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Thread: solving a mystery
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14th February 2011, 12:57 PM #16
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14th February 2011 12:57 PM # ADSGoogle Adsense Advertisement
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15th February 2011, 10:02 AM #17
Brian,
I'm not an expert in racing in small boats - you are an expert.
I'm careful recreational sailor than I always gybe really careful - similar like this:
[ame="http://www.youtube.com/watch?v=X-joHCA-o04"]YouTube - Sailing Canoe BETH YuanFen - Dziekanowskie Lake (Poland) - gybeing[/ame]
(Wind up to Force 4 in gusts (smooth and flat water - tiny lake), first reef on mainsail taken. I've kneel a midship.)
Probably Raid41 has a bit less of stability than BETH - because the first one has higher sides and higher positioned center of weight and larger sail area than second one. Whenever BETH is a bit smoother (she has less of breadth a bit).
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15th February 2011, 05:10 PM #18
I think the point about feedback is an important one.
The RAID has much more initial stability than BETH, but a higher centre of gravity.
The height of the centre of gravity doesn't have a lot of effect if you are in the cockpit
Beth is so close to the water that you get a pretty good feeling of her balance and trim at all times. It could be that the big wide flat cockpit and high sides hides the water to some extent on the RAID.
Also in a gust and when a gust ends BETH will respond by heeling pretty quickly as the righting moment from the hull is relatively small - so you can easily have momentary excesses of windward and leeward heeling as the wind pressure, direction and sail trim fluctuate.
MIK
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15th February 2011, 08:58 PM #19SENIOR MEMBER
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Robert, thanks for posting the video.
lets say you and I weigh about the same, rigs probably about the same and hulls were probably not much different. Raid41 was very light for her size.
Ok. Your much narrower hull is carrying the same weight as my hull. So your Beth is immersed more, she is sat in and in contact with the water. So the hull is actually doing it's job, supporting the boat. As she moves around bits of hull buoyancy are performing.
Now, we also know that when RAID41's tanks were full she also felt completely different, secure, I never capsized her once with the tanks full.
So, Robert imagine your boats made of mylar film and carbon tubes, sails of parashute silk and you weigh 30 kg say. Everything is different.
Hulls are designed to sit on their designed water lines. With nothing in the tanks, and only one on board Raid was sitting so high her hull shape, wasn't even in the water, she sat like a leaf on the surface.
Why not just sail with full tanks? Well Chris and I ended up putting in side tanks which when tested drove her over and the mast under after a trial capsize in shallow water. Why not fill under floor tanks - Bryan capsized with full under floor tanks, although yet again poor Raid had double the amount she should have had.
poor raid41. Fed up.
Brian
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15th February 2011, 10:33 PM #20
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16th February 2011, 12:25 AM #21
Capsizes in gybeing very often unexpected are - did you seen this video (especially 6:35 - 7:04)?
[ame=http://www.youtube.com/watch?v=p-M4cpLUoDw]YouTube - RAID Finland 2008[/ame]
Wind gusting irregulary, due to interference on the shore. Apple with two of crew (both my friends are! Peter and Wojtek), and main sail well-reefed. Running and rowing but main eased over 90 degree (I think). Gust!!! And a really short moment of inattention at helm and behaviour (moving to port side) of skipper than boat passed the stern to wind (gybe, but wihout a turn of sail!). Sail eased over 90 degree caused a capsize.
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16th February 2011, 01:25 AM #22SENIOR MEMBER
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Hi Robert, Peter will kill you for posting that. I will certainly tees him when we next are in contact!
Such a simple yet unexpected capsize. Busy rowing out, rig blowing free, gust comes and quietly capsizes. Shows how easy it is.
Thanks, Robert.
Brian
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16th February 2011, 01:51 AM #23
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24th February 2011, 11:38 AM #24Intermediate Member
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Hi all,
Dumb question:
Phil Bolger used to talk about a "sea of frozen peas" hydrodynamic theory, where the ideal chined boat shape had a longitudinal curve to the sides identical to the bottom panel's rocker. If these curves weren't equal, water (or frozen peas?) would curve up over the chine from the greater curved panel to the lesser, creating chaotic vortices at the chine, and squirrelly sailing behaviour - especially downwind, when the stem digs in.
Chappelle also writes about keeping the foot of the stem above the waterline in sharpies, because of tripping & uncontrolled erratic bahaviour caused by chaotic currents around the chines.
Is there any credence in this theory? I have always been a bit suspicious of the frozen peas idea, just because it doesn't seem to take a boat's leeway into account - the perfect situation where there are no vortices at a chine sounds a bit platonic/perfect world.
