Ok guys, I will probably get slammed for this question but I am going to ask it anyways....

I saw the post about the 89 mph pontoon boat...

My question is, if you put two motors on a boat, that are the same horsepower, will it in fact make the boat go faster?

I was thinking that since the motors are the same size, that they will not turn anymore rpm's then the other, so in turn just go the same speed.

I was thinking that the only reason people put 2 motors on their boats is mainly because when they get out away from shore, mainly saltwater, that is one broke down they could get back in, plus have the added thrust to get through the larger wake....

Good question. Doesnt really make sence does it?? This would be a good question for those salt water boys.

You have double the propulsion.

but they are turning at the same rate...

same rpm's and all that

yes but say its a 250 hp, you get 2 250's and now you have 500 hp. more horsepower is better

that's true, but they are turning at the same RPM's, so it's not like they are making you go faster, just double the thrust...

atleast that's the way I see it.

OK...let's compare a 12/24V trolling motor when it's only running on 12v. It'll run in a 10 mph hour wind for 4 hours. That same motor, when hooked to 24V, will run in the same wind, at the same speed for twice as long.

When you double HP, you cut the load in 1/2 for each engine. Each engine is now doing 1/2 as much work as it was previously. So consider a 20 foot boat will run 50 with a 150 on it at 5500 rpms. Now put 2 150's on it and each engine would have to run about 2700-3000 rpms to hit that same speed. If you run each engine up to the 5500 rpm mark, the speed will increase greatly. (no, it probably won't double)

Now, if you're asking if 2 150's will run faster than a single 300 then answer is no. Salt water boats run 2 engines in case one fails way off shore! It's called 'redundancy' and it's cheap insurance.

You got 2 props delivering the samh hp 250X 2 =500

This makes the most sense...

BTW, another reason for multiple outboards is because some boats have such a high HP rating that a single motor hasn't been built yet to accomodate it. An example is the big Kingfish boats with 3-300's on them. No one makes a 900 hp outboard yet!

...and I know that's off topic, but wanted to fill in the gap on my explanation.

2 engines can push a boat faster than one,..just like an on an airplane. two engines provide more thrust...thrust pushes weight, the more push power the faster you can go...or the stronger you can push, depends on the applied torque...an in an airplane 2 is always better than one...

NitroRick, hope you understand now, but if not I'll give you another example. You have 1 hourse pulling a wagon, that weighs 2500 lbs. loaded. Harness up another horse, and they will be able to pull that wagon faster. Not twice as fast, but faster. Thus double the horspower to weight ratio = more speed.

Just joking, I know you understand, from all the good explainations above.

Tough question because of the variables involved, but the short and sweet answer is that a boat with 2 engines "can" go faster.

We are conditioned to think that the speed of the boat is directly tied to the horsepower it puts out. Answer this: What makes a boat go forward? Horsepower ? No.

The horsepower of an engine is what limits how aggressive a prop we can run. And what makes a boat go forward fast is the amount of water we can get the prop to displace in a given time.

Let's take a 300 hp engine and put the biggest most agressive prop we can on it. It can turn around 5500 rpm. Imagine the prop as a solid disc. Say 15 inches in diameter. That gives you a visual of how much water it can move. I'll make up a number, say it can shove 2 gallons per second.

Now let's put 2 150's on the boat. We cant run a 15" diameter prop, but let's say we could put a 13" diameter on it. 5500rpm.

We'll assume all the props are the same pitch so I don't have to think too much. The 300hp has a theoretical prop area of about 176 inches. The 150 with the 13" prop is sporting a total area of 132 inches...times 2, for a total prop area on that boat of 264 square inches.

Same amount of HP on both boats turning the same rpm, but one has a 264 sq. in. prop and the other has a 176 sq. in. prop. Which will go faster?

The reason to have more horsepower is so you can sling a bigger prop.

You however won't get the full benefit of all 264 inches of surface area because of the extra drag of having another engine in the water, and the extra surface area of the prop itself. The faster you go, the more parasitic drag you'll have.

Speaking of drag, the reason that pontoon can hit 80mph. is because of the design of the tubes its riding on.. and the fact they are dispacing a lot of water with an insane amount of power.

I'm solidly in the camp of "hull design makes a boat go fast more than raw horsepower"

I'm not a boat person, but an airplane guy. I'm sure the boat experts can explain better.

very good explaination....

this one makes sense to

That's what I was gonna say! haha just kiddin', that was an interesting read...