Sailboat on River

Been there, done that. The Hudson River is a prime example. (1) Beat down river 'till you get the hell out of the shipping channel, anchor, have lunch, swat flies, go for a swim. (2) Iron jenny. (3) Get run over by a tanker.

Hope this clears up your moot point.

Hm! Interesting problem. No wind, but drifting straight downstream the current creates a wind of 2 m/s.

I say tack. Whatever net forward velocity you gain will be added to the 2 m/s drift velocity. If the boat can beat upwind in a 2 m/s breeze (pretty light air) on a flat lake, then you can increase your net forward velocity on the river.

Figuring the exact speed gain would require the exact performance specs of the boat in question.

Great timing for your post... I had a discussion about this today with a friend that was sailing with me who had never been aboard. But we also had 15knot winds all day... plenty of trimming for the apparent angle.

Really... in your example you might as well drift since the sails are just going to flog around. The one only value tacking is going to give is a bit of steerage way but it all comes down to apparent wind.

Straight from wiki: http://en.wikipedia.org/wiki/Apparent_wind

Apparent wind velocity is the vector sum of the true wind and the headwind an object would experience in still air. The headwind velocity in still air is inverse of the object's velocity, therefore the apparent wind can also be defined as a vector subtraction: the Velocity of the wind minus the Velocity of the object.

Hi Switchman, if "the sails are just going to flog around" in the ~4 knot wind caused by my river's flow, lets quadruple that flow to ~16 knots (8 m/s), very rapid for a river, but just to illustrate the point...

What speed would your boat then average downstream (bank coordinates), given that you optimally tack and beat?

-J

Jorrie,

I just got into my office and only have a moment ... I'll come back to this as soon as I have both of these plants running smoothly.

I'll have to think about the boat speed part of your question.

The apparent wind will be 4knots (causing one's sails to luff or "flog" around) and 16 knots respectively. 16 knots I can make headway and increase spead.

I know you want the calculations involved and am interested to follow your line of thinking, but must return to this after a while.

Sailing on a river on a still day is identical to sailing on a lake on a windy day if you ignore the scenery moving past on the shoreline. In either case you can tack into the wind.

Interesting discussion at http://terrytao.wordpress.com/2009/03/23/sailing-into-the-wind-or-faster-than-the-wind/

As the river flows carrying the sailboat across the still air, it will generates thrust that will allow a sailboat to indeed travel at a ground (bottom of the river) speed higher than the river flow velocity. However faster that may be, in this case, the course of the boat along the river banks will be always be slower with the sails up by virtue of the drag of the boat in the water and the drift that the sailboat will endure.

If you imagine an ice boat (minimal drag) traveling on a 200m wide ice patch traveling down that river, at various angle against between 0 and 90 degrees relative to the flow of the river, I think it becomes obvious that it will never beat the river flow in terms of speed against the river banks (even though the 90 degrees could yield "unlimited" speed and obviously 0 degrees will get the ice boat to actually "back up")

Maybe a nicer way to put it : Reaching into the wind at a speed exceeding the wind velocity is not the same as gaining distance upwind at a rate higher than the wind velocity.

Hi Aviator, your ice patch drifting down the river with the iceboat on it makes for an interesting analysis. The ice patch should have very close to the actual flow speed of the river. If the ice boat can make any headway in a downstream direction relative to the ice (by tacking), it must be going faster than the water speed - as measured relative to the river bank.

I suppose an efficient sailing boat in the water could do the same, not so?

-J

Hum ! You are obviously right and I may have to remove my left foot from my mouth. HOWEVER, for the sake of the discussion, I believe the ice patch is flawed in one respect : One would expect a great deal of inertia from a large ice patch. On the other hand, if that tacking was going on for any lenght of time, one could reasonnably expect the ice patch to slow down as the force of the wind on the sail would really act to slow that patch down. In effect, the ice patch analogy temporarly removes the drift of the "sailboat". If we were to imagine a self-regenerative small ice patch under the ice boat, it would be drifting immediately and the result boat displacement along the river banks would indeed be slower then the river flow. Hence, I maintain the No Sail recommandation.

The vector representing the "ground trajectory" of the boat is the result of various forces : one is the apparent wind (river flow). The others are many but are all resulting from the first : Thrust generated by the sails, drag in the water, drag in the wind from the boat superstructure, opposition to any sliding by the keel... While the trajectory vector can be many times the lenght of the apparent wind speed vector, I maintain that there can be no geometric solution that would provide a resulting trajectory vector that would end "windward" of the apparent wind vector.

One can effectively tack and make way against the wind, at a hull speed even in excess of the wind but one cannot make way against the direction of the wind at a speed exceeding the speed of the wind itself which I believe represents an absolute limit. If this is indeed correct, one does not need to know the specifics of the boat as even a zero drag boat won't change the outcome.

Nonetheless, just in case, I am now getting ready to remove my second shoe...

Hi Aviator, you said: "One can effectively tack and make way against the wind, at a hull speed even in excess of the wind but one cannot make way against the direction of the wind at a speed exceeding the speed of the wind itself which I believe represents an absolute limit."

