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Old 08-21-2010, 11:44 AM   #1
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Subject: What draft & why?? I'm probably much to new here, in these 'forums' to start asking questions & making comments & have several handicaps; 1/ Male, 2/ Over 70, 3/ Dyslexic as all heck,4/ Semi retired - ex waterfront person for over 50 years (owned Fibreglass business' in 3 countries, 5/ going back to the sea, 6/ Am going to rebuild/design a GBE cat - Darwin aprox. Sept/Oct/10, 7/ Wish to 'cruise' - short & long distance, race (but only in the warmer cilme's) - to old to want to get cold (anything under 20*c).

Am sure I need mentoring by the likes of MMNETSEA. Richard, your guidance & advise is needed here, I'm sure.

What a great 'forum' - y'all have here!!!!! My special congrats to; 'Auzzee', 'Harborpilot', 'Istiopploos', 'JeanneP', 'Lighthouse', 'Magwas', 'Mmnetsea', 'Nausikaa', Oldsquizzy',

'Redbopeep', 'StormW', & of course many, many others in this great sight !! Truly unique & well above all the others. Thank Gawd I finally found this wonderful place & and all it's 'world-wide' SUPA people - here-in.

My question is: Why do so many 'yachts' have such great (deep) draft? Is it because people think that 'deep' makes for stability (keeping upright) or because our modern thinking has not filtered down to 'cruising' yachts? These are 'usually' - of 'older' yacht design philosophy, not that there is anything wrong with that thinking in any way. I have observed that mono's & multi's have undergone a 'major shift' design wise in the last 20 or so years, again, not that that in itself is necessarily 'for the better'. Example - I worked on a 56' mono, cruising in the 'around the world regatta' - on the hard in Cairns with 5'6" draft & a 'wing-keel' that was more stable (when sailing & at anchor) than many yachts with 6, 7 & 8' draft. It was also smoother sailing & quicker 'to' & 'off' the wind. Bob Millar - 12 mtr America's Cup.

As far as muitihulls (which is my personal choice, but not 'one-eyed-fixed') - getting a 30, 40, 50 or bigger multihull to go cruising (near land is a more enjoyable experience both because of the people one meets & the wonderful scenery to be enjoyed) - then have - fixed mini-keels, spade-hung rudders & deep-draft hulls thus 3, 4 & more foot draft IS something I can't seem to get my head around in any way, shape or form. For example I would like those interested to look up 2 multihulls that were designed & built in New Zealand. Designed by David Barker & both catamarans. These yacht were designed to be & go 'cruising' however proved very fast over a 'race course'. They are both light, quick, seaworthy & safe - that is if any yacht in the water is safe. Adrenaline might be a yachties 'Liebraumilch'. I admit to having a distinct liking for both.

I sure am looking forward to positive 'feed-back' form 'knowledgeable' yachting people in these forums. Please for the sake of the "FORUM" keep replies focused on 'positive', 'informative' & with - some basis in fact - to the fore. I would like to again say the GREATEST - Thanks to all. James - the geriatric fossile in Far North Queensland, Australia (where ever that might be). Ciao

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Old 08-21-2010, 12:27 PM   #2
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Thanks for the kind words. I am pleased you enjoy this forum. As you say, it is truly a world wide forum, which only enhances the degree of knowledge the forum members have available to them.

Onto the subject of draft.

Obviously cat sailors will have shallower draft vessels than mono-hull sailors but, as I pointed out in another recent post, it is all horses for courses. I sail in the north where we have deep water harbours and fjords and where cats are not so popular although, to be fair, I think they are on the increase even here. For my owm part, I have always favoured mono-hulls with long-keels and deep drafts simply because I think they have better seakeeping qualities than the more modern, flatter and wider hull form with fin keels. I am also a traditionalist so my choice is probably not based entirely on rational thought.

Interestingly, you mentioned the wing-keeled boat that you worked on in Cairns as being more stable (at anchor)than many deep draft vessels. There is a very large but false perception concerning stability. A vessel which is stable is usually not stable. Let me explain. By describing a vessel as stable a seaman means that a vessel when heeled by an external force will return to her upright position. There are different degrees of stability. A very stable vessel will have a large righting moment when heeled and will therefore quickly return to the upright position. Anyone who has sailed in destroyers will confirm this as these ships are very stable and, hence, very lively in a seaway. The classic, very stable object is the doll found in bird cages which, due to its round, lead filled base, bobs immediately upright again after the bird pushes it over. Having observed how such a doll behaves no-one would want to be in such a stable boat.

