Only 5000RPM at WOT with new F250's!!

[bjflink]

Guys, I think this is a prop issue, so any help/advice would be appreciated. I just purchased a 2000 GW 306, which was repowered a few years ago by the last owner with a pair of the new 4.2L F250's. At WOT, I'm hitting 44mph, but only turning 5000RPM. If I trim all the way to the top, I can squeeze out another hundred rpms, with no gain in speed, but the props start to cavitate. The current props are Yamaha SSII 15 1/4 by 19s. Does anyone else have a similar setup but with different performance #'s at WOT. I've been informed that at WOT, I need to be turning close to 6000 RPMs.

Thanks in advance for any info!

 

[JJ2468]

Way wrong props. As per Grady's website they are suggesting 15 3/4 x 15.

 

[bjflink]

Does Grady's website list recommended props for repower applications? I couldn't find anything on their website. Thank you!

 

[JJ2468]

No. Same exact hull all these years later

Had a 2003 with the old 3.3 F250s. Ran the 17 pitch props but it struggled under a heavy load with these props

Try both but I would go with what Grady uses

 

[bjflink]

I don't have anything to compare the current performance to, but aside from not winding out to 6000, the boat seems to perform well...44mph top speed and 1.5mpg at a 30mph cruise. Will the performance change much by going to a smaller pitch? How will it change...top end? Cruise efficiency?

 

[LastChance]

I don't have anything to compare the current performance to, but aside from not winding out to 6000, the boat seems to perform well...44mph top speed and 1.5mpg at a 30mph cruise. Will the performance change much by going to a smaller pitch? How will it change...top end? Cruise efficiency?

The new 250's have a different gear ratio, which means they are now over propped and overloaded. Grady customer service should be able to provide you with the correct pitch.

 

[everwhom]

I have those exact motors on my 330 Express. I'm running 15 pitch (Mercury Thunderbolt / Mirage) and getting about 5500 at WOT and about 41mph.

On your 306 you probably want either 15 inch or 17 inch props. The full throttle range is 5000 - 6000 RPM, so you're not ridiculously off... With lower pitch props, you should have even more low end torque (though I imagine you have plenty with those motors -- my 330 leaps out of the hole if I gun it!). You might gain a mph or two at the top end, but I would not expect your cruise efficiency to change.

 

[bjflink]

Thanks everyone for the feedback! I called Grady and they did confirm I should be running 15 3/4 Diameter by 15 pitch props. Seems as though the SDS props in this size are hard to come by and back-ordered until October! I guess I'll try ebay.

 

[NikM820]

Try Ken at Prog gods

http://www.propgods.com/Content2/Default.aspx

 

[bjflink]

Thank you all for the suggestions. I changed the props to 15 3/4D x 19P, and I am now turning 5900 RPMS and have gobs more torque. If anyone is interested on the SDS props I took off, let me know. They are SWSII SDS 15 1/4 x 19P.

 

[trapper]

Hi BJ, something does not seem right here, you kept the same pitch went up 1/2 inch in Diameter and gained close to 900 ( from 5000 to 5900) rpm? Did I miss read the change? What make of prop? Cheers, trapper

 

[bjflink]

Sorry, typo on my part. I went from 15 1/4 x 19P to 15 3/4 x 15 P. Same brand/model prop...SWSII SDS