sprit Sail (TSRASS - haha).

Below is a picture of the boat on the Ammersee near Munich, Germany where I sail. The boat does not sit on the rocks, it weighs only 40 kg at about 4,4 m length. Everything, including the mast, boom, sprit and daggerboard are made of 1.8 cm thick glue laminated paulownia boards. The planks are 1.8 cm thick, the two frames are each 5.4 cm thick, as is the coaming made of paulownia and placed on the outside of the planks. A friend of mine, a sailor, is good at woodwork and was able to do the woodwork with Paulownia (KiriKiri). He did a good job.

(Sorry if some terms are wrong, they are automatic translations from German using DeepL.)

The boat sails like a flat-bottomed boat, noisy in light winds and waves, brisk in lighter waves. Sail area is about 6.1 sqm after some modifications. The sail has two sides, runs around the mast. The leech and underside are closed with Velcro, the area from the top of the mast to the top of the sprit is open and serves as a ram-air opening.

The sprit sits between the halves of the sail, as does the boom. The point: I used to be a hang glider pilot in the 70s. Early hang glider sails were rigged with an exposed cross bar under the sail (sprit), glide performance was poor. With the crossbar in the sail, performance immediately improved.

I wanted this effect for the sprit sail as well, because it could be that the saying should not be, the sprit runs equally well on each side, but, it runs equally poorly on each side (windward or leeward).

Whether it is so, I do not know, because I have no comparison boat, but it was a lot of work, to get this sail to run. In the beginning, the leeward side just hung down like loose cloth, even with a lot of wind. No differential pressure built up. Now, with the Ram-Air cut, it works. With little wind the sail is double convex and only with more wind concave (windward) and convex (leeward).

So much for boat and sail. What is bothering me at the moment:

After capsizing in a lot of wind, I have righted the boat four times and have not managed to bail it empty. If I move just a little from the center, it runs completely full over the sides or over the bow or over the stern. That it would be so bad I had calculated, but not really expected. I have already full wooden boats bailed out with the bucket, but this was bad or I simply became too old by now, 72.

Since I like the lines of this boat because it's not chopped to pieces by transoms and watertight compartments, I don't want to retrofit something like that either. I'm thinking more of a 15 cm diameter foam roller that sits over 3.2 m in length in the middle section of the boat outside under coaming and would deliver about 50 kgf each when fully submerged, which should reduce wobbling when full.

In any case, I do not want to capsize in the fall in the current state of construction. I tried a pump yesterday, it pumps strong like a good garden hose, but you would need 6 of them to get around 950 liters of water out of the boat quickly.
Below the waterlines of the boat solo, with me (85 kg), with 2 people, boat after capsizing and righting over the daggerboard (not shown :-().

Has anyone ever tried such foam rollers in canvas outside on such a boat?

 

[messabout]

Pretty little boat. I like it. The sheer clamps are too big but aside from that it is one I could learn to love. It appears to be a wee bit narrow for boats of its type but that should not be a problem for experienced sailors.

The sail might very well be the problem with the capsize bit. The spars are brutally heavy and the two sided sail is problematic. Get yourself a conventional (one sided) 6 meter sprit boom or peak boom sail. The capsize problem will almost surely be mitigated if you reduce the weight of any and all the spars. The vertical spar on the double sail is going to be partly responsible for your swimming sessions. The picture shows a mast that is not, but should be, tapered toward the top. Drastically reduce the aloft weight and you will have more fun. Outriggers of any sort would spoil the honest lines of such a pretty little boat.

P.S. the conventional sprit boomed rig will drive the boat as well or better than what you have.

 

[rnlock]

Sheer clamps? I'd call them gunwales. Considering that they're made out of paulownia, which is very light, that for most of the hull's length they're not supported by frames or bulkheads, and that the stays are fastened to them, I'd guess that they were just big enough.
I had a Bolger Nymph once, a project I took over from someone else. I thought the frames would be in the way. For a gunwale, if I remember correctly, I used two pieces of wood (of unknown provenance) that were 3/4 inch square, capped with one that was something like 1-1/2 (i.e. 0,1,4) by 3/4. This had the rigidity of a significantly heavier gunwale, because there was a bit more material where it was needed, and a bit less where it wasn't. The interior was unobstructed with those frames removed. I glassed it, so I might have used it for a bathtub! (but I didn't) Anyway, I think Dieter has done essentially the same thing here.

