Prop diameter

[DUKE1]

Gents,
I just read the prop article in the FAQ's and I still have a question. I understand that the pitch is the theoretical forward movement of the boat in inches, but what I don't understand is what the importance of the diameter is. Does it have to do with the weight of the prop so it does not over rev the motor? Sorry for the simplictic question, but I want to go from a 14.5 X 19 to a 17" pitch but it looks like I'll have to go with a 15" diameter to get the pitch I want. Will this diameter change affect the RPM's as well as the pitch change, or is pitch the only factor in determining the RPM's of the motor?

Thanks in advance for the info-
DUKE1

PS-its on a 86 V4 140 Johnson looper (w/V6 GC)

 

[Dhadley]

Re: Prop diameter

There are 14 1/4, 14 1/2 14 3/4, 15, 15 1/4 etc diameter props for your motor (also some smaller too). Diameter has a lot to do with design. Some props use a large diameter and little rake. Some use a large diameter and lots of rake. For years the "standard" V6 style prop was 14 1/4 or 14 1/2.

On big heavy boats diameter helps with thrust. On faster hulls more diameter allows a higher engine mounting (X dimension) which can help with speed. You have to consider design when setting up your boat.

Diameter is only part of the whole package. Personally, I like lots of diameter.

 

[DUKE1]

Re: Prop diameter

Thanks for the reply. What's considered heavy? My boat weighs about 3200 then 160 gallons of fuel plus two people is the usual load. I have twin 140's. Right now as is, my WOT is only around 5500, and I'm at 37MPS on GPS. I want it more in the 6000 range or closer to it. So a 15" diameter at say 15x17 would give me better WOT and more speed. WOT is most important to me....I dont want to lug anything.
DUKE1

 

[Dhadley]

Re: Prop diameter

Have you experimented with the height of the motors?

 

[DUKE1]

Re: Prop diameter

No I have not, I'm interested in learning though. What height should they be at? Is 5500 RPM ok...or should I change that prop? If so, should I go to 15" diameter then down in pitch from 19 to17?