RPM Gain? Alum. vs. Stainless.

[andy1canada]

Howdy!

I'm on the cusp of dropping some $$$ for a new SS/prop for my 19'-cuddy.
I'm almost done breaking in a new Merc 4.3/MPI/Alpha. I was advised to wait until the break-in period was over because the engine would likely 'loosen-up' a little in the process. Apparently it has.

I'm currently swinging a stock Merc 14 1/2 x 18 -alum/4blade.

Initially she was hitting 47 to 4800/rpm @wot and now she's creeping closer to 5000 at wot. She has decent holeshot and is hitting about 45 mph trimmed up with three on board. I'm happy with that as she's intended a general purpose runabout for ski/fish/tube/cruise.

I like the way the 4-blade seems to 'grip' the water and it also seems to lift much of the boat out of the water when we're moving at a good clip. Although I've not tried a 3-blade to make any comparison.

The boat bottom is still quite rough and I'm certain once that's addressed and I find the right prop for her she'll move her 4000/lbs near or better than 50mph. I'm leaning towards a 'vented' prop to maintain or improve the holeshot.

My question is this: If I'm currently on threshold of MAX/rpm's (with the alum) will the switch to an SS-prop of the same pitch and diameter likely push me over and engage the rev-limiter?

From what I've learned it seems reasonable to expect rpm gains of 2 to 3/hundred after the switch to SS. Am I on track here?

If so, can someone recommend a good SS-4 blade (vented) in a 19-pitch?
As per WH's (and T-GUN's) recommendations I was considering a Stiletto BP-4 blade but I don't think they come in a 19p.

I live here on Vancouver Island and Stiletto's are not readily available; though I could order one if needed.

Any and all help greatly appreciated.

Cheers,
Terry

 

[Tail_Gunner]

Re: RPM Gain? Alum. vs. Stainless.

18 pitch Stilletto should drop the rpm to 4800, they have enough cupping to act like a 19 pitch 3 blade ss. Your awful close to a 20, now if you were running a 19 pitch 3 blade alum and touching the rev limiter id say go for it a 20 that is. What has my attention here is your running a heavy load and hittting 5000 which should be hitting the rev limiter, you never hear a slight miss @ wot?

Here's what i would do get the 20 clean the hull and have a go at it, if your running say 4600 with a heavy load that's would be ok one would think. If it drop's you to much, i can tell you Stilletto let me switch for 25.00 as long as i stayed in the baypro 11 line, it should take one 5 minute run to determine if your over propped

The 4.3 in my boat does not have a lot of Tourqe at the top of the rpm band, i have tried a 20 and 22 4 blade BayPro and until you get above 4000 rpm the 22 outran the 20. It hit the wall though @ 4400 rpm @ 53.4 mph while the 20 does 54.5 @ 4900

I never got to caught up as in why, i just accepted the fact the engine was running strong and i was happy with midrange punch and ride qualities, and @ full Wot id get a bit of chine walking and this old man has to remeber his age and where not to go.....

See if WH will respond here, maybe that current prop has some good cupping and a 20 would take you to 4700 could be perfect....

Umm that hull could be causing a lot of drag

 

[andy1canada]

Re: RPM Gain? Alum. vs. Stainless.

Thanks TG!

You have, as usual, been quick to respond to my questions now and in the past.

Again, thanks!

I get the gist of what you are saying to me and NO, the rev-limiter has not kicked in yet. My mechanic brought his shop tack out with him during sea-trials after the new install and we determined that my old tack is within about 100/rpm accuracy. So, in reality, I'm probably now hitting around 4900 at wot. Which is precisely where I'd like to be.

You didn't answer my question about whether or not one should expect any rpm gain when changing (same pitch/dia.) from alum to stainless. I thought, since an SS prop is sharper (less friction) that it would spin slightly faster (than a similar alum) and - if I'm correct in my assumption - that I should add another degree of pitch or so when I go SS in a 4-blade to avoid over revving.

However, now I'm even more confused.

Since I posted yesterday I stopped in to see my mechanic at his buddy's shop - where he also sells and repairs props - to pick up my new trim-indicator gauge; I took the opportunity to bounce a few questions off the owner/operator of the prop-shop.

I described my new engine and the current prop ( a stock alum/merc/14 1/2 X 18p/4 blade) as well as the current performance details. He asked me what I wanted to do with the boat and I explained - fish, cruise, ski/tube ect.

I told him I'd been doing considerable research on props (mostly here on iboats) and had learned a fair bit and liked the idea that a 4-blade will usually lift more of the boat out of the water (less friction) and also hold the water better in turns.

Then I asked him if it was true that an identically configured SS prop would spin faster than an aluminum. He said no. Because they are heavier.

Is this true?

He's recommending - same as my mechanic did - that I go to a 17/pitch/3-blade/SS Mirage. (only $966.00... ouch!) If she revs too high with the 17 he said to bring it back and throw a 19 on her.