The boom certainly looks oversized, even in paulownia. I once had a boat with a hollow, bamboo sprit boom. It was so light it didn't hurt if the sail was luffing and I got my head close enough for it to hit. It seems to me that the mast is somewhat oversized, since it's held up by stays. Tapering wouldn't hurt here either. Also that vertical sprit, or whatever you want to call it. The top of the Bolger Brick's 15' 9-1/2" long mast is only 3/4 of an inch in diameter, and it's 2-1/4 at the partners. It was very easy to carry around. Mine held up very well, though I admit I didn't measure the builder's work. The Brick has a tremendous righting moment, it's 4 feet wide, and if you measured the prismatic coefficient sideways, it would be almost 1. (It would be 1 if the chine logs were on the inside.) Obviously, a paulownia mast for the same boat would have to be a little larger, maybe by 30 percent to match douglas fir. OTOH, if this mast was bendy, it would probably bend the wrong way in high winds, so it probably shouldn't be TOO bendy. The Brick's mast bent at the top in wind gusts, flattening the sail and relieving some of the heeling moment. That's bending the right way. Paulownia is supposed to be pretty soft, so if it was my boat I'd be looking for areas subject to mechanical wear and thinking about covering those specific areas with something harder, whether a wood veneer or some glass and epoxy. I think wood veneer would be much classier.

I'm curious about how that vertical sprit is rigged. I'll make no guesses as to whether this sprit rig is better than a conventional sprit rig, I think I'd have to see it in action. Certainly the mast and the sprits create a fair amount of drag, as do the stays. Looking at that eyebolt at the top of the mast, cantilevered out into space, it's either too heavy or too weak. A bent metal strap with a few holes might be better.

I think the only paulownia I've seen was in old propellers for rubber powered airplanes. It must really help when building light boats.

I've seen the use of buoyant foam, or something like that, on the outside of the gunwales, but I can't remember where. I think it would have a rubber boat look. (I.e. like a Zodiac or other inflatable.) I also think it would work. Dieter, when you're on the water, do you really go forward of the mast or within a couple of feet of the transom? If not, you could lash some buoyancy bags there. When I was in college, the Outing Club attached closed cell foam in the sides of their canoes. On the inside. I've seen closed cell foam that was actually soft and comfortable to lean on. It was being sold for backpackers to sleep on. But that's probably a little expensive. Possibly there are cheaper ways to get it. I imagine something like this could keep the boat high enough to avoid taking on much water. Nevertheless, I'm sure foam on the outside would also work.

 

[Dieter51]

Pretty little boat. I like it. The sheer clamps are too big but aside from that it is one I could learn to love. It appears to be a wee bit narrow for boats of its type but that should not be a problem for experienced sailors.

The sail might very well be the problem with the capsize bit. The spars are brutally heavy and the two sided sail is problematic. Get yourself a conventional (one sided) 6 meter sprit boom or peak boom sail. The capsize problem will almost surely be mitigated if you reduce the weight of any and all the spars. The vertical spar on the double sail is going to be partly responsible for your swimming sessions. The picture shows a mast that is not, but should be, tapered toward the top. Drastically reduce the aloft weight and you will have more fun. Outriggers of any sort would spoil the honest lines of such a pretty little boat.

P.S. the conventional sprit boomed rig will drive the boat as well or better than what you have.

 

[Dieter51]

Thanks for the answers, I may answer them both together? First a word about the paulownia:

Below is a picture of the test of my "woodworm": 1.2 m long, 0.06 m wide, 0.018 m thick, weight 50 kg. And holds! The wood is soft, but still enormously stiff. It does not rot in water, does not need painting, just a little oil now and then. We took a very expensive oil, which also makes a very rough surface, inside. Unfortunately, the wood panels from the hardware store were glued only D3, so had to put another layer of glass and epoxy on the outside, I really wanted to avoid that.

