Prop Diameter Question

[Hoss 350]

I have a 1996 Four Winns 170 Horizon with a 130 HP Johnson outdrive. Ratio 2.25 to 1. In most references I can find, the recommended prop size for this motor is a 13.5 inch diameter. As far as I can tell, it came from Four Winns with a 13.5 x 22 pitch (this is from the specs. published online by four winns..)

All three props that came with the boat when i bought it are 14.25" diameter, however. I can only assume that since the specifications for my boat call for a 13.5 x 22 that these 14.25s are aftermarket.

I have two 14.25 x 21 and one 14.25 x 19. Haven't run the 19 yet.

The spec for my boat say top speed should be in the 48 mph range, yet I can only get about 43 out of her, according to the pinwheel-style speedo readout on my fishfinder/depthfinder. This is with the 21 pitch.

This is at WOT at 5250 RPM, so I calculate about 9.3% slip with the 14.25 x 21 (if I did it right???)

I guess my question is twofold:

1.) If it is at 9.3% slip, is this good?

2.) what are the ramifications of going bigger diameter on my prop? 5,250 seems pretty low for WOT, I am wondering if the bigger prop is too much for it? I don't know much about prop diameters, so am hoping for some help/recommendations.

I don't know if the previous owner found the 14.25 to be a bunch better through tiral and error, or if he got a good deal on them and figured they'd work, you know? anyway, any insight you can lend would be appreciated.

The reason that I am asking is because i am looking to get a good stainless prop and want to get the right one...

 

[steelespike]

Re: Prop Diameter Question

I can't help you with selecting a specific brand or style of prop.
I can say that prop diameter is generally a result of the prop design and you wouldn't normally select a prop by its diameter.I looked up some applications in your hp. Diameter ranged from 13 3/4 to 14 3/4 boats varied aluminum and fiberglass roughly your type generally smaller boats 17' or so would be smaller diameter but again diameter is a result of prop design.

 

[rndn]

Re: Prop Diameter Question

Your rpm's at wide open throttle (WOT) should be closer to 6000 rpm's. To achieve that you need to lower pitch about 1" for every 200 rpm's. As stated above don't worry about the diameter and focus on the pitch and performance.

 

[Hoss 350]

Re: Prop Diameter Question

Thanks, guys. The more i got to thinking about it, the more I realized that a 21 pitch prop shoud be more or less the same, regardless of diameter. You confirmed that, so I thank you.

I think what I am going to do is try the 19 pitch on it and see what kind of performance changes i get out of it.

I am just concerned that the cruising speed at 33 MPH at about 4K RPM (the "sweet spot" for cruising with this boat, IMHO) will be lost, that I'll have to run the motor up higher RPMS to maintain 33 MPH. This is important, one of my favorite fishing holes is a 63 mile round trip, and if i have to run 4,500 RPM to maintain 33 MPH (where I like to be) that will just ruin fuel consuption to the point to where I won't be able to make the trip without refueling...

The trip I'm talking about is putting in at the dam on teh Spokane River at Post Falls, then running upstream 10 miles to Lake Coeur d'Alene, then turning south on the main lake another 15 miles to harrison, Idaho, then hitting the mouth of the Couer d'Alene River and boating about 8 miles or so upriver to the canals for Killarney Lake, where I catch northern Pike. It is an awesome trip.