pAm I thinking correctly??

[bobhol]

Hello all .It has been awhile since i have needed to post .Typically I can find answers to my questions by searching. Here is my dilemma.I purchased a new to me pontoon .It is a Crest 20 ft powered by a 40 Bigfoot .I am happy with the speed ,about 17 mph with a 3 person load and a full tankof gas ...my typical scenerio...I can easily hit redline ,6000rpm,and a little more after 15 seconds or so and the right trim...about 6200 .I typically cruise around our inland lake about 6 mph and 2500 rpm.The prop on it is a mercury 854340A33 9P,which is a 14 by 9P 3 blade. I am wondering if I would be happier with a 4 blade 13.8 by 11P like this Quicksilver Nemesis 4506189402 or something similar . I realize that going to a 4 blade will cost me rpms and increasing pitch will do the same .I am GUESSING that WOT would be in the 5600 -5700 range .Is that feasible and would that be an ok range? My reasoning is I would get better response with the four blade and lower my cruise rpm which is quieter .... Also the guy at the prop shop says I should just recup my original prop (which he does ) and I will get the same result cheaper .....what do you think??....thanks for your valuable insights...

 

[Chris1956]

In theory, adding 2 " of pitch decreases RPM abut 400. So an 11P prop should give about 5800RPM.

 

[bobhol]

Yes,I agree but going to a 4 blade also decreases rpm ,right??...what about recupping my original prop?...and is 5600-5800 good enough?

 

[Scott Danforth]

look at the specs of the motor for WOT

pontoons do not always follow the rules of thumb.

cupping reduces your WOT by increasing effective pitch

a 4-blade will also decrease you WOT

something else may be wrong, my bud has an 18' Crest with a 30hp and hits 22mph with 6 people on board while plowing like crazy what is your motor height?

 

[bobhol]

Hi ,I tried dropping the motor a bit but it started spraying water over the transom as it was too low ...I dont believe my height is wrong after that experiment...in fact I am not unhappy with the speed ..more would be nice ,lol...but my concern is lowering the rpm closer to 2000 at my normal around the lake speed of 6 mph to make the motor a bit more quiet .It seems I have a few hundred rpm to play with. Also a 4 blade or recupped prop would give me better response at low speed for docking etc...My only disappointment with the motor is its noisy....even at idle ,it burbles quite loud as exhaust comes out at the waters surface till it gets above 1000 rpm ....I have a friend with a larger merc ,i think 115 on his 20 fishing boat and you can hardly hear it run ... I also have noticed very little speed difference between 5500 and 6000 but a lot of noise difference ..MERCURY motor specs call for 5500-6000 rpm at WOT ...

 

[Scott Danforth]

you cant compare a pontoon to a standard V-hull

if your motor is too low on a pontoon, the motor plows and you loose speed. if you are too high, the motor ventilates and you loose speed.

motor height to water lever is more important than most people realize, and on a pontoon, simply moving the cooler from the back to the front may be all it takes to raise the motor enough. of any boat needing a powered jack plate, its a pontoon.

back to the question. how far below the water surface is the AV plate?

 

[bobhol]

My av plate is about 2 inches below the waterline with no one on the boat I just went and checked