Logo
FULL SITE MENU

Under pitched prop...

B

briang0

Regular Contributor
I trashed my prop last weekend. My old prop was 13 x 13 3 blade but should have been 13 x 12 according to the prop shop. I never could get to WOT of 4000 rpm. I found a great used prop for short money but it is 13 x 11 I install it today and as expected at WOT I am over 4000 rpm around 4300. Since I never operate at WOT I would like to leave it on for the rest of the season and have it fixed over the winter. My question is provided I don't run the boat over 4k will I cause any problems to the motor. I was out for a while today and she ran well came up on plane and the engine seemed to be working less harder than it was with the older prop.
 
If you are not running at WOT all the time you should not have an issue. Keep track of how she run's and what changes you note with your fuel consumption and who knows, you might have found your new prop without the expense of going new.
 
If you can get that prop off again without a hassle, you can "have your cake and eat it too" (so to speak). THe answer is cupping the blades. A good prop shop will beat in enough cupping on the tips of those blades to make them act (at speed) like they had another inch of pitch. Magic!

A case in point: My boat once had a 16 x 15 uncupped prop on one side and a 16 x 14 heavily cupped prop on the other side. THe motors reacted the SAME at all speeds.

Jeff
 
Brian,

I don't see where you specify what engine you have, but you need to know what RPM the manufacturer recommends for WOT. Say is is 4200-4400. Cruise speed is about 75-80% of WOT. So if you can only get a WOT of 4000, 80% of that is 3200 whereas if your WOT is 4400, 80% would be about 3500. You will be going faster and using about the same fuel...theoretically of course.

Erich
 
My engine is a Crusader CS280 which is a 280 HP Chevy 409 like the Beach Boys song. The manual that I have which I got from this site THANKS says that the WOT should be 4000 RPM. I have a 1974 25' Chris Craft Express Cruiser which weights out with engine and fuel at about 4500 lbs if you add 2 people and some gear I figure 5000 lbs. The prop shop that I use which is a Prop Scan shop said I should have a 13 x 12 prop if I want to get 4000 RPM. Right now with the 13 x 11 that I just purchased if I go to WOT she'll go to about 4300 - 4400 so it's looking like this winter I'll get it re pitched up to 12. I just wanted to be sure I will not mess up the engine as long as I don't run her above WOT which I never do. I seldom run her at 3500 RPM. I don't like what I see on the fuel flow gauge.
 
Well I found out today that the 11 pitch prop will not work for me. I tried to take myself, 3 other adults and 2 kid for a ride and when we headed out into the river with a 4 knot current and about a 15 knot head wind I had to really crank up the engine to make headway something I never had to do before. So I pulled the boat out this afternoon and pulled the prop I'm sending it to the prop shop tomorrow to have it pitched up to at least 12 I'm considering 12.5 maybe. I know that 13 is too much from past experience but just barely.
 
Whoa! Ain't you got that assbackwards? If the 11 pitch is too great (analogus to having too high a gear in a car) than going to a 12 or 12 1/2 pitch will make it worse!

Think on it before you act.

Jeff
 
Sorry the prop was spinning but nothing was happening... Went to the prop shop this morning explained what was going on. he look at my old prop and the new "used prop and said it was a combination of 2 things one was the lack of pitch the other was cupping. I'll know if a few days well hopefully by the end of the week. It's amazing how much aggravation a rock can cause.
 
Ah-hah! A friend of mine ran into that same problem a few years ago. In his case, he repowered his Marinette with high performance engines that 'blew away' his stock props.

WIth straightened blades and some nice cup, she'll "be real fine, that 409!"

Jeff
 
got prop back!!! God bless Prop Scan and the Prop Shop... WOT of 4000 which is what he told me I would have with a 12 pitch and the increase in cup. I had 3 adults and one child in her today and no problem.... Hopefully I can keep from dinging up this one.......
 
Back
Top