duncan
Active member
there is a precise mathmatical formula based around the square root of the waterline length - and then a load of variables!
you can generally see it clearly from the behaviour of the boat in response to increased throttle - although with turbo diesels you need to beware the imact of the turbo coming in at particular revs that can confuse! Most are set up so that the turbo comes in before the maximum displacement speed for the hull is reached - and thus is there to get you over the 'hump'.
For planning hulls in the 6-10 metre range it's not far away from the length in metres as knots. For pure planning hulls it tends to be a quiet distinct speed but for semidisplacement it is naturally a wider range as the hull shape itself is designed to avoid the sharp transition.
So for Dave's boat he will start to see a lower return on boat speed for engine revs approaching 7 knots and then a return to a (different) linear relationship above about 10/11 'ish - all through the water speeds.
I have a v.slightly longer hull, distinctly planning, and for me it's 7.5 knots (1500rpm) through to 9.5 knots (1800rpm) when the boats reasonably light or 10.5knots (1950rpm) fully loaded. Then it's reasonably linear through to 28knots (3100rpm) at just under 2knots per 100rpm untill drag kicks in over the last 500rpm to max out around 34knots (3600rpm). The good news is that I can cruise happily at anywhere between 12 - 28 knots depending on conditions with the engine equally happy/loaded.
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you can generally see it clearly from the behaviour of the boat in response to increased throttle - although with turbo diesels you need to beware the imact of the turbo coming in at particular revs that can confuse! Most are set up so that the turbo comes in before the maximum displacement speed for the hull is reached - and thus is there to get you over the 'hump'.
For planning hulls in the 6-10 metre range it's not far away from the length in metres as knots. For pure planning hulls it tends to be a quiet distinct speed but for semidisplacement it is naturally a wider range as the hull shape itself is designed to avoid the sharp transition.
So for Dave's boat he will start to see a lower return on boat speed for engine revs approaching 7 knots and then a return to a (different) linear relationship above about 10/11 'ish - all through the water speeds.
I have a v.slightly longer hull, distinctly planning, and for me it's 7.5 knots (1500rpm) through to 9.5 knots (1800rpm) when the boats reasonably light or 10.5knots (1950rpm) fully loaded. Then it's reasonably linear through to 28knots (3100rpm) at just under 2knots per 100rpm untill drag kicks in over the last 500rpm to max out around 34knots (3600rpm). The good news is that I can cruise happily at anywhere between 12 - 28 knots depending on conditions with the engine equally happy/loaded.
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