Prop Slip - What are good numbers?

[OldSchoolBoats]

I picked up a 25P MAX 5 (15 diameter) for my 400R

Current slip with that prop, according to the calculator, is between 22 - 24

I know that I need to get that number down, so please educate me on how to do that and if / how the motor height, trim, weight....etc.....might affect the slip

 

[SKIDMARC]

Maybe a stupid question but what do you mean my "Slip" ?

Is it an outboard thing?

 

[zx14]

Boat?
My 26 Daytona I’m getting 8% at 110mph.

 

[sintax]

Maybe a stupid question but what do you mean my "Slip" ?

Is it an outboard thing?

its an efficiency thing... Prop, hull, and drive all play into it.

 

[OldSchoolBoats]

Boat?
My 26 Daytona I’m getting 8% at 110mph.

22 Shockwave Deck

 

[Flatsix66]

Sounds high. I'm far from an expert...calculated mine at around 12 last week after feeling I found a good height and trim level. 28p, 5k, 72mph gps. 300xs. When I had the 300x and 26P I calculated it around 8%.

 

[SKIDMARC]

its an efficiency thing... Prop, hull, and drive all play into it.

How is it measured? Sorry I really don't know.

 

[UltraLucky]

How is it measured? Sorry I really don't know.

Prop Slip Calculator | Mercury Racing

Quickly determine slip for Mercury Racing propellers using our prop slip calculator.

www.mercuryracing.com

 

[SKIDMARC]

Prop Slip Calculator | Mercury Racing

Quickly determine slip for Mercury Racing propellers using our prop slip calculator.

www.mercuryracing.com

Thank you!!

 

[Dirtbag]

i Always wonder about this.....because it makes no sense.

Lets say joe is driving my boat. Joe gets my boat up to 85 mph. I run the exact same setting except the weight of the boat is obviously much heavier. i only hit 78 mph. the slip calculation is now completely different based on speed. but a heavier boat wont go ass fast and thuss slip is not calculated correctly.....help

 

[Flatsix66]

A prop will move a given amount of water per revolution, anything less would be slip. Like a tire, if its hooked up it will go whatever the distance is due to diameter, if it's spinning it wont go as far.

 

[RCDave]

As speed increases and the engine height raised, slip should decrease. Up to a point....

Does the 400R sit on a hydraulic jack plate? Can you raise it so the propshaft is even with the middle sponson? I've read the lowers on the 400R are more efficient with higher propshaft heights

 

[Uncle Dave]

Boat?
My 26 Daytona I’m getting 8% at 110mph.

What type prop?

UD

 

[RiverDave]

I picked up a 25P MAX 5 (15 diameter) for my 400R

Current slip with that prop, according to the calculator, is between 22 - 24

I know that I need to get that number down, so please educate me on how to do that and if / how the motor height, trim, weight....etc.....might affect the slip

You sure you put the right drive ratio in there because those numbers are terrible. If it’s dialed in as an outboard the numbers should be 5-6%? I have heard of some outboards getting down in the 2-3.

 

[blue wonder]

Funny you just posted that Dave, on my Mach 22 W/ twins I’m @ 3%

 

[RCDave]

I'm in below 5% slip on my 22 Cougar MTR (300xs) as well. (at higher speeds semi surfacing the prop)

 

[sintax]

You sure you put the right drive ratio in there because those numbers are terrible. If it’s dialed in as an outboard the numbers should be 5-6%? I have heard of some outboards getting down in the 2-3.

haha i wasnt going to say that, but i was thinking it. Those are similar numbers to what i see people post with poorly setup v-drive boats....

 

[ONE-A-DAY]

I was at 15-17% and as of last night I have it now at 6%. Went from four blade bravos and humpback cleavers to the new Hill 5 blade cleavers and changed from spinning out to spinning in. The prop change alone took me to 10%, engine height up and down in 1/4 inch increments and out to in got me to 6%. I think I can get it slightly lower but at the expense of water pressure dropping below 10 psi, with all this 6% is plenty good in my book. The numbers below work out to 6% with a 1.62 ratio.

 

[lbhsbz]

The other part of the question is the REAL prop pitch. 2 years ago, I sourced a 26P and a 28P version of the eact same prop I have in a 24P. Both made the boat go slower and turned higher RPMs than the 24. It’s a crap shoot...I go for single digit slip numbers, but since you don’t really know what you have for a prop, it’s a fuzzy number at best.

 

[Uncle Dave]

How is it measured? Sorry I really don't know.

Props are sold many ways but the main way is "pitch" which is measured in inches.

If you put the prop in a block of gelatin and made a complete revolution it would go forward that amount of inches as there would be no slippage.

Put that prop in water and some % of that forward momentum is lost due to the prop not biting but slipping in the water.

multiply the pitch / rpm and gear ratio and you get a 100% theoretical number with no slippage (online calcs you can use for this)

start reducing by %.

