need help with prop size

[michael37659]

I have a 1988 Fisher marsh hawk 2v alum boat that has a dry weight 453lbs,has 1972 johnson 50 hp on back.the plate is setting level with bottom of the transom.Right now it has a sst 13 7/8 x 17p prop on it.I am getting 28mph with speedo and 4200 rpms.I was thinkin about dropping down to huster 14x13p prop. would 4'' less pitch be to much to get it up to the 5500 max rpms.Theres usually just me, my fishin gear and about 10gal gas onboard.Thanks.

 

[steelespike]

Re: need help with prop size

Just based On my limited experience you should be getting more speed and rpm with that setup.Speedos can be inaccurate a gps would be better.Is the motor in excellent tune?good compression?Prop good? bottom clean?
Carbs opening all the way?Antivent(cavitation) plate even with the bottom is considered a starting point. Usually end up about 1" above.I think you should be closer to 35 mph and in the 5,000 rpm range.Standard prop in 72 was 13 3/16 X 17 (of course depending on the application)rpm range 5,000 to 6,000.Your boat with that setup should allow that rpm range in my opinion.With everything right I think a 19" pitch would be possible.

 

[michael37659]

Re: need help with prop size

my speedo very well could be inaccurate.The motor is in very good tune.I took a digital photo sensor tach and calibrated my tach yesterday evening before i took it out and ran it.The bottom of the boat is alittle dirty not bad though.The prop im running is alittle dinged up but not bad, it is a stainless 13 7/8 x 17.I think i might have problem though with my fuel line though.I noticed this morning that where it plugs into my tank, it has quite abit of play in it than the other end that plugs into the motor.I might be pulling alittle bit of air in line.I will replace it today and try it out again before i go changing props.The boat has a good holeshot,comes up on plane good and runs great to 4200rpm. Thanks Admirl.

 

[steelespike]

Re: need help with prop size

Something is screwy.They are running 17" on alumiunum boats
that weigh 700 to 900 lbs.dry weight. turning about 5,500 and speeds about 29.
a 13" prop is close to what they run for a pontoon setup.
Are you sure the throttle opens all the way?

 

[michael37659]

Re: need help with prop size

You are right,my throttle wasnt fully opening,Ithought i had checked it.I am goin to go try it out in alittle bit.Thanks i really appeciate you helping me out.

 

[michael37659]

Re: need help with prop size

I adjusted my linkage and took it for a run and ive got it up to 5100 rpm and 32mph but i dont think my speedometer is correct.im a whole lot happier with the way its running.Once again thanks.