slippage

[BRADLEYDWIGHT]

Ron I like the way you describe prop terminology in simple terms. So how about slippage. What is it really? And is their an easy way to determine if you've got it? In my case in a 17'-10" RaysonCraft flat.

[BRADLEYDWIGHT]

and a stelling 11 1/2-15 fyi. Sorry, maybe an RH prop in the near future :D

[Ron Hill]

One of my old students, Ron Lewis, made a book for all gear ratios and pitch sizes.

He also made one for Al Stoker.... My prop shop lost a lot of business because of the chart... BECAUSE some guy would come in with an 85 MPH boat and want to go faster. After we checked the speed chart, if the guy was running at 125%, he'd only be going 75....

Chad had an on line calculator but I don't think he still has it....

This could be dumb, but I just take the RPM and multiply times the pitch and come up with a number... If you KNOW for sure it is a 15, then you can figure out 100% speed in theory....Then, test, figure your speed, and slip...Add pitch, see if you gain speed...

My Flat Bottom guys are running right at 100%, no slip, but, as I say, I don't add the cup to the pitch...

So, no I don't know how to figure slippage....

I know that under good conditions an inch of pitch is 200 RPM...but this is more a rule of thumb for outboards and sterndrives.

I know the guys that run Super Stock like to turn, say 76 hundred, so they change the gear to get there... outboarders change the prop to get there..

Who is right or wrong? I don't know... but to help people win races or improve performance you need to have DATA.... RPM, SPEED and FAIRLY accurate prop size.... I can compare my props, really only to mine without measuring and testing...Testing is always where it is "AT"...that is one reason even NASCAR limits testing for their teams...

Quick story: Gil Suiter owned a Super Stock and Julian Pettengill drove it and he also owned and drove the amazing K Boat, "COLD FIRE". Julian would use the Super Stock prop on the K boat, just different gears...

Well, they liked to turn their Super Stock engine 7,200 and in the early 1980's I met Gil and Julian at an APBA Convention and we had lunch together, between meetings. Gil asked if I ever worked on Inboard wheels? I said, "NO." He asked if I'd like to work a couple for him? I said sure...

About February, two props arrived at my shop, both Stellings, that looked like "War Clubs" to me. Big ears, no cup, thick as hell. I narrowed the blades, cupped them, thinned them back and sent them off COD for $180.00. (Big money in those days for prop work).

About two days later, Gil calls, mad as fire. He says I ruined his props. I said, "Well, what do they do?" He said, "They will only turn 6,500 RPM." I said, "SO, how fast do they go?" He said, "WE DON'T know". I said, "You dumb JackAsses are National Champions and you don't know how fast your boat goes?? Only the RPM?" I said, "Take the prop off and turn any RPM you WHAT!!!!! and hung up."

Next day, Gil calls, said they rented an Arizona Highway Patrol's radar and found out an interesting fact....My props did lower the RPM's 700, but they gained 7 MPH and they were leaving for the KILOS in Modesto....They got blown out, but Gil had me do more work...

Bud Gilbert remembers when lowered Brad Miller's Crazy Horse" a THOUSAND RPM and gained almost 10 MPH....

Times have changed, but testing props is the ONLY way to find out things...

[bathtub888]

This could be dumb, but I just take the RPM and multiply times the pitch and come up with a number... If you KNOW for sure it is a 15, then you can figure out 100% speed in theory....Then, test, figure your speed, and slip...Add pitch, see if you gain speed...





Ron I'm having trouble with this theory... i may be reading this wrong, but say my pitch on my propeller is 13pitch, times my rpm at the Prop, 3000rpm. that would equal 39000, wouldnt i have to find out how many inches in a mile? then do the whole thing that way?
help, im lost.

[Mark75H]

There are 63,360 inches in a mile, but between miles per hour and revolutions per minute you have to divide by 60 to even out the hours and minutes.

63360/60 = 1056



Prop RPM (not motor RPM) times pitch
1056

that will give you the theoretical amount the boat will move forward if it were moving thru dirt or maybe Jello

But that is not what moves a boat.
Boats move as a reaction to the thrust being shot out the back by the prop. The difference between the boat's forward movement and the calculated pitch is called "slip"

Simply multiplying pitch times prop RPM divided by 1,000 will give you speed with 5.6% slip .... a pretty good indication of how fast that prop will never go

[bathtub888]

how does prop diameter affect this? i am just having a hard time getting it..... too many factors in a prop i suppose.