IF the boat should hit 100MPH in theory with no slippage goes 90 - you have 10% slippage.

(its more complex than that but those are the basics)


UD

 

[OldSchoolBoats]

You sure you put the right drive ratio in there because those numbers are terrible. If it’s dialed in as an outboard the numbers should be 5-6%? I have heard of some outboards getting down in the 2-3.

Yeah, 1.75.

Maybe I am recalling the numbers wrong in my head. Will pay more attention next time.......lol.

The other part of the question is the REAL prop pitch. 2 years ago, I sourced a 26P and a 28P version of the eact same prop I have in a 24P. Both made the boat go slower and turned higher RPMs than the 24. It’s a crap shoot...I go for single digit slip numbers, but since you don’t really know what you have for a prop, it’s a fuzzy number at best.

It's a brand new merc racing MAX 5 25p. For the $$$ they get for these things, I hope the pitch is correct.....

Sent from my SM-N960U using Tapatalk

 

[Dirtbag]

Props are sold many ways but the main way is "pitch" which is measured in inches.

If you put the prop in a block of gelatin and made a complete revolution it would go forward that amount of inches as there would be no slippage.

Put that prop in water and some % of that forward momentum is lost due to the prop not biting but slipping in the water.

multiply the pitch / rpm and gear ratio and you get a 100% heretical number with no slippage (online calls you can use for this)

start reducing by %.

IF the boat should hit 100MPH in theory with no slippage goes 90 - you have 10% slippage.

(its more complex than that but this are the basics)


UD

how does weight of the boat calculate into this? same boat same set up two different weights run drastically different

 

[lbhsbz]

Yeah, 1.75.

Maybe I am recalling the numbers wrong in my head. Will pay more attention next time.......lol.
It's a brand new merc racing MAX 5 25p. For the $$$ they get for these things, I hope the pitch is correct.....

Sent from my SM-N960U using Tapatalk

You hope, but you have no idea. Too many variables.

 

[Uncle Dave]

how does weight of the boat calculate into this? same boat same set up two different weights run drastically different

Weight doesnt factor as an entry variable, only in the slippage result at given MPH.

If you go look at the Merc online slip calc (or any other) there is no entry for weight.

Prop Slip Calculator | Mercury Racing

Quickly determine slip for Mercury Racing propellers using our prop slip calculator.

www.mercuryracing.com

UD

 

[Dirtbag]

Weight doesnt factor as an entry variable, only in the slippage result at given MPH.

If you go look at the Merc online slip calc (or any other) there is no entry for weight.

Prop Slip Calculator | Mercury Racing

Quickly determine slip for Mercury Racing propellers using our prop slip calculator.

www.mercuryracing.com

UD

i understand that perfectly but it has to be a variable.....if i drive joes boat that adds weight. Weight iss harder to move in water. so if joe goes 80 at 5k rpms and i only go 75 at 5k rpms the slip becomes a different number...... to me that makes no sense

 

[RiverDave]

i understand that perfectly but it has to be a variable.....if i drive joes boat that adds weight. Weight iss harder to move in water. so if joe goes 80 at 5k rpms and i only go 75 at 5k rpms the slip becomes a different number...... to me that makes no sense

If the prop isn’t slipping then the boat would just turn slightly less rpm and run 75 mph.

Props will slip less as drag is reduced though. By adding more weight you are adding more drag

 

[twocents]

Just a suggestion, if you really want to get your boat dialed-in and be able to compare different props, try this. Start at 3000 rpm which should be a comfortable planing speed. Check the speed on GPS at what you believe is your most efficient trim setting, record that speed and double check that the rpm is steady at 3000rpm. Then increase the throttle to 3500 rpm and do the same thing, and do it again at 4000 rpm, 4500 rpm, 5000 rpm and at WOT. Now you have a set of numbers ranging from 3000 rpm to WOT. Drop that data into the standard formula to calculate efficiency. What you'll most likely see is that your efficiency percentage will change at each rpm level. Usually less effiicient at the lower rpms and more efficient (less slip) at the higher rpm range. Depending upon how you want your boat to perform, you can then make an informed decision about which propeller is best suited for your desired application.

 

[Dirtbag]

If the prop isn’t slipping then the boat would just turn slightly less rpm and run 75 mph.

Props will slip less as drag is reduced though. By adding more weight you are adding more drag

so basically my fatasss will never have low slip %?

 

[Uncle Dave]

so basically my fatasss will never have low slip %?

Prob not...

UD

 

[ToMorrow44]

i understand that perfectly but it has to be a variable.....if i drive joes boat that adds weight. Weight iss harder to move in water. so if joe goes 80 at 5k rpms and i only go 75 at 5k rpms the slip becomes a different number...... to me that makes no sense

It makes perfect sense. More weight = more drag = more prop slip. Prop slip IS the variable in boat performance. It accounts for weight, drag, aerodynamics, etc. complied into one variable. That’s why if you’re testing props, to compare performance accurately it needs to be done on the same day, same body of water, same weight etc.

