Low-speed sailboat hull

Thanks for the sketch and explanation. Your flotation solution is neat. But if I understand correctly, it doesn't add lot of structural support. It's a good idea, but I think, why not keep these side decks which add: 1. Flotation; 2. Structural support; 3. Seating in higher winds.

I haven't yet started working on grating. Hull is now 22.4kg dry weight, so I can still add non-removable stuff, or trade remaining weight for length.

Please take a look at my update later in this post.

So, I updated the design, got rid of stern air box, and turned bow deck box into dry storage space by cutting out part of the bulkhead. Side decks are 4mm plywood, supported by 6.5mm plywood bulkheads, 6 on each side, spaced apart by 50cm, with first and last coinciding with transom and bow bulkhead. I will add some holes in these bulkheads later, so that each section doesn't become isolated and can ventilate properly with only one hatch.

Some screenies:

http://static.dyp.im/9tjMqPRIVN/380451b405ece29ed5c6f4508959f0bf.JPG
http://static.dyp.im/73bjL4L6AL/67e50341b6cd1fb1417c8c4f72af5b57.JPG
http://static.dyp.im/x621icxXIh/d3c649145315966ac57baef603f359eb.JPG

I tried fitting virtual people inside, and it seems that if crew is sitting in proper position for correct trim, there is very little space for legs, and the daggerboard case would get very annoying. I haven't been able to solve daggerboard problem, since now it's impossible to insert it to side decks. But I increased beam to 1.2m.

http://static.dyp.im/5EFGNcAWZL/15a40e0230a35de94e97ac223e7b5102.JPG

And some more views with people inside:

http://static.dyp.im/0N3OlQQle1/28dba043cd8b62da3b445060e74180ca.JPG
http://static.dyp.im/yulhiqcVcF/9e7632d732571e6b4b3f4c95556e6a8c.JPG

As you can see from the first screenshot, daggerboard case is really getting in the way. I can't make it lower, or it will leak (the top is only 5cm above waterline now). I can't move it to side decks, since they are narrow, and their curvature won't allow for seamless installation.

As you can see from the second screenshot, the bow crew member will have a hard time when tacking. I could make boom higher, but that will lessen sail area (which is 6.7m^2 now). I'd rather increase sail area instead. Is it possible to raise the boom higher, and compensate in other dimensions? In which direction should I enlarge the sail so it sets well?

 

P.S. Problem with crew position solved. I increased sail area by lengthening boom and yard, peaked yard up a little more, and since sail area moved aft, daggerboard box also moved aft. Now, crew can fit around the box.

Screens:

http://static.dyp.im/TGA9sjI84Y/66adb29f91578620644a69a2925612a4.JPG
http://static.dyp.im/GED3hXPEFm/fab5704c27615287ee66916b92b94555.JPG

Doesn't solve the boom problem, though.

Anyway, how does this look now? Are hull updates good? Is the sail looking fine (7.3m^2, by the way)?

 

Solving "the boom problem" is easy: frigging duck when tacking or gybing. Basic SOP for dinghies anyway.

The new sail looks like a better shape, although you may need a bit more of the yard forward of the halyard attachment point. Be aware that to get a lug like this to set well, the luff needs to be very strong. That means plenty of reinforcement with tape and/or luff rope. If you can get one of the modern low stretch lines over there (like Dyneema or whatever the latest cool thing is) that would make an excellent luff rope.

I notice you said you increased the beam to 1.2 metres. TBH that's not a lot, so I was surprised it was less before. What's your freeboard?

 

Ducking... Yeah, that's fun. The last time I was sailing with my girlfriend, she was begging me not to tack because of low boom. We had very unpredictable winds, and with unweatherly boat, I tried to use every wind shift to get to our destination. I was like "prepare for tack!", and she was like "oh God, please, don't...." me: "tacking!" she: whyyyyy...." (nevertheless, we both loved the experience). So maybe you're right, it's not worth sacrificing sail area for comfort.

I'll modify the sail for proper halyard attachment point. Maybe lengthen the mast to 4m too.

And sure, I can get that rope. Our local yachtclub sells it at exaggerated price, but I don't need much of it, so...

 

Yeah you can always raise the sail up a taller mast in light winds. That'll help get it up in what breeze there is.

 

Here, I extended mast to 4m, and drew a halyard, as it goes from mast top to theoretical attachment point (40% of yards length aft of front end).

http://static.dyp.im/vUZZCAihlC/556041d381d4f1b2b661da85ded73347.JPG

Considering that yard isn't lashed tight to the mast, just with loose hoop (from the same halyard probably, as in GIS), will the yard be balanced like I drew? Or would it swing forward (I mean not the angle, but the whole yard moving towards the bow)?

I don't really know how to predict what angle will result from the mast top to the halyard attachment point. How can I calculate that?

 

One more thing. While designing removable seat for rowing, I realized that (unless I forgot something) with current design, it doesn't have to be removable. It only adds 1.35kg to a total of 23.75kg (as always, dry weight) which is still under 25kg limit. It is located above daggerboard case. Not only that, but it shouldn't get in the way of sailors, since they sit around daggerboard case anyway.