Overall, I was also concerned with not having to use plywood. That has become very expensive if it's good, and despite it always starts to delaminate after a while if the boats are left outside in the winter. Even the pro boats when they get damp and hot under the tarpaulin in the spring.

The rigging of our boat is relatively light despite the double canvas (170 gr/m^2). The spar weighs maybe 0.35 kg, the mast can be put on the boat with one hand, the boom is also quite light. Overall, the rig should not be heavier than that of a Laser, which I also sailed for some time.

However, it is built very thick because for the new sail I wanted to avoid all bending under load so you know the cause of a problem is with the sail and not the bending. With the weight of the rig, I also don't see the cause of the capsize, but with the narrow boat, a typical unfavorable stability curve of such boats and with the unfamiliar sail.

I am used to hearing the wind strength from the fluttering of the sail in the tack. But this sail doesn't flutter, zero, nada, it just turns completely with the mast and boom like a weather tack on the steeple - a wing, in fact. And when you tighten the sheet, you know how strong the wind currently is. And of course you can see it on the water if you look attentively into the wind direction, of course.

I was looking at the GPS and at my sheet and rudder after a tack when the boat picked up speed very quickly and showed a little over 7 knots. That was also the case before, no problem. But then came a boe that threw me into the sail that was on the water in front of me. Not a single boe was that strong before, they had announced 3-4 bf, with boes around 6 bf. But I hadn't experienced those.

The capsize itself wasn't bad either, such a narrow, light boat that drifts habfully on its side and has no tendency to capsize is actually harmless. It's easy to get on the centerboard and slowly straighten it out, with the bow turning into the wind. And since the boat is very tidy, it's easy to transfer to the inside while doing so, That's where previous planing dinghies of mine were much more critical.

But all this is of no use if the boat is full and every second wants to capsize again to the side or to the front or to the back when you move from the center. I did not know that so and had not expected it. But some builders of flat-bottomed boats made of plywood must have experienced this. Or do they all have large buoyancy tanks? Our boat behaves like a GIS (youtube) without such tanks, but is much smaller and much, much lighter.

So my question is how others have solved this problem.

I have now found capsize tubes that are 3.6 m long and 0.2 m in diameter. A bit too much, but 100 "kg" of buoyancy per side if I strap those under the gunwalls(?) might help for now.

I righted the boat a total of 4x after I capsized, tried to bail from the inside and outside with a bucket, each time it immediately tipped over again. I was exhausted after an hour of trying. And it wasn't until later that two SUPS and a diving boat with oars came out and helped. The SUPs stabilized on the sides, the dinghy rowed the three of us together to the beach. Thanks!

But what if I was out on my own, as usual? In the fall, when the water is 14 and not 24 degrees. That's possible when you're 30 years old, but not when you're over 70. That's why I'm looking for solutions to this problem that I may not have come up with yet.

And by the way I wanted to show boat and sail, because maybe someone else wants to build a boat from Paulownia. I think it is the building material of the future for self builders. I sewed some dinghies in the 70's from plywood with copper wire and glued with polyester and glass. This is gluing paulownia without sewing much better. I would even plank the hull crosswise next time, in strips and with PU glue, because it will be even stiffer. The way it was done with classic wood a long time ago.

I would keep my hands off the twin skin sail, because it is too much work and I have to lay the boat on its side to be able to set and tension it properly. This is not a problem on my lawn, but it is a bit stupid.
Greetings - Dieter

 

[Dieter51]

Sorry, I forgot to write that the solid but lightweight gunwales of paulownia are the minimum, since the planks are, after all, only supported front and back. You can feel the plank slightly work in the wave and wind gusts, but together with the epoxy and glass over the chine it is well tolerable.

Overall, you have to rethink a lot with paulownia, you can not put large point loads. But: A Paulownia planked boat is finished when it's put together. With a boat made of foam, the work would just begin - and I don't want to do it.

 

[rnlock]

Dieter,
I was just at the library today, and saw a bit on the Derby Dory, which has flotation placed on the gunwales, or just outside them. The piece included some photos of a guy with the dory upside down, flipping it back upright, and climbing in. It's a chapter in Still More Good Boats by Roger Taylor, published by International Marine. The Derby Dory was designed by Garry Hoyt and Robert Henry. I'm not sure it's worth an extensive search, though. I was only flipping through and didn't read much of the article.

