by BruceK » 11 May 2016, 18:30
As I said before a semi displacement hull is the most fuel inefficient. It is neither as efficient at displacement speed as a displacement hull and requires more force to get it to plane and so is less fuel efficient than a planing boat. You want to read the fuel stats for lifeboats. The only advantage of a Semi D is they are more sea kindly than a planing boat but not as much as full displacement. They work well with big boats of 50 and up but do not scale down well to the 30 foot range. A semi D really should be used mostly at hull speed with a sacrifice in economy to get it to a low speed plane, if it really planes at all. Remember a semi D can have a displacement or planing bias.
As for economy what you have to remember in a 30 foot displacement you may only achieve 7 knots efficiently but burn 6 litre an hour to do so. So to travel 70 nm will take you 60 litres. Now a full planing will cruise at 25 knts using 20l per hour, but it covers 20nm in just under 3 hours having used just under 60 litres. So tell me which now is more economical. (yes my KAD42's use 6 litres an hour on no load idle. i.e. neutral)
Next what you have to realise is the fuel economy graph is not linear with revs/ speed but logarithmic and describes a parabolic curve. So it may well suck 20l per hour at wot say 4000 rpm but say only 12 for cruising at 3200. If you go balls to the wall all the time you have to pay the fiddler, but rarely will you see top fuel consumption figures. Last year when I audited everything up, my fuel figures worked out at 2.7 litres per mile annual average.
Now the last word. Your fuel bills are actually the smallest percentage of running and maintenance costs over a year. If fuel is frightening you, then you really should be considering a return to sailing. Not that it's much cheaper either. And that you can take to the bank. I had a rough year last year and a lot of maintenance to do. My total budget is 12 grand a year all in (I hit 24.4 but I cocked up) of which 2 grand was in fuel or just £250 a month for the season used. Hardly a deal killer for the power boat hobby.
Also remember boating diesel prices are not forecourt prices either. I pay less than 80p a litre on a 60/40 split
Last edited by BruceK on 11 May 2016, 19:00, edited 1 time in total.