John Passmore's amazing Heavenly Twins storm-story from 2000

Seajet said:
remember the surface drift set up by a strong wind has the whole top 1 metre layer of the sea going to leeward
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Now Dr Jin Wu working at the College of Marine Studies, University of Delaware in his paper titled "Sea Surface Wind Currents Induced by Wind and Waves" stated that only the top few mm are affected and then move at only 3.5% of wind speed. But oh no. All-Knowledge-Andy claims it's the top 100cm that are affected, and to a sufficient velocity that it affects a sailing boat.

Now who do I believe? Hmm, let's think...
 
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Very interesting observations, thanks to all. I hadn't realised there's much difference between the 26 and 27 ft versions of the HT.

The accommodation of cats between about 25-30ft seems to be a combination of low bridgedeck headroom and narrow full-height areas. I like the amount of space on cats' layout drawings, but I might end up pining for the limited floor area and better headroom of a Centaur, after a week.

The ability to anchor in shallows & dry out with impunity is part of the appeal; but maybe a Centaur's bilge keels are no less able, give or take a few inches of draft.

I reckon the HT's reputed durability - with the exception of John Passmore's ill-fated, hard-pressed boat - must add something for the adventurer too...though her terrible sloth when carrying world-girdling stores and fuel, must feel like an awful drawback to those setting out with trans-oceanic ambitions.

Maybe it's worth remembering that few monohulls at this size are really roomy or really fast; certainly none are both; or if they are, they can't dry-out like a cat.
 
AngusMcDoon said:
Now Dr Jin Wu working at the College of Marine Studies, University of Delaware in his paper titled "Sea Surface Wind Currents Induced by Wind and Waves" stated that only the top few mm are affected and then move at only 3.5% of wind speed. But oh no. All-Knowledge-Andy, claims it's the top 100cm that are affected, and to a sufficient velocity that it affects a sailing boat.

Now who do I believe? Hmm, let's think...
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Without wishing in any way to disparage Dr Jin, would a tiny movement of the top few mm explain the significant variations in sea level caused by on shore and off shore winds?

I keep a boat on a swinging mooring in a local freshwater loch, and recently spent an afternoon on the mooring in quite gusty conditions. It was noticeable that each gust was accompanied by a substantial flow of water past the boat.

Of course these both might be pressure effects.
 
NP66, the Admiralty Pilot, gives a surface current of around one fortieth of the wind speed, at an angle of about 30 deg to the right of the direction the wind is blowing to.
The current may lag 'some hours' after the onset of the wind, and persisist afterwards for longer.
The Hydrographer doesn't say how deep the current runs.

1.112, 15th edition
 
Angus,

10/10 in your efforts to rubbish anything I say, but as usual you're barking up the wrong tree.

I have been out quite a few times after a gale has blown for days, and the leeward set of the sea has been noticeable; funnily enough when I allowed for it with DR it worked out right.

Even more disappointingly for you, I didn't make up the ' one metre deep ' bit myself, I read it in books on navigation and was also taught it by very experienced Yachtmaster Instructors.

Try applying common sense...
 
Seajet said:
Even more disappointingly for you, I didn't make up the ' one metre deep ' bit myself, I read it in books on navigation and was also taught it by very experienced Yachtmaster Instructors.
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Dr Wu has been a busy chap. Here's another paper from him that shows that at 7.5mm depth the wind induced drift, which is not very great to start with, has halved...

http://www.dtic.mil/dtic/tr/fulltext/u2/770145.pdf

And here's another that says that the wind induced water velocity at 5cm is zero, and below that it actually reverses...

Simulation of wind-induced currents using a multi-level model, H. Tsuruya, K.P.P. Pathirana, Y. Nakagawa, Transactions on Ecology and the Environment vol 12 1996

Now I'm not an oceanographer, so I don't know who is right - these academics who publish in refereed journals and conferences or the authors of some unspecified books you claim to have read and the second hand quotes of some anonymous instructor you might have met. Any references?

Seajet said:
Try applying common sense...
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Is that your answer to rigourous science? Does the finding that wind induced drift reverses direction a few cm down agree with common sense?
 
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Angus,

all you are proving is what a thoroughly unpleasant person you are !

Funnily enough I have read more than one book in my time, including several on navigation so I don't remember which ones I learned this from.

One Yachtmaster Instructor among several who mentioned it was called Colin, I didn't remember his surname as it's a while since I did the evening classes he ran at Horsham West Sussex.

You could also ask Pete Misson and Mike Dymond, who instructed on my YM offshore practical.

Why do you think deep fin keelers do so well to windward in heavy weather then, Sherlock ?

As I say I have put this one metre deep surface drift to the test and found it correct; you can believe what you like, with a bit of luck you will rely on it being a few mm which is idiotic and end up somewhere ghastly where you'll be right at home; it doesn't take Einstein to consider surface drift just a bit of experience and imagination.

Incidentally I have always thought one metre a suspiciously round figure, it must vary a little; but it seems a good realistic figure to use in navigation.
 
Seajet said:
Angus,

all you are proving is what a thoroughly unpleasant person you are !

Funnily enough I have read more than one book in my time, including several on navigation so I don't remember which ones I learned this from.

One Yachtmaster Instructor among several who mentioned it was called Colin, I didn't remember his surname as it's a while since I did the evening classes he ran at Horsham West Sussex.

You could also ask Pete Misson and Mike Dymond, who instructed on my YM offshore practical.

Why do you think deep fin keelers do so well to windward in heavy weather then, Sherlock ?

As I say I have put this one metre deep surface drift to the test and found it correct; you can believe what you like, with a bit of luck you will rely on it being a few mm which is idiotic and end up somewhere ghastly where you'll be right at home; it doesn't take Einstein to consider surface drift just a bit of experience and imagination.

