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I was just wondering, I usually run my engine around 2000rpm , which is roughly 75% of the max rpm, as advised by some before.
I get roughly .5g/hour for 7nm/h on my Tartan 37.(this is approximate, it sounds low to me).
I was wondering, did anyone know the most efficient rpm when speed is not an issue, but distance is ?

Appreciated.
 

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Every engine has a "power curve" torque/HP output vs. rpm.

Take that curve, and develop your OWN plot by overlaying Gallons/hour vs. rpm. Will take some time to do if you dont have a flow-meter (GPH or cc/min) on the fuel delivery system. Just like the 'peaks' that you find on the power curve, the GPH vs. distance travelled plot will also be found to optimize as a 'peak' when you draw the overlaid plot on that power curve.

With this recorded data, you can then construct a new plot/curve for rpm vs. distance travelled. The 'optimum' or most efficient will be where that curve 'peaks'.
This of course assumes that the hull remains clean, the boat's weight and the seastate remains constant.
 

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It's complicated. The boat's resistance through the water is non-linear, so is the specific fuel consumption vs. bhp, and then the prop doesn't behave linearly either!

But if I back my Universal 5424 off to 1500 rpm from 2000, the fuel consumption is almost halved while the boat goes about 1 knot slower - 5kts vs. 6.

That means the range is nearly doubled.

I think you need to experiment. Other factors to consider, at least on my boat, are vibration (1500 to 1800rpm is a no man's land of strong vibration), engine temperature (below 1500 it runs too cool), and your own patience.

This means I cruise at 1500 or 2000rpm, which are sweet spots in terms of smoothness.
 

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On a windless day with glass for water and no current chart you boat speed (I'd use both the knot meter and GPS) at 50 or 100 RPM increments. At the point where your speed starts to disproportionally increase less than your RPM increases it should be an estimate of where you get diminishing returns as far as GPH.
 

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Old soul
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With our Perkins 4108, which I think is the same as a Westerbeke 40, we get our optimal efficiency around 1600 RPM. We ran our engine around 2000 to 2200 (based on "expert" advice) for a season. The fuel efficiency dropped through the floor. I don't have a fuel gauge so never quantified it exactly, but my estimate is that we worsened our efficiency by nearly half, while only gaining 3/4 knot.

At around 1600 we get something close to 1/2 gallon/hr moving at about 6 knots (max. hull speed is around 7 for us).
 

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make sure you load your engine at full rpm against a head wind or something as part of your routine maintenance mike...

your rpms while great for fuel efficiency arent great for soot and carbon deposits and stuff that kill diesels, especially marine ones...

diesels that are never loaded or idled too much will eventually get chronic overheating and prone to cracking in or around the exhaust manifold areas as well as block simply from excessive deposits causing higher than normal compression and therefore excessive heat.

im sure you know this but for anybody else...this is key to good diesel maintenance
 

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make sure you load your engine at full rpm against a head wind or something as part of your routine maintenance mike...
Yes, agreed :). We ramp it up to around 2500+ every couple of week or so to blow out any build up. We often do see a dump of carbon and crap after that time.
 

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Yes, agreed :). We ramp it up to around 2500+ every couple of week or so to blow out any build up. We often do see a dump of carbon and crap after that time.
awesome:)

throw a can of seafoam when you do this and your engine will kiss you back...youll notice plenty of crap coming out when loading the engine however manual cleaning of manifolds, elbows and overall exhaust system is always best

I know there are many NON fans of seafoam out there but honestly on a marine diesel in a boat, a sailboat for that matter I think it should be standard maintenance item...plus it does help "stabilize" the fuel too, mostly fights water ingress.
 

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awesome:)

throw a can of seafoam when you do this and your engine will kiss you back...youll notice plenty of crap coming out when loading the engine however manual cleaning of manifolds, elbows and overall exhaust system is always best.
I've never used Seafoam but I've heard good things. I'll give it a try next time Christian. Thanks :).
 

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My Westerbeke is a 55b, Isuzu engine. With our hull and prop we found 1800 to be the start of our curve and runs up to about 2300. At 1800 we do about 1.25/hr and 2300 about 1.5/hr. Manual says 2000 to 2500 so that is pretty close. I have done everything I can to avoid Westerbeke parts and the fiscal trauma they induce. I don't think I will need to purchase anything from them again!
I have been using Desl-Shok with good results and have used BG products in the past that I was very happy with. One thing that I do religiously is biocide. Haphazard application can develop resistant bacteria in your tank, much like not completing a regimen of antibiotics.
 

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isuzu engines are awesome

honestly I think they are the most underrated engine out there

however in latin america they are worshipped, especially diesel pickups like the pup...

My father in law has an isuzu pickup with diesel that has close to 500k miles with only an injector pump change, and injector swap midway...THATS IT

with crap fuel we have down there, and bad roads etc...

I have an isuzu rodeo now(my first isuzu) and I got to say I love this v6 tremendously!

very underrated in the west

fwiw
 
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Christian, I have no complaints with the Isuzu (4LE1-A). It has preformed extremely well, puts out great power for it's size, reasonable fuel economy and we have pushed it to 3500 RPM for intermittent periods. I only regret it was Westerbeke that marinized it. Who paints an engine, wiring harness, hoses and all? I like to keep my engine compartment pristine (well, clean anyway)but every move results in flecks of red paint shooting around! I did strip and powdercoat the manifold, grey. I no longer get parts through them, almost everything can be had through Isuzu or the manufactures that provided Westerbeke with the parts for half or even a tenth of the price. You get the parts faster too!
 

