10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

[Newhook]

I have a 10 7/8x12 on my motor now and the motor revs too high at WOT. I can swap the prop for a 10 3/8 x13 or even a 14"pitch to get the extra pitch to slow the motor but I wonder if I am ahead in doing so. The seller says that diameter does not matter except for "holding power", but cannot explain what that means. It will cost a bundle for the 10 7/8 in the higher pitch and nothing to do the swap. <br /><br />What should I do and what is "holding power"?

 

[Capn Mike]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

In a perfect world, with no slip, etc., one degree of pitch equals 200 rpm @ WOT. Of course, there is slip, but that's a pretty good rule of thumb, and seems to work on my 200 Johnson. I am told 1" of diameter equals 100 rpm, but I can't verify it personally. I've heard other formulas for diameter as well, but hear it has to do with HP. What rpm are you running on what kind of boat, motor?

 

[Dhadley]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

Generaly speaking a larger diameter will carry the boat better. And you can run the motor at a higher X dimension too.<br /><br />What motor do you have? I may have something in that range.<br /><br />Good luck!

 

[Newhook]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

Capt Mike,<br /><br />The motor runs at ~6100 rpm WOT and I'm told that is a bit high. I am told that the motor (a 25 hp Merc big foot 4 stroke pushinga 16' Princecraft utility) will not allow for over-rev and will mis-fire when it gets too high - don't know if it's true. I just need to know if I lose anything in going with the smaller diameter. I can get up to a 14" pitch in the 10 3/8 diameter on a staight swap.

 

[Newhook]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

Dhadley,<br /><br />What is "X dimension"?

 

[Capn Mike]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

You might have "kinda" the wrong engine for your kind of boat. If my often-faulty memory serves me right, the Big-Foot is a geared-down prop, and the motor was intended for pontoon, houseboats, and works great as a kicker on larger fishing boats. You'll find that a non-geared engine would run the proper prop on your light boat and give you better performance. The reason your BigFoot may be running too fast may be because you have a "runabout" prop on it; the BigFoot is designed to turn a deeper pitched prop.

 

[Dhadley]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

X dimension is motor height (up and down), not to be confused with trim angle (in and out). IE -- where the propshaft (or cavation plate) is in relation to the true bottom of the hull when the shaft (plate) is parallel.<br /><br />Good luck!

 

[trevorcday]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

All merc 25 4-strokes are called big foots for some strange reason. I believe it is because they use the same size gearcase as 30/40/50/60 in the two stroke line. Any way there is no "little foot" 25 4-stroke.

 

[Newhook]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

Mrmerc,<br /><br />I noticed the same thing in the Merc pamphlet; that the 25hp has no "small-foot" version. They are all geared at 2.4:1 vs. 2:1 for most other motors in the same hp. Is there a performance issue to your mind? <br /><br />Merc did some testing on a variety of boats (results on web page), and they used either 10 3/8 x 12 or <br />10 3/4 x 12 or <br />10 3/8 x 13<br />props on a variety of small alum. boats. All of them were heavier to some extent but not otherwise unlike mine (i.e.Klamath 16'Explorer). None of the testing was done with the 10 7/8 x 11" prop, the stock prop that comes with the motor, which is both larger diameter and less pitch than all of those used in testing. All of them used a higher pitch, as you can see. What I cannot figure is why they simply did not increase pitch and keep the diameter the same? Why would they do that? Any theories? <br /><br />Thanks

 

[what200]

Re: 10 7/8 x 12 vs 10 3/8 x 13pitch. What's the diff.?

here is the link to a prop calculator:<br />http://www.go-fast.com/Prop_Slip_Calculator.htm<br />you can use this to estimate your slip, speed, pitch, and rpm.<br />Slip is the difference between the theoretical distance a prop should traven and the actual distance traveled. (i.e. a 20" pitch prop travels 18" therefore has 10% slip)<br />A larger diameter prop will tend to slip less at the expense on extra drag and needed hp to turn the prop. Typical slip #'s might range from 7% to 15% depending on application. You might try adding 1% of slip to account for the smaller prop.