fenwayfear
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Someone explain the cause and effect of prop slip and how to correct it.
What determines prop slip?
- Thread starter fenwayfear
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XXXR2001
Active Member
Prop design, boat type, boat weight, engine height, gearcase design, etc.... all contribute to prop slip. Having a propeller worked by a good prop man for your particular setup can improve performance and reduce prop slip. "Worked" means changing the pitch, cup, etc. of the propeller blades by hammering them into a different shape.
Prop slip isn't necessarily a bad thing. In drag racing, we need the prop to slip somewhat on holeshot to get the motor rpms up quickly and then they bite hard and stay hooked up (i.e. low slipage on the top end).
Based on a mathematical formula, a prop with x number of pitch can only reach a max speed at a given rpm. The further under that max speed you are, the more prop slip you have.
Prop slip isn't necessarily a bad thing. In drag racing, we need the prop to slip somewhat on holeshot to get the motor rpms up quickly and then they bite hard and stay hooked up (i.e. low slipage on the top end).
Based on a mathematical formula, a prop with x number of pitch can only reach a max speed at a given rpm. The further under that max speed you are, the more prop slip you have.
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Everything effects prop slip hull, prop, height, weight, ect. I believe 7% is normal for our hulls. A prop has to have slip to work. Unless you have a prop pitch measuring tool you'll drive yourself crazy using a prop calcaulator. I've seen many times where used props run different than what is stamped on them. Some 4 blades and props with lots of blade cup, you have to add 1 pitch size to get your correct slip.
whipper
Well-Known Member
Slip is the differance between theoretical and actual travel. If a prop is say 15'' pitch and moving forward travels only 12'' inches or pitch then that prop is only traveling 80% of its theoretical max. Hence giving that prop a slip of 20%. Its common to have ventilation or cavitation but this is generally considered excessive and mainly happens with to small a diameter for the load or power of the engine. But as XR states common among drag racers and lots of us rec boaters also.
As mentioned 5-7% even 8 is a good {great} actually, slip for any highly efficient boat like ours. You know you have the right sized prop for your load and power when your in those slip percentages.
To little slip could be caused by to large of diameter prop.Your really waisting power over coming friction {slip} rather than moving forward. The positive and negative pressure on the blade faces relies on the angle of attack. By not having sufficient blade angle {rake+angle} there would be no thrust or slip. Imagine turning a plate it has no thrust or slip. Then mold that plate by angeling the outside edges of that plate to more of a cone shape. Now you have slip and thrust sorta. Not much until you cut some blades out and angle them some more. Now ya have a prop shape. That will move you forward. :big grin
There are virtually thousands of different examples of shape and sizes of props. The trick becomes how to identify what works for your boat and motor. Load and driving style also come into play. You can get very close but there is really, when ya get down to it, no two props that will be {perfect}. You may find what you think is the perfect prop then you go have a big Mac, fill up full of fuel. Raise your motor 1/4 inch more than normal,winds out of the south rather than north, temps increase 10 deg then all of sudden your perfect prop isn't the same as last night when it was cooler your stomach had less in it,lower fuel {load} and the water was 15 deg cooler.:help Thats the basics of prop slip. Like said again, find one to get ya in the 6-8 range under a few different conditions and load scenario's and you'll have found a great combo prop for your needs and setup that will be good on most days.
Thats why guys have several props. They know what each prop does with the purpose of the intended use of there boat for that day or even moment. They will pick the one they feel is best for that time and purpose to maximize performance.
I guess to answer your question how to {fix} prop slip. To much or to little? Again the differances of high or low percentages are determained buy some of the variables above. Most complain about to high rather than low. To small of diameter for power and load? What hull and power you have? Guys can tell ya what works for them. Diameter can be a setup question as in what prop shaft hight you run on what hull with what load. These are the variables that will help on what dimentions over all in a prop might be best suited for you.
As mentioned 5-7% even 8 is a good {great} actually, slip for any highly efficient boat like ours. You know you have the right sized prop for your load and power when your in those slip percentages.
