View Full Version : Slip Calculator ?
When using these on line slip calculators are there adjustments you need to make between a three blade vs a four blade prop.
Sam
blackhawk
05-08-2005, 08:11 AM
No, slip is slip.
Curious how do you compensate for the extra blade, I was told the Calc's are set up for 3 blade. Plus I would think a 4 blade will have less slip maybe I am wrong.
Phil
mattyboy
05-08-2005, 12:33 PM
in theory doesn't a
22 pitch 2 blade
22 pitch 3 blade
22 pitch 4 blade
all travel 22 inches in one rev
so why would you need to know how many blades
I had a busted leg once ;)
Hmmmmm ? good thought mattyboy, a gold star for you :D .
Sam
blackhawk
05-08-2005, 08:36 PM
If your leg is broken..
Your leg is broken :) :) :boggled:
LOL, exactly!
Normaly you drop the pitch when you go fro 3 to 4 blades, some get away with 1" but many have to go 2"'s to keep the RPM up where it was with a 3 blade. With less blades you have more slip. If you put the same pitch on you can lose 200 to 400 RPM or more. Below a post that goes into it a bit.
http://www.donzi.net/forums/showthread.php?t=906&highlight=blade+prop
Pismo
05-09-2005, 12:41 PM
More blades = less slip but more drag.
Not that I don't apprecaite all your help and comments but I called Julie at Throttle Up Propellers today. She said that you do not have to compensate for a four blade prop when figuring slip. She did say that a four blade prop will generally have less slip than a three blade prop.
Sam
blackhawk
05-09-2005, 02:15 PM
Not that I don't apprecaite all your help and comments but I called Julie at Throttle Up Propellers today. She said that you do not have to compensate for a four blade prop when figuring slip. She did say that a four blade prop will generally have less slip than a three blade prop.
Sam
:confused: If a four blade slips less than a three blade then it will show less slip on the calculator. What's there to "compensate" for? :confused:
slip is slip
Rootsy
05-09-2005, 02:24 PM
maybe what you are "looking" for is that a 4 blade has more mass to spin (inertia) which takes more hp... that is why... for equal slip for 2 props (3 and a 4 blade) the 4 blade will turn a lower rpm... cause torque is being used to spin the prop vs that torque being used to generate added thrust as it is with the 3 blade...
that is why you as a rule of thumb drop 1 inch of pitch...
the 4 blade having less slip will also lower your rpm when compared to an equally pitched 3 blade. This is because you are not wasting energy in the added slip...
some 4 blades will have more slip or equal slip to a 3 blade... slip = traction... no different than spinning the wheels on your car...
That's the point Scott, you don't have an option to choose between a three and four blade prop when computing your slip. Also keep in mind that a four blade in therory will slip less. According to my numbers I'm running @ a 9% slip level with a four blade. On a different three blade set up I was at 7%...................go figure.
Anyway have you begun to test props yet. I went for a spin on Sat and got a face full of nasty little bugs, yum :biggrin.: . I ran into a guy who just picked up a Blackhawk from McMachan. Needless to say he was getting used to the quirks of a Blackhawk.
Sam
Ranman
05-09-2005, 03:48 PM
While I agree with the "additional mass" theory Jamie put out there, I always thought that generally a 4 blad prop will have more total surface area than a 3 blade prop. Each blade of the 4 blade may be slightly smaller than a 3 blade, but the total surface ares will be more. This is why you lower the pitch an inch or so from a 3 to 4. More surface area would mean more "bite" in the water causing your motor to have to work harder to turn the same speed since it's puching more water. Slip simply gives you an indication of how efficient your prop/boat hull combo is. If you have the right prop for the boat, slip will be reduced. Other props may not work as well and thus generate more slip (which is wasted energy). Lastly, you'll never get slip to 0 and you can't have a negative slip number.
blackhawk
05-09-2005, 03:53 PM
That's the point Scott, you don't have an option to choose between a three and four blade prop when computing your slip. Also keep in mind that a four blade in therory will slip less. According to my numbers I'm running @ a 9% slip level with a four blade. On a different three blade set up I was at 7%...................go figure.
