Basic rules for a 38' power cat design

I'd like to draw up a 38' lightweight power cat and have it close enough to correct for balance, and performance that the navel architect didn't have to make wholesale changes to make it work.

Is there a software program for this or a rule of thumb type set of rules for balance power etc. I'm only looking for a rough guide.

The end result is a power cat
Aluminium
about 35-38'
Lightweight - 3t dry
Outboard motors
15kn cruise, 20kn wot
Flybridge
1 master
1 head
1 shower

TIA
Dave

 

Don't worry about the length, the NA will handle that.

Don't worry about the dry weight, the NA will handle that.

Don't bother drawing stuff up for the NA. They will handle that. By all means draw stuff for your own purposes, but spare the NA your efforts.

20 knots w/ master, head and shower. Okay.

Now write down about twenty pages of where, when ,why, and by whom the craft will be operated. That's what the NA needs. Include business plans, construction and delivery constraints, budgets and payment constraints, etc. Will the NA be overseeing construction? Will there be class approvals? Will there be sea trials?

Any hard dimensional limits should be accompanied by an explanation of where they come from.

The size, weight, and accommodation list so far seem to counter indicate a catamaran, and aluminum wouldn't be my first choice, either. The Munson 35' aluminum landing craft cat weighs about 15,000 pounds wet. It's an open boat with a decent cabin but no master or shower.
https://dmna.ny.gov/nynm/manuals/MILBOATS_TECHMAN_BK071.pdf

It's difficult to find off-the-shelf outboard options for your size-weight-speed combo. The boat is too big and light and slow to keep the props in the water in a seaway. 18 ton, 38' cats that do 45 knots with 4 X 300 hp outboards are no problem, but what you have suggested doesn't work in practice unless it is in extremely protected waters.

So don't be surprised if an NA suggests a 25' fiberglass monohull that looks an awful lot like the thousands of other 25' monohull pocket cruisers out there.

 

Thanks

I get what you're saying, I want the layout clear in my head before approaching someone.

Interesting point about sea trials. When negotiating with a NA is in common to make the contract performance based.

 

I agree about the props coming out of the water in rough weather. Are inboards an option?

 

I hear what you're saying but not an option for me, I have a 50' sailing cat now with outboards and apart from the prop grip of inboards Id rather the OB's. I'd like to beach it often and I'd like the steering control in the marina, and I don't want separate rudders and I like the weight reduction and to be able to lift them clear of the water when moored or at anchor.

 

I understand that outboards will be a constraint. In that case they should be included in the Statement of Requirements (SOR). This is what the NA needs to start the design. Afterwards they get separated between constraints (these can't be changed, like outboards) and the rest. The rest are, for example, light weight, offshore capable, styling, shallow draft, etc. The latter are goals to be acheived, but don't have specified dimensions or details. They will be introduced after the constraints are defined. Drawing the boat is the final step and is only a graphic representation of the design.

 

Run Yamaha F70s; lightest in class, I went to the 90s and regret it.