Low-speed sailboat hull

Here are some links on your board question, they are articles by Jim Michalak, who knows what he is talking about.

Board placement:
http://www.jimsboats.com/webarchives/2000/1jun00.htm#SAIL AREA MATH

Sizing boards:
http://www.jimsboats.com/webarchives/1998/15may98.htm#Sizing Underwater Boards

Board shape:
http://www.jimsboats.com/webarchives/1998/1jun98.htm#Underwater Board Shape

Find more articles here:
http://www.jimsboats.com/webarchives/date.htm

 

Laukegas.

I am impressed with the amount of diligence you are putting into this design project.

My comments are in your text.

 

I think you should really consider eliminating the center air box.

Sometimes you might want to sit in the bottom of the boat. The center air box will be in your way. It will deprive you and your guest of leg room. With just a support bench, you and your guest can put your legs under it, on either side of the dagger board trunk.

Just a thought.

 

Thanks a lot, tdem. I have never found theory on foils as detailed as in the links you provided. I only knew that basic "4% rule", but not formulas that explain the physics involved.

Thank you. If even your instructor couldn't answer this dilemma, I guess the only way to find out is by practice - build the boat based on my best guess, and then modify the rig until the boat is balanced. I plan to make my sail out of Tyvek or polytarp, and see how it works. If balance will be unbearable, I can always make another sail. I can make a sail of such size in 2-3 days and for ~30$. Maybe bit more for Tyvek. Not a big loss expense.
And when I see that the rig works to satisfaction, I'll save up and go to sailmaker, asking to make a lugsail of size and shape that I know that works.

That's the general idea. As for raking, it would introduce a lot of complexion and probably cost to the build. I hope I can get away without that...

Well, they say slight weather helm is desirable. You see, since my sail is big, it requires large board area. As you can see from screenshots, centerboard is big alright, and even so, it is only 3.5% of the sail area (I'd like to go up to 4% at least). Rudder is ~50% of the board area. It'd be great if rudder also contributed to preventing leeway. If I'm not mistaken, I think I remember having red that with slight weather helm, rudder is at a better angle to create lift, while with lee helm it creates lift in the opposite way, which is a terrible thing.
I try to picture it in my head, and it seems to me that if boat tries to make leeway, then the rudder receives waterflow at an angle even with neutral helm, aiding the board.

So I'm not sure about this. Slight weather helm seems a good thing, but such a delicate thing seems impossible to predict with all the variables you mentioned. I guess I'll just have to guess.

Well, thank you As you helped me for the past few years in my previous projects, you can see that I often failed because of lack of knowledge and/or poor preparation for these projects, as well as cutting corners with "that's a bit below borderline, but I hope it'll work" philosophy, which often ended up with lots of frustration. After this summer, I'm still having nightmares about being unable to sail upwind.

"You can't take carriage, mount an engine, and make it into a rocket", they say in my country. In all my previous projects, I always tried to convert something that has nothing to do with sailing into a weatherly craft. All these attempts were crude efforts to make it work somehow with as little expense as possible. If I would add the cost of all these little projects, I could have a fine boat by now. I try not to think about it.

So, this time, I want to cut corners no more. It's time to make a sailboat that is actually designed for sailing. From scratch, not in limitations of previous failures. And I realize that my knowledge is still very far from perfect, and sailboat design is not a science to be taken lightly.

For example, take a look at this boat. I found it while searching for design solutions. It is pretty much the same dimensions as my design, just slightly bigger. A fine boat, I'm sure. Yet it weights almost three times as much as my design. As I scrolled through the pages on the link I provided, I shuddered with horror, thinking "do you REALLY need that additional crossbeam?".

I realized that if I want to make an ultralight boat that won't break up on a gust of wind, I must design it perfectly.

That's why I'm investing so much time into learning, re-designing, questioning you, and hopping for more critique. Once the boat is done, there won't be a lot I can change. But I can change a lot now.

I'm not exactly sure what you mean here. Do you mean that I should remove the air box, but leave the center seat, or remove it too? I'd be great to have a seat for rowing. Not sure what you meant by "support bench".

