3 vs. 4 blade prop on 15 hp?

[BF]

Hi all,<br /><br />I've got a late '70's 15 hp evinrude on a 16' aluminum boat. It has the stock 9x10 3 blade on it. I've<br />been pretty pleased with how well this little motor pushes this boat. Sure, with 4 adults in it, it<br />slows down a lot, but it still planes easily (it's not as underpowered as I feared it might be). <br />I even took the kids tubing behind it this weekend. With 2 adults in the boat it had no problem <br />pulling the tube as fast as my kids care to go (they're small).<br /><br />I wanna get a 2nd prop (as a backup)... I can get either 9 1/4 X 10 or 9 1/4 X9 both sizes available <br />in either 3 blade or 4 blade. All are the same price. I thought maybe I'd go with the 9 1/4 X 9 in 4 blade, <br />because it would be different pitch than what I have now, and might be a bit better (?) for pushing <br />heavier loads (either weight in boat, or tube). But, maybe I should even go to a 9 1/4 x 8??<br /><br />So, for a little 15 hp, how much difference would 1 inch in pitch make?<br /><br />How much difference would a 3 vs. 4 blade make?<br /><br />Would a 1/4" difference in diameter be enough to notice, like more load on motor? (I doubt it).<br /><br />Any thoughts? <br /><br />Thanks!<br /><br />Brent

 

[james1]

Re: 3 vs. 4 blade prop on 15 hp?

Howzit Bp. I'm not sure about your boat/motor setup but with the rigs that I work with the smaller the pitch the bigger the load I can push.The bigger the pitch the faster I can go.4 bladers seem to give us better performance all round(fuel consumption and thrust)<br />Can't help with anything else.<br /><br />Regards

 

[BF]

Re: 3 vs. 4 blade prop on 15 hp?

Rogue,<br /><br />Has your experience with 4 blade props been on small outboards? Most of what I've seen about their advantages talks about a better holeshot by getting less slippage and transmitting more power... this is usually getting a big/heavy boat on plane fast (using a big motor). But, I'm not sure whether there would be much benefit on a 15 hp. (?) Maybe it won't have enough torque to handle more bite anyway?<br /><br />Brent

 

[Hooty]

Re: 3 vs. 4 blade prop on 15 hp?

Talking about props without talking rpm is difficult at best. Recently, a friend of mine "figured" his engine was turning about 5500rpm and that was about where he wanted it. He finally broke down, spent $40 for a Tiny Tach and found out he was seriously lugging (4300rpm) his engine, which is a killer. My advise is put a tachometer on the thang and find out where you are, engine speed wise, now and go from there.<br /><br />g'luck & c/6<br /><br />Hooty

 

[BF]

Re: 3 vs. 4 blade prop on 15 hp?

good advice...<br /><br />You're right. I don't know what it's turning, but it's running real strong with a light load (me and my Lab) in the boat. With 4 adults and a cooler rev's are noticably down (and of course speed). So, eventhough the main thing I want is a spare prop on hand, I thought dropping down in pitch to get a bit more rpm's with a loaded boat might not be a bad idea. If it's a weekend of luggin' folks around, I'd switch to the "load" prop... otherwise I'll use the "speed prop" (such that it is!! )<br /><br />Until I called the shop today for a price, I didn't know they made 4 blade props for small outboards. Also, I didn't realize that they had 1" increments in pitch. If the "stock" 9x10 was the only one available, it would've been OK... the problem is now having to make a choice! <br /><br />Anyway, I was particularly curious whether a 4 blade would have any advantage.<br /><br />Brent

 

[BF]

Re: 3 vs. 4 blade prop on 15 hp?

^

 

[BF]

Re: 3 vs. 4 blade prop on 15 hp?

^last bump before I let it go^