 

[Dieter51]

Dieter,
I was just at the library today, and saw a bit on the Derby Dory, which has flotation placed on the gunwales, or just outside them. The piece included some photos of a guy with the dory upside down, flipping it back upright, and climbing in. It's a chapter in Still More Good Boats by Roger Taylor, published by International Marine. The Derby Dory was designed by Garry Hoyt and Robert Henry. I'm not sure it's worth an extensive search, though. I was only flipping through and didn't read much of the article.

Thanks for the tip, I will try to find the photos and will show my own solution next week. But I'm afraid the blue capsize tubes on the Gunwales won't look very pretty. But if they don't interfere with sailing and eliminate the problem for now for this year, then it would be ok.

Greetings - Dieter

 

[sharpii2]

I think your flotation tube idea will work wonders. They will.pretty much keep the boat from fully swamping from a capsize.

This is because the buoyancy on the low side will force much of the hull out of the water. As an added benifit, they may well prevent a complete turtling.

This is because water on the low side may cascade over the flotation tube on that side then pour into the bottom of the boat, where it will act as somewhat swishy ballast. If enough comes on board, it may actually support much of your body weight while no where near filling up the boat.

The trick to bailing this water out is to sit closer to the down wind side of your boat an hurl the water over that side. You want the up wind side higher, because that is where the waves are likely to be coming from.

 

[Dieter51]

I think your flotation tube idea will work wonders. They will.pretty much keep the boat from fully swamping from a capsize.

This is because the buoyancy on the low side will force much of the hull out of the water. As an added benifit, they may well prevent a complete turtling.

This is because water on the low side may cascade over the flotation tube on that side then pour into the bottom of the boat, where it will act as somewhat swishy ballast. If enough comes on board, it may actually support much of your body weight while no where near filling up the boat.

The trick to bailing this water out is to sit closer to the down wind side of your boat an hurl the water over that side. You want the up wind side higher, because that is where the waves are likely to be coming from.

Thanks for the hope and the recommendations. I will show a photo and report my experience after a test. Maybe someone has the same problem.

 

[rnlock]

I was just looking through Chapelle's American Small Sailing Craft. I don't think I can tell the difference between a sharpie and a flattie. Does that mean I have a tin eye*? ;-)






*FYI, having a "tin ear" means someone can't distinguish musical tones very well.

 

[Squidly-Diddly]

I'd install a bunch of these along entire gunnel, running a 3/4" I.D.,7/8" O.D. PVC pipe through the hole to give some structure, then a rope through the pipe for safety because the pipe might shatter. Secure at rear of gunnel and bow and one or two mid points on gunnel. "Secure" should include attachment of inner most Safety Rope. Maybe wrap the Noodles with a spiral of rope in addition or instead.
I think full length floats on gunnels would look better and could defect waves at the bow.

I'd suggest some sort of flotation at the bow and stern to keep things on an even keel if swapped. Ice chest and/or seat tied in place.

Not related but IMO someone should make a low-pressure, high volume foot operated bilge pump. Just some sort of box with a slightly weighted intake hose (weighted so it finds and stays at a low point) and a discharge hose with a big "U" end to hang over the side. It would be just a box with pedal that could be operated either standing or sitting with box up against a backing wall or if the box was tied in place to resist foot pressure. The idea would be that it would be hands free and your foot (really leg) will have a lot more power and stamina that any hand (arm) powered unit. A foot powered unit would probably work best with a Return Spring rather than be "dual action" like hand pump, and rather than attempt to make Dual Action Foot Power with bicycle style Rat Traps, because of danger of getting trapped and drowning. Foot-powered Bilge Pump for Kayaking ~ SeaKayaker.org https://seakayaker.org/foot-powered-bilge-pump/

 

[Dieter51]

I'd install a bunch of these along entire gunnel, running a 3/4" I.D.,7/8" O.D. PVC pipe through the hole to give some structure, then a rope through the pipe for safety because the pipe might shatter. Secure at rear of gunnel and bow and one or two mid points on gunnel. "Secure" should include attachment of inner most Safety Rope. Maybe wrap the Noodles with a spiral of rope in addition or instead.
I think full length floats on gunnels would look better and could defect waves at the bow.

