Originally Posted by David M
It would be interesting to hear back from Midland with more details.
Hi David - I haven't got the exact figures with me as I am on my boat until tommorrow evening but I had referred to them from the sea trials documentation
of one of the vessels prior to making my earlier posts. The vessel was a commercial
one so while a very powerful vessel it was not an especially light one (it was actually required to carry quite high deadweight compared to a pleasure vessel and constructed to Lloyds so scantlings were not light) and a catamaran
(but I know monohulls that perform the similarly).
The following comments are based on miles per gallon, as were my earlier posts, (actually the sea trials figures were gallons per mile as it was a thirsty boat
) which as you say gives the true measure of efficiency.
From the time the boat I used to get some example figures was properly on the plane right through to maximum speed the fuel use in miles per gallon was essentially constant ie going faster increased fuel consumprion per hour as one would expect but the miles per gallon did not increase, they were essentially constant.
In the period from slow speed up until the boat was fully on the plane the best fuel consumption in miles per gallon was nearly 25% worse than the consumption on the plane. That is it was most economical to operate the vessel, as far as fuel consumption was concerned, on the plane. And once on the plane then from a miles per gallon point of view it did not matter how fast the boat went fuel consumption in miles per gallon was essentially constant (within experimental error).
In this case the fuel flow was measured by the engines' ECS's supplemented by equipment
on board with the engine
manufacturer's representative and speed measured by GPS
on reversed runs. This vessel had an exact sister vessel which I was also on for sea trials some weeks apart from the other one and the overall results were essentially identical and the fuel flows for each of the engines on both boats matched that of all the other engines within minor variances, so validating the results.
I have not been in the position to be part of such measurements for heavy deep vee hull
forms but I would wonder if they would be able to produce figures more efficient at planing speeds than at displacement
speeds as the above vessels, for example, do.
I would also wonder if low powered ones, whether monohulls or cats) would be able to too. For example, I am quite familiar with some cats which do reach "planing" speed but really they just make it whereas the boats above, for example, can nearly double their speed as between getting on the plane to maximum speed and pretty much sit on the water
while doing so rather than in it.
But perhaps such deep vee'd or low powered vessels would be better classed as semi displacement and when in full displacement mode probably approximate a displacement hull
and show similar fuel efficiencies as those. Of course, a semi displacement vessel becomes less efficient in terms of miles per gallon the faster it is driven.