Prop - Part II

[skibug]

I have a 2003 Malibu RLX, 335 Monsoon Engine.  It is a great ski boat.  That said, I also ski behind a 2005 SN 196; which is also a great ski boat for other reasons than my RLX. 

Skiing behind the RLX - 3 blade CNC, my baseline RPM @ 34 is 2900, really low RPM, which is probably why the pull is so soft.  I run my KX @ + to firm it up a little bit. The wakes are the soft and flat, almost unnoticable.  It is more forgiving than the '05 SN 196.  It seems to give a little more (i.e I can pull it around more) than the '05 SN 196.  This can be good and bad.  The bad part is I seem to get a little more slack at times; and, I think that is because of the ability to move the boat and the pull being softer.  The good part is that when you are scrambling (which is probably more often then not) it lets you, without punishing you.

 Skiing behind the SN 196 - 4 blade prop, baseline RPM @ 34 is about 3700, less forgiving; but, when skiing good, line stays tight, KX @ n.  I can't move the boat as much, wake is a little more noticable, harder/taller.  I have had my (2) best ski sets ever behind this boat.  Don't know if that is a coincidence or not.

 All that said, would going to a 4 blade prop on my RLX firm up the pull and keep it from being pulled around as much?

 Any comments, thoughts, or first hand experiences?

[eleeski]

I replaced the 4 blade on my old Nautique with a 3 blade (prop damage not premeditated). It slalomed great on either prop. Three bladed props give a better trick table. I wish Nautique would go with a 3 blade prop and a bit of tuning of the back of the hull to make it a great trick boat again. According to the popular whining, GPS has killed slalom anyway.

 

Eric

[DW]

2900 rpm is probably a little below the peak torque level for that engine.  First thing would be to take out a little pitch on the prop, or reduce a bit of cup.  I would shoot for more like 3200 - 3400 rpm at 34 mph.  That can be easily accomplished with a 3 blade prop.  Running in an area under the power curve with more available torque, you will pull down the rpm less and the pull will be more firm.  The same thing would happen if you change to a 4 blade prop, and have it spin at 3200-3400 rpm at 34 mph. 

[ForrestGump]

The boat rpms at 34mph do seem odd. The malibu is wayyy low.  With that same engine on the one we ski behind, we're seeing 3450 at 34mph.  And my 05 Nautique 196 is 3400 at 34mph with the 330hp engine and stock 422 Acme 4 blade prop.

[skibug]

I re-confirmed the RPMs on the 05 SN to be 3450 not 3700.  My mistake. 

 Not that this would make a huge impact; but, I do run with 70 lbs in the nose of RLX.

 

DW, thanks for the input.  Which would you try first, pitch or cup, and how much?  Also, I am assuming this can be done, by a propeller shop, to my existing 3 blade prop? 

[eleeski]

My old American skier's engine got so tired it wouldn't pull 36. So I bought a new boat from a high altitude owner. He had a prop pitched to run a few hundred RPM faster. Since my lake is below sea level, I didn't need that - so I switched to the recommended prop on that boat. I moved the high altitude prop onto my American skier. Was I bummed when the American skier would once again pull nice 36s! I just wasted a bunch of money when a couple hundred dollars in a new prop would have done the trick.

Most boats seem to run 100:1. 36mph is about 3600 rpm. The peak of the power curve is probably 4500 rpm. There is a lot of room for rpm increases. I reprop a bit higher as it makes tricking more responsive. My PP classic Nautique runs about 3600 rpm at 34 and feels much better than my PP classic MC running 3300 rpm - but there are a lot of other variables. And Stan's new ZO MC feels as good as the Nautique (but I don't have to think about rpms at all so I'm not sure where that runs).

Throw a new higher rpm prop on your boat. If the boat doesn't feel any better you'll at least have a spare prop to save the day when you ding it.

Eric 

[DW]

Power peaks on the newer engines are around 4800-5000 rpm (including Monsoon, Hammerhead is a bit higher), with torque peaks at 3800-4000 rpm (which is what you feel).

If you are looking to raise your engine speed from 2900 to 3400, you will need to reduce pitch.  You won't have enough cup in the prop to take out to get you where you need to be.  The prop manufacturer can actually tell you how much pitch you will need to remove to accomplish your change, I am going to make a guess that the prop is an Acme, it should be a 515 for that boat.  If so, Bill Weeks at Acme is a very helpful guy.  500 rpm will take a fair amount of change. 

You might also want to check the accuracy of your tachometer before you repitch the prop.  If a buddy has pretty much the same boat, you can take a ride and listen to the engine pitch to see how different they are as a quick check.

[skibug]

I did call ACME earlier this morning and talked with some very helpful and knowledgable gentleman "Jim".  He told me I need a 515 - 13X12.  I currenlty have a 13X12 5/8; which was typical on the 03 RLX.  He is quite certain this will give me the extra 300 - 400  or so RPMs that I need.  I have had the prop refinished once about 2 years ago and he seemed to think between the original pitch and the strong possibility that the cupping was altered in the refinishing process, this would account for the lower RPM.  He did inform me that the wakes would proabably get a little firmer.  Anyway, thanks to everyone who responded with the good advice.

 

[skibug]

Check that last entry.......it is all back to ground zero.  I have a 515 13X12 on my boat, just checked it.  I am back to where I started; but, I will be calling ACME tomorrow.

[boarditup]

The Acme 911 would be my choice for your application.  We use it for wakeboarding applications on your boat.  However, given what you told me, it should be what you need to move the motor into the rpm range you desire.

 www.boarditup.com