what is a good diameter and pitch ffor my motor?

[mdrussell2]

Currently I have a 1973 17' Sleekcraft with a 1975 merc 1500 inline six. Im not getting the speed or the whole shot I want. Im at 5400rpm wot and have a 17 pitch quicksilver prop 12 1/2 diameter and only running around 36 mph with nothing in boat but two full 6 gallon tanks any suggestions would be greatly appreciated.

 

[Bondo]

Re: what is a good diameter and pitch ffor my motor?

Im not getting the speed or the whole shot I want.

Ayuh,.... Welcome Aboard,...

That's sayin' yer Underpowered...
To fix a lack of Speed, 'n high rpms, you'd go with a prop of Higher pitch...
To help the Holeshot, 'n increase rpms, you'd go to a prop of Lesser pitch...

Has yer motor had a Tune Up,..??
Is it makin' power like it should,..??
Compression test,..??

 

[mdrussell2]

Re: what is a good diameter and pitch ffor my motor?

compression is good all six at 120 psi +-2 and I just got done tuning it up my self have all the power i need I am at 5400 rpm in about 4-6 seconds just no speed I know after doing some research that i need to go up just not sure how much 19p or 24p or in the middle? the manual says wot should be between 4800-5800rpm. does that help at all
?

 

[Bondo]

Re: what is a good diameter and pitch ffor my motor?

the manual says wot should be between 4800-5800rpm.

Ayuh,... At 5400rpms, you can't afford to go Up in pitch,...
You'll be Lugging the motor, 'n the holeshot will get worse...

What's yer Prop Slip figure out too,..??

 

[mdrussell2]

Re: what is a good diameter and pitch ffor my motor?

I believe prop slip is around .25 if my calculations are right but if i go up to 19 pitch that should bring rpms down to about 5000 which is still in range per the manual or am i calculating wrong?

 

[steelespike]

Re: what is a good diameter and pitch ffor my motor?

You need to prop to the top end of the rpm range.
A 77(couldn't find a 75) 17 ft Sleekcraft weighs about 850lbs with a 150 I figure you should run 50 to 60 mph.Unless its just a tub of a boat.If your prop is in rough shape I would try a new prop in the same pitch.Not likely to get much more speed but could eliminate the prop as a problem.
More later.

 

[mdrussell2]

Re: what is a good diameter and pitch ffor my motor?

its not its real sleek kinda like a speed boat i know i should be getting at least 50 could thr diameter be the problem?

 

[steelespike]

Re: what is a good diameter and pitch ffor my motor?

No; diameter is a secondary consideration mostly involving being able to clear the lower unit.
Is the antivent plate(just above the prop) about even with the bottom of the boat.
Most endup up to an inch above the bottom.Isd the prop kindof beatup?
Does the boat get up on the water and run with about 1/3 to 1/4 or less in the water.
Being a reasonably light fast boat I would think it might run with more pitch but your rpm
seems low for a 17" pitch.You might check your tach.There may be a selector switch on the back ;note the setting and rotate it a few times to clean the contacts.or check against a known good tach.

 

[mdrussell2]

Re: what is a good diameter and pitch ffor my motor?

yes anticavitation plate is about an inch above the bottom of the boat is it the same thing as an anti vent plate im guessing. and as far as running on the water after i trim up a little i get to about 1/3 of the stern still in the water. Ill get a new tach about time i got a new one anyway thanks for the help by the way.

 

[steelespike]

Re: what is a good diameter and pitch ffor my motor?

Yes antivent plate is the same.Actually cavitation is different animal dealing with low pressure areas on the prop blades erroding the surface.Actually cavitation can indicate a really efficient prop.