what prop?

[Roguelawyer]

I have recently acquired a 1967 Mercury 200, 20hp outboard. I took it out on the water and it ran pretty well. It just couldn't muster the RPMs you would want at WOT.

I have an old 14 ft aluminum boat (1964 Sears Traveler to be exact). Weighs in at about 150 - 175 would be my guess. I was running it with about 350 lbs of people and gear and just couldn't quite get it running at the RPM I want. I dont have a tach but through experience I would say it was running at 75 - 80% of where it should be. I was running regular gasoline, haven't put new plugs in it since I bought it (it seemed to run smooth), or anything else really. The one thing I did was adjust the screw that limits the throttle. I may be able to back it out further but I think it is at its max.

I had a relatively beat up three bladed 9 pitch prop (9P A4 is the marking). I have shopped around and can only find 9, 11, and 13 pitch three bladed props. There is also a 2 bladed 11 pitch prop out there.

I was thinking a lower pitch would help but can't seem too find a 7 pitch prop. Any suggestions?

 

[Volphin]

Re: what prop?

First things first. Hook up a tach and see what your WOT actually is.

V

 

[Silvertip]

Re: what prop?

Unless you know what the manufacturers recommended WOT RPM is and what you actually OBSERVE in your application, selecting a prop is a crap shoot. There are too many variables and one size does not fit all. Install a tach or use a shop tach for testing with what YOU consider an average load.

 

[Mi duckdown]

Re: what prop?

Without tach and speed info, you would be chasing your tail.
There aren't to many options in props for a 67 20hp.
Beat up prop??
HOW beat up?
Pics of prop?
And what is the motor compresion?

 

[Roguelawyer]

Re: what prop?

Unless you know what the manufacturers recommended WOT RPM is and what you actually OBSERVE in your application, selecting a prop is a crap shoot. There are too many variables and one size does not fit all. Install a tach or use a shop tach for testing with what YOU consider an average load.

Ok , can any of you Internet boating gurus help me with Mercury's manufacturer's recommendation WOT rpm is for a 1967 merc 20 hp? My guess would be around 5300. Maybe a little higher if it were new

 

[steelespike]

Re: what prop?

67 Merc 200 is rated fpr 5000-5400.Generall;y you should prop close the the max rpm.
for best overall performance.

 

[Roguelawyer]

Re: what prop?

Great, thanks. That is what I thought. I haven't had a chance to get the boat out and run it again. I picked up a new (old stock) Mercury prop at a local Merc dealer. He gave me a great deal on it. My guess is that it has been sitting on his shelf for a couple decades. It is a 9 pitch, which is the lowest made for the motor. Seems to have a little less surface area and quite a bit different shape than the one I finally removed.

I found a tach that I can borrow to check the RPMs at WOT. If they are too low then I guess I can have the prop I removed re-pitched and repaired.