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Following on from recent correspondence on our forum I thought that it was time for me to make a stab at effective power utilisation in Seadog propulsion.
In order to do this I have had to make a few assumptions.
1 A water line length of 26 foot, this may be slightly high and will therefore show higher speeds than are possible.
2 That they all weigh 6 Tons. That will obviously vary, I believe that Taliesin used to weigh in higher than that when fully laden but I carry lots of stuff.
3 I have assumed max engine revs of 3000 and a 2:1 gearbox ratio. This does not affect the horse power or speed but would change prop diameter and pitch along with gear ratio if higher revs were to be selected.
4 A clean hull so that max displacement speed can be attained.
5 10% loss of power in gearbox.
6 A displacement speed ratio of 1.4 x square root of WL this may be on the high side, but again has little effect on the results.
I have done rough calcs. On a horse power range of 25 hp to 80 hp with the following results.
Engine Horsepower 25 Giving at the shaft 22 hp Maximum Hull Speed 6.42 kts.
Engine Horsepower 30 Giving at the shaft 27 hp Maximum Hull Speed 6.88 kts
Engine Horsepower 35 Giving at the shaft 31 hp Maximum Hull Speed 7.14 kts
Engine Horsepower 40 Giving at the shaft 36 hp Maximum Hull Speed 7.14 kts
Engine Horsepower 45 Giving at the shaft 40 hp Maximum Hull Speed 7.14 kts
Engine Horsepower 50 Giving at the shaft 45 hp Maximum Hull Speed 7.14 kts
Engine Horsepower 60 Giving at the shaft 54 hp Maximum Hull Speed 7.14 kts
Engine Horsepower 80 Giving at the shaft 72 hp Maximum Hull Speed 7.14 kts
From the above it can be seen that the Maximum hull speed in displacement mode is 7.14 kts. Once that is reached any extra power is just wasted on digging a hole for the transom and creating wash.
Reg Freeman got it right; anything above say 40hp is wasted.
Eric Richardson
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