POWER TO WEIGHT AND ALL THAT

Barry Greenwood

Barry.Greenwood@BTinternet.com

One of the interesting offshoots from having my engines overhauled last year was the performance data supplied from the dynamometer engine testing. This was performed after the engines were rebuilt and tested the engines at all speed ranges and allowed them to be run-in subsequent to being refitted in the boat. It also gave me peace of mind that I would not have to remove them again to rectify a fault occurring in the first few hours of running.

On test, the engines were run at progressively increasing engine speeds and the power output was recorded from the dynamometer. This was proportional to the load being applied and was recorded as brake horsepower (bhp) against revs. Per minute (rpm). The gearboxes were not fitted at this stage. The maximum-recorded power was 196bhp at 2600rpm. An allowance of about 4% must be allowed for transmission losses to give a more accurate figure when fitted in the boat. This would bring the maximum power output available down to 188shp (shaft horsepower).

From this engine data some interesting speed calculations can be made. In his book on naval architecture, K. Barnaby uses the formula: V=K Ö BHP/D

where V = boat speed in knots, K = constant, Bhp = engine power (in this case shp), D = displacement in tons. From tank testing and previously published data on series ‘C’ hulls, K = 3.5 (I would not expect this to alter by more than a point or two for the entire Moonraker range.)

The following graph is useful to determine power output at all engine speed ranges. It is based on actual data recorded from the port engine, a Perkins T6354 175hp. The starboard engine gave virtually identical results.

From this graph and using the above, one can build a table of theoretical boat speeds against varying displacement and power.

It can be seen that more than a knot can be lost by the addition of one ton. This could be the difference between full and empty fuel and water tanks and number of crew, etc. Our own boat, Golden Sands, is always heavier at the start of our annual holiday with extra supplies, food, clothes, etc. (and if we go to France, with beer, by the end of the holiday also.) If your boat has been constantly afloat, it may also have taken up a certain amount of water through the gelcoat. In one case I heard about up to half a ton was removed during osmosis treatment.