Which prop is best for multiple footers?

[Michigan boarder]

We have four barefooters on our team and we all like some speed.  Our team boat is a 2019 Txi and our driver is instructed to bury the throttle when it's one or four of us behind the boat, he says it runs 46mph but it doesn't feel like it.  I don't know what prop is on it though.  I can see us using my Echelon occasionally and I am thinking of putting a different prop on it, since we rarely weight the boat down heavily anymore and mostly use it for everything except surfing.  What prop will get me some more speed?  Here's what I have:

Engine: 388 stroker,

383hp at 4,800rpm

460ftlbs tq at 4,200rpm

(445ftlbs tq at only 3,000rpm)

Here is the performance with an ACME 425 prop 13x13 .80 cup with nobody behind the boat and just driver & spotter:

RPM      MPH

2080      20

2355      25

2785      30

3180      35

3760      40

4900      49 WOT

I don't know what the speed is with me footing, or multiple people, my wife just rolls the hammer down once we are on plane and off we go.  I know I can call ACME, and I probably still will, but thought I'd ask the crew first.  Should I put the 515 on for more speed?

[jk13]

Did Sarge give you a max RPM you should run that built motor?

[justgary]

1 hour ago, jk13 said:

Did Sarge give you a max RPM you should run that built motor?

If Sarge will allow it, you might want to try a 13x12.75 or a 13x12.5 to get the RPM up to maybe 5200 or so.  You can play with cup also, but it looks to me like you could use just a touch less pitch.

I foot longline at 42 or less.  I'm not a big guy, but I can't imagine going 46....

[Michigan boarder]

14 hours ago, jk13 said:

Did Sarge give you a max RPM you should run that built motor?

No, but I think the limiting factor is the intake, up to 5,400 RPMs would be about it.  During its first water test with the original Dymex prop we were hitting 5,300 RPM's.

[Michigan boarder]

13 hours ago, justgary said:

If Sarge will allow it, you might want to try a 13x12.75 or a 13x12.5 to get the RPM up to maybe 5200 or so.  You can play with cup also, but it looks to me like you could use just a touch less pitch.

I foot longline at 42 or less.  I'm not a big guy, but I can't imagine going 46....

46 feels the best!  We have footed as low as 38, and actually at around 30 during our one-act-show act (which is brutal).

I talked to a friend of mine last night and he's got a 515 on the shelf, I might just put it on and see how it runs.  Or, I suppose I should first get some GPS readings of some footers behind my boat and figure out exactly where we are at with the current prop, that's probably the best idea to start with.

[justgary]

1 hour ago, Michigan boarder said:

No, but I think the limiting factor is the intake, up to 5,400 RPMs would be about it.  During its first water test with the original Dymex prop we were hitting 5,300 RPM's.

So how fast did it go with that prop?

I really don't know how much extra stroke you get from the tall crank, but it does make the pistons move faster for the same RPM compared to a standard crank.  That would be the only thing I could see that might make you want to limit the RPM, but the ECM will do that for you at some point.  I think the rev limiter will kick in at 5500, but I'm not sure on that engine. 

[Michigan boarder]

1 hour ago, justgary said:

So how fast did it go with that prop?

I really don't know how much extra stroke you get from the tall crank, but it does make the pistons move faster for the same RPM compared to a standard crank.  That would be the only thing I could see that might make you want to limit the RPM, but the ECM will do that for you at some point.  I think the rev limiter will kick in at 5500, but I'm not sure on that engine. 

It's a carbed engine, so it doesn't have an ECM or rev limiter.  Good point on the RPM limits though, and creates some reservations for me.

Here is the data with the original prop and the updated engine (GPS):

RPM      MPH

2290      20

2650      25

3130      30

3750      35

4450      40

5330      45 WOT

[justgary]

Interesting.  You didn't say what pitch the original prop is, but it seems like it is just on the other side of where you want to be.  It might not lose as much speed under load.  Perhaps a bit of cup on that one....

And I knew you had a carb, so I'm sorry about that. 

I used to think 42 felt right until I spent some time on a short line.  38 is fine now.  I have gone as low as 29, but I had to concentrate to keep everything above water. 

[TomH]

My $0.02; you're fighting a losing battle running multiple barefooters behind an inboard, and I have a hard time believing you were truly at 46 mph with all 4 footing (maybe while still on skis, but probably not after a step-off).  If this is for your ski shows, find a way to reorder the acts to use the twin-rig and you'll get all the speed you need/want.  We used to fairly easily pull 11 guys with twin 150's, and have run up to 15 on the triple 150.  

