Acme 668 or 422

[jsta281]

I recently purchased a 206 with the ZR 409 motor. This boat come factory with the 668 which I understand is the same prop as a 422 with added cup.

I am in Utah where the elevation is 4200 ft and the DA can frequent be 7000-9000 ft. For you that may not be familiar with DA it is a calculation used in drag racing (values being actual elevation. air temp, barometric pressure, and humidity). This extremely high DA is a major factor in making power. As an example my Mustang in "bad air" (DA a of 7000+) runs 11.3. In good air here (DA of 4500) runs 10.7. Theoretically, at a DA of 0 or sea level my car could run as low as 10.2ish. (I have never run at sea level).

When I took the boat out this week basically empty with 3 adults am two kids with an outside temp of 65 and a water temp of 59 the boat topped out at 47-48mph and WOT was near the ideal 5600 rpm.

I am speculating that with the boat loaded in a more real world scenario and much warmer temps it is likely that the boat motor won't get the the magic "5600" rpm.

The other issue is if I swap to a 422 does anyone know what "red line" is on these motors and where the rev limiter kicks in?

I have read that not being able to get the RPM's at WOT will both negatively affect hole shot and top end.

I recognize I am asking for the theories and opinions here, so please speculate.

[TRBenj]

Im surprised to hear that the 206 has that much slower of a hull than the 196. Did you, by chance, forget to put the hydrogate up (trick/jump)? 5600rpm on a 409 powered 196 with a 668 and the gate up is easily 52-53mph. I assume youre reading off the ZO tach and speedo and not relying on the other gauges? They can be skewed pretty good at WOT.

The other thing that seems strange is that youre seeing very strong performance (rpm wise) considering your altitude. One would expect to see reduced RPM due to the higher elevation (and lower hp levels). The only other guy I know of with a 409 206 was stuck in the 5300-5400 range... and that might have been with the 422 (not even the 668). He went to an even smaller 3-blade to get the revs up.

There is no "red line" per se. 5600rpm is where the 409 makes its peak hp, and is also the point where the rev limiter will set in. I would agree that you'll see reduced speeds and RPM when warmer temps and higher loads are in play. The 422 will run about 200rpm higher, so if youre RPM's with the 668 come down to the 5400 range next year, going to the 422 would be smart. To make a prop recommendation prior to getting back good data based on these new conditions would be a little premature, however.

Propping a boat to turn the ideal RPM (where the engine makes peak hp) at WOT generally results in the best all around performance. That is to say, a good balance between holeshot and top end. By running a prop that is too large (keeping you from reaching max RPM), you give up top end performance- speed should increase if you can get your RPM to where they need to be. This would entail running a shorter prop (less pitch), which would have the added benefit of improved holeshot. I think this is what you meant with your comment. Of course, a significantly shorter prop will improve holeshot even more- at the expense of top end speed (it would cause you to overshoot your ideal RPM's at WOT, or hit the rev limiter, both of which will lower top end).

Long story short: IF your RPM and speed numbers are accurate, then youre right where you want to be with the 668... report back with new performance numbers when conditions change.

[jsta281]

Ya 48 with the gate up using ZO. It might have still been creeping up but no way was going to hit 50. I attributed the 2+ mph difference from others post to altitude, so that was what I expected. What I did not consider were the favorable weather conditions for making power compared to what I will see in the summer. This was with about 650 pounds of "people weight" but otherwise an empty boat and a 1/2 tank of gas.

I wish I would have more accurately documented the RPMs.

I called ACME and the person I spoke to was surprised I even saw near 5600 rpm with the amount of cup that comes I'm the 668. He actually speculated that the prop had some of the cup taken out by the previous owner.

In regards to the rev limiter, is it for certain that it is set at 5600? I know in cars peak hp usually comes lower in the rpm range then actual red line, and then the limiter is usually set a bit higher then red line.

It seems like the 422 is the best candidate for my application. If I actually had to go "less prop" anyone know what the "next step down" from the 422 would be?

[TRBenj]

I attributed the 2+ mph difference from others post to altitude, so that was what I expected.

