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Old 01-09-2015, 04:41 PM   #1
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GPH/MPG/RPM Albin 36

Ok, I have an 81 Albin 36 DC with a single Lehman 120. Based on a 31 foot 3 inch LWL, I figure a 7.5 knot hull speed. At an indicated 1725 RPM in calm water I get an indicated (via knotmeter and GPS) speed a bit over 7 knots. 1: does that make sense? 2: Do I need to run the engine harder now and again to avoid "underloading" problems? 3: am I thinking reasonably that I should be burning about 2.5 GPH (and thus getting a bit under 3 nautical mpg)? I see so many folks talking about 4 mpg and I just don't see it. I also see a lot about need to properly load the engine - and it seems to me that with 120 hp on my boat 80%+ load would require 2000+ turns which would be very inefficient.
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Old 01-09-2015, 05:39 PM   #2
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Try running at 6 knots. And disregard any discussions containing the terms "underloading" or "overpropping" when a Lehman 120 is involved.
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Old 01-09-2015, 06:23 PM   #3
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I am not sure either...

I have a 40 with a 120 Ford and best measurement I have is about a 34 foot waterline.

I am overpropped and live aboard with 2 so the boat is well loaded...but I dropped to a max of 120 gallons of fuel.

I run around 1650/1700 rpm and go 6.3 knots and burn 1.9gph for around 3.3 nmpg.

that's numbers averaged over the last 4 years and a little over 6000 miles/1200 or so hours.
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Old 01-09-2015, 11:18 PM   #4
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I propose that operating one knot below hull speed is an optimal cruise speed for a full displacement boat. If the engine isn't working hard enough to maintain optimal engine temperature at that speed, one may not have the correct engine or thermostat or whatever. ... You seem to have more HP than necessary. An 80-horsepower John Deere is just more than enough to push my 14-ton boat.
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Old 01-09-2015, 11:44 PM   #5
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Mark that SD act is an entirely different world than FD. You and I are on the same page it seems. I usually run about 50% and w half the power of the Coot I probably have the displacement. 8 tons.

It takes about 20hp to run our boat at 6 knots and it probably takes about 40hp to run the Coot a knot below cruise. At least w FD we know how much power we need.
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Old 01-10-2015, 12:21 AM   #6
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At normal cruise of 6.3 knots at 1800 RPM, I'm using 68 horsepower at 1.7 gallons an hour: 42% of maximum fuel-consumption rate at 2400 RPM.
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Old 01-10-2015, 11:59 AM   #7
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I'm not familiar with the Albin, but with the FL 120. We had one on our old boat and have now two on our AMS trawler, so we have got some operation experience with them.

Every continuous operation in the range of (1250) 1400 to 1800 rpm is fine for the FL120.
We NEVER operated them outside of this range for more than 5-10 minutes!

In the engine handbook a minimum continuous speed of 1250 rpm is indicated. However we found that after ~ 10 hours running at 1250 rpm the engine was starting to soot (1 hour at 1700-1800 rpm recovered that). Above 1400 rpm we have never had that sooting effect.

At 1725 rpm I would expect a fuel consumption of ~7.8 l/h (normal propulsion load curve). With a speed of 7-7.1 knots you would have something around 3.4 nmpg.

I'm not sure whether your readings 1725 rpm @ 7.1 kn is normal for an Albin 36 with a single FL120.

Recalculating some standard figures on Albin 36 I found in the www I wouldn't regard these 1725/7.1 as very unreasonable. From that point you should be able to cruise 6.3 kn @ ~1400 rpm with 5.2 nmpg without any negative effect on your engine.




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Old 01-10-2015, 12:26 PM   #8
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Forget all the hull seed baloney you read. The gy who first applied that term to pleasure boats should be shot. any faster than Sq root of wl will be pushing water and wasting fuel. About 5.5 knots in your case.
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Old 01-10-2015, 02:32 PM   #9
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Hull speed is matter of principle of the hydrodynamics, it's just the propagation velocity of a wave with a length equal to hulls water line LWL.

Whether it is smart to cruise at hull speed, above or below, is another question.
Cruising at SQRT(LWL) knots (LWL given in feet) which is 75% of hull speed will typically need only 25-30% of that power you would need to travel at hull speed - that will for sure save fuel.
The most fuel efficient speed of a full displacement hull is the lowest speed you can run without harming your engine.
(Different story for planning hulls...)


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Old 01-10-2015, 03:01 PM   #10
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All boat hulls are different and all FD hulls are different so the best speed to run them will also vary. But for a typical FD trawler w average proportions and displacement that is not overpowered .5 to 1 knot below hull speed delivers a speed that is very efficient and not over driving the hull. There's a very narrow speed range for FD boats. Mine is about 5.5 to 6.5 knots. I'm not saying the above is w/o bias or subjective opinion but IMO it's so close there's little room for variations.
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Old 01-10-2015, 05:07 PM   #11
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Quote:
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But for a typical FD trawler w average proportions and displacement that is not overpowered .5 to 1 knot below hull speed delivers a speed that is very efficient and not over driving the hull.

