1980 Sylvan (Starcraft) 18ft SS, 140hp IO prop comparison question

[Alumarine]

Hi there, I have a 1980 Sylvan (StarCraft wannabee) 18ft. SS open bow 140hp Mercruiser IO, I think it weighs between 1800 and 2000 pounds (not sure).

This is new to me and I only had a chance to make two 25 mile runs before freeze up.
I will just use it for cruising but I would prefer speed to grunt.

I'm currently running an old Black Max 48-832832-21 14.25" - 21p aluminum prop.
I'm getting 37mph on GPS at 4200rpm, calm water, 460-500lbs of people and gear with 10 gallons of fuel. WOT range is 4200-4600.
Other speeds are 21/2700, 25/3000, 27.5/3200 and 35.4/4000.

I'm looking for a spare prop and found an old Stainless 48-73982A, 13.75", 21p.
How will this compare to my current prop for performance?

With my current prop it pops out of the water.
The motor seems strong, has great compression and runs well.
I plan on removing my deck this spring to see if I have any wet foam.
I'm hoping I'm running heavy so I can get the rev's/speed up a bit.

Thanks, hopefully I've included all the needed info.

 

[steelespike]

If the ss prop is cupped they usually suggested to calculate it like a 22. Assuming 1.98 gear ratio,and the same 4200 rpm
guessing 12 % slip you might pick up 1 or 2 mph.Problem is the motor will likely struggle to maintain rpm.
If the motor is from 1980 it may be a little tired.You may want to check the timing is set right and advancing as it should.
Be sure the throttle opens all the way.A compression check may be a good idea.Is the bottom clean and smooth?
If the prop is visibly worn just going to a new prop may help.Though your 12% slip calculation indicates it seems to grip well.
Check these suggestions see how we make out.

 

[Alumarine]

The motor has 150-152 lbs on all four, I can't recall if I checked to see if the carb opened fully.
I did set the timing and dwell but don't know about the advance. I have a new distributor sitting on the shelf so I'll just put that in.

Will also check the bottom.

Good to know that the SS is more like a 22p compared to the aluminum I have. Definitely don't want to go that direction unless I can get some more rev's out of it.

Thanks for the help. Won't be able to try anything for a few months at least. Will look for a different spare.

 

[SkiDad]

i don't believe you can get any more speed. but a 19p would be a better prop for your setup.

 

[steelespike]

When I mentioned a cupped prop it doesn't mean all ss props are cupped.
A cupped prop loads the motor more so your likely to lose some rpm which is already near the bottom.
I'm hoping you may find some speed in your setup.
Another thing to check is the trim.Does the boat respond to up trim.Does the up trim work correctly.
I agree a 19" prop will make the boat more lively and get the rpm up but top speed will likely be less.

 

[Alumarine]

When I mentioned a cupped prop it doesn't mean all ss props are cupped.
A cupped prop loads the motor more so your likely to lose some rpm which is already near the bottom.
I'm hoping you may find some speed in your setup.
Another thing to check is the trim.Does the boat respond to up trim.Does the up trim work correctly.
I agree a 19" prop will make the boat more lively and get the rpm up but top speed will likely be less.

Trim works. Not sure how it responds.
Only ran WOT for a bit as it sounded like the u-joints were bad (they sure were).
I was trimming up though while testing it.

We'll see what happens in the spring. I've got a few props to try.