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I do not really understand why some of the forum members regard the F 350 as good for large boats because it has a lot of torque, as I see it, as torque is directly related to displacement or even to the gear ratio I do not clearly see the claimed benefits of this high torque, I would say ( very probably wrongly) that you very probable have the same torque available running 3 verados with their 18 cylinders, 900 hp and 3 x 2.6 liter = 7.8 liters ( naturally as being non atmosferic they have some extra torque available) or 3 suzukis DF 300 with their 12.6 liters and 900 hp. if you wanted that extra torque that could reduce the propeller speed and increase its diameter for efficiency you could even take Zuke´s way by reducing the gear ratio so I dont really see the benefit of the F 350´s torque which is obtained via two extra cylinders making this engine mega heavy for its power ???żżż reliabilty issues apart I would much rather have 3 zukes which combined have more power than 2 F350 ( 200 hp more ), 18 cylinders (2 more), 12.6 liters (2.2 liters more ) and very probably more torque when combined and very likely at less price so I do not see why would someone prefer 2 x f350s.
For the same reasons, ( without including price or reliability as an advantage) the very best would be to have 3 verados SCSI 350 instead of 3 yamahas F350 or would those three combined lack torque to move a large boat ?? I believe not
I think you are absolutely right that focusing just on available torque is not very helpful. A real issue with most outboard setups is the limited ability to generate useful thrust due to the small propellers. This is exacerbated with heavier boats. I suspect that a primary reason for the "torquey feel" of the F350 is that the gear case can take a larger diameter prop. Three engines instead of two increases the prop area markedly, but at a cost in efficiency. It may be that the reason people feel the F350 is better for heavy boats is actually more due to the bigger props than anything else.
This is also one reason that there is such a startling difference in feel on heavier boats that have Diesel inboards and large diameter props versus some of the new large outboard-powered express style boats.
Horsepower is torque x rpm/5252. Most rated horsepower is a maximum at a specific rpm, say 6000 rpms. The big question is..how much torque/hp do they make at other rpms? An engine that makes less hp, but has a wide torque band will feel much stronger than a "peaky" engine that is weaker at lower rpms.
Nomans is on the money. Although every engine has a torque curve (good luck finding one on an outboard) using that torque to your advantage is often times a function of propeller size and gear ratio. It is an engineering dilemma to match a propeller/gear ratio to an engine that spins from 700-6000 rpms and maximize the engine's torque curve to both cruising and WOT performance. You typically land on a prop that satisfies your hole shot, cruising speed and WOT needs as a mix. A Suzuki 300 with it's larger diameter prop will feel more torquey at the lower to cruise rpms but isn't going to win a top end race with some other engines available because the larger prop becomes a negative as speed approaches WOT all other variables being equal.
Although looking at outboard torque curves might be fun, it's an excercise in curiousity only because they all perform very close to equal and your OB brand buying decision should surely be made on other factors besides a torque curve, even if you knew it..
Imagine for a moment though the ad nauseum posts we could read if OB torque curves were readily available.....it would be a whole new "this OB is the best" world.
i think you need to understand engine dynamics a bit more. Displacement doesn't rule the torque curve of an engine. Engines with the same ci can have different strokes and compression ratios that will negate torque curves. Gear reduction ratios only multiply torque. Torque is what moves a boat hp is what sells it.
Torque has nothing whatsoever to do with moving a boat. Or moving anything for that matter. Torque is simply a twisting moment. You can have torque without any movement at all.
Put a three foot long lug nut wrench on one of the lug nuts on your truck. Stand on the end of the lug nut wrench. If you are 200 pounds in weight you are no applying a torque of 600 foot pounds (three times 200) to the nut. Is the nut, the wheel or the truck going anywhere? I don't think so.
HP is the measurement of work being done. Work as in moving a boat of many thousand pounds at some velocity over a period of time.
Now, do you need torque to turn a propeller in order to get the work done? Absolutely. But torque is not a unit of work. HP is the unit of work.
i think you need to understand engine dynamics a bit more. Displacement doesn't rule the torque curve of an engine. Engines with the same ci can have different strokes and compression ratios that will negate torque curves. Gear reduction ratios only multiply torque. Torque is what moves a boat hp is what sells it.
a big boat weighs a lot,like 15,000 lbs for some of the~ 35-36 foot outboard powered boats.
like grady White 36.6, Big Whaler etc.
