Help With 20' Pontoon Prop/speed

[cbedd]

I have a 20' pontoon boat with a mercury 50hp bigfoot 2002, It came with a 14dia by 13 p prop. The mercury web site says that the motor should run at max5500-6000rpm, the most we can get at full throttle is 4200rpm and top speed 15. We were told to use a 14x10 pitch to get the rpm's up. So we put one on and we now are getting at 15mph the rpm's are 5500 but we still have more throttle if we increase the throttle speed it will go over 6000rpm , not recommended by manufacture. What can we do to have max rpms 5500-6000
with full throttle , and will this give us any more speed. Any info please.

 

[steelespike]

Re: Help With 20' Pontoon Prop/speed

You should really put this in the prop section.I think a 13 would normally be pretty close for a big foot.Are you sure your getting full throttle?Are the tubes clean and prop in excellent shape.How is the compression.
Is the speed by gps:much more accurate than a speedo.I think your setup with a 12 or 13 should get about 18 to 22 mph. and about 5,500 rpm.Is the motor running on all cylinders?

 

[Silvertip]

Re: Help With 20' Pontoon Prop/speed

14x11P four blade pontoon prop is pretty standard for the 40/50/60 big foot motors. If the props you've tried are Mercury props thats one thing. But you need to understand that (for example) a 14 x 10 prop from two different manufacturers will very likely perform differently. The reason is the prop "design". One prop may have more rake than the other, the blade shape may be different, and the amount of cupping may be different to name just a couple of features. Pontoon props have very large, more rounded blades which provides more thrust, especially in reverse for docking manuevers. It is the prop that puts the observed WOT rpm in the manufacturers recommended band. Speed will be whatever that prop provides. More pitch than that lugs the engine and less pitch than that causes over reving.

 

[cbedd]

Re: Help With 20' Pontoon Prop/speed

You should really put this in the prop section.I think a 13 would normally be pretty close for a big foot.Are you sure your getting full throttle?Are the tubes clean and prop in excellent shape.How is the compression.
Is the speed by gps:much more accurate than a speedo.I think your setup with a 12 or 13 should get about 18 to 22 mph. and about 5,500 rpm.Is the motor running on all cylinders?

Yes I have full throttle the tubes and prop are in great shape. The motor is near new and runs perfect. The speed is by speedo, today we are taking out our gps to get a speed reading. Thanks.

 

[cbedd]

Re: Help With 20' Pontoon Prop/speed

14x11P four blade pontoon prop is pretty standard for the 40/50/60 big foot motors. If the props you've tried are Mercury props thats one thing. But you need to understand that (for example) a 14 x 10 prop from two different manufacturers will very likely perform differently. The reason is the prop "design". One prop may have more rake than the other, the blade shape may be different, and the amount of cupping may be different to name just a couple of features. Pontoon props have very large, more rounded blades which provides more thrust, especially in reverse for docking manuevers. It is the prop that puts the observed WOT rpm in the manufacturers recommended band. Speed will be whatever that prop provides. More pitch than that lugs the engine and less pitch than that causes over reving.

The prop that came with the boat we think is mercruy, The 14-10p is after market and it does have larger more rounded blades. We can buy a 14-11p used for $100. do you think that well bring up the rpm's to were they should be at full throttle. Also the prop shop said that if we take the 14-13p that came on the boat and change to dia from 14 down to 13 3/4 it may help with the rpm issue and still give us the push we need.

 

[Silvertip]

Re: Help With 20' Pontoon Prop/speed

As suggested eariler the 11P prop should put the engine in the proper rpm range and is a very common prop for that series of engines. I think you will find that using GPS your speed is a little faster than you are seeing on the pitot speedometer. Those don't start registering until boat speed is somewhere around 10 mph.

