Got some prop questions...

[Saginaw Bay]

OK I have a 1972 slickcraft ss195 19.5'

I have a 15 3/4" x 17 prop on her now which get me about 32MPH GPS.
It takes about 5 seconds to plane out.
I am not sure what RPM I am running due to bad tach.

I look in the manual and it lists the 888 15 3/4"x17 prop for being for a 20-25' boat. and it running from 32-40 MPH

I see that for 17-19' the manual lists a 15 1/4"x21 pitch and listing 41-49MPH

and for a 19-21' boat a 15 1/2"x19 pitch prop for 37-44 MPH.


I am thinking about getting the 15 1/4"x21 pitch prop to get more speed.
Am I correct saying that the boat will plane a lot faster?
Am I correct in assuming that the reduced diameter of the prop will still let me operate close to my correct RPM range?

What would going to a 4 blade prop do for me?


Thanks for the input!

 

[Bondo]

Re: Got some prop questions...

I have a 15 3/4" x 17 prop on her now
I am thinking about getting the 15 1/4"x21 pitch prop to get more speed.
Am I correct saying that the boat will plane a lot faster?
Am I correct in assuming that the reduced diameter of the prop will still let me operate close to my correct RPM range?

Nope,... It'll be Lugging badly, may not plane out at All...
Pitch determines the rpms, which you don't even know what they are...
Fix the tach 1st....

 

[Saginaw Bay]

Re: Got some prop questions...

OK,
Not trying to be a smart alleck here, just trying to understand.

Why would it be lugging with the 21 pitch prop?

I would be going from a 15 3/4" to a 15 1/4" diameter, wouldnt the smaller size account for the RPM loss of the higher pitch ?

Also, the manual has a 15 1/4"x21 listed for a 17-19' boat. My boat is a 19.5'

the prop I have on currently is listed for a boat up to 25' as per the manual.

I just think the boat, who just got a fresh tune up, including cap,rotor,points,condenser,plugs,wires,&PCV. should have a better top end speed.

The carb has been gone through also and other than the electric choke being disables and tied open, is in good shape.

Bond-o what is your opinion of how what the top end speed of this boat should be?

 

[Bondo]

Re: Got some prop questions...

Ayuh,... Without the rpms, it's pretty much unanswerable...
Unless you're turnin' about 6 Grand now, you'll Never pull a 21" prop,...
Regardless the diameter....

 

[Saginaw Bay]

Re: Got some prop questions...

What then would a 19 pitch prop act like?

I guess I am looking for some sort of rule of thumb to go by here?
(smaller diameter= more rpm, more pitch faster planing but less top end, etc.)

I assume( I know what assume means) that my rig is running at the proper RPMs
The tach seems to pick up a "fast idle" and registers it att 1K rpm while I think, by the sounds of it would be about 1K short. And when I got the boat last year I remember the tach being a touch over 4K before I "fixed" the tach. lol
.
Now,Running WOT I am reading between 2400-and 2600 RPM which I am guessing is about 4200-4600 RPM.

The boat runs GREAT!, I just think it shoukld produce more speed.

My thinking also is that I think I am running currently what the manual says I should be for the prop listed ( 15 3/4"x17 prop for 32 MPH ) , I am on the short side of the vessel size listing (19.5' vs 20-25' listed in the manual).

The maual also states 19-21' boats having a 15.5"x19 pitch prop, is THAT more inlne?

 

[steelespike]

Re: Got some prop questions...

What motor are you running?
Its very important for the health of your motor to be able to operate within its rpm range.
To make it basic; diameter is not usually considered in prop selection if the prop is for your motor/outdrive the diameter will work.Pitch is the key. Very basically assuming a motor can operate within its rpm rating; lower pitch is more power less speed higher pitch less power more speed.The motor must be able to reach its rated rpm Without your rpm and speed there is no way to accurately select a prop.
Normally a 17" pitch produces excellent hole shot but would likely over rev
with a light load. Typically for speed you would run a 19- 21 motor would be well within its rpm range and run 40+ mph. Hole shot would be slower.Higher number props are kind of like overdrive on a car.Again without the tach there is no way to know about your present setup. and no way to know the results of a prop change.