A racing Brain teaser

[Guest Rocket]

Slacko believes in lee bow effect..... I don't - we sail on an escalator.

 

On that basis the tacking boat will win so long as they can make a positive VMG in 5km/h of wind - which is easy for some boats but not others...

 

Maybe the boat with the biggest motor wins.

[Guest]

I agree with Nev

I beg to differ.

No matter how much sail you have up, drifting with the tide is drifting with the tide. Take it to the extreme. whack up an acre of spinaker off the stern in a dead calm, are you saying that this will stop the boat? Of course not, it will still do 5km/hr downstream. The parachute effect will be infintessimal. Still think there's some tricky little equation to do with the air in the sail drifting as well. I notice that wheels bugger isn't saying much.

[Jon 360]

If it's dead calm on land the flow of the river will create a down stream breeze ?

[NevP 0]

No matter how much sail you have up, drifting with the tide is drifting with the tide. Take it to the extreme. whack up an acre of spinaker off the stern in a dead calm, are you saying that this will stop the boat? Of course not, it will still do 5km/hr downstream. The parachute effect will be infintessimal. Still think there's some tricky little equation to do with the air in the sail drifting as well. I notice that wheels bugger isn't saying much.

 

Nah, I'm not saying you'd stop dead but wind drag is wind drag no matter how infinitesimal and must have some effect and slow the streamlined boat. Remember your drifting in a 5 km/h current and no wind "over the ground" so you now have a 5 km/h breeze over the deck. That has to slow you some.

[Guest]

There's also some questions about lift off the keel/centreboard. Don't you need it to sail fullstop? Would there be any? What about the fact that the river will flow more slowly at the edges/ ends of each board? Should probably disregard that I guess. Also, would you not expect a certain amount of air to stay attached to the water and move with it? Therefore less apparent wind? The more I think about it the more I think that the lazy buggers are going to clean up.

[Clipper 343]

I agree with Nev

I beg to differ.

No matter how much sail you have up, drifting with the tide is drifting with the tide. Take it to the extreme. whack up an acre of spinaker off the stern in a dead calm, are you saying that this will stop the boat? Of course not, it will still do 5km/hr downstream. The parachute effect will be infintessimal. Still think there's some tricky little equation to do with the air in the sail drifting as well. I notice that wheels bugger isn't saying much.

 

The density of the air vs the density of the sea means the current will of course have a far great impact than the areodynamic drag

 

In simple terms and from memory:

 

Force=0.5 x density x drag coefficient x velocity squared.

 

Density of air = 1.2kg/m^3

Density of sea = 1020kg/m^2 (at a guess)

 

so same shape, at same speed, drag would be 850 times more from the sea than from the air.

 

Which is different from your point I think steve, but comes to the same conclusion.

[Guest Dry Reach]

In a real world, with all the information, a sailing boat could potentially do 5 knots (river flow velocity) plus and VMG it made towards the finish line.

 

what we don't know is...

 

 

what boat type and its speed/VMG

 

 

How many times it tacks... 1 or 101? Due to unknown river width!

 

How slow through that tacks it is. (time deduction)

 

and most of all - most you are all presuming that the 5 km/h river flow will push the "unknown type " of boat at 5 km/h and give it a true wind of 5km/h which will then generate a apparent wind due to its boat speed.

 

but that is assumption! as the boats potential speed in a presumed 5km/h wind may be slow (dog) or fast (racer).thus anybody who can make a prediction or finish time is...

 

 

Dreaming or guessing.

[Guest]

Last month we put 3 RIB's in at Cambridge, stopped at Rangiriri pub for the night, then carried on to port waikato the next day. All good fun. At times we had a flow of up to 5 knots. The water stayed glassy smooth which sort of indicates that a fair bit of air (low down at least) was attached to the water and moving with it. Also don't remember a 5 knot breeze in my face, I would have noticed that. Top of the mast might be a different story.

Thanks for the physics lesson Ken, always knew you were smarter than me.

[wheels 543]

Blimey!, I didn't realise sailing was so complicated

[Guest]

Blimey!, I didn't realise sailing was so complicated

 

yeah right

[Murky 1]

Last month we put 3 RIB's in at Cambridge, stopped at Rangiriri pub for the night, then carried on to port waikato the next day. All good fun

I like your version much better Broke Mariner. All you need to do is package it into some kind of question like how many pints were downed at the Rangiriri pub or did you catch a fish at Port Waikato or which RIB hit the top speed. That seems much more enjoyable.

[Guest]

Well after the driest summer in yonks, how many times do you reckon we went aground. Was my birthday leer up so had to do something other than sit around with a bottle of rum. Booked the whole pub out and had a ball.

[SloopJohnB 322]

Wheels have you been reading CR4?

 

I was going wait for the answer be posting this question.

[SloopJohnB 322]

Blimey!, I didn't realise sailing was so complicated

 

Some good theories on CR4.

 

Like this one.

