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This is lifted from the boat design net,,Boat Design Net - the Boat Design and Boat Building Site forum.
Fun contemplation ,
a startling discovery on required power.
I have long been obsessed with accuracy in maths. Applying maths to power prediction and you come up with Wymans formula, Gerr's formula and Kieth's formula. It is the latter i want to talk about. If you search the net for "repowering Tortuga" you will find a very interesting article on these three formulas, how and why the owner chose his engine. His maths more or less proves that the power required for a speed on a flat calm times 1.36 gives the power for average conditions, and times 2.5 give the requirement for rough water. Now I know rough water for the Bismarck would keep Tortuga in harbour but: I decided Kieth's formula seemed to predict flat water conditions so I applied it to boats from "Voyaging under power". I then multiplied the result by 1.36 and got results very close to the installed power. If you apply the formula to WW2 destroyers and multiply by 2.5 you get very close to the installed power.
Little Sinbad 9.2kts 150hp predicted 151 hp
Nordhaven 46 8.75kts 101hp predicted 107.6hp
Diesel Duck 38 9kts 80hp predicted 85hp
Neville 39 9.1kts 105hp predicted 101hp
Neville 48 10kts 200hp predicted 193.7hp
Seaton 256 9.8kts 195hp predicted 180.7hp
Willard 40 10.5kts 130hp predicted 140hp
It looks like these 6 designers used Kieth's formula for power and added 36% then fitted an available power unit as close to that number as they could fit in their engine space.
, Mik the stick
Senior Member
Join Date: Dec 2012
Rep: 6 Posts: 175
Location: Devon
Fun contemplation ,
a startling discovery on required power.
I have long been obsessed with accuracy in maths. Applying maths to power prediction and you come up with Wymans formula, Gerr's formula and Kieth's formula. It is the latter i want to talk about. If you search the net for "repowering Tortuga" you will find a very interesting article on these three formulas, how and why the owner chose his engine. His maths more or less proves that the power required for a speed on a flat calm times 1.36 gives the power for average conditions, and times 2.5 give the requirement for rough water. Now I know rough water for the Bismarck would keep Tortuga in harbour but: I decided Kieth's formula seemed to predict flat water conditions so I applied it to boats from "Voyaging under power". I then multiplied the result by 1.36 and got results very close to the installed power. If you apply the formula to WW2 destroyers and multiply by 2.5 you get very close to the installed power.
Little Sinbad 9.2kts 150hp predicted 151 hp
Nordhaven 46 8.75kts 101hp predicted 107.6hp
Diesel Duck 38 9kts 80hp predicted 85hp
Neville 39 9.1kts 105hp predicted 101hp
Neville 48 10kts 200hp predicted 193.7hp
Seaton 256 9.8kts 195hp predicted 180.7hp
Willard 40 10.5kts 130hp predicted 140hp
It looks like these 6 designers used Kieth's formula for power and added 36% then fitted an available power unit as close to that number as they could fit in their engine space.
, Mik the stick
Senior Member
Join Date: Dec 2012
Rep: 6 Posts: 175
Location: Devon
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