design issues

[jorgoz13]

Hello y'all,

I'm no boatdesigner, but a future boatbuilder and have a question about a certain design i've found on the net.

It concerns a step hydro from the 30's i think. But i find it a little on the small side, specially the beam.



I would like to know if it would be safe to increase the measures overal by something like 10% (which is an absolte max.). Which would mean wider beam and bigger depth. Lenght could be proportianaly adjusted by spacing the frames evenly further apart. I've been told so far that it would probably not make that big a difference if i change the beam but didn't touch the depth.

I would like to have your feedback on this.

George

[Raggi_Thor]

One common problem with rebuilding old designs is that the boat is lighter today thanks to modern materials like epoxy, plywood, foam, glass etc and (often) lighter engines..
If you oncrease the size 10% that means that the designed underwater volume, or displacement, increases by 1.1^3 or approx 30%. If the boat also is built lighter it will float very high on the water.

So you should at least do a weight calculation and compare with the intended displacement.

[D'ARTOIS]

The remarks of Raggi Thor are certainly correct, but in principle it can be done as to your initial idea. The increase in beam, where you will loose a little in prospected speed, will be made good by the lower weight of the vessel you intend to build.
Do as Raggi says and compare the weights and make a solid weight calculation before you start working.

[jorgoz13]

I can't calculate the weight yet, i haven't received the plans. But i'm curious, i'm not quite sure the boat that much lighter, as there is more material in a bigger boat. The original also didn't use glas or canvas i think. And i intend to use a light engine, say 135 hp. Speed is not that vital either. But someone told me the resale value would diminish if i messed around with the measurements, and this is a very important point for me, as i would build it to sell it.

George

[D'ARTOIS]

If you have a program like Max Surf where you can import frames you can automatically increase in % the overall dimensions. Or, the beam. To get additional length by spacing the frames a bit farther, is not a very good idea. If have done so with a sailing yacht design and the result was not to my likeing although the boat performed good. Basically it is never good to meddle with an original design, specifically not with those oldies. In that respect, I would take the exterior line from a particular design, and draw it anew, making my own weight calculations and whatever comes next.
I have played around with a tank-model that looks a bit like your design although it is longer and more slender, with a L : B ratio of 5 : 1;
The boat has he same or close to the same profile as your boat has, quite common in those years, including the slightly hollow flat bottom.
Even going fast, it leaves almost no wake, quite good in the busy waters of Holland, where lots of houseboats and moored boats clutter the sides.
An ideal configuraion for closed waters that are reasonably flat.

Remember a few things: If you broaden the beam does not implicate that you automatically increase draft. On the contrary.
The second thing is that if you broadens the beam, you will loose a bit of performance. The reason that so many fast boats are so slender.