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Old 27-02-2022, 17:12   #1
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Repower- Torque or HP?

Hey guys,



Looking at an older boat with a pair of Perkins M135 engines. Moderate hours, solid shape- but WOW do they use fuel.



Looking at the torque curve:
http://www.transmarsa.com/wp-content...-M130-M135.pdf


It looks to run 430nm at 1,600rpm and has a 135hp rating.



A modern Yanmar 4JH3-DTE:
https://www.yanmar.com/marine/product/engines/4jh3-dte/


It looks at about 260nm at 3,400rpm and a 150hp rating.





So Im wondering what the discrepancy is about?

Yanmar claims 150hp and so I would think this is the more powerful engine, but looking at the torque curve says the opposite, that its significantly less powerful.

If I were to purchase this boat, I would want to repower with more efficient engines (eventually) and so would like to understand this.
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Old 27-02-2022, 17:19   #2
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Re: Repower- Torque or HP?

Hp is a measure of work done over time. So that's what determines how powerful an engine is. Torque can be multiplied with gearing. So a higher revving engine with less torque may need different reduction gearing and a different prop to match the same boat optimally.
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Old 27-02-2022, 17:21   #3
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Re: Repower- Torque or HP?

I knew a new prop set would be a part of moving up to the high speed diesels. I am still feeling a bit lost about why the difference in torque is SO big. I mean its nearly double.
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Old 27-02-2022, 17:23   #4
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Re: Repower- Torque or HP?

Quote:
Originally Posted by BenBowSirocco View Post
I knew a new prop set would be a part of moving up to the high speed diesels. I am still feeling a bit lost about why the difference in torque is SO big. I mean its nearly double.
HP is a function of torque vs RPM. So for a given HP, an engine that makes it at higher RPM will be making less torque.
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Old 27-02-2022, 19:21   #5
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Re: Repower- Torque or HP?

Quote:
Originally Posted by BenBowSirocco View Post
I knew a new prop set would be a part of moving up to the high speed diesels. I am still feeling a bit lost about why the difference in torque is SO big. I mean its nearly double.
The RPM is nearly double on the yanmar as well.
Horsepower = torque x RPM / 5252
If you run the yanmar through a 2:1 gearbox, the output of the gearbox will have (approximately) Half the RPM and double the torque. Very similar to the other engine.

If you are doing a repower, it is worth a discussion with the engine manufacturer, and probably also a propeller manufacturer. It is important that the engine, gearbox, and propeller are all selected to work with each other, and with the hull design of your boat.
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Old 28-02-2022, 08:16   #6
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Re: Repower- Torque or HP?

"If you run the yanmar through a 2:1 gearbox, the output of the gearbox will have (approximately) Half the RPM and double the torque. Very similar to the other engine."


That makes sense.



I suppose any discussion involving a repower would need to be done in concert with a NA (hopefully the original), Engine Manufacturer and Propeller Manufacturer..



Certainly not a small task it seems.



Those Perkins use so much more fuel than the Yanmars. Makes me wonder if its mostly a technology advancement, or perhaps those Perkins M135 are desperately under-rated.
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Old 28-02-2022, 11:13   #7
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Re: Repower- Torque or HP?

Quote:
Originally Posted by BenBowSirocco View Post
Hey guys,



Looking at an older boat with a pair of Perkins M135 engines. Moderate hours, solid shape- but WOW do they use fuel.



Looking at the torque curve:
http://www.transmarsa.com/wp-content...-M130-M135.pdf


It looks to run 430nm at 1,600rpm and has a 135hp rating.



A modern Yanmar 4JH3-DTE:
https://www.yanmar.com/marine/product/engines/4jh3-dte/


It looks at about 260nm at 3,400rpm and a 150hp rating.





So Im wondering what the discrepancy is about?

Yanmar claims 150hp and so I would think this is the more powerful engine, but looking at the torque curve says the opposite, that its significantly less powerful.

If I were to purchase this boat, I would want to repower with more efficient engines (eventually) and so would like to understand this.

===


A re-power with new Yanmars will probably cost upwards of $80,000. You can buy quite a lot of fuel with that so I'd be in no hurry as long as the Perkins are in good condition and running well. All things considered, an engine running at lower RPMs will be more lightly stressed and have longer service life than an engine with comparable horsepower at high RPMs.
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Old 28-02-2022, 11:28   #8
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Re: Repower- Torque or HP?

I just looked at the documents in the OP. I'm not sure where the idea that those Perkins are inefficient comes from. Based on the full power fuel consumption spec, they make about 18.2 hp-hr per gallon of diesel burned (7.4 gal/hr to make 135hp). For a naturally aspirated diesel, that's actually not bad. Turbodiesels will do a bit better, but even the best modern stuff is only around 21 hp-hr per gallon.

For comparison, the Yanmar mentioned in the OP is rated at 125hp (not 150). Based on the charts given, the Yanmar appears to burn 6.9 gal/hr to make 125hp, giving 18.1 hp-hr per gallon. So at least at full power, it's not any more efficient than the Perkins.

Doing a comparison at lower RPM for cruising, if we throttle the Perkins back to 2200 rpm, the prop curve given in the book shows 85hp at that point and 3.8 UK gallons per hour, so 4.56 US gallons per hour. Rounding up to 4.6 as a worst case, that puts efficiency at just shy of 18.5 hp-hr per gallon. The Yanmar throttled back to 3200 RPM is putting out 80hp on the prop curve. At that point, as best I can read the chart, it's burning 4.3 gal/hr. That puts it at 18.6 hp-hr per gallon.

