The value of diesel hp and torque ratings for comparison and repowering

Toolate

Admiral
Joined
Aug 6, 2013
Posts
6,355
Likes
2,274
Location
Southwestern CT
First Name
Ben
Boat Make
1998 36' Sabre Express CAT 3116TA's, 2015 ROS Scotia 16 Etec 50
Just reading the thread about engine hp and cruise speed and wondering about why diesel engines are constantly referred and compared primarily by their hp ratings. Often there is mention of the weight of an engine around here but almost never any specific talk about the torque at a given rpm (cruise) of one engine vs another that each have the same hp rating.

If I were to repower using diesel, after choosing what hp I was after, I would find the engines in that range and I think I would want the most torque per pound of engine at the slowest rpm available. Think we should leave maintenance/fuel consumption/sound levels/physical size out of this? Might just get too technical.

For example- the info I found on the internet (questionable) for two engines with the same hp rating is as follows (sorry about the Nm ratings):

Cummins QSB5.9 [email protected], 1156 Nm torque, 1360lbs which equals .26 hp/pound and .85 Nm of torque per pound. Torque to hp ratio of 3.25.

Yanmar 6YLA-STE 350/3300, 980 Nm, 1349lbswhich equals .26 hp/pound and .73 Nm of torque per pound. Torque to hp ratio of 2.8.

Interesting that the torque from the Cummins is 17% more than the Yanmar at almost the same weight. Also interesting that the torque to hp ratio for the cummins is so much higher than the yanmar. I would be curious to see how this shakes out for a bunch more engines. Anyone have this info for another 350 hp rated engine?

Now I am no diesel guy so please take it easy on me if I am out of line but it seems like the torque ratings are what should be looked at first especially when most people are taking a 2:1 gear and reducing the speed by half and doubling the torque to the shaft and relatively slow turning wheel. The real reason I am curious about this is because there are so many ratings for many of these engines depending on turbos, rpm ratings etc. that it seems to me that, in a repower situation, you would want to select an engine that makes the same torque as your current engine (at a similar rpm if you want to keep your gear and maybe just change your prop) and make your comparisons from there. Seems like a more direct route to finding the engine that will best reproduce the performance you had would be to compare torque per pound of the new engine to the same spec on the old engine. Not sure I am explaining myself very well but think about it- doesn't this make more sense than just looking at hp numbers?
 

Fish220sr

Captain
Lite User
Joined
Sep 3, 2011
Posts
528
Likes
218
Location
Long Island
Boat Make
29' Webbers Cove

Keelboater

Admiral
Joined
Oct 3, 2013
Posts
6,167
Likes
4,459
Location
Clinton Harbor
Boat Make
35' Bruno & Stillman
Torque in foot pounds= (5252 x HP) / RPM

Fill in the blanks for your motor. Notice that it is all RPM related. 300 HP @ 3300 RPM does not have the same amount of torque as 300 HP @ 2600 RPM.
 

Blitzen

Admiral
Joined
Sep 1, 2011
Posts
5,134
Likes
4,755
Location
Sumday Isle, RI
Boat Make
Flowers Boat Works 46 hull #1, Flowers Boat Works 36 hull #1, Wayne Beal 28 Hull #1, Repco 30 1968,
I think you need to start looking at bore and stroke of the engines to get a fair comparison. Once the torque curve flattens out for a given motor you can always feed it more fuel for hp and increased rpm but not torque. Most commercial rated engines will run at the top of the torque curve and no higher. So the hp ratting is a bit lower but it is strong as it is going to be.
 

djmarchand

Captain
Joined
Jul 24, 2012
Posts
664
Likes
290
Location
Litchfield, CT / Punta Gorda, Fl
Boat Make
Atlas Pompano 23 outboard
Talk to a repower specialist. They will first look at what you want to use the boat for. Recreational, short term fast cruising, fishing in the canyons where you have to go fast for a number of hours and then putter around for a while and then fast back, or a commercial lobster boat where you stop and go fast, stop and go fast.

Then when they have zeroed in on a couple of engines they will look at the torque curve. You want the fat part of the curve where the boat is climbing up on plane. That is where the engine needs torque to push the boat over the hump.

High rpm, high output engines like the Yanmar 6LP (hp peaks at 3,800) can be geared to turn a prop at reasonable rpms at cruising speeds. But they do not have the grunt torque of a Cummins 6B or QSB. Cubic inches/liters almost always means more torque.

David
 

Keelboater

Admiral
Joined
Oct 3, 2013
Posts
6,167
Likes
4,459
Location
Clinton Harbor
Boat Make
35' Bruno & Stillman
If Torque is great :grin:

and torque is king.

Then; king = great

Now we're really getting somewhere!
 

Jon Boat

Lieutenant
Joined
Jan 29, 2013
Posts
18
Likes
3
interesting thread

so what is moving the boat forward ?

the torque of the engine measured a dyno, or the propeller under the boat ?

i think i read something related to that a few years ago that was not quite what some are thinking here, and what i read made more sense that anything else I had ever read

and yes, i am old, been boating longer that most of you, and do mostly read much now as my boating days are long gone


jon
 
Last edited:

Toolate

Admiral
Joined
Aug 6, 2013
Posts
6,355
Likes
2,274
Location
Southwestern CT
First Name
Ben
Boat Make
1998 36' Sabre Express CAT 3116TA's, 2015 ROS Scotia 16 Etec 50
Torque in foot pounds= (5252 x HP) / RPM

Fill in the blanks for your motor. Notice that it is all RPM related. 300 HP @ 3300 RPM does not have the same amount of torque as 300 HP @ 2600 RPM.


Where does this equation come from? I dont think the torque to hp relationship/ratio is a linear relationship for all engines. I think this is a major misconception. Am I wrong?

I am not considering a diesel repower just talking about comparing engines. I have been into cars for half my life and with any gas engine (I am more familiar with them), there are a million ways to build them for more hp or torque and a chevy 350 can be built to produce 180 hp or 1400hp and anywhere between 150 lbft of torque and 1000 or so. Same is true for diesels but it surprises me that the talk around here seems to be hp focused rather than the torque and weight of an engine- all the while keeping in mind the cruise rpm.
 

Toolate

Admiral
Joined
Aug 6, 2013
Posts
6,355
Likes
2,274
Location
Southwestern CT
First Name
Ben
Boat Make
1998 36' Sabre Express CAT 3116TA's, 2015 ROS Scotia 16 Etec 50
Look at the rpms; that's why Cummins kicks Yanmar's ass.

I am not looking to compare these two engines specifically as much as to look at the relationship between hp and torque for all Diesel engines.

Can anyone provide some info on another 350hp diesel that turns the same rpm as either of these engines?

I think if we found a Cat or Steyr or Sisu that was rated at 350hp, the torque output would not be the same as either of the engines I posted at a matching rpm. See where I am headed? Wish I could explain myself a bit better.
 

Toolate

Admiral
Joined
Aug 6, 2013
Posts
6,355
Likes
2,274
Location
Southwestern CT
First Name
Ben
Boat Make
1998 36' Sabre Express CAT 3116TA's, 2015 ROS Scotia 16 Etec 50
Talk to a repower specialist... Then when they have zeroed in on a couple of engines they will look at the torque curve. You want the fat part of the curve where the boat is climbing up on plane. That is where the engine needs torque to push the boat over the hump.

David

This is the kind of analysis I would think every person should do- I hear a lot of talk about this engine or that engine and the hp ratings/weight but seldom any mention of the torque and I am wondering if people understand that not all diesels rated for XXX hp make the exact same amount of torque and to me, the torque numbers hold more value that the hp numbers.
 

Keelboater

Admiral
Joined
Oct 3, 2013
Posts
6,167
Likes
4,459
Location
Clinton Harbor
Boat Make
35' Bruno & Stillman
interesting thread

so what is moving the boat forward ?

the torque of the engine measured a dyno, or the propeller under the boat ?

i think i read something related to that a few years ago that was not quite what some are thinking here, and what i read made more sense that anything else I had ever read

and yes, i am old, been boating longer that most of you, and do mostly read much now as my boating days are long gone


jon

My morning coffee thoughts before the caffiene kicks in:
Without torque nothing would happen. Motors generate torque. Props use it to generate thrust. The motor creates the torque. The gearbox reduction multiplies the torque. The shaft transmits 100% of the useable torque; Torque in from the motor gearbox = torque out at the prop. The motor & gearbox may be capable of producing more torque than is required at the prop, but that does nothing for efficiency. If you remove the prop from the shaft what happens? No torque is transmitted through the shaft. It is all used to drive the motor. Motor RPM's get extremely high as a result. The boat goes nowhere. The prop uses torque to create the thrust. The amount of torque necessary to turn the prop at a given RPM depends on the diameter and the pitch of the prop. The larger the prop and the greater the pitch, the more torque required to turn it because more thrust is generated to push the boat. Something like that. Now, back to diesel talk........where the torque is generated. ;)
 

F/V First Team

Admiral
Joined
Jan 10, 2012
Posts
6,146
Likes
2,481
Location
Narnia
Website
www.otisenterprisesmarine.com
Boat Make
Northern Bay 36 - Modified
Sisu's turn much slower, for example my cruising rpm is 1700 producing close to 1400 NM.

So trying to compare engines with much different rpm ranges can be difficult when considering repowering. Now one has to take into consideration gearbox changes, running gear changes, propeller changes. Every time we've swapped out an engine and slipped a Sisu in we've always had to repitch the propeller and half of the time a different gear ratio was added to the mix. We've never lost speed though, always increased.

Take the example of the 40 Young Brothers. 375 hp Cat @ 2800, 2.5:1 gear reduction spinning a 27x25 (barely) which pulled 12 knots WOT. Slipped in a Sisu 410 @ 2100, 2:1 (and a brand new shaft, increasing diameter from 1.75" to 2", wound 4" of pitch into the propeller and she went down the bay doing 20+ mph over turning the wheel a couple hundred rpm. Propeller has since been changed to a 28x31 and the boat does over 22 knots WOT. It's a big boat built like a tank back in the day when resin was cheap, over an inch thick in places that really didn't need to be that thick.

End result slowed down the shaft rpm by 70 and required an inch of diameter and six inches of pitch for the propeller for a ten knot gain in speed. Pretty impressive I thought.

So it isn't a case of what engine can turn what rpm, but rather how you can harness that power and turn it into something you can do with the shaft rpm. If your engine turns 3300 and you want to replace it with one that spins 2100 you need to get your calculator out and figure the shaft speed. If your 3300 rpm engine needs 2.65:1 to bring the shaft rpm down to 1245 rpm then you would need a reduction gear around 1.65:1 to bring the 2100 rpm engine to the same speed. But since the engine is turning slower, there is more torque produced so you will be overturning your wheel. If you don't want to swap propellers now you need to do fancier math to dissect your running gear and find which gear reduction you will need, maybe you'll wind up with one of those elusive 0.86:1 gear boxes so you don't have to change your propeller. Not the way I would go personally, but to each their own.
 

Keelboater

Admiral
Joined
Oct 3, 2013
Posts
6,167
Likes
4,459
Location
Clinton Harbor
Boat Make
35' Bruno & Stillman
This is the kind of analysis I would think every person should do- I hear a lot of talk about this engine or that engine and the hp ratings/weight but seldom any mention of the torque and I am wondering if people understand that not all diesels rated for XXX hp make the exact same amount of torque and to me, the torque numbers hold more value that the hp numbers.

Like I said earlier: HP = (T x RPM) / 5252
How much torque do you want, and at what RPM? The HP is just a number that measures the amount of work being done. Some motors get the work done "faster", some get it done "slower", but they both get it done. The performance curve of the motor tells all. Yes, the torque AND the RPM are the most important. The HP just gets tagged on for ID.
 

Keelboater

Admiral
Joined
Oct 3, 2013
Posts
6,167
Likes
4,459
Location
Clinton Harbor
Boat Make
35' Bruno & Stillman
".......how you can harness that power and turn it into something......"

Words of wisdom from the mad doctor himself. ;)
 

Members online

Latest Posts

Top Bottom