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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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Old 05-17-2011
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current theory

Several times when motoring or sailing into a strong current I noticed that the helm became more tender than normal.
At first blush it would seem that it wouldn't matter if the boat was moving through the water or the water was moving past the boat.

My experience is that as soon as the current speed gets close to the boat speed through the water the helm gets very tender.

I think I have figured out why.
When motoring directly into a 4 knot current at 5 knots you get an over ground 1 knot progress. If however due to wave action or even slight steering adjustments or even current eddy's you get even slightly off dead ahead the vectors change rapidly and the bow gets pushed off affecting the helm.

Have you experienced the same thing?
Does my explanation make sense?
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Old 05-17-2011
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Another thought

OK, I haven't really thought this through but... maybe you actually lack momentum. That same boat is only moving at 1 knot. The faster you go, the more stability you have. Look at a bicycle. Hard to balance when stopped, the faster you go, the more stable you are. The better you can withstand side forces.

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Originally Posted by davidpm View Post
Several times when motoring or sailing into a strong current I noticed that the helm became more tender than normal.
At first blush it would seem that it wouldn't matter if the boat was moving through the water or the water was moving past the boat.

My experience is that as soon as the current speed gets close to the boat speed through the water the helm gets very tender.

I think I have figured out why.
When motoring directly into a 4 knot current at 5 knots you get an over ground 1 knot progress. If however due to wave action or even slight steering adjustments or even current eddy's you get even slightly off dead ahead the vectors change rapidly and the bow gets pushed off affecting the helm.

Have you experienced the same thing?
Does my explanation make sense?
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Old 05-17-2011
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I've definitely noticed a change in 'feel' as you pass through and in & out of eddies in the presence of strong currents, but through-the-water speed is what it is, I don't think it should change the helm's feel if it's a consistent current.

However truly consistent current is pretty rare in our experience.. any time we're in significant current (say over 2 knots) there seems to always be random turbulence so I suspect what you're feeling would be the ebb and flow of eddies on the rudder.
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Old 05-17-2011
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ebb & flow

That makes sense.

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Originally Posted by Faster View Post
I've definitely noticed a change in 'feel' as you pass through and in & out of eddies in the presence of strong currents, but through-the-water speed is what it is, I don't think it should change the helm's feel if it's a consistent current.

However truly consistent current is pretty rare in our experience.. any time we're in significant current (say over 2 knots) there seems to always be random turbulence so I suspect what you're feeling would be the ebb and flow of eddies on the rudder.
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Old 05-18-2011
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Yep. Your boat's foils don't care what speed they're going over ground, only through the water. We've got some strong rips around these parts. They're great to get into as you can use them to get squirted through an adverse tide, but the flow around the rudder and keel is disturbed and things feel bumpy. It's possible that your rudder can even stall momentarily. It's very much like a plane experiencing 'turbulence', but thankfully not quite as 3 dimensional.
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Old 05-19-2011
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OK another thought experiment.
Imagine a large tank with a consistent artificial current flowing through it.
Now imagine a model boat placed in this stream, bow upstream.
You hold one finger against the center of the transom so the boat is stationary over ground.
With this model I thing it is pretty easy to see that keeping it perfectly balanced would be very hard. The bow will want to fall off very easily and quickly.
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Old 05-19-2011
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Speaking theoretically, I don't know. As mentioned above the water flow over the keel and rudder would be the same if you were sailing along at X knots (kind of like being in a wind tunnel). The finger is SOG (i.e. - keeping you in one place relative to the ground). But the water is moving past your boat at X knots...not SOG. So the foils and forces should work.

That said, I could also see what you're saying about the bow wanting to fall off...like a kayak in a river (which I actually have experienced).

So are there different dynamics at work here?
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Old 05-20-2011
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Quote:
Originally Posted by smackdaddy View Post
That said, I could also see what you're saying about the bow wanting to fall off...like a kayak in a river (which I actually have experienced).

So are there different dynamics at work here?
Yes there is.. the weight distribution in your average yacht is centered above the keel - and you have a deep keel and rudder adding their own lifting forces to keep the boat in line.

The weight distribution in a kayak is all down the stern - and you have a long, thin, keel with no appreciable lifting forces.

A yacht's rudder turns the boat around the axis of the keel, so, to get the "thought experiment" right, the proverbial finger should be placed in the center of the boat - not the transom. ...but if you hold the rudder dead center in a current with the boat stationary, you'll find it will eventually fall off one way or another as soon as the keel's "lift force" (generated by the angle of attack to the average current flow direction) counteracts the rudder's smaller "lift force".

..all highly technical really.
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Last edited by Classic30; 05-20-2011 at 12:15 AM.
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Old 05-20-2011
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Quote:
Originally Posted by davidpm View Post
OK another thought experiment.
Imagine a large tank with a consistent artificial current flowing through it.
Now imagine a model boat placed in this stream, bow upstream.
You hold one finger against the center of the transom so the boat is stationary over ground.
With this model I thing it is pretty easy to see that keeping it perfectly balanced would be very hard. The bow will want to fall off very easily and quickly.
Probably a reiteration of what was said in the previous post, but hold the boat from a point in the center (where the boat's CE is located), and it should track very well. Boats pivot around a location just above their keels, not sterns.
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Old 05-20-2011
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Actually the finger in my thought experiment was designed to simulate the propeller. I was visualizing the boat while motoring. Sorry I wasn't explicit.
The force while motoring is at the prop.
While sailing it is at the mast.
It is easier to visualize how tender the helm will be while motoring as the propulsive force is at the stern.

Less easy to visualize is when motoring with a fast current. Again the helm becomes very sloppy. But instead of tender it becomes sluggish.
I have experience this when running with the current through hell gate in nyc for example.

Last edited by davidpm; 05-20-2011 at 01:19 AM.
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