General Prop Theory Question

[tractoman]

On my '95 Sea Ray 180, 4.3LX Alpha Gen II 1.81 ratio, I had been running an aluminum 19p x14.5 prop. Switched to a stainless Mirage 19p x 15.75 (I think that is the diameter) prop. The Mirage just feels like it's too big. The boat gets up and goes but feels like the larger prop is putting too much load on the engine. It sounds different also but that is probably due to the different length exhaust passage of the center of the prop. Could this be too big? This weekend I was running with 4 people, 3/4 tank of gas, about 37 on the speedo at 4000rpm. This was not WOT. I don't normally go WOT with the family in the boat. Any thoughts on this. Is the mirage just not a good prop for this application? I want good pull and decent hole shot. Not so concerned with top speed but I figure I can get about 43-45 mph out of this set up. Any advice welcome.

 

[mpsyamaha]

Re: General Prop Theory Question

Without accurate WOT numbers there is no way to know how well that new prop is working on your boat.

 

[steelespike]

Re: General Prop Theory Question

First of all Prop numbers are allways written diameter X pitch. 14.5 X 19
The diameter seems really large I could only find one 19" prop bigger at iboats at 16" and the rest were smaller
Diameter has a secondary affect on rpm. maybe 100 rpm.I'm more inclined to think your old prop was less aggressive either from age or design. We need the true wot rpm and a gps speed with the al. prop to evaluate your setup.If the ss prop is only available it will have to do.
Partial throttle readings unfortunately don't mean much.You need actual carefully trimmed wot rpm and speed.
The motor needs to be able to rev freely within its wot rpm range. Of course once setup you can run at what ever speed you like..

 

[gm280]

Re: General Prop Theory Question

On my '95 Sea Ray 180, 4.3LX Alpha Gen II 1.81 ratio, I had been running an aluminum 19p x14.5 prop. Switched to a stainless Mirage 19p x 15.75 (I think that is the diameter) prop. The Mirage just feels like it's too big. The boat gets up and goes but feels like the larger prop is putting too much load on the engine. It sounds different also but that is probably due to the different length exhaust passage of the center of the prop. Could this be too big? This weekend I was running with 4 people, 3/4 tank of gas, about 37 on the speedo at 4000rpm. This was not WOT. I don't normally go WOT with the family in the boat. Any thoughts on this. Is the mirage just not a good prop for this application? I want good pull and decent hole shot. Not so concerned with top speed but I figure I can get about 43-45 mph out of this set up. Any advice welcome.

Back a few decades ago (actually more decades then I want to admit) I bought a very good book all about props and prop design and theory. It was a book written be Mercury Motor company and it explained everything anybody could ever want to know about props and how they were designed and work. It explained diameter, pitch, cup, rake, and everything else concerning props. It even gave very good examples about how to figure a boat's top speed (with slip figured in) for most any setup...within reason. And after reading that book (a few times) I have a much better understanding of props. But with that stated, props have so many variables concerning how they perform that it is hard to understand exactly how each prop will work for every situation. Between diameter, pitch, cup, hook, rake and everything else it gets really hard to pick one that is the best for what you want the first time...