A racing Brain teaser

[wheels 543]

Here is a Brain Teaser. It is from an engineering Website and it is possible a couple of others here also view this website. The actual answer is going to be posted on the 6th, so even those that have seen this before, can still offer there opinions. Although they most likely have been privy to many of the replies.

So here it is.

 

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?

[Guest]

The streaming boats will be head to the apparent breeze and should make it in 2 hours.

The tacking boat will eventually go aground.

The more I think about it the more confused I get as there is no mention of how well this particular boat sails in 5 knots, so I assume we need to leave that out of the equation. I also think that the answer might be a formula based on 2 hours rather than a specific time.

However, I reckon the tacking boat will win as it's adding to it's VMG, by how much, dunno, not yet.

[Guest]

Would the yachts floating with the current be slowed by the windage of apparent wind holding them back?

I thought the VMG close hauled info would be required to work out the time taken if the tacking option was taken, but think it would be quicker (if done well).

Is the correct answer to retire to the bar as there is no wind?

[MrWolf 0]

Here is a Brain Teaser. It is from an engineering Website and it is possible a couple of others here also view this website. The actual answer is going to be posted on the 6th, so even those that have seen this before, can still offer there opinions. Although they most likely have been privy to many of the replies.

So here it is.

 

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?

 

They all arrive together in a winning time of two hours

[Guest]

Would the yachts floating with the current be slowed by the windage of apparent wind holding them back?

No.

[col j 0]

what about the guy who hoists his kite underwater? theres always one

 

how many corners do they go round?

[John B 106]

Got to be a trick question but just to play the game, how fast did Oracle go in 5 knots of wind again?.. twenty something wasn't it.

[Adrianp 119]

Tacking Yacht wins in 1h, 20min!

[Guest]

Love to see your calculations HF.

[B00B00 310]

I would say the guy tacking will win, he is using the apparent breeze created by the boat moving through the air (5kms an hr) to make headway so will be moving thru the forward thru the water.

If he can do a VMG thru the water of 1km an hr he would be going 6kms vs 5km so I would say about 1 hr 40 for the tacking boat to take the win.

 

something like that anyway......

Its really no different to racing in the ditch outside westhaven, we would get up to 1.5 kts of tide on a good day!

[Guest Dry Reach]

Here is a Brain Teaser. It is from an engineering Website and it is possible a couple of others here also view this website. The actual answer is going to be posted on the 6th, so even those that have seen this before, can still offer there opinions. Although they most likely have been privy to many of the replies.

So here it is.

 

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?

 

I presume their is 0 knots breeze on the river?

 

and the opponents stream lined boat will do 5 knots with the river flow? (how streamlined is he?)

 

Also time taken for tacking option would depend on type of boat chosen (eg a p class versus a 40' keel boat and there respective speeds / VMG's) and how they get pushed along with the river flow versus how they generate Apparent wind.

 

to many variables.

 

without these details being confirmed there can be no calculation or comparison.

 

However if i was to guess?....

 

Since the race is going with the river flow (@5km'h) the guy who streamlines his boat and travels straight down the river will take 2 hours if he travels at 5km/h in a straight line

 

Any boat with sails will be slowed by the friction of the apparent wind versus the river flow and the extra distance sailed.

(eg- they could be (will?) be pushed downstream at 5 knots but the apparent wind will not generate the extra speed to off set the extra distance! plus the time to do tacks is dead time lost.)

 

also the width of the river is a key figure here! as this will determine how many tacks / dead time!!

 

 

 

 

soooo the streamlined boat will be faster.!

[Adrianp 119]

Love to see your calculations HF.

 

I just used a simple assumption that the tacking boat will make a VMG of 2.5kmh to windward in 5kmh of wind and hence has a total VMG of 7.5kmh meaning they will take 1.33 hours or 1h 20m to cover 10k.

 

I did have to draw a diagram to double check! 2.5kmh may be a bit optimistic thou

[Guest Dry Reach]

Love to see your calculations HF.

