New SS prop

[kajnjim]

I have an '06 2 stroke Mercury 50hp. Was running with 10 3/8 X 14 aluminum prop at 5400-5500 rpm at WOT. Consulted very reputable prop shop before switching to SS. Suggested I stay with same diameter and pitch anticipating losing 100-200 rpm at WOT. Replaced with Mercury Vengeance in same diameter and pitch and now running 6400-6900 rpm at WOT depending on trim. No change in top speed. Kinda wondering how I'm getting such a large change and in the opposite direction anticipated. Any ideas out there. Thanks in advance.


Jim

 

[charleso]

It should not make that much of a difference. The only reason I could think of is that the Vengance prop is ported. The ports allow for slip at lower rpm's so that the motor can turn up quicker, therefore reaching higher rpm's

 

[b.gagnon]

There must be something wrong with your tach.... A ported prop only allows the motor to gain rpm's faster.

 

[kajnjim]

Consensus says next step: Check tach. Will do. Thanks.

 

[jimmbo]

Was the old prop also a mercury prop? If not, the thrust washer may be the wrong one and there is a gap between prop and gear case alllowing exhaust gases out. But that usually results in major slippage when getting on plane

 

[Texasmark]

It should not make that much of a difference. The only reason I could think of is that the Vengance prop is ported. The ports allow for slip at lower rpm's so that the motor can turn up quicker, therefore reaching higher rpm's

My guess too. Prop slip went up allowing rpms to rise with boat maintaining the same speed. Not familiar with a Vengeance prop but a quick check could answer that question for sure.

Trim it down till it bites harder. Or, if you don't need a dazzling hole shot, get out some JB weld and plug the holes. Personally, I'd leave them open and just tuck her in a tad.

 

[Texasmark]

There must be something wrong with your tach.... A ported prop only allows the motor to gain rpm's faster.

It also does what he is mentioning....BTDT Gotta size the holes for the application. Too small and useless, too large and rpms jump up too high (in the hole) and stay too long (after the bow has come back down and you are at speed), blow outs can occur in tight turns and riding a chop at high throttle settings.

 

[kajnjim]

Prop is not ported. Original merc prop. New prop came with thrust washer as it has flo-torq hub. Waiting on tiny tach to compare with current tach. Thanks.

Jim

 

[kajnjim]

While waiting on Tiny Tach, I replaced impeller and accidentally broke upper linkage at horizontal shift shaft. Replaced it and checked RPMs against the Tiny Tach. With Tiny Tach set at 120, RPMs matched the boat tach. 2 new problems now: 1) Engine periodically and transiently will rev spontaneously jumping from 1800rpm to 3200rpm for about 5 seconds every minute or two. 2) Shifting into forward and neutral are nice and smooth, but reverse is harder and grinds a little. I did set the nuts at exact same position when I put the LU back on, so I'm a little puzzled by the shifting problem. Any ideas/input appreciated.

Jim