3 to 4 blade

[Ridewright79]

I have a 95 stingray 698zp 5.7L and need some advice for a prop change. When I bought the boat it had a 21P SS prop and would run 52 mph (on GPS) @ 4300 rpm. I found the previous owner used a automotive distributor so I had DUI build one for my application. This fixed my timing issue however I purchased a solus amita aluminum 21p and it runs 56-57mph @ 4300. I want to go to a SS 4 blade and not too sure what pitch to use. I have been researching this for a bit and think a 20p should give me the same performance as the 21P amita. I'm worried about the flex aspec and cup effect going to SS.

 

[Texasmark]

Re: 3 to 4 blade

I have a post on this prop forum from Mercury "What You Need to Know About Props" that helps you with your question. Last hit was a day or two ago. Check it out.

Mark

 

[Ridewright79]

Re: 3 to 4 blade

I did look at that and a lot of other articles regarding prop geometry. Technically going from 3 to 4 you should drop 1" in pitch. Also for every 1/4" of diameter increase should drop your rpm 200 +/-. This is why I am having a little issue choosing a SS prop. It's not a cheap purchase to be wrong. Technically if I did use a 4 blade stainless at 14" diameter and drop pitch to a 19 the rpm should be the same considering the rpm loss with the diameter and going to a 4 blade.

 

[Texasmark]

Re: 3 to 4 blade

You are correct "should be" but only a real test will tell the tale.

Mark

 

[jestor68]

Re: 3 to 4 blade

A 1/4" of diameter change will not affect rpm like you think it will. That's like saying a 1/4" of pitch will change 200 rpm.

A 1/4" of diameter will cause maybe a 50 rpm difference.

Have you not figured out by now how well Solas aluminum squeeze cast props perform? The Solas 4 blade squeeze cast aluminum prop will perform just as well. Reduce one inch of pitch compared to the 3 blade to MAINTAIN RPM.

Since you're not turning enough WOT rpm now, I'd recommend a 19 pitch Solas HR Titan 4 or Rubex 4 to get your rpm up to around 4500-4600 and provide better water sports performance(if that's what you're trying to accomplish).

 

[Ridewright79]

Re: 3 to 4 blade

Yeah I fixed my original rpm issue with the distributor. I put the stainless 21p on yesterday and run 54 mph at 4500 rpm trimmed. I have a solus alum 4 blade now and it's a 21p. Obviously it dropped the wot rpm to 4300. It was somewhat of a test prop to see if I would like the 4 blade performance and love it. I was just concerned with the difference of a stainless vs aluminum. They make a 20p ss and that should take me back to 4500 rpm considering my results from yesterday. Everything besides the wot I love with the four blade.

 

[Ridewright79]

Re: 3 to 4 blade

As far as the rpm change from diameter that was kinda strange for me. I read that on a couple of websites. I could see going a inch change would make a difference. Just what I read thanks for clarifying that for me.

 

[Alwhite00]

Re: 3 to 4 blade

This was from my last boat but here is some info....

I recently switched from a 3 blade aluminum 14.25 x 21 to a stainless 14.125 X 19 Boat has a 260 V8 / Alpha 1 - it's a Sea Ray Seville 210 MC

Aluminum 3 blade prop - 1,000 rpm (no wake speed) 3.3 mph 3,500 rpm - 25 mph and topped out 4,400 rpm 40 mph - Very slow out of the hole (I'm guessing 10+ seconds to get on plane)

Stainless 4 blade prop - 1,000 rpm (no wake speed) 4.6 mph 3,300 rpm - 25 mph and topped out 4,500 rpm 41 mph - Jumps out of the hole (I'm guessing 5 seconds to get on plane)

All speeds are per the GPS

All in all I am EXTREMELY happy with making this swap, It's like I have a new boat. - I see no down side in this case. I know this will not be the same result for everyone but I thought some would be interested in the information.

LK

 

[Ridewright79]

Re: 3 to 4 blade

Good info. I know the 4 blade made a big difference in hole shot and cruising speed. I am leaning to the 19p just to keep my rpms. Thanks for the input. All boats respond alittle different based on weight, hull design and Hp.

 

[Ridewright79]

Re: 3 to 4 blade

Ok found a friend with a rev4 19p to do a test run. Very impressed. Kept my rpms at 4600 wot and tops out at 58 mph on GPS. Noticeable grip on hole shot and excellent response. Cruising speed is about the same as the Amita but seems to be more effective over 50 mph. Now just have to make the purchase.