 

I just used a simple assumption that the tacking boat will make a VMG of 2.5kmh to windward in 5kmh of wind and hence has a total VMG of 7.5kmh meaning they will take 1.33 hours or 1h 20m to cover 10k.

 

I did have to draw a diagram to double check! 2.5kmh may be a bit optimistic thou

 

Very! and not an average that takes into account how many "down speed" tacks and how wide the river is!

 

these brain teasers are written by people who don't comsuder all the facts and details required to make an accurate calculation.

 

If the river was 10KM wide and you only had to tack once then that is different to a 80 metre wide river and 20 plus tacks!

 

look at the question and ask how wide is a "VERY WIDE RIVER"???

 

the answer is

 

Who cares.

[NevP 0]

I'd say the tacking boat will win in a little under 2 hours and the streamlined boat will take a little over 2 hours. But by how much is anyone's guess without a bit more info on performance, etc.

 

Looking at it a different way. If you couldn't see the shore or any other features fixed to the land this situation would appear exactly the same as;

- Zero current

- 5 km/hr wind blowing directly from the finish line

- finish line moving up the course at 5km/hr

 

Unless the tacking boat is a complete brick it should be able to make some VMG relative to the water towards the finish.

No matter how "streamlined" the other boat is to the wind it will be blown backwards relative to the water. And how they do they keep the boat "streamlined"? Ever tried holding a yacht head to wind?

[Guest]

I agree with Nev

[Guest Dry Reach]

I'd say the tacking boat will win in a little under 2 hours and the streamlined boat will take a little over 2 hours. But by how much is anyone's guess without a bit more info on performance, etc.

 

Looking at it a different way. If you couldn't see the shore or any other features fixed to the land this situation would appear exactly the same as;

- Zero current

- 5 km/hr wind blowing directly from the finish line

- finish line moving up the course at 5km/hr

 

Unless the tacking boat is a complete brick it should be able to make some VMG relative to the water towards the finish.

No matter how "streamlined" the other boat is to the wind it will be blown backwards relative to the water. And how they do they keep the boat "streamlined"? Ever tried holding a yacht head to wind?

 

egg zachary!

 

if the streamlined boat had 99% of its surface underwater and next to nothing making friction with the 5 km "wind' then it should hit close to 5km/h.

 

But i think there is no answer to this without all the info and wheels knows this and is sitting back having a right royal laugh!

[JK 28]

Assuming that the current is constant over the width of the "wide" river then both boats would make the same 5km downstream no matter where they were on the river.

 

The boat that is straighlining it will make only 5km/hr VMG to the mark, in practice it would be less as pushing into a 5km/hr headwind would slow their current induced progress and there would be some slippage of wind/current past the keel/sails, think sitting head to wind in 0knots of current they would be going backwards.

 

The tacking boat would be making additional VMG due to the 5km/hr apparent wind appearing to come from dead ahead - assuming they have to be capable of making some windward advantage in 5km/hr as it would be a sorry boat that made negative ground to windward in 2.5knots of wind with the current up their arse. Again it would be a bit less due to the less than 100% transfer of the current speed to the hull.

 

If it were just a wide finishing line they had to cross then it would always be the tacking boat, if they end up sailing a heap of extra distance due to overlaying the mark then it may well be the boat that goes straight for the mark that comes out ahead.

 

So, not enough information to be sure.

[Murky 1]

You are in a sailing race on a very wide river. On land there is no wind; it is a dead calm.

But we are on the river. And it is a very wide river, therefore the breeze could potentially be doing anything.

 

Think we are still short of a detail or two here...also think that many of us are going to be more than slightly irritated when the answer finally emerges.

[Guest]

With the current up the chuffer the keel isn't going to give as much lift so the VMG to the apparent wind going to be compromised.

It could be that you just go sideways and the drifters are quicker.

[Grinna 2]

Who wins the race? What is the winning time?

 

The correct answer is, of course, Knot Me will win the race because despite having a dodgy paintjob on the topsides and despite being 20 minutes late for the start, he somehow and quite annoyingly manages to go faster in the light than anyone else and just pip them at the line .... the bastid!!