Low-speed sailboat hull

I don't mean it to sound like I disregard your advice. It's just that almost everybody advises me quite differently, and sometimes it ain't easy to decide whom should I trust in either case.

So I'm just asking to make sure I understand why you suggest this or that, so that I can choose.

I hope I didn't offend you.

 

Yup. It's like that.

People regularly go sailing in boats that I consider to be completely bonkers. If you're determined to do some particular thing, go ahead and do it. If it doesn't work, at least you'll know why and what to do about it.

IMO it'll just make recovery easier if you have a bit more buoyancy. With 70 litres in triangular tanks, she'd float very low. Also, when standing on the board to bring the boat up she'll sink right down, since one tank will be out of the water, meaning she'll come up full. Result will be a lot of bailing, particularly if there's any chop (when it will probably be coming back in).

 

Understood. Well, I'll raise the sheer, and we'll see how it looks.

By the way, I'm now working on a rudder.

My rudder design:

http://static.dyp.im/aSDmAsuUZ0/cd56b3d7c7491935cbe61b0536956143.JPG

http://static.dyp.im/O5269qOBhP/74c8363e849284e8a4b8c267a35a7534.JPG

Do you or anybody else know of a system that would keep the rudder blade down in the water for as long as the tiller is lowered? Like in the first screenshot. And when the tiller is raised, the blade would be free to float up or kick up when beaching.

I'd like to do this without adding weights to the blade.

 

Too much stress on the case and pivot when the blade is up. It'll break, sooner or later. Give the blade more support when raised (ie: move the pivot). Just use a bit of shock cord to hold a pivoting blade down. It can just loop over a thumb cleat. This will absorb shock from grounding and be easy to free in a hurry.

 

Okay. I increased size of semi-circle of the rudderblade. Is this better?

http://static.dyp.im/XBMLeFsAHY/b8e8f7c9fc466171cea34b16ca6072fc.JPG

(this is view with one of panels removed)

Shock cord would work, I know this solution, and I'll probably use it, but I was wondering if there is some way to connect angle of tiller with angle of rudder blade. Is there?

 

I also designed oars and their placement now.

Paddling setup

Sailing setup

As you can see, there is enough space in paddling setup for both persons, although transom will be somewhat submerged. Can't have everything, I guess. However, since this boat is designed even for very low winds, I don't expect much rowing. Just need to have a back-up propulsion.

In sailing setup, oars can stay in oarlocks and be raised on deck (if sailing in light winds, when hiking on deck isn't necessary), or, they can be stowed on bottom of the boat, partly under the seat, as seen in screenshot. They shouldn't get too much in the way when sitting on the same side oars are at if there will be these styrofoam blocks I mentioned earlier.

There is of course, another option - I could arrange some holders to hold oars on bow deck, protruding outwards in the direction of travel (more or less). That way, they wouldn't get in the way at all, but it might look a bit awkward, and cause problems when docking.


Now, the oars themselves are designed based on this tutorial and plans, with some changes. The body of oar is 8-sided pine beam, 2.4x2.4cm, the paddle is 8cm wide, 28cm long, 1cm thick. Handle is 10cm long, rounded to 2cm diameter. Oar is 120cm long in total.

Here's how it looks:

http://static.dyp.im/C0bZy4MHC7/c6378029da6ea5158dc7f1864a8ce16f.JPG
http://static.dyp.im/CwxMtpkHqU/06c3a7517c61e31a9220e67c7698be2d.JPG


I'm not sure if I should make it any larger/thicker. It seems proportional to such a small boat. I checked if it's comfortable for paddler's hand height, that there is enough of it in the water, and that it clears the sheer at all paddling angles:

http://static.dyp.im/NgLqG3cnzo/898fcaa5e63aad3886e0f8ec82c381f1.JPG

(Blue line is the waterline at maximum load)

Of course, the sail will be lowered and daggerboard will be raised and stored forward of the paddler.

Any concerns that I should consider about this solution?

 

I don't want to be rude, but if you want people to look at a succession of images could you please resize them to something reasonable? There's no need for images four times the linear dimensions of most people's screens, and they take ages to load.

