Propping 18' Checkmate with Tower

[sludgeguy68]

I've got a 1979 Merc 115 (inline) on the back of an 18' 1985 Checkmate bow rider... this was the first year Merc rated HP at the prop. Power trim and jack plate. Running a 19 pitch SS prop it came with. WOT operating range for the motor is 5000k-5500k

I'm getting 5100k, reaching 48mph on GPS on a good day. Also having trouble planing out with a bunch of people on the boat - gotta get someone to sit way up on the tip of the bow.

I'm guessing a smaller pitch would get me a little more speed and higher RPM, but I don't want the planing issue to get worse. What do you guys think?

 

[steelespike]

Re: Propping 18' Checkmate with Tower

I'm no expert on the Mercs but I don't think anyone was rating at the prop until about 85.
If you go to a similar prop in a 17 it should give you about 5500 rpm and better hole shot.
Do you know the gear ratio? Using 1.86 and 19" pitch I think 48 would make about 5600 rpm.
We know the gps should be accurate so I may have the wrong gear ratio or perhaps the tach isn't accurate.Not unusual at that age. Perhaps you can borrow or rent a shop tach to check things out.

 

[sludgeguy68]

Re: Propping 18' Checkmate with Tower

Thanks; I've read in the merc forums that the 1978 140hp became the 115hp in 1979, but they both pushed about 120hp at the prop. The gear ratio is 2:1, does that change the calculation? Tach is brand new (Faria)

The 17 pitch seems to be a good match if it'll help with hole shot, although I guess I'll loose a bit of top speed. Any recommendations on models that would fit this motor? There seems to be a lot of different spline/hub configurations out there.

 

[steelespike]

Re: Propping 18' Checkmate with Tower

Using 2.00 the boat can't make 48 at 5100 rpm.
If we assume the gps is right and use 12% slip, rpm would be about 6000.
If we assume tach is right 5100 produces about 40 at 12%.
Perhaps the tach is set wrong?
If the motor is making 6000 it would be unusual to struggle with hole shot.
Also limits what we can do to solve hole shoot.
If the present prop produces a lot of bow lift it may slow hole shot.
Also obviously 6000 isn't good for the motor.
One or more of our numbers may be wrong.

 

[hwsiii]

Re: Propping 18' Checkmate with Tower

sludgeguy68, these numbers backup exactly what spike is telling you, so I would listen to what he says.



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