View Full Version : through my research it seems rpm affects wake
DooSPX
01-21-2010, 12:45 PM
I have been doing a lot of research on Props for my boat. It seems from many sources that lower rpm also affects the wake as much as it does the firmness of the pull that the skier feels. It seems that for slalom, the wake will be softer as well as have a softer pull to the skier if the rpm's are lower. It also seems that there is a fine line on how soft or firm the pull is so the boat does not run with or try to pull the skier out of form in a pre-turn.
For me, it seems as though the OJ XMP #454 13.7 x 17.5 4 blade .110 cup is going to give everything that a Power Slot owner could want. It also seems that on the 91-94 PS hull, there is a top end increase as well. soft/lower wake and rooster tail, killer hole shot and a steady soft "feeling" pull that will not cause the boat to run with the skier has they are in their pre-turn. as soon as I button up my head gasket replacement, it will be ordered.
Interesting observation. I went back to a 13" 4 blade from a 16" 4blade because I thought the wake was better for slower and longer lines. I had been using the 16" OJ to lower rpm and get more ski miles per gallon. 500rpm difference for 34mph skiing. No noticeable difference in wake at -32 or -35 loops, but the bump seemed a more significant at -28 loop and longer. 32mph down seemed like harder bump but that may just be a function of speed. Same hull as the 95-97 PS190.
DooSPX
01-21-2010, 02:02 PM
Interesting. I think the goal for slalom is run the smallest prop in both pitch and diameter so it has the smallest prop column which would affect the bump. I believe that is why they lower the pitch but add some cup on most of the slalom props.
It was explained to me that the reason for more cup is to kind of act like having more pitch, so you can have great hole shot from lowering the pitch and not kill topend.
Example, the recommended and king prop for Power Slot guys is the OJ #454 13.7x17.5 .110 cup 4 blade and does not scrub speed, in fact some with the 91-94 hulls said they got better topend.
To compare, Acme's 4 blade prop is the #275 13.5x17.5 .090 cup and has scrubbed off about 2 mph from the topend end on the same 91-94 hulls. Now, Acme is testing a new 3 blade for the Power Slot guys its the Acme 1749 Propeller 3 Blade 13.5 x 17.5 LH 1 1/8" .150 Cup (notice the cup) It should have the same hole shot as the #275 4 blade yet actually improve topend over stock too. Bill wanted me to test it, but im not sure I want to dish out 400 dollars even though i can return it if im not happy with it. just do not have that kind of cash around just to test props. I would love to if I could.
Skipper
01-21-2010, 02:02 PM
I notice the difference in the rooster tail and slope of the approach to the wake between my 3 blade and 4 blade ACME (1:1 trans). I believe the rooster tail is samller and the wake flatter with the 4 blade. There is a big difference in RPM's. Never equated the change to the RPMs? Good observation.
Jesus_Freak
01-21-2010, 02:04 PM
It seems from many sources that lower rpm also affects the wake as much as it does the firmness of the pull that the skier feels. It seems that for slalom, the wake will be softer as well as have a softer pull to the skier if the rpm's are lower.
Makes sense. I expect the wake effect would not apply to wakeboarding or other lower-speed sports, because prop inertial influences on the wake become less important at lower speeds.
I went back to a 13" 4 blade from a 16" 4blade because I thought the wake was better for slower and longer lines. I had been using the 16" OJ to lower rpm and get more ski miles per gallon. 500rpm difference for 34mph skiing. No noticeable difference in wake at -32 or -35 loops, but the bump seemed a more significant at -28 loop and longer. 32mph down seemed like harder bump but that may just be a function of speed. Same hull as the 95-97 PS190.
13" to 16" is a monster change! Wow! Did you see a statistical, measurable change in "ski miles per gallon"?
DooSPX
01-21-2010, 02:09 PM
I edited my second post to clarify more.
Jesus_Freak
01-21-2010, 02:10 PM
Interesting. I think the goal for slalom is run the smallest prop in both pitch and diameter so it has the smallest prop column which would affect the bump.
Definitely diameter, but not certain about pitch/cup.
It would be difficult to isolate all of these effects, because the precise placement of the skier relative to the peak in the rooster tail confounds the data. For example, at -22' a change in diameter and/or pitch could move the skier forward or aft relative to the peak. It may "feel" different, but you havent changed the peak height. I guess what the skier feels is all that matters, so who cares what caused it, right?:)
DooSPX
01-21-2010, 02:24 PM
Definitely diameter, but not certain about pitch/cup.
