Fuel Consumption Question

[sunchaser]

MV Darlin wrote;
“ I have twin 120 Ford Lehman's in my OA 43, 52 ft overall. I burn less than 5 gph at 80% power, 8.2 knot's.”

An FL burns 6gph at WOT.
That would be 12 gph w two engines.
“Less that 5gph” .. no way

Eric
With our 225 rated HP twin engines, in reasonably flat water, we burn between 4.5 - 5.5 gph combined at slightly less than 8 knots and 1650 RPM. The engines are curve rated at 10.2 gph at full load, 2500 RPM.

In heavy seas, at the same RPM, fuel consumption rises dramatically as we climb Mother Nature's hills.

 

[Hippocampus]

Newbie question. Does anyone follow exhaust temperatures? Do you change rpm as a result of that reading?

 

[Nomad Willy]

Hippo,
Exhaust temp is the best way to keep from overloading if overpropped.

 

[Hippocampus]

Knew that thanks but asking a different question. Hear some use exhaust temperature to adjust pitch of an adjustable prop in order to improve mpg. Was wondering if folks with a non adjustable prop use exhaust temperature to adjust rpm in order to get optimal loading and mpg?

 

[Alaskaflyer]

Knew that thanks but asking a different question. Hear some use exhaust temperature to adjust pitch of an adjustable prop in order to improve mpg. Was wondering if folks with a non adjustable prop use exhaust temperature to adjust rpm in order to get optimal loading and mpg?

Interesting question. For owners and pilots of small aircraft with adjustable pitch propellers, EGT is an important measurement for efficiency. Those of us flying fixed pitch propellers have largely rejected it as a useful measure.

 

[rslifkin]

With adjustable pitch props, yes, EGT would be useful for finding optimal engine load at a given RPM. Otherwise, it's only useful as a warning for "something is wrong and I'm either overloaded or having a mechanical issue and should back off and check things out"

 

[slowgoesit]

Interesting question. For owners and pilots of small aircraft with adjustable pitch propellers, EGT is an important measurement for efficiency. Those of us flying fixed pitch propellers have largely rejected it as a useful measure.

We plan on using the installed EGT as our primary measure of loading on our new to us boat. (Which has an adjustable pitch propeller as well). Seller stated that he set the alarm to go off at 500 degrees EGT. He normally didn't exceed 400 degrees EGT, even though Mike at Gardner said the engine is rated for 600 degrees. Better safe than sorry, and the boat doesn't need the additional power anyway. It will be interesting playing around with throttle/pitch settings. I wish I had a realtime fuel flow gauge though. Unfortunately I don't see any way to do it it without having TWO sensors, one for pumped fuel, one for returned fuel, and some electronics to subtract returned from pumped to get the actual burn rate. Anyone got any ideas?

 

[FF]

"I wish I had a real time fuel flow gauge though. Unfortunately I don't see any way to do it it without having TWO sensors, one for pumped fuel, one for returned fuel, and some electronics to subtract returned from pumped to get the actual burn rate. Anyone got any ideas? "

You have described just how a FlowScan works. They are very accurate and reliable once dialed in , but the modern all electronic engines have made them obsolete.

 

[Alaskaflyer]

But could you operate on the lean side of peak with your Diesel engines to save even more fuel?!? [emoji6]

(Answer is no unless you can manually adjust the fuel air mixture.)

 

[Alaskaflyer]

Does the power setting you mentioned - 400° or so - correspond with the cruise speed expected with the hull length? I remember he claimed about a 7.5 knot cruise in the advertising. What is the max speed?

 

[Fish53]

Agreed. Twins may cost a little more, but nowhere near double. If it's truly double in the same boat, then either then twins are over-powered or the single is underpowered, as one of them is the wrong engine choice.

I've owned a couple of twins and while twice the expense is an overstatement "a little more" is dubious unless >30% is what's considered "little".

 

[Art]

I've owned a couple of twins and while twice the expense is an overstatement "a little more" is dubious unless >30% is what's considered "little".

There's nothing wrong with a running single engine [except greatly reduced maneuverability when operating at slow speed in close quarters]... as long as nothing BIG mechanically fails on a single engine; while cruising, that is.

Let's say, in general, that $500 per year [closer to $100 annually if we do DIY] is needed to maintain a good condition engine. That means twins eat-up $1,000 a year. = just over $83 per month [about $200 annual, under $17 per mo. if DIY]. Fuel cost is nearly a push [a bit increased for twins], due to the fact that equal hp. of single or twins is required to move the boat at any speed.

Sooo... twins is the affordable name of the game for me... I love what they accomplish for handling and their safety of redundant propulsion on boats!

 

[backinblue]

There's no right or wrong answer when it comes to singles vs. twins. They both have pros and cons. No need to justify either IMO. If you like what you have, enjoy it/them!

 

[slowgoesit]

Does the power setting you mentioned - 400° or so - correspond with the cruise speed expected with the hull length? I remember he claimed about a 7.5 knot cruise in the advertising. What is the max speed?

1400 rpm, pitch such to give 400 degrees EGT gives 7.5 knots through water. Fuel burn (with hydraulic pump on the main running the 12kw genny) is 2.4 gph.

LWL is 47.5', so Square root of 47.5 x 1.3 = 8.96 knots, so we are not traveling at hull speed with the above settings. I'm okay with that, as 7.5kts is plenty fast for us! Especially when with full tanks, we have a zero reserve range of 6562 nm! That's Seattle to Key West with a very 12% reserve . . . probably best to add some fuel in Panama, just to be on the safe side!

 

[Alaskaflyer]

1400 rpm, pitch such to give 400 degrees EGT gives 7.5 knots through water. Fuel burn (with hydraulic pump on the main running the 12kw genny) is 2.4 gph.

LWL is 47.5', so Square root of 47.5 x 1.3 = 8.96 knots, so we are not traveling at hull speed with the above settings. I'm okay with that, as 7.5kts is plenty fast for us! Especially when with full tanks, we have a zero reserve range of 6562 nm! That's Seattle to Key West with a very 12% reserve . . . probably best to add some fuel in Panama, just to be on the safe side!

Cool. Curious how they determined the default RPM?

 

[slowgoesit]

Cool. Curious how they determined the default RPM?

Max RPM for the engine is 1500. Min RPM for the variable pitch prop to have hydraulic pressure in order to change pitch is around 1050, they use 1100 as minimum with prop engaged. So if they are just putzing along, they use 1100 rpm, and run pitch up to 400 degrees EGT, but I'm not sure what speed through water that gives the boat. I will have to play with it . . . when we get to the boat . . . which will not be until probably early part of June, darn it!

 

[Alaskaflyer]

It would be interesting to go out and experiment and eventually graph the curves, but it won't be nearly as useful without a Floscan.