Low-speed sailboat hull

D'oh. I have a digital camera! Ok, I'll get you some shots.

 

Maybe. If anyone reading this have "Elements of Yacht Design" by Norman L. Skene, 8th edition, please, contact me...

Oh! I love you.

 

These pages cover the basics for timber masts, as well as booms and spinnaker poles.

You'll have to do a bit of common sense extrapolation since Skene's doesn't concern itself with lug rigs.

 

Thanks a lot. I calculated that for a 7.31m^2 sail on a 4m mast with safety factor of 2, the recommended thickness is 7.8cm. I'll have to see if I can source 8x4cm timber for both parts of the mast.


By the way, I've been meddling with hull shape again, fairing lines even further (I discovered that chine and sheer line curvatures are not planar, so I leveled them). I also took the liberty to increase beam a little, especially in bow, to make more room for forward crew member. As a result, boat looks far better now, and the righting moment is substantially increased. I get the feeling that this might be the final hullshape. At last, it looks beautiful to my loving eye.

Here go updated calculations and some screenshots.



LWL: 3m
Beam: 1.31m
Displacement: 210kg
Prismatic coefficient: .51
Wetted surface area: 2.78m^2
Longitudinal center of buoyancy: 1cm aft of midships
Maximum heel angle: 23.5°
Freeboard: 25.5cm
Dry hull weight: 26.5kg
Finished hull weight: ~31.5kg
Sailing weight: ~45-48kg











Stability:

Setup 1: both crew members sitting on the bottom of the boat, windward side

a) Boat fully upright

Righting moment: 303nm
Equilibrium wind speed: 5.3m/s (10.3kt)

b) Boat at maximum heel angle

Righting moment: 690nm
Equilibrium wind speed: 8.3m/s (16.1kt)

Setup 2: helmsman sitting on the deck, foreman sitting on the bottom

a) Boat fully upright

Righting moment: 517nm
Equilibrium wind speed: 6.9m/s (13.4kt)

b) Boat at maximum heel angle

Righting moment: 775nm
Equilibrium wind speed: 8.8m/s (17.1kt)

Setup 3: both crew members sitting on the deck

a) Boat fully upright

Righting moment: 696nm
Equilibrium wind speed: 8m/s (15.6kt)

b) Boat at maximum heel angle

Righting moment: 826nm
Equilibrium wind speed: 9.1m/s (17.7kt)


These numbers increased substantially from the last time, yet they are pessimistic - a crew can always lean out more, release mainsheet, etc. But I'm just giving an idea of what winds would it take to capsize the boat with unreefed sail.
The maximum heeling angle of 23.5° isn't very impressive, but as you can see from screenshots, boat is very beamy, and righting moments allow for quite high winds, considering oversized sail. So I guess that will be alright. Freeboard is 25cm.

Well, I don't have anything else to add. Please comment if you see any more flaws. I'm in love with this boat, and can't wait to start building

P.S. A boat without a name is a disaster waiting to happen. Any ideas on how to catch inspiration?

 

What's the wetted surface when board and rudder are included? If you want good light weather performance, you should be aiming for SA:WS to be around 2.5:1.

(I can just tell you're going to love that)

Beef up the bulkhead at the mast. You don't need to cut that much away. Leave a much bigger radius at the corners.

And you are still having backing for your pintles, right? Reinforcing the top edge of a 6mm ply transom doesn't mean getting rid of backing for the pintles.

 

Um... I've got a feeling this won't end well!
2.8 for hull, 2*.27 for board, 2*.13 for rudder comes to a total of 3.58. Sail area 7.31. So, ratio is 2:1...

Well, I don't know what can I do about it. Changing hull shape to get lesser wetted surface area will ruin other advantages, and make little change.

I remember having been told that Lasers usually sail heeled in light winds. I ran Inclines Hydrostatics with Delftship, and it seems that with 20° heel, wetted area for the hull comes down to 2.3, 3.1 in total, making ratio almost 2.4:1. Not ideal, but better.

I don't know what else could I do about this. Spray hull with Ultra Ever Dry to reduce viscious resistance, maybe...

Okay, I'll modify bulkhead. And yes, I forgot the backing. Will add one now.

 

Yup, heeling boats in light airs is an old trick. Bow down trim is another (lifts the wider stern sections out more).

It'll be ok with what sail you have. It just won't be absolutely stunning in light airs.

You can always add a ringtail, just for fun. Inefficient as hell, but will look impressive. Ditto a watersail.

 

How much built in buoyancy do you have? Enough to support the boat and both crew?