Keep in mind that when you play with propellers trying to get rpm up or down, that boat speed does not necessarily increase or decrease significantly even though rpm may change significantly. That because of the distance a prop moves forward with each revolution of the engine. Just using your numbers for example, the 13P prop results in a "theoretical" forward movement of 13 inches with each engine revolution and with that prop you observed 15 mph at 4200 rpm. To force the engine up into its recommended band you switched to a 10P prop so the movement is now only 10 inches (theoretically) with each rev. So to achieve the same 15 MPH speed the engine now needs to rev to something over 6000 (again your number). So the increased rpm and the same speed is to compensate for the less forward movement provided by the prop with less pitch. The effect is the same as shifting down or up a gear in a car.

 

[cbedd]

Re: Help With 20' Pontoon Prop/speed

As suggested eariler the 11P prop should put the engine in the proper rpm range and is a very common prop for that series of engines. I think you will find that using GPS your speed is a little faster than you are seeing on the pitot speedometer. Those don't start registering until boat speed is somewhere around 10 mph.

Keep in mind that when you play with propellers trying to get rpm up or down, that boat speed does not necessarily increase or decrease significantly even though rpm may change significantly. That because of the distance a prop moves forward with each revolution of the engine. Just using your numbers for example, the 13P prop results in a "theoretical" forward movement of 13 inches with each engine revolution and with that prop you observed 15 mph at 4200 rpm. To force the engine up into its recommended band you switched to a 10P prop so the movement is now only 10 inches (theoretically) with each rev. So to achieve the same 15 MPH speed the engine now needs to rev to something over 6000 (again your number). So the increased rpm and the same speed is to compensate for the less forward movement provided by the prop with less pitch. The effect is the same as shifting down or up a gear in a car.

Thanks for the info, went out today with my gps and with the 14-10p prop at 5500 rpm we were doing 26km which is around 15mph so my speedo is accurate. At 5500 rpm I still had alot of throttle left. I think I will buy a 11p prop as suggested. What's another $100 , what is it they say about owning a boat????

 

[Jeep Man]

Re: Help With 20' Pontoon Prop/speed

14x11P four blade pontoon prop is pretty standard for the 40/50/60 big foot motors. If the props you've tried are Mercury props thats one thing. But you need to understand that (for example) a 14 x 10 prop from two different manufacturers will very likely perform differently. The reason is the prop "design". One prop may have more rake than the other, the blade shape may be different, and the amount of cupping may be different to name just a couple of features. Pontoon props have very large, more rounded blades which provides more thrust, especially in reverse for docking manuevers. It is the prop that puts the observed WOT rpm in the manufacturers recommended band. Speed will be whatever that prop provides. More pitch than that lugs the engine and less pitch than that causes over reving.

Silvertip, I have a 60 Bigfoot. I'm not familiar with a 4 blade being standard. Mine is a 14x11 three blade and I am going to a 14x9 to get the WOT rpms into the proper band. Am I going with the wrong one?

 

[Jeep Man]

Re: Help With 20' Pontoon Prop/speed

Cbedd, where are you located. When you said KPH I figured your north of the border. THE reason for my asking is that there was an ad in July on Kijiji London for a 14x11 Bigfoot prop for $40. I have the guys email address if you need it.

 

[Silvertip]

Re: Help With 20' Pontoon Prop/speed

Just to add a point of clarification -- there is no "standard" prop for most outboards. They are shipped from the factory without them for the simple reason that the manufacturer has no idea what boat the engine will be installed on. As you are finding out, certain boats require a specific prop to operate properly. That's why prop selection is such a crap shoot. There is no "one size fits all" in this regard.

 

[cbedd]

Re: Help With 20' Pontoon Prop/speed

Cbedd, where are you located. When you said KPH I figured your north of the border. THE reason for my asking is that there was an ad in July on Kijiji London for a 14x11 Bigfoot prop for $40. I have the guys email address if you need it.

I live in Salmon Arm BC Can. Thanks anyway for the info

 

[cbedd]

Re: Help With 20' Pontoon Prop/speed

Silvertip, I have a 60 Bigfoot. I'm not familiar with a 4 blade being standard. Mine is a 14x11 three blade and I am going to a 14x9 to get the WOT rpms into the proper band. Am I going with the wrong one?

What is the motor on a pontoon and if so how big?