RIVER SAILING
This is an extract and condensation of some recent emails on the Grampian Discussion group concerning River Sailing and the effects of currents on boats in rivers.
"Speaking of a trip up the Mississippi,, has anyone here done
much river sailing? My concerns are river current versus Hull speed. My
understanding is that hull speeds are pretty much set in stone, for a certain
hull design and length. And forward movement is essential for steering control.
So, say you have a forward current of 7-8 mph,, would you in essence be sitting
still? I have only lake sailed in the past and have recently considered putting
my G30 on the Mississippi,, and am concerned about the currents which in the
"Alton Pool" only run about 3-5 mph ahead of the locks and dams. still a current
though.
I have my pilots license and I know if I have an indicated wind speed of 120
knots, and a headwind of 50 knots, my ground speed is only 70 knots. My
question, I guess, is the current speed on the river the same as having a
headwind in the air,, which could null out your net movement? and void your
steerage?
Also, sailing down river, could you lose steering if your tail current was 7 knots or better? I read a blog about a couple who have left the St. Louis area, and motored down the Mississippi, then to set sail in New Orleans and have navigated through the Panama canal,, I am guessing you still have current issues to address on the Ocean as well. That blog is www.larabeck.com Very interesting stuff." (Jeff - skid0023)
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"If you were in a plane heading 40kts AIS into a 40kt head wind, your ground
speed would be zero. However your rudder, ailerons, and elevators would still
work fine, and other then issues with bicyclists (who make fun of you as they
pass you by) the plane will fly perfectly fine.
The same thing occurs in the boat. If your heading at 5kts into
a 5kt head, your air err... water speed is still 5kts, even if your ground speed
is 0. Since there is still water flowing over your rudder, it can still steer
you.
The real trick is crab angle. Imagine doing a circuit with a high wind.
Certainly you might have difficulty keeping a nice square pattern, and your
heading on the X-wind and base legs will have little resemblance to your track.
Maneuvering your boat into a port can be fun in a high current. Much like
landing in a stiff cross-wind.
The hull speed is the water speed, so current doesn't affect it.
As a pilot, you know when your cruising the wind doesn't affect how the plane
flies. It only affects navigation. A plane flies just as well in a tail-wind as
a head wind. It only gets dicey when you have to negotiate the boundary between
air and the ground (Like approaching a runway)
If you were on a motor boat in the middle of a body of water, the current would
have little effect on how the boat handled. (Again - it would affect navigation,
but not operation). The tricky part is when you have to negotiate between rocks
in a channel and the current. (just like ground and
air above)
Sailing adds a third dimension. Since you now care about the wind (To trim your
sails, and stay out of irons), the water (keep your keel and rudder
working) and the pointy jagged rocks that like you make holes in nice boats.
Each of these mediums move differently and affect your heading,
your course, and point of sail. Now here's something to noodle on:
You're motoring through the water (Your sails are up, but producing no thrust)
The current is 5kts from the North.
Wind (as measured on land) is 5kts from the East.
Your GPS reports a speed of 5kts and a track of due North.
What is your speed through the water?
What speed and direction is the wind traveling over your sails?
Now, turn the boat 180degrees, and keep the same engine power.
What is your ground (GPS) speed?
What speed and direction is the wind traveling over your sails? " (John Ross)
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"I am taking a stab at this, with the intent of learning,, so No
Laughing... sailing is still new for me.
On the north track with the GPS saying 5kts, I would assume with the motor
steady, the knot meter on the sailboat would read 10kts. Though the true ground
speed would only be 5kts indicated by the GPS. The wind, I am only assuming,
would be 0kts as we would be matching wind speed,, But,, since it is coming from
the east,, maybe the wind speed would still be the indicated 5kts from the east?
If I were flying I would have the nose in the wind and crabbing as you mentioned
before, though I don't think I would be doing that sailing.
180 degrees,, Down river with the flow.
Engine speed the same, basically providing 10kts of forward thrust, but this
time with a 5kts tail thrust. Hmmm... the GPS I would assume would give an
indicated ground speed of 15kts, though the knot meter on the sailboat would
give an indicated water speed of 5kts.. 10kts provided less the 5kts given.
Again with the wind speed, Just guessing here... nose not into the wind,,
indicated wind speed would be 20kts...5kts provided plus 15kts created...
blowing across the anemometer..." (Jeff - skid0023)
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"The wind wd be 7.07 knots from NE or 45 degrees of the compass - assuming as you say sailing North @ 5 knots and a 5 knot wind from East. Magnitude is Sqrt(25+25), direction Tan-1(1). That is, if I got my trig right :-). If you're going north at 5 knots you have both a 5 knot headwind as well as a 5 knot east wind. " -( Nick)
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Jeff - No worries. I found these concepts hard learn as well.
One the first one - your water speed is indeed 10kts.
The wind however is not 0. It is about 7kts from 045 (Nick gets the cookie!!!).
After the 180 degree turn, you are correct. Your GPS would indicate 15kts, and
your engine would be providing 10kts of thrust. Why would your knotmeter not
agree with the engine?
The Apparent wind would be about 16kts from 162degrees.
One thing that helped me was one of those disk flight computers (Such as:
http://sportys.com/pilotshop/product/12644, or a Jeppesen E6B). There's
nothing like making the pencil marks then spinning the wheels to help you
visualize what's going on. I was never good at trigonometry, but that method was
easy for me to pick up. (John Ross)
MORE of this discussion will be added later