V-drive formula to calc prop slip

J

jerjrvette

Member
I liked the spread sheet that was posted a while back. So that you don't have to guess what your prop slip is you can use this formula. SLIP= Speed(MPH) x1056 x Drive Ratio
          -------------------------------------- x 100
                        RPM x Pitch (in.)


Good Luck, JR
 
73 Sanger Flat

73 Sanger Flat

Active member
Here is what I use and it pretty close to the GPS

I will try and attach the excel file

This work really good for me

It also has another formula at the bottom to compare

Mine is in blue on the page as to what i ran a few weeks ago ..
 

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  • V-Drive prop slip.xlsx
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C

Carnivalride

New member
jerjrvette said:
I liked the spread sheet that was posted a while back. So that you don't have to guess what your prop slip is you can use this formula. SLIP= Speed(MPH) x1056 x Drive Ratio
          -------------------------------------- x 100
                        RPM x Pitch (in.)


Good Luck, JR
Click to expand...

Engine RPM x Gear ratio / (1056 / prop pitch) = theoretical MPH

So 7200rpm, 1.18 gears and a 16 pitch prop

7200 x 1.18 / 66 = 128.7

Now depending how effecient your boat is greatly affects this. I hear 10% or 15% slip all the time, personally I call BS, most lake flats run around 20% and old heavy true flats are often worse.

So 128.7 x .8 = 103

Now if you have a GPS take you GPS mph and divide by you theoritical mph to get you effecincy.

Say the boat ran 105 on GPS, 105/128.7=81.6% so 100-81.6=18.4% slip

Clear as mud?
 
73 Sanger Flat

73 Sanger Flat

Active member
Loren -- i would agree most boat slip number are a lot higher than they think ..
Mine is right about 19% on the last run i did .. It matches my spreadsheet pretty close ..

Did you see that video of Galen Hydro on facebook .. It ran goooodd !!! 
 
C

Carnivalride

New member
Brazos River Rat ( Billy ) said:
Loren -- i would agree most boat slip number are a lot higher than they think ..
Mine is right about 19% on the last run i did .. It matches my spreadsheet pretty close ..

Did you see that video of Galen Hydro on facebook .. It ran goooodd !!!
Click to expand...

With a 2 blade prop around 10-3/4" to 11" diameter I think 19% is in the ballpark from my experience.

Yes I did it looked like a very successful maiden voyage!!!
 
73 Sanger Flat

73 Sanger Flat

Active member
I think with a little seat time , Galen will be flying .. What an awesome ride he has .. I was impressed . First time a drove a Hydro and I got to run it up to 108 myself with Galen in the boat .. The boat handle great .. .. JJ work is top notch .. Can't wait to see what it will do .. I took it to 6600 rpm and it was pulling so freaking hard all the way from the time I stabbed the throttle to the time I let off .. It is awsome how it yanks the left front sponson about 2 foot out of the water on the launch ..
 
C

Carnivalride

New member
Brazos River Rat ( Billy ) said:
I think with a little seat time , Galen will be flying .. What an awesome ride he has .. I was impressed . First time a drove a Hydro and I got to run it up to 108 myself with Galen in the boat .. The boat handle great .. .. JJ work is top notch .. Can't wait to see what it will do .. I took it to 6600 rpm and it was pulling so freaking hard all the way from the time I stabbed the throttle to the time I let off .. It is awsome how it yanks the left front sponson about 2 foot out of the water on the launch ..
Click to expand...

I've only driven 2 hydros and they were very different than what i'm used to. They seem to sit up higher and then that outboard steering was hoorible for the lake. I think Galens is inboard so it should steer better than the 2 I drove. So how was the handling compared to your flat Billy?
 
H

HellinnFrnt

New member
beaverfab said:
Need to know RPM of engine, pitch of prop, gears in v drive,and gps speed. Then I can help. j.j.
Click to expand...

It's up there bubba :)  and it's a Menkins 10 3/4 x 15  3 blade cupped  55 mph 18%  4800 rpm
 
73 Sanger Flat

73 Sanger Flat

Active member
On the hard straight runs it was fun to drive .. Line it up , shoot , turn rudder left and hang on .. This thing never stopped pulling hard all the way through the test run ..
His boat took a set and started blowing the tell and it felt really good ..

I am inpressed with how it ran and handled the water ..
It is completely different from my flat .. My left foot wanted to push down on something .. Plus his throttle is hand controll , so that was cool ..

The first 5 minutes I was trying to just get use to driving it and getting some  heat on the aluminum rods , and I tried to make a right turn about 15-20 mph and the boat spun completely 180* and almost sunk .. We had about 2 feet of water on the deck and the back almost swamp , so I just nailed it and jumped it back on top of the water .. Not sure what happen , I think it caught the skag on the right sponson deep enough in the water to spin us out .. It was not fun ..
We had a least a foot of water in the boat when we got back on plain .. pretty dam scarry first 5 minutes ..
 
B

beaverfab

New member
With no slip you would be running 80.45 you are runing 55 so your slip is 29.5 %. IT will slip more at 4800 rpm than 5800 so if your boat will turn more rpm then the slip my be less. j.j.
 
C

Carnivalride

New member
beaverfab said:
With no slip you would be running 80.45 you are runing 55 so your slip is 29.5 %. IT will slip more at 4800 rpm than 5800 so if your boat will turn more rpm then the slip my be less. j.j.
Click to expand...

??? 55/80.45= .683 effeciency, 1-.683 = 31.6%
 
C

Captin Hazlewood

New member
jerjrvette said:
I liked the spread sheet that was posted a while back. So that you don't have to guess what your prop slip is you can use this formula. SLIP= Speed(MPH) x1056 x Drive Ratio
          -------------------------------------- x 100
                        RPM x Pitch (in.)
Click to expand...

There is an app for that. If you have a smart phone look for Hering Propellers.
 
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