Looking for a 4 blade prop. Am I on the right track?

[supr_fly03]

Here is the information:

1) I have no performance issues really, but I would like better hole shot for pulling since that is what my boat does 70% of the time
2) Current blade is the stock Mercruiser 3 blade aluminum.
3) Current prop is a 21p x 14.25 (might be a 14.5).
4) Wide open throttle RPM 5000 (according to tach) and 48mph (according to gauge) with an average load of two adults, and two to three kids.
5) 5.0/Alpha 1 Gen II, 1.62 ratio, 1999, and 220HP.
6) Boat is a 1999 StarCraft 2010GT, 20 ft long and I am unsure of the weight

Last year I hit a crab pot and trashed the edges of the stock blade. I dont want to have it reapired since props are on sale and only $30 dollars more for new. I would like stainless, but cant afford it.

I was thinking about a Solas Amita 4 blade aluminum prop in 21p x 14 flavor.

Does that sound like a good fit? What is the difference between a Amita blade and a Rubex blade?

I am not worried about losing a couple of MPH up top since most of my time on the water is either cruising or pulling tubes/skiiers. I hardly ever run it at top speed.

Thanks in advance!

 

[Mordekai]

Re: Looking for a 4 blade prop. Am I on the right track?

What's your max RPM?
Here are my numbers.
Merc Cruiser 3.0 Alpha 1 Gen 2 2.0 Ratio

Stock 14 x 21p 3 blade from the factory
3500 RPM 25MPH
4000 RPM 30MPH
4800 RPM 40MPH WOT

Solas Rubex 14x21p 4 blade aluminum (I suspect this prop is lugging my engine a little bit)
3500 RPM 30MPH
4000 RPM 35MPH
4400 RPM 40MPH WOT

Solas Rubex 14x19p 4 blade aluminum
3500 RPM 25MPH
4000 RPM 30MPH
4800 RPM 40MPH WOT

Impressions:
The stock 3 blade sucks, vents easily during turns, no stern lift and slow to get on plane.

The solas 21p was my first foray into 4 blade props. However it is too much prop and was lugging my engine a little bit with 4400RPM WOT, great hole shot however.

The Solas 19p gave very similar RPM/MPH characteristic to the stock 3 blade prop BUT I could not get it to vent during hard turn or acceleration, so the extra blade is doing its job.
Doing WOT take off, the boat jump out of the water like a stabbed rat. Over all, this is the blade I will keep as my main one.

 

[craze1cars]

Re: Looking for a 4 blade prop. Am I on the right track?

I think you'll find you need to drop to a 19 pitch if you want to maintain your 5,000 rpm in a 4 blade.

Extra blade usually requires you to drop some pitch or it will slow the motor down.

Of course I believe you are over-revving a hair...isn't your rig supposed to be 4,800 max? So maybe dropping some RPMS is not such a bad thing. In which case staying with a 21 might be smart. But if you do so and rpms drop too far, it may negate any gain in holeshot that you would have otherwise felt with the change from 3 to 4 blade. I don't think you'll lose any holeshot, though.

And the other school of thought is that you are over-revving becuase you're running a chewed prop that has lost efficiency and is therefore slipping more. So you may drop more RPMS than you think going to a 4 blade without any damage...so if that's the case, I wouldn't be surprised if you spin 4500 or 4600 with a new 21 pitch 4 blade...

I believe Amita & Rubex blades are identical. Only difference is in the hub...Rubex interchanges and Amita is fixed. Someone more knowledgeable on Solas stuff may correct me on that...

IMO you're in the ballpark. Try it and see if ya like it. But don't be too surprised if you wish for a 19 when all is said & done...and then find the 19 spins too fast...

That's what happened to me! I had a 19 that slightly over-revved, and an identical 21 that slightly under-revved. Of course the same mfr didn't make a 20 so I could split the difference. So I just paid to have my 21 repitched to a 20 and I picked it up from the shop today. In theory I am now going to be perfect but we shall see after a test-drive...but assuming I have finally succeeded in reaching my target RPM I had to buy 2 stainless props and then pay extra to have one repitched in order to get there LOL...only about $800 so far...Thankfully I will have one that can be sold very soon, and then I will get some of my money back AND will have a perfectly tuned prop for my ride...

 

[supr_fly03]

Re: Looking for a 4 blade prop. Am I on the right track?

Well keep me posted on your performance. My max RPM is 4800. I was thinking of sticking with the 21P blade. I am afraid of over revving the motor on the high end when I do let it hang all out. But then again, when jerking people around on a tube or ski's I will probably be wishing I had gotten a 19p blade. In all honesty, the tach actually has hit 5200 several times but I was not sure if it was correct because sometimes it tachs all the way out, then settles back down.

I like that stats you posted and that makes me kinda steer to the 19P as I believe it will give me the best performance for my boat usage. I guess I would have to watch my tach.

 

[craze1cars]

Re: Looking for a 4 blade prop. Am I on the right track?

Oh....well if you're at 4,800 that's different. Your original post said 5,000....

So if it's consistently holding 4,800 with a nicked up 3 blade now, then I REALLY think you should just get a 19 pitch 4 blade as a replacement. I believe a 21 will pull your RPMS down more than you realize it will. You won't like that.

Your tach is doing what all tachs do when they get to be more than 10-ish years old on a boat...minor internal corrosion causes random drifting. Replace it...glad I did on mine....and my boat is a 1999 as well. No more random drifting, and I KNOW exactly where I'm at. Nice that I also have PerfectPass now as it includes an integrated digital tach...so now I have TWO accurate tachs to confirm where I'm at.