Acme 668 or 422

[TRBenj]

Starting with a 668 prop if you had to just venture a guess based on our conversations what do you guess would be the proper cup for a that prop up here in Utah?

Oh geez, no idea how to answer that question. Too many variables to even wager a guess- hull, engine, prop, load, ambient conditions. It all comes down the numbers. Get those and we can go from there.

In order of most important to least important, when propping your DD to maximize top speed (which usually results in best all around performance, too):
1. Get WOT RPM to match the hp peak of the motor as closely as possible
2. Select a prop with more pitch in place of more cup (.105 or less cup, preferably)
3. 3-blade over 4-blade, if available

I dont think you bought the wrong boat, btw... for a family who does it all (including skiing), I think its a great choice. 409 is a great engine!

Nothing wrong with optimizing performance either... especially something as simple as selecting the best prop for your boat and use. Many others have taken it to an extreme- trying to make these watersports tractors run well beyond their intended operating speeds. Of course, I dont think theres anything wrong with that either!

[NCH2oSki]

This boat is purer recreation, with maybe an equal use between skiing wake boarding and even wake surfing. Although now that I own a boat, finding a club or private lake to run buoys is something I would like to look into.

Before anybody says anything, I know I know, I bought the wrong boat :-). However since it's my boat, and I'm paying for it :-) and I love skiing as my passion that's why I bought a direct drive. I'm the only "purist" in the group At least that's how I refer to myself tongue-in-cheek meaning I'm the only skier. I also just could not get around how awesome direct drive boats drive.

Sounds to me like you bought the perfect boat. I live and ski near the MC factory, and a couple years back I was spending a lot of time behind their 197s. One of the things I found that I prefered as a skier, was that I really prefered the feel of a slower turning prop. I was skiing behind a 197 with Zero off and the ly6 motor, they changed the prop several times on it, and I was really surprised at how much easier it was for ME to ski 34.2 at close to or below 3400 rpms than it was at 3900-4200 rpms. The pull is much softer and more forgiving, with the slower rpm setup.

It was fairly entertaining watching my buddy bang some cup into the prop with a hammer, he offered to do it to my prop also...but I passed.

Probably worthless information, but I thought I'd share.

[jsta281]

Thanks for the insight. I dont think i would let anyone take a hammer to my prop either So when you say it was easier to ski at 34.5 with the slower rpm was that due to the boat not "yanking" you out of the turn? I don't have any real experience with competition level skiing.

[jsta281]

TRBenj,

So it seems we have covered just about all the possible props in my "wheel house" in terms of what might be the best prop for my application. In looking at the Acme list of props there is one other possibility. The Acme 224 is a 13 diameter with 15.5 pitch and .O9 cup. Based on our prior discussions, if diameter has minimal impact on top MPH, then this prop has the same pitch as the 668 but alot less cup. Any speculation on what kind of reduction, if any, in RPM would occur with this prop as compared to the 668 or the 422?

[NCH2oSki]

Thanks for the insight. I dont think i would let anyone take a hammer to my prop either So when you say it was easier to ski at 34.5 with the slower rpm was that due to the boat not "yanking" you out of the turn? I don't have any real experience with competition level skiing.

Pretty much, it felt like the boat was running away with me all the time.

[TRBenj]

TRBenj,

So it seems we have covered just about all the possible props in my "wheel house" in terms of what might be the best prop for my application. In looking at the Acme list of props there is one other possibility. The Acme 224 is a 13 diameter with 15.5 pitch and .O9 cup. Based on our prior discussions, if diameter has minimal impact on top MPH, then this prop has the same pitch as the 668 but alot less cup. Any speculation on what kind of reduction, if any, in RPM would occur with this prop as compared to the 668 or the 422?

Looks to be 1/2" bigger in diameter with .015" less cup than a 422... my guess is that the 2 props would run pretty darn close. Honestly, the 224 wouldnt be on my radar since you have a 668 already... if you just needed a small tweak, you could take some cup out of it to make it run like a 422. If you wanted to go smaller, then there are lots of options in the 15" pitch range.

[jsta281]

I took the boat out for some more test runs. I actually had two props to run, the 668 and the 422. The results are curious. Here is the back story. I took my prop in for a repair. According to the prop guy he repaired it back to factory specs. So I had a repaired to factory 668. While I was there I noticed a 422 on the shelf. He said I could borrow it and try it out. I think it had been repaired too but it must have been a very minor repair because virtually all the tooling marks were still showing. Anyway on to the results.

2009 206 ZR409 I filled the tank with fuel, had 3 men weighing about 190 each and a teenager weighing about 110. I did load some gear in the boat to try and simulate a more realistic weight probably 200 pounds of gear in the boat, distributed in the front 1/2 of the boat.

