Prop Help

Walkers2011

Cadet
Joined
Apr 1, 2017
Messages
13
Hi everyone!
I currently have a 89 Bayliner Trophy 21? hardtop cuddy. This was my first year with her and love her to death, it was a great year! The problem I am having is that it is significantly underpowered. It has a mercruiser 3.0 which is 140 HP on a boat that weighs with current fishing buddies gear and fuel roughly 3500 lbs. The engine was brand new installed when purchased, it had 7 hours on it. The biggest improvement I am hoping to make is getting on plane. It gets on plane at about 4800 rpms which pretty much at max throttle... I know the right prop pitch/diameter/material/blades can all help out with this....to a point obviously. I was wondering if anyone has any ideas on the prop I should be using to hop on plane faster, I don?t care about speed so much just love my fishing and want her to plane without screaming the engine as much.
Current prop 15x25 aluminum probably Walmart brand
Stern drive alpha one

And no I don?t care about price I?ll pay to make her happy and yes I?m aware price can be over a grand for quality.

Thanks for the advice or tips!!!!
 

thunder550

Petty Officer 3rd Class
Joined
Sep 6, 2017
Messages
83
To make educated recommendations, we will need to know pitch and blade count of your current prop, gear ratio of the drive, and what your current wide open throttle RPM and speed are.
 

Walkers2011

Cadet
Joined
Apr 1, 2017
Messages
13
Current prop
15x25 aluminum 3 blade
Wide open throttle is 5200 rpm
Planes currently at around 4800-5000 rpm
Sadly the gear ratio I do not know it?s a alpha 1 stern drive newly rebuilt I would imagine it would be the default ratio if there is one, it?s a earlier model of the alpha one if that helps even slightly
 

thunder550

Petty Officer 3rd Class
Joined
Sep 6, 2017
Messages
83
Something isn't right with your numbers. 15x25 prop as in 15" diameter, 25" pitch? I'm wondering if you have a spun hub. 5200 RPM and 25mph don't line up if your prop is good. 25 pitch seems a bit steep for a 4 cylinder too, at least with my limited knowledge.
 
Last edited:

thunder550

Petty Officer 3rd Class
Joined
Sep 6, 2017
Messages
83
For S&G I put my best guess at your specs in Mercury's prop selector and it's recommending 13.5-14.25" diameter 21-23" pitch props for acceleration, using a 2.0 ratio drive.
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,296
biggest prop you should have with your hull/motor/drive is a 17 pitch

as thunder550 stated, something is not right with your numbers

3.0's dont like to go above 4600 for very long. they have a tendency to warp the head when spun that fast.
 

Walkers2011

Cadet
Joined
Apr 1, 2017
Messages
13
Yep it was my only real confusion personally, I figured the heavy weight and underpowered engine combined is what?s keeping it from flying, the 25 mph with a 3.0 on a 21? when max spec is 19? for the 3.0. The trophy hard top caddy and 3 passengers is a lot of weight, roughly 3500 lbs which is much heavier than anything that 3.0 likes. I was hoping a 4 blade SS with the same pitch and dim would make a difference but it?s sounding like it won?t?
 

thunder550

Petty Officer 3rd Class
Joined
Sep 6, 2017
Messages
83
25mph and 5200 RPM is not a viable combination. Your engine doesn't have the torque to turn that steep prop at those RPMs. I think you have multiple problems happening...first, something mechanically wrong, maybe the prop hub is spun, but I would think that would get progressively worse. Second, that prop isn't a good match for your boat.

In order for anyone to give a good recommendation you will need to figure out the drive ratio at some point. Either mechanically by turning the engine crank by hand and counting prop rotations or by finding a serial number and looking up specs.
 

Walkers2011

Cadet
Joined
Apr 1, 2017
Messages
13
Ok ty so much I?ll pull the current prop and have a shop look at it and see if that was the issues I?m having then I?ll just have them order me one since I?ll be paying them anyhow

Thank you so much for the help and guidance!!!! :)
 

Scott Danforth

Grumpy Vintage Moderator still playing with boats
Staff member
Joined
Jul 23, 2011
Messages
47,296
Step one..... Verify the tach with either a shop yach or a photo tach

Step 2, verify the speed with GPS
 

thunder550

Petty Officer 3rd Class
Joined
Sep 6, 2017
Messages
83
Step one..... Verify the tach with either a shop yach or a photo tach

Step 2, verify the speed with GPS

Good call, I was assuming the numbers were accurate. That is definitely the first thing to verify.

Assuming numbers are correct, a good shop should be able to pick out a prop for you that will work well with your combination. But verify those numbers first.
 
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