'72 Starcraft Nova 15'

[St Clair musky]

I have '72 Nova 15' with a 50hp merc on the back. The prop is pretty beat up so I'd like to get a new one this winter. The old one is a 13 pitch. Does anyone know if this is the correct pitch for this boat? The most I could get it up to was just over 25mph with just me. Seems slow? No tach on it
Thanks

 

[Sea Rider]

Without a tach would be impossible to dial a prop for OB to run at least middle to max wot rpm range as factory stated. What need to do before going for any other prop is repair current one at a prop shop, install a tach and go for a wot spin as usually loaded, depending on the wot run can go for a prop maximization to get all the horses out of that 50.

Current beaten prop could be a nice performer, but without a tach won't possibly know. You can have it repitched to pull wot revs to some extend in case it's needed. Will need to invest some money, dialing best prop is not rocket science...

Happy Boating

 

[Watermann]

Welcome aboard, :wave:

Yes 13p is pretty normal for that size of boat and motor. I have a 55HP on a 16' and use a 13p as well. New props have some changes from old styles such as aggressive rake and substantial cupping which will lower RPMs. I tried to go to a newer style 15p which was the original prop's pitch and the boat wallowed about like a hog in mud trying to get on a plane.

Do you have power tilt and trim?

 

[St Clair musky]

No power tilt and trim, I'll probably just try another 13P and see how it runs? Don't really want to put a tach on it unless I need to. The wiring is at the control box. But if it runs good I might just go with it
Thanks for the replies

 

[Sea Rider]

Dialing best prop by means of a tach is a night-day difference water performance.. You don't know what you're missing by not having one. Selecting props on a wild guess without a tach to check max wot revs is definitely no bueno for any OB.

Happy Boating

 

[St Clair musky]

I think I'll pick up a tach over the winter. I put a new lower unit on and when I was doing that I checked the gearing. My book says that a '72 50 hp should be 1.68:1 I believe? But the one I took off and the new one I put on are both 2:1?
I think putting a tach on is a good idea.
Thanks for the replies

 

[jimmbo]

If the gear ratio is different, a tach is essential to determine the proper prop. How did you check the gearing?

BTW a 72 Merc 500 had a 2:1 gear ratio

 

[St Clair musky]

I checked by marking the drive shaft and the prop then turning the shaft. Both were exactly 2 turns to 1. Then counted the teeth on the forward gear and it was 26 teeth I think? That's consistent with 2:1. The book says it should have 23 teeth

 

[jimmbo]

Well my 1972 mercury brochure says 13:26, so 2:1

 

[jimmbo]

A 1972 650 has a 23:14 gear ratio which is 1.65:1

 

[St Clair musky]

Thanks for the replies, I've been busy so I haven't had time to reply.
I'm glad your manual says 2:1, I was going by the seloc manual and I don't think they are always completely accurate. I checked again and it says the 500 from 69 - 78 (I think ) is 1.68:1
I'll trust your manual
But I'll still get a tach installed for next spring to see where I am

 

[St Clair musky]

I checked again, it says from 65-79 is 1.64:1
I have a terrible memory

 

[jimmbo]

I think your book has a typo, or incomplete info

 

[Sea Rider]

For the prop gurus : Been wondering why is it so important to ask the gear ratio of any given engine when somebody inquires about prop issues, in other words, most recreational boaters will not change their current gear ratio for other, will simply dial a prop that achives middle to max wot rpm range factory stated as currently loaded, right ? So what's the sole purpose of asking ?

Happy Boating

 

[jimmbo]

knowing the gear ratio can permit calculating what speeds can be expected at a given rpm for a particular pitch, taking slip into account. Engine manufacturers have changed ratios in engine lines, for reasons only known to them. Generally I/O makers will use a variety of ratios over a line of engines that permits a smaller pitch range of props to fit high and low power engines.
Mercury in the past did offer a High Altitude gear case option on some outboards and I/Os, again probably to reduce the need for lower pitch props to be required