Incidentally I have always thought one metre a suspiciously round figure, it must vary a little; but it seems a good realistic figure to use in navigation.
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I'm not quite sure why giving references to 3 scientific papers is unpleasant. I am questioning an unsubstatiated claim of yours that disagrees with scientific literature.

I'm also not sure why the fact that deep fin keeled boats being better to windward that shallow draft boats has anything to do with wind induced surface drift. My understanding is that they are better because they are a more hydrodynamically efficient shape at resisting sideways force because they are better at creating lift. Are you saying it's because they can reach below this wind induced drift? That's a theory new to science if you are.

If the wind induced drift does extend to 1m below the surface, how do you use this value in your navigation calculations? The direction of this drift is not parallel to the wind, nor is the direction constant as depth decreases. Your DR calculations must be fiendishly complex if you do use it.

I don't rely on the depth of this drift being mm or metres deep because I don't take account of it in my sailing calculations. But if I did believe it to be only a very shallow effect, why is that idiotic? I would not have guessed that the direction reverses with depth, but I am quite happy to accept Tsuruya et al's findings. Is that also idiotic?

And when you claim to have put your theory to test, how did you take account of leeway to distinguish it from wind drift? Leeway is a notoriously difficult value to measure or even estimate with any accuracy in sailing boats. It is very dependent on seastate, hull shape, sail efficiency, point of sail and more.

When you say you want me to end up somewhere ghastly, are you wishing shipwreck on me?
 
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Being called Colin doesn't in itself, necessarily, imply infallibility.

Seajet said:
Angus,

all you are proving is what a thoroughly unpleasant person you are !

Funnily enough I have read more than one book in my time, including several on navigation so I don't remember which ones I learned this from.

One Yachtmaster Instructor among several who mentioned it was called Colin, I didn't remember his surname as it's a while since I did the evening classes he ran at Horsham West Sussex.

You could also ask Pete Misson and Mike Dymond, who instructed on my YM offshore practical.

Why do you think deep fin keelers do so well to windward in heavy weather then, Sherlock ?

As I say I have put this one metre deep surface drift to the test and found it correct; you can believe what you like, with a bit of luck you will rely on it being a few mm which is idiotic and end up somewhere ghastly where you'll be right at home; it doesn't take Einstein to consider surface drift just a bit of experience and imagination.

Incidentally I have always thought one metre a suspiciously round figure, it must vary a little; but it seems a good realistic figure to use in navigation.
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Angus,

no of course I'm not wishing shipwreck on you; I hope your boat is fine !

I calculate leeway depending on apparent wind, boat & wind speed - separate to surface drift which is significant after a strong wind in the same direction for I'd guestimate over one day, more for longer periods.

It's your attitude trying to attack me which is unpleasant;you come across some obscure Japanese paper and see it as ammo' to have a go at me, not passing it on as information you think might be useful.

You asked for references and I gave them; I can safely say any of the instructors mentioned are far better than you or I will ever be.

If you choose to believe surface drift after prolonged strong winds is only a few mm deep, well I question your judgement - and I don't know what context this scientist I've never heard of said it in - but if you think it's right then try it, as I have tried what I was told and found it worked, quite a few times.
 
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Seajet said:
If you choose to believe surface drift after prolonged strong winds is only a few mm deep, well I question your judgement - and I don't know what context this scientist I've never heard of said it in - but if you think it's right then try it, as I have tried what I was told and found it worked, quite a few times.
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Professor Wu, alas now departed, is hardly obscure...

http://www.udel.edu/PR/UDaily/2008/jan/jinwu011808.html

He was a leading expert in sea surface and wave motion. There are plenty of his papers around here where I work. The autopilot people use his findings extensively. I am not an autopilot expert, I hasten to add.
 
Angus,

if he's departed, was he going on a few mm surface drift ?!

Please note I never said I'm the expert who came up with ' one metre deep ', that was a lot of VERY experienced sailors.

Be careful with ' experts ' though; the people I trusted proved right in spades and their sailing accomplishments such as getting through the 1979 Fastnet without a scratch and Cox'n of the Jersey Lifeboat as well as commanding submarines and container ships makes me think they probably know what they're talking about !

However I have known other experts such as really brilliant Test Pilots who still took a chance once too often and flew into the ground, other Commercial Pilots ( sort of a more arduous & expensive Yachtmaster ) who navigated us over the wrong town and / or ran out of fuel mid-air, and laser experts ( Scottish and American consultants ) who mistakenly lasered Scottish towns 2 miles away with a laser whose blinding range was 9 miles...

So don't believe ' experts ' over those who have proven what they say and common sense; Custer was a qualified General, but he still said " leave the gatling guns boys, we won't be needing them " !
 
You can't have lots of surface drift and no return, or the sea is going to end up lopsided. So what drifts one way must drift back the other, underwater probably. So I'm going with the common sense theory that if the top x mm of the sea surface is going one way, then there's a layer of y mm subsurface going the other way, and that thus they cancel each other out on my keel and I needn't give a toss.

(where x and y are numbers between 3 and 1000mm, depending on your reading material, your mates' opinions and your inside leg measurement.)
 
In case anyone's still reading this for references to Heavenly Twins catamarans, can I ask if anybody knows the approximate headroom, in the saloon at the spot between the sofas?
 
Pete7 said:
But isn't that exactly what does happen, doesn't the HW time and height in the Solent change with a good SW blow?

Pete
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Isn't that what they call storm surge, not surface drift? Attached pic shows one odd result of storm surge, at Felixstowe, about five winters back.

bench.jpg
 
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