So as much as that Perkins isn't the most efficient diesel in the world, it's really not terrible (better than a 2 stroke Detroit will manage). And the specific Yanmar mentioned isn't really any better in the efficiency department. Most of the diesels that will get closer to the 20 hp-hr per gallon range are going to be bigger than this power range. The Yanmar is likely cleaner burning, will smell less, and will leave less soot near the exhaust outlet, however, being a more modern engine.
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Old 28-02-2022, 12:03   #9
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Re: Repower- Torque or HP?

Quote:
Originally Posted by rslifkin View Post
I just looked at the documents in the OP. I'm not sure where the idea that those Perkins are inefficient comes from. Based on the full power fuel consumption spec, they make about 18.2 hp-hr per gallon of diesel burned (7.4 gal/hr to make 135hp). For a naturally aspirated diesel, that's actually not bad. Turbodiesels will do a bit better, but even the best modern stuff is only around 21 hp-hr per gallon.

For comparison, the Yanmar mentioned in the OP is rated at 125hp (not 150). Based on the charts given, the Yanmar appears to burn 6.9 gal/hr to make 125hp, giving 18.1 hp-hr per gallon. So at least at full power, it's not any more efficient than the Perkins.

Doing a comparison at lower RPM for cruising, if we throttle the Perkins back to 2200 rpm, the prop curve given in the book shows 85hp at that point and 3.8 UK gallons per hour, so 4.56 US gallons per hour. Rounding up to 4.6 as a worst case, that puts efficiency at just shy of 18.5 hp-hr per gallon. The Yanmar throttled back to 3200 RPM is putting out 80hp on the prop curve. At that point, as best I can read the chart, it's burning 4.3 gal/hr. That puts it at 18.6 hp-hr per gallon.

So as much as that Perkins isn't the most efficient diesel in the world, it's really not terrible (better than a 2 stroke Detroit will manage). And the specific Yanmar mentioned isn't really any better in the efficiency department. Most of the diesels that will get closer to the 20 hp-hr per gallon range are going to be bigger than this power range. The Yanmar is likely cleaner burning, will smell less, and will leave less soot near the exhaust outlet, however, being a more modern engine.
Right on all counts. He just saved you $80,000.
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Old 28-02-2022, 14:51   #10
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Re: Repower- Torque or HP?

Quote:
Originally Posted by wayne.b View Post
===


A re-power with new Yanmars will probably cost upwards of $80,000. You can buy quite a lot of fuel with that so I'd be in no hurry as long as the Perkins are in good condition and running well. All things considered, an engine running at lower RPMs will be more lightly stressed and have longer service life than an engine with comparable horsepower at high RPMs.


My experience with high reving, low torque engines in a boat is that as soon as the prop gets dirty they tend to lug down a lot whereas the longer stroke higher torque engines are not so prone to do so.
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Old 28-02-2022, 16:52   #11
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Re: Repower- Torque or HP?

Chalk and cheese, you are looking at a 2 litre turbo inter cooled high speed engine verry good for lightweight planeing hulls,and I mean lightweight low water plane,compared to a 6cyl 6liter non turboed non inter cooled heavy engine ,verry long life ,good for heavy planing or displacement hulls check output at prop shaft ,also the injectors may need a service ,will often improve fuel burn .⛵️⚓️
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Old 02-03-2022, 01:31   #12
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Re: Repower- Torque or HP?

rslifkin has it. We have a pair of the M135 and they are within a bees di...very similar to most other diesels. The difference between power-consumption for diesels in the last 40 years is pretty small - 18.2 vs 21 for the best modern is about 15%: not bad for 25 years old and a lot less costly and complex!

Where modern diesels, inc common rail turbo and aftercooled, wins easily is the weight and size. The M135 is 600kg without transmission, and big. Your example (not really the same as others have pointed out) is 229kg or just over a third the weight. The size is also entirely different:0.73 cu.m vs 0.38cub.m or nearly half the size.

When you say "moderate hours", how many? If the Perkins have been looked after decently then "moderate hours" may be 10,000 with a life of 20,000+. I seriously doubt (and won't be around!) if the Yanmar you compared to would get anything like that - "moderate" may be 2000hrs.
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Old 23-05-2022, 11:06   #13
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Re: Repower- Torque or HP?

365 cubic inch ( 6 ltr.) 6 cylinder vs. 122 cubic inch (2 ltr.) 4 cylinder explains the difference in torque numbers and at what RPM peak HP/ Trq. is created. Simply put, higher displacement engines tend to produce more torque at lower RPM's.

Larger displacement engines typically have longer life spans than that of high revving, smaller displacement, turbo charged ones. I have a Fischer Panda generator on my boat that has a 3 cylinder Kubota that runs at 2600 RPM with a light load on out.....I am curious to see how many hours I get out of it before it has to be replaced.
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Old 23-05-2022, 11:33   #14
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Re: Repower- Torque or HP?

Don't confuse torque with hp...torque is the true measure of an engine's power..

Diesel engines typically burn about one gallon of fuel for 20 hp produced....give or take, don't bombard me with other stats..

Most marine diesels are designed to operate at a given rpm for a given time..running an engine at max rpm is a sure fire way to destroy it in short time.

My advice...get the " Propellor Handbook" authored by Dave Gerr. In it, you will find anything and everything related to marine diesel propulsion. It's a complicated read, and may require several readings to get a good grasp of the situation.

The book will show how to calculate the necessary engine requirements for your boat. this will require several pages of calculations. Well worth the effort.
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