50hp Merc 13' Whaler - What size prop?

[Tom P]

I own a 1970 13’ Boston Whaler with a 50hp Mercury Merc 500 motor Serial number (A131168 (1985)). The current prop that is on the motor is a C8 73136 A40L 13P. The top speed of the boat is 30mph (GPS). I hit something last weekend and bent the prop a little. I also believe that the prop that’s on there now is not the right prop for this boat motor combination. I believe that a 13’ Whaler with a 50hp on it should do close to 50mph, 40mph at min (hang on tight). I’ve tried using some of the online prop selectors and none of them seem to have the motor/boat combination that I have. I have a Vortex on another boat that I’m real happy with. Can anyone recommend a pitch for this boat/motor combination?

Thanks,

Tom

 

[Silvertip]

Re: 50hp Merc 13' Whaler - What size prop?

Without some wide open throttle RPM and maximum speed data its hard to tell what you need. I tend to agree however, that you could probably use a 15P prop.

 

[Tom P]

Re: 50hp Merc 13' Whaler - What size prop?

Currently the boat doesn’t have a tach. I’m planning to put one in. But that’s a ways off. From what I do know about props, it kind of seems like guess work without a tech. Sounds like it might be worth the effort to ridge something temporary to get RPMs. A 15P-16P being a good start.

Thanks Again,

Tom

 

[roscoe]

Re: 50hp Merc 13' Whaler - What size prop?

tinytach dot com

Small, portable, easy, under $40.
Just wrap a wire around a sparkplug wire, and attach the ground wire.

 

[Tom P]

Re: 50hp Merc 13' Whaler - What size prop?

I checked out the TinyTac. Looks cool. Will do.

Thanks,

Tom

 

[b.gagnon]

Re: 50hp Merc 13' Whaler - What size prop?

My friends whaler 13' does 28mph with a 35hp...Get the tach and take it out for a run to get the rpm...After that it should be easy to figure it out! My guess is that you will end up at about 42-44mph.

 

[Windykid]

Re: 50hp Merc 13' Whaler - What size prop?

That engine isnt a long shaft by any chance?

 

[Tom P]

Re: 50hp Merc 13' Whaler - What size prop?

Actually it is. I’m also at some point going to add a lift to make the transom higher

Tom

 

[snoking]

Re: 50hp Merc 13' Whaler - What size prop?

Well, my 40HP 4 cylinder 2 stroke Merc with 14" prop pushed my 12.4 rib to around 40 mph, and was slightly over rev'd. It is a light - fast boat. I would think that 14" is the most that you would want. SNOKING

 

[Tom P]

Re: 50hp Merc 13' Whaler - What size prop?

Well, I spoke with Michigan Wheel. They recommended a 10”. I spoke with Mercury at the NY boat show. They recommended a pitch of 17 or 19. I wanted a Vortex so I ordered a 10” with a pitch of 16. A pitch of 16 was the highest the make in a 10” for my motor. The plan is to hook up a tech and get RPMs with the new prop. I believe in having a spare prop. So If this one is close to being correct, great. If not, it becomes the backup.

Tom