going from al to ss prop?

[jedimaster]

What would you expect the rpm drop to be going from a 19pitch aluminum to a 23 pitch ss?

 

[steelespike]

Re: going from al to ss prop?

Without info on boat, motor,normal load,present rpm and speed it is difficult
to be accurate.Not an expert but I would think the drop would be in the neighborhood of 600 rpm

 

[jedimaster]

Re: going from al to ss prop?

Well its 3.0 mercruiser,
with a 19 pitch 3 blade aluminum its sitting at 4600rpm wot. I have read people run 23 pitch ss 4 blade props with 3.0 mercruisers. Just wondering how far down my rpm's will go. Out of the hole is not a big concern of mine, I have a 17 pitch ski prop for those times. Looking for one to makimize top end. I was thinking of running a 3 blade 21ss or a 4 blade 23 ss any thoughts?

 

[jedimaster]

Re: going from al to ss prop?

also its a 17 foot bowrider.

 

[rndn]

Re: going from al to ss prop?

You need to keep the motor running in the recommended rpm range. Increasing the pitch to a point to where your motor does not reach the recommended rpm range will do serious permanent damage to your motor.

 

[rndn]

Re: going from al to ss prop?

One comparison is to try to take off on a 10 speed bike while the bike is in 10th gear or go up a hill without downshifting. Thats the kind of effort you will ask of your motor.

 

[tommays]

Re: going from al to ss prop?

It would be like draging a few 5 gallon buckets behing the boat

You just getting the RPM with the 19"



Tommays

 

[jedimaster]

Re: going from al to ss prop?

The rec is for 4200 - 4600 at WOT, so should I just be happy with the speed from being at 4600 with a 19 or try and get it to 4200?
What advantages would I get from say a stainless 19 3 blade versus a 3 blade AL 19? I was under the impresion that a stainless will increase the rpm's by about 100 rpm's and a 4 blade will also increase, thats why I was thinking a 21 4 blade ss would be ok, perhaps a 5 blade 23?

 

[Sherminator]

Re: going from al to ss prop?

The stainless steel will bite in the water better and doesnt flex like aluminum does, a 4 blader will help with low speed and midrange but your top end will suffer, maybe loose 3-4 mph.. also same with a 5 blader.


going from alum 19p to an identical 19p ss should help you all the way around. it will hop out of the hole faster, plane out faster, and maybe net you a few MPH on top. but as far as RPMs go, youll have to play with different pitches of props.. but a ss19 is deffinetly going to give you different results compared to a 19 alum. just to keep that in mind.

 

[jedimaster]

Re: going from al to ss prop?

When comparing say a 19 SS3 blade to a 21 SS 4 blade I have read that the 4 blade 21 will have equal top end and better hole shot.

 

[Crownie2]

Re: going from al to ss prop?

No way. IMO, both will suffer.

Your best improvement will be to just get a SS 19"

Bob

 

[jedimaster]

Re: going from al to ss prop?

Ok sounds good, I'll look at 19 pitch ss then, what about a SS 3 blade 19 versus SS 4 blade 19?
better hole shot with the 4 blade and worse top end? or worse both?

 

[ParallaxBill]

Re: going from al to ss prop?

Here's some actual test that may give you an idea of what changes happen when trying different prop metals and pitches.

My boat is an 18 foot trihull powered by a Mercruiser 165 I/O

With a Mercruiser SS 13.75" x 21 pitch it will run 3,900 rpm WOT @ 43 mph.

Same load and conditions,

14.5" x 19 pitch the results are 4,300 rpm @ 42 mph.

With the smaller diameter but longer pitch ss prop I gained a little speed, get better ride quality (can trim up the nose higher for a better ride without cavitation) and get a better hole shot. But I'm only at the bottom of my recommended WOT rpm range of 3,900-4,300 rpm.

In my opinion a direct 19" pitch swap from aluminum to SS you will gain a little speed but lose a little rpm, maybe 100-200 which should suit your boat just fine.

 

[ParallaxBill]

Re: going from al to ss prop?

I'm actually thinking about a 14" x 20 pitch 4 blade as my next prop. Because I want more cruising power but still want to run close to my WOT rpm of 4,300.

 

[Crownie2]

Re: going from al to ss prop?

Bill -I am not sure what you mean be "cruising power.

I am assuming you know that your top speed will be down somewhat and that rpm's at a certain cruising speed will be somewhat lower (which some people like) and your acceleration may be a little slower.

Keep in mind that four blades usually offer more stern lift which suits some boats but not others...

Bob

 

[ParallaxBill]

Re: going from al to ss prop?

What I mean is more efficient performance at cruising speed 20-30 mph. Stern lift will help in this area and I'm thinking the way to get it is going to a 4 blade. I'm wanting my boat to be able to run wot in the rpm range it should (per factory specs) while maximizing cruising speed efficiency.

WOT means nothing to me other than to establish the proper rpm range for any particular boat. It has to be right or the boat will only be efficient in a narrow rpm range.