Changeing Diameter

[MSmith3615]

Re: Changeing Diameter

I believe its 1.51 0r 1.47 I'm not sure I will post question in I/O forum to find out.

 

[walleyehed]

Re: Changeing Diameter

I posted one as well, but I was using the correct ratio...<br />With that being said, we still have a problem.<br />Either the tach is off a fair shake or the prop is a 19 pitch, not a 17...the numbers work out real well for a 19.<br />Have you verified the last 2 numbers on the prop???17P it should be..

 

[MSmith3615]

Re: Changeing Diameter

Prop is 17pitch, rpms were taken from a ACT-3 Engine RPM Detector, which uses electromagnetic induction that senses the flux field. I'm really confused, numbers on GPS were 46-47 depending on with current or without.

 

[walleyehed]

Re: Changeing Diameter

I'm not sure how to respond....with a 17 , the RPM and GPS speed figures, the slip ratio is negative, and that's impossible.<br />I noticed in another post, a gent with a 21 wellcraft, 260 and an alpha1, with a 14-1/2X19 he's geting 44-45 MPH at 5000RPM.<br />I think we need to dig deeper into the total gear ratio, as I think it's not what it should be...

 

[MSmith3615]

Re: Changeing Diameter

I'm lost for words myself, its not a stock 260. The hp is around 300 if that matters, the WOT on a 260/350 Magnum is 4200-4600 so the gent with 19 pitch on the Well Craft has to have a tach that is off. My boat is a 21ft too. When I tried the 19pitch I couldn't get about 4000 rpms. Its possible that the person who owned the boat before me altered the gear ratio but I don't know. How can I find out?

 

[walleyehed]

Re: Changeing Diameter

Here's another thought...I have a buddy that has an older (maybe early 80's) wellcraft cuddy in 21 ft. It has a 5.0L 228. We spin a 19 6000RPM and a 21 at about 5500.<br />Continuous, we run 5200-5500, and have for 20 yrs...BUT, I'm not smart enough to know the gear ratio on that unit, so we may have a difference there.<br />I will research this farther so we can figure this out.

 

[flashback]

Re: Changeing Diameter

If it helps, let me throw another wrench in the mix.. I have an old (79) 21 ft chris scorpion. it has a 350 SBC turning out about 275 HP. the prop is 15.50 x 19 alum.. WOT is about 4300 rpm at aprox. 48 mph. sometimes if the water is just right it will do 50 mph..

 

[MSmith3615]

Re: Changeing Diameter

Thanks alot for info guys, flashback your numbers are close to mine. With the 19 pitch alum mine turns 4100 as of today about 49-52 with good water conditons.