2mph due to altitude makes sense... but WOT RPM and speed will track together. Lower hp due to altitude = less rpm = less speed. 5600rpm with a 668 on a 206 is very strong, which should push the boat faster than 48... so Im inclined to agree that something isnt quite right with your numbers. Either the prop has been reworked with less cup (or even pitch), or your RPM figures are off, or some combination of the 2.

What I did not consider were the favorable weather conditions for making power compared to what I will see in the summer. This was with about 650 pounds of "people weight" but otherwise an empty boat and a 1/2 tank of gas.

Cooler water and air will definitely make the boat run faster. Again, speed and RPM will trend together. What youre also missing is the fact that the boat will run even faster with extra weight in it... IF the weight is in back. It will get the nose up a bit and reduce drag. It wouldnt explain a higher RPM unless extra speed came with it, though.

I called ACME and the person I spoke to was surprised I even saw near 5600 rpm with the amount of cup that comes I'm the 668. He actually speculated that the prop had some of the cup taken out by the previous owner.

I agree. Are you even sure its a 668? I could have sworn the other guy with the 409/206 said his boat came with a 422.

In regards to the rev limiter, is it for certain that it is set at 5600? I know in cars peak hp usually comes lower in the rpm range then actual red line, and then the limiter is usually set a bit higher then red line.

Yup, I promise. Throw a smaller prop on there and see where the RPM's peg if you dont want to take my word for it. Bet it runs 5600rpm on the nose. Like I said, there is no "red line" per se... that would simply be an arbitrary number based on your convention anyways. Certainly the 6.0L block can handle more RPM than the 5600 PCM limits it to... if I had to guess, I would say that the lower limiter is due to cooling concerns at sustained high RPM running. Its not a hard rev limiter, btw... PCM controls rpm via the throttle- so the ECM will simply dial back the throttle to keep the revs at 5600 if it would otherwise keep climbing. Very smooth operation- you wouldnt have any idea its up against the limiter based on its behavior unless you looked at the tach.

The fact that RPM is limited at the same point where peak hp occurs makes it tricky to prop optimally. Basically you want to get it to 5600 on the nose, but you sometimes only get *so close* based on the prop options available. +/-100 rpm is usually close enough to elicit the best performance, but you cant err on the high side (which I tend to do since a prop that would do so would also improve holeshot). A 5800rpm redline would be nice, but you'd really have to sweet talk someone at PCM in order to get yours changed.

It seems like the 422 is the best candidate for my application. If I actually had to go "less prop" anyone know what the "next step down" from the 422 would be?

I still think thats premature without more accurate info. I would highly recommend testing the boat out in the conditions you plan on running in... air/water/altitude/load/etc first, and get precise speed and RPM info. Then determine which prop will get you to where you want to go. The 470 or 1442 would be the 2 Id be considering if you want to go even smaller... both are 3-blades with 15" of pitch that will turn 200rpm or so more than a 422 at WOT.

[jsta281]

Thanks so much for your reply, it is very helpful. Yes I do know for certain it is 668. However it appears as if it is been repaired for some minor damage so the chances that some cup was taken out is certainly possible.

So help me understand this, if I go with a "smaller prop" like the 422 or one of the three blades you suggested and I successfully get my RPMs back to 5600 does that mean my mile-per-hour will go up? In cars it wouldn't work that way. Going to a smaller wheel would allow me to get to my RPM quicker but I would not be going the same speed obviously as that of a larger wheel.

[TRBenj]

So help me understand this, if I go with a "smaller prop" like the 422 or one of the three blades you suggested and I successfully get my RPMs back to 5600 does that mean my mile-per-hour will go up? In cars it wouldn't work that way. Going to a smaller wheel would allow me to get to my RPM quicker but I would not be going the same speed obviously as that of a larger wheel.

The speed vs rpm comparison to a car (or anything with gears) holds true. For a given rpm, a shorter wheel (or prop) will translate to lower speed, but will get there more quickly. When discussing max speed in this application, however, it's a little trickier. Rpm is not constant... A shorter prop will allow the enging to spin a higher rpm at wot. If, at that higher rpm, the engine is making more hp, then the boat should go faster. It's all about gettin maximum hp to the water at the point where hull drag and forward thrust are equal (ie, max speed occurs when you can no longer overcome the drag and you stop accelerating).