[Mark75H]

Not really. Just forget about diameter and rake until you understand pitch and slip.

[Mark75H]

Have you ever looked behind a boat and seen the water being pushed back behind the boat or seen a roostertail flying up behind a boat? That water is "slip"


Should we do the calculations for some particular prop and see how it works out? Do you think that would help?

[bathtub888]

Have you ever looked behind a boat and seen the water being pushed back behind the boat or seen a roostertail flying up behind a boat? That water is "slip"


Should we do the calculations for some particular prop and see how it works out? Do you think that would help?


heh heh, i think i got it.

[Ron Hill]

You save me here... Area, diameter and Rake.....are other problems... but slippage is the first step to knowing what is going on!!!

[bathtub888]

You save me here... Area, diameter and Rake.....are other problems... but slippage is the first step to knowing what is going on!!!


well i did some math, which i hate...

my buddy and i were talking today, he has a 13pitch, spins 3000 rpm, so that is 39000. divided by 1056 = 36.9 so in theory that is what it should be... blah.. i will read some more.

[Mark75H]

well i did some math, which i hate...

Well, duh! The only time I enjoy Algebra or Trig is when I'm doing boat stuff. The Trig comes in handy calculating the megaphone tapers on exhaust systems.

Your calculations are correct.

Now if that boat runs 32 mph, your slip is 15% --- because the water from the prop that is jetting back behind the boat is going back away from the boat at about 5 mph.

[bathtub888]

Well, duh! The only time I enjoy Algebra or Trig is when I'm doing boat stuff. The Trig comes in handy calculating the megaphone tapers on exhaust systems.

Your calculations are correct.

Now if that boat runs 32 mph, your slip is 15% --- because the water from the prop that is jetting back behind the boat is going back away from the boat at about 5 mph.

haha ok.. well i will hand it to you, you have patience with a newbie........... being only 21 i guess i could always go back to highschool and upgrade some math.. hahaha :eek:

[Ron Hill]

15% slip isn't bad, considering your boat is a PLANING Hull...to go faster, you'd need to fly....but getting a bath tub to fly won't be easy!!

[bathtub888]

15% slip isn't bad, considering your boat is a PLANING Hull...to go faster, you'd need to fly....but getting a bath tub to fly won't be easy!!


I need some wings... i gotta juice up my motor some more.... :D

[Ted March]

Formula for MPH at zero slip is:
MPH = RPM X PITCH X GEAR RATIO divided by 1056

Example:
MPH = RPM X Pitch X Gear Ratio / 1056
105.339 = 8000 X 26 X 0.5348 divided by 1056

This is the maximum speed that could be obtained using
a 26 pitch prop turning 8000 RPM with a 1:87:1 gear ratio
in the lower unit.

Take your present mph reading, speedometer, GPS, radar or clocked
speed. Subtract it from your theorectical maximum speed. Divide
the diference by the theoretical maximum speed. The number you
come up with is your percentage of slip

Example:
105.4 mph as in the example above
Your GPS reads 97.5 mph
The difference is 105.4 minus 97.5 = 7.9 mph
7.9 divided by 105.3 = .075 = 7.5% slip

[capnzee]

The only time your GPS is correct is when you are running in "dead" water; that is water with no current and no tide, then it is usually a minute or so behind. You might be better off using the old Keller if you're running on the Colorado or Mississippi. The simplest way I have found to find a hypothetical speed at 100% efficiency is simply take the RPM's(at the prop), subtract 1 inch from the pitch of the propeller you are running and multiply the two. Forget the decimal point or put it where it makes sense, you will be within a mile an hour or so of all the complicated formulas you are hearing about. Example, you are turning a 21 inch prop at 5000 RPM's......multiply 5000 X 20 (21" minus 1 equals 20)and you get 100 mph at 100% efficiency. If you have a light boat properly set-up, you should operate with 10 per cent or less loss of efficiency(90 mph or better). The difficulty is knowing what pitch you are actually running. The number stamped on the prop, especially the modern day "progressive pitch" doesn't mean much any more and usually depends on the individual manufacture of the prop. So, the only way you can actually find out if a prop works better is to try it on a given day, then try another and another. Buy the one that works best and use it as a standard against the next prop you want to buy (buying the same prop from the same manufacturer guarantees nothing). If you think you have the perfect prop, make a plaster of paris or glass mold of it (both blades, there is a good chance each one is a little different) ;) Capnzee