Bigfoot prop selection

[bgaup]

Recently purchased and installed a used 50hp Four Stroke Mercury Bigfoot for my 20 ft pontoon boat. Mounted motor so that anti-ventilation plate is even with bottom of pontoons. <br /><br />Took boat out this weekend for a test run. Since new motor is 100lbs heavier than old two stroke back of toon now sits nearly six inches lower in water than before.<br /><br />Motor at WOT I was running at 4,500 rpms. Prop is 13inch 13pitch standard prop. Before I start changing props should I adjust the height of the motor? (It can go up about 2 inches) Is motor too heavy for boat? (It's rated for up to 115hp) If I change to a larger diameter Bigfoot prop, won't that lower my rpm's even more? <br /><br />Thanks for your help.

 

[Triton II]

Re: Bigfoot prop selection

Hi Kiwi2, I have a high thrust Yam 60 and have found the original mounting position set up by the dealer was about an inch too low. When I set the motor one hole higher (the AV plate is now level with the bottom of the chine (i.e at the V join) and about 1/2 an inch higher than the skeg. The prop doesn't ventilate unless the boat is thrown into turns with the motor trimmed right up, and the prop now turns at 5600 instead of 5400 and I got about a knot of extra speed. So, yes, if it were me, I'd play with motor height before changing props. A larger diameter big foot prop will lower rpms, however by how much is something the prop experts will help with (I know 200-300 rpm per inch of pitch is a rule of thumb, but not sure about diameter). I believe the Mercury big foot and Yamaha high thrust motors are similar (in principle at least). Good luck, let us know how you go.

 

[Silvertip]

Re: Bigfoot prop selection

Diameter on the prop has little effect on performance with any given prop since its figured into the design of the prop to achieve the pitch rating. a 13-1/2 x 17 prop has the same theoretical performance as a 13-3/4 x 17 and will not alter RPM unless the prop design characteristics result in it have less or more slip.