Please sanity-check my 5'6 dinghy design!

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markgoespop
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Please sanity-check my 5'6 dinghy design!

Post by markgoespop » Mon Jul 31, 2017 5:57 pm

I would like to sanity-check my first design with you folks. I will be building a dinghy to go on my tiny 18ft trailer sailer in the next 3 weeks, come what may.

I have the following design constraints:
- cheap as possible
- *really* easy to build
- carries 2 (we're ~150kg)
- won't flood in rough-ish costal water
- max length 5'6
- max width 3'6

My current design is a flat bottomed pram. It is fore/aft symmetrical. It has extreme rocker: 7" deep at the centre, so that the waterline at 150kg sits at the join between the bottom and the bow and stern transoms. The sides are 14" deep, and at 150kg there is 7" of freeboard at the lowest point in the centre. The bow and stern reach 7" higher. There will be a 7" deep skeg on the back half of the hull.

I will build it from cheap 6mm ply, following the construction method of the 'Elegant Punt', i.e. external rails, glue and screw:

https://www.youtube.com/watch?v=P1FXt9-h0NQ

Does this sound sensible enough, given the design constraints? Can you see any refinements?

Here is a paper model to help you visualise:
Photo on 31-07-2017 at 22.21.jpg



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Re: Please sanity-check my 5'6 dinghy design!

Post by terrulian » Mon Jul 31, 2017 10:22 pm

If you use an inferior design, inferior materials, and inferior construction methods, what do you think the result will be?
Why re-invent the wheel? The designs on this site can be assembled very quickly if you are not particular about the level of finish. You can use cheap plywood but I would certainly not go with construction grade. The boat in the video will not last a season absent fiberglass and epoxy. There is a contest like this here in San Francisco every year where assembly must be finished in a day. The boats have to make it around a course of about 100 yards in protected waters. Most cannot do this.
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Re: Please sanity-check my 5'6 dinghy design!

Post by fallguy1000 » Mon Jul 31, 2017 10:32 pm

Okay, first of all, I just need to sanity check the purpose of the design.

Why would you want a dinghy with an 18' trailer sailer?

Assuming the 18' boat uses a centerboard and has a shallow draft, I can't imagine using the dinghy to get off the vessel, other than if you are mooring. And then is the dinghy cartopping?

Finally, I don't know the propulsion plan, but short boats with lotsa intended or incidental rocker tend to cork or bobber or spin like a top in less nautical terms. This means everytime you row imperfectly straight, the boat will tend to spin. Now, I think the skeg will help with that some, but I'm not wise enough to be sure. The dimensions of the craft and the rocker make me very nervous about corking. A longer skeg might help.

I would probably model the design in 1:10 scale first. Realizing that it is very difficult to scale down 6mm ply to .6mm ply; you'll have to go a little heavy on the thickness, but if you did it in some light frp; it would be pretty simple with some mold wax I'd say. To test for corking, just apply some force to one side of the model (blow on it) and see if it moves forward or starts to turn quickly.

I think you'll have better luck with responses on boatdesign.net as well. This site is more oriented (not entirely) toward boat building.

Your picture has me thinking of rub a dub dub.....just sayin.
My boat build is here -------->

viewtopic.php?f=12&t=62495

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Re: Please sanity-check my 5'6 dinghy design!

Post by Marshall Moser » Tue Aug 01, 2017 10:39 am

Why the dimensional constraints? Storage on the big boat? What about the D5 or the FB11? or either of those two designs scaled down a bit?

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Re: Please sanity-check my 5'6 dinghy design!

Post by Evan_Gatehouse » Tue Aug 01, 2017 6:24 pm

won't flood in rough-ish coastal water
That's where your wee bathtub will have problems. It probably will. Just too small.

Also you won't be able to bend the plywood sides to meet your bow. Radius is too tight.

I'm guessing the trailer sailer is on a mooring? If so, can you leave the dinghy behind on the mooring?

If you are cruising the sailboat then almost any place you can dock a dinghy you could dock a 18' boat.

if you are cruising and want to anchor the sailboat and go ashore, decide to get a bit wet up to your knees or so, anchor the sailboat in deeper water with a clothesline arrangement, and then go ashore that way. When you want to get to the sailboat, pull the line and retrieve the sailboat.

Like this:
http://navigatorjoel.blogspot.ca/2011/1 ... sited.html

I'd go with the FB11 before I'd built a 5-1/2' dinghy. Not much difference in work, only 3 sheets of plywood.

The smallest practical dinghy I know is the Bic Sportyak - and it's marginal for anything other than sheltered waters.
https://www.jamestowndistributors.com/u ... ?pid=56175#
designer: FB11/GV10,11,13/ HMD18/
SK17,MM21/MT24

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Re: Please sanity-check my 5'6 dinghy design!

Post by jacquesmm » Tue Aug 01, 2017 6:32 pm

That Sportyak is a death trap. A good friend of mine almost drowned in it: swamped by a big wake then unable to get back on board because the boat was to small.
Good to know. I admit my experience with it was as a much younger (lighter) teenager with a friend. Total distance we would row it was from a sailboat on a mooring to shore - about 100' maximum.
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Re: Please sanity-check my 5'6 dinghy design!

Post by deuce_454 » Fri Aug 25, 2017 3:05 pm

there is a folding dinghy in the free plans section that could be fun to build... or some of the single sheet designs out there

personally id build a skin on frame row boat but that just because its insanely light and really beautiful

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