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Old 01-05-2021, 10:02 AM   #41
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I have a question about another aspect of running low power

I've always wondered about the overall engine efficiency when a boat has more HP than needed to drive it.


I re-powered a 12,000 lb 32 ft trawler from a 60 hp Nissan to an 85 hp Perkins. The normal rules of thumb for need hp vs speed and fuel burn vs displacement are assuming an engine running some where around optimum torque. With the 85 hp engine producing ~30 hp i'll be pretty far off the optimum for torque so will my fuel consumption be much worse than if i had a closer matched engine. The original poster has the same issue in that he has engines unmatched to his load as he uses the boat.
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Old 01-05-2021, 10:46 AM   #42
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I don't think the original poster has any problem, at all. As I recall the post they liked to run at 1200-1300rpm. Although this produces less than the max horsepower, it is actually at the top of the torque curve. See repost of the attached.

It is really hard for me to imagine that the engine is anything but happy and close to as fuel efficient as it can be, especially as compared to significantly more throttle and higher rpm.
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Old 01-05-2021, 11:06 AM   #43
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Replying to gkesden,



I hadn't looked at the curves for the OPs engines but when i do i see peak torque around 1300 rpm so that's an efficient point and so you are right that he's good there. I'm also in the camp that engine life is rated in total pounds of fuel, spend it how you like, cooling system excepted.


My question then is more general, if his peak torque have been at 2000 rpm would he lose much efficiency running at 1300?


I can't find a torque curve for the 4.236 but i think peak torque is up around 1800 rpm and i'll be running at 1400 probably to get hull speed.


It's really a question of whether there's an efficiency penalty for having reserve power, i.e. an engine much larger than needed to reach hull speed.. Displacement hulls only obviously.
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Old 01-05-2021, 11:25 AM   #44
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Torque has the ability to accelerate.
HP has the ability to do work.

Face it .. if you have an engine twice as big as it needs to be there’s no way you’re going to make it efficient. All you need to know is that the slower you run it the less fuel it will burn.
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Old 01-05-2021, 11:28 AM   #45
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Esheuwfelt,

Others here may be better able to reason about this than me. I'd stay tuned to see if there are better replies.

I have a really hard time thinking about fuel efficiency from rpm alone. If an engine is propped more aggressively, the load on the engine will be higher at the same rpm than if less so. Think about it this way, would you expect your car to use more or less fuel at 2000rpm going up or down a steep hill? In one case the hill helps spin the engine, in the other it fights it. Then there is the space in between.

When it comes to moving a boat through water, just like moving a car through air, I suspect it is the air/water resistance that matters most.

In other words, even if an engine might, in isolation, be more energy efficient at a higher rpm, moving a boat at a faster speed seems less likely to be less efficient.

Tuning the gearing and prop to the desired cruise speed seems like it would help, but I can't say I have the data to do that on these engines. I usually just tune the props such that the engine performs sensibly across the full range of throttle (low vessel speed at idle speed, makes at least close to WOT rpms at WOT, doesn't make blacknorbdark smoke, except,possibly, very briefly on hard acceleration). Then I assume I am close to good.



If I really knew I'd never want more power, I might over prop a little to lower rpms at cruise to get cruise closer to that torque sweet spot. But, I just hate tovmess with things like that too much, personally.

Basically, I just try to make sure my engines start quickly, cruise no leaa than somewhere near peak on the rpm torque curve and run clean, in the normal temp range, and smooth with good oil pressure.

Again, there are others here with much more experience and intuition about these things than me.
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Old 01-05-2021, 11:44 AM   #46
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Imagine a wrench on a bolt.

Torque is a measure of the force applied to the wrench to turn the bolt.

Work is how much effort goes into turning the wrench. The harder it is turned and the more it is turned the more work is done.

Horsepower is a unit of work. It is torque x rpm. How hard it is to turn the wrench multiplied by the number of times the wrench is turned per unit time.

Applied to engines, torque is how much force the engine can apply. At any rpm, the work the engine can do, e.g. it's hp, is its torque x rpm.

