The Blue book is the yacht racing rule book and it stipulates windward performance under power. And the requirement is that the boat can make the nominated speed into 12 knots of headwind.
Waterline length (Lw) is the basis of the calculation and for the Dash 750 it is 7.32 and so that works out at 1.8 x √7.32 = 4.8 knots. This afternoon the we have a nice breeze of 10-12 knots so its just what we need for this test.
Todays trial is with Dave Renouf who is a dealer for Corsair trimarans based at Birkenhead Pt Marina in Sydney – click here for more info.
For the test we used a Cruise 4 and 4x Optima 55 ah blue top batteries. The all up weight of the boat, batteries, Cruise 4, the petrol outboard and Dave and I is 1250kg. We tested on Pittwater just out from Crystal Bay and did two runs for each speed at right angles to the wind. And we also did the conformance test with her heading straight into the wind.
We used the Cruise 4 because that allows us to simulate the Travel 1003, and the Cruise 2 as well, by simply running at a lower throttle setting.
You can see on the graph the green line is the conformance trial and you’ll see first two data points are at 1000, 2000 watts, representing the Travel 1003, and the Cruise 2. Our target speed is 4.8 knots and you can see that the Travel 1003 would not be powerful enough on this boat—it does about 4 knots at this power. The Cruise 2 would be sufficiently powerful to push the Dash 750 at the required speed, and with a little in reserve.
When using the Cruise 2, only two of these batteries would have been required, and so the weight would be reduced by about 36 kg. (The Cruise 2 weighs more or less the same as the Cruise 4)
Even though its 13kg total weight makes it especially appealing for a racing boat, its upwind performance is not quite enough to meet the racing rules rules.
So this boat would need to be fitted the Cruise 2 with a couple of deep cycle batteries mounted down below. For weight conscious racers, Lithium batteries are interesting alternative. They also have the advantage of not suffering voltage sag under the high current loads that you see with electric outboards.
They also offer advantages for boats that are sola powered because they are virtually no losses in charge and discharge, so all the energy harvested from solar panels can be used for driving the motor.