I made a screenshot. Is this what you had in mind?

http://static.dyp.im/I59OEQx5xp/b1606bd8822dd596239c014d57dadd71.JPG

 

Laukegas.

Your latest drawing is exactly what I had in mind.

You might need a pair of gussets for the seat, so it can help stiffen the boat, and do the job the two bulkheads did.

Attached is one of my construction drawings which shows gussets.

With your design, it is probably best to put them under the seat, at about mid length, fore and aft.

You are correct about weather helm. You want some, but not too much.

With some, the boat has a "feel" to it, so you can sense when the boat is really starting to sail. The end of the tiller, the end of the push-pull stick, or the steering line, will give a slight tug.

Too much weather helm and you're wrestling with a brute.

 

In that case, why not use a proper NACA foil shape on the board? If you make it from real wood instead of plywood, and with a good shape, it will be a much better end product.

Remember you also use the board for righting the boat after a capsize. Imagine climbing on a 16mm plywood board. Snap.

A thicker board with gentle changes in shape will also be more resistant to stalling, which is quite important at low speeds.

 

Thanks, sharpii2. I'll add the supports you described.

I've actually tried to design a NACA foil. First, for the chord length of 35cm, the maximum thickness would have to be 4.34cm. Which is pretty thick. I'd have to use 45x45 timber beams, glue them together, and probably fiberglass after shaping to provide sufficient strength. That's a lot of complications.
Also, NACA shaped foil wouldn't sit well in centerboard box, since the slot is a simple quadrangle. I'd either have to make the upper part of the board square shaped (which would heavily complicate shaping the lower part of the foil), and this would prevent sailing with half-raised board, OR, I should make the slot shaped as the foil profile. The only way that could be done, I reckon, is gluing together a lot of precisely cut plywood quadrangles with NACA-shaped cutout. Very complicated.

Weight is also a consideration. 6kg for NACA shaped board, 3.3kg for simple one.

So, in the end, of course NACA profile would give better performance, but at what cost...

As for righting after a capsize, you're sure that 16mm plywood board would break? Sounds mighty thick for me. I once stood on 6mm unsupported plywood, and it didn't break.

 

you can use a thinner foil section if you do not want to carve up a thinker dagger board. Making it fit tight is no problem, you put a foil shaped hole (or a plate over the dagger board box bottom) at the place where it pokes through the bottom of the hull, and one at the top of the dagger board box (which can be just a rectangle). I have done it, no problem, you just need to have the upper half of the dagger board a constant section so it stay in top and bottom plate tight. You also use a lanyard or a cord and cleat to hold it in place since it wants to float up out of the slot. with a clean foil shaped cut out that matches the foil section it will have the least amount of drag and deliver the best performance.

 

Exactly, Petros has a simple solution. You could use a 008 with a total thickness of ~2.8cm. I don't see why you would have to fibreglass it to prevent breakage. It's thicker than the ply foil, won't that need fibreglass too then?

 

16mm Ply will snap as a board, even a rudder. A NACA 008 type with maybe a slightly softer front nose is a good shape for the length in my experience. 28mm is thick enough to not break even with an unglassed foil over 1 meter long under water (unless you bounce on the tip!), if in a decent timber ie laminated Spruce or similar. I would not expect the foil to weigh more than 2.75Kg max finished. It's not hard to get a board to be square at the top and tapered below. A profile guage and accurate plot of the section are your friends here.

Petros and tdem have given you good advice, you would do well to follow it. Good luck with the build.

 

I also think a thicker board is called for, because your board is a dagger board and sticks so far below the hull.

As far as NACA foils go, I think such would definitely be an improvement, but how much of one, I'm not sure. Of all the boats I have owned, I have never had one that had a NACA foil for its board. All have sailed to windward quite readily.

I would at least round the nose of my board, and taper its tail.

Its flat sides will fit nicely inside a flat sided trunk.

16 mm does seem a bit thin for such a deep board.

