Prop diameter vs. pitch

[Capn Mike]

I use a 15.5 X 15 prop on my 2300 Seaswirl WA (200 Johnson) and it works fine in the summer, with canvas down. In the winter, the extra drag drops the rpms down 800+. Seems as though a different prop would work better for winter, but no one makes a 15.5 X 14 for my engine.<br />My question is: what difference does prop diameter make? It's easy to figure out pitch effect, but I've never seen much on diameter. Is a, say, 17 X 13 the same as a 15 X 15?

 

[jegervais]

Re: Prop diameter vs. pitch

Typically as pitch increases, diameter decreases. Smaller diameter contributes to allowing the engine to spin up faster (less blade surface = less drag). Johnson offers the SST prop (stainless steel) in 15.5 x 14. What is your WOT rpm now? You should be up around 6000.<br /><br />-John

 

[Capn Mike]

Re: Prop diameter vs. pitch

WOT in winter is 4700; summer 5600. My question is how much difference does diameter make? 15 X 14 = (say) 14 X 16 (or whatever). I don't want to spend $$ for the wrong prop!

 

[jegervais]

Re: Prop diameter vs. pitch

As far as I know, there's no formula for the effect diameter has - you need to be concerned with pitch, diameter only matters from a clearance stand point (i.e. prop hitting anti-ventilation plate). I don't think 1 inch of pitch is going to make up 800/900 rpm of difference either.<br /><br />-John

 

[Hooty]

Re: Prop diameter vs. pitch

Hey, Capn Mike,<br />As a rule of thumb, 1in. of pitch = 400rpm + - .<br /><br />g/luck & c/6<br /><br />Hooty

 

[Hawkeye1]

Re: Prop diameter vs. pitch

Hooty every OB prop I have tested 1" of pitch + about 200 rpm. 2" gets you 400. I am a little concerned that haveing the top up cost you 800 rpm. I only lost 400 with a full with hardtop/ curtians on a 23'er. My old WaC only dropped about 200 rpm with the top up. Are you sure that horse is in good running condition. As far a diam vrs rpm goes, it will depend on what brand or type of prop. Wont make much difference in most cases. Power tech props might be able to help, they make some lower pitch props but I am concerned about the 800 rpm drop just from the top.

 

[Hooty]

Re: Prop diameter vs. pitch

Roger 200 rpms per 1" pitch.<br />Hawkeye1 iz right on.<br />Where wuz I??<br />Thanks Hawkeye.<br /><br />c/6<br /><br />Hooty

 

[Pursuit2150]

Re: Prop diameter vs. pitch

Once you decide from above, if you want to change the prop.That's you'r decision, but take my advice do not go to 6000 rpm, it is not the recommended rpm per OMC. I use a 14-3/4 X 17 SST.WOT 5500 is plenty for the boat!<br />I don't think its a big deal with all the canvas up.