Turning a wrench with the engine and turning an engine with the wrench are basically the same thing, but with the force Police from opposing sides of the lever arm.

Needing to move less mass inside an engine is more efficient than needing to move more mass inside an engine.small friction surfaces are normally more efficient than larger ones. Smaller engines generally are more efficient than larger ones for what they can accomplish.

Having said that, that doesn't mean that those of us with big engines can't make sensible choices about how to run them and balance things like noise, time, fuel cost, and engine life as suits are own style and budget. And, understanding how a little bit about how a hull moves through the water and how an engine works and how a boat is actually performing can all help.

I think. At the least.
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Old 01-05-2021, 12:17 PM   #47
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Here's a thread about engine efficiency that may interest others on this thread...

https://www.trawlerforum.com/forums/...ency-8496.html

In post 15, TF Member and Naval Architect Tad Roberts did a brief analysis using my numbers.

It's important to note in this discussion that this dates back to 2013 and at that time, my tach readings were off by +200 RPM at cruise so the references to my "2000" Rpm are actually 1800 RPM. I only discovered this error after changing alternators and purchasing a digital tach. One tach was wildly erratic and I was going by the other as my reference.

Tad also included this link to his article at http://www.tadroberts.ca/about/pdf/p...d-function.pdf
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Old 01-05-2021, 02:31 PM   #48
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Quote:
Originally Posted by Nomad Willy View Post
Face it .. if you have an engine twice as big as it needs to be there’s no way you’re going to make it efficient. All you need to know is that the slower you run it the less fuel it will burn.
Ummmm not necessarily all true and let me explain.
I had an original 1978 Mainship model 1 with the oem Perkins T6.354 diesel rated at 160 HP.
After owning it for close to 10 years I repowered it with a Cummins 6BTA at 270 HP.

It was way more efficient with the Cummins engine if I cruised at the same speed as I did with the Perkins, about 8 to 8.5 knots. That is going by nautical miles per gallon.
If I cruised at about 11.5 knots it got the same MPG as the Perkins did at 8 knots.
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Old 01-05-2021, 03:25 PM   #49
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esheufelt,

I think you mentioned not having the curves for the 4.236. Here is what I have for that engine.
Attached Thumbnails
Perkins4236Curve.jpg  
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File Type: pdf Perkins4236Hp.pdf (311.3 KB, 2 views)
File Type: pdf Perkins4236Curve.pdf (2.46 MB, 2 views)
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Old 01-05-2021, 10:50 PM   #50
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This is all great stuff, much thanks and appreciation to all, but now after reading Al's post above, i believe a digital tach may be in order:

FlyWright-
(It's important to note in this discussion that this dates back to 2013 and at that time, my tach readings were off by +200 RPM at cruise so the references to my "2000" Rpm are actually 1800 RPM. I only discovered this error after changing alternators and purchasing a digital tach. One tach was wildly erratic and I was going by the other as my reference.)

Although both the upper and lower helm readings on my tachometers seem to be close; I will check with a second person on board on our next trip on the Delta.
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Old 01-05-2021, 10:52 PM   #51
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This is all great stuff, much thanks and appreciation to all, but now after reading Al's post above, i believe a digital tach may be in order:

FlyWright-
(It's important to note in this discussion that this dates back to 2013 and at that time, my tach readings were off by +200 RPM at cruise so the references to my "2000" Rpm are actually 1800 RPM. I only discovered this error after changing alternators and purchasing a digital tach. One tach was wildly erratic and I was going by the other as my reference.)

Although both the upper and lower helm readings on my tachometers seem to be close; I will check with a second person on board on our next trip on the Delta.
Lemme know if you want to borrow my tach. It's sitting onboard in the Delta.
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Old 01-05-2021, 10:54 PM   #52
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I wouldn't assume a digital tach would be more accurate. It reduces some error modes, but senders and pickups and electrical noise are also factors.
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Old 01-06-2021, 12:07 PM   #53
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Just to throw another log on the fire.....
Here's a good article on Props, Engines and efficiency..

