will 17 in prop make boat goes faster than 19 in i have now

i got a new to me searay 215 express cruiser and it has a 19 in prop on there now..i am taching about 4500 at wot and it is running about 45-47 mph..it has a great hole shot and it is good on gas..i was thinking of going to a 17 in prop because my mechanic said it should be taching at 5500 rpm..will going to a smaller prop make my gas milage worse and will it give it better top end and how much if so..also there is an aluminum prop on there now will a stainless make a big difference? please let me know

Re: will 17 in prop make boat goes faster than 19 in i have now

What engine and drive do you have?

Re: will 17 in prop make boat goes faster than 19 in i have now

If yo have a 21 foot cruiser I will assume this is an I/O. That would also mean 5500 RPM is a tad high for most I/Os. If it is an outboard, then 5500 may be more reasonable. Yes the engine will use more gas for the simple reason that it will be running faster. Will the boat be faster than it is now? the answer is NO -- it will likely be slower but will have even better hole shot. Your thoughts that just because the engine is running faster must make the boat go faster is wrong. The engine is running faster because the prop is taking less "bite" on the water so it is moving the boat forward roughly 2 inches less with each revolution that with the 19P. That translates to more engine rpm but less or possibly the same boat speed. The only time your orignal theory holds true is if the boat is severely over propped and the engine is lugging terribly. Yours is not in that situation but then we don't know what the power plant is.

Re: will 17 in prop make boat goes faster than 19 in i have now

Silvertip said:

If yo have a 21 foot cruiser I will assume this is an I/O. That would also mean 5500 RPM is a tad high for most I/Os. If it is an outboard, then 5500 may be more reasonable. Yes the engine will use more gas for the simple reason that it will be running faster. Will the boat be faster than it is now? the answer is NO -- it will likely be slower but will have even better hole shot. Your thoughts that just because the engine is running faster must make the boat go faster is wrong. The engine is running faster because the prop is taking less "bite" on the water so it is moving the boat forward roughly 2 inches less with each revolution that with the 19P. That translates to more engine rpm but less or possibly the same boat speed. The only time your orignal theory holds true is if the boat is severely over propped and the engine is lugging terribly. Yours is not in that situation but then we don't know what the power plant is.

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it has a 5.0 mercruiser and it is a sterndrive with a gen 2 alpha 1 outdrive..thanks for ur input and it makes perfect sense..would going to a 21 make it have better top end? also will going bigger give it better gas milage

Re: will 17 in prop make boat goes faster than 19 in i have now

Just going up or down in prop sizes doesn't make the boat fast or slower, there are too many factors involved.
It's a mathematical equation to figure theoretic speed: Pitch X RPM = MPH. A prop is designed to move a boat so far for each revolution so in theory your 19P prop should move your boat 19" for each revolution and a 17P prop will move you boat 17" for every revolution. With mathematics (as in life?!) there are variables and the type of hull, the gearing in the lower unit, the ideal RPM range of the motor.

Some props are designed as bow lifting (typically have more cupping and larger surface area on the blades), there are choppers (more for the really fast boats, smaller surface area and can spin up faster but lack the bow lift needed for the deep-v hulls and have more resistance cause more of the hull is in the water), rakers, 2-blade, 3-blade, 4-blade, 5-blade (many, many variables) ideally I would think you'd want one that offers the best for all uses of your boat. So, just changing pitch is not the right way to approach, maybe you can borrow a more aggressive 19P prop that give more bow lift. There's a website/company BBLADES that can give you tons of info on selecting the right prop, I'd try them...plus I believe they have a "date a prop" before you "buy a prop" program so you can get it right?! Cruise through their website and see if that helps

I forgot to mention trimming...are you using your trim or trim-tabs? by lifting the bow up with the trim if lifts more of the boat out of the water; therefore, offer less resistance which can equate higher speeds?

Re: will 17 in prop make boat goes faster than 19 in i have now

What year is this setup? Just based on your rpm and description I'd say your setup pretty good.
A 21 ft cruiser that approaches sub 50 speeds with great hole shot is working right.
A simple pitch reduction will likely get you a reduction in speed.
The Gurus may be able to suggest something that squeezes out a little more but you may
lose your great hole shot.

Re: will 17 in prop make boat goes faster than 19 in i have now

bajaunderground said:

Just going up or down in prop sizes doesn't make the boat fast or slower, there are too many factors involved.
It's a mathematical equation to figure theoretic speed: Pitch X RPM = MPH. A prop is designed to move a boat so far for each revolution so in theory your 19P prop should move your boat 19" for each revolution and a 17P prop will move you boat 17" for every revolution. With mathematics (as in life?!) there are variables and the type of hull, the gearing in the lower unit, the ideal RPM range of the motor.

Some props are designed as bow lifting (typically have more cupping and larger surface area on the blades), there are choppers (more for the really fast boats, smaller surface area and can spin up faster but lack the bow lift needed for the deep-v hulls and have more resistance cause more of the hull is in the water), rakers, 2-blade, 3-blade, 4-blade, 5-blade (many, many variables) ideally I would think you'd want one that offers the best for all uses of your boat. So, just changing pitch is not the right way to approach, maybe you can borrow a more aggressive 19P prop that give more bow lift. There's a website/company BBLADES that can give you tons of info on selecting the right prop, I'd try them...plus I believe they have a "date a prop" before you "buy a prop" program so you can get it right?! Cruise through their website and see if that helps

I forgot to mention trimming...are you using your trim or trim-tabs? by lifting the bow up with the trim if lifts more of the boat out of the water; therefore, offer less resistance which can equate higher speeds?

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thanks for the info..yes i am using the trim..i trim it out till i hit the sweet spot and am getting the max out of it..i am going to go to that site that u said

Re: will 17 in prop make boat goes faster than 19 in i have now

my 190 SeaRay with a 5.0/Alpha and 19P Solas prop would hit 49mph. So your 45-47mph is good. to go faster you need more power.