Finding data with shaft RPMs at the end of watch - SailNet Community
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Old 03-22-2011 Thread Starter
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Finding data with shaft RPMs at the end of watch

This is more for the older mariners here.
Using prop pitch, slip and Shaft RPMs.
What is the difference between positive slip & negative slip?
Finding RPMs for a specific speed?
Finding distance with pitch, slip & RPMs?
Finding speed with pitch, slip & RPMs?

As you can see this is for those of us who have sailed on the ships for a living, working as Deck Watch Officers.

Yeah I know that you have Modern electronics... but some of us still do it the old tried & true methods.

1600 Ton Master, 2nd Mate Unlimited Tonnage

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Old 03-22-2011
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Risky, but I'll try it

OK, I'll bite, but disclaimer, my ticket is MUCH more limited than yours, but I'm ready to muster up and take my punishment for being incorrect in the attempt (hoping there is credits for being close at least)

I guess, at the end of a watch, if you knew how many RPM's the shaft was turning during the watch period, how long the watch was (say 4 hrs), what the pitch of the prop was (inches per rotation), what the slip was (a fraction of those inches you loose), you could do some simple math and figure out how far you went.

Please correct the following equation:

Distance = (4 hours) * (60 Min/hour) * (rotations/minute) * (inches of pitch) * (1- (fractional slip))

This would give distance travelled in inches. To translate inches to nm:

Distance in nm = (6076 ft/nm) / ((Distance in inches)/12 feet)

Not sure how you get negative slip?

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Old 03-22-2011
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all these calculations can be off if the helmsman keeps over correcting. this slows the ship down just like a fowled bottom would.

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Old 03-22-2011
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If there is enough current it will change the boats speed over the ground a good amount and mess with the slip numbers BUT its so high in the first place

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Tommays
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If a dirty bottom slows you down what do you think it does to your boat
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Old 03-22-2011
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waiting for the professor to correct the work

captbill- I think you are right, if the helmsman steers a wiggly course

tommays- I'd assume we do set and drift calculations on top of the math suggested. So the math is only covering speed over the water, not speed over ground.

still nervously waiting for the professor Boasun.

Negative slip???
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Old 03-22-2011
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I'm used to timig a run over a measured distance (average both ways) to get make a RPM/speed table. Figure distance from the speed using the table. Calculations assuming a slip value are infreior. Slip can also change at different rpms
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Old 03-22-2011 Thread Starter
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I've used the end of watch revolutions to check the navigation of other means and with the assumed slip it was usually right on the mark.
So crossing the ocean (before electronic tools) This was one of the means to determine the distance that they've traveled in a four hour period. Along with Course steered and known currents in the area, ships have DR'd themselves across the Alantic, the Indian and the Pacific Oceans, along with several seas to boot.
The navigator will determine the slip by observation over several watches. applying any changes that may appear due to a dirty bottom or if the ship is trimmed wrong. Little details such as those makes the difference of being 'Spot on' or 'Fairly close'.
A current pushing you may give you an observed slip that gives you more distance that what the pitch alone will get you. And a retarding current can slow you down, making the slip appear worst then it actually is. The pilot charts of the areas you're transiting will help with known currents unless there have been a contrary wind blowing for a period of time.
This is a small part of the Art of Navigation that we should all be practicing.

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Last edited by Boasun; 03-22-2011 at 02:19 PM.
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Old 03-22-2011
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We go through hell gate a few times a year and SOG goes from 6 knots to 12 knots for a pretty good amount of time

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Old 03-22-2011
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Boasun,

So it sounds like you determine the slip on a given voyage by comparing the turns over a number of watches, and doing the math on pitch. This way, you account for dirty bottom, ship loading, etc. You also try and compensate out set and drift.

So negative slip would appear to occur for instance, if I had a strong fair current?
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Old 03-22-2011
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Normal slip may change between %35 to %45 of the pitch on a sailboat.

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