# Calculating the speed

Discussion in 'Propulsion' started by huntermike32, Oct 19, 2005.

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### huntermike32Junior Member

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

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### JangoSenior Enthusiast

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

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### RANCHI OTTONaval Architect

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

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### cyclopsSenior Member

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.

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### huntermike32Junior Member

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

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### JangoSenior Enthusiast

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

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### cyclopsSenior Member

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

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### RANCHI OTTONaval Architect

the inputs are of the above formula are:

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

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### JangoSenior Enthusiast

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.

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### racedayJunior Member

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

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### cyclopsSenior Member

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

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### JangoSenior Enthusiast

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### RHoughRetro Dude

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.

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### cyclopsSenior Member

How is the acceleration?

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### cyclopsSenior Member

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.

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