New Boat Prop

[ChrisAW]

I have a 15'6" Smokercraft aluminum V that I got a deal on last winter. Found a complete Mariner 30E with controls for $600, runs great after a little bit of work.

 

I did a little measuring and found that my prop has around a 11.25" pitch. Using one of these online calculators, factoring in my RPM's and and all that, it calculates pretty much exactly what my highest speed was during use this last season, around 26mph.

 

Whats funny is that a friend of mine let me use his 9.9 on this boat before I bought my motor, and it would push this boat to 16mph, with me and my tackle. The only thing that changed with the big motor is a console and controls, adding maybe 20 pounds besides the weight of the motor itself. It seems to me that a 30hp motor should push this faster, but I'm now seeing that it is probably the pitch of my prop that won't allow it to run any faster.

 

 

I've found that a 12 or 13 pitch is available for this motor. It really doesn't have any trouble getting on plane when I have the motor about the middle of the trim angle available. I do not have a hydraulic trim though, so I can't adjust on the fly.

 

Should I avoid such a steep pitch, in fear of losing a lot of low end torque? Or would a boat such as this not really have that problem?

[slonezp]

How close are you to max rpms?

[ChrisAW]

How close are you to max rpms?

 

I wasn't able to find get a tachometer last year, but I have no reason to believe it isn't getting close. It sounds healthy, I would say atleast 5000 of the 5500 max.

[Hi Salenity]

If your not at or above max rpm at wot don't get a bigger prop

[ChrisAW]

Ill have to find a tach and see. I'm sure it is. It doesn't struggle at all, but I will make sure before I go any further. I need a new prop either way.

[Brian Needham]

a bigger prop is going to drop your RPM 200-400..... so you really need to know what your starting RPMs are because if you are already say 300-400 from max, then a bigger prop will drop you below intended operating range which is not suggested or good for your motor

[Brian Needham]

is your speed GPS?

if not then do not rely on non GPS speed for any math based equations.

[Way2slow]

First major flaw to your theory is as the speed of a boat increases, so does the hull drag. A 225 hp motor will push my boat 76 mph, a 325 hp motor pushes it 83. That's only a seven mph gain in speed for a 100 hp gain in power. Using your logic, the 325 should push it over 100 mph and that ain't gonna happen.

If you take the nut off the prop and look around the edge, under the washer/nut, there should be a bunch of numbers. If it doesn't have the diameter and pitch in it, then a dealer can convert the part number for you, provided it's a merc prop. If it's an aftermarket, it still should have the size there or stamped into the side of it.

You really need the rpms to know what the motor is doing and before you start trying props. I've been messing with, rebuilding and racing boats and motors over 50 years and I still can't tell you what rpm a motor is turning within several hundred without a tach.

[Hi Salenity]

Secondly you say you need a new prop either way, how bad is yours? Is it stainless? Just courious

[ChrisAW]

No, probably the original aluminum prop. It's a little beat up, but not missing any chunks and has no cracks that I noticed. It would make a good spare. I'm just not a fan of leaving my equipment in beat up shape until it gets worse.

I realize it won't pick TONS of speed, but it isn't struggling and if I can get more, I would mind any little bit it can get.

[Hi Salenity]

I'm with you 100%. First we are going to need a tach. It's vital to know the RPM at WOT, then we will need a GPS speed. I'd try for a top speed with different trim angles.

The problem with just throwing a larger prop on it is if its not over revving now it probably won't turn enough RPMs to gain any speed and it may have holeshot problems and actually loose top speed.

[Way2slow]

A stainless prop will give you more speed and better hole shot than the same pitch in aluminum. The stainless prop is more efficient because it takes less horse power for the thinner blades to slice the water. The blades are a hellavalot more rigid on the stainless prop so they don't flex back as much when under a load, like hole shot or WOT, therefor they don't loose pitch like the aluminum, giving more speed.

Not knowing rpm makes it rather difficult to fine tune a prop to a boat.

Go one pitch to much and it may very well run slower and the hole shot will totally suck.

[ChrisAW]

Thanks for the info guys. We are still frozen up here in Michigan, so our light be a little while before I can get an rpm reading.