[inspektorgadget007]

Ahoy,


Saiboat Challenger 7.4

http://sailquest.com/market/models/chall24.htm


powered by a two stroke Mercury outboard motor, Serial No. 7066273

Question:

-fuel consumption per hour at half RPM, three quarter RPM and full RPM, expressed in litres.

Any idea?

[achris]

Originally Posted by inspektorgadget007

Ahoy,


Saiboat Challenger 7.4

http://sailquest.com/market/models/chall24.htm


powered by a two stroke Mercury outboard motor, Serial No. 7066273

Question:

-fuel consumption per hour at half RPM, three quarter RPM and full RPM, expressed in litres.

Any idea?

None whatsoever!!!

Let's start with a horsepower....

Rule of thumb for 2 stroke consumption is GpH@WOT=hp/10

Without actually measuring had much is being used nobody can tell you...

Chris.........

[Silvertip]

Put the pencil away, forget the formula, and put a decimal point one position to the left of the last number in the horsepower rating. 40 HP = 4.0 gallons per hour at wide open throttle.

[achris]

Originally Posted by Silvertip

Put the pencil away, forget the formula, and put a decimal point one position to the left of the last number in the horsepower rating. 40 HP = 4.0 gallons per hour at wide open throttle.

Isn't that what dividing by 10 means????

[inspektorgadget007]

Chris and Silvertip,


Thank you for your answers.

Chris, can you translate WOT?

I assume when you both talk about gallon it means American gallon(3.78 l.) not Imperial gallon(4.54 l.)

Is that right?

[achris]

WOT, Wide Open Throttle.... For the purpose of this rough rule of thumb, use 4.

Er, you're new to boats aren't you?

Chris..........

[inspektorgadget007]

Hi AChris,


Altough I bought this sailboat in 2005, I don't have much free time to go on the water.
So, more or less stll I'm kind of new to boating.

Again, what gallon is? American or Imperial?

Any rough rule for a diesel engine fuel consumption?

[achris]

Use US gallons.... No rule for diesels, sorry.....

[chrisg]

so a 115HP will get 11.5GPH? I assume this is for 2-stroke, 4's are different.

So with a 6gal tank, I can run for 1/2 hour. Mmmm... ther is something wrong with my motor!

[Silvertip]

Achris -- in case the decimal point thing slid past you, it was "tongue in cheek". There are people that go faint when they see a formula regardless how simple it is.

[achris]

Originally Posted by chrisg

so a 115HP will get 11.5GPH? I assume this is for 2-stroke, 4's are different.

So with a 6gal tank, I can run for 1/2 hour. ...

At WOT (Wide Open Throttle), yes.... Now if you know how far you travel in 30 minutes at WOT, then you have a mile per gallon figure to start working with for speeds other than WOT...

Chris.............

[mthieme]

My 35 HP Merc burns about 1 gallon per hour at 3/4 throttle.
My 25 HP Merc burns 3 gallons in 1 hour 40 minutes WOT.
But neither is pushing a displacement hull,

[emckelvy]

Actually, the old HP/10 formula was thought up in the days when 2-stroke outboards were rated at the crankshaft.

So, if you use this on a later-model outboard rated at the prop, the value you come up with is gonna be a bit on the low side.

But, HP/10 is sure a lot easier to figure out in your head than HP/8.21 or whatever correction factor you jack into the equation!!!

Here's a good example of this:

Old Merc 1150 rated at crankshaft, approx 11.5 GPH WOT;

Old Merc 1400 rated at crankshaft, approx 14 GPH WOT;

Later-model Merc 115 (which was for all intents and purposes a 140 rated at the prop as 115HP), approx 14 GPH WOT.

And so on, and so on.

Still lots easier for my Pea-Brain to divide HP by 10 and then just jack in a bit more GPH to come up with a S.W.A.G.!!!

And of course the original formula works great on all the Antique Fossil (er I mean "Classic") Mercs we're still playing with!

BTW it shouldn't matter what type of hull you're pushing, if the motor is truly running at WOT, in its rated operating rpm, it's gonna consume the max amount of fuel.

Of course, the type of hull you're pushing will greatly change the Speed and MPG values, as will cruising at the most efficient speed for the hull/motor configuration; but we're not talking about those, only specific consumption at WOT.

Cheers........ed