Analogy . Lets take a Big Dump truck. It has a diesel engine that probably generates 900 to 1000 ft lbs of torque but only produces 450-500 HP.It has lots of sand/rock load.
To get that big ass heavy load moving you need lots of torque plus multiple speed transmissions.Peak Torque on a deisel may be ~2100 rpm.
get behind an 18 wheeler leaving a stop light,see how many times he will shift to keep keep ~2100 rpm until he reaches 70 mph.
Now lets put in a Sports Car engine that has 500 hp and low torque,500 ft lbs or less, ie not 1000 ft lbs in the truck.
Good luck moving that loaded dump truck or 88,000 lb.18 wheeler.
So that is one of the reasons those big ass 50 foot Bertrams ,Hatteras,Vikings without multiple speed transmissions use Diesels. TORQUE
Fire safety also.
__________________ FL Upper Keys,33 foot World Cat w controlled upper bridge,Yam F250 w mech shift and throttles
i think you need to understand engine dynamics a bit more.
i agree. torque doesn't move anything. power is what moves your boat, your car, your feet...etc
the hp/torque thing has got to be the most misunderstood thing on this board.
you want a never-ending supply of torque? hang a hammer off the end of tree branch. it will sit there, applying that torque all day. not going to move your boat. in order to move your boat you need energy. the rate at which you apply that energy is called power. that is what moves your boat and keeps it moving.
you can get all the torque you want from a small 12v motor. put it through a series of reducing gears and you can turn 1 in-lb @ 12000rpm into 1,000 ft-lb at 1rpm. tons of torque. not going to move a boat very fast.
the only thing that matters is HP. and as some other posters stated, it is not peak HP that matters (except when operating at that particular rpm), but rather HP at your cruise rpm.
...assuming it is geared & propped correctly, it doesn't matter if it is 10,000rpm & 100ft-lb or 5,000rpm & 200ft-lb. what matters is the amount of HP
Torque has nothing whatsoever to do with moving a boat. Or moving anything for that matter. Torque is simply a twisting moment. You can have torque without any movement at all.
Put a three foot long lug nut wrench on one of the lug nuts on your truck. Stand on the end of the lug nut wrench. If you are 200 pounds in weight you are no applying a torque of 600 foot pounds (three times 200) to the nut. Is the nut, the wheel or the truck going anywhere? I don't think so.
HP is the measurement of work being done. Work as in moving a boat of many thousand pounds at some velocity over a period of time.
Now, do you need torque to turn a propeller in order to get the work done? Absolutely. But torque is not a unit of work. HP is the unit of work.
I think your getting torque with confused with force. You are applying force to the wrench reguardless of whether or not it's enough to move anything. HP and torque are simply units of measurement. You need force to turn a propeller to get the work done... how much force? Would you like that measured in HP or torque?
So explain to me...
My express has 355 hp diesel engines. Quick to plane, cruises 22 knots.
The same boat with 350 hp gas engines would barely be able to get out of it's own way.
If you say horsepower is the only thing that matters, and torque is irrelevant, why is that so
I believe that the only advantage of a torquey engine is its ability to move a boat relatively fast without high rpms ( like indeed in any moving vehicle it being a car or a motorcycle) but for the same Hps the boat will get the same speed only that the torquey engine will give those hps at lower rps and thus make less noise have longer life or better reliability ... but the boat if weights the same, when proped right should get the same speed it having the more torquey engine or the less torquey one ..... as many people say here "just my 2 cents", and Mr Topbnr i believe that if proped right your boat would be faster due to the weight reduction caused by using gasoline engines instead of heavy diesels .......
Tobnpr asked the pertinent question. The answer.."horsepower" without the corresponding rpm's is sorta meaningless. A 350 hp engine, rated at 5000 rpm's, has exactly 1/2 the TORQUE of a 350 hp diesel rated at 2500 rpms.
So explain to me...
My express has 355 hp diesel engines. Quick to plane, cruises 22 knots.
The same boat with 350 hp gas engines would barely be able to get out of it's own way.
