I have a question about props

What works best big prop turning slow or smaler prop turning faster ?

I have a 38 Top Gun with twin 700 HP 572īs and #3 drives
max rpm on engines are 5200 rpm drives are 1-1.5 ratio and props are 16 x 28 in order to get top speed I have to turn the engines to 5600 which I donīt like plus it seems like they donīt pull much after 5200. I have a set of drives with 1-1.62 ratio and a set of 17 x 31 props that I want to try instead. I tried the 17 x 31 on the 1-1,5 drives but could only get 4600 rpm out of the engines and the engines was blowing the spark plugs. Do you think the other drives and bigger props are the way to go ?

Cigaretteman

I don't know anything.

But people on here who seem to say, " When prop pitch approaches 30"s you need to consider "gearing up" to keep the pitch down.

Also, the threads seem to suggest speeds in the 80 to 100 mph need the higher gear to permit lower prop pitch.

Just another boater passing on what I've heard.

MAV

p.s. Go to the Baja web page Team Baja Marine and select props and talk to Houston Props on that site. He would have a better idea.

IMHO and from what I've found smaller pitch turning harder does
much better in the boats we've tested ,,I don't truth fully know
on a small boat what would work best but on a 388,and a fountain ect it relly helped everything and I heard the same thing
if you go larger than 30 you need to re -gear to keep R's up and use a 28 or 30

I apriciate your inputs ! My budget right now tells me to try out the 1.62 drives and the 17x31 props to go from 1.5 to 1.62 should give me 8% more rpms on the engines which should give me just under 5000 rpm on the engines. With a max of 5200 that should be close enough. I will give it a try to see what happens. I will keep you posted.

Thanks

Cigaretteman

What year is your 38 Cigarette and what kind of speed were you seeing at 5,600 with the 1.48's and 18 X 28's?
We're getting ready to dunk mine in in the next two weeks, I have a '94 (notched transom) with the exact same props, 3A SSM'a, and about the same power. So obviously I'm reallly curious.

O.C.

My Gun is an 1989 with kevlar hull also notched transom. The props are 16 x 28 not 18 I have corrected that now. The speedometer says 78 mph in dead flat water and around 80 mph with 2 fotters at 5500-5600 rpm I havenīt tryied a gps yet.
Oh and engines are naturally aspirated with tunnelrams.

Cigaretteman

CigMan/ I am no prop expert but from what I can tell, if you are running 1.50 with 5200 rpm, 28p and running 78, with a 1.62 and 31p, at 78 with the same slip factor you would be running around 5050 or so. However, at 5050 you may be developing slightly less hp which may bring speed down slightly. With the 16 x 28 you would probably have slightly more slip than with the 17 x 31, which again may bring the 31p rpm down some more.

I have a pair of 17.5 x 28's (3 blades) with addt'l cup added by Merc which may work better, staying with the 1.5s. I could use the 17 x 31's. Let me know if your interested in trading. I also have one right 18 x 30 3 blade, but need a left. Thanks, Bob