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28-04-2011, 22:27 #1
how much fuel does a 25 hp engine use
hi fellas just wondering how much fuel do a 25 hp engine use. i know it all depends on how fast or slow you go. but a rough idea please for say a 25 mile round trip. with a 13 ft dory. many thanks john
29-04-2011, 00:36 #2
A rough rule of thumb is 1 gallon per hour per 10hp at nearly full throttle - modern efi engines can beat that - so that equates to 2.5 gallons an hour. I imagine you're getting around 20 knots? - if so, not allowing for tides, your journey will take you 1 hour 15 minutes, so 3.12 gallons. However - if you have a top speed of 20 knots, in a 2 knot tide, one way you would be doing 22 knots and the other way you would be doing 18 knots - that's a 20% difference (10% either way) - so add 10% to start (more if you're in an area with really strong tides) - takes you roughly to 3.5 gallons.
Then there's the thirds rule - 1 third to get there, 1 third to get back, 1 third in reserve so that takes you to 5.25 gallons.
And will you definitely go straight there and back - if you watch your log it's surprising how easy it is to do extra miles - I've had the log read twice what I've guessed I've done when I've been on a 'here, there and everywhere' trip, so perhaps a bit more in reserve?
Were it me with an engine I haven't yet got to know, I'd be taking two five gallon tanks and I'd be keeping a very careful eye on the fuel on the outward journey to check all is well.
29-04-2011, 00:39 #3
A 25hp 2 -stroke o/b uses approx 2.5 gph at wot.
29-04-2011, 11:26 #4
Best scenario would be lightly loaded boat running at a speed that's best suited to hull and engine (will only really exist if the right powered engine is fitted in the first place) - in your case this is likely to be about 12 knots. Next you need an absolutely flat water, and no wind (it takes more away going into it than it gives back when behind you). Finally you have the underlying efficiency of the engine with about 40% difference between old 2strokes and the best modern units (either 2DFI or 4stroke).
With everything 'right' in your favour, and without consideration of reserve, you could make 25miles on 1 gallon.
The opposite would be a choppy sea, wind and a heavy hand ie trying to push along around 5-6 knots and constantly adjusting the throttle. Old engine. You could set out with 5 gallons and not get there...
As the boat and engine get bigger the variables have a lower % impact - and diesel engines are more consistent in their fuel consumption over a wider working range too - providing more opportunity to accurately predict fuel consumption in the real world.
29-04-2011, 16:03 #5
I think you would need an exceptional set of circumstances to get to those extremes. If you had 40% better fuel consumption and a 2 knot tide in your favour for the whole trip, it would still take 2 gallons - I can't see anything else giving equal help to the engine to take that down to 1 gallon. The most effect wind has ever had on my boat when a it's been right on the stern of my open backed cuddy giving a 'sail' effect has been to push it along at 1.5 knots - with something as low profile as a dory the wind will only have a tiny effect.
At the other extreme, even with the 2T carb engine if you were doing 5 knots at 80% throttle (very unlikely - quarter to half throttle would be the most you'd need to hold that speed even in the most adverse conditions), then the 10 gallons I suggested he carry would still just about do it. But then who would do the whole trip in such conditions? - Even if the forecast was so off and bad weather came in so suddenly to give conditions like that near the start or even well into the outward leg, wouldn't you just turn round and go back or head for a nearer safe haven rather than risk carrying on into such challenging conditions in a 13ft boat?
30-04-2011, 02:29 #6
thank you very much guys you have been very helpfull. john