Lower HP for outboard vs inboard.

[bruceb58]

Re: Lower HP for outboard vs inboard.

While this is true both engines have similar HP but the IO is more powerful because of mor torque. Th OB has a weight advantage but strap both boats together and the IO will pull the OB all over the place. Less torque equals less twist to spin the engine to create HP.

You can't make that blanket statement unless you have the torque curves of each engine.

HP = Torque * RPM/5252

 

[chambers1517]

Re: Lower HP for outboard vs inboard.

Look at rpm at max Hp. If HP is equal the slower turning engine at rated HP will have more torque. This is almost always the larger displacement engine plus the torque builds at a lower rpm.

 

[agallant80]

Re: Lower HP for outboard vs inboard.

Now that you are switching trucks again, does that mean you are switching boats now too?

No, I was talking to a coworker about the boat he wanted. On the truck front I shot piston rods through both sides of the block.... Epic destruction.

 

[QC]

Re: Lower HP for outboard vs inboard.

Look at rpm at max Hp. If HP is equal the slower turning engine at rated HP will have more torque. This is almost always the larger displacement engine plus the torque builds at a lower rpm.

So? What are you going to do with that torque in a marine application?

Your I/O dragging the OB around comment is also potentially false. If geared right, prop-shaft torque could actually be the same. Also true of your Ferrari example: if geared right, 500 bhp from either engine will do the same amount of work over the same amount of time (power). Torque is a force, there is no time in the equation, so with torque you can get things done (force) but there is no limit on how much time you have to do it. When you add time (rpM) to the equation you are dealing with horsepower, and it is always what you are actually using. Yes, a higher peak torque engine, if all else is equal, will accelerate faster, but nothing else is ever equal

I am sorry to belabor this point. I always do :redface: With that said, this is definitely a semantics thing as mass marketing has destroyed the word "Torque" and somehow convinced us all that it is all that we want. The only reason you want torque is to make horsepower.

 

[GA_Boater]

Re: Lower HP for outboard vs inboard.

Actually it is torque then Horsepower. A 200 hp outboard powered boat will smoke a 220 hp powered i/0, until you add 8 people then the I/0 pulls ahead. The I/0 has more torque. Race a Ferrari against a diesel truck, no contest then strap 20000lbs behind both and see what happens.

What happens is the Ferrari blows it's guts all over the road. These types of comparisons are silly.

 

[chambers1517]

Re: Lower HP for outboard vs inboard.

Silly yes but my point is a 200hp OB is not the same as a 200hp IO. When the IO has a larger flatter torque curve then it is substantually more powerful than the OB even though HP is the same. A 200 hp engine with 176ft lbs of torque is not equal to a 200 hp 245ft lbs of torque engine. I'll use it to pull tubes and skiers.

 

[bruceb58]

Re: Lower HP for outboard vs inboard.

Silly yes but my point is a 200hp OB is not the same as a 200hp IO. When the IO has a larger flatter torque curve then it is substantually more powerful than the OB even though HP is the same. A 200 hp engine with 176ft lbs of torque is not equal to a 200 hp 245ft lbs of torque engine. I'll use it to pull tubes and skiers.

Do you have torque graphs that show this? Where did you get these numbers? How do you know that a 200HP outboard 4 stroke has less torque than a 200HP I/O engine at lets say 2000 RPM. Show your data!

 

[QC]

Re: Lower HP for outboard vs inboard.

And again... what matters is torque and power at the propshaft, nowhere else. If you have 200 lb/ft at 3000 RPM and 300 lb/ft. at 2000 RPM, they are the same at the propshaft if the former has a 3:1 ratio and the latter a 2:1. Both examples are 600 lb ft at 1000 RPM propshaft torque

 

[ssobol]

Re: Lower HP for outboard vs inboard.

I'd say that has actually flipped around now. I/O's need every component to be marine rated. You can't just safely stuff a chevy 350 in the engine compartment and hope it works. Hence, fuel injection only took off when the EPA forced it, and there is no direct injection marine I/O on the market that I know of. Fuel injection has been out for a long time on outboards, and DI has existed for quite a while also. DI 2 stroke outboards can be more efficient and pollute less than a 4 stroke.