It would be difficult to isolate all of these effects, because the precise placement of the skier relative to the peak in the rooster tail confounds the data. For example, at -22' a change in diameter and/or pitch could move the skier forward or aft relative to the peak. It may "feel" different, but you havent changed the peak height. I guess what the skier feels is all that matters, so who cares what caused it, right?:)
good point of the pitch and the placement of the skier, and feel is what matters to the skier. I just thought this would be a great discussion to have and some science and math about our hobby. :cool:
would love to have detailed discussion from many members' opinions as the thread grows. :D
DooSPX
01-21-2010, 02:26 PM
I notice the difference in the rooster tail and slope of the approach to the wake between my 3 blade and 4 blade ACME (1:1 trans). I believe the rooster tail is samller and the wake flatter with the 4 blade. There is a big difference in RPM's. Never equated the change to the RPMs? Good observation.
thank you for you input skipper. Your findings about the 4 blade seem to back up with OJ is trying to tell people about slalom wake differences that he is hearing from hardcore and pro skiers alike and their findings with the 4 blade props on the slots. I do not have the science behind this, but I would guess the extra blade with how it overlaps slightly have a great affect on the column which would affect the rooster tail.
It would great if Eric from OJ Props could come in and shine some science on this. or at least pick his engineers brains on this topic.
russlars
01-21-2010, 03:11 PM
I worked through this issue big time last year. It seems like there are a lot of variables that affect the ski wake and boat performance. Many of which are subtle and depend on line length and personal preferences, etc. The bottom line is you have to take the time to test multiple props under the same conditions so that you can make an educated decision.
Eric at OJ is great to work with this way. I ended up testing 3 OJ's with absolutely no strings attached (just return shipping). I also tested 2 ACME props, but as stated, these must be paid for in advance with a guaranteed refund if not happy. It is also helpful to have someone else testing these with you under the same circumstances to provide additional insight. What you may like, someone else may hate. Go test them with your boat, it is the only way you will know for sure!:twocents:
DooSPX
01-21-2010, 03:18 PM
well stated Russlars, I am getting the OJ454 per many many conversations with Eric. Right on about testing many different props to find what fits your needs the best. If I had the money to put out to test the acme's I would in a heart beat. But Eric said the #454 4blade will make my lowly 240hp with the 1.52:1 power slot feel like it has a much bigger motor yet have a super friendly slalom wake and pull as well. and that prop has proven it self many times on power slot boats.
Makes sense. I expect the wake effect would not apply to wakeboarding or other lower-speed sports, because prop inertial influences on the wake become less important at lower speeds.
13" to 16" is a monster change! Wow! Did you see a statistical, measurable change in "ski miles per gallon"?
Jesus_Freak, there is a very noticeable change in "ski-mpg". We free-ski about 2.7 miles down to the course and usually let drivers/younger skiers get some time in. The low rpms are great no matter who is freeskiing because of the fuel conservation. 5-6 gallons turning the 16" prop would get me tired out in the course and a couple of freeskiers their exercise for the day. Not with the 13" prop. Running the 16" also made the SG slower to settle for the course.
east tx skier
01-21-2010, 03:32 PM
Look at the MC boats approved this year and the difference in the various engine/prop combinations. They are dialing in an rpm sweet spot on each one.
DooSPX
01-21-2010, 03:34 PM
Look at the MC boats approved this year and the difference in the various engine/prop combinations. They are dialing in an rpm sweet spot on each one.
on usawaterskis website?
it list the prop and rpm ?
have a link?
DooSPX
01-21-2010, 03:35 PM
Jesus_Freak, there is a very noticeable change in "ski-mpg". We free-ski about 2.7 miles down to the course and usually let drivers/younger skiers get some time in. The low rpms are great no matter who is freeskiing because of the fuel conservation. 5-6 gallons turning the 16" prop would get me tired out in the course and a couple of freeskiers their exercise for the day. Not with the 13" prop. Running the 16" also made the SG slower to settle for the course.
very interesting findings.
PM Eric from OJ and tell him exactly what you want. I will pick the perfect prop for your needs. I'm sure he can find one or make one that will do everything you are looking for.
russlars
01-21-2010, 03:38 PM
well stated Russlars, I am getting the OJ454 per many many conversations with Eric. Right on about testing many different props to find what fits your needs the best. If I had the money to put out to test the acme's I would in a heart beat. But Eric said the #454 4blade will make my lowly 240hp with the 1.52:1 power slot feel like it has a much bigger motor yet have a super friendly slalom wake and pull as well. and that prop has proven it self many times on power slot boats.
What I did for my ACME test was just put the props on my credit card. They will give you 30 days to test and if you don't like it, return it for a credit back to your card. If you are like me, you will want to do a thorough test, otherwise you will always be second guessing your decision. Either way you are going to be spending $400-500 for a prop. You might as well get the one you want.
DooSPX
01-21-2010, 06:34 PM
true.......