I have drawn even thinner ones, but they weren't true dagger boards, and they didn't go down as deep.

In your case, I would start at 18 mm and go up from there. The extra 2 mm of extra thickness goes a long way toward strengthening the board.

24 mm sounds even better. 28 mm may be a bit of over kill, but maybe just a bit.

 

Is this what you had in mind?

http://static.dyp.im/SQT1YIOV7K/d0fd7a37718504a40ff4409fb499b830.JPG

With removed seat and one of the side panels for better look inside:

http://static.dyp.im/uAzIwaN2By/4a37a3459ef07da7af5153d88ef79272.JPG

You're right, I guess. Solid wood would be stronger than plywood anyway... I designed a centerboard with 008 profile, chord length 36.5cm, maximum thickness 2.9cm. However, I trimmed off 1.5cm from the trailing edge, since it gets very thin there. So now minimum thickness is .3cm, at the trailing edge. So, the board is 120x35cm, weights 3kg, and the part that sticks out of the bottom of the hull is 85cm long.

Here is profile screenshot:

http://static.dyp.im/rdUv5m6Y0r/6f1e0d47c777c5296c3ae4b6e68c8e60.JPG

If you zoom in, you can see the filet on theoretical trailing edge. Grey line represents NACA shape. As I said, this cut-off is made 1.5cm from that edge, and gives .3cm minimum thickness.

Yes, I guess I should heed this advice. You see, it's just that I cannot fiberglass the board because fiberglass cloth is not available in my country except the 2 inch rolls. I can epoxy-coat it, no problem, just no cloth...
As for weight, my software tells me that this board of 35x120cm with 008 profile (2.9cm maximum width) will weight exactly 3kg if made from 340kg/m^3 pine or other wood of same density. Of course, that is, discounting glue and coating.

So, I've settled with 2.9cm. Maybe an overkill, but I can't make this board smaller, I need enough area to negate drift. With advice Petros gave about centerboard box (if I understood it right), it shouldn't be too hard to make the box.

As for building the board, my idea is this: buy 45x45mm pine balks, lacking knots and selected for straightness, cut each to 120cm lengths, mill each to a certain width, glue together by centerlines, and then plane down with hand plane until NACA shape is achieved.

A screenshot of how it would look like:

http://static.dyp.im/6aY4JSLxqQ/398900e2b53db97ea22e5562c6344d31.JPG

I think I saw this method in some instructable, I can't remember. Will this method provide a board strong enough? I can run several 15cm long screws from trailing edge along the centerline every 10cm or so, to provide more strength. I have done this this summer when I made my leeboard from butt-joined planks, if you remember. Just have to drill it right.

 

That looks fine, for construction, you will not need any screws, the glue is strong enough. Epoxy or urea formaldehyde is just fine. If you plane correctly and accurately you need almost no pressure to join the blanks, even masking tape will hold them fine if you glue one by one or say 3 at a time. If you try the whole lot you will need a brace on at least one side because the blanks slide over each other under pressure. If you do the lot as one make sure you have a polythene sheet covering the clamps to stop glue getting in the threads....

Remember to reverse any direction/angle of grain each block in the laminate to keep the board flat over time. Unless you can resaw to perfection you will have a degree or five which is perfectly acceptable.

I've built longer boards (1.3m) down to 2Kg (unfinished) with Sitka Spruce and WR Cedar so you may be a little lighter. The finish coats ie epoxy sheathing adds a bit though but worth it and protective lower beading (with microfibres and/or other tough fillers. Nominal Euro Spruce weight is around 420 Kg/m³ but you can get away with 'light' pieces fine. The better the lack of run out the easier to plane. I heavily recommend using a hand power planer to 'rough' the blank prior to using a hand plane. It saves a lot of work, or you will be removing approx 2Kg by hand - a lot of very hard planing, I've done a couple this way and never again!. Though I find a good hand plane is very good for getting the shape correct quite quickly and accurately once the bulk is off. A No4 and 5 maybe a 6 are the most useful. A spokeshave and chisel are handy where you are changing from being fully in the box to true section - a fast transition.