I would argue with "I/O's need every component to be marine rated". Perhaps this is the case with the outdrive, but most of the parts of the Chevy 350 in your boat are identical to the 350 in your car (for example). That is a big reason why they use them, there is a big supply of whole engines and individual parts. Only things that are specific to a marine installation are different (e.g. risers for cooling water).

The choices of what engines are available for a particular boat are made by the builder. While they will size the motor for the boat, some of the decision is driven buy what motors are available and at what cost.

 

[bruceb58]

Re: Lower HP for outboard vs inboard.

Both examples are 600 lb ft at 1000 RPM propshaft torque

Yep!

 

[chambers1517]

Re: Lower HP for outboard vs inboard.

I can build 600ft lbs with a 5 hp motor with hardly any hp left. No matter how you multiply or divide it, a higher torque equal hp motor has more power.

 

[QC]

Re: Lower HP for outboard vs inboard.

I can build 600ft lbs with a 5 hp motor with hardly any hp left. No matter how you multiply or divide it, a higher torque equal hp motor has more power.

Those statements actually make no sense. Torque is a force, Horsepower is power. They are different, but you cannot have one without the other. Please understand this is what I do for a living. You are right, that a higher peak torque engine may have more power at that point (peak torque RPM), but it doesn't have more power unless the peak torque is at such an RPM that it develops more horsepower. torque x RPM/5252 = hp. The point is that it is only more powerful, if it makes more horsepower. So if they are identical in every other way a 200 hp engine at 4600 with peak torque of 300 lb/ft at 2000 RPM does indeed have more "power" than the same engine if it only made 200 lb/t at 2000 RPM, but those statements only mean more "power" at 2000 RPM, not at 4600. The latter would have a more linear horsepower curve, while the former would have a nice hump around 2000 RPM with more horsepower there because it makes more torque there. VERY useful for a wheeled vehicle that encounters hills, and great for planing, and great for tackling swells, but in my example it would be exactly the same at top speed (4600 RPM if propped correctly).

Listen, I am not trying to tell you that torque doesn't matter, of course it does, but the only thing we have here (Web) is semantics. Words. The words have to be right or we can't make any sense. You cannot make a blanket statement that the engine with more torque makes more power because it is literally incorrect. Without RPM none of this is even discussable. I can make 300 lb ft with my hands and body, but I can't push any boat faster than about 2 MPH because I can't make any horsepower, because I can't continue to make 300 lb/ft for more than 1/4 of a revolution, let alone multiple revolutions per minute.

 

[wrench 3]

Re: Lower HP for outboard vs inboard.

op2:

I think we need more popcorn!

 

[Bamaman1]

Re: Lower HP for outboard vs inboard.

I read on the pontoon forum all the time, and those with 320 hp Mercruisers might see the same top end speed as a 150 hp tritoon. And the 150 hp will get much better fuel mileage.

To get the speed out of a tritoon, going to the high performance hulls and 250 hp 4 stroke outboards is a seriously expensive proposition vs. a standard tritoon with 150 hp. I'm talking $8K for maybe 3-4 mph improvement. But the big engine will push a heavy load of people much better.

I've had 2 I/O's, and had trouble out of both. I'm only going outboards in the future.

 

[chambers1517]

Re: Lower HP for outboard vs inboard.

Marine Engine Torque Vs. Horsepower | Performance Product Technologies

 

[chambers1517]

Re: Lower HP for outboard vs inboard.

I know this article is pushing diesel engines but the HP Torque comparisons are still valid. Top speed aside, torque is more important for overall performance and larger displacement engines are better at it.

 

[UncleWillie]

Re: Lower HP for outboard vs inboard.

What gives with HP ratings on inboard and outboard boats? I am seeing that some boat makers are offering the Same Boat with an inboard or outboard option but the HP difference is huge. You can get a 330HP I/O but only a 250HP Outboard. Are outboards more effecent?

The boats may be similar in the mind of the marketing department but they are not the Same Boat.

You can't pull the OB off a boat with a splash well and drop in an I/O any more than you can pull the I/O and Mount the OB on a boat with an engine housing.
The structures are different, the weight balance is different.
If it isn't Apples to Oranges, it is at least Oranges to Limes.