Controllable Pitch Propellers

He advocates controllable pitch propellers. Claims a fixed pitch prop is only most efficient at one point. Rpm, loading, etc.
Some other articles from on the home page are interesting too.
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Old 01-06-2021, 12:13 PM   #54
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Running your Perkins

Purchased my PT35 in 2000 with 2100 hrs. Have run her at 1400-1750 rpm since now have 5400 Hrs. Do my own work on the engine. The mechanic from whom I purchased some parts said that the lobster fishermen on LI beat the crap out of them and do an upper end rebuild at about 12,000 hrs. Re low RPM use should be no problem. The engines were used as the base for the Allis-Chalmers tractors and are almost indestructible.
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Old 01-06-2021, 12:26 PM   #55
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Solly that’s absolutely true.

BUT nobody wants them as they are very expensive, not as efficient (blade wise) and they are also troublesome. Some are not though. But mostly they are just too expensive.
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Old 01-06-2021, 01:52 PM   #56
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RPM is only part of the story. He may have less prop pitch and even though he runs at higher rpm his load may be less. If he is pushing water and has the bow up he is probably running too hard.
In any event running at a speed that keeps the bow down will be a good fuel use speed and not overload the engines.
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Old 01-06-2021, 11:25 PM   #57
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I wouldn't assume a digital tach would be more accurate. It reduces some error modes, but senders and pickups and electrical noise are also factors.
I included a link to this inexpensive laser digital tach above. It's less than $20 from Amazon and eliminates any wiring/sender/pickup errors. Just a piece of reflective tape on the flywheel.

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Old 01-06-2021, 11:41 PM   #58
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I included a link to this inexpensive laser digital tach above. It's less than $20 from Amazon and eliminates any wiring/sender/pickup errors. Just a piece of reflective tape on the flywheel.

Oh. I thought he was talking about a helm-mount tach gauge with digital display vs a handheld photo tach.

I have exactly that phototach. It isn't perfect, though. It gets fooled by shiny spots on pulleys , changes in ambient light, wear or dirt or curling of the reflective tape, etc.

I generally get very useful reads from it. It is a great way to double check my helm tachs. And, because I can move it from one engine to another, and also from main pulley to alternator to check it shows the right ratio, it is easier to verify and build confidence.

But, at least in my use, it gives more variation than my gauges unless I am super careful.
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Old 02-06-2021, 07:49 PM   #59
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Perkins 6.354's naturally aspirated with reasonable oil changes and maintenance (follow old manual intervals) will last from 8,000 - 12,000 hours before a major rebuild.

Typically, 1,800 RPMs is standard with a Velvet Drive. Prop pitch and 3 vs. 4 blade can be adjusted for loaded vessel weight and desired speed. If you get better economy and a satisfactory speed, don't change a thing. The old diesels actually prefer lower RPMs, and last exponentially longer than modern high-RPM diesels. With that said, I have heard every 2-10 hours of low RPM use, it is advisable to run at 1,800 - 2,000 RPM for a few minutes to blow out any carbon buildup that may be developing on the upper edge of the cylinder wall where the piston ring stroke ends (read long-term engine life killer if you don't).

Regarding transmissions, everyone seems to differ on opinions, however, a Velvet Drive properly maintained, and always warmed up before heavy work, is a workhorse IMO.
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Old 02-06-2021, 09:26 PM   #60
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"With that said, I have heard every 2-10 hours of low RPM use, it is advisable to run at 1,800 - 2,000 RPM for a few minutes to blow out any carbon buildup that may be developing on the upper edge of the cylinder wall where the piston ring stroke ends (read long-term engine life killer if you don't)."

I've heard that running them a little harder periodically is only relevant with turbo / Mani cooler engines and that natural Perkins are happy to just hum along like a tractor.

Let's face it, that's what they are...tractor engines modified successfully for the maritime market.

Ever hear of a farmer running his tractor at high RPM to 'blow out the carbon"? I always thought tractors had 3 speeds...ON SLOW and OFF. That's how I treat my Perkins twins. At 4000 hrs, they haven't complained yet.

YMMV
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