Most boats would have a higher Cp, but most are designed to go at hull speed or more and you don't want that. Just guessing that you have optimised for maybe 3 knots boats speed?

Your foils look huge. People always try to minimise their area because they have two sides

Richard Woods

 

I agree with Richard about your foils being a bit on the large side. You could knock 20% off their area while keeping the same aspect ratio. That should still give a decent amount of stall resistance.

Ok, well as one possibility, if you give the midships a bit more deadrise and a bit more keel depth, you can get the same section area with less girth. If not carried to extremes, this shouldn't have a bad effect on other properties.

This would also mean your freeboard could go up a bit more, since the chine height would be rising.

 

sorry...but i think you lost the initial intent:
the boat that is draw here in the last post is a very small variant of a square boat, and square boat is not able in the slow wind.
For slow wind must to have a hull with U sections, and the sail have to be a modern sail, no a square sail.

like this:

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By "ringtail" you mean a studding sail that is set on boom and yard, parallel to the leech, such as seen here?
(Damn google. It took me time to find what ringtail sail looks like, because 95% of the results are variations of this) Okay that's off topic.

Buoyancy is about 26 liters per side tank, 52 liters in total. Not outrageously lot, but the the boat is all plywood and timber, and has an additional volume of 40 liters. Average material density is around 650kg/m^3 in this boat, so there would be 26 of these 40 liters usable for floatation.
52+26= 78 liters in total.

I could increase the volume of these side tanks by increasing horizontal distance between chine and sheer, but that would also increase weight, and as you can see, the boat is already 2kg overweight.

As for hull speed, yes, you're right, prismatic coefficient of .51 is optimum for 3 knots. At least according to this relationship between CP and S/L ratio. 3m waterline equals 9.84 feet, and since CP of 52.5 and lower equals S/L ratio of 1, then the square root of 9.84 feet equals 3.13 knots. Accounting that .51 is somewhat lower than .525, I'd say 3 knots. Your guess was impressive!

Yes, foils are huge, but so is the sail. I sized them by this rule: centerboard must be 3.5-4% of sail area, and rudder area must be 50% of centerboard area. My centerboard is 3.72% of sail area, and rudder is 47% of centerboard area. Both NACA 0008. So, are they too large? It seems to fit these rules. I'm a bit afraid to have lots of leeway in low winds, where due to low speed foils have little lifting power. Am I being too careful?

I'll see, maybe I'll experiment with this. I'd need to increase beam even further to compensate for displacement.

Come on, my boat is not square. Not at all. How can you say this? Look at the top-down screenshot. It is shaped like a droplet. The water would love to flow around it.
You're right that U shaped hull would be better instead of hard-chine, but then I'd have to plank or multi-chine it (or make from composite materials). It would be incompatible with other design requirements, such as low weight and low cost. Also, this will be my first independent, full-scale project. I don't want to go over my head, you know.
If I went with modern Bermudian rig, I'd have to order up Dacron sail, instead of making my own of Tyvek, because due to stretch, it would not set well. I know because I tried. Not even battens could help it. Maybe if sail was multi-layered, then it could set better, but that is very laborious project. Lug sail has a better chance of setting well with stretchy material, I believe that is one of the reasons why square sails were used in days of cotton and other stretchy material sails.
Also, lug rig allows a lot of sail area up high, which will help to get the most out of low-wind conditions.
Also, the mast is now 4m, while boat itself is 3m. If I used Bermudian rig, I couldn't deliver such high sail area without even longer mast.

So yes, if the only requirement was top-notch performance in low winds, then I'd go with totally different approach. However, there are lots of other limitations, and I have to compromise something.

 

is a square boat, not in longitudinal, but the sections;
one boat 3 meters long in plywood cross laminated i'll be built in the very few more time like hard chine, coz the chine more of 90° is not easy to achieve as you belive: you have all the disadvantages of squere boat and chine boat.
for the sail just buy, new or used, a rig of any small boat modern series.

this is, if you want to sail in light wind, otherwise you'll have a boat as all the other square.

 

Everything is a compromise and the successful designs find the right balance. The faster you go the more "power" the foils have, so they can be smaller. Having big foils will be slower, because of their WSA

I know you sail in a light wind area, and won't ever capsize - of course not! Even so, I strongly recommend that before actually sailing your boat you do a capsize test with the boat in shallow water and see if you can get back into it, and where the flooded WL is.

https://www.youtube.com/watch?v=CC5okX21AB8&list=UUhKTQtbKN5BaXFTg2BjcbqA

Richard Woods

 

ah ... and maybe you do not know the technical 'development conical'.
I think in the forum there will be someone who knows and will explain it to you well in English