Speed vs Engine Revs

[DBA Forum (B) - Colin Stone]

Intrigued by Charles Fitzhardinge's comment on Sp vs Engine Revs - erpm - in a recent post, (8+kph/1200erpm) I thought that a small survey would be of interest to see what the figures are for the same 8kph.

I guess LWL, Displacement, prop size and gearbox reduction are also important and overall fuel consumption.

So for KEI:
LWL 20m (BWL 4.85 Draft 1.1) 70 tons 30x20inch 4 blade reduction 2.5:1. 800erpm 8kph. Approx 1 kph/100erpm. Fuel 2.66ltr/hr this year.

Previous same size prop with much thicker blades was 2kph slower for same revs.

Any other stats? Will longer LWLs have higher speeds/erpm?

Colin Stone
KEI

Sent via BlackBerry® BIS

[DBA Forum (B) - Tim Hackett]

Maria is approx 28m at w/line, 80 tonnes and has a three blade prop of unknown provenance! 900 rpm gives 8kph. Can be an issue when low speed needed as tickover is 550 rpm and we have no trolling valve.

Tim

[DBA Forum (B) - Tim Hackett]

G'box is 2.5:1 TwinDisk marine. (Can you edit posts after posting when you are a ***@t like me and forget to include things!!!!)

Tim

[DBA Forum (B) - Charles Fitzhardinge]

Of course, the critical rmp is the propeller speed, not the engine speed.

Comparing results for one vessel works, because the relationship between engine speed and prop speed gearing remains constant
(in my case 1.91:1 reduction) so the propeller rotates at a little more than half engine speed.

To make any sense of comparison between boats we need the reduction ratio as well as engine rpm.
There is a well known formula (albeit approximate only) for estimating the theoretical "Hulls Speed" (the maximum boat speed given waterline length and without approaching planing). More complex formulae, requiring inputs of displacement, thrust (engine torque, propeller speed and prop characteristics) - some also require draft and beam and even approximate hull shape.

Charles Fitzhardinge
De Vrouwe Anna Maria