Prop "breaks" at high RPMs

[Capt Al]

I have a 50hp Merc on a 20 foot Pontoon. At near max RPMs (5500-6000) the prop "breaks loose". I know this can severely shorten a motors' life so I avoid going over 5000 RPMs but I was wondering if it would wiser in the long run to change props. If 'yes', should I go with a larger or smaller pitch?

 

[fone_man]

Re: Prop "breaks" at high RPMs

Prop breaks loose??? Do you mean RPMs rise with not significant increase in movement... like a spun hub on prop???<br /><br />Please explain....<br /><br />

<br /><br /><br /><br />------------------<br />"Keep It Between The Bouys" & "Good Luck"!!!<br /><br />fone_man

 

[Walter]

Re: Prop "breaks" at high RPMs

That's what I was thinking...spun prop. Maybe we're talking cavitation here...<br /><br />Walt

 

[kevlar3000]

Re: Prop "breaks" at high RPMs

Is that too much rpm for a 50? Not sure of year ect... but it sounds high. Just curious.

 

[kevlar3000]

Re: Prop "breaks" at high RPMs

I forgot, is it possible that your prop is coming out of the water at higher throttle? Over waves ect.... 20 feet is a lot of boat.

 

[Capt Al]

Re: Prop "breaks" at high RPMs

Sorry guys ... I should have been more specific. This is a new 2002 50hp Merc 4-stroke EFI with a max RPM range of 5500-6000. The prop cavitates as I approach 6000 RPMs. This doesn't present a major problem for me as I rarely, if ever, have a reason to exceed my ideal cruising range of 4500-5200 RPMs. Prop is not clearing water, it's just classic cavitation. I was wondering if a larger or smaller pitched prop would correct this problem.

 

[Fouled Plug]

Re: Prop "breaks" at high RPMs

Is your Merc the "Bigfoot" model? It is ideal for pontoons because of the oversized anti-cavitaion plate and taller gearcase to get the prop lower in the water.

 

[Capt Al]

Re: Prop "breaks" at high RPMs

No. It is not the "Bigfoot". I ran this pontoon for the past 14 years with a "normal" Merc "Classic 50" and could not justify the extra expense of the "Bigfoot" which, among other modifictions, requires a very specific prop made for these motors. My old "Classic 50" never had this cavitation quirk but, then again, this new 4-stroke delivers more top end than this boat has ever experienced.

 

[Fouled Plug]

Re: Prop "breaks" at high RPMs

I'd guess you've just reached the limits of performance with this particular motor/hull combo. Pontoons create a lot of turbulence in the water, which is forced toward the prop. You may have to just run at max of 5000 RPM, and avoid the cavitation. (As well as probably making the motor last longer!)

 

[Walter]

Re: Prop "breaks" at high RPMs

Look...You should NEVER attempt to put a Pontoon boat on a plane. It's simply too dangerous...<br /><br />Someone could easily spill their cold adult beverage.<br /><br />Walt

 

[fone_man]

Re: Prop "breaks" at high RPMs

Walt... I see your into the "wine" again tonite... but I agree with you on the never plane "pontoon"!!!!!!!<br /><br />

<br /><br />------------------<br />"Keep It Between The Bouys" & "Good Luck"!!!<br /><br />fone_man