Low-speed sailboat hull

What's wrong with copying dimensions that work?

If you want comfort for hours, you need to design the proportions like a good quality chair, which you aren't. You just have a box at a height which doesn't match any chair ever built. Your pictures show the crew sitting with their knees up, which is going to be hard on the arse bones. Personally I can't see it being all that comfortable anyway. In light winds I think I'd rather be lounging around in the bottom of the boat, using the topsides as a handy backrest. My 2c.

Either that or turn the seats into narrow side decks, where at least they'd be at a more comfortable height. You'd still be able to lounge around in the bottom between them.

Yes, the dagger board box is a problem if the middle of the boat is completely open. Rowing thwart isn't, since it could just drop onto a chock on the topsides.

Why do you want a standing lug? Balanced seems better to me, since it is self-vanging. If you use a standing lug you'll need a vang anyway. Still the only real difference is where you place the tack.

Question: how much experience have you had doing what you want this boat to do?

 

Nothing - except I want to know why and how it works So that I can adjust, even if slightly, to my specific requirements without losing what makes that shape so good.

Good point. Maybe I'll go with sitting on bottom of the boat, then. Would also solve the boom-headache problem.

So, where should I place the daggerboard box then? And how would I support it? Any suggestion?

Well,
1. Standing lug can be dipped (by lowering yard a bit, rotating to vertical, and raising back on the other side of the mast) on long tacks. Would take half a minute, but would improve windward performance;
2. Bad tack is less bad on standing lug than on balanced one, because less of the sail is in front of the mast;
3. Can be rigged boomless if that is needed for any reason;
4. Sets better than balanced lug (in terms of leech tension);
5. Todd Bradshaw said that standing lug absorbs gusts better, because being non-self-vanging, it allows sail to twist when needed. And we have a lot of gusts in where I sail;
6. Easier to rig;
7. Allows higher peaked yard (better windward performance);
8. Emiliano Marino (author of "Sailmaker's Apprentice" favors it over balanced one, so it must be true.

Good question. As a matter of fact, I sailed this summer with balanced lug rigged flatbottom fishing boat with my girlfriend. So I had some experience with balanced lug. Due to horrible hull shape and poorly made, undersized sail boat was extremely unweatherly.
And we sat on the bottom of the boat. Our backs weren't thanking us after some 4 hours of sailing

 

Basically, higher peak reduces twist, as long as you have enough yard in front of the mast to support it.

I know where I'd put it, but you wouldn't like it (in the middle ).

I'd dispute 4. I'd say without a vang it will be worse, since that applies to every other type of sail.

5 is a bonkers way of dealing with gusts. Use yard flex instead, and you'll have a sail that sets properly in the lulls too. Beats me why anyone would use a sail that had no vanging capability. It's just asking for poor behaviour, particularly once the sheets are eased a bit. It can be deadly on a run in a breeze.

6 is not true, since both types really need a downhaul for the boom/luff, and if you want it to set right you'll then need to add a vang to the standing lug.

7 isn't true either. See GIS and OzRacer sails.

I bet your backs wouldn't be thanking you if you'd spent the same time sitting on a low box with your knees in the air either. Closed-cell foam cushions are great.

 

You say I wouldn't, but by the time you wrote this, I just finished alternative design, without seats, and... Guess what

http://static.dyp.im/H4Dm8K5CWe/ad87ecbc525527e5f511e44c5025f82d.JPG

Weights only 19kg. So I could actually scale it back up to like 3.5m. But maybe I should add something to strengthen the structure. The middle section looks like it's missing something.

My idea is that sailors would sit around this box. One in front of it, one behind. I'll might need to move box around until it's possible and doesn't screw up balance.

How does this look?



As for the lug, well, nothing forbids me using boom vang when required. Just one more line.

If I understand it correctly, it's not so much about how much of yard is in front of the mast, but how much of is in front of the halyard attachment point, right? I mean, yard is lashed to the mast very loosely anyway. It pivots around the point where halyard is attached anyway, not the mast. True?

Sharpii2 said not to exceed 45° yard angle. He says that if I do, there will be problems with leech tension. What do you think about that in standing lug?

 

Yes, it's about the forces around the halyard attachment point. You need enough yard in front of that to give you the leech tension you require. There are high-peaked sails around that demonstrate Sharpii2's rule isn't absolute. The Solent lug is the ultimate example of this.

You could always just put the board out against the topsides, following their angle. The boat should be sailed fairly flat anyway, so having the board tilted to one side a bit shouldn't noticeably affect leeway. The loss from a 25 degree tilt is only about 10% of side force, so you might make an extra half a degree or so of leeway on the wrong tack.

 

Now that is quite a peak. Well, you see, this summer, when I rigged that fishing boat I mentioned, I had a very bad leech sag. I tensioned the yard as much as I could, as well as downhaul, but it just wouldn't set straight enough. Sail bagged when going upwind in a breeze, and there is nothing I could do.

I'm afraid of that happening again. This is that sail.

And I really don't understand what influences twist and leech tension in lug rig. Different people tell different stories.

