How water ballast works

[Michael Rogers]

I've now read Teacher's marking. Bah! Humbug! Spoiled my day. I would sulk, but the weather outside is too nice, and I was fettling 'Cavatina' yesterday - not long now! I'm off to my music lesson.

[Michael Rogers]

Just in case it wasn't obvious, I was joshing. And the weather here continues early-Spring-like. Doesn't half lift the spirits!

On the train back from Bath last night, we sat opposite a young man with a calculator and a pad covered with Maths scrawling. He told us he was studying for an exam to be a (merchant) naval engineer so, perhaps a bit mischievously, I gave him the Homework. He didn't get the answer right either! But he agreed it was a Good Question.

[Andy Dingle]

Peter. Just sat down in a quiet room for a hour or two to read your explanation of how water ballast works (better late than never!), excellent stuff and I would like to thank you for the work you put into it. Very well done - the half pint of Milk Stout I promised you is very well earned! (The boy's got an 'ology!).

Now, when I was sailing fast catamarans, I used to earwig the bright, younger and fitter sailors talking of 'getting into the groove' and sailing off the apparent wind, in effect sailing faster than the wind was blowing... always had me walking away thinking their roll ups contained more than 'Golden Virginia'... !
But is this possible? If so, how is it possible? I do have an understanding of apparent wind but if you were so minded to want to consume that other half of Milk Stout at my expense, then I would welcome your thoughts on this one too!!

Regards

Andy
Baycruiser23 No.25 'Equinox'

[Michael Rogers]

Hallo Andy

The subject you raise came up in 2013, started by yours truly actually when I was totally gobsmacked by the extraordinary America's cup TV coverage, and (like you) unable to understand how those amazing 'boats' could do 40+ knots in a 20 knot wind (they called off racing if the wind went over 20kn!). Peter (Cockerton) provided a link to an excellent article which I printed off and sort-of understood (any obstacle being my limited abilities rather than the quality of the article). It has all sorts of practically useful stuff about real and apparent wind.

I should be able to, but can't, move these links about. If you use the (absolutely brilliant! - thanks, Jonathan) Search facility on the Forum, put in 'The America's Cup', it'll come up - I think Peter C's contribution is the second one.

If, however, Peter (Taylor) feels like having a go at something on the subject for the Library, I'm sure that would be appreciated by one and all.

Writing as someone with an ale-drinker's opinion on/contempt for half pints, couldn't you be a tad more generous with your Milk Stout, Andy??!!

Michael     (Trouper 12 'Cavatina")

[Graham W]

Peter (Cockerton) provided a link to an excellent article which I printed off and sort-of understood (any obstacle being my limited abilities rather than the quality of the article). It has all sorts of practically useful stuff about real and apparent wind?

Here's the original article http://www.sailnet.com/forums/learning-sail-articles/18848-understanding-apparent-wind.html

[Andy Dingle]

I thought to incite (provoke!) another interesting sailing related discussion point, seems to me at this time of year we ought to be girding our loins and getting all excited at the start of another sailing season, I should have known better not to realise that this subject had already been thrashed out!

The quoted link is to quite an excellent article on an amazing subject, I bought it up in the pub the other day, but quite quickly found myself standing on my own ... Goodness knows why. Or perhaps it was due to the half pints I kept buying... !

Anyone got any other suggestions for a discussion point..?

Now, I did read of this chap who came up with a mathematical formulae to convert the power of sail into horse power... very interesting I thought..!  No?  ..  probably not..

[Graham W]

This one? http://forum.woodenboat.com/showthread.php?119686-Formula-for-quot-Sail-area-Horsepower-quot

On the basis of some of the figures quoted (30-50 sq ft in an F4 = 1hp), the BR's rig generates 4-6hp, which sounds about right.

[Andy Dingle]

Ha! .. Isn't there anything original under the sun anymore... !

[Peter Taylor]

Re. sailing faster than the wind:
it's a misconception that yachts can sail faster than the wind because of the apparent wind effect - although the increase in apparent wind *speed* does help the yacht to go faster, the change in apparent wind *direction* acts to limit the ultimate speed that can be reached.

An object (let's call it a yacht) will continue to accelerate as long as the force pushing it forward is greater than any resistance to its movement.  The yacht's sails are like a "motor" which works by making air blow over a wing shaped object (the sail) which generates a force which accelerates the boat in a forward direction.  As long as that force is greater than the water drag on the boat the speed will continue to increase.  If you can reduce the water drag enough (by planing, using foils to lift the hull, or simply making it very streamlined as e.g. a C class cat) then (as long as your "motor" keeps working) there is no reason the boat should not keep accelerating until it's going faster than the wind.