Anyway have you begun to test props yet. I went for a spin on Sat and got a face full of nasty little bugs, yum :biggrin.: . I ran into a guy who just picked up a Blackhawk from McMachan. Needless to say he was getting used to the quirks of a Blackhawk.
Sam
And that is my point Sam, slip is slip! ;) When you plug your numbers in and your actual speed is lower with one prop than the other with the same pitch and rpm, then it has higher slip. Doesn't matter if they are 2, 3, 4 or 5 blades. That being said, I do see your point, especially when you are working "backwards" trying to compute what your actual top speed SHOULD be.
Also, the prop experts I have talked to say a 4 blade is more efficient but a 3-blade has higher top-end. Hah? What? :confused: From the homework I have done, it appears that a 4 blade is more efficient at cruise but not at WFO when you are fully trimmed out.
And the only prop I have tested was the brand new Turbo 1 I bought and it did not work on my boat. Lifted the stern too much and felt out of control. :( Back to square one!!!
Out of control like bad chine walking ?
gcarter
05-09-2005, 04:08 PM
Generally speaking, three blade have less parasite drag (less surface area) so they theorettically have a higher top speed. However, four and five blade props vibrate less, have more blade area (diameter filled with metal), and can absorb more HP. If you have a high HP application, and are limited in diameter (like an outdrive) and don't have the luxurey of changing the ratio very much, then a multi blade prop may be the only option you have.
But in a perfect world, it's hard to beat a three blade for top speed.
blackhawk
05-09-2005, 04:18 PM
I don't even know what you would call it! :rolleyes: But for lack of a better word yes chine walking. Julie at Throttle-Up thinks a Mirage Plus will work much better. Which is what I was going to try first until several people said the props were almost identical.
blackhawk
05-09-2005, 05:29 PM
Scott, I did a few WFO blasts with it. With two people out of control. With a third person in the back(about 220lbs) it was much better. It feels like the Turbo lifts the whole boat(including the stern) out of the water too much.
I talked to Julie at throttle up and asked her what prop would "stick" the stern better but still lift the bow and she said the M+. Similar surface area more more rounded ears and no "lip" on the M+(something like that anyway she was talking fast!)
Didn't bother with the GPS cause I had some "skittish" passengers. :D Was planning on going out after work with my brother and take some rpm, speed and trim angle notes from 4-5000 rpm with the T1 and the Viper back to back. I MAY be overtrimming the T1, which means I need a smaller prop. But if memory serves me correctly the trim gauge was reading pretty close to the same(and this gauge actually works!) I'll take some good notes and go from there.
knots2u
05-09-2005, 07:51 PM
Does the weight of the boat account for the prop slippage? IE 5000 rpm, 1.5 ratio, 32 pitch won't make the Queen Mary do 85 mph. :biggrin:
mattyboy
05-09-2005, 08:22 PM
although all that has been posted is true
slip is only a measure of effiencency of the prop
diameter surface area all are reflected in the numbers you plug in
rpm speed drive ratio
GG Donzi
05-11-2005, 02:20 AM
I know this is not what your looking for though I found this link a while back, sure beats to old method!
http://www.csgnetwork.com/marinepropcalc.html
Doug G
05-12-2005, 05:34 PM
OK....this post has gotten me thinking. I have a 1988 Ragazza 23, 350 Merc/Alpha. I am currently running a 14 1/2 x19 4 blade aluminum prop. At WOT, the motor is turning 4400 RPM's, which is the bottom of the operating range of 4400-4800 RPM's, which is good for 40 MPH on GPS. I was running a 14 1/2 x 19 3 blade at 4700, which got me to around 48ish MPH.
I plugged my numbers into the slip calculator and came out with 24% slip. :shocking: I'm no rocket scientist, but I think that's a bit high :biggrin: My question is, what should the slip % be? I see some guys are running under 10%, but I think that is in a totally different application (go-fast vs cruise). I know I need to drop the pitch an inch, but what else should I be looking at doing, or, is 24% where I should be? :confused: Engine and drive are serviced regularly.
As always, the input is appreciated.
5 days til launch!!!
