RE PROP

[Mr Honey Do]

Hi
I have a 1991 Marlin 28 with 2002 Twin Yamaha 250hp OX66's
I'm estimating the weight is around 8500 lbs.
It currently has SS 3 blade 21 degree props.
I'm only getting 4900 rpm WOT
My mechanic & I agree that we need to move to 19 degrees and 4 blade.

Any recommendations on props?
Is there any difference in manufacturers?
A SS Prop is a SS prop, right? or is it?

Thanks, I really appreciate any comments/thoughts

 

[NikM820]

I had merc rev 4s 17 pitch and loved them. 2005 yamaha f250s. Contact Ken at prop gods for more information. If you search this forum you will find more about this topic.

 

[REBThunderroad]

I have a 2005 marlin with F250's. I spin Yamaha SWS II 15.5 x 19" props and get 46 top end light at 5900 rpms & 42 mph loaded @ 5700 rpms. Cruise is 4200 rpms 31 mph.

I just listed my spare Yamaha SWS II 15.5 x 17" props (for sale) on Great Grady. As I recall these props would hit 6100 rpms ar 44-45 mph and cruise at 31 mph 4400 rpms. They have approximately 95 hours on them.

 

[ocnslr]

Pitch is measured in inches, not degrees. If your mechanic doesn't know that then perhaps a new mechanic would help.

Your 4900 is quite low. Rule of thumb is 300RPM change for every two inches of prop change. Going from 21" to 19" would be a start, but you need to go to 17" to get close to the 5,500-RPM WOT point on those OX66's.

Going from a 3-blade to a 4-blade will also reduce RPM, all other things being equal. So going from a 21" 3-blade to a 19" 4-blade will still leave you "over-propped".

I would suggest a 17" 3-blade, perhaps the Yamaha series, or a PowerTech OFS series.

Brian

 

[fishbust]

Rule of thumb is 200 rpm for every inch of pitch. Going from 21" (way over pitched) to a 19" will bring you from 4900 rpm's to 5300 rpm's, same diameter, same number of blades.

You need 17's, for same diameter, same number of blades = 5,700 rpm's. You need to set pitch so you can load up heavy. Top end is for Checkmate's and Formula's.

Going to 4 blades complicates things because of reducing diameter. You can also further complicate things by changing engine height.
If you reduce diameter, you should be ok with Rev 4's in 17" pitch. Best guess-timate. Confirm with Ken at Propgods, he is good.

Over pitching is overloading motors. You can snap a driveshaft that way. When a driveshaft snaps and immediately removes all load, rev limiter cannot react fast enough. A snapped driveshaft at high rpm = also a powerhead that self destructs in the worst way possible, hyperextending piston rods to take out both sides of the block = total basket case. Over pitched is not a good place to be.

P.S. = you need a new mechanic.

 

[onoahimahi]

These new props from Mercury sound interesting:

http://www.boattest.com/engine-review/M ... a-ECO_2014

 

[tiderunner]

Your propped wrong. I had a 1993 marlin 28 with twin 2000 ox66's on it. Ran 5200-5400 lightly loaded @ 40 knots. This boat also had a real full tower on it on that was custom-ed built . Was running yamaha 15 1/4 X 17 props. I actually have an extra set that are in near perfect condition as they were my spares $400 + shipping their yours. email me logey_pogey@yahoo.com for pics.

R.