I'd suggest some sort of flotation at the bow and stern to keep things on an even keel if swapped. Ice chest and/or seat tied in place.

Not related but IMO someone should make a low-pressure, high volume foot operated bilge pump. Just some sort of box with a slightly weighted intake hose (weighted so it finds and stays at a low point) and a discharge hose with a big "U" end to hang over the side. It would be just a box with pedal that could be operated either standing or sitting with box up against a backing wall or if the box was tied in place to resist foot pressure. The idea would be that it would be hands free and your foot (really leg) will have a lot more power and stamina that any hand (arm) powered unit. A foot powered unit would probably work best with a Return Spring rather than be "dual action" like hand pump, and rather than attempt to make Dual Action Foot Power with bicycle style Rat Traps, because of danger of getting trapped and drowning. Foot-powered Bilge Pump for Kayaking ~ SeaKayaker.org https://seakayaker.org/foot-powered-bilge-pump/

Yes, that would be possible and cheap and safe, but the low volume is critical.

In the meantime, my inflatable buoyancy tanks have arrived, but only 30 l volume, each. Which is not enough to make a difference when capsizing and keep the boat min 5 cm higher in the water after capsizing.

In the wintertime I could widen the sides of the boat to the width of the Gunwales at the top and just a little at the bottom, with foam, which I then laminate over in winter. Not very much, but still some work and the boat then nearly has the flare of those well known rowing boats.

To be able to decide, what to do, I ordered two inflatable small beds of Bestways for 15 euros each, I will fold those once crosswise and then directly lash them behind the mast and once crosswise in the stern. Not completely inflated. Then capsize with me and the sail and see if this buoyancy, not reaching over the gunwales, improves things for me. Dirty trial.

Yes, people will laugh, but it doesn't matter, I want to know if I can sail alone in the wind in the fall without having to worry about capsizing. Photo will come.

Thanks for all the replies!
Dieter

 

[Dieter51]

The story goes on. Regrettably, I injured my previously robust knee during a voluntary capsize attempt, making sailing unthinkable. It's still not fully recovered, but hopefully, it will hold up.

The first improvement: I reconfigured the sailing rig. It now operates with an 'open mouth' behind the mast, accommodating the new slender sprits which are affixed to ball sockets. This alteration has improved the sail performance, even though it has become slightly smaller, approximately 5.8 square meters.

The second improvement: I introduced a new, slightly curved thwart to the hull. Now, it is possible to sail while sitting on the thwart rather than just from the hull floor. This modification is effective.

The third improvement: I attached three fixed flotation tubes (or transport rollers) to the hull, providing nearly 180 liters of volume. This capacity is sufficient to empty the boat with a pump within 5 minutes after capsizing. I used the most powerful electric pump I could find, which has a capacity of nearly 10 cubic meters per hour or 3000 gallons per hour. This hasn't been tested yet.

A question has been bothering me: My boat, with a bottom width of only about 1 meter, possesses good initial stability. However, when it leans over, the stability quickly becomes negative. Many boats with a length of about 4.6 meters and a bottom width of only 1 meter have been built. Bolger suggests sailing such boats while sitting on the windward side on the bottom. This works, and so does sailing from the thwart, though the latter is less comfortable than sitting on the sides of, say, a Laser (I owned one a few years ago).

So how do owners of flat-bottom sailboats with a 6 square meter sail area manage? One fact is evident: If two or three people sit at the bottom, the boat sails very stably and calmly even in high winds. If I am alone on the floor, I must ensure to move to the outside of the thwart timely during a gust. At 72 years of age, the necessary agility is waning.

How do others handle it? Such boats are uncommon in Europe, and there are none where I live, so I can't ask locally. I have plenty of sailing experience with slender dinghies, but they all had some form of ultimate stability, which this boat lacks without hiking out far, as anticipated.

 

[Dieter51]

This is, how the Paulownia-Boat now looks with its "new" sail.