 

[thumpar]

Re: Lower HP for outboard vs inboard.

The boats may be similar in the mind of the marketing department but they are not the Same Boat.

You can't pull the OB off a boat with a splash well and drop in an I/O any more than you can pull the I/O and Mount the OB on a boat with an engine housing.
The structures are different, the weight balance is different.
If it isn't Apples to Oranges, it is at least Oranges to Limes.

The load points are different also. An I/O pushes low on the transom. An OB push high on the transom.

 

[QC]

Re: Lower HP for outboard vs inboard.

Marine Engine Torque Vs. Horsepower | Performance Product Technologies

I reference things from the net all the time, but that doesn't make them right. This article actually makes my point which is: despite all of the hype regarding torque, ultimately you need horespower to make a boat go (they conclude the opposite ). I will repeat that if you have two engines, and two boats that are identical in every other way (gear ratio, weight, shape, prop, etc. etc. etc), that the one with higher peak torque will plane quicker and pull harder at that point (peak torque RPM). But the examples we are talking about are not the same. They make peak torque at different RPM, so the "power" is different for each at peak torque. Annnnnd, what you really care about is power at the propshaft, nothing else matters. So this is why gear ratios are so important to this discussion, gearing changes torque in direct relation to the ratio. Propshaft torque = engine torque x gear reduction ratio. Buuuutttt, horsepower stays the same because the combo of RPM and torque, while changed, results in the same output (horsepower). If you divide the RPM in half, and double the torque, you get the same horsepower. Period.

So the example from the link above is actually really good to clarify this discussion.

This is the targeted low torque engine to make their point:"a 300 Hp GM small V8 engine that makes 300 Hp at 5,000 and 375 Lb-Ft torque at 3200 rpm"

And this is the suggested diesel engine: a diesel engine that produces 300 Hp at 3600 rpm and makes 500 Lb-Ft torque at approximately 1600 rpm

Now with the gasoline engine let's use a 2:1 ratio to keep things simple:

Gasoline example at Propshaft: 300 bhp at 2500 RPM and 750 lb-ft torque at 1600

So let's use a 1:1 gear ratio for the diesel which would get us the same at the propshaft as the flywheel (not calculating any loss through drivetrain)

Diesel example at Propshaft: 300 bhp at 3600 RPM and 500 lb-ft torque at 1600

Yes, the Gasoline example would have to use a higher pitch prop to compensate for the lower RPM at the propshaft (2500 vs. 3600), but it has 50% more torque to help overcome that bigger wheel at planing RPM. So which boat is faster? Which combination has more power? Which combination has more torque? Hmmmmm.

 

[UncleWillie]

Re: Lower HP for outboard vs inboard.

QC has it correct!

All 300hp engines have different Torque than others and the max torque occurs at different RPMs.
The point of Max Torque does not occur at Max HP which also does not necessarily occur at Max RPM.
It is not all that simple.

Adding some more math to QCs examples.

a 300 Hp GM small V8 engine that makes 300 Hp at 5,000 and 375 Lb-Ft torque at 3200 rpm

300hp @ 5000 rpm = 315Lb-Ft
375Lb-Ft @ 3200rpm = 228hp (Max torque @ 76% Power)

300 hp at 3200rpm would produce 492Lb-Ft.
That is what gear boxes are for.

A diesel engine that produces 300 Hp at 3600 rpm and makes 500 Lb-Ft torque at approximately 1600 rpm

300hp @ 3600rpm = 437Lb-Ft.
500Lb-Ft. @ 1600rpm = 152hp (Lots of Torque, Not Much Power)

300hp at 1600rpm yields 985Lb-Ft. (Now we're Talking!)

2:1 Gasoline example at Propshaft: 300 bhp at 2500 RPM and 750 lb-ft torque at 1600

300 hp @ 2500rpm = 630Lb-Ft. (Double the Torque)
750Lb-Ft @ 1600rpm = 228hp (Same HP)

Never forget that Power(HP) is the product of Force(Torque) and Velocity(RPM).
You can always gear for higher torque, or higher rpm, but it is always about the POWER!

Your 2hp, 6000lb Tailer Winch has way more torque than the Boat, But I wouldnt exactly call it fast.