But (you say!), my yacht's "motor" makes use of the wind in order for it to work... so if I go faster it won't keep working! That is where the apparent wind effect limits the speed you can reach.  The faster you go, the more the apparent wind direction comes from ahead.  You need your wing shape to be very efficient so that it can generate enough forward force even when the apparent wind is coming from a direction close to the boat's bow.  If your "motor" can generate enough forward force to overcome drag up to about 30 degrees to the apparent wind you will be limited to around 2 times the wind speed.  You will achieve that speed on a course which would be a broad reach if you were not moving (i.e. wind on the quarter) but which turns into a very close beat once you are up to speed!  If you try to go any faster than 2 times the wind speed then, once you are up to speed, the apparent wind will be closer to the bow than your 30 degree limit and your "motor" will not be producing enough force.

Apparently the 2013 America's Cup boats managed about 1.8 times the wind speed with peaks of just over 2.  They did that by cutting down the water drag by foiling and by having very efficient "motors" - aerofoil shaped wing sails - allowing them to point close to the apparent wind .

Ice yachts manage several times the wind speed because the frictional drag they have to overcome is so low. That means that although the apparent wind is almost from ahead - so the sail is generating little forward force - what little force the sail can generate is enough to overcome the friction of the ice plus the air drag on the sail and ice yacht.

Peter

[Graham W]

Ha! .. Isn't there anything original under the sun anymore... !

Indeed!  "What has been is what will be, and what has been done is what will be done, and there is nothing new under the sun."

Speaking of the sun, wouldn't it be good if it was warm enough to put some of this theory into practice?

[Michael Rogers]

Thanks again, Peter T

As always with your contributions, you have an enviable knack of making complex ideas seem simple. Taken together with the article Peter C referred us to, your explanation has me feeling that I probably now understand the general idea - quite a novel situation for yours truly around such matters!

You have also indirectly explained why the America's Cup boats steered what seemed to me such weird courses: and why they needed computers on board to work out the optimum balance between 'chasing' maximum apparent wind speed and getting first to the mark or finishing line.

Fascinating to watch how Ben Ainslie et al get on. For myself I'm happy to stay with Swallows and Amazons- style sailing.

Michael

[Andy Dingle]

Peter. I want you in my pub quiz team....

(And not the quiz we entered during the last Morbihan week where strangely we managed to win 100 euro's and various other goodies, despite the quiz being in French, where not a single member of our crew could barely say 'voulez-vous couchette avec moi ce soir' or something, without getting a slap around the face. (Apologies to Lady M).
Very odd that one?! -   (edited, as an afterthought, in case any frenchies read this!) Very, very odd......

Do you even like Milk Stout?

[Peter Taylor]

Andy - re. Milk Stout: I seem to remember that Mackeson didn't taste that bad... however your pocket will be overjoyed to learn that nowadays, having imbibed far too much in the past, my drinking is limited to lime and soda which costs all of 50p a pint at our local pub! And now you also know why I can afford Seatern!

Returning to somewhere close to the thread (although nothing to do with water ballast):  I wonder how many of you know that it's possible to build a wind powered vehicle that can travel directly forward into the wind, and even travel directly downwind faster than the wind.  I remember seeing a small model demonstrating the possibility many years ago when I was a physics student.  The model was of the cotton-reel-wheels and model-aircraft-propeller type of construction but worked very well.

Since those long ago days it appears that the technique periodically gets forgotten and then reinvented. See http://en.wikipedia.org/wiki/Blackbird_(land_yacht)_  for a recent incarnation.  It works fine for a land vehicle driven via wheels, in theory it should work for a boat driven by an in water propeller, however I suspect that, in practice, the inefficiency of the propeller plus the drag on the boat would be too great.  However might it work for a propeller driven boat once lifted onto foils? America's Cup here I come!

Peter

ps just to confirm "there's nothing new under the sun" I've discovered it has already been done for a model boat - see http://www.sailwings.net/windspinner.html - At least this demonstrates the upwind case, the downwind case is more challenging.

pps Andy, your Morbihan quiz reminds me of a TV sketch (?Not the Nine O'clock News?) which proved that penguins were as intelligent as humans. The premise was that penguins only fail IQ tests because of language problems. A penguin was lined up with a number of non-english speaking people (French, German, etc.) and asked questions in English and the humans scored no higher than the penguin - theory proved!

[Julian Swindell]

I think a Red Wing yacht was once built which had a huge wind mill instead of a mainsail (the argument being that only the sail area was restricted, not it's shape, although it had to be red). The windmill powered a prop and it could go directly into the wind. Apart from that, it did everything else really badly and so was abandoned. There is a small fleet of Red Wings in Poole Harbour. Beautiful wooden boats with tall thin Bermudian sails (red of course)

[david]

Peter/Julian, Looks like people have moved up from the model stage! Here is a full size catamaran that uses your "windmill"  type propulsion.

https://www.youtube.com/watch?v=IzGCYaJbf0A