| Rossiter Pintail Mortagne sur Gironde, near Bordeaux |
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List classes of boat for sale |
Speed Bumps |
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TonyL 
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Joined: 28 Jun 05 Online Status: Offline Posts: 57 |
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 Topic: Speed BumpsPosted: 23 Sep 05 at 11:24pm |
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Well I'd just like to recommend a viewing of Mr Jim Saltonstall MBE's
sail training DVD "Breeze On" which has some positive things to say
about Speed bumps. If its good enough for Jim, it's good enough fo
me.
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m_liddell
Really should get out more 
Joined: 27 May 05 Location: United Kingdom Online Status: Offline Posts: 583 |
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Posted: 24 Sep 05 at 11:37am |
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Does his out of place and overuse of the word "ferret" disturb anyone else? |
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Calum_Reid
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Joined: 09 Apr 04 Location: United Kingdom Online Status: Offline Posts: 59 |
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Posted: 24 Sep 05 at 12:33pm |
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Along the lines of what yellowhammer said its the dimples in golf balls that reduce drag and make them fly so far.
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TonyL
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Joined: 28 Jun 05 Online Status: Offline Posts: 57 |
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Posted: 24 Sep 05 at 12:48pm |
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No, but as someone who did several of Jim's blue badge/white badge race training weeks as a kid the sound of his voice takes me straight back to getting up at 7am to jog round the boat park as part of his ruthless daily PT regime! |
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Stefan Lloyd
Really should get out more
Joined: 03 Aug 04 Online Status: Offline Posts: 1599 |
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Posted: 25 Sep 05 at 8:24am |
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Yes but they are not laminar-flow wings. They fly too fast for laminar flow. On the other hand, glider pilots go to huge effort to maintain a smooth wing finish in order to maximise the laminar flow over the wing and laminar flow is definitely faster for them - raindrops on the wing can halve their performance. Similarly Bethwaite's has done experimental work on foils and found a smooth finish is very important for performance. As it happens, as part of my degree I did some work on how smooth glider wings needed to be to maintain laminar flow. The answer turned out to be similar to human eye resolution i.e. if it looked smooth, it was smooth enough. However you can't apply this finding to other situations because, technically, it is all about something called the Reynolds number, which is a scaling factor for fluid flows. At a substantially different Reynolds number, the situation can be completely different. So it's dangerous to generalise. A golf-ball is not an aerofoil but wings and sails are. A glider wing is laminar flow but an airline wing is not. Highly polished foils are laminar and according to Bethwaite, parts of the flow over a sail can also be, especially in light airs. Regarding hull finish: seen any AC boats with rough bottoms? I suspect not. |
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jeffers
Really should get out more
Joined: 29 Mar 04 Location: United Kingdom Online Status: Offline Posts: 3048 |
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Posted: 25 Sep 05 at 8:00pm |
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Wasn't there something trick about the Aus or NZ boat quite a few years
back when they went for a 'sharkskin' finish on the hull?
This is a long long time ago..... We had everything today from 'speed bump' conditions to flat out hard hiking and planing (I have never used my outhaul so much in 1 day!) Paul |
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Paul
---------------------- D-Zero GBR 74 |
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Isis
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Joined: 01 Sep 05 Online Status: Offline Posts: 2753 |
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Posted: 25 Sep 05 at 8:28pm |
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There was definantly some experimentation with matt paint jobs on some
of the aus moths a while back but it doesnt seem to have caught on so
it cant have been too sucsesfull
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49erGBR735HSC
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Joined: 30 Mar 05 Location: United Kingdom Online Status: Offline Posts: 1991 |
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Posted: 25 Sep 05 at 8:35pm |
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I reckon that the smother you can have a hull, the better. It boils down to the fact that a rough surface causes friction which is a force that acts against the movement of an object. Its standard practice for airlines to highly polish wings and it's for the same reason, to reduce friction. I'm still thinking about the sail stuff, but you do really want a smothe hull.
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CurlyBen
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Joined: 17 Aug 05 Location: Southampton Online Status: Offline Posts: 539 |
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Posted: 25 Sep 05 at 8:43pm |
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I'm not sure about all the theory and stuff (I seem to remember the
dimples on gold balls are to make them spin in the air, which is faster
for them, but not sure how easily you'd get a sail to do that!) but I'd
try both ways and see which is faster. Sorry it's not very scientific..
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Rupert
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Joined: 11 Aug 04 Location: Whitefriars sc Online Status: Offline Posts: 8956 |
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Posted: 25 Sep 05 at 8:44pm |
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The concept behind the sharkskin finish was that it trapped a layer of unmoving water, which the water flowing over the surface then had far less trouble getting past than any solid surface, however smooth. No idea if it really worked, I'll leave that up to 3M, who came up with the idea.
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