First Timer prop question

Begester

Petty Officer 1st Class
Joined
Oct 23, 2006
Messages
203
Hello everyone. I have recently purchased a 1994 Sea Ray 180 Signature 18.5 ft Bowrider with a 5.0 V8 and Alpha I outdrive and I'm looking to swap the current SS prop out for an aluminum one. The prop I have on there now is a Mirage 3-blade SS and I can get about 45 mph at WOT 4100rpm. The reason that I'd like to go aluminum is because I'm relatively new to boating and I want to make sure that if I accidently bottom out this prop, that I dont run the risk of damaging the outdrive/lower unit. I'd much rather pay $60 to get an aluminum prop remachined than have to pay $600 to repair the lower unit.

I am wondering where to go from here though. How does efficiency factor into prop selection? I'd rather save a few bucks at the gas pump than be able to get 50mph out of my boat. I read somewhere that a 4 blade prop is more efficient than a 3 blade, but havent seen much about that on this forum.

I like the way that the boat handles with the prop I've got now, I can pull a skiier with 7 people on the boat and it handles quite well. But everyone keeps telling me that SS is a bad idea for beginners like myself.

Any suggestions or ideas would be greatly appreciated! Thanks for the help!

BJ
 

Ron G

Commander
Joined
Apr 28, 2005
Messages
2,905
Re: First Timer prop question

The fact that your already aware that at times you hit bottom is a plus because everbody does it once in awhile.i personally would stay with a ss myself most have a rubber hub that should slip if you do hit and will less damage ,you didnt mention what pitch the mirage was and we need to know that.and also is the 4100rpms at wot with the full load of people?
 

Ron G

Commander
Joined
Apr 28, 2005
Messages
2,905
Re: First Timer prop question

and i forgot welcome to iboats theres alot of great guys with alot of wisdom here on this site!
 

Crownie2

Petty Officer 1st Class
Joined
Jul 26, 2006
Messages
378
Re: First Timer prop question

Are you really hitting bottom? Is there a pretty good chance you might??

If the answer is no, then I wouldn't worry about it. You seem to be doing pretty well so far...

By the way - your RPM's at wide open seem a little low and that could be causing you to lose a little efficiency. What pitch is the prop?

Bob
 

Begester

Petty Officer 1st Class
Joined
Oct 23, 2006
Messages
203
Re: First Timer prop question

Thanks for the quick responses to my question.

The pitch on my prop right now is a 19 (3-blade), and yes, the WOT is full of people.

As for the bottoming out, being here in the Arizona desert, I have heard that there are a number of shallow spots and places where its easy to drag across the bottom. Also, because everything is rock out here, the chances of dragging the prop along something soft like sand or mud are pretty low.

From what I gather, maybe I should think about keeping the SS prop, but would I be better off getting a 4 blade from a gas saving perspective?

Thanks again for the help!
 

Dhadley

Supreme Mariner
Joined
Feb 4, 2001
Messages
16,978
Re: First Timer prop question

4 blade props have different characteristicts than a 3 blade of the same blade configuration. Typically a 4 blade prop likes to (generally) run at a higher transom height (X dimension) than 3 blades. There are 4 blade props made specifically for sterndrives because we can't easily change the X dimension.

You have to look at how the boat is running now. If you need a lot of positive trim to get the nose up and "loose" then a typical 3 blade may be better.

Since you have all that weight in the back the nose may be "loose" already. A typical 4 blade will tend to lift the tail. If you're using a lot of positive trim a 4 blade may work against you.
 

Ron G

Commander
Joined
Apr 28, 2005
Messages
2,905
Re: First Timer prop question

Also try running it with just you and see what the wot rpms are.
 
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