Fuel consumption - 50 hp diesel in a yacht...

[cliff]

Re: If the short answer is \'you don\'t\'...

Well, I am now totally confused coz in my wee DV24 I never burn more than 2 litres per hour even on full throttle @ 3200rpm and pushing my wee 34'er along @7kts (clean bum). The other posts seem to indicate I should be burning around a gallon / hour or a bitty over. Dropping her back to 2000 rpm drops the fuel consumption to a little under 1½ lph even with the eber howling away.

There are 4 S34's based where I am, one with a Bukh, one with a Beta, one with a Nanni and the last one with a V@lvo. All have different average fuel consumptions.

Guess the rules of thumb are a vague "guestimate" at best.

IMHO the only way to determine fuel consumption is to measure it for your own paricular boat / engine / prop setup..
--------------------

"Artificial intelligence is no match for natural stupidity"

 

[Robin]

Re: If the short answer is \'you don\'t\'...

[ QUOTE ]
IMHO the only way to determine fuel consumption is to measure it for your own paricular boat / engine / prop setup..


[/ QUOTE ]

Yes absolutely correct, because the amount of bhp used is going to be different in every case. However the formula for amount of diesel burnt per bhp used is pretty well constant.

 

[cliff]

Re: If the short answer is \'you don\'t\'...

Don't see it - 4 identical yachts with 4 different engines all around the same HP, 4 different props and 4 different consumptions. I am more inclined to conclude that make of engine has some bearing on the consumption with prop matching having a greater effect. I still think the rule of thumb is only a "guesstimate" at best and with the low hp engines we are talking about has a comparatively large margin of error. ie for a 24hp we are looking at between ½gph to 1¼gph on full throttle - too large a difference to leave to a "guesstimate" - better to measure then one knows how much fuel to take on board.
--------------------

"Artificial intelligence is no match for natural stupidity"

 

[Robin]

Re: If the short answer is \'you don\'t\'...

The real killer difference is the waterline length and therefore hullspeed. It takes very little hp (and therefore fuel) to run at sq root of waterline length in feet and a helluva lot more to run at 1.34 times that speed (hull speed). The consumption curve goes almost vertical as you get near to hull speed. We always set the engine at constant revs of 2000rpm, not normally adjusted if the speed varies because of a weedy bottom, headwind/tailwind etc. If you don't use constant revs the calculation is hit and miss.

I agree that individual measurement is the only sure way, but it is only subsequently relevant if the engine is always run at the same revs, isn't run light for charging on a mooring and there is no extra confusion from an Ever Busted. We achieve our figures of 0.58gph very consistently and enough to trust them for fuel remaining calculations, but we always nonetheless use 2/3rds gph as the figure in practice (easy mental calculation too), that way we have a pleasant surprise on refueling and not a nasty one at sea!

 

[cliff]

Re: If the short answer is \'you don\'t\'...

I consistently return 2lph but use 2/3 GPH for calculating fuel requirements - easy calc 10nm/gallon or 1½hours/gallon so always err on the side of caution.
What I was getting at is the rules of thumb seem to say I should be burning a little over a gallon/hour when in fact I burn ½ of that and also 4 identical Sadler 34's all with different engines/props all return considerably different fuel consumptions so water line length does not come into it -same hull shape.
--------------------

"Artificial intelligence is no match for natural stupidity"