DooSPX
01-21-2010, 11:17 PM
Where did you find that info ETS? Any details?
Also, lets get back to the science of these huh? LOL
Jesus_Freak
01-22-2010, 02:03 PM
...What you may like, someone else may hate....
Never met anyone that likes a good 'ol rooster tail slapping their ski around. :D
Jesus_Freak
01-22-2010, 02:13 PM
thank you for you input skipper. Your findings about the 4 blade seem to back up with OJ is trying to tell people about slalom wake differences that he is hearing from hardcore and pro skiers alike and their findings with the 4 blade props on the slots. I do not have the science behind this, but I would guess the extra blade with how it overlaps slightly have a great affect on the column which would affect the rooster tail.
It would great if Eric from OJ Props could come in and shine some science on this. or at least pick his engineers brains on this topic.
OK, so we have four independent prop variables*: diameter, pitch, cup, and N blades. Blade RPM is not an independent variable, since all combinations of these 4 variables fix the blade RPM required to reach 34.2 MPH. If we were to try 4 variants of each independent variable, that would produce a testing matrix of 4^4, or 256, tests. Who wants to start?:)
*This assumes a constant blade design, water conditions, hull design, passenger/equipment load and distribution, etc.
DooSPX
01-22-2010, 05:08 PM
OK, so we have four independent prop variables*: diameter, pitch, cup, and N blades. Blade RPM is not an independent variable, since all combinations of these 4 variables fix the blade RPM required to reach 34.2 MPH. If we were to try 4 variants of each independent variable, that would produce a testing matrix of 4^4, or 256, tests. Who wants to start?:)
*This assumes a constant blade design, water conditions, hull design, passenger/equipment load and distribution, etc.
assuming same boat, same conditions for weather, water, and passenger/equipment load and distribution... only taking into account the number of blades, the shape of the blades, the rpm and diameter of the prop.
I love all the math and science stuff... even though a lot of it is over my head. :rolleyes:
I am going to try to get a hold of Eric and see if he can chime in with what he has seen on the MC hulls over the years.
DooSPX
01-23-2010, 02:14 PM
I sent Eric a message. Lets hope he can chime in when he gets a chance.
east tx skier
01-23-2010, 03:32 PM
on usawaterskis website?
it list the prop and rpm ?
have a link?
Link to 2010 Approved Towboats (http://www.usawaterski.org/pages/TournKit/AWSA/Chief%20Driver/2010ApprovedTournamentTowboats.pdf)
DooSPX
01-23-2010, 10:51 PM
awesome! thank you Doug!
east tx skier
01-24-2010, 03:12 PM
No problem. There was some discussion of these variances in Engine/prop combinations on Ball of Spray a few months back.
DooSPX
01-25-2010, 10:56 AM
Eric should be in here with his two cents today sometime. Looking forward to what he has to say. :D
EJ OJPROP
01-25-2010, 02:24 PM
This is a tough one, no simple answer, as the right prop for one boat/skier/driver may not be the same prop for the same boat with different skier/driver. I have always said it is a personal preference as to which size works best for the individual. If you prefer a softer feel behind the boat then a prop that turns a lower RPM at your skiing speed is what you would look for. A firmer pull will be felt with a prop that turns higher RPM. The RPM difference can be a small as 20 to 50 RPM. Not sure the RPM directly affects the wake, I would say the RPM dictates the feel of the pull.
OK, so we have four independent prop variables*: diameter, pitch, cup, and N blades. Blade RPM is not an independent variable, since all combinations of these 4 variables fix the blade RPM required to reach 34.2 MPH. If we were to try 4 variants of each independent variable, that would produce a testing matrix of 4^4, or 256, tests. Who wants to start?:)
*This assumes a constant blade design, water conditions, hull design, passenger/equipment load and distribution, etc.
assuming same boat, same conditions for weather, water, and passenger/equipment load and distribution... only taking into account the number of blades, the shape of the blades, the rpm and diameter of the prop.
I love all the math and science stuff... even though a lot of it is over my head. :rolleyes:
I am going to try to get a hold of Eric and see if he can chime in with what he has seen on the MC hulls over the years.
The sad part of all this is that non of it matters...no two hulls are ever close to exactly alike, so while you may be able to narrow down your personal search by testing on your boat it will not translate to the next guy/boat. Then there is fuel consumption and weight...and other constantly changing variables, but I digress.
Oddly enough pro skiers (dying breed) and serious weekend warriors worry about none of this since the boat is the great equalizer. At tournaments everybody is skiing behind the same boat with the same gear. On a weekend your buddies are behind your boat and your gear (or theirs as the case may be). The single largest controllable variable is YOU...
