Do I need a different prop?

[trenth]

At WOT I'm running about 7000 RPMs. I have a 17ft aquasport center console (1500lbs) with a 1995 Johnson 88 SPL. Range is 4500-5500 at WOT. The current prop is a 3 blade 15 3/4 x 15. Do I need a different pitch to get me down to 5500 at WOT ?

Thanks

 

[Dhadley]

Re: Do I need a different prop?

In stock form that motor won't turn 7000 rpm. Let's verify the tach first. Make sure it's set on position 6. By the sounds of things it might be set on 4.

 

[trenth]

Re: Do I need a different prop?

I just checked the tach. It is indeed set to 6P. Sounds like the tach is running high?

 

[steelespike]

Re: Do I need a different prop?

Yes something is up. What is your speed?
A 17" pitch is probably closer to what you need but the 15 isn't "that" far off.

 

[trenth]

Re: Do I need a different prop?

Speed was in the upper 30s - close to 40 mph clocked with GPS. I didn't stay at speed very long cause the tach was reading 7000. The original literature for my boat indicates WOT speed of 19mph with that same prop at 5500 rpm. Sounds like things are pointing to a high reading tach. Guess I need to find a shop tach to test against.....

 

[180shabah]

Re: Do I need a different prop?

Did you move the switch to 4?

 

[Dhadley]

Re: Do I need a different prop?

Position 6 is correct for a V4 crossflow. On those boats with a V4 crossflow we typicaly use a 17" prop and get way more than 19 mph. Also the V4 can't use a prop that's over 15" in diameter.

 

[trenth]

Re: Do I need a different prop?

Just realized my typo. I meant to say 39 mph not 19.

 

[180shabah]

Re: Do I need a different prop?

Position 6 is correct for a V4 crossflow.

Don't move the switch then

 

[Tacklewasher]

Re: Do I need a different prop?

At 7,000 rpm, 15p prop, you get a theoretical speed of 50 mph.

At 6,000 rpm, 15p prop, you get a theoretical speed of 43 mph.

At 5,500 rpm, 15p prop, you get a theoretical speed of 39 mph.

Assuming around a 10% slip, it really sounds like you are closer to 6,000 rpm.

Is there a way to try another tach? What does it read at idle?

 

[trenth]

Re: Do I need a different prop?

Idle in the water in gear reads about 1000. That seems high to me as well. My idle speed seems to be set as low as it'll go without stalling (just rebuilt both carbs. everything there is running well). I'm trying to get my hands on a shop tach or a lazer tach or something to verify, but it's all pointing to an inaccurate tach.