What do you smarter / more experienced people think?
Jack
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24th February 2011, 12:29 PM #25
i think, like most of Bolger's writings that it bears some consideration because of the point ...
Some similarity in the curves might reduce vortices.
However, the question is how much.
The example falls down because it is a static point of view. As soon as the boat is moving it will generate waves. That will throw the peas out of alignment.
My sense is the example means ... don't do anything stupid. I do think the water tends to want to align with the chine more than the peas would suggest.
Generally systems tend to move to the dynamic that will minimise energy.
MIK
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24th February 2011, 06:56 PM #26
Bolger was talking more about "perfect fluid" dynamics then reality, with his floating peas thing. Fluid dynamics in either fluid type, is a pretty sophisticated subject and this was an attempt to offer some understanding to the masses, which the peas idea does fairly well, unless you're the type that knows more then you should or needs to know, then the peas hit the fan and it gets messy.
The same is true of his elevation and plan curve ideas. In the size boats he was discussing, of the general shapes he employed and most importantly at the relative S/L ratios he expected from these designs, the similar curve theory is true (mostly). Venture very far from any of these variables and everything goes out the window, concept wise.
In the video of the boat above, a substantial and clearly fast moving gust caught them and then they "rolled" it in with inappropriate actions. This is a skippering error. It's obviously a blustery day and they're reefed. Even the guy on the shore could see what was happening before it did, proclaiming a capsize while it was just occurring, obviously before the skipper of the boat knew what was going on, suggesting his (the guy on shore) expertise at least.
As far as buried bows, rocker and peas, it's a very convoluted set of concessions and compromises that we live with for specific design attributes. All play a part, some more important then others, none singly common amongst other designs. Bolger tried hard to bring a common man's understanding to many of the principles involved, though by his own admission he glossed over some aspects to define others simply.
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27th February 2011, 10:09 AM #27
A bit off topic, but I have a question about BETH's stability if swamped and recovering.
Is she stable enaugh if recoverered and swamped for skipper's return on board? I'm a bit over 75 kg + suit. My BETH "YuanFen" is circa 50 kg (birch plywood and pine timbers) and spars are heavier than designed also (pine).
Look there (wooden sailing canoe - I.Oughtred's Rob Roy - capsizing - ca 1:35 - without side flotation chambers):
[ame="http://www.youtube.com/watch?v=uUT_hpklx2Y"]YouTube - Swift's Swim[/ame]
and there (Japanese GRP or polyethylene sailing canoe capsizing with side flotation chambers):
[ame="http://www.youtube.com/watch?v=3vUJ-Rfmdfw"]YouTube -[/ame]
[ame="http://www.youtube.com/watch?v=YHGSqWpGh0Q"]YouTube -[/ame]
Can I use additional side flotation - for example styrofoam bloks under side decks or air chambers? Or it is unnecessery that stability swamped BETH would be OK for me at sea???
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27th February 2011, 10:24 AM #28SENIOR MEMBER
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Hi Robert, realise MIK will answer this but I just want to mention my experience with side buoyancy bags on my first open sailing canoe.
When laid on her side she floated very high, very little water in the boat. I righted her by pulling down on the gunnel as you do. This downward pressure pushed her much lower and as she righted she pulled up an enormous amount of water. She sort of scooped up loads of water. She was very full when upright. Totally unstable. This was when I realised I needed a decked canoe, which I guess you have in Beth.
The thing is to think about righting her without all the downward pressure. perhaps lifting the lower gunnel from below as well as pulling over from the top gunnel.
RAID41 proved some water in the boat is important, so Beth will be much heavier which is what you want, but not too heavy.
The other thing I have learnt is that there is no comparison between capsize testing in flat shallow water and actually in the rough deep water you capsize in in a realsituation. You are going to have to practice deep rough water capsize recovery - after learning in the shallow water first!
Brian
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27th February 2011, 10:43 AM #29
Thank you Brian! Your information is very helpful for me.
And what is above exactly is planned - probably in the first weekend of July I will be able to practice on a larger shallow lake (Jamno) and sea (Baltic Sea).
Jamno
There is a planned second rally of all Polish crafts of type proa
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27th February 2011, 12:56 PM #30
Comparing the Japanese canoe's and the "Rob Roy/Swift".
The Japanese canoe's sit so much higher when on their sides, and seem very much wider and curved in the sides.
Seems that the Japanese canoes float on their sides 'almost dry', and then when the lower gunwale is pulled down during righting, due to the curve, there is less 'mouth' in the water too scoop in water.
The Rob Roy/Swift seems to be the opposite, minimal curve = a long part of the lower gunwale submerged and scooping up water.
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