Yeah, I thought about that. Somehow my instinct says not to do it. What about a centerboard box like in the screenshot in my previous post? It seems not so terrible to me.
Only that something is missing in this boat, and I can't tell what it is...

 

What's missing in your seat-less version are side decks. These should be about 12-15 cm across, following the curve of the boat. These can easily be cut out of plywood in either single pieces, or smaller pieces scarfed or butted together. These side decks should come with two or three pairs of side frames. These side frames can be as wide as the side decks, on top, and taper to a point, just before they hit the chine. These can be made out of plywood.

Another alternative is an outside gunwale of, say 8 cm deep, held out from the side of the boat by a bunch of evenly spaced wooden blocks that are about 2 cm or more thick.

This might be the more comfortable solution for sailing in light winds, as you will have the smooth sides of the boat to rest your backs against. These outside gunwales also make handy handles for lifting and carrying the boat. These outside-gunwales should best be made out of timber, rather than plywood.

Keep in mind that, if you use either one of these solutions, all the rigidity of your hull is going to come from: the bow and stern air-boxes, the outer-gunwales, or side decks, and, to a lesser extent, the bracing for the Dagger Board case.

If you go with the outer-gunwale proposal, you will need temporary frames to control the curve of the tops of your sides, until the outer-gunwale is installed. (see attachment)

If any water comes aboard, you will be sitting in it. For me, this causes a very irritating itch. This is why all my designs have either a false bottom or grating of some kind.

 

I'm not keen on relying on sending your boom skywards to spill wind. The problems are that first, it will mean an inefficient set whenever the sheets are eased at all, which you probably won't want 99% of the time and second, that running in a blow without a vang is, as I said before, rather deadly. It's a great way to get the death rolls or an accidental gybe, or both, which can be a great way to go swimming. It's better to spill wind by letting the yard flex under extreme loads.

 

I think side decks are a must in this kind of boat without them if you heel your boat a bit too much you will flood it in no time.

 

laukejas,

the prismatic coefficient is a mathematical calculation to describe the boat, but says nothing about the rate, if you calculate the cefficiente prismatic central hull of a trimaran will be a very high value, while being very fast.
To have a fast boat with a light wind must:
1) that the outputs of the stern are strained and always clean, even with a full load;
2) that the sail is very well designed and built;
3) that the ratio of the sail area and the wetted surface is very high;
4) that the sailing center is properly calibrated to use the minimum angle of the rudder;
5) that the main section is a semicircle.

 

Sharpii2, of these two options you provided I decided to go with side decks. It may be a bit on a heavier side, but I think it would accomplish two things: provide structural integrity AND allow seating on the floor in low winds, with option to sit on these decks in high winds.

But I didn't manage to get them to be 12-15cm wide due to the hull shape. I might modify the hull, increase the angle of side panels, so that sheer line and deck line are further apart horizontally. Right now, these decks are 8.7cm at widest point, and 4cm on narrowest. Too narrow to sit on, I imagine.

Here are two screenshots, second one is with side panel removed for better look of side frames. Hope I got the number right.

http://static.dyp.im/1UELO2anfT/998421be337656c2c731c3b37e79eaff.JPG

http://static.dyp.im/VYcQlcNQSv/80a942ffc68857e020b5b66f3a86ca82.JPG

I will add holes in each of the frames later for better ventilation of the whole compartment.

Weight now reads 25kg dry. Still within limit.



Thank you all for clarifications on lug sail. I will work on it next, try to squeeze out as much sail area as possible with yard set high. I won't be racing, but since 90% of the time I end up beating upwind, it would be good to have better performance there.


NoEyeDeer, I understand the dangers you describe. That's why I think I could rig a boom vang, and use it whenever necessary (especially in gust conditions) so that the boat and it's crew wouldn't have to experience that which you warn me about.

 

I did what I mentioned earlier. Increased horizontal distance between chine and sheer, and thus the side decks are 13.5cm wide at widest point (just where they connect to aft deck), and 8.5cm at narrowest point, where they meet bow deck. Is that enough, or do I increase the width more?

A screenshot:

http://static.dyp.im/vRKS6bRRcJ/70c3fabc627d4f7b71cf483a60ac1a8c.JPG


By the way, I suddenly started worrying whenever daggerboard case won't spill water when boat is fully loaded. It turned out that it would, so I raised it up a bit. Now the top of the case is about 5cm above waterline at theoretical maximum load (210kg). Is that enough, or should I make it even higher?

http://static.dyp.im/CCskQAX1js/15c7882bbf83e95e06d0aacb0bdcc59f.JPG

 

Should be fine, but do you need to box in under the decks? That stern tank is massive. What are the volumes of the bow and stern tanks?

Oh and the side decks should be a fair continuation of the fore and aft decks. You don't want that step there.

 

Well, that vertical wall under side decks is not so much for buoyancy, but for comfortable surface to rest our backs against.

Aft tank is about 50 liters, bow tank is around 30.


I will try to filet that joint between decks. not very easy, since aft and fore decks are cambered.

 

So camber your side decks. That's how it's usually done.

Are you going to be carrying oars? If so, where are you going to store them?