[jsta281]

So is there some advantage of a 3 blade over a 4 blade assuming both hit the magic 5600 rpm? I have read, I even think by acme that a 3 blade is more efficient. I am not really sure what that mean. It also seems that 4 blades are more popular or at least more common on the newer boats. Why would Acme make a 3 blade (470) with almost identical specs (minus .5 pitch) to a 4 blade (422). Why not just make a 4 blade with .5 less pitch?

[TRBenj]

So is there some advantage of a 3 blade over a 4 blade assuming both hit the magic 5600 rpm? I have read, I even think by acme that a 3 blade is more efficient. I am not really sure what that mean. It also seems that 4 blades are more popular or at least more common on the newer boats. Why would Acme make a 3 blade (470) with almost identical specs (minus .5 pitch) to a 4 blade (422). Why not just make a 4 blade with .5 less pitch?

The more blades, the less efficient a prop will be. A 1-blade prop would be pretty tough to balance though... The more efficient a prop is, the higher top speed it will allow you to achieve (it turns RPM into forward motion). Look at the performance v-drives and racing inboards and you'll notice many run 2-blade props. 3's will generally be faster than 4's, all else being equal (other parameters of the prop design, like diameter, rake, etc will come into play... but assuming 2 props are pretty comparable...). 4-blades became popular on ski boats in the 90's because they improved holeshot due to their greater blade surface area, and were a good bit smoother than the hand finished 3's (4's are easier to balance). The new-school CNC props have narrowed the gap to the point where the 4's dont hold much (if any) advantage on light ski boats. The 3's have a lot of blade surface area too and come out of the hole great- and the more precise machining results in props that are very smooth.

The difference of 1/2" of pitch is not insignificant... as I said, it will change WOT RPM's by about 200rpm on a 1.23 boat, which is a lot when youre trying to dial in performance. Acme does make a 4-blade with 1/2" less pitch than the 422 (the 654), but given the choice between the 654 and 470 (same dimensions, 3-blade vs. 4-blade) I'll pick the 470 every time for my lighter ski boats. If I were pulling heavier loads more often and didnt mind giving up a touch of top end, maybe Id lean towards the 4-blade. Acme and OJ seem to think that 4-blades work better on the reduction transmissions, but that simply has not been my experience.

The unfortunate thing is that Acme doesnt offer any 3-blades for the 1.23 that have more than 15" of pitch. A few options in the 15.5" to 16" range would be nice for the HO boats, in order to keep the revs down in the 5000 range. Dialing in WOT rpm's to the point you want them is more important than blade count though... so sometimes your hands are tied as to what props you can run for best performance.

[jsta281]

Thanks again for the response. If at some point you tire of answering all my noob questions I understand, but this has been very educational. My next question is regarding diameter. What is the advantage or disadvantage of a larger diameter? It seems that many/most of the DD even going back a few years had 13 in diameter props, and it appears that all the V drive boats do as well. Some I think are even 14 in diameter. Is there some advantage to a 12.5 diameter? If I replaced my 12.5 with a 13 all other factors being equal what would happen? My guess is it would be similar to adding cup do to a larger surface area? Is entertaining a larger diameter prop, just a bad idea?

[TRBenj]

Thanks again for the response. If at some point you tire of answering all my noob questions I understand, but this has been very educational. My next question is regarding diameter. What is the advantage or disadvantage of a larger diameter? It seems that many/most of the DD even going back a few years had 13 in diameter props, and it appears that all the V drive boats do as well. Some I think are even 14 in diameter. Is there some advantage to a 12.5 diameter? If I replaced my 12.5 with a 13 all other factors being equal what would happen? My guess is it would be similar to adding cup do to a larger surface area? Is entertaining a larger diameter prop, just a bad idea?

Not a problem... fun discussion.

Props that are smaller in diameter tend to be a little faster. Larger diameter props increase holeshot and are better at moving large loads (tug boats have large props). That is why there is a push on wakeboarding boats to go to larger diameter props, especially on larger boats carrying a lot of ballast. The larger diameter does have some tradeoff with top end speed, but its not huge. One of the fastest ski boats I know of (flirting with the 60mph mark) is swinging a big 13.5" diameter prop.

