Prop or tach issue

[legend 550]

Finally got my Wellcraft put back together 21' 305 ci OMC Cobra ratio 1.59:1 Michigan wheel recommended a 14.5 x 21 prop. Here's the problem, terrible out of the hole, tach only shows 2100 rpm @32.9 mph by my calculations even figuring 15% slip I'd have to be turning 3100 to get to 32.9. wot only 2500 on the tach 40.1 mph.
I know my motor is way off and I need to work on that ( 15 years in storage didn't do it any favors) but the numbers I'm getting from my gps and tack don't make sense and the motor doesn't sound like it's twisting hard at all. Question is, Is my prop that far off or is my tach that far off?

 

[Starcraft5834]

Engine compression check for starters.....if thats all the gusto you can muster id say your engine is ill.

 

[Starcraft5834]

If your getting expected gps speeds with low tac. Your tac prob is not correcr

 

[jestor68]

It appears that you have both issues.

Firstly; using your gear ratio and a 10% slip percentage, it takes 3518 rpm to go 40 mph. So it appears the tach is wrong.

Secondly; the 3518 rpm is too low with the 21 pitch prop, so you'll need to reduce to a 19, or possibly a 17 pitch prop to get the rpm up where it should be ( 4200-4600?).

 

[bruceb58]

With my 21' Bayliner Cuddy that I had years ago, I had an OMC Cobra with the 5.0L like you have and I used a 15 x 17 prop. You are WAY over propped.

 

[legend 550]

I thank you gentlemen, that's the exact information I was looking for. I have a couple of 151/2 x 15 props laying around I could have re pitched to 17.
Now I need to diagnose the sick motor and defective tach and it'll be happy sailing for me. I think i'll keep the 21 in case I put the 5.7 in.

 

[legend 550]

Update A buddy gave me a 141/2 x 19 Solas prop to try.Tach was set on the wrong setting so that was easy. Compression check shows 155 to 160 on all cylinders,fuel pressure 6 psi@ 3000 rpm, manifold vacuum at 15 in at idle, total timing set to 34 deg (initial plus mechanical advance), found a cracked rotor was causing a floating miss and a rough running condition. Water wasn't perfect, had 18" rollers on lake Erie, hole shot was good with the 2 of us and all our gear and
34 mph cruse @ 3300 rpm. Wot with the 18" rollers was 43.2 @ 4200 I'm sure 45 to 4600 could be hit on flat water. All and all I'm happy with it now so thank you sirs for your help and the prop I'm having repiched to a 17 will be handy for when I have 4 to 5 people on board.