Ship's officers always try to maintain their vesels with an adequate stability. A vessel which is too stable is described as stiff, is lively and uncomfortable at sea. This condition can also put excessive strain on the vessel's structure.

A vessel which has less stability but still remains stable has an easier movement.

The art is retain sufficient stability to counter heel induced by external forces (waves) whilst not being excessively stiff.

This brings me back to my claim that a vessel which is stable is not stable. Many a ship's passenger has described a ship as being unstable as the ship has been lively and, being a landsman, the person in question associates stability with stability or, to malke it clearer, associates the inherant stability of the vessel with a lack of rolling.

To get back to the wing-keeled boat, she was obviously stable in that she retained positive stability but, it sounds to me, that she was less stable than the boats she was compared with as they had a livlier movement.

Draft and stability are related to some extent. A greater draft permits the further lowering of the centre of gravity. So long as the centre of gravity remains below the metacentre then the vessel will remain stable with a positive righting lever when heeled. The greater the metacentric height (i.e. the distance from the centre of gravity to the metacentre) the greater the resultant righting lever.

Should the metacentre and the centre of gravity be in the same position then the vessel will have neutral stability and should the centre of gravity move above the metacentre she will be unstable.

Complicated - not really but it takes a little though. All well explained here on wikipedia.

Once again - many thanks for the kind words.

Aye // Stephen
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Old 08-22-2010, 05:51 AM   #3
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Nausikaa - THANKS SO MUCH, big time! Now to good matters; Stephen that's seriously good looking yacht you have there. The 'Colin Archer' credentials she has sure do show-up & I'll bet 'London-to-a-brick' She sails like she looks. With 3.5 t of disp. she must move 'smoothly' (with a 'sea-kind' motion) through the water & be quite 'stable' (ha - there I go again). I'm sure we all envey you - your 'mistress' I sure do. With just under 3 years to go I'll bet you're 'champing-at-the-bit' to finally get to sea. Do keep us in touch so 'our souls can stay alive' through your eyes & experiences, Please

Now on the the 'Draft' subject. When I can get (the dyslexia-side of) my head around what you have said (& I thank-you) I'll try to ask many more questions. I regret using the adjective 'stable' which was obviously not correct. Maybe I should have used - 'comfortable motion' - both at rest & at sea ?? Or even to quote you - 'easier movement' - which is what I was trying to get across. This 'horse's' course is in Australia, New Zealand & South-east Asia where very large areas of the navigable cruising grounds call for approx. 300mm (more or less) of draft. My problem then is that designers, builders & so called 'experts' (& we all know that joke or should) do not allow 'cruising people' to purchase suitable vessels usually because they have little or no practical experience nor 'common-cruising-sense' of an kind or don't choose to use it. Gawd help a cruiser if they approach a new anchorage either - before 0700 or after 1600 - for they WILL do so at their own peril which can spoil your day, wreck the boat & spill your 'sundowners' - all NOT GOOD. Does the 'stable/stable' rule apply to small vessels as well as very large ones (destroyers)? Does the 'stable/stable' rule equally apply to modern yachts like yours & the one I'm about to redesign/rebuild? or are there new & different considerations? Sure hope 'JeanneP' joins in this discussion as they have 'encyclopedia's of knowledge' to assist everyone. Re 'parrot-dolls'. He He - couldn't stop laughing about your description as I've been breeding 'Australian parrots' for some 30 years & still do. Sure did get the point however & still laughing. I have 6 flying around in/outside of the house. Thanks!! Re; 'sufficient stability to counter heel induced by external forces (sea conditions & wind strengths). If you go into David Barker - New Zealand - artist/yacht designer - you can find sailing catamarans 'Sundancer' & 'Sundreamer' where David designed & built these 2 boats with 7 to 9 * of cant-out of the hulls which gave the same stability as a boat of 20% more beam which would weigh 35% more & be equally more difficult to sail, motor, berth, slip & beach. The net gain in his doing this was a gain of at least 30% in every function a boat has to preform. All of that at less cost & a more sea-kindly vessel in every way, even extending to - sailing fast & efficient both on the 'race-course' & cruising which is what both boats were designed to do. By the way 'Sundancer' (12.4 lwl x 2400 kg) draws 330mm of water - ready to go cruising, 'Sundreamer' (18 lwl x 4770 kg) draws 450 mm in the same cruising mode. Light weight/strength was achieved by smart design & meticulous building rules. It will take me quite some time to get my head around "metacentre" but I'll keep trying. Finally the 'kind words' were in 'fact' just that - - a statement of fact - which all of you should be very proud!! Ciao, from down-under, james aka jj aka Silver-Raven. Great Sailing all. PS - If JeanneP - should read this - - Have we heard anything further about Mike Harker?? You might open-up this subject again as it is MOST important (life threatening - in fact) to all of us that want to go cruising in these ever-increasing dangerous times. jj

