My formula for power/length on a planing boat - seems to work

[Sundays_Child]

Length in feet squared, divided by 2. Nice and simple.

At pretty much any length you have a fast but manageable boat.

 

[peterb26]

So for a 30 foot boat:-

30 x 30 = 900

900 / 2 = 450

450 hp on a 30 footer. Yes I think that will plane......... /forums/images/graemlins/tongue.gif

 

[Whitelighter]

hmm, think you power needs are a little ott. Depends on what you class as a fast boat of course.

My BMB was 34 feet, so by your reckoning it should have 34x34/2 = 578 Hp. It in fact had 2 x 230 Hp diesels, so some 118 Hp short of your figure. Yet it managed around 38 knts.

My W24 has a single D4 225. Again, by your maths this should require 24.9x24.9/2 = 310 Hp. With 225 Hp available she hits just under 40knts.

Of course, you might want to go faster...

 

[benjenbav]

Maybe waterline length or length excluding bathing platform? Otherwise my 32 ft boat is seriously undercooked with 400 hp.

 

[Lakesailor]

Nah, think you may be wrong.
My 15.3ft boat will do 37 mph with a 70hp. It'll climb on the plane at 14 mph at anything over 2000rpm.
Interesting to know what it'd do with the 138 your formula recommends.

 

[Channel Ribs]

I was working out some recommendations for a chap last week and I think you need to figure weight in as well, after all the engine needs to propel the boat over the water in order to plane nicely; merely setting up interfering waves is not usually enough.

I suspect some of the less scrupulous engine retailers may be really keen on your method. /forums/images/graemlins/smile.gif

 

[duncan]

way to complex

80 hp / 1000kg

as good as any 'simple' approach - ie pretty unrefined and inaccurate

 

[Pinnacle]

I have 270hp pushing 1.8 metric tons....... /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

 

[duncan]

and you can't get on the plane????

 

[nyx2k]

80 hp / 1000kg
as good as any 'simple' approach - ie pretty unrefined and inaccurate

thats about right as my 250kg day boat has 25hp on at the moment and hits 28 knts no problem

 

[Hurricane]

If I round mine up to 52 feet then
52 x 52 /2 = 1352
1352 divided by two engines is 676hp per engine
My D12s are rated at 675hp

There you go - it works!!!

 

[Pinnacle]

/forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif It's too easy!!! 25 kts at c3100rpm. 35 kts at 4000rpm. I have not done WOT ( about 5000rpm, but it should be good for +45 kts )

Over /forums/images/graemlins/grin.gifpowered, I'd say!! /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif /forums/images/graemlins/grin.gif

 

[Andrew_Fanner]

Apparently I need 512HP not the measly 290 Mr Perkins so thoughtfully provided, and that assumes none of those horses have wandered off to the knackers Plane at 12-14 or so depending on load, water and tabs.

Isn't it Cranchi's Formula or summat?

 

[Sundays_Child]

I should have clarified that I like boats with quite a bit of poke.

Many people are understadably happy with a BMW 520, I'd prefer a 540 or an M5.

 

[Dave_Snelson]

28 ft = 392HP and I have 410HP.

80HP / tonne = 280HP

Either way OK

 

[Sundays_Child]

Jez,

MBY finally made it over here. Nice to see that you'l be offering twin D4 300's on the 34! Sounds like a lot of fun.

 

[Whitelighter]

oh yeah. Should do about 45 knts ish. Haven't got our hands on a proper set yet, but I beleive we have a hull waiting for a pair in Sweden!!!!