Prop 668-

Water temp 62, air temp 71

With the hydro gate in the up position (lever down for trick)

36 mph at 3750 rpm
48.9 mph at 5450 rpm
43.4 mph at 4780 rpm

Hydrogate down (slalom mode)
36 mph at 3760 rpm

422 prop
Water temp 62, air temp 71

With the hydro gate in the up position (lever down for trick)

36 mph 3730 rpm
48.4 mph 5370 rpm

hydro gate down (slalom mode)
36 mph 3730

certainly not what i expected based on our discussions.

TRBenj, what say ye?

[TRBenj]

Something is wrong with one, or both of the props... my guess is the 422 is off from factory specs. It should run a ~150-200rpm higher at slalom speeds and at WOT (versus the 668). This one looks to be running fewer RPM, so it either has extra cup or pitch than a stock 422. It should have had you flirting with the 5600rpm rev limiter if the 668 would pull 5450.

Are you sure your hydrogate is functioning properly? Both props ran identical (or near identical) rpm with the gate in the up or down position... that does not seem right. Should be a ~2mph difference if you held the RPM constant and tried both gate positions. You do notice the obvious difference in running attitude when adjusting it, right?

Give me a little more info on where you had the weight distributed... I dont quite understand where you had people sitting.

[jsta281]

Yes the gate is functioning properly, as I was changing props I showed one of the guys with me what it was and how it worked, and yes you can feel and see the nose drop when engaging at speed.

As for the weight, one adult on the back bench the rest of us up under the windshield. Virtually all the gear weight was between the motor and the front of the boat in the walk through area,

[TRBenj]

Very strange that the 36mph rpm's were consistent between gate position... that makes no sense to me.

Id toss out the data from the 422 you tried, I think its off. That 668 seems like its running decent numbers... if you want to try and maximize performance, have a bit of cup taken out of it. I would guess that bringing it closer to the 422 specs (.105 cup) would allow you to pick up some RPM and run closer to 5600. In this case, Id say 5590rpm is about as ideal as you can get- that way you know your revs are maxed out, and youre essentially operating at your hp peak, yet are not up against the rev limiter. Maybe try an incremental step first (~.130 cup) and see how many revs you pick up and then decide whether you want to do another iteration or not?

[jsta281]

Just so you know, the prop guy said the same thing you did about the 422 that being that something must be off about it. He's going to Measure it and then let me know what he finds.

Would you care to speculate how much weight impacts max RPM?

Also, Can you speculate what a difference in 20° air temp would do to RPM All other things being equal?

As I think about it, the way that I had in the boat for this test is probably pretty close to the average weight during regular use with the exception of early-morning ski trips with a buddy where we would unload everything possible.

Hindsight is always 20/20, but I kind of wish I would've run one more test and that is one more TopSpeed run with only me the driver and no extra gear in the boat. Care to speculate on what that would have been mile-per-hour and rpm wise?

[jsta281]

One last request for speculation what would another 1700 feet in elevation do? We commonly spend a week in Bear Lake Utah which is 5900 feet elevation.

[TRBenj]

It has not been my experience to see much change at all with max RPM's when adding weight. Especially if it is evenly distributed. Top speed will be minimally impacted as well... wetted surface remains largely constant. What I have seen is if you move all the weight to the rear, you can pick up a little bit of speed (as much as 1-2mph in these larger hulls) by getting the nose up in the air a little bit. RPM still stays pretty much constant, though. Where you see the effects of weight the most is during the holeshot. Though, if you add a lot of weight primarily in the front of the boat, you'll slow down incrementally (maybe 1-2mph).

20deg of ambient air temps shouldnt have a huge impact either, at least that hasnt been my experience. I do plenty of running at 70*F and 90*F and cant say the boat performs measurably better when its cooler. If youre talking 50* air temps, along with significantly cooler water temps, sometimes you can pick up a little bit of performance (~100rpm). But not a ton.

I dont have a lot of experience changing elevation... cant comment on that much. I do recall one time going to a lake about 500' lower in elevation, and it was early fall (so cooler air and water temps than we see in the summer) and I picked up 200rpm up top. Thats the most extreme difference Ive ever noticed due to ambient conditions.

[jsta281]

Well based on all that I think when I buy a prop I will look at either the 470 3 blade or the 224 4 blade.

The 224 (13 inch dia) has the same 15.5 pitch but the least amount of cup (.09) of all my acme prop options.

I think the 470 would be best when heading to our lakes at higher altitude then 4200 ft (and there are several) but the 224 would be better at 4200 ft.

[jsta281]

It would have been nice to see a 422 "should have" changed. But I don't thing it would be enough of a change.