I would like some critique on a boat I'm designing

Lately I've been working on plans for an 8' sailing digney and was hoping for some advice before I go any further. Particularly on hull shape and the width of plywood used for it.

In the picture below I have two cross sections at the beam, the bottom one being my original draft. However I think it's too rounded, I need the boat to be stable enough for 5 people to get in. Do you think the one above it is a better shape for the boat or something else?

For the hull I was thinking of using 3/4" plywood for the bottom three panels and the back with the other panels being 1/2". Is that a good selection?

Those are the sections I would like some advice on before I continue planning, but I would appreciate any other critique you have.

 

3/4" for the bottom seems excessive to me.

5 people in an 8 ft dinghy... why?

 

Its very unlikely that five people would be riding at the same time but I wanted to make the seats wide enough for the possibility. Its would probably only be 3 people riding at once at the most. And mainly used by one person.

 

This is a challenge then as the boat weight will change hugely between one passenger and 5.
What production 8' dinghys can carry 5 people?

 

Yeah... that probably won't work. It looks like 5 people boats are around 17' or so.

 

Why are you designing your own boat when there are so many dinghys available, used, for 1/10 the cost and anguish of building/designing your own?

 

It's a bit of a personal challenge. I wanted to make one with higher walls and seat so it would be comfy to sit in and look slick with the covered storage.

 

3/4" is too heavy for such a small vessel. As a monocoque, stitch and glue structure, I think you could cut all of your hull planks by 1/2. Five people is a lot for an 8' dink unless you're thinking of crew of a diminutive size. You need some rocker in your hull as it's going to drag its butt horribly as you have it drawn right now. You might want to look into a scow type or pram bow. This should provide additional stability and capacity on such a short waterline and restore some displacement that you will lose by adding rocker.

You've got a bit of refining to do before cutting any material.

 

Unless they are large people, you can get 5 into an 8' dinghy. It will have the freeboard and buoyancy for it. But it will have marginal stability at that load. If they all get bumped to or decide to look over one side, over she'll go. Production 8's are limited to 3 in order to pass USCG notional stability and sea worthiness standards and resultant liability reasons. And yeah, it would be astonishingly heavy with 3/4 and 1/2 ply.

 

But... this is boat design dot net. Its all about the cost and anguish.

 

A deep hull (high sides) + heavy build + and somewhat narrow bottom = you could have some stability issues. Also, you have 7 panels whereas the same could be accomplished with 4. .... Keep it simple !
For this size I would say 4 people maximum and be prepared to go 'swimming'.
Why don't you take a look at other 8' dinghy designs that have proven out. The 'Sabot' (sample enclosed) has been around for years... I have built one in plywood (as a kid, at school) and later, one in FRP. Both worked very well. You could raise the sides a bit (3" ?) but leave the thwarts as is. Be aware- the higher the sides the longer your oars will have to be.

 

Agreed.
So lets look at some good, proven designs.

 

3 would be the sensible maximum for an 8ft dinghy, and that would be with a pram bow, not a stem and unless 2 of the crew were children probably only when rowing, not sailing. Certainly you won't get 5 adults in an 8ft sailing boat, you'd need something like the 16ft Wayfarer to do that

My 10ft Duo dinghy is built in 4mm plywood, as is my 8ft Crayfish. Any thicker and it won't bend round the curves

Richard Woods of Woods Designs
www.sailingcatamarans.com

 

Dear all,

just for my understanding which calculations you do to know how many persons the boat can have? Do you know the free board?

These questions are only for my understanding

Thanks,

Raffaele

 

Dear Raffaele,

I use the number of persons times 4.25 ft. ( 1.3 meters ) for a rough estimate.

How about you, what formula do you use?

Kindest regards.