 

Hi Guru, it all depends on the class of yacht in the race. Each class has design specifics such as waterline length and weight. being straight up and down with no wind means the least amount of hull is in the water. to answer this question I can see a "Class" of yacht must be applied to the question. the it is simply a matter of findinding out the 'below waterline area' of the design, including any keel and not forgetting the area of the rudder when side to the current too before we can work out drag from the curve of the hull. Another factor would be weight when considering momentum and drag. Lets take it that we are floating sideways in the deepest part of the river away from the shallows which drag the hull down (and where the current is slower). Now the last factor of great importance is the material used in the building of the hull. Magnetic attraction to water can slow things up considerable. Is the hull made of a material which rides on the emissions of current put out by the H2O itself. great speed is gained here.

 

Now we see why it is easier to put a yacht design sideways in a Current Tunnel and measure time in relation to drag than to work out the maths manually.

 

it is posible to get a correct answer to this question and when you do please give it to the ownwers of Americas Cup Class Yacht Oracle. Be nice to see a good challenge from them.

[wheels 543]

Yeah mate. I get a regular Global Specs newsletter online and the link was on that to CR4.

Man talk about differeing scenarios, both here and most especially on CR4.

I can't wait for the official answer now. I though the answer was simple and straight forward, but now I am confused.

[Murky 1]

I think MrWolf came through with the answer they will have been looking for.

 

Possibly not the most tidily-constructed question but perhaps the red herrings left lying around are all part of the fun.

 

Broke Mariner, I am guessing seven groundings in total with one RIB having been responsible for three of them. Am I close?

[Guest]

The fastest RIB (2 stroke) was always planing and so was necessarily further out of the water, so although it led the way and was supposed to warn others, it missed tree branches and shoals so that the second one often grounded heavily in the same water. Many times more than 7. The channel was very difficult to find at times and we spent a lot of time pulling the boats even when in the middle of the river. Obviously the wider it was, the shallower it was, so Rangiriri to Mercer was a minefield. The last few miles where the delta formed meant wading a lot. 2 weeks later would have meant a clear run. Really interesting around Puni/Tuakau where there are dozens of squatter type shacks which are developed maimais, some quite flash with ranch sliders, wharves, decks even power generation. For a bunch of yachties and fishermen/women it was an eye opener.

[bene_sails 0]

You are in a sailing race on a very wide river. On land there is no wind; it is a dead calm. The race is 10 Km downstream, and the river is running 5 Km/ hr. Your opponents decide that to make the best time, they will streamline their boats to the wind and float downstream as fast as they can. You decide on a tacking strategy, sailing back and forth across the river's width. Who wins the race? What is the winning time?

 

here is my first answer.

 

Assuming:

boats are class and identical

5km/h current is constant across the width of the river.

a 5km/h current generates a 5km/h head wind in the opposite direction

river is in a straight line (negate shortcuts on inside of bends)

no time is lost through tacking (hey its hypothetical)

a 5km/h headwind produces a 1km/h drag force when head to wind

a 5km/h wind at 45 degrees off the bow produces 2.5km/h forward motion

 

oponent

points with the current, directly into apparent wind and does not alter course or do anything to alter forces acting upon it (rock, skull, row.)

 

so forces would be

5km/h towards finish (river)

1km/h against (wind)

resulting in 4km/h VMG toward finish.

time to finish 2.5 hours

 

yourself

points at 45degrees to the apparent wind, towards the finish and does not alter course or change sail trim other than tacking.

 

forces are

5km/h towards finish (river)

2.5km/h at 45 degrees to the finish (Vector equation resulting in 1.75km/h towards the finish, 1.75km/h towards the bank. )

resulting in 6.75km/h toward finish.

time to finish 1.48 hours

 

you win.

 

now there are a lot of things i dont agree with in this equation.

 

1. i dont believe there would be any apparent wind on the river. at 5km/h i think the surface friction between the water and air would result in an air flow with the river. and considering rivers flow downhill there would be no resistance for the air to stop flowing with it.

2. i cant concieve that if there was an apparent wind that it would make a boat go slower than the current when there is no wind on land.

3. obviously there are a lot of things i have assumed dont or do apply, bearing in mind that i have taken this as a purely hypothetical situation and couldnt think of anything more boring to do... a boat race in no wind on a river?

 

theres my first take on this anyhow.

[Guest]

Grinna is right But lets assume we didn't start or started on time

 

Only Danaide as alluded to a possibly important factor everyone has seemed to have missed so far. It's something Pilots have to keep in mind and the reason the AC weather boats have tall sticky up thing for their spinning around thingies.

 

I'm picking the tacking boat, the other is just a tree stump or the equivalent of. Timewise???? knot enough info.

 

[edit] just posted only to spot Benny has picked up on it also, nice work.

[Guest]

Ha!

I love these things but usually fail miserably.My view is that there is insufficient information to calculate a finishing time for the tacking boat, therefore the tacking boat can be discounted. The boat that streamlines itself can presumably stay in the middle of the river therefore stay in maximum tide. As the tacking boats near the sides of the river they will fall out of the tide and their vmg will decrease. I don't believe wind resistance would make a substantial difference to the streamlined boat.

Therefore I believe the finishing time will be 2 hours. I'm looking forward to the correct answer Wheels. This is a bit like those videos you see on youtube of land yachts sailing faster than the wind directly down wind. B@&%t !!!!!