He said the Mirage is in a class by itself and would provide better cruising economy, an instant 5-mph top end gain, but might not have as good a hole shot as the 4-blade.

Then I asked him if the Mirage was vented because I've learned that venting a prop enables the engine to bring it up to speed quicker, which usually translates into improved hole-shot.

He then told me that vented props are usually only applicable to outboard engines that need help to get revving from the get-go.

I'm trying to find something useful in what he told me before I dump a bunch of $$$ on a new prop.

Please, if any of you guy's (WH...?) want to dive in, feel free...

Cheers,
Terry

 

[QC]

Re: RPM Gain? Alum. vs. Stainless.

You didn't answer my question about whether or not one should expect any rpm gain when changing (same pitch/dia.) from alum to stainless. I thought, since an SS prop is sharper (less friction) that it would spin slightly faster (than a similar alum) and - if I'm correct in my assumption - that I should add another degree of pitch or so when I go SS in a 4-blade to avoid over revving. . . .

Then I asked him if it was true that an identically configured SS prop would spin faster than an aluminum. He said no. Because they are heavier.

Is this true?

No and No . . .

Stainless props usually drop RPM slightly because they do not flex at high loads like an Aluminum. The weight is not an issue at all. SS will also have slightly slower hole shot for the same reason, but higher speeds because of less slip. Based on the info you have posted I would suggest same everything, but stainless. However, if the bottom is really bad, you have been selecting props based on bad data which is what you have. I can't imagine you will pick up 5 MPH, but no matter what more speed is more RPM . . . Get the bottom right and then start over

 

[Tail_Gunner]

Re: RPM Gain? Alum. vs. Stainless.

Thanks TG!

You have, as usual, been quick to respond to my questions now and in the past.

Again, thanks!

I get the gist of what you are saying to me and NO, the rev-limiter has not kicked in yet. My mechanic brought his shop tack out with him during sea-trials after the new install and we determined that my old tack is within about 100/rpm accuracy. So, in reality, I'm probably now hitting around 4900 at wot. Which is precisely where I'd like to be.

You didn't answer my question about whether or not one should expect any rpm gain when changing (same pitch/dia.) from alum to stainless. I thought, since an SS prop is sharper (less friction) that it would spin slightly faster (than a similar alum) and - if I'm correct in my assumption - that I should add another degree of pitch or so when I go SS in a 4-blade to avoid over revving.

However, now I'm even more confused.

Since I posted yesterday I stopped in to see my mechanic at his buddy's shop - where he also sells and repairs props - to pick up my new trim-indicator gauge; I took the opportunity to bounce a few questions off the owner/operator of the prop-shop.

I described my new engine and the current prop ( a stock alum/merc/14 1/2 X 18p/4 blade) as well as the current performance details. He asked me what I wanted to do with the boat and I explained - fish, cruise, ski/tube ect.

I told him I'd been doing considerable research on props (mostly here on iboats) and had learned a fair bit and liked the idea that a 4-blade will usually lift more of the boat out of the water (less friction) and also hold the water better in turns.

Then I asked him if it was true that an identically configured SS prop would spin faster than an aluminum. He said no. Because they are heavier.

Is this true?

He's recommending - same as my mechanic did - that I go to a 17/pitch/3-blade/SS Mirage. (only $966.00... ouch!) If she revs too high with the 17 he said to bring it back and throw a 19 on her.

He said the Mirage is in a class by itself and would provide better cruising economy, an instant 5-mph top end gain, but might not have as good a hole shot as the 4-blade.

Then I asked him if the Mirage was vented because I've learned that venting a prop enables the engine to bring it up to speed quicker, which usually translates into improved hole-shot.

He then told me that vented props are usually only applicable to outboard engines that need help to get revving from the get-go.

I'm trying to find something useful in what he told me before I dump a bunch of $$$ on a new prop.

Please, if any of you guy's (WH...?) want to dive in, feel free...

Cheers,
Terry

That put"s a different light on the subject, after a clean hull the 18 pitch ss might be too much.

 

[andy1canada]

Re: RPM Gain? Alum. vs. Stainless.

Howdy,

Thanks again guy's.

QC... I know I need to clean/paint my bottom (the boat's bottom) but it's not in the cards right now. I'm too busy with work and my holidays are coming quick. I don't really want to head off without a spare prop; the one on there now will be the spare when I replace it.

All I'm trying to do is make a reasonably informed choice on a new prop based on the current performance of the boat.

I can't believe how expensive these SS props are.

Cheers,
Terry

 

[Tail_Gunner]

Re: RPM Gain? Alum. vs. Stainless.

Is there a big import fee your dealing with if so PM me

 

[QC]

Re: RPM Gain? Alum. vs. Stainless.

I can't believe how expensive these SS props are.

I just see you needing yet another SS prop when the bottom is right. I think an 18 SS will bring you down some. What about a vented 19? This should give you decent hole shot with most if not all of the vents open and maybe towrads the bottom of the RPM range with the bad bottom. Then if the bottom makes a big difference you may not overshoot the 4800 . . .