Help! An evaluation of a small boat hydrostatics?

Clinker ply would work.

 

Looks to me like the best sort of compromise is something like a Mirror dinghy even if it is very slightly longer. The OP's original sketch is somewhat like a poor version of the Ian Proctor Gull design but without the benefit of being a better all rounder, and the Gull is a 'tub'! with respect.

As many have said, the shorter the boat, in many ways the harder to get right as each part has to work properly. As a general purpose all rounder the Mirror is still hard to beat even after 50 odd years. So something along those lines would give a better result in the real world, or a similar take on the idea. There simply is not enough length to get a hollow entry (and stern) more traditional shape to work properly.

 

ya, ok guys thank you all. Suppose you've right. Well by saying it can be built by two (maybe three I dont know) sheets of plywood I ment using the lapstrake method for boatbuilding. I have built three prams my friends like optimist - the best was SeaMidge (freeplans are available). Prams as you said -and I can say that- are practical and useful under many conditions and many of them are developed for even rough waters...but I want something new and I won't give up.
thanks for your tips

 

If you just want it light enough to be easily handled on land, you can get that in a larger (and better) hull. As an extreme example, I've just built a clinker ply hull that is 5.8 metres by 1.2 and weighs about 28 kg.

Even if you use heavier scantlings (which would probably be advisable if it's going to be knocked about) you should still be able to get something like 3.6 metres at under 40 kg.

 

You're absolutely right! Clinkers give a light and solid construction even at longer lengths than 7 or 8 ft. But don't u think that is a great and interesting challenge to design a small boat close to two meters which is reliable enough as a tender?. Just to go 100 feet away from the shore some good days and enjoy fishing? I think that tiny boats two meters - or if u want less than 7 ft- long are not been exploited enough by designers. Τhey have not paid proper attention to them. But I strongly believe that those small boats have enough abilities to discover

 

Let´s be careful because a boat for the sake of being small does not have to have abilities to discover. The boat may be small and be a disaster as a float. But I think that what I just said is very obvious.

 

Well, my opinion is that a 7ft boat could be prove to be a remarkably successful tender..

 

Agree with you, kavos, but a boat with a big breadth in relation to length is likely to be a very low balance period, making it uncomfortable for passengers and perhaps even dangerous.
Maybe that's why the OP is asking for advice in relation to the stability of the boat.
I think it's no good solution to cut the length of a boat that has been designed with certain measures. If that boat worked well, the smaller boat (same breadth and depth) is unlikely to work well.

 

The "built by eye" method is truly just that, but also built by people so familiar with their shapes, that that can build by eye. Judging by your renderings, you clearly haven't this level of experience with the shapes involved, so you'd be best advised to use a set of plans and adapt the aesthetic considerations to it afterward.

The best thing you can do with a 7' boat is carry yourself and a guest, maybe a picnic basket and that's it. It's not going to row well with this kind of load, it's not going to sail or power well either. Simply too small to do much, given the volume you have to work with.

 

my friend TANSL, I'm telling you that there are many successful designs in the same dimensions. Do u really think that this boat has a big breadth in relation to length?
I think the question is the hull type.
And yes PAR I agree with you. It is a good advise to use a set of plans but not a creative idea...

 

My dear kavos, yes, I do "think that this boat has a big breadth in relation to length".
But this is wishful thinking because without doing any calculations, I can not know how far it is inconvenient.
Think, for example, two people standing in the boat, the difficulties that will have to maintain equilibrium.

 

The Tiny Ripple. It is indeed tiny...
6 feet over all
4 feet 9 inches on the water line
3 feet 6 inches in breadth
3 1/2 inches draft
freeboard at the bow is 1 foot 2 inches
the least, 9 inches
and at the stern, 11 inches.

 

Dear friend kavos, I am very happy that you have put that example.
Have you seen how is the bow ?.
Have you seen how is the waterplane area?
Have you seen how deep is it?
You think, really, that this boat has something to do with what we have attached to post # 1?
Have you ever stopped to think about the relationships between the dimensions that exist in boats ?.
Why do you think the designers start by defining these relationships when they want to project a boat?
I advise you to let yourself be guided by someone who understands boat, for your sake and that of your passengers.
But, most importantly, make the boat you want and be happy.
Cheers.

 

At most he will be carrying one passenger. Short dinghies are OK for protected waters where the vessel they tend is anchored or moored. They won't perform well on any other characteristic other than load per length.

 

Oh dear TALS you are surprising me!
I didn't post that example as the same one.
only to notice you the relation between breadth and length
OK here another example
just for you
The Auklet
LOA:2.2 m - 7' 2"
Beam:1.2 m - 3' 11"

and thanks God I let myself be guided by someone else who anderstands ship geometry
Cheers