If you say horsepower is the only thing that matters, and torque is irrelevant, why is that so
it is a good question.
to try to keep things as simple as possible, let's look at why you need 'less hp' with a diesel for the same cruise as a gas motor with 'more hp'
again...what matters is hp (and only hp). but not peak hp but hp at cruise rpm (for cruise speed at least).
so, in your example of a 355hp diesel vs a 350hp gas, why will the diesel engine cruise faster than the gas? at what rpm does the diesel cruise? the gas? how about compared to peak rpm (and peak hp)?
the reason is that diesels cruise at much closer to peak rpm than do gas engines. the closer cruise rpm is to peak rpm, the closer cruise HP is to peak HP. it really is this simple. never mind anything else
now, let's talk about why, when comparing 2 engines with the same peak HP one might have better acceleration than the other...we have to simplify and ignore things like prop slip, etc and assume that for the example they are equal.
the answer is simple...the one that has more HP at any given time is going to be accelerating faster. it does not matter how this HP is broken down into torque & rpm. whichever one has more HP at a given time/speed is going to be accelerating faster. it is that simple.
so if you have an engine that puts out 500hp at 5000rpm but has (let's just say) so much lift & duration on the cams that it has no power at 2000rpm, then at 2000rpm you aren't going to have any acceleration. this is what you might refer to as a 'peaky' HP curve. but as far as why you don't have any acceleration, it is because you don't have any HP. now, that is not to say that a torque curve is useless. quite the contrary, in the example i give, you are going to see the issue i describe by a dramatic low in the torque curve and then an up slope as the rpm's increase (and a corresponding increase in HP).
the point is that 'torque', by itself, is useless in moving your boat. it is only when torque is delivered at an rpm that it is useful. and the measure of its usefulness is the product of torque and rpm, aka POWER. torque is no more important than rpm and rpm no more important than torque. i could show anyone a perfectly flat TORQUE curve starting at 1,000ft-lbs and staying there. but without knowing at what rpm the torque is being delivered, it is useless. let's say you have an engine that delivers a perfectly flat 1,000ft-lbs from 1rpm to 100rpm. how is that going to accelerate compared to a 'wimpy' gas engine that delivers a flat 100ft-lbs from 1,000 to 10,000rpm? answer? (neither makes a great boat engine), but the high revving engine is going to out accelerate, out cruise, out everything the lower revving engine. how, when the lower revving engine has 10TIMES as much TORQUE as the higher revving engine is this possible?! answer: HORSE POWER.
i am reading some of the responses in this thread...and i don't know why i get involved in these things...but...
to those who say that it is torque that matters and a diesel is more powerful because it has more torque (ha ha)...i really don't think even you guys believe this. let me ask you this:
2 engines, one gas, one diesel. the diesel turns a max 2,000 rpm with 400ft-lbs, making ~150hp. the gas turns 5,250 rpm with 390ft-lbs of torque, making 390hp.
so, we have two boats, one with the 390hp gas engine set up (gears, props) properly and the other with the 150hp diesel engine set up properly. the boats are otherwise identical.
question to the 'torque' guys: which boat will accelerate faster, cruise faster, run WOT faster? keeping in mind the diesel has more torque, why?
In your hypothetical scenario, the big gas HP engines will have higher cruise and WOT speeds. The two scenarios would be close in terms of hole shot acceleration, and quite possibly the diesels would be better.
The question about torque vs hp is interesting, but the key to understanding the implications is to look at the torque curve.
A diesel will generate max torque in the 1500 to 2500 rpm range. A gas engine will generate max torque in the 3500 to 4500 rpm. These are typical ranges, and exceptions can be designed.
When that diesel boat starts at full throttle there is sufficient torque to enable the engines to get into peak torque range immediately. Max horsepower in a diesel engine is generated at RPM not a lot greater than the RPM at peak torque RPM.
In contrast, the gas engines are unable to get close to peak torque out of the hole because the gas engines deliver low torque at low RPM. Hole shot is relatively weak given the HP, but acceleration will really start to kick in on the gas engines as the RPM reaches mid RPM range as the gas engines approach peak torque. The gas engines will generate more HP at high RPM, and gas engines will typically redline at least 2000RPM higher than diesel.
There isn't necessarily a better engine torque curve. One should recongize that the torque curve will create characteristics that are better or worse than a different torque curve for a given application.
Just for point of clarification in my mind, let's throw a third engine of the same hp into the fray. Please compare a diesel, a 4-stroke gas and a 2-stroke gas engine of the same hp on the same make, model and year boat, and assume all 3 are properly set up. What would be the biggest performance differences and what would be (if any) similarities in performance?
Importance of torquey engines ?? 
