MacGregorSailors.com

Two sailboats are going to race from Marina Del WalMart (MDW) in California to Catalina Island. To make the test controlled, blue hull boats are chosen over slower white hull slugs.

Captain Rich fires up his 50 horse Honda up and Captain Randy fires up his 90 horse Tohatsu. Both Captains slam their throttles down at the same instant. Captain Rich's mast falls down and his VHF radio falls overboard, making his boat lighter.

Who reaches Catalina first?

Sorry Rich, I could not resist. I am sitting with a boat in cold storage.

Heres an easy way to see the flaw in the 'delete the current' argument

Lets not let anyone tack - only run. 5 knot current, both boats.

Boat one has no wind. Takes one hour to go 5 nm. Easy.

Boat 2 has 5 kt wind dead astern. Apparent wind 0 kts. Takes one hour to go 5nm. Easy.

Now 'take away the current' from each boat.......

I think everyone will agree the outcome changes. Its a new problem.

Good puzzle Richard. I think you got me. If one considers that relative to the water's frame of reference there is actually a wind on a calm day due to the current and the boat should be able to tack "upwind" in the direction of the current. I don't think this violates Newton's laws. If we move everything to the water's frame of reference, there is kinetic energy supplied by this phantom wind. Thinking about it anymore makes my head hurt especially after having a few beers so I'm going to shut up and go to bed!

BB

Catigale,

Lets not let anyone tack - only run. 5 knot current, both boats.

Boat one has no wind. Takes one hour to go 5 nm. Easy.

Not correct. Boat one has apprent 5 knots wind blowing in direction opposite to the current and it pushes the boat back from finish line. (Let say 1 kn) It takes more[b/] then one hour to go 5 nm. (One hour and 12 minutes to be exact with four knots speed over ground)

Now 'take away the current' from each boat.......

Boat one has no wind and sits dead in the water at the start line not moving toward finish

Boat two has astern wind and make it drift toward finish. Let say 1 kn. it takes 5 hours to cover 5 miles and pass finish.

There is no flaw in in the 'delete the current' argument

adm wrote: There is no flaw in in the 'delete the current' argument

Almost but not quite, the flaw lies in "the argument" to begin with

Hello:
OK, so the boat with the 5 knt apparent wind has to tack so he must cover a greater distance added to the leeway the boat is going to have reducing his VMG. In effect he will no longer be going 5 knots parrallel to the current flow. It might just even out because the energy being added is resistive or opposite to the current flow. To extract energy, the wind has to resist the boat's movement downstream. Add drag to that cuz' there is no free ride. Sailboats can tack against the wind because of an excess of energy. The closer it points, the less energy is useful. Since the only energy available is in the form of resistance to forward movement there is no excess. Any energy extracted from the wind will be from it's forward or current movement.

and allowing him to take advantage of the airfoil shape, thus increasing his hull speed just slightly more than the existing 5 mph.

You're stating this as fact while providing no explanation. It's simply not so.

A boat starting from rest in a 5 mph current with zero mph wind will be traveling less than five mph. The windage against the boat will actually hold it back slightly, and it will be traveling backward in relation to the current. Assume for a moment you'll be able to figure out how to steer and while travelling in reverse, you'll be able to get enough steerageway to point the boat and properly to fill the sails. But don't forget, A boat traveling the same speed as the current will have no steerageway. Even assuming that all you have to do it throw up the sail and the airfoil shape will give you some speed cross current, you will never be able to accelerate through the zero relative water speed to the point you're traveling enough faster than the current to regain steerageway.

I think your bug guy should have stuck with bugs.

I've actually tried this and proven it to myself on the Hudson at flood. Even with winds substantially higher than the current speed. Every time, as the boat attempted to transition though the speed of the current, I completely lost directional control and the boat would not stay on the wind. As the centerboard stopped moving through the water, it became ineffective and with no lateral resistance, the wind rolled the boat over on its side.

As I attempted to recover from about the third time this happened, I smacked a bridge abutment. At that point I stopped trying to sail faster than the current and fired the motor.

Here's the next problem. How fast can a Mac with a 50 HP Tohatsu travelling with a 5mph current go?

RichandLori


That is why most adults can not help sixgrader with homework.

The boat with no wind will have an "apparent wind" of something shy of 5 knots coming directly from the front of the boat. Assuming a boat can generate boat speed of at least 1/2 the apparent wind this boat should be able to slowly tack downstream.

The 5 knot wind boat will experience an apparent wind of close to zero and will not be able to depend on the sails for power. As others have pointed out this boat will not be able to turn since no water is flowing over the foils.

In both examples I am forgetting about speed differences due to friction of the wind and water.

I'd say the no wind boat will win.

If Chip fires up his Tohatsu, his SOG will be his STW + current speed.

Chip,

I do not know how to post matematical equation so I am resorting to verbal description.

Value of combined speed is equal to square root of following sum: motoring speed squared , current speed squared and negative value of two times motoring speed times current speed times cosine of angle between speed vectors, where angle is less or equal 180 degrees.


Can we kill this tread and do not make fools out of ourselves.

Kaz - I think the WMD were hidden somewhere in your description...

In your post above you added windage and friction to the equation to refute the logical position of 'deleting current'. Thats a logical no-no.

If you neglect windage and friction, I think you have to agree that 'deleting the current' changes the outcome of the race???? My point was exactly that......the current is the same for both boats, but that doesnt mean you can delete it and not change the problem.

(im so disappointed no one took my bait for the three sailboats problem)

I love this thread as least as much as President Bush Likes Macgregors....please dont stop, Ill have to go back to work...

BB - you are correct, no laws of Physics are being violated for the case where there is no wind from the shore reference point.

In the frame of reference of the water, it is NOT a calm day, there IS a 5 mph REAL wind blowing!!!!! The shore is also moving backwards at 5mph with this wind.

STILL wondering HOW does one "SAIL" in "calm-no-wind" conditions...HA. I know what I do...1. turn on switch, 2. be sure fuel lines connected, 3. be sure everyone on board is knuckled down, 4. CRANK UP the 50 HORSES in my NISSAN....VAARRRRROOOOOOM !!! (ALL DONE AFTER LOWERING ALL SAILS)... ....

Catigale,

WMD - For my defence they were not armed - at least for now.
My point was that the entire discussion on public forum where stranger may lurk is about the problem which should be resolved by smart sixgrader or any sevengrader eligible to move to eight grade. Those strangers may get unfavorable (but generally unwarranted) impression about this forum. Because as adults we forgot long time ago what was taught in the schools at our tender age, I am pleading that we move to things we are good at as adults.

We can play with wind speed and direction, current speed and direction and even add extra factors.
As long as the changes are the same for each boat and we maintan the same reference point for both the outcame did not change in respect to this rece. Depending on the changes the boaters may not finsh at all or will have to circumnavigate and cross finish line from opposite side. I agree that this can be called a change of outcome of the race.
From abstract point of pure physics, one racer always has additional source of energy which he can use to win in the common sense.

I can make myself available to discuss it further off line by forum PM, email, telephone or even face to face.