AW or "apparent water"

[Quoddy]

At the end of last season I was trying to sail to the mooring which was upwind and up current. My success was limited to zero.
Reason? Let’s say I’m on a starboard tack and the current is coming from the starboard. The normal sideslip of ½ knot to port that would create lift from the keel to starboard was neutralized by the ½ knot current from the starboard. The apparent water (new term) was zero, therefore no lift and no go. If this is right, is more speed the only answer?

 

[Maine Sail]

At the end of last season I was trying to sail to the mooring which was upwind and up current. My success was limited to zero.
Reason? Let’s say I’m on a starboard tack and the current is coming from the starboard. The normal sideslip of ½ knot to port that would create lift from the keel to starboard was neutralized by the ½ knot current from the starboard. The apparent water (new term) was zero, therefore no lift and no go. If this is right, is more speed the only answer?

Yes. I have picked up moorings for years by knowing the "momentum" of my vessel and when to turn her into the wind to "glide" up to the ball. This changes with tide & wind but once you get the hang of it is not difficult. I always bring the boat into the wind at the last moment I think she wil still have enough momentum for the wind & tide to just glide up to the ball but not overshoot it to fast.

 

[Bill Roosa]

Apparent current

I'd agree the current carrying you to starboard will increase your effective sideslip. This would force you to "point higher" (to port) to compensate to get to your goal. We get this all the time with the tides and shoals in the Chesepeake Bay. The boat is pointed at the place to round the shoal and enter the river but the GPS says you are on a course to hit the shoal before you reach the turn point. Since you are already close hauled you HAVE to tack and imporve your angle to make the turn. don't know how the current always knows you are close hauled but it seems to happen a lot.

 

[Alan]

No, not necessarily. I've faced this problem many times. At my mooring location the wind and current can be factors for mooring pickup. In these situations you can sail in below the mooring and shoot the wind at the last minute or sail above the mooring under jib only make a turn a few hundred yards above the mooring and furl the jib allowing the current/wind combination carry you back to the mooring. At this point you can steer a course that will allow you to pass the mooring 2 boat lengths either side. As you pass the mooring ball turn the boat into the wind and allow the boats momentum carry it back. If timed correctly the boat will come to a stop directly behind the mooring ball and you use these few seconds to grab the pickup stick.
Admittedly it does take a little practice but provided you have the time to play with it's a skill worth mastering. Judging your boats ability to bleed its momentum under varying conditions is the key to success.

 

[COOL]

If this is right, is more speed the only answer?

More speed will be better, because the faster you sail
the less time you will spend in the current.
So you do not want to 'pinch'. If the current and wind are coming
from the same direction, the current is reducing your wind speed
and your SOG. As far as your AW, the hull and foils do not know
they are in current. The boat will still sail through the water as if the
current was not there.
As far as making headway towards your mooring,
you need to look for areas of current relief, or even
back eddies in the current.
This will generally be in the shallower water near a shore.

 

[Quoddy]

Apparent water revisited

Thanks for the specific advice on mooring pickup and maneuvering in currents.
The related question is something like this.
The all important lift created by sideslip to leeward is negated when the water is moving the same way you are slipping. One knot slip over ground to port in still water creates lift to starboard. One knot slip over ground to port in water moving one knot to port creates zero lift. Hence going upwind is much more difficult. Seems right, or does it?

 

[Alan]

Quoddy, "One knot slip over ground to port in still water creates lift to starboard."
To create a one knot side slip in still water you would have to be sailing in a serious blow.

"One knot slip over ground to port in water moving one knot to port creates zero lift." That is only true if you are drifting. If sailing then the forward movement of the boat does create lift on the keel equal to the lift you outlined in the first scenario.

In short, any time the keel is moving forward through a fluid, lift is created.

 

[Todd Smith]

Yes Quoddy,

Apparent water is what matters to the hull, keel and rudder. Same as apparent wind to the sails.

Todd

 

[Bill Roosa]

keel lift

The keel does not produce "lift" in all cases, when you motor straight ahead the lateral forces on the keel sum to zero. There is a hydrodynamic force due the keel's curvature (always pulls to port or starboard) and a drag force (always pushes to the stern)
When the boat is stationary with no wind you drift with the current with no force on the keel at all. As the wind increases the sails supply a forward force and a lateral force. The boat begins to move forward (and drag begins to be created) and to leeward (and a lateral resisting force is generated as the water is pushed out of the way sideways). As the boat gathers speed the angle of attack of the keel becomes something more than 0 and the water must flow over a longer path on the windward side than on the leeward side of the keel. This is a lift type of force exactly like an airplanes wing. The difference being the keel is symmetrical and an airplane wing is typically not. Stunt airplanes that fly upside down do have symmetrical wings for the same reason sailboats have symmetrical keels. You have to be able to operate in either direction effectively. The symmetrical wing/keel is not the optimized solution but unless you always sail on one tack it is the only one you have.
Nowhere in here have we talked about what the current does. That is because the current does not do anything as far as the keel is concerned. The whole boat is carried with the mass of water and the keel acts as if it was in still water. The boat however does get moved with the current so you notice it when you compare your direction with the current to one without it. Just like drifting with the current you change your course without any additional expenditure of energy.