prop diameter

[moterboat]

what will changing the diameter of the prop do to the rpms and speed of the boat? I see that the props avalible for my rude are 12.75in. - 13in. -
- 13.25in. - 13.75 - 14.0 all these are avalible with various pitches..

say if I went from a 13x17 to a 14x17 what changes would that make to rpm and speed? and whats the best way to determin the best diameter for my set up

 

[MikDee]

Re: prop diameter

Usually the diameter is ratio preset by the pitch, the more pitch the less diameter, and vice versa. Generally the more pitch you need, the less diameter needed, as it only causes extra drag (designed for Higher powered lighter boats). Different manufacturers use different guielines, you'll usually find a Michigan wheel has a larger diameter, per pitch ratio. Sometimes you need a larger diameter prop then stock, if your boat is heavy (falling off plane, unable to hold speed or plane at lower rpm's) that's why I/O props are usually larger in diameter then outboard props. This is also another reason for 4, or 5 blade, props. Generally with larger dia. props, you lose a slight amount of speed, but gain some power.

 

[Cricket Too]

Re: prop diameter

MikDee, I have had the same question for a while concerning prop diameter and that was a good expalnation that helped out.

So generally if you are dropping pitch size you will want to drop diamater also, correct, at least with an outboard?

My biggest dilemna with diameter is that I don't care all that much about losing some top end speed, I am more concerned with hole shot and WOT.

I would like to have a nice high WOT, and good hole shot so that I don't need to fully open the throttle to get on plane....under normal load.

Obviously to do this I am planning on dropping pitch, so I would guess that I should look to drop some diameter also, correct?

Running a 14X17 now and want to drop to a 15" pitch, most of what I'm seeing in diameter is 13 3/4, but I have seen some 14X15 props out there, so I wondered if I should stick with the bigger diameter.

 

[MikDee]

Re: prop diameter

As I said the norm is as you drop in pitch, the diam. usually gets bigger, but yes, if you want to get a better hole shot, the extra diameter on some brands of props does help.