Prop Help!! Need help with sizing after trying a couple props

I know this is probably an annoying question as everyone asks this and never bothers to do a search. I have done research but when I put it into effect I haven't achieved the desired results so I thought I'd ask.

I have A 1997 Mariner 150 on a 19.5' Prosports CC weighing around 2000lbs. The motor is rated up to 5600 rpm. When I purchased the boat, it had a 15X21 4 blade SS prop and was turning 4500 rpms max. After doing the math, it was determined that a 15X17 would do the trick and get me to the 5600 rpm range. A buddy of mine happened to have an aluminum 15X17 3 blade brand new so I tried it and now i'm turning 6000 rpms. Not sure what to try next. 19P?

Re: Prop Help!! Need help with sizing after trying a couple props

if you ran at a normal load the 17 may be to low and a 19 might be perfect.

Re: Prop Help!! Need help with sizing after trying a couple props

Is the alum cupped? Does it have progressive pitch? Both can make a difference both to load up engine and to increase mph. Thin blades of SS are on the other side however. The 4th blade is said to need 1" of pitch reduction vs a 3. I run my Mercs over the upper limit every time I am at WOT. Been doing it since '89. My 115 tower was rated at 5500 and it ran at 6k with a Laser prop.

As George said, a 19 4 blade is as good a guess as any. But if the rule of thumb that everyone seems to like, 150 - 200 rpm per inch, it may still be too much. I run my Merc 2 strokes at or over the top end of the rating. They just all around perform better if they are allowed to "RUN". I have had my 90 with the tach pegged at over 7k one day when diddling the linkage and it still runs like a top today.

Only way you will know is to run it. One thing is for sure. Your hole shot will be better.

Mark

Re: Prop Help!! Need help with sizing after trying a couple props

Thanks for the reply's. Trying to find that happy medium. Right now the boat hops right up on plane with no effort but my top end is 35 mph. Trying to get closer to 40 but still hop on plane. I probably will call my prop guy and see if I can try an 18 and a 19. I was worried the motor was going to pop at 6k. Sounds like it would be fine?

Re: Prop Help!! Need help with sizing after trying a couple props

lots of people run those old mercs at higher than rated. I don't normally but I have a few times with no bad results. usually the better the hole shot the less the top end. you have to find a happy medium for how you use your boat. I use my boats for a lot of things and I change props to suit my use at the time. I use a higher pitch for fishing (light load)and a lower pitch for water sports (heavy load)with the grankids. So I'm at the high end of the RPM band for sports and the lower for fishing. It only takes a couple on min to change props, and I've always got one or the other for a spare on board.