Low-speed sailboat hull

or, if you just can not live without the stich and glue, the right section is this

 

Yeah, and these used sails are more expensive than the cost to build this boat. I looked.

So maybe I could just partially raise the centerboard in lower winds? I mean, if centerboard is too big, I can always use part of it. If it's too small, then there is nothing I can do to prevent leeway. Anyway, the recommended value of 3.5-4% of sail area, is it too large in your opinion?

Of course I will capsize, the question is when! The video you've shown is amazing. Almost no water in Duo after righting. I'm wondering if that's because the floatation bags are on the outside, or because of their volume. I'm not sure I understand. Could explain how it works?

Yeah, what is that? I've tried searching, but since I have no idea what you mean...

I've chosen stitch and glue because of availability of materials in my country.

This boat you've shown is closer to U-shape than mine.

 

Yup, one of those ringtails. I did occur to me that you'd get mixed up with the possums too, but as a good Australian I'm into promotion of native fauna.

No. You'd have 14 of those 40 available, because 26 is taken up by material weight. So, available buoyancy is around 66 kg, which is minimal.

It depends what you want to optimise for. If you want to maximise stall resistance for adverse situations, like tacking in ghosting conditions, large area is good. If you want to minimise drag for normal sailing, even in light winds, a smaller area would help.

Required area depends on aspect ratio, as well as section shape. With a good section, and with the moderately high aspect ratio you're using, I think you'd be safe heading down towards 3% of sail area for the board. Incidentally, regarding section shape, years ago I had a brief chat with Julian Bethwaite about the laminar flow NS14 board sections mentioned in High Performance Sailing. His recommendation was to not use an NACA 00xx series. Unfortunately I cannot remember precisely every detail of what he said, so I'm a bit hazy about the forward 10% of the section. I should probably try emailing him and seeing if he'll cough up the relevant details.

NO! Read carefully.

I said IF you INCREASE keel DEPTH. In other words, make the hull deeper. IOW, more draft. More rocker. More depth of keel. This will increase the area of the section. You then reduce it back to where you want it by raising the chine. You do NOT increase beam. Think about the geometry here. Increasing beam will obviously increase wetted surface. This would be a waste of time if the aim is to reduce wetted surface.

OTOH, if you increase the height of the chine while lowering the height of the keel, the length along the deadrise will be very slightly longer (Pythagoras and all that) but you will be able to reduce the amount of immersed topsides. The reduction in immersed topsides area will be greater than the increase in the hypotenuse along the deadrise. The result will be a reduction in wetted surface. You could safely go up to around 10 degrees deadrise without adverse effects.

You should play around with various midship section geometries and compare their immersed girths. It'll give you a better understanding of some of the basic principles here.

Hang on. Tyvek? You've got bugger all chance of getting any sail to set well if you want to use Tyvek.

 

This is a good suggestion if you really want to optimise for light winds. Some of John Welsford's boats use a section similar to this and it can work very well. What it does is give your immersed sections a rough approximation of a circular arc, which will have less wetted surface for the same displacement compared to the flat bottomed shape you have drawn. The trade off is a reduction in initial stability.

 

Oh, sorry. Got mistaken in my math. I'll try to increase volume by offsetting sheer line transversely away from chine. It will increase the triangular section area amidships.

Required area depends on aspect ratio, as well as section shape. With a good section, and with the moderately high aspect ratio you're using, I think you'd be safe heading down towards 3% of sail area for the board. Incidentally, regarding section shape, years ago I had a brief chat with Julian Bethwaite about the laminar flow NS14 board sections mentioned in High Performance Sailing. His recommendation was to not use an NACA 00xx series. Unfortunately I cannot remember precisely every detail of what he said, so I'm a bit hazy about the forward 10% of the section. I should probably try emailing him and seeing if he'll cough up the relevant details.[/QUOTE]

I'll consider this. Maybe I'll go down to 3.5% for starters. I'm really afraid to have too small board, because in my experiences with fishing boats, I always had horrible leeway. Left me with a phobia.

Understood! I'll try to adjust the hull in the way you described.

By the way, why is it called deadrise? What's so dead about it?

Well, yes. Or polytarp. I mentioned this many times in this thread... Remember, my budget is 1000$ tops. Sail of such size would cost 500$ alone at our cheapest sailmaker. And I doubt I can find used sail of such dimensions. Even used ones are much more expensive than in US.

I remember reading that general recommendation is not to go over 10m^2 sails with Tyvek. Smaller than 10 should set acceptably. I plan to reinforce luff with non-stretch rope, because that's most loaded edge in this type of sail. Or maybe even all around the perimeter.
Anyways, stretch happens usually in higher winds. While winds are low, I doubt there will be much difference between Tyvek and Dacron sails...

Or is there some kind of intermediate step between these two materials?

 

P.S. If, for example, I use 10° deadrise angle amidships, what would be proper deadrise angle at transom? Twice that?

 

TBH I'd just make it 10 degrees all the way from stem to transom. Or whatever the transom is now.

 

BTW, the lug rig for the OZ PD racer is 8.3 m². The board is 1.1 metres long and 0.28 wide, with 0.87 available as span below the hull. This gives LA/SA of 2.9%.

You really would do well to take a close look at how these boats are set up. The hull shape is a basic box, as per PD racer rules, but a lot of thought has been put into structure, rig and foils. You can pick up the complete plans for $20. It's worth it if you're a beginner and want to learn stuff. Mik is a smart bloke with a lot of experience.

 

I'll check it out more closely. It's just that these boats vary in structure a lot. Some weight 5 times more than others, I reckon.

 

Yeah but the one Mik drew up is really light and stiff. It's not one of the no-brainer Fred Flintstone PD racers.

 

Here: http://www.pdracer.info/index.html

 

Yes, Mik is a good guy, I've met him a few times, most recently in Texas when I was sailing one of his Goat Island Skiffs, he wasn't - Ha!

Be careful with foil choice discussions. Many foils only work effectively with a mirror finish. If you cannot see your face in it (like you would on a 49er board for example), then stick with the 00 series

I guess you are still "using your eyes". So check out the Laser foils, a slightly smaller rig than yours. A Laser is too small (but still works OK except in big winds) while the daggerboard is oversize

RW

 

All right, I've modified the hull. Made the deadrise 10° amidships, as you suggested. Wetted surface went down to 2.55m^2. Apart from that, I can't say I like these changes. I've faired the lines, etc, but the hull looks weird to me. Especially on the side view. One of the reasons is because the deadrise angle appears to be going lower towards transom, even though there it's the same 10°.



Side tank volume increased to 45l each, 90l in total. However, there is far less space onboard now. Stability has reduced. I guess I'll have to increase the beam again.

It looked way better in the previous update... Please advice where I got it wrong.

 

Looks fine. Has 10% less wetted surface. Stability, in practice, will be little different since you'll be relying on crew weight to hold the thing up if there's any breeze. If you want to give it more deadrise at the transom, go ahead. That'll generally give you a bit more speed in light airs and a bit less in a breeze.

Pointless increasing beam again. That'll just send you backwards in terms of light airs performance. If you want more interior space in combination with more buoyancy, think of another way to get those results.

 

That's the thing, I can't think of anything. Do you have any ideas?