Does more boat engine hp equal faster speed

[rockman7892]

I'm not sure if this is the right forumn for this question but since is engine related I'll ask it here.

I was having a conversation with a friend the other day who stated that a boat with a larger hp engine or multiple larger hp engines would not be any faster than a boat with smaller hp engine(s). I argured that this statement was not correct but cant seem to put my finger on the evidence.

I'm an electrical guy so I'm familiar with electrical motors and the relationship between speed, torque,and hp. However I'm not quite sure with some of the concepts of a mechanical engine.

My thought is that with a boat engine the speed of the boat is directly related to the pitch of the prop and how fast the prop is spinning in rpm. These two factors being the determining factor that determine speed the diamater of the prop is related to providing the torque required to move the boat at a given prop speed.

I'm not sure if what I just stated is correct but assuming it is, hp = torque x speed. So for a given prop speed the prop pitch and prop diamater will require a certain torque. This torque times the speed of the prop equals the hp the motor must put out. So if we keep increasing the speed assuming the torque stays constant which I'm not sure it does then with increasing speed wee will need increasing hp. So with more avaliable hp we are able to operate the prop at higher speeds because we can produce the required torque at these higher speeds.

My friends arguemnt however was that if both motors spin the prop at lets say 5000 rpm for example this determines the speed of the boat and having additional hp will not cause the boat to go any faster.

Can anyone please help me with an explanation here and possibly some supporting equations.

Thanks

 

[patprimmer]

The propellor and gearing must be matched to the hull and engine and this is often the cause of considerable experimentation to get correct.

Generally for higher speed you use a smaller dia propeller at higher pitch or higher speed. Blade area also comes into it.

Insufficient blade area or to higher speed difference between the speed of the boat vs the speed the propeller is pumping the water back and you will get cavitation on one side of the blade. Cavitation is the water boiling at ambient temperature due to reduction of pressure at that point.

Shaft angle also plays a part.

Designed speed for the hull also plays a very big role and trying to push a boat past its design speed with brute power can be problematic in several ways.

Ideally, an engine should just reach maximum power when the boat is at maximum speed.

In reality, slightly shorter gearing gives better overall performance as the boat must always have reserve power to accelerate to the maximum speed.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[MikeHalloran]

If both motors weigh about the same and both can spin the prop at 5000 rpm, the boat speed will be the same, regardless of the engines' capability.

... but adding power and optimizing the prop will make a boat go faster.

... it would be stupid not to change the prop; it's the easiest and cheapest change to make in matching a boat to its engine(s).

... so you're both right, sort ot.

It's not a nice easy problem.

For a displacement hull, the power requirement at the prop is a cubic function of hull speed. So doubling the engine power doesn't double the speed; not even close.

The props slip quite a bit, so assuming they knife through the water in proportion to their pitch is just a first order approximation.

... and then it gets messy.



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

A prop knifing through the water and moving the boat forward by a mechanical thread is not at all how a prop works.

It works by pumping water backward and the equal and opposite reaction pushes the boat forward. Straight from Isaac Newton and his laws of motion.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[dicer]

Putting to process of linking the HP to what ever medium you are linking it to. More HP has the potential to make what ever go faster. Just hang an afterburned jet fighter engine on that boat and it will literally fly. We are talking thousands of HP.
So the simple answer is yes it will go faster. And that is why you see some boats with 3 or more outboard motors mounted on the rear.

 

[dicer]

This forum needs an edit button. (Putting process aside.)

 

[Lou Scannon]

As Pat mentioned, hull speed plays a major role. Every hull has a theoretical limiting speed when functioning as a displacement hull, which is proportional to the 1/3 power of the waterline length, if I'm not mistaken. I'm sure the equation can be readily found in a search.
A hull that is designed as strictly a displacement hull can be propelled close to its hull speed, but as the hull speed is approached the power requirement rises exponentially. Perhaps this is what the OP's friend was referring to.
A hull that is designed as a planing hull can only begin to plane if there is enough power available to allow the hull to climb its own bow wave. Once it's on plane, less power is required to continue planing.