Strange one 1990 F90

[hotrod071964]

I took off my alum prop and replaced it with a stainless(rapture same pitch .25 inches bigger in diameter). I estimated that I would loose 200-300 rpm which was ok I was turning 5300 last year with the alum prop.At wot it would only turn 4400 and I dropped 10mph. Not happy with this I put the alum back on the following day. Again we headed out and I discovered at wot I now turn 6600rpm with the same prop from last year that turned 5300 rpm.
Any ideas?
Thanks
Randy

[Ken]

Randy - indeed a strange one .
Aluminum prop blades will flex under high rpm/load - so that an aluminum prop with the same pitch as stainless will rev up higher than the stainless because it's blades are "giving".
I would not expect the rpm to drop as much as you are experiencing with the stainless, and you are definitely out of the "max" rpm hp range. Sounds like you will have to go to a lower pitch ss prop.
With the old aluminum prop giving you 6600 rpm now instead of 5300, then I would suspect either a bad tachometer, or the aluminum prop is slipping on the hub.
To check the prop hub for slippage, put a mark on the prop and one on the hub or prop shaft nut so that the two marks closely line up with each other. Run the boat and then check the two marks
to see if their relative position has changed.
A prop shop can replace a bad hub bushing.
Good luck [img]smile.gif[/img] ,
Ken K

[hotrod071964]

Hi Ken, I am grasping at straws here...would an under-torqued propeller nut cause the bushing on the alum prop to slip. I know it can't slip with the SS.
Thanks
Randy

[Ken]

Randy,
With my limited experience of props, I would say no - the aluminum prop is pressed onto the hub bushing just like the ss prop and the amount of torque on the prop nut will have very little effect on slippage.
Ken K [img]smile.gif[/img]

[hotrod071964]

The alum prop is a year old, so I really can't see it slipping. The pitch of the engine is higher so I would have to rule out the tach and the speed does seem alot faster and the speedometer tells me 12mph faster. GPS with the SS prop on matched the speedo, but I didn't take it on the following day with the alum prop on.I am left with 2 issues, why did the SS prop drop my rpm 900 from the 5300(which is what the alum prop ran 6 months ago) and why did the rpm go up when I put the alum back on. Is there a govener on this motor? or and adjustment that controls max rpm

[Ken]

Randy,
At this point I think you need to get in touch with a prop expert - he is the Power Tech rep for Louisiana. You can reach him (email) at marcus@ptprop.com. Tell him what boat and motor you have and what kind of performance you want (hole shot vs. top speed vs. bow lift, etc.) and he can recommend the prop you need. He will give you his phone number and he will tell you what to expect from that aluminum prop you have.
Good luck [img]smile.gif[/img] ,
Ken K