If it were me I would like to see it in the 10% range, give or take a couple %. 24% seems way high even for a cruise type boat. Just think of all that wasted forward thrust / motion you are not using not to mention the fuel you waste. Just my two cents !
Sam
Rootsy
05-12-2005, 06:51 PM
just to pitch (no pun intended) some more fuel on the fire... different hull designs work differently and more or less efficiently with different styles of props and different amounts of slip... there is a HELLUVA lot more to a prop than pitch and diameter... you ahve rake, cup, the progression of the pitch across the blade, blade area, all that good stuffaroo...
it is a matter of testing and keeping good notes.. and getting some data across the rpm band... so you cna figure overall efficiency...
JR
GG Donzi
05-12-2005, 07:44 PM
I'm not a big fan of aluminum prop's. They flex alot say good by to the pitch. Just my 2 cents. Roosty I could'nt have said it better!
Mudball
05-12-2005, 08:34 PM
The prop doesn't matter for slip calcs! Be it 2,3,4,5,6 blades or whatever!
What matters is what the prop does with your boat!!
According to everything I have learned, the 3 blade Mirage/plus props are mostly accurate in pitch. The Bravo's are 1" less than advertised, according to TU. So a 24 Bravo=23 mirage plus and on and on. I don't know about other props.
So, it purely depends on what you see with RPM, Drive ratio and the given prop at =GPS= speeds for slip!
Mudball
05-12-2005, 08:37 PM
Go to www.go-fast.com and check out the prop slip calculator.
You will see all then...
bob haver
05-12-2005, 09:06 PM
Hey Guys,
I know you are prefer the turbo props, but I am going "outside the box" and trying a Bravo 1, 26 pitch, lab finished prop on my 502 MPI. I am currently running a Mirage 25 3-blade. The boat handles fine with the 3-blade, no chine walk. I have not GPS'd the speed but I am pretty sure this boat with the 3-blade is doing 74-75 MPH. This is only because I run with guys that have GPS. My buddy has a Formula that is GPS'd at 72.8. I ran faster than him by 2-3 MPH.
I will definitely post what the lab finished 4-blade prop does. Merc racing assured me that if this does not increase bow lift and add a little more speed They will re-do at no charge. Lets see if it (the prop) and Merc will come through on their promise. I'll keep ya posted.:beer:
Rootsy
05-12-2005, 09:20 PM
The prop doesn't matter for slip calcs! Be it 2,3,4,5,6 blades or whatever!
What matters is what the prop does with your boat!!
According to everything I have learned, the 3 blade Mirage/plus props are mostly accurate in pitch. The Bravo's are 1" less than advertised, according to TU. So a 24 Bravo=23 mirage plus and on and on. I don't know about other props.
So, it purely depends on what you see with RPM, Drive ratio and the given prop at =GPS= speeds for slip!
actually the michigan wheels are the worst in quality control... mercury is middle of the road and precision prop (hydromotive) are some of the tightest... merc is +/- 1 degree on pitch... precision is +/- 0.5 degrees...
JR
blackhawk
05-12-2005, 09:52 PM
What matters is what the prop does with your boat!!
Exactly, these guys making props can tell you whatever they want but bottom line until you try it on YOUR boat you won't know for sure!
smoothie
05-13-2005, 09:56 AM
Hey Guys,
I know you are prefer the turbo props, but I am going "outside the box" and trying a Bravo 1, 26 pitch, lab finished prop on my 502 MPI. I am currently running a Mirage 25 3-blade. The boat handles fine with the 3-blade, no chine walk. I have not GPS'd the speed but I am pretty sure this boat with the 3-blade is doing 74-75 MPH. This is only because I run with guys that have GPS. My buddy has a Formula that is GPS'd at 72.8. I ran faster than him by 2-3 MPH.
I will definitely post what the lab finished 4-blade prop does. Merc racing assured me that if this does not increase bow lift and add a little more speed They will re-do at no charge. Lets see if it (the prop) and Merc will come through on their promise. I'll keep ya posted.:beer:
I have a Bravo 1 26p for my 20 cig and it lifts the bow quite nicely, need to get it labbed to get to 5200 rpm...
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