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Old 23 October 2017, 12:40   #1
eze
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Country: UK - England
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Boat speed predictions

Hi,

Maybe you old sea dogs have seen this before, but if not then enter details & lets see how accurate it is

Speed Calculator




ps - not sure if this is designed for outboards, maybe just inboard stern legs?




Mercruisers only , sorry about that
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Old 23 October 2017, 13:48   #2
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It works for both o/b and sterndrive, and any make...
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Old 23 October 2017, 14:03   #3
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Either that is miles off or my gps is on the boat for mine
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Old 23 October 2017, 14:21   #4
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Bang on for our hard cruiser says 42 assuming that's mph we do 36kts flat out
It's way over for the delta says 55mph using 4000lbs and 52 at 4500lbs we get 40kts
And that's using the heavy deep vee cabin constant
Not 100% sure on the weight of the delta but I'd imagine 2000kg ish 7.4m with 240 yam inboard
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Old 23 October 2017, 14:39   #5
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RIBase
It is a standard formula irrespective of boat make, engine type etc.

It is called Crouch's Planing Speed Formula

Boat Speed in Kts = A Constant divided by (Square Root of (Weight of the Boat in LB / Shaft Horse Power))

The constant is the key to getting accurate results.

I differ with the Mercruiser constant and I'd say for a standard RIB the constant should be about 150.

As an example take a 1300kg (2860lb) boat with a 150HP engine. Assuming the engine puts out 135HP at the prop, then the speed would be about 33 knots. This is with a constant of 150.

150 works for my Pac 22. For Beamishken's Delta, 175 seems to give 40 kts with a 2 tonne boat.
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Old 23 October 2017, 15:23   #6
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I plugged in 2 ribs, a 5m and a 9m. A constant of about 200 is closer for my boats than their recommendation of 225.
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Old 23 October 2017, 16:19   #7
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Quote:
Originally Posted by GuyC View Post
boat with a 150HP engine. Assuming the engine puts out 135HP at the prop,
Have all outboards not been "prop rated" since the 1980s
Prior to that it was only Yamaha that used this system.

(I think your constant of 150/160 is probably realistic, gives about the right result for mine)
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