T&T: Prop Pitch

[Mark Bliss]

Brent and Rob,
I agree that you are already most likely at or near optimal economy for the 
speed you want with that boat and those motors.  However, the only way to 
know is to experiment with some different props. There are no hard rules 
because there are too many variables from boat weight, hull design, 
transmissions, engines, shaft sizes, etc.
Some general rules I found are;

* One inch of diameter equals about two inches of pitch as for amount of 
water moved at a given RPM.

* In my research and experimentation I came to a truth (others had already 
found and I confirmed).  There is no such thing as the best prop for a 
particular boat. There is only the best prop for a particular application or 
goal.

* Prop shop people recommend a prop that allows those particular gas engines 
to reach 4200 to 4500 RPMs for best overall performance....and that may be 
true for off the shelf applications, but it's dead wrong when the goal is 
best economy at displacement speed.

I can't remember now what size props were on that boat before and after. But 
it would turn 4300 RPMs at WOT.  After trying several different pitched 
props, it would only turn about 3000 RPMs at WOT. But my economy at 7.5 
knots went from just under 1 mile per gallon to about 1.5 per gallon. 
Conversely, the economy at the higher planing speeds went to hell.  But my 
goal was the same as yours. So that didn't matter.


----- Original Message ----- 
From: "Brent Hodges" <vbhodges at gmail.com>
To: "'Trawlers andTrawlering'" <trawlers-and-trawlering at lists.samurai.com>; 
"'Phil & Kate'" <pkpennington at comcast.net>; "Rob Dorsey" 
<Skipper at bestrevenge.us>
Cc: "Mark personal" <mbliss at simple.net>
Sent: Wednesday, September 12, 2007 10:48 AM
Subject: Re: Prop Pitch


>
> From: "Rob Dorsey" <Skipper at bestrevenge.us>
> We have the odd combination of a pretty much displacement hull with
>> two huge Crusader 454s spinning 22x22 props.
> we operate said 454s at 1,500 rpm and get our 7.5 kts of hull speed at 6 
> gph.>
>> BUT, we've been told that a re-pitch of our props, given that we don't 
>> care
>> about the top speed, for better economy. We'd love to see 5 gph without
>> slowing down to 6 kts to stretch the range as we do now.
> Rob Dorsey
> Carver 3607 "Best Revenge"
> (endsnip)
>
> Rob, I have a buddy who I'll cc on this that went through this exersise 
> with a boat similar to yours. He was happy with his results, but if I 
> remember right, his end result wasn't any better than what you have now. 
> Sounds to me, from my old gas boating experience, that 6gph at 7.5 knots 
> (almost 1 1/2 mpg statute) is pretty darn good for your boat with those 
> engines. I doubt that all the repropping in the world isn't going to 
> improve much on that!
>
> Brent Hodges
> Friendship
> 43 Albin Sundeck
> Seabrook, Tx