If you use any image editor to resize them to something like 1024 wide, in medium quality .jpg, which would still provide plenty of detail, they would load in a second or so. This would be a vast improvement, compared to chewing up several megabytes and several seconds with each image view.

Anyway, plans for single bladed canoe paddles (real ones) are readily available. The blade sizes are commonly much larger than you indicated. Here's a good example: http://paddlemaking.blogspot.com.au/2009/12/canoe-paddle-plans.html

 

I second this, takes like 30 seconds per image for me. On a good day.

 

Sure, I can post them smaller. I though that posting larger images will allow to zoom in more, see more details. But if large images are an inconvenience, then I'll size them down from now on.

 

Your pictures haven't had significant detail that requires zooming. They're very simple, and easily accessible at much smaller sizes.

 

Okay, I'll keep them small.

I have started working with the freeboard raising. Just before I go through with it, I made some calculations:

With the latest updates, the righting moment for 0° heel with both crew members is 253nm, which means that boat will heel more only when wind exceeds 4.85m/s (9.42 knots).
At maximum heel angle (when the waterline meets the sheer line, and boat would start taking water if heeled any more), which is 18.5°, due to shifted center of buoyancy, the righting moment is 538nm. That means only a wind of 7.26m/s (14.11 knots) would overcome this righting moment.

So, in other words, with both crew members and an unreefed sail, this boat should theoretically avoid capsize up to gusts of 14 knots. Of course, they may be more variables, such as crew jumping on the deck to right the boat before it capsizes, releasing the mainsheet, etc. But this figure of 14 knots covers some of the worst-case scenarios.

I'll try to raise it as much as I can, but I'm not sure what is the minimum. So I've these calculations to put things in perspective.

So, 538nm righting moment at 18.5° heel, after which boat starts taking water. If that's not enough, I just want to know what numbers should I aim at when raising freeboard, since now raising freeboard from 20cm to 30cm as suggested by NoEyeDeer would probably go over the weight limit, since there is only 1kg to spare.

I'd appreciate any further comments, and thank you all for helping me out so far

P.S. I calculated, that raising freeboard to a minimum of 30cm would produce a maximum heel angle of 30° with righting moment of 679nm, which translates to 8.16m/s (15.86 knot) capsizing wind speed, and increases boat hull weight by 3.5kg, from 24.2kg to 27.7kg.

 

Ok, my 2c (anyone is free to disagree of course) is that 30 cm freeboard would be nice, but you can probably get away with a bit less, but personally I'd be liking more than 20 cm. Get as much as you think you can get, without screwing everything else up. Just as a reference, my rowing boat has 25 cm with one person aboard, decreasing to around 22 with two people. I consider this minimal, and that's not a sailing boat either.

Do your righting arm calculations take into account the crew leaning out? You'll do that instinctively once a low freeboard, heavily canvassed dinghy starts heeling. It adds a fair amount to the righting arm. Anyway, with two adults on board it should be ok for sail carrying power.

 

All right, I'll try to raise it as much as I can. Maybe I can play around with other parts, see if I can reduce some weight somewhere.

No, the righting arm calculations I mentioned earlier were made with crew sitting on the bottom of the boat, on the windward side, like in one of previous screenshots. This moves CoG 12cm to the windward side. If crew were to sit on side deck and lean out, CoG would move out much further, although higher as well. This wouldn't be too comfortable to do, but could save some wet clothes if crew reacts quick enough.

I just calculated basic "cruising" setup - what would happened if boat sailed peacefully in moderate wind, and then suddenly a gust hit, leaving no time to jump on deck and lean out (not very probable, but possible).

I can make these calculations if desired.

 

I made these calculations, just for comparison. Screenshots, being rescaled to a proper size, show how each setup looks.

