Prop Pitch

[gfincati]

I have an 18' Crestliner pontoon boat with a 2003 50 HP Bigfoot. I am purchasing a new prop. What pitch and how many blades should I get for best top end speed? How much speed can I expect??

Thanks
GF

 

[Silvertip]

Re: Prop Pitch

It would certainly help if we knew what the current prop diameter and pitch was, what your current top speed is and what the engine rpm is at wide open throttle with what you consider an average load. By wide open throttle engine rpm I don't mean what the book says. We know what that is. We need to know what you OBSERVE on the tach at wide open throttle (and not in neutral please!).

Mercury makes an excellent elephant ear pontoon prop. Buy one. It will very likely be the same as the one you have now and will likely be a 14 x 9 if the selling dealer set things up correctly. Four blade pontoon props are best for that appplication as they provide better thrust in reverse as well as great forward push. Won't hazard a guess on top speed because we have no data to base a guess on.

 

[gfincati]

Re: Prop Pitch

Pitch is 15 and diameter unknown. Wide open RPM under load is 4200 and top speed is 17mph according to speedo. I would like to get about 25mph

 

[Silvertip]

Re: Prop Pitch

You have the wrong prop which very likely came off a "V" hull boat and was installed on a pontoon that requires a very different prop. You are about 1800 rpm lower than what is required (6000 rpm). The engine is lugging so bad it is stressing the engine to point of hurting it. As I said, the fairly common prop for that rig is a 14x9 (14 inch diameter, 9inches of pitch). And shooting for 25 mph is the wrong way to approach prop selection. The proper prop is one that puts the engine at or very near the top end of the recommended rpm band with an average load. Maximum speed will be whatever that prop provides. Increasing pitch from that point pulls off rpm and the engine lugs, and decreasing pitch from that point adds rpm and causes overreving. In your case a drop in pitch of 6 inches would result in an increase of about 1200 rpm which also gets the engine onto its power band so it would at that point begin to pick up a little speed. 25 MPH might be possible but I wouldn't bank on it.

 

[EGlideRider]

Re: Prop Pitch

As usual, Silvertip provided top notch response.

And I agree that 25 might be unreachable.

 

[gfincati]

Re: Prop Pitch

Thanks for all the help. I will do as you suggest - next summer that is!

 

[Jeep Man]

Re: Prop Pitch

I've got a 20 ft with a 60 hp Bigfoot. Late in the season, I switched from a 14x11 to a 14x9. I found I was overrevving (above 6200). I'll likely switch back to the 14x11 in the spring.

 

[Silvertip]

Re: Prop Pitch

An extra 200 rpm won't bother anything and besides, you can easily throttle back 200 rpm. But load up that pontoon and you will wish you had that 9P again to gain back that 200 RPM. What works on one boat may not work as well on another. A 9P prop from Merc may be a better performer than one from another manuacturer. Lots of variables to consider.