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gordon20mxz

Malibu Boat Horse Power Questions? Benefits? Explanation?

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gordon20mxz

I have what may seem to be a stupid horse power question with boats. I tried to search the forum and the internet but I can't get a straight answer.

I know I am missing something, so I want to be educated.

Lets' say you have Malibu 350 Monsoon engine in your boat. The engine, via the transmission, turns the drive shaft to the propeller. Your in your boat with NO ballast, you give it gas and the propeller moves you forward and you get on plane. Next, same boat, you fill up all of your ballast, now your boat weighs more, you give it gas and it takes longer to get on plane. That is logical.

But why does it take longer to get on plane? And how would more horse power help you?

My thought is the reason it takes longer to get on plane with the heavy boat is because the propeller is "slipping" in the water more. It's harder to push the boat forward because it weighs more. There is no more "friction" in the water to turn the propeller in the water, just because your boat weighs more. The propeller just can't advance the boat forward as efficiently because of "slipping" of the propeller with a boat that now has more resistance.

So it seems to me the only way to get on plane faster is to have a bigger propeller to grab more water or to spin the propeller faster. Are our biggest diameter propellers are 15" for Malibu Wake Setters, correct? (without getting into the nuances of cup and pitch). Spinning the propeller faster has to do with the transmission not the horse power. Unless different transmissions require different horse powers?? Is that what I am missing??

So, when people consider buying a boat with more horsepower is the benefit really that that engine can drive a different transmission with a gear ratio that spins the propeller faster? We discuss the horse power of upgraded engines more, but should we really be discussing the transmissions more (that come with different engines)?

What's the story? Thanks

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oldjeep

More horsepower doesn't actually help you, it is the additional torque that the higher horsepower engines have that allow it to come up to RPM faster rather than bogging down under the additional load.

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MadMan

The advertised horsepower is always peak power, at a high rpm. This helps with top speed, not a heavily loaded boat getting on plane.

What happens when getting on plane is, at wide open throttle, you might only reach 3000 rpm initially. At this rpm, your 350 horsepower engine is only making 200 horsepower. When you switch to a lower pitch prop, your initial rpm might reach 3800 rpm, where it is making 250 horsepower. This is why these props get your sacked out boat on plane quicker, it gets your engines rpm higher into the power band.

Another way to do this is build engine the produces more torque (horsepower is torque x rpm), most often done by building a larger engine. This way, instead of making 200 horsepower at 3000 rpm, it's all reading making 250 hp.

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Technicallyabu

Physics. It takes Work to move an object. Work is calculated as mass x acceleration x distance. HP is a measure of the rate at which work can be done. The more HP, the more work you can do in the same time period. HP is calculated as torque x rpm / 5252.

When people say you need an engine with more torque they are oversimplifying. Its more accurate to say you need more hp. An engine with 500 hp at 2500 rpm necessarily makes more torque than a engine that makes 500 hp at 4000 rpm.

The transmission and propeller transmit the engine hp, the rate at which it can do work, to the water pushing the boat through it. The transmission ratio and propeller pitch need to be selected so that the engine is working in its usable rpm range.

Increasing the trans ratio or reducing prop pitch slows the boat speed at the same engine rpm requiring less HP and a "smaller" engine.

And this is an oversimplification.

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gordon20mxz

OK. I understand torque, in terms of a vehicle on the pavement where you have tires with traction and 100% of their rotation gets transmitted to forward movement. And a vehicle has a transmission with gear changes (ratios) to help you use that torque to move a load with 100% traction to get going forward.

But there is a huge variable with a prop moving through water. A prop does not have 100% "traction" like tires on pavement. The friction to spin the prop in the water should be constant no matter what the weight of the boat. So my question is why do the bigger engines "feel" better or stronger in boating with a a wake board boat.

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justgary

The friction to spin the prop in the water should be constant no matter what the weight of the boat.

No. Viscous drag is *very* complicated, since it relies on angle of attack and the relative velocity of the blade to the fluid, to name a few. As the boat speeds up, both the relative velocity and the effective angle of attack change. Your prop is very efficient at idle, but much less so at max speed.

The same thing can be said of the hull as well, so when you put them together, all of the drag forces conspire against you as you attempt to go faster. When you add weight to the hull, it only gets worse.

You could prop your 350 to get what you seek when sacked out, but then you would complain that you can only go 30 MPH before hitting the rev limiter when running empty.

https://en.wikipedia.org/wiki/Reynolds_number

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Rednucleus

Ahhhh, now I remember why I chose veterinary medicine over engineering!

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catalac

Cliff Clavin would have the answer.

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MadMan

So my question is why do the bigger engines "feel" better or stronger in boating with a a wake board boat.

Having the bigger engine is like having a 2 for 1 special. Not only does it have more torque (and horsepower) at say 3000 rpm, because of this torque it is able to spin the prop to a higher rpm initially. So, it just starts at a higher rpm (maybe 3800) where it is making even more horsepower. The bigger engine is making way more horsepower at 3800 than the smaller one at 3000.

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TallRedRider

I have what may seem to be a stupid horse power question with boats. I tried to search the forum and the internet but I can't get a straight answer.

I know I am missing something, so I want to be educated.

Lets' say you have Malibu 350 Monsoon engine in your boat. The engine, via the transmission, turns the drive shaft to the propeller. Your in your boat with NO ballast, you give it gas and the propeller moves you forward and you get on plane. Next, same boat, you fill up all of your ballast, now your boat weighs more, you give it gas and it takes longer to get on plane. That is logical.

But why does it take longer to get on plane? And how would more horse power help you?