Freeboard: 20cm
Maximum heel angle after which boat floods: 18.5°

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

a) Boat fully upright

Righting moment: 271nm
Equilibrium wind speed: 5m/s (9.71kt)

b) Boat at maximum heel angle

Righting moment: 544nm
Equilibrium wind speed: 7.3m/s (14.2kt)

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

a) Boat fully upright

Righting moment: 489nm
Equilibrium wind speed: 6.75m/s (13.12kt)

b) Boat at maximum heel angle

Righting moment: 660nm
Equilibrium wind speed: 8m/s (15.55kt)

Setup 3: both crew members sitting on the deck

a) Boat fully upright

Righting moment: 728nm
Equilibrium wind speed: 8.23m/s (16kt)

b) Boat at maximum heel angle

Righting moment: 801nm
Equilibrium wind speed: 8.86m/s (17.22kt)


After I find maximum freeboard that doesn't make boat go overweight, I'll re-run these calculations.

 

So, after some time spent elsewhere, I'm back here again, with updated design. After lots of experimentation, I've arrived at freeboard height of 25cm at full load. It gives a dry weight of 25.16kg, which is slightly above desired one, but I guess it will be okay. It also increased maximum heeling angle from 18.5° to 24°, allowing GZ to move sideways 4cm more in case of an unexpected gust. That is quite a lot if you keep in mind that previously GZ was 23cm at maximum heel with crew sitting inside.

Here are some screenshots:




I also modified rudder design. Now, rudder box has a roach, allowing tiller to be used within 0-20° with full support. I guess that should make it somewhat stronger.



Also, I decided to offset upper and lower rudder hinges by 2mm. I remember reading somewhere that this would make it much easier to install the rudder when on water - you don't have to align both hinges at the same time, you fit the lower first, and then the upper.



The transom plate is 6.5mm plywood. I'll definitely won't mount an outboard on it, but I was wondering if it would be strong enough to withstand forces from the rudder. So, just to be sure, I though I'd add another layer of plywood as reinforcement from inside:



I also increased oar length and size. They look really big now.



None of the principal dimensions have changed, apart from the freeboard.

If this looks okay, I have several questions.

1. How much epoxy should I buy for this boat? I will not fiberglass it, just fillet edges of plywood when stitching and gluing. I estimated that there will be around 40m of edges that will require both filleting with epoxy and wood flour and covering with epoxy and fiberglass tape. Then, about 10m more edges that will require only epoxy and wood flour. Then, several square meters of epoxy only for gluing stuff together (layering plywood, joining timber for boards, gluing the mast). These are very rough estimates, it would take me days to figure out exact numbers. Epoxy is very, very expensive here, but the price diminishes quickly if you buy big cans. For example, there is 1kg, 2.5kg, 5kg, 10kg. Since this is my very first time I work with epoxy, I have absolutely no idea how much of it will be used. My gut tells me to buy 5kg can for 100$ to be on the safe side. Can someone advice?
2. You saw the screenshots. The sail is 7.3m, maximum heeling moment boat can withstand is 800nm. How thick the mast should be, and how much taper would be safe? Right now, I designed for 6cm solid round spruce mast, glued from two planks, 4m length. Is that thick enough?
3. How thick should spars be? I'll mill them from single, knot-free spruce beams. Right now, I'm thinking 3cm for boom (2.5m long) and yard (3m long). Sail will be loose-footed, but lashed to the yard. Is 3cm thick enough?
4. I managed to contact the guys who sell water and boil proof birch plywood. They confirmed the density is 700kg/m^3, all sizes of sheets are readily available, and they also sent me a pricing list. Prices are listed for each thickness, expressed as money per square meter. I'm now wondering whenever I should buy highest quality ply, which is described as "knot and patch free", and costs (for 4mm) 13.3$/m^2. Second-best quality is described as "may consist knots and patches, no more than one per square meter", and costs 9.5$/^2. And price goes down with each step of quality down to "with knots, may have holes" which costs 7$/m^2. So, should I invest in the best quality, or maybe it isn't worth it for such a project?
5. I haven't though about this until now, but do I need a skeg? I know it helps tracking when rowing, but it adds weight, and I don't plan on rowing much. Also, it may ruin my balance calculations, because it works very differently than NACA foils.

I'd appreciate if anyone could help me out on these final stages