If you find a lot of reverse grain try diagonal patterns and allow a bit more thickness for sanding fair.

I am assuming you will not be standing on the trailing edge alone when righting the boat and also not being too harsh grabbing it when trying to get onto it, but yes the edge should be strong enough. The epoxy sheathing really helps for that little bit extra strength at that point. BTW painting white helps reduce warp (heat from sun) protects the epoxy (UV), and allows you to see any weed better so you may consider that.

 

yes Laukejas, that is exactly what I was describing. you got it. easy to make, light weight and excellent performance. do not make the box portion larger than necessary because it will contain a bit of water (up to the water line) that adds some weight. The less empty volume in the box the better.

I used a combination of glue and screws, it just speeds assembly since you can keep building even before the glue has cured.

The dagger board plan is excellent as well. If you have use of a table saw you can even reduce the amount of hand planing by tapering each block of the laminate to better approximate the foil shape.

 

Thank you for these tips, SukiSolo. I will write them down to remember when I start building. And yes, I'll be painting white. The whole boat, maybe except some parts for looks.

As for the painting, I just had a conversation with a fellow boatbuilder. He said to me that I should never paint both sides of the hull with sealing (non-breathing) paint. He says that this way, the moisture is trapped in the wood, and will lead to rot. He suggested that if outside of the hull is epoxied/glassed and sealed with paint, the inside should be only impregnated and painted with different kind, breathing paint.
Is this true?

Thank you. I won't have access to table saw, but maybe I should invest into orbital sander. They say that with some rough sandpaper, it can remove lots of wood.

Actually, I'm thinking of building a workbench for this project. You know, just a 9mm plywood sheet with some underside reinforcements and legs, maybe 1x2m or so. I could set up jigsaw underneath, and have a tablesaw. If I could also fix a drill underneath, I could make a milling machine also. Not sure if this can be done, I'll have to search for some information on this.



As previous issues have been solved, I've moved on to the next stage. I added some gussets to the seat/daggerboard case, since I've still got a lot of spare plywood on my 9mm sheet:

http://static.dyp.im/JAm1PO50y4/a6be5db45335b7686147227ece6263e3.JPG
http://static.dyp.im/dQs5KK92Jt/66b160035b99057e14df5c9028ccf49d.JPG


Now, I've turned to the hull shape again. I included models of two people into my model, each weights 80kg (worst case scenario), with two seating arrangements. At this stage, I think the model is accurate enough already to calculate where LCG will be located, and check it against LCB.

First arrangement:

http://static.dyp.im/2FqfcuZx1U/73b504d93c4626e3818d13a6fda92440.JPG
http://static.dyp.im/k2y81lf33w/20be413874c11fadba002734d066a222.JPG

Second arrangement:

http://static.dyp.im/9UqsRdmnsg/6f985780f0e23cd0730cc710da56b35a.JPG
http://static.dyp.im/XBTntYF1AZ/e4e0458d7e28886c078913c2bd40c475.JPG

Now, the first arrangement would be preferred, because it is more comfortable, allows to hike out more, and permits proper human interaction. However, with it, the LCG is about 12.5% aft of midships (calculated as (LOA/2 - LCG(distance from transom)) / LOA)) . For proper longitudinal balance LCB should also be 12.5% aft of midships. I tried modeling such hull, and I just can't get it so far aft.

The second arrangement places LCG 6.5% aft of midships. And I actually managed to modify the hull to place LCB at this place:

http://static.dyp.im/yetit6iBeN/cdaa2e3276e0c9ab6a0b46fe3741dc89.jpg

And it looks somehow wrong to me. Way too much rocker aft. Yet the numbers are correct, I checked twice and then again.

This hull would work for second arrangement, yet not for the first one. For first one, I'd need to transfer LCB twice as aft as now.

Can anybody give any advice on how I could solve the balance issue?

P.S. Before you say it, I considered moving center seat forward. But it messes up COE/CLR balance.