The thing you want to keep an eye on is hull clearance. 10% of prop diameter is the general rule of thumb to prevent hull burn. Personally, I am comfortable running down to 1" of clearance on my direct drives, but pushing it much beyond that is asking for hull damage. CC went to a shallower strut angle in '97, which dictated a change from the 14" diameter props used from '89-96 to 13". Most of the 1.23 Acme props are 12.5" diameter. The 97-01 boats have the least amount of clearance- the 02+ (TSC2 and TSC3) boats have a bit more due to the tunnel on the keel (capped by the hydrogate at the stern). I believe its been confirmed that you can run up to a 13.25" diameter Acme and still have plenty of room... though I havent attempted this myself. Most of the Acme's (save for their "pancake" props like the 1868) have a good bit of aft rake to the blades, which helps hull clearance too.

[jsta281]

Will an increase in diameter, all other things being equal, result in RPM drop? And in regards to a three blade prop being more efficient than a four blade Is that saying that if you had a three blade prop that was as "big" As possible And still hit 5600 RPMs And a similarly "perfect" or "big" on a four blade prop getting to 5600 rpm that the mile-per-hour would be faster with a three blade?

So correct me if I'm wrong based on this "summary" of much of this discussion. If only considering Top Speed as a performance factor the smallest in diameter but "biggest" in pitch and cup 3 blade prop Still reaching optimal RPM would get you the most mile-per-hour?

[NCH2oSki]

Just out of curiosity what is your primary use for you boat?

[TRBenj]

Will an increase in diameter, all other things being equal, result in RPM drop?

Yes... but diameter plays a much smaller role than pitch. For instance, the 1/2" difference in pitch between the 422 and 654 will result in about a 200rpm difference. The 3/4" difference in diameter (along with the .015" difference in cup) between the 470 and 1442 have them running pretty darn close to even.

And in regards to a three blade prop being more efficient than a four blade Is that saying that if you had a three blade prop that was as "big" As possible And still hit 5600 RPMs And a similarly "perfect" or "big" on a four blade prop getting to 5600 rpm that the mile-per-hour would be faster with a three blade?

Correct... assuming all else is equal. A good comparison would be the 654 and 470, as both are 12.5x15 with .105 cup. They'll run near identical RPM and the 470 will be a touch faster up top.

If only considering Top Speed as a performance factor the smallest in diameter but "biggest" in pitch and cup 3 blade prop Still reaching optimal RPM would get you the most mile-per-hour?

Correct. A few other tidbits:

- At some point, if blade surface and diameter are reduced too much, the prop will fail to hook up properly. We tried a conventional 3-blade 12x16 OJ with small little blades on our '09 196 and it just spun up to the rev limiter and only ran 48mph. No bite out of the hole either- just cavitation. Those little 12" OJ Legends work great on the 1600-1800 lb, 16-17' classic CC's we have, though. We're seeing lots of cavitation on our warmed up Barefoot Nautique as well- not enough prop to keep the slippage down to a reasonable level. So you need a certain amount of prop to keep the boat moving forward as it accelerates.

- Pitch can help dial in WOT RPM to a point (more pitch will bring down RPM), but it comes at a price- efficiency. If at all possible, keeping cup in the .080-.105 range would be ideal. Excessive cup, as seen on props like the 1458 (.135) and 668 (.150) seems to work against top end, to a point. If that extra cup were replaced by a bit more pitch (to keep the revs constant) then the prop would likely run faster... though it would come at a holeshot penalty (additional pitch reduces intial RPM out of the hole more than cup does). The 668 works great on our 196/409 at keeping the revs where they need to be, but I bet that a 3-blade 12.5x16 with .080 pitch would be a bit faster and keep the revs about the same level. Its too bad that no such prop exists!

[jsta281]

Just out of curiosity what is your primary use for you boat?

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.

I realize that most of my questions in this post must seem like I'm trying to build a race boat only concerned with Top end. No doubt speed is a factor for me, I'm a bit of a speed freak as I have mentioned or eluted to, I'm a drag racer. But TopSpeed for this boat is not an overriding factor of its overall performance. What I'm trying to do is in my boat/prop education, is dissect or analyze the variables independent of each other so that I can make a more educated decision on what Prop would be best.

[jsta281]

TRBenj

I'm going to go out and do more testing. Hopefully as soon as next week because sooner or later it's going to be ridiculously cold here in Utah. 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?