Transom height

just had a used mariner 75hp p

just had a used mariner 75hp put on my 20ft pontoon. after checking different prop charts they tell me to use a 13x19 SS. that is what i'm running. i get around 3000 rpm top. do you thnk that i should look at a different prop or the height of the motor on the transom? i think that the cavitation plate is about 2inches or so below the water line.

"Harold, the only good use for

"Harold, the only good use for a printed prop chart is starting a fire in the fireplace.

You can not use a chart to figure out what prop to use on your rig.

The prop must match the boat in the manner in which you use it. I understand that it would be nice to look at a piece of paper that says for a 20 foot boat with a 75 horse you use prop "x" - but that may be for an aluminum planing model that weighs 1000 pounds with a 500 pound passenger load (see what I'm getting at).

A pontoon boat has all the planing characteristics of a lead weight.

Additionally, it sounds like your motor is mounted either way too high or you are not running enough shaft length. Although a motor on a "toon" is not mounted with the same accuracy as either a flat or v-bottom boat, the cavitation plate should still be close to inline with an imaginary line drawn from the bottom of one tube to the other (which I would figure would be at least 5 or 6 inches below the water line).

Just at guess off the top of my head I would expect a 20 footer with a 75 to be running something closer to a 13 pitch prop, depending on how many people are routinely aboard - this is not a 20 foot planing hull...."

"Thanks for the reply Graham.

"Thanks for the reply Graham.
My Toon weights 1300, motor 280.
With 2 on its 370lbs, and with 4 its 620 plus ice chest, battery,gas, etc about another 100lbs.
Have been thinking about a 13 3/4 x 15 but I don't think that will get the R's up close to 5200. Maybe raise the motor.
The Toon is 100miles away, will be going there Friday & Saturday."

"Harold, I threw all your numb

"Harold, I threw all your numbers in my prop calculator, and when running "heavy" it does spit out a 13 pitch.

The 13 3/4 should probably serve you well, but as you mention, you may have to play around with getting it mounted just right and then figure out how best to trim it.

I believe the low end of your wide open throttle range is 4750, so if you can get it over that at full throttle (even if you aren't going to run it that way) all should be fine.

Based on what you noted above, I would think just the prop change alone would take you well into the mid 4k rpm range...."

"thanks again, was going down

"thanks again, was going down this weekend but my daughter that is at college decided to come home for the weekend, so it will be another week. thanks"

"back to the top of the page,

"back to the top of the page, dans discount prop and the mercury site tell me that i should run a 13x19 SS for top performace. wich is what i'm running, being this is on a mariner motor and i think its has the hub kit system can they slip?

could this be part of my problem?"