Would be made out of 4mm ply. Second screenshot shows how there are 4 pine beams, 1x2cm to support it.

http://static.dyp.im/ccMGh0VRy1/53660a72526c283f49d39d00feec92b4.JPG
http://static.dyp.im/lRa9v04GD9/76a5105646af4a3a49742dd84ec76e8d.JPG
http://static.dyp.im/DtFdckkvZI/774dc93d5fcb2d4b61b4b639167b85dd.JPG

I see no real disadvantages to having this seat non-removable. Would add strength to structure as well, although there is enough of it already.

Sorry for so many updates and questions, but I'm working hard now.

P.S. I figured out that each side tank has 35 liters of volume, so that's 70 liters in total. Maybe not bad in case of capsize, yet most of that buoyancy is by the sheer line.

P.P.S. I also ran some calculations, and according to them, righting moment of this boat is 230 nm with two persons on board, allowing them sail flat (not heeled) in 4.8m/s, or 9.3 knots, or 10.7 mph of wind. Usually dinghies are designed to take reefs in 15mph (6.7m/s, 13 knots), but this boat is designed for low winds, so I guess this kind of figure is ok. Please comment if I'm wrong.

Sorry, I forgot the question. Freeboard is about 20cm in maximum load.

 

20 cm isn't much freeboard. I'd generally expect 30'ish on a boat like this. She'll dip her rail pretty easily with 20.

 

I see you are making real progress here.

I like what you did about the dagger board problem, by designing the rig around it, so it could be moved aft. This is an excellent foray into the problems of really small boat design. I've enjoyed this thread.

The 35 liters per side tank sounds about right. A swimmer could easily push that under him, unless he is quite small himself.

With my scow design, I went with somewhat less, but not much.

 

That may be right. I could raise sheer line a bit, or make it less concave. That would probably result in slightly higher weight, probably come up to 25kg limit. Not a problem, but there is another:

As I tried ordering all plywood parts on 4 sheets (three of 4mm, one of 6.5mm), right now, they fit just barely:

http://static.dyp.im/iOoHCniy1l/93af284eba73c0b0d08989486c6ab444.JPG

If I raise sheer line even by one centimeter, it won't fit anymore on these standard 1250x2500mm sheets (which correspond to 4x8 feet US standard). Most of plywood manufacturers in Europe provide different dimension sheets as well. So it would be possible in my case, although more costly. Also, if this design proves itself, I'd want to post plans for it on the internet, free of charge. And since most of boatbuilders are located in US anyway, it would make a bit of an inconvenience for this project to call for non-standard sheets.

So, are you quite sure that I must sacrifice this in favor of higher freeboard.

P.S. As you can see, I used just half of 6.5mm sheet. I plan to use the other half for workshop table top, since I have no workbench. I would construct it before starting to build the boat.

Thank you for your compliments. I'm really trying this time.

As you can see from the screenshots, I haven't designed any removable grating. I though that I could just find some styrofoam, as hard as possible, join several pieces together to a thickness of 5cm, and enclose in PVC fabric. Something like what was done in Featherwind, the boat tdem shown (here).
Or I could just put some inflatable pillows, if I find one of dimensions that would fit and not move around too much.

Do you see any flaws I might have overlooked? Apart from the freeboard, people stopped criticizing. I wonder if that's because they are tired, or because design has been perfected (less likely)

There are just few things remaining: rudder hinges, ventilation openings for buoyancy tanks, oar locks, oars themselves, and so on. After that's done, I can produce plans, determine amount of materials needed, source them, and start building.

So, any major issues I need to think over before I go into finalizing the design?

 

No, it doesn't. 8 feet x 4 feet is 2440 mm x 1220 mm. 2500 x 1250 is approximately 8'2" x 4'1', so it wouldn't fit on those sheets anyway. Also, in Australia and NZ the metric conversion was rounded down to 2400 x 1200, which is the standard sheet here (although other sizes are available, depending on origin). So, if you really want it to fit on the same number of sheets for everyone in the world, you'll have to design for 2400 x 1200 sheets.

Personally I wouldn't worry about it. My view is that materials exist to serve me. I don't exist to serve the materials.

I am quite sure that you have very little freeboard for a open sailing dinghy. I'd consider this amount adequate for a flat water rowing skiff at maximum load.

Double it.

 

All right. I'll raise it to 30cm.

Um... There is no way to do that, except for complicating design a lot and adding lots of weight, as well as reducing space for crew.

 

Raising the freeboard will automatically increase the amount of volume in your side tanks, even if you don't do anything else. However, you had them triangular in section last time I looked. That means you can get a substantial increase in volume with little or no increase in weight, simply by moving the base of the tanks inwards a bit.

 

Well, yeah, raising sheer will do that, you're right. But triangular makes a lot more sense to me. Much more easier to build. And if I move them inside, there will be less room for crew.

Are you really sure there won't be enough buoyancy with raising of the sheer line? Sharpii2 thinks that 70 liters is about right. With sheer line raised, it might be 100 or more.

 

Ok, do wotcha like.