Prop 'Slip' gauge, who wudda thunk it ...

[DaleH]

Wow, new one on me ...

What is Prop Slip?
Pitch is the distance the propeller/boat progresses thru the water in one complete turn. Every inch of boat that's in the water induces drag, and therefore the distance traveled is not always the same. This results in the propeller "slipping" behind the theoretical distance it should travel with each rotation. The difference between theory and reality is called "propeller slip".

How it works ...
Inside the DataTachTM is a computer that receives GPS speed & RPM info and it calculates the slip of your boats propeller. It does this 5 times per second, to give you an accurate reading. See the equation below.

Slip = ([Speed (mph) x 1056 x Drive Ratio x 100] divided by [RPM x Pitch (in)])

A few things that affect prop slip...
* Pitch of propeller
* Propeller size
* Condition of boat bottom
* Condition of propeller
* Engine mounting height
* Weight distribution
* Jack plate mounting
* Trim angle

 

[96TL]

Thats awesome.

 

[Megabyte]

I'm sure that the driver of that go-fast is watching his slip gage at 100 mph. :shock:
Oh, and only a go-fast would measure speed on the water in miles per hour. :roll:

Interesting concept though...
Anyone here going to install one of these gages?

 

[DaleH]

Oh, and only a go-fast would measure speed on the water in miles per hour. :roll:

Then I must have a go-fast boat then? So where am I going wrong using my GPS speed? My GPS also reads out 'speed make good' as some other acronym (I can't remember ) if I'm vectoring to a set course heading.

Or are you and I into another GPH debate ;? ?

 

[Megabyte]

Naaaa.... not a debate, but it is a pet peeve of mine.

If you never have occasion to compute time and distance over water using a nautical chart, you can easilly use mph which is based on statute miles.

However, nautical charts are based on nautical miles via Lat/Lon, so if you are going to do the math you have to use nautical units of measurement. Otherwise, your calculations will be off because you cannot mix apples and oranges... unless you want fruit salad.

Distance - Speed

Statute miles - Miles per hour
Nautical miles - Knots

To see the difference between knots and miles per hour, use this calculator:
http://www.paulnoll.com/China/Metric/number-knots.html

Lets face it, when operating small boats in local waters, the unit of speed that you use is really a non-issue.
However, if you ever had to plot a course over a distance figuring in set and drift, you cannot do it using miles per hour on a nautical chart (unless you want fruit salad).

 

[DaleH]

Kevin:

I agree that KNOTS is a better indicator than MPH.

But IMHO ... like when trimming tabs for the best efficiency, to me ... MPG or nMPG is significantly better than GPH ... as unless you know the speed ... that number is of limited value ... to me .

 

[Megabyte]

Gotcha... I misunderstood.

 

[TheOtherLine]

Wait till Grouper Jim sees this thread. He's going to have to get one...

 

[grouperjim]

I'm slipping along at about 10 percent depending on whose formula you are using. I guess 10 percent is about average(?)

 

[Megabyte]

I guess 10 percent is about average(?)

I believe that is better than average.