My thought is the reason it takes longer to get on plane with the heavy boat is because the propeller is "slipping" in the water more. It's harder to push the boat forward because it weighs more. There is no more "friction" in the water to turn the propeller in the water, just because your boat weighs more. The propeller just can't advance the boat forward as efficiently because of "slipping" of the propeller with a boat that now has more resistance.

So it seems to me the only way to get on plane faster is to have a bigger propeller to grab more water or to spin the propeller faster. Are our biggest diameter propellers are 15" for Malibu Wake Setters, correct? (without getting into the nuances of cup and pitch). Spinning the propeller faster has to do with the transmission not the horse power. Unless different transmissions require different horse powers?? Is that what I am missing??

So, when people consider buying a boat with more horsepower is the benefit really that that engine can drive a different transmission with a gear ratio that spins the propeller faster? We discuss the horse power of upgraded engines more, but should we really be discussing the transmissions more (that come with different engines)?

What's the story? Thanks

Reading your post carefully, I think the question is really framed with one faulty assumption. You assume that the prop spins with minimal resistance, and that by just putting a bigger prop or higher geared transmission, you can get the boat moving forward easier with the same engine you have now.

But the fact is that there is a LOT of resistance to your prop spinning in the water. It is a little more complicated than just putting a heavy trailer behind your diesel truck and going uphill because of prop slip and fluid dynamics. But nonetheless, the engine does feel plenty of resistance through the drivetrain. That is why boats with identical props, V-drives, and trannies will behave differently when you have different engines. An engine with more torque will spin the prop faster against the same resistance.

With my Malibu, I could not go faster than 18 MPH with my surf ballast all full, and the RPM's would get stuck at 3600 RPM or so. It was unable to spin the prop fast enough to reach the rev limiter because of the resistance. Even though it is in fluid, it still behaves a lot like a loaded truck going uphill.

Edited by TallRedRider

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williemon

Like tallredrider said, the rpm would stall out at a particular rpm due to the loaded boat. If the engine made more torque, and as a side effect it would end up making more hp due to the increased torque, it could have made it past that particular rpm. Propping down also increases torque. Propping down has a trade off of lower top speed, whereas a bigger higher torque engine could have kept the fastster prop

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gordon20mxz

Guys, this has been helpful to me, thanks. I guess what I understand now is: a prop that is in a non-loaded boat can move in the water efficiently and push the boat forward. In a loaded down boat, the prop becomes in-efficient and there is a lot of resistance to spin a prop that can't move itself along in a forward direction. So a larger engine with more torque can over come that resistance and more effectively spin the prop.

I now understand the factors I was missing. Thanks.

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UWSkier

Yes the prop is "slipping" some. It always is. More torque and HP lets you spin that slipping prop faster, all else being equal.

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DarkSide

Another thing to consider as you add weight. You are pushing the hull deeper into the water, more hull in the water means more resistance.

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gordon20mxz

Smoothwater,

Thank you for your explanation, that is helpful also. I think it is a good thing that the 2016 Malibu Wakesetter's will be coming with a larger base motor.

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williemon

Try this one, ride a bicycle with gears. You get the concept real fast since you are the engine.

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gordon20mxz

Try this one, ride a bicycle with gears. You get the concept real fast since you are the engine.

Willie,

Your idea a great way to look at it and I have thought about it it a way that is close to this. So, your on the bile alone and you maybe able to get going in 3rd gear. The you put another person on the handle bars and another person on your back (ballast). Now you can't get going in third gear, unless "you" get much stronger (bigger engine).

The difference with the bike example is that with the bike you have 100% traction with the pavement. If you can make the tire turn your not going anywhere. But. the prop doesn't have 100% traction with the water, it is going to spin. But what I learned is that a prop spinning against a big load and not being able to push things forward is very inefficient and the water does create a lot of friction in that scenario and a bigger engine does help over come that friction and spin the prop.

Thanks for your input.

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SunriseH2OSkier

You had me at 'Peterbilt', Peter... :lol:

Nothing like a good stump puller. :thumbup:

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MadMan

I think it was MB that produced wake boats with a 2 speed transmission a few years ago. Seemed like a good idea to me. Use the lower gear for ballast sports, the higher gear for everything else. Wouldn't even have to shift while moving.

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MadMan

- Your boat trailer winch has a 12" handle that you are strong enough to exert 100 pound feet of force upon to crank it. Your strap moves 12" using the 12" handle. If you can crank it at a constant 1000 rpm, the Power you are producing is 19 hp. (100 lb ft x 1 foot distance) x 1000 rpm / 5252 = 19 hp.

I wish I could generate 19hp with my arm(s), I would make Arnold look like a wimp...

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justgary

An excellent description, Peter!

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DarkSide

I think it was MB that produced wake boats with a 2 speed transmission a few years ago. Seemed like a good idea to me. Use the lower gear for ballast sports, the higher gear for everything else. Wouldn't even have to shift while moving.

This is being tested again now. The new version will not allow you to shift if your RPM is over a certain point, like 1000 RPM. The first version allowed shifting similar to a car, so when you got to a certain speed it would change. I guess it was very clunky.... New design sounds much better.

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williemon

It would be great if the shift point were custom configurable. Maybe with 2 actual shifts. That way I could customize it for shifting when just cruising or ski speeds, and then a second mode would be for surf and wake speeds. The shift point could be adjustable in each mode to compensate for prop, altitude, ballast and driving style. Great!! Now we just need a retrofit for older Bu.

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DarkSide

I believe it will be sumilar to what you are saying. Example a 1:1 setting for cruising, skiing, i.e. no ballast. Then a 2nd 2.5:1 for board sports where 25 would be absolute top end.

Ratios were example only...do not take as actual.

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