How much hp to plane?

[Preformer 40]

I recently purchased a 40 ft go fast boat,and im turning it into a center console.I will be installing a outboard bracket and would like to know how much hp it will take to plane.40 ft 9000 lb 24 deg dead rise,9.6"beam.Everyone i talk to says i need at least 3X200hp =600hp,im thinking less,speed isnt important to me,i want to go fish and take family out for the day.I dont know how to mathmaticaly calculate does anyone know?
Im thinking 1X300hp pitched at 11 inches?

[DaveB]

Hi,

I've got access to NavCAD at work and I know how to use it just enough to be dangerous... not very well...

Anyhow, thought that this might start an interesting discussion...

Note that PE=effective power which is often approximately half of the installed power, or you only get about 1/2 of the shaft power propelling you forward. A good start would be to assume the sticker on the motor would have to be double the effective power.

I've attached a pdf report from NavCAD... it includes as much of the input as I could... there's a good chance that I've made some mistakes/innapropriate assumptions and would be happy to correct them if pointed out...

Cheers,

Dave

[Willallison]

What power was installed in the boat originally?
How fast did it go?
Is the 6000# you quoted, the weight without engines, or with the old ones?

Boats like this often don't like to go slow - ie anything less than say 30 knots many tend to bog down. If you underpower a rig like this you may turn it into a dog...

[fede]

Hi, I did some quick math, and what I get is that a boat like the one you describe needs 450 HP for a top speed of 20 knots so I guess 3x200 is reasonable if you want to get to the 30 kn.You might go with a twin 250 config. I think thogh that if a hull has been designed to go fast it performs well when it goes fast...

[Preformer 40]

Hey dave thanks for the calculations!! I agree with will about the thing being a dog at slow speeds and i will have to think this one out,The boat is 9000#.Im leaning towards twins now,may be 200x2 15pitch.Thanks for helping me on this one,im clueless on the physics.The boat originally had 3 small blocks and was said to go 80+ mph.

[tom28571]

If this boat were the usual modified V hullform with about 10 to 12 degrees deadrise at the transom, it should plane and easily make 20 knots on your 300hp single. The 24 degree deep V hull makes me side with the thoughts of the others though. 9,000 lbs sounds light for a 40 foot 24 degree V hull. Is that hull weight or all-up displacement? As was said, these boats are not happy underpowered and running slow. I would guess that 20 knots might normally be considered slow for this boat.

[fede]

24° deadrise makes it a high performance deep V hull with no doubt, 80 mph... may be with a total of around 1100 hp if not more,I don't see how with outboards though.

[Preformer 40]

I was told that the hull is 9000 pounds,im going to put it on a scale in a week.
The more i think about it,a single is not the solution,I know i need prop area to push this pig.My latest thoughts are 2 x 250 at 15 pitch that has got to move it along pretty good? Im looking for torque and 38 mph will this work?
The mercury guide says 47 mph,but i think thats on a low deadrise.

[Willallison]

If we assume that the boat did indeed weigh 9000# fully rigged (and that it will remain at this weight), and that it ran 65 knots with 1000hp, then a quick run through with Crouch's formula suggests that you would need 380hp to do 40 knots.....

[blowhole]

I put 2 v-8 omc outboards wot 46 mph,blew one up, took off and got 30 mph out of one motor.
This boat was hard to tell when on plane,it didnt nose lift and speed up it just lifted slowly and felt better.i think it was planing around 27 mph?
props 15x15 ss,one motor 15x11 alum.mpg-.7 it was a pig! its gone now,but it would ride over 5 footers at 46 mph! you could barly feel any bumps due to its speed,lenght and weight.once you got to the spot to fish boat rolled alot and made fishing difficult.

[bhnautika]

Just keep in mind, some of these formulas use SHP not hp, sometimes the difference is a fair bit.

[RANCHI OTTO]

My estimation based on coeff. KR= (BHP^0.551)/(Displ^0.476) for similar boats and based on my previuos designs is as follows :

Vk (knots) / BHP (hp.metric)
---------------------------
20 / 200
25 / 262
30 / 345
35 / 432
40 / 540
45 / 672

[bhnautika]

[quote=RANCHI OTTO]My estimation based on coeff. KR= (BHP^0.551)/(Displ^0.476) for similar boats and based on my previuos designs is as follows :

I think that last formula is Eugene Clements and it should be
V=124.7 X (P^.551/W^.476)
P=SHP
W=weight in pounds
V=mph multiply by .8684 for knots
If you don’t mind me asking how do you use the coeff in your estimation

[RANCHI OTTO]

The KR coeff. is derived from SEA TRIAL RESULTS based on designed boats. From the speed vs power sea trial results, I calculate a factor KR for each speed.
So doing I have various KR vs speed curves according to waterjet/immersed screw/surface prop./outboard propulsion systems.
This KR is not "empirical a formula" but is based on REAL SEA TRIAL RESULTS of a lot of boats.
This is a very usefull fast estimation method that permit you to evaluate in few minutes the performances of a boat knowing only the displacement.
The E.Clement estimation is valid only for immersed screw...

[RANCHI OTTO]

This is an example of my KR coeff. for surface piercing propulsion system: