why do marine engines not ever talk about torque?

[aggiedave98]

Why do marine engines only tell horsepower numbers and not torque numbers (or a curve) like automobile engines?

 

[Boomyal]

Re: why do marine engines not ever talk about torque?

Good question considering that torque, in the proper place, is so important for marine engines.<br /><br />Maybe they want to keep us in the dark or at least not bore us??

 

[Chaps]

Re: why do marine engines not ever talk about torque?

I saw a discussion on this subject on another board a couple of years ago and as I recall the torque spec is not relevant due to the way a propellor interacts with the water medium.

 

[seahorse5]

Re: why do marine engines not ever talk about torque?

Here is GM's site showing the torque and HP curves for each of their motors. Just click on any motor at the bottom of the page.<br /><br /> http://www.gm.com/automotive/gmpowertrain/engines/specialized/marine/index.htm

 

[Don S]

Re: why do marine engines not ever talk about torque?

When you look at the curves at the link given by seahorse read the disclaimer under the chart.<br />Those engines do not have the ECM attached, so all the Marine companies do is program the ECM which can change the torque curve some.<br />But it's probably fairly close.

 

[QC]

Re: why do marine engines not ever talk about torque?

This happens to be a favorite topic. First Chaps is right on on this one. It is the prop load curve that matters NOT the engine's torque curve.<br /><br />Remember that horsepower (bhp) is a calculation of work i.e. moving something. Torque is twisting effort. bhp = torque X RPM divided by 5252. Propeller curve is a funny term because in actuality it is linear. The bhp required to turn it goes up in a straight line relative to prop RPM. With that said, then peak torque as we all think we know it is absolutely meaningless to a propeller, right? Wrong. There is one point that prop curve cannot describe or define. The point at which the boat climbs out on plane. What happens there is that the engine actually does "lug" briefly and then comes out of the lug condition when the boat fully planes. Then the engine accelerates merrily up the prop load curve.<br /><br />The most important part of this whole post is the calc I wrote down above. Stare at it (I have been for 20 years) what it eventually tells you is that we have all been mislead about torque. Some say "torque pulls loads, horsepower wins races" both are true and both are false. You can't have bhp without torque and torque is nothing without RPM which turns it into bhp. The last little note on this is to think how much torque you can apply with your little own body. Around 300 lb/ft. The peak torque for the 4.3 GM from the site noted in Seahorse's post is 267 lb/ft. Does anybody think he can hand crank a prop to 40 MPH in any boat? Of course not, becasaue you can't make any RPM at 300 lb/ft. But you can easily make some RPM turning the jack on your trailer, right? And what happens? The front of the trailer rises; voila, you just made horsepower i.e. work.

 

[Chaps]

Re: why do marine engines not ever talk about torque?

Now isn't that exactly what I said already? <br /><br />Sheesh . . . these people that just repeat the same info over and over in posts . . .

 

[QC]

Re: why do marine engines not ever talk about torque?

At least I gave you credit Chap-dude!

 

[Chaps]

Re: why do marine engines not ever talk about torque?

That you did . . . and I'll fess up, what I said in my first post is absolutely everything I know about the subject . . . which isn't much!

 

[DHPMARINE]

Re: why do marine engines not ever talk about torque?

I always thought,and maybe incorrectly, torque spins the wheels,horsepower makes the speed.With prop slippage,is torque relevant in a boat ?<br /><br />DHP

 

[MrBill]

Re: why do marine engines not ever talk about torque?

Torque is very relevant...and all the boating rags talk about it, graph it, etc. especially in comparisons between two-strokes and four-strokes. <br />Forget about all the equations that try to explain torque, except the one that shows the relationship of torque and horsepower in the same equation (Quietcat mentioned above)...<br /><br />Torque = (Horsepower X 5252) divided by RPM.<br /><br />What does this mean to boaters?? First, know where your power curve is, second, acceleration follows the torque curve, and third...it's better to make torque at high rpm because you can take advantage of 'gearing'. Hence, lower units are "geared" accordingly.

 

[aggiedave98]

Re: why do marine engines not ever talk about torque?

MrBill,<br /><br />you said:<br /><br />"it's better to make torque at high rpm because you can take advantage of 'gearing'."<br /><br /><br />But since in boats, you need a lot of torque to get planed out and less (maybe?) to keep you going, wouldn't you want a low of low end torque?

 

[Dunaruna]

Re: why do marine engines not ever talk about torque?

Originally posted by aggiedave98:<br /> wouldn't you want a low of low end torque?

Which is exactly what a marine cam is designed to do.<br /><br />Aldo

 

[Silvertip]

Re: why do marine engines not ever talk about torque?

QuietCat -- you said "You can't have bhp without torque and torque is nothing without RPM which turns it into bhp. There is one exception (just worthy of note) -- a steam engine develops maximum torque at nearly "0 RPM". If you have ever seen an old 10 to 16 horsepower steam tractor at a "tractor pull" pulling against our modern 400+ horsepower gas engines, you will understand. The steam tractor usually wins.

 

[QC]

Re: why do marine engines not ever talk about torque?

Just remember, you cannot separate the two. DHPMarine is correct, torque is twisting effort, not the resulting motion that occurs. Horsepower is the result of applying that torque at some speed rate i.e. RPM. Prop slippage is just wasted bhp: fuel was spent, bhp was made, it just didn't make the boat go any faster. One of a boat's inefficiencies when compared to a similar sized car.<br /><br />I don't agree on the gearing comment. The gearing is simply to take engine RPM that would blow a propeller apart and bring it down to an RPM where the size and physical properties of the propeller work better. Theoretically, you cannot lug a marine engine unless you have some sort of shifting transmission or it is propped incorrectly. The only issue is the bhp required to turn the propeller a given speed. That requirement goes directly up as prop speed increases, so there is very little bhp (or torque) required to keep a prop spinning at low rpm. The only time a correctly propped marine engine is at full torque (along the lug curve) is at full speed. Not peak torque, peak torque is the max the engine is capable of making. That same little 4.3 makes 267 lb/ft at 2800. This equals 142 bhp, but the propeller at this speed only needs approximately 60 bhp to keep spinning.<br /><br />That reserve of 60 required vs. 142 available does help you pop on plane. The abiltiy to jump up on plane is not accounted for on the prop curve. It is basically a gravity issue resulting from your boat sticking it's bow up and then pushing that weight over the bow wave. Other than under this condition the engine is not near peak torque, and if you accelerate gradually you never get close. The car discussion is VERY different, and high peak torque levels are much more important.<br /><br />BTW, the boating rags usually have this wrong . . .

 

[QC]

Re: why do marine engines not ever talk about torque?

I missed the steam comment. The steam engine still is nothing if it doesn't complete a revolution (at nearly 0) . . . Then that torque DOES become horsepower. You're right, torque starts things rolling, but unless it rolls you don't have work, if you have work you have horsepower. I am not familiar with the steam vs. gas at the tractor pull thing, but there are some other things going on if the 10 bhp tractor "wins". If it is simply breakout of the sled then I could see it, but then this becomes a gearing and traction discussion.<br /><br />Aggiedave is right about the benefit of peak torque at a low RPM when we are talking about a car. It is not very relevant with a marine engine unless the boat is so underpowered and over propped that it simply won't plane.

 

[Silvertip]

Re: why do marine engines not ever talk about torque?

No arguement gents -- just presenting a little twist on an interesting topic. If you think about it, whatever maximum steam pressure is exerted on the piston at zero RPM is maximum torque. Granted, at maximum torque, the steamer has zero horsepower because its doing no work.

 

[QC]

Re: why do marine engines not ever talk about torque?

Good debate, no argument. Cylinder pressure in a gasoline engine translates directly to torque too.

 

[cc lancer]

Re: why do marine engines not ever talk about torque?

Cylinder pressure in a gasoline engine translates directly to torque too.<br /><br />Does this mean we are not going to talk about rod length?

 

[QC]

Re: why do marine engines not ever talk about torque?

I was at a bar this weekend at Lake Havasu. The restrooms were marked "Inboards" and "Outboards" . . . cclancer's question made me think of that, sorry.