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Old 08-22-2010, 06:21 AM   #4
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Hi,

good article on keels :CLICK

Remember friend who ran aground in mud in Phangnga Bay - Andaman Sea(Mud from decades of tin mining) The wing keel guaranteed that the normal way of releasing the boat from mother earth would not work - the wing was deep. Had to get a diver with a spade to remove the mud. Never had to do that with a cat.

Richard
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Old 08-22-2010, 02:07 PM   #5
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Hi,

My response in mauve


Nausikaa - THANKS SO MUCH, big time! Now to good matters; Stephen that's seriously good looking yacht you have there. The 'Colin Archer' credentials she has sure do show-up & I'll bet 'London-to-a-brick' She sails like she looks.

Again, many thanks for the complement. She suits me and, being a little selfish in that respect, that is all I can ask for

With 3.5 t of disp. she must move 'smoothly' (with a 'sea-kind' motion) through the water & be quite 'stable'

She is

With just under 3 years to go I'll bet you're 'champing-at-the-bit' to finally get to sea. Do keep us in touch so 'our souls can stay alive' through your eyes & experiences

Of course I will keep in touch - through these fori

Now on the the 'Draft' subject. When I can get (the dyslexia-side of) my head around what you have said (& I thank-you) I'll try to ask many more questions.

Ask away. That is what a forum is for. The more questions the better. Of course there are many others here who can give their views and opinions too

I regret using the adjective 'stable' which was obviously not correct. Maybe I should have used - 'comfortable motion' - both at rest & at sea ?? Or even to quote you - 'easier movement' - which is what I was trying to get across.

Don't regret it. Very many people would have said exactly that too. I am probaby being too pedantic

This 'horse's' course is in Australia, New Zealand & South-east Asia where very large areas of the navigable cruising grounds call for approx. 300mm (more or less) of draft. My problem then is that designers, builders & so called 'experts' (& we all know that joke or should) do not allow 'cruising people' to purchase suitable vessels usually because they have little or no practical experience nor 'common-cruising-sense' of an kind or don't choose to use it.

In general, I concur. However, there are still some very good cruising craft out there. One US design I particularly like is the Nor'Sea. It too is a 27-footer although I believe a 37-footer is on the cards too.

Does the 'stable/stable' rule apply to small vessels as well as very large ones (destroyers)? Does the 'stable/stable' rule equally apply to modern yachts like yours & the one I'm about to redesign/rebuild? or are there new & different considerations?

The rule applies to all vessels. There are, however, other considerations too such as form stability. A cat is a good example of this. The GM of a cat is not very large in comparisson to a long-keeler of similar length but the cat achieves stability through her form stability. Let me try to explain. A vessel's righting lever is a result of two things, the distance GZ (see the Wikipedia page I refered too) and the force of buoyancy (which is the same as displacement if the vessel is to remain afloat). The position of G (bentre of gravity) remains the same even when heeled but the position of the centre of buoyancy moves outwards and downwards towards the side to which the vessel is heeled. On a narrow-hulled vessel B can, as a result of the hull's dimensions, not move so far as it can in a wide hulled vessel so, assuming all other conditions remain constant, a wide hulled vessel is more stable than a narrow hulled vessel. Although a cat has two hulls, the effect remains the same although the centre of gravity will be between the hulls and the centre of buoyancy may be. Confusing? Yes but, like a jigsaw puzzle or differential calculus, everything will suddenly drop into place.



Again, I hope this helps

Aye // Stephen
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