TMCNo1
01-25-2010, 04:05 PM
The sad part of all this is that non of it matters...no two hulls are ever close to exactly alike, so while you may be able to narrow down your personal search by testing on your boat it will not translate to the next guy/boat. Then there is fuel consumption and weight...and other constantly changing variables, but I digress.
Oddly enough pro skiers (dying breed) and serious weekend warriors worry about none of this since the boat is the great equalizer. At tournaments everybody is skiing behind the same boat with the same gear. On a weekend your buddies are behind your boat and your gear (or theirs as the case may be). The single largest controllable variable is YOU...
That makes more sense than anything I have read yet. Mike, you have a way with words.
That makes more sense than anything I have read yet. Mike, you have a way with words.
Thank you sir
DooSPX
01-26-2010, 05:19 PM
very true about the hulls never the same. that would have more of an affect than other small details mentioned. I was actually trying to refer to say the same boat. i.e. my boat or JF's boat.
and thank you very much Mike for your input. That is what I wanted out of this thread was peoples input.
DooSPX
01-26-2010, 05:21 PM
sorry, double post! :rolleyes:
cbryan70
01-26-2010, 05:26 PM
How often does MC maintain/wax the molds? Hull should not be THAT different boat to boat. weight yes...
TMCNo1
01-26-2010, 06:30 PM
How often does MC maintain/wax the molds? Hull should not be THAT different boat to boat. weight yes...
Usually they are waxed between items and polished/buffed every 4 to 6 depending on when it was used last IIRC. But, there can be one or more different molds in rotation for the same structure, possibly resulting a slight difference in two boats of the same model coming out of production on a given day that may only be realized by a trained professional who would know the differences if any after assembly and extensive testing.
Jesus_Freak
01-27-2010, 01:34 PM
At tournaments everybody is skiing behind the same boat......
Yes, but I dont have to like it do I? :)
The single largest controllable variable is YOU...
Yes, even more so than fin adjustments, etc. But that is for another thread...
cbryan70
01-27-2010, 01:40 PM
Usually they are waxed between items and polished/buffed every 4 to 6 depending on when it was used last IIRC. But, there can be one or more different molds in rotation for the same structure, possibly resulting a slight difference in two boats of the same model coming out of production on a given day that may only be realized by a trained professional who would know the differences if any after assembly and extensive testing.
Yeah I thought it was most manf were about every 5 boats out of the mold it would be waxed down. Very few buff/polish after every boat but I do know of one that still does this. If I were paying the MC$$ i would request to be the first boat down the polished/buffed mold.
How often does MC maintain/wax the molds? Hull should not be THAT different boat to boat. weight yes...
A mold is built from a a prototype bottom, a hull is built and tested, then modified until all are satisfied. The bottom is then made into a buck so that molds can be pulled from it. Differences are inevitable due to heat, humidity etc as well as damage/repair that goes on while molds are being made.
Lastly, you hit on weight which is a huge factor in a slalom boat, but don't forget placement of that weight. A 190/197 that is heavy in the transom makes for a poor long line boat, where if the weight were in the bow it could be the best thing you ever skied behind.
Since about 1997 and the Nautique TSC1 hull the boat really is no longer a factor. The wakes get smaller and softer, the engines make less actual power (vs advertised), the hulls have more drag etc and so on. I have actually heard pros saying the wake behind the boat was too soft...this after everyone on the start dock ran into 39.
DooSPX
01-27-2010, 03:39 PM
Interesting MYMC.
one of the reasons I am getting a aftermarket intake... weight savings!! :cool:
Interesting MYMC.
one of the reasons I am getting a aftermarket intake... weight savings!! :cool:
Great idea, nothing better than having your cake and eating it...increased engine performance and efficiency along with weight reduction = win+win+win
Nice job.
DooSPX
01-27-2010, 07:31 PM
Great idea, nothing better than having your cake and eating it...increased engine performance and efficiency along with weight reduction = win+win+win
Nice job.
my point as well! :D
thanks MYMC!!!
Jesus_Freak
01-28-2010, 01:38 PM
...a hull is built and tested, then modified until all are satisfied.
It does not have to be done this way, now does it? ;)
It does not have to be done this way, now does it? ;)
No...the truth is it doesn't; however, sometimes you can lead a horse to water and then you need to repeatedly bash him in the head to get him to understand just how much easier it would be if you listened 5 years ago...but I digress. ;)
Jesus_Freak
01-28-2010, 01:58 PM
No...the truth is it doesn't; however, sometimes you can lead a horse to water and then you need to repeatedly bash him in the head to get him to understand just how much easier it would be if you listened 5 years ago...but I digress. ;)
;) Laughing uncontrollably at my desk. Thank you. ;)
;) Laughing uncontrollably at my desk. Thank you. ;)
Anything I can do to brighten your day...:D