[Woodski]

@Michigan boarder - looks like the Dymex is slipping a fair amount (no surprise) and the Acme 425 might be just under the optimum when pulling extra ballast (footers) as it appears to hit the wall at 4900 right after the power peak.  I would certainly try either a 515 or 449 if available to cross check all three data sets.  Also, a good test will be to add a footer and plot the data to see which one slips more or can pull peak RPM and perhaps with slightly less pitch it will pull a bit more RPM.

[Michigan boarder]

On 8/16/2019 at 12:43 PM, TomH said:

My $0.02; you're fighting a losing battle running multiple barefooters behind an inboard, and I have a hard time believing you were truly at 46 mph with all 4 footing (maybe while still on skis, but probably not after a step-off).  If this is for your ski shows, find a way to reorder the acts to use the twin-rig and you'll get all the speed you need/want.  We used to fairly easily pull 11 guys with twin 150's, and have run up to 15 on the triple 150.  

We are OK with 4 footers behind the Txi, it's a little soft when there's some chop but when it's glass it's fine.  I agree, doubt it's 46 with all of us, but with just me behind it feels about right, it's fast and the water is hard.  That's why we've asked the driver to simply hammer down and don't worry about the guage.

For our shows we have multiple footers behind the twin rig (twin 250's) so we have plenty of power.  The down side is holding speed steady at that speed is difficult for our driver, and it seems to vary 2 or 3 mph either way during the pull, which is really tough.  We only have 2 acts where the Txi pulls a footer, and it's just me around the jump or it's me being pulled while pulling a jumper via strap then stepping off (jump/barefoot choo choo).

So really this is about the Echelon, which is being looked at as a separate training boat, but this year is quickly slipping away (last ski show was Saturday) and I just destroyed my barefoot suit, so I'm bailing on this and switching to slalom to finish the year.  I'll revisit next year.

[Michigan boarder]

11 hours ago, Woodski said:

@Michigan boarder - looks like the Dymex is slipping a fair amount (no surprise) and the Acme 425 might be just under the optimum when pulling extra ballast (footers) as it appears to hit the wall at 4900 right after the power peak.  I would certainly try either a 515 or 449 if available to cross check all three data sets.  Also, a good test will be to add a footer and plot the data to see which one slips more or can pull peak RPM and perhaps with slightly less pitch it will pull a bit more RPM.

Yeah the original Dymex (13x13) was slipping, I could feel it when rolling the throttle.  I think I'll try the 515, change it on the lift, and see how it feels next year.

[Michigan boarder]

On 8/16/2019 at 12:28 PM, justgary said:

Interesting.  You didn't say what pitch the original prop is, but it seems like it is just on the other side of where you want to be.  It might not lose as much speed under load.  Perhaps a bit of cup on that one....

And I knew you had a carb, so I'm sorry about that. 

I used to think 42 felt right until I spent some time on a short line.  38 is fine now.  I have gone as low as 29, but I had to concentrate to keep everything above water. 

The original was 13x13, but when side by side with a new Acme 13x13 they were a lot different.  Anything under 30 is crazy, it was just a steady waterboarding for me, but I just had to make it a short way thru the show area.  Hardest part was I could not see anything and didn't want to collide with the jumper to my left or the pyramid to my right.

[justgary]

I think I would definitely play around with cupping the Dymex.  Back in the day, I would cup every prop I could get my hands on just to see the difference.  A small hammer and a trailer ball works great, and you can even leave the ball on the drawbar.  Just go slowly and do all three blades the same.  Start about 1/3 of the way out the trailing edge, and end about 1/3 to 1/2 of the way around the curve.  If you picture the prop rotating around the hub and ignore the shape of the leading edge, it's pretty easy to see where the cup should be.

[Michigan boarder]

38 minutes ago, justgary said:

I think I would definitely play around with cupping the Dymex.  Back in the day, I would cup every prop I could get my hands on just to see the difference.  A small hammer and a trailer ball works great, and you can even leave the ball on the drawbar.  Just go slowly and do all three blades the same.  Start about 1/3 of the way out the trailing edge, and end about 1/3 to 1/2 of the way around the curve.  If you picture the prop rotating around the hub and ignore the shape of the leading edge, it's pretty easy to see where the cup should be.

You have better skills than I do!  I have it as a spare for now, the most I'll do is sell it as is, maybe to help pay for a 515 or 449 if those test well.

[Woodski]

@Michigan boarder - as JustGary noted, adding cup is actually pretty easy on a nibral prop and I would also add cup to the Dymex.  I don't think the Dymex is quite as efficient as the Acme, but worth playing with.  I also use a straight 1/2" round bar on a prop with a straight section that needs cup.  Another trick is to use a small sand bag and always go slow.  Use a straight edge to measure the gap to understand how much cup you added, or to get all three blades the same.