SO 30i performance Jul 4, 2019 20:39:45 GMT
Post by bernabox on Jul 4, 2019 20:39:45 GMT
The fuel consumption graphs are quite interesting and carry a huge amount of information.
The right hand graph on that spec sheet gives the fuel consumption with rpm as you have noticed, but the thing that is not clear is whether that graph assumes driving a boat propellor or is it at maximum power output. The issue is clearer if you look at the left hand graph. Maximum power is shown, before and after the gear and the power drawn by a properly matched propellor. Note that when you reduce engine speed to 2500 rpm the propellor power requirement is considerably less than the maximum available power at that rpm. Similarly at any other rpm less than the rated rpm. I interpret this to mean that the engine governor is reducing the fuel supply to that needed to drive the propellor. So it is not surprising that you are getting better fuel consumption at lower rpm. Note also that this is Yanmar information, it applies to marine leisure craft of all types, not just Jeanneau.
It it may be that there is another interpretation of these graphs, if so I would be most interested to hear about it.
As a comparison, our 30i is lift keel, twin rudders and with the centreboard mostly up, we regularly use about 1.75 l/hr. This is made up of 2400 rpm to give the 5 knot chanel speed limit out of our marina, about a mile, plus cruising at 2900-3000 rpm for 2 to 4 hour motor cruising when there is no wind, or a short time on slow speed while we haul up sails if there is wind for sailing. So this mixture of our typical activities seems to give that 1.75 l/hr over long periods, which I check each time the engine hours appears at somewhat random intervals.
With hull and propellor clean and no wind, we get 5.8 to 6 knots cruising at 2800 to 3000 rpm. Flat out at 3600 for a few minutes as Yanmar recommends, we get to about 6.8 knots but with huge increase in noise and wave making. And those fuel consumption graphs indicate more than double the fuel consumed. That is the inevitable effect of a displacement hull moving through water at the boundary with air and making waves. When the hull speed catches up with its bow wave speed, a tremendous increases in power is required to get the hull to climb up the back of that bow wave and get on the plane.
From these figures looks like our boats have similar performance considering probable differences in weight of cruising gear on board, propellor, bimini and dodger details etc.
PS Apple has highlighted that rpm range, it’s not my emphasis.