 

[Dirtbag]

Prob not...

UD

if i have a shirt on its like a damn parachute UD......fml

 

[Dirtbag]

It makes perfect sense. More weight = more drag = more prop slip. Prop slip IS the variable in boat performance. It accounts for weight, drag, aerodynamics, etc. complied into one variable. That’s why if you’re testing props, to compare performance accurately it needs to be done on the same day, same body of water, same weight etc.

but throughout the day the boat gets lighter and lighter with gas going bye bye.

i want my boat to run perfect...i definitely want it to run efficiently. but im not about going superfast....my boat can do low 80s but i like her at mid 60s....hmmm

 

[4Waters]

That prop slip calculator makes no sense, 19p, 4900rpm, 1.65 gear ratio 62mph on gps= -16% slip

 

[LargeOrangeFont]

so basically my fatasss will never have low slip %?

Correct.

 

[Rayson1971]

Don't forget cats are weight sensitive also.

 

[LargeOrangeFont]

That prop slip calculator makes no sense, 19p, 4900rpm, 1.65 gear ratio 62mph on gps= -16% slip

Sure your tach is right? I was spinning way more RPM than that doing 62MPH in my Caliber 1

 

[LargeOrangeFont]

but throughout the day the boat gets lighter and lighter with gas going bye bye.

i want my boat to run perfect...i definitely want it to run efficiently. but im not about going superfast....my boat can do low 80s but i like her at mid 60s....hmmm

Don't take this the wrong way, but you have a bus, not a sports car. You can lower the slip somewhat with a good 5 blade prop and likely get some better performance but it will also be harder on the drive.. The weight of your boat, especially up front, kills the efficiency.

You can gain a couple MPH (less prop slip) just by moving people and items around in the boat. Your setup may be running close to perfect for your particular combination.

Honestly trying a different prop is the first step, just be aware it will be harder on the drive.

 

[4Waters]

Sure your tach is right? I was spinning way more RPM than that doing 62MPH in my Caliber 1

I've got a 305, it's WOT is 4600-5000 I've got a way to check though

 

[Dirtbag]

Don't take this the wrong way, but you have a bus, not a sports car. You can lower the slip somewhat with a good 5 blade prop and likely get some better performance but it will also be harder on the drive.. The weight of your boat kills somewhat kills the efficiency.

yep got it. and no i dont take it the wrong way. I tell people all the time shockwaves are the cadillacs of the lake. Built heavy and robust for a smooth ride. i have a 5 blade now and like it a lot better. way easier to get on plane. and yes i take it super easy out of the hole dont want to blow the drive off the back end with the 600. i do have an imco lower now tho for a little more beef

 

[zx14]

What type prop?

UD

A 3 time worked by BBlades 28 bravo made to 29.with diffuser cut off.

 

[Uncle Dave]

A 3 time worked by BBlades 28 bravo made to 29.with diffuser cut off.

Daddy like.....

UD

 

[LargeOrangeFont]

I've got a 305, it's WOT is 4600-5000 I've got a way to check though

Either that or the prop pitch you are running is wrong or mislabeled.. sounds closer to a 22 honestly.

 

[LargeOrangeFont]

yep got it. and no i dont take it the wrong way. I tell people all the time shockwaves are the cadillacs of the lake. Built heavy and robust for a smooth ride. i have a 5 blade now and like it a lot better. way easier to get on plane. and yes i take it super easy out of the hole dont want to blow the drive off the back end with the 600. i do have an imco lower now tho for a little more beef

You are probably pretty close to dialed.. That's not to say there can't be small tweaks to yeild some gains though.

 

[lbhsbz]

That prop slip calculator makes no sense, 19p, 4900rpm, 1.65 gear ratio 62mph on gps= -16% slip

Common core?

 

[lbhsbz]

I've got a 305, it's WOT is 4600-5000 I've got a way to check though

As my uncle used to say, GM built a great motor with the 305, they just forgot 2 things...torque and horsepower. What kinda boat are you playing with and how are you measuring the speed?

 

[guest hs]

Just did mine on the calculator

 

[guest hs]

Doesn’t seem right to me I always thought it had a lot slip.

 

[LargeOrangeFont]

Doesn’t seem right to me I always thought it had a lot slip.

Negative slip is not a thing.. One or more of the inputs is wrong then.

 

[Bigbore500r]

So you HAVE tried a different prop!

 

[LargeOrangeFont]

I've got a 305, it's WOT is 4600-5000 I've got a way to check though

Mine had a 350. I forgot what the peak RPM I was seeing though. It did over 65 MPH.