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As has been noted, this is complicated and there is no simple answer. Engine efficiency is normally measured by the manufacturer by measuring fuel consumed and power developed - i.e. lbs/HP-hr or gms/kW-hr. However the power to drive the boat through the water basically varies with cube of the speed (and then increases even faster as you approach hull speed). What that means is that if you reduce speed by 20%, the power is cut about in half. That has a big effect on the fuel burned per mile. So in general slower is better, but at loads below half power, the engine efficiency starts to drop off, and its not good to run it at low loads for long periods. So in general, operating the engine at about half power results in the best balance between fuel efficiency and long term reliability.
 

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Christian, I have no complaints with the Isuzu (4LE1-A). It has preformed extremely well, puts out great power for it's size, reasonable fuel economy and we have pushed it to 3500 RPM for intermittent periods. I only regret it was Westerbeke that marinized it. Who paints an engine, wiring harness, hoses and all? I like to keep my engine compartment pristine (well, clean anyway)but every move results in flecks of red paint shooting around! I did strip and powdercoat the manifold, grey. I no longer get parts through them, almost everything can be had through Isuzu or the manufactures that provided Westerbeke with the parts for half or even a tenth of the price. You get the parts faster too!
pet peeve as well

to many "rebuilders" of old engines do this too and it pisses me off even more!

however westerbeke isnt the only one that paints everything too, there are some well respected engine brands that do this, and it does irk me...:)

regarding parts isuzus are well stocked worldwide and VERY cheap, next for me would be a mitsubishi block or based engine...like kubota, vetus etc:)

oh if only I had the money! jajja:laugher:laugher:laugher:laugher:laugher:laugher
 

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As has been noted, this is complicated and there is no simple answer. Engine efficiency is normally measured by the manufacturer by measuring fuel consumed and power developed - i.e. lbs/HP-hr or gms/kW-hr. However the power to drive the boat through the water basically varies with cube of the speed (and then increases even faster as you approach hull speed). What that means is that if you reduce speed by 20%, the power is cut about in half. That has a big effect on the fuel burned per mile. So in general slower is better, but at loads below half power, the engine efficiency starts to drop off, and its not good to run it at low loads for long periods. So in general, operating the engine at about half power results in the best balance between fuel efficiency and long term reliability.
easy way to uncomplicate what in essence has much science behind it( using the half max rpm rule...I like 2/3rds max rpm to set a good cruising speed in general terms)

(so say a yanmar with 3600max rpm I would say 2200 is a solid cruising speed rpm, however many engines benefit going higher, especially smaller ones, just like you high rev a small jap 4 cylinder car and benefit from not lugging said engine, you actually get better fuel efficiency

however a detroit diesel would be the opposite)

this is of course for engines that to BEGIN with are correctly propped and sized for the boat

for example I cruised all day long on a yanmar 2gmf(the18hp version) at 2500rpm on a very heavy 28 foot wooden boat for its size plus 2k cruising weight.

however in a racer those rpms would go down by at least 400rpm or more...simply because of the displacement reduction.

as soon as you get out of one range say boat size or prop size or displacement getting average rpms by asking on a forum is a great way to get horrible results.

assume you are not correctly propped or pitched or sized in order to get better results

meaning know for sure what you have down to the T including prop shaft size, cutlass bearing design, dripless or standard etc...

theeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeen start comparing with other users of your setup:)
 

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A three (or a four) blade prop will greatly increase your efficiency. Your prop is incredibly important to what you actually get from your engine. It is where the power meets the water. I was told, correctly or not, the easiest/simplest way to see if you were propped right was if, in calm water, you made your top speed at the top of your recommended cruising RPM range. For me that is about 7.5 knots at 2600 RPM (3500 RPM is max, no load) with a 3 blade MaxProp at 18°. I made the mistake of resetting my pitch to 20° and achieved that at about 1800 RPM which is max torque. I gained nothing at higher RPM and had no slow speed.
I like the MaxProp not only for its feathering but also that I get full thrust in reverse. On the occasions we have found ourselves in a "depth minus situation", reversing has always gotten us off, knock on wood!
 

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The formula I came up with years ago was that a diesel engine was at maximum efficiency at 40% load at peak torque RPM.

So if your engine peak torque is say, 1700RPM, then you want to be at 1700RPM at 40% load. Any more load then 40% will decrease efficiency.
 

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Thevdeub :

It is not a lot of work to do your own experiment.
Get a long tall measuring glass, and a stopwatch.
Scrub your hull, and choose a very calm day.
Disconnect the fuel feed from the tank and run an improvised fuel take-off to the measuring jug.
Bring a friend along to do the steering and throttle.
Run your motor at various rpm settings and measure the rate of fuel consumption.
Convert it to the rate of fuel consumption per mile covered.

You will find that lower speeds are surprisingly easy on fuel consumption. Higher speeds are very thirsty indeed.
 
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