Agree - usually we are doing the same, knowing that there were some 100 nm of extra range if we would slow down further ...


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Old 01-10-2015, 06:07 PM   #12
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Quote:
Originally Posted by PAJones View Post
Ok, I have an 81 Albin 36 DC with a single Lehman 120. Based on a 31 foot 3 inch LWL, I figure a 7.5 knot hull speed. At an indicated 1725 RPM in calm water I get an indicated (via knotmeter and GPS) speed a bit over 7 knots. 1: does that make sense? 2: Do I need to run the engine harder now and again to avoid "underloading" problems? 3: am I thinking reasonably that I should be burning about 2.5 GPH (and thus getting a bit under 3 nautical mpg)? I see so many folks talking about 4 mpg and I just don't see it. I also see a lot about need to properly load the engine - and it seems to me that with 120 hp on my boat 80%+ load would require 2000+ turns which would be very inefficient.

Can't speak to your speeds or fuel consumptions, but...

How accurate is your tach? If you're relying on an analog tach, it may or may not be displaying true flywheel RPMs... so you might want to check that.

Disregard if you've already sorted through all that.

-Chris
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Old 01-10-2015, 06:49 PM   #13
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RPM is revolutions per minute. In only one minute (by the way). And engine speed is not a good representation of engine load that is a critical part of fuel consumption. You could compare 5 to 10 Lehman powered boats and their fuel consumption at any specific rpm and find considerable differences because rpm does not tell engine load.
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Old 01-10-2015, 09:21 PM   #14
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Quote:
...How accurate is your tach?...
Quick hijack.

Easy to check and less than $12/delivered:

Hot Sale Digital Laser Photo Tachometer Non Contact RPM Tach Tester RPM Motors | eBay
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Old 01-10-2015, 09:23 PM   #15
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Old 01-11-2015, 03:11 AM   #16
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Quote:
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RPM is revolutions per minute. In only one minute (by the way). And engine speed is not a good representation of engine load that is a critical part of fuel consumption. You could compare 5 to 10 Lehman powered boats and their fuel consumption at any specific rpm and find considerable differences because rpm does not tell engine load.

Exactly. RPM tells you only something about engine load and subsequently fuel consumption if you know additionally your load curve. And these may differ from boat to boat depending on the screw sizing. If your screw is oversized you will operate the engine with higher loads at "part load rpm" compared to a screw whose load curve at max rpm fits exactly max engine power.

However since the load curve of a given screw installation follows more or less the hydrodynamics law of similarity and knowing that most of our boats don't come with extraordinary propulsion lay outs you will get at least a "house number" for your specific fuel consumption. But for sure not exact enough to rely on without further proof ...


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Old 01-11-2015, 03:30 AM   #17
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How accurate is your tach? If you're relying on an analog tach, it may or may not be displaying true flywheel RPMs... so you might want to check that.

-Chris

We checked it with a laser tachometer - all our engines tachs (the old one and the two "new" ones) had / have an offset.
The installation on our old boat took the tach signal from sensor coupled mechanically to the engine, the failure was simply in the instrument at the helm.
Now on our AMS the signals are taken from the alternators. Here we have a constant factor between true fly wheel rpm and tach indication (0.9 / 0.8 for port / starboard).


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Old 01-11-2015, 11:00 AM   #18
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Sadden,
My post #13 was directed to those that may be comparing fuel consumption of a specific boat w a specific engine. At the same rpm those boats may have very different fuel burn rates. It is a common practice to over prop trawlers (unfortunately IMO) and an over propped trawler will have a higher engine load at a specific rpm and thus a higher fuel consumption. So such boats can only be realistically compared (fuel comsumption wise) only if their WOT rpm is the same.
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Old 01-11-2015, 02:06 PM   #19
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Eric,
no contradiction, this is what I tried to say: engine load and consequently fuel consumption will differ if you have the same engine (and hull) but different propulsion load curves i.e. different WOT rpm due to different screw designs. (I guess that's what you mean with "over propped": engine will not reach its rated max or WOT rpm because the screw / prop is not properly sized resulting in a load curve above engines rated power at engines rated max rpm.)
The over propped trawlers I have seen here in Northern Europe had typically a WOT rpm 5 - 15% below engines "rated WOT". My assumption is that in these cases a comparison of s.f.c. to a vessel with properly sized screw may differ by some 10-20% - that's what I meant with "house numbers" - and if the comparison is outside of that range one should start to think about.


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Old 01-11-2015, 02:10 PM   #20
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Fuel consumption rate is the best measure of engine load.
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