Semi-planing boats

but is there a calculation for cats?

Yes, there is an immersed transom. Hopefully, not too immersed, my engines are heavy 122#.

 

Your optimum speed will reveal itself under actual use, and may vary as circumstances vary, for example the loading condition of the boat, and many other factors. These days with metered fuel use and GPS, mpg can be right in front of you as you motor along. The transom immersion won't matter at the kind of speeds you will be cruising at., I'd say when water breaks cleanly from the transom, will likely be the optimum speed of the hull, but might not be the optimum speed of the hull/motors combination, only testing will show that. As an extreme example, if it had small motors and it had to be given full throttle to get transom break-out, it certainly would not be the optimum situation in terms of speed vs mpg.

 

darnit...I figured some magic, but probably the slower I go; the better the fuel economy

more worried about a heavy stern than anything

 

Ref the power cat in my avatar, when they replaced the 115 hp O/B motors with 150 hp outboards, they made significant savings on fuel, as they could achieve their service speed at less revs with the bigger engines.
And when the 150 hp outboards were replaced about 8 years later with a new pair (same 150 hp), the miles per gallon improved again quite noticeably.

 

You will have to see how that turns out, but your choice of engines indicates you desire a pretty lively cruise speed in the high teens (knots), so if that turns out a little expensive to feed, that is all part of the deal you make to have that speed up your sleeve, at least you can always reduce speed, whereas with small engines speeding up would be largely out of the equation.

 

we chose the engines to support autopilot; the alternators in the next model down engine are 50% alt output, from 32 to 16 or so amps

 

I think you will appreciate having the option to run faster, what do you expect top speed to be, 22-23 knots ?

 

In a round about way yes. But data is needed.

Lwl =?
Displacement (full load) =?
WSA at design displacement = ?
L/B ratio =?
B/T (at transom ) ratio =?
Hull Spacing (as %'age of Lwl) =?
Design LCG (as %'age of Lwl) =?

 

thanks!

 

So this 2034kg per hull...when doubled to 4.61 tonne, am I correct in assuming this does not account for the raft/bridge structure outfitting and pas/fuel etc?
Since im after the expected full load displacement.

Same for the design LCG... what is the expected LCG in full load?
If it end sup as your figure of roughly 50% this is no ideal... it should be further aft.. around 5-8% aft of midships.

 

The hull weights with engines and fuel are about
1600+400+fuel of 64 gallons full.

The cabin should be about 4000# (est)...hoping less

beams and cockpit 500#

I know RW did the hydrostatics mainly on the hulls.

The engines and fuel tanks likely force the lcg aft as they are, of course, aft. No?

Best I got

 

It'll do for now.
So Ive made some "guesstimates" based on your data:



This is "roughly" what the numbers suggest.

The 9 - 11 knots is your hump..so avoid that region if possible.
Good cruising suggests around 8 knots....with a top speed in a light load around 15 knots.

Make sure you can get your LCG around 5-8% aft.

 

Ad Hoc, I'm pretty sure his boat is considerably lighter that 9 tonnes ? Was that the figure given ? Couldn't be that heavy.

 

My bad.

I mixed kg and pounds some.

The estimated vessel weight would come in around.
1600# hulls times
400# engines and batts
400# fuel
x2
4800#
700# beams and cockpit
4000# cabin
4700#
Total 9500# or 4500 kg rounded up
4.5 tonnes

The boat has about 1000# available full; so real conservative to use 4.5/5 tonnes.

We expect to see low to mid 20s for top speeds. And I am a bit nervous about being heavy astern; so I actually am a bit glad for the lcg remarks.

 

No worries.
Ok... revised, with 'corrected' displacement:



Again, based upon many 'assumptions' to fill in the gaps.