Calculating the speed

[huntermike32]

how do i calculate the speed of a boat compared with its horsepower and boat weight and size. thanks

[Jango]

I have found the following Formulas quite accurate.


MPH = Sq rt (Prop HP divided by Gross weight) X Hull Factor


Hull Factors

180 - Heavy Deep V bottoms

200 - Fast shallow V bottoms

220 - Stepped bottoms or Fast Cats

Hope this is of some help, John

[RANCHI OTTO]

This method is based on several sea trial results.
The speed coeff. is KR=(BHP)^0.551/(Displ)^0.476
This is only a rough estimation but I hope interesting for you

1 / SOL = immersed screw
2 / Waterjet
3 / Piercing screw
4 / Outboard

[cyclops]

The Glen-L boat design pages had a simple curve for various horsepower and weight V hulled runabouts. It predicted my 220 hp 19' bowrider to within 2 mph.

[huntermike32]

ok so on glen l they did not have my exact boat so to estimate the speed i would find the square root of hp divided by weight then multiply that by the 180 for the deep v design

it would look like this square root of (50hp/800lbs.) multiplied by 180 for the deep v design = 45

[Jango]

Yes you are correct, but keep in mind.

1. Gross weight includes Boat, Motor, fuel, gear and passengers.

2. Prop HP is HP at the output shaft, not Brake HP. ( I believe for outboards, B.I.A. Certified HP is Prop HP)

Also, calculated results assume Optimum Prop and All conditions Ideal. In other words - Max. Speed.


Note: My GlenL "Monoco" with a Hull Factor of 200 calculates to within 1 mph of actual speed.

sq rt ( 350 / 2350 ) X 200 = 77.2 MPH

[cyclops]

That's because they used your boat to make the chart.

[RANCHI OTTO]

the inputs are of the above formula are:

Vk = speed (knots)
Displacement (tons)
BHP= metric horspower

[Jango]

The GlenL Chart does NOT at all come close to the calculated and actual Speed of my Monoco.

Using the chart shows a Speed of 62 mph, whereas both calculated and Actual Speed is 77+ mph.

The chart does NOT take into consideration various Bottom configurations. Appears fairly close for Deep Vee's only.

[raceday]

you can go here to use this calcutator.. http://www.racedaymarine.com/speed.htm

[cyclops]

Jango. Your bottom at the rear becomes almost flat. Right?

[Jango]

Yes, Cyclops. About a 3 1/2 deg deadrise.

[RHough]

Gotta love Glen-L.

I'm building a Dyno-Mite with a 15HP Merc. Glen-L says the boat should weight about 125# ...

125# Hull
125# Engine, controls, etc
330# two people

580# all up with 15HP = 32MPH
415# with one person = 38MPH

Woo-hoo!

The reality is that a 10.5" pitch prop is the highest pitch that I can find, so at max RPM with 9% slip the engine makes about 27MPH ...

For 15HP and 27MPH the boat can weigh 800#

It isn't going to go any faster no matter how light I build it.

[cyclops]

How is the acceleration?

[cyclops]

Jango. At the speeds you can run. Have you ever heard of Chine Walking or Hooking? They are very real and common where you are in speed. And not a live to talk about it after it happens. ------Surf over to --www.iboats.com --register in the forums and do a search of chine walking. First hand report from another overpowered boat driver.