Changed prop and lost power?

[Frank115]

I have a 18' bowrider with a 1979 115hp I6.The old prop was a Michigan wheel 13x19 and the top speed was 42mph@5500rpm.I was sold a new michigan vortex xhs 13x19 part 3992105 and now top speed is 36 mph@5300.Now the boat wants to stall when i start to pull a skier.both props are the same size .does this sound right?new prop is also 1/4inch from the trim tab.Thanks for your help

 

[walleyehed]

Re: Changed prop and lost power?

Was the old prop 1/4" from the trim tab?

 

[walleyehed]

Re: Changed prop and lost power?

I run your numbers and they don't add up... the original prop (19P) at 5500, showing 42mph is 15% slip....not bad.
The new prop (19P) at 5300, showing 36mph is almost 26% slip....not good, infact, bad.
How are you measuring speed, and have you verified the tach?

 

[Frank115]

Re: Changed prop and lost power?

The old prop was 1/2inch away from trim tab.the newer prop the blades are more swept back. tach has not been checked but has always work o.k.

 

[Frank115]

Re: Changed prop and lost power?

also i have 2 speedometers on boat and they read the same.thanks for helping me.maybe i should use old prop.

 

[Crownie2]

Re: Changed prop and lost power?

Could you remove the trim tab and try it just to eliminate it as a problem?

 

[Texasmark]

Re: Changed prop and lost power?

Ok, lets get some more info.

The orig prop; was it al, SS, or Bronze?

I see the al is cupped (pulled it up on the www). Was the orig cupped?

Both 3 blade?

Are either ported? Can't see that the al is.

Do you have PTT on your engine? This is very important as if one of the props has more bow lift than the other, without PTT the boat could be plowing and that could be your problem.

Compare the leading edge of both blades. Are they similar in thickness, or is the old one much thinner?

The al is said to be high rake and progressive pitch. That rake may be more than your orig and that could be the reason for the 1/4" tab spacing. Don't know if your orig was high rake and progressive pitched or not. Gonna guess no.

I think your slip numbers are correct and are caused by prop efficiency, for lack of something else to blame it on.

I think my SS Ballistic was much much more efficient at WOT than the al Hustler. However, the Hustler was more efficient at the lower rpm's and the Hustler was ported, Ballistic wasn't. Ports help you oudda de hole with da skiier.

What happened to you is similar to what happened to me in going from a Ballistic SS to a Hustler al. After lots of thought and looking around the site (and following Kenny's conversations with me and others), I am about convinced that at WOT the al is not as efficient as the SS.

That means I loose effective hp which drops my rpm's and speed. Exactly what happened to me. I lost 600 rpm's and 6 mph with the Hustler and it was 1" less pitch (and slightly larger in diameter).

However, my trim cannot go all the way up (to where I start loosing speed) because my transom has a 25 degree dead rise. So maybe I was getting less bow lift with the al and I was dragging boat. Had I been able to trim out more I maybe could have recovered my rpm's and hence speed.

Course I was running 50+ mph with a 90 hp on a 17' boat at 5600 rpm's. Maybe I shouldn't have been going that fast with that setup.......maybe I had my cake and was eating it too. 8)

Get back with the answers.

Mark

 

[walleyehed]

Re: Changed prop and lost power?

The question I have is "do we have a higher Rake prop design, or do we just have a prop with more cup around the tips?" Obviously 2 very different designs, but the numbers make no sense at all...a higher efficiency prop should drop slip...this one went the other direction.
And, as Mark asked...did we go from steel to aluminum?

 

[Frank115]

Re: Changed prop and lost power?

Both props are alum. older prop blades seemed rounder and not swept back like the new one.i have power trim on boat and engine trimed for highest rpm at top speed.Seems like blades are a little longer.now my engine wants to stall when i start to pull a skier and my top end dropped rpms.do you think this new prop is taking a bigger bite and causing more resistance?i think new prop has deeper cups and more rake

 

[walleyehed]

Re: Changed prop and lost power?

Higher resistance, more cup, and possibly even Rake depending on design, all lead to a more efficient prop...In your case the slip went up by around 10%. None of this makes sense...the old prop with, as you say "round ears" is an older design we don't see "alot of" anymore, but the new prop dropping speed and RPM leads me to a mis-matched set-up.

 

[Texasmark]

Re: Changed prop and lost power?

My 2c. He is loading his engine with the higher performance prop. It's eating more water than the orig; more efficient....like the effect (on the engine) he would have going up an inch or two with his old prop.

This loading up drops engine rpm's which probably causes his speed to drop off and in doing so more boat is in the water which is the reason for his 36 mph number.

On my boat, very light, padded alum, I can trim the bow down (plow.....increase the wetted area) and cut 10 mph off top end and hold right at the same rpm's. Since at WOT and trimmed out my slip is so slight, my (halffast....grin) Ballistic just can't handle the extra load and slips.

I know my slip changes because I can be putting along at 35ish and slam it to the firewall. The rpm's immediately go to 5600 and then the boat picks up speed to 50. Rpm's hold constant the whole time.

So the numbers may be confusing because the boat (load) is changing on us and throwing in an unknown variable affecting slip.

Mark

 

[walleyehed]

Re: Changed prop and lost power?

10% gain in slip is excessive.

 

[Frank115]

Re: Changed prop and lost power?

So what do you recommend? don't want to use old prop,should i get a different size newer style prop?What scares me is if i go to a 13 1/4x17 it might hit the trim tab which i don't want to take off.you guys are great help

 

[walleyehed]

Re: Changed prop and lost power?

The MWC P/N you give at the top for the new prop is a 13-3/4X15P.
This fits 36mph at "6300" RPM not 5300.
anyway, check your P/N...it shows to be a 15 pitch.

 

[Texasmark]

Re: Changed prop and lost power?

If the trim tab is in your way there are a couple of fixes. You can notch it, as I did, or you can buy a thin one at your Merc dealer, or you can buy an offset one on this site. Dhadley I think posted it once.

Mark

 

[Frank115]

Re: Changed prop and lost power?

o.k thanks .i'm going to run both props again just to check.i'll get back with the results

 

[walleyehed]

Re: Changed prop and lost power?

Frank, can you double-check you numbers? The P/N you give for the new prop is 15 pitch....

 

[Crownie2]

Re: Changed prop and lost power?

Frank, can you double-check you numbers? The P/N you give for the new prop is 15 pitch....

I found the data sheet on this prop on the Michigan site and it says it is a 13" x 17" al Vortex for 4 1/4" hubs.

That still doesn't explain the numbers...

Bob

 

[walleyehed]

Re: Changed prop and lost power?

I've got the book right in front of me and I did make an error in the numbers...it's not a 15...or a 17..in print it's a 13X19 alum.

 

[Frank115]

Re: Changed prop and lost power?

the box said 13x19..i will try to test again this weekend