What SS prop for my Merc 470?

[montejw360]

I'd like to get an SS prop for my Glas Ply. My main trip will be 30 miles or so, Bellingham to Friday Harbor. The boat is trailered, and has bottom paint that I want to remove since it's trailered.

1) Performance issue you are trying to correct.

Better cruise speed (currently 22-23mph gps'd on a calm day) and fuel consumption (don't know, might not want to know). Or better fuel consumption at the same speed.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.

Mercury Marine 48-79576-A4, Aluminum

3) Current prop diameter and pitch (required).

14-1/4" (best I can come up with on the internet, measuring it that seems right), 21P

4) Wide open throttle RPM and speed with an average load (very helpful)

4000, 33mph, 2 people

5) Engine/drive make, model, year, and HP

Mercruiser 470 Sterndrive, 1980, 170hp

6) Boat make model, year, length and weight

1980 Glas Ply Cruiser, 20', haven't weighed it yet

Thanks,
Monte

 

[SparkieBoat]

Re: What SS prop for my Merc 470?

probably about the best you will get with that set up. you could experiment with a 19 pitch prop if you can find one that someone will let you borrow. I think you should be more into the mid/upper 4k RPMs for that motor. check your motors recommended RPM range and prop for the top end of it.

 

[montejw360]

Re: What SS prop for my Merc 470?

@SparkieBoat: recommended RPM WOT is 3800-4200, so it's pretty good where it's at for WOT. I know it's not going to get loads better, but in the long run it might help. thanks

 

[steelespike]

Re: What SS prop for my Merc 470?

While tabs will help stay on plane with less throttle and improve ride the additional drag can increase consumption.
4 blade props can also improve hole shot(a fuel savings) improve the ability to stay on plane at lower throttle settings( a fuel savings)and improve handling at speed and at the dock.
Usually a slight loss of top speed.
The amount of savings would depend on how well your present setup is working.
Assuming present prop is right on it usually is suggested to drop 1" pitch when going to a 4 blade
I checked the 470 the gear ratio is 1.7 based on that and your rpm(4000) I think the your present prop is a little too much. Unconfirmed the 470 turns 4600 max. It appears you may want to drop 3" if you go to a 4 blade.
Solas amita make a good predictable al 4 blade.

 

[montejw360]

Re: What SS prop for my Merc 470?

@steelspike 3800-4200 is the spec'd wot I've found in shop manual.

 

[SparkieBoat]

Re: What SS prop for my Merc 470?

if you are in the right RPM range, then thats about all you can do..if you change props you will get out of recommended RPM range. Sooo not much you can do other than get a bigger engine