Perkins 135HP Fuel Consumption

Does anyone who owns a 38+/- foot trawler powered by twin Perkins 135HP diesel engines have a fuel consumption (vs. RPMs) chart, or and other fuels consumption statistics? Thanks!

At 1650 RPM (phototach), my single Perkins cruises at about 1.8 gph at 6.7 - 6.9 knots. Two of them at the same RPM obviously would be 3.6 but the speed would be a little more. Your resulting speed may need more or less RPM to cruise how you want. 3 GPH at a 7 knot cruise would be a pretty good SWAG.

The Perkins is right there with most regarding specific efficiency at trawler speed, not the best, but not the worst.

A 38 with a single 354 at 7kts, I'd figure about 1.5gph. Depends on the size and weight of the boat. Big, wide and heavy with full keel (lots of wetted surface area) it could be 2-2.5gph.

I have a light and skinny 38 with a 505cid Cummins, at 7.7kts it burns 1.9gph. The engine is not that efficient at that speed (950rpm), but the light boat is easy to push. So I lose with the engine but gain with the boat. But it can also cruise at 20kts.

Just saw twins. Does not make a huge difference as the hp to push is basically the same. What does change is the parasitic losses related to what it takes to spin two and pump all their juices vs just one. It takes about 0.5gph just to spin a 6liter diesel at 1500. So same boat with a single might burn 2.0, a twin 2.5.

Just a wild guess on my part, but likely pretty close.

I recently sold my Grand Banks 36CL with twin Perkins 135 HP. Very dependable, burned about 3GPH at 1700-1800 rpm cruising at 7 knots on average. To be honest, unless one is planning a long passage with no fuel stops, for displacement speeds-not pushing a big bow wave, the fuel burn is irrelevant relative to other boat costs. Most cruisers spend more at happy hour each day than they do on diesel. Hope the Perkins work well for you.

High wire:

" Two of them at the same RPM obviously would be 3.6"

Sorry but that is just not true. The fuel used depends on work done not how many engines. Two engines at the same speed would each be doing less work and therefore use less fuel. As explained above parasitic losses would increase but work done and fuel use by each engine would decrease.

Hi, this link is perkins sabre 135hp, you can find power curve and consuptions diagram this engines.

Note the readings in the UK gallon per hour on the right and liters per hour on the left.


https://www.transdiesel.com/app_docs/PERKINS%20SABRE%20M135.pdf

Thanks to all who responded to my post/inquiry, especially to North Baltic Sea for the URL link to Perkins' engine spec .PDF.
Peace and blessings,
Larry Buchman

I have read that older engines lose some of their power over time in a linear fashion (while others say it's not so if maintenance is done!), so your consumption may not match the manufacturers curves very well. We have twin M135's but in a power cat so it won't help your situation...

bayview said:

High wire:

" Two of them at the same RPM obviously would be 3.6"

Sorry but that is just not true. The fuel used depends on work done not how many engines. Two engines at the same speed would each be doing less work and therefore use less fuel. As explained above parasitic losses would increase but work done and fuel use by each engine would decrease.

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For the same boat speed I agree. Each engine does approximately half the work plus losses.

For the same engine RPM I disagree. An engine dissipating say 50 hp of energy into the water will use X amount of fuel. Two engines dissipating 50 hp each into the water will use 2X. Assuming correct props to reach full power at rated RPM.

We've got twin t6.3544m in our 40' mainship. We do about 7.5 kts at 1500 RPM and get about 2mpg. Back that out and you get just shy of 4 gallons per hour total or just under 2gph per engine. This is averaged over a few thousand miles and includes a couple hundred hours of generator run time.

Gabe and Em

Gabe n Em said:

We've got twin t6.3544m in our 40' mainship. We do about 7.5 kts at 1500 RPM and get about 2mpg. Back that out and you get just shy of 4 gallons per hour total or just under 2gph per engine. This is averaged over a few thousand miles and includes a couple hundred hours of generator run time.

Gabe and Em

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That's damn good! I've got 4.236 naturals (same as the 6.354 with 4 cylinders) on my 34 and get 7.5 kts at 3.2 gph total at 1800 RPM. If I slowed down closer to hull speed (1600 RPM/7 kts) I suspect we'd be close.

But you guys are right...in the end, the fuel burn difference really doesn't matter...unless you have gassers on a planing hull.

FlyWright said:

That's damn good! I've got 4.236 naturals (same as the 6.354 with 4 cylinders) on my 34 and get 7.5 kts at 3.2 gph total at 1800 RPM. If I slowed down closer to hull speed (1600 RPM/7 kts) I suspect we'd be close.

But you guys are right...in the end, the fuel burn difference really doesn't matter...unless you have gassers on a planing hull.

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Thanks Al, it does go up quickly from there. I haven't crunched the numbers but as soon as you try to go above 8 kts, over 1600 rpm, the gph seems to go through the roof.

No

Gabe n Em said:

Thanks Al, it does go up quickly from there. I haven't crunched the numbers but as soon as you try to go above 8 kts, over 1600 rpm, the gph seems to go through the roof.

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Ordinarily, I do not like it when folks respond to posts who do not have on-point experience but, in this case, my experiennce may be of some value. I have twin Lehman 120s in my DeFever 44. Lehmans are almost a 1:1 comparison to Perkins. My boat weighs 56,000 pounds with a full load-out of fuel (720 gal) and water (350 gal). We are presently doing the Loop and have logged over 4,000 miles so far. We have averaged 3.7 GPH at 1,650 RPM which yields 8.5 MPH in flat conditions, including a modest amount of generator time.

Some of the responses mention boat weight which, I think, has a direct correlation to fuel consumption. Engine model, hull shape, running RPM all are variables affecting fuel consumption but weight being moved also must be considrred when evaluating and comparing. Moving a 36,000 pound 36-foot Grand Banks equipped with twins through the water will not consume as much fuel as my DeFever weighing 50% more equipped with the same engines.

I suspect that many of you don't understand the significance of the link to the Perkins data sheet that was posted above which contains a fuel consumption curve for that engine. Let me explain:

The curve uses a theoretical relationship between engine rpm vs hp required WHILE DRIVING A PROP. At wot it is the same as the full load power/fuel consumption curve but at any lesser rpm the hp required is less than what the engine can put out at wot.

The manufacturer then applies his knowledge of the fuel consumption required to produce that hp at each rpm to produce the fuel consumption curve. That curve is approximate because it is based on theoretical hp since it would be impossible to measure each boat and propeller's hp requirement, but it is pretty close, particularly at or below displacement speeds.

So for your own boat first run the engine up to the speed you want to know the fuel consumption for and note the rpm. Then go to the curve- the bottom one of several on that data sheet, and read over to the right for the fuel required. Note that Perkins being British, that gph is in Imperial gallons per hour. An Imperial gallon is 20% more than a US gallon.

For example, at 1,600 rpm- a good moderate rpm for that engine, it burns about 1.7 Imperial gph or about 2 US gph. That is consistent with the individual boat data reported above.

And if you want to go fast, say maybe 10-12 kts at 2,200 rpm then the engine will burn 3.8 Imperial gph or about 4.5 US gph. And of course multiply by 2 for the total fuel consumption for a twin.

David

"If you have to ask the price, you can't afford it." J.P. Morgan, when asked the cost of keeping a yacht by another banker.