New Lower Unit, less acceleration

[Danny C]

[FONT=&quot]I clipped a submerged wingdike with my '83 merc 115. It slightly bent two flukes of the SS prop and slightly bent the prop shaft. I had a shop retune the Prop and it looks like they bead blasted it as well. The guy that was going to replace the prop shaft for $200 ended up just swapping the lower unit with a newer one in better shape for $300. Since the swap, my acceleration has decreased. When I hit the throttle, I can hear the engine respond (rpms increase), but I do not get the thrust like I was used to. I will eventually get up to speed, but it takes awhile to get there. It almost feels like the prop isn't gripping the water right away. Could it be from a different gear ratio in the lower unit? Does the prop need a rough side to grip the water? The mechanic that swapped it suggested drilling vent holes in the prop. I am a little hesitant about just drilling holes. Any suggestions/thought?

-Thanks[/FONT]

 

[Chris1956]

Maybe your prop is partially spun? If so, it will shortly fail completely. A prop shop should be able to check it out. BTW your gear ratio is 2::1. You can check that fairly easily. Other gearcases that will fit your motor could be 2.3::1.

 

[Faztbullet]

If a 2:3:1 ratio was installed the rpm will be higher(about 600rpm) hole shot will be slower and top end will be slower, Easy test is to mark prop and rotate crankshaft with rachet and count how many times you turn crank for 1 revolution of prop.

 

[enginepower]

Are the rpm's now higher than they used to be at top speed? If so, prop doesn't have enough pitch.

 

[Danny C]

Thank You for the responses. RPMs seem higher but my tach does not work so I am guessing. I will have to check the ratio. The prop pitch should not have changed from what I am used to. What does it mean for the prop to be spun? what are the symptoms?

 

[Texasmark]

Spun prop means that the hub is slipping inside the prop housing. Rubber hubs are designed to slip upon impact and reengage upon a shift to N and then back to F and underway. The plastic and brass replaceable hubs available today have no way to provide for slippage. The plastic is keyed onto the prop housing and the brass containing the spline is molded in. Personally I won't run one because I think if you whack something serious you will rip the guts out of the thing and are dead in the water.

If he put a 2.33 lower unit in your engine and the same prop, your prop is turning 15% slower for a given rpm. On effect on hole shot, I feel that it would be like cutting 15% of the pitch out of a prop. The rpms would be higher and speed at WOT slower. However, I feel that running a 2.0 prop on a 2.33 LU will make the hole shot stellar. Reason being that the engine could develop it's HP in the hole much faster and as a result could spin up the prop faster which would get you out of the hole faster.

I have a 2.33 on my 90. I have 3 Ballistic SS props with a 17, 21, and 24 pitch with the 24 being ported....did it myself. The porting is to allow the prop to spin up in the hole faster getting me a faster hole shot with a prop pitch made for the top end. Running the 17 on my 2.33 is like you running your 2.0 prop on a 2.33 gearbox. The hole shot on my boat with the 17 will throw you out of the boat if you aren't hanging on; but the WOT rpms can go out of sight and instead of pushing 50 mph 35 is tops.

If your curious about the props, the PO of my boat had it for 2 years and used the 17. The dealer that sold it to him apparently didn't realize it was a 2.33 gearbox and used the pitch he used on 2.0 gearboxes of higher HP engines he sold. I took the boat out on the first outing and after the first hole shot and a little looking at what I had, plus a stuck thermostat problem, I loaded up the boat, took it home and came on here and bought a 21. That was better but still not enough pitch so I bought a 24 on here, ported it and now I have what works.

To check your ratio, just mark your flywheel and prop positon. Turn the flywheel through 2 complete revolutions. If the prop has completed one revolution you have a 2.07. If it hasn't completed one full turn you have a 2.33.

HTH,
Mark