To little slip could be caused by to large of diameter prop.Your really waisting power over coming friction {slip} rather than moving forward. The positive and negative pressure on the blade faces relies on the angle of attack. By not having sufficient blade angle {rake+angle} there would be no thrust or slip. Imagine turning a plate it has no thrust or slip. Then mold that plate by angeling the outside edges of that plate to more of a cone shape. Now you have slip and thrust sorta. Not much until you cut some blades out and angle them some more. Now ya have a prop shape. That will move you forward. :big grin
There are virtually thousands of different examples of shape and sizes of props. The trick becomes how to identify what works for your boat and motor. Load and driving style also come into play. You can get very close but there is really, when ya get down to it, no two props that will be {perfect}. You may find what you think is the perfect prop then you go have a big Mac, fill up full of fuel. Raise your motor 1/4 inch more than normal,winds out of the south rather than north, temps increase 10 deg then all of sudden your perfect prop isn't the same as last night when it was cooler your stomach had less in it,lower fuel {load} and the water was 15 deg cooler.:help Thats the basics of prop slip. Like said again, find one to get ya in the 6-8 range under a few different conditions and load scenario's and you'll have found a great combo prop for your needs and setup that will be good on most days.
Thats why guys have several props. They know what each prop does with the purpose of the intended use of there boat for that day or even moment. They will pick the one they feel is best for that time and purpose to maximize performance.
I guess to answer your question how to {fix} prop slip. To much or to little? Again the differances of high or low percentages are determained buy some of the variables above. Most complain about to high rather than low. To small of diameter for power and load? What hull and power you have? Guys can tell ya what works for them. Diameter can be a setup question as in what prop shaft hight you run on what hull with what load. These are the variables that will help on what dimentions over all in a prop might be best suited for you.
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fenwayfear
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Xb2003/225PM/sporty/10" setback/3-batts(1 on ea side for troller, stater in bilge port side).
Darth VMAX
Well-Known Member
Not sure how accurate it is. But on the prop slip calculators the Twin Prop set-up on my XB2002 is only 1-2%
I believe its close, especially on my hole-shot (I NEED more slip)
I believe its close, especially on my hole-shot (I NEED more slip)
whipper
Well-Known Member
Lots of guys with your setup run Todds props. Hydromotive Quad4 in a 27-28 for all round use fishen load ect.. They run in that 7-8% slip for your setup. If your having troubles getting out of the hole that might be the prop. A 28 or 30 chopper style for instance will spool up as mentioned bringing the rpm up to 4500-5000 then grab and go dropping the r,s down to 2500-3000 or so and your off. I personally dont like that but lots of guys love that.
When running wot are you encountering to much slip? Numbers on the calculator adding up to 10++? What prop are ya using? i just love useing smaller props. I think most guys do. Unfortunatly its the bigger wheels that get your top speed higher unless you have the rpm. A great all round pitch for my setup which is the same as yours though my hulls an XS not an XB but the bottoms are the same is 25. With a 225PM on a XS or XB2003 a 25 or 26 4 blade prop is awsome!! Youll run 80 and change even with a stock limitter and youll be fast out of the hole. Slip will be sub 10 unless theres somthing wrong somewere.
fenwayfear
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I am in the break-in stage of the motor. I am just gathering info it avoid buying 26 wheels
. I have a 27 trophy plus to test and a 28 hoss. The boat gets on plane with the 27 will very little effort with just me in the boat.
. I have a 27 trophy plus to test and a 28 hoss. The boat gets on plane with the 27 will very little effort with just me in the boat.
whipper
Well-Known Member
I am in the break-in stage of the motor. I am just gathering info it avoid buying 26 wheels
Those two props are awsome and you should have your needs covered for a while.
Bobalouie
Active Member
That 28 hoss will be a great prop for you. I have a 26 Hoss T1, and I love it. It carries a load great, hauls the mail, and has very good handling.