I agree, but does this still mean one can beat the river flow speed by tacking with a sail? In other words, will you win a downriver race against me that is just difting with the current?

-J

The problem I see with traveling (straight) by the river current alone is that the boat will not have any steerage at all when moving at the same rate as the water flow. Hence you will be subject to the currents and resulting eddies and may find yourself floundering along a bend. Hoisting a sail will create a resistance and allow you to tack to the apparent wind thus providing tiller control of the vessel.

Jorrie, I know you are looking for a numerical answer (and in all honesty I'm not the one to give it to you) but there seem to be so many potential variables (size of vessel, drag coefficient, mast size, keel size and type, etc.) that unless one could fill in all the blanks, (and knew what to do with them once filled in), it would be impossible to provide.

But I'll take that chance. My numerical answer is...........9192.

Hi mareng, I am waiting for someone to have the time to stick in the specifications of an actual boat and do the sums, at least approximately.

BTW, unless it is coded, I think your answer is out by some orders...

-J

My 48' vessel has a waterline of 36.9 ft and has a calculated hull speed of 8.14 knots based on the formula 1.34*√36.9. Interestingly enough though, experience shows me a consistant 8.7kt.

Figuring that with a 16kt apparent wind on 825sqft of wing I can make rated hull speed then I'm going to run down your river (with a maniacal grin ..because I LOVE speed)tacking at 50°.

I should be running a bit over 22.1 knots. Hope there's no logs floating in this river of yours... I don't want you having us calculating the drag induced from rapid sinking

Based on the formula Speed = √(c²+d²)
C= angle of current to vessel
c= sine C * current
d= vessel speed -(cosineC * current)

Hi Switchman, thanks for the figures!

So are you saying your boat will do 8 knots relative to the water, traveling at 50 degrees of the current, in a zig-zag pattern? This means you can gain 5 knots on this 16 knot river flow. Wow! But I think it's a bit optimistic...

-J

Jorrie,

It really is not optimistic. Once hull speed (speed over water) is achieved then it adds to the current with speed losses based on the angle. If I had that same 16knot apparent directly at my back on a vessel doing 10 knots I would be running the river at 26 knots.

Here in San Francisco bay we regularly have high winds over 20knots and one must reduce sail area to get a balance and match tide and wave conditions, speed, yet keep from overpowering the rig.

Its pretty typical here to ride on a 5.5kt tide and be making 14 to 15 knots on the gps.

Ga. Steerage being the problem. I've tried this on my Lightning (#14542). At such low relative speeds frictional losses, momentum, sectional resistance, displacement, viscosity, drag, etc. will confound even the best skipper. "Shoulda stayed on that tack" is enunciated at least once every sailing. In the days of canvas sails, wetting them would add to some efficiency. Not so with Dacron or Mylar.

My previous answer still stands. Stay on the tack that gets you out of the shipping channel.

Ya wanna crunch numbers, go to the 'Cosmology' section.

Jorrie, having given some more thought to your quest, I believe that most all standard calculations are assuming that the vessel will have way on.

That's funny, Mareng.

I was just doing the numbers on my vessel... in this exact page, when my email bleeped at me showing your post. As long as nothing breaks in the the next few minutes of my morning I will post them.

Oups ! Sorry : I guess I should have added my last post at the end of line. Anyway, have a nice day everyone !

i think you will always go slower than the river current, but if you have a government grant and have a HHO powered leaf blower on board all bets are off........

the truth is gong to cost you...... the sarcsm is free.....

I note that nobody ever supplied realistic numbers for your stated condition. 2 m/s is close enough to 4 knots to use that as a figure, and the sort of boat that can sail in a river 200 meters wide is small, like a Laser. The performance of a Laser can be seen from its polar diagram:

This polar does not show performance at 4 knots windspeed, but you can extrapolate, after noticing that when sailing close hauled (45 degrees to the wind) the boat speeds are relatively close (relatively insensitive to wind speed). This is because the laser will not easily plane upwind, and so is roughly limited to its hull speed (about 4.8 knots). (Off the wind, it can plane and do far more than hull speed: 16 knots in 25 knots of breeze, for example.)

So the Laser can do about 4.5 knots through the water at 45 degrees to a 8.5 knot wind. Extrapolating to 4 knots windspeed, we'd expect about 4.0 through the water, and a VMG of about 2.9 knots.

So, drifting, you'd expect about 3.8 knots across the ground. (A 4 knot breeze would cause a Laser to be pushed at about .2 knots, with the sail luffing and a skilled skipper keeping it head to wind.)

Sailing, you'd expect about 6.9 knots across the ground.

If it were a very foggy day, and the sailor could not see the river edges, (and somehow magically arrived there, not knowing he was in a river) he would sail the boat exactly as he would in a lake with a 4 knot breeze. If the river is deep and has a smooth bottom, even the ripples seen around the boat would appear just as they would on a lake with a 4 knot breeze.