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Message board > Engines > Fuel consumption: displacement v planing

Message 5 of 5
Posted by member Simon Kidd on Thursday 2 January 2014

Hi Nicolas. My Experiance with the Pilot Hull (Albeit the River Pilot and Bosun) is planing starting around 15knots - thus the wetted area is reduced around this speed - and thus the resisitance and thus load on the engine is reduced. I'd think the "economical displacement Cruise" would be around the 5 knot mark...ish... Motorboats Monthly did a test of the Pilot 20 years ago using the Honda 45hp fourstoke. I lightly loaded basic boat at a cruise of 17 knots on the plan bunred (according to thier fuel meter) approx 2.3 Gal per Hour. At 5 knots ish - river speed - it was about 1.2 litres per hour ( I find the 75hp Honda burns under 2 litres per hour around 5.5knots). Interestingly, a Viking 20 on the same test with a 15hp fourstroke burned approx 1.5 litres per hour at 5 knots... So - a few ideas.. In short I;d guess the engine would have a good cruise in the 15 - 18 knot mark - anything more and the engine would burn more. Again - your engine dealer may have fuel burn figures per RPM to give you a guide to work with. Every boat is different, every prop can differ - and every day with a different load changes fuel burn again - so be cautious and perhaps do some testing..

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Message 4 of 5
Posted by member Simon Kidd on Thursday 2 January 2014

Hi Peter. It would probabaly be better to contact 32 Owners directly and ask though I'm unsure of how manay had the 250hp Yanmars.. The boats I've come across were likely earlier - with the 200hp Volvos and 220hp Mercurisers. I have seen a number of annomalies in the performance data in the HOC section also - perhaps a review is needed - the Pilot and Navigator figures are very useful - but need careful interpretation. Alternatively - MotorBoats Monthly (Sept 2000 I think) published a range of figures for the 32 (albeit using the Mercruiser 220's). Yanmar should publish fuel burn figures for revs - these can uaully be used to work out fuel burn by recording your speed against revs...not ideal - but probably a good starting point for reliable figures. Best Regards Simon.

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Message 3 of 5
Posted by member Peter Cox on Saturday 21 December 2013

Does anybody have a fuel consumption table of RPM/Speed/Litres Per Hour for a 32 with twin Yanmar 250 HP engines? The information asked for on the Members' database is not clear for this model and for other Hardys; are the RPM and fuel figures at top speed only? Some of the quoted RPMs and top speeds are suspiciously low and look more like cruising speeds.

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Message 2 of 5
Posted by member John on Friday 20 December 2013

Nicholas - There is a performance page in the Members Only pages of this site where many owners have detailed the performance of various models which might be interesting.

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Message 1 of 5
Posted by member Nicholas Duffin on Friday 20 December 2013

Has anyone measured fuel consumption at higher displacement speeds compared to on the plane? My family pilot with a Mercury 80 does 10 to 11 knots at 3800 rpm. Increasing to 4200 rpm gives a smooth plane at 15 knots with plenty of power in hand. I have been up to 20 knots so far. My impression is that the engine is under more load at 10 knots than at 15 and I wonder if 15 may be more economical in terms of miles per gallon. Has anyone actually measured whether this is the case? Best wishes for Christmas and the new year! Nicholas

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