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Wave Rider View Drop Down
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Post Options Post Options   Quote Wave Rider Quote  Post ReplyReply Direct Link To This Post Posted: 07 Mar 05 at 9:18am

Dunno i think they should make a younger person's style boat e.r single hander with trapeze and fast but easier to sail than something like the 700?

 

Maybe they could call it the 500 seeing as they have all the others and 100 would mean it was the lowest performace they made !

Maybe there isnt much call for that kind of boat in the market but it would be good !

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Post Options Post Options   Quote carshalton fc Quote  Post ReplyReply Direct Link To This Post Posted: 06 Mar 05 at 9:52pm
wonder what rs are going to make next?  the rs range is the only range i think that almost has a boat for everyone!
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Phil eltringham View Drop Down
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Post Options Post Options   Quote Phil eltringham Quote  Post ReplyReply Direct Link To This Post Posted: 02 Mar 05 at 11:29pm
I cant remember how it works on the 800 but my favouite trick for that in the 29er was to make the mainsheet strops really long in light winds so that you could not sheet the main all the way to the centreline.  Seemed to work well in the really light stuff.  Only problem was that if the wind built you had to remember to change them. 
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Post Options Post Options   Quote Yann Quote  Post ReplyReply Direct Link To This Post Posted: 02 Mar 05 at 11:11pm
i sail an 800, that happens, its HORRIBLE 
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Post Options Post Options   Quote redback Quote  Post ReplyReply Direct Link To This Post Posted: 02 Mar 05 at 9:52pm

From what I have read you are quite right.  Its much better to use a foil designed for the turbulent flow within which they will operate and as a bonus they are sections which are stronger and so can be used to right the boat after a capsize.  Another factor is that a very thin section is poor at recovery from a stall, as too are sections with a short cord, the ability ot recover from a stall is very important at low speeds or situations with high lateral forces.

I have encountered stalling in 2 very different types of boat. 1. the Laser where it is possible to stall the foil by using a vigorous roll tack and not let the main out enough, and 2. the RS800 where in lightish winds if you oversheet the main the foil gives up and the boat slows very noticeably - it feels like you've just hit a muddy bottom.

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Post Options Post Options   Quote Phil eltringham Quote  Post ReplyReply Direct Link To This Post Posted: 02 Mar 05 at 7:22pm

'Laminar sections'

I would be a little reluctant of these.  In order to produce a section that will do that, firstly it would have a fairly short chord (I found my book and at 4 knots in still water the lamina layer will last at most 50cm along a surface, and that is in still water, with a very smoothe surface).  Given that the water, especially near the surface where most of a foil is operating, is never still the chances of having any truely laminar flow is unlikely at best.  The other thing is that in order to maintain the laminar layer the section has to change in thickness very slowly along its chord so as not to get seperation.  The upshot of this is a section that is very thin for a large amount of it's chord (towards trailing edge).  This makes for an inherently less stiff foil (not good for performance in waves) and also more likely for you to put a foot through it when you capsize. 

I would like to be proved wrong about this but from what little I know on the subject my personal view is that they are a bit of a waste of time. 

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Post Options Post Options   Quote Blobby Quote  Post ReplyReply Direct Link To This Post Posted: 02 Mar 05 at 12:39am

Don't forget laminar flow sections for dagger boards - specifically designed sections that operate with a laminar boundary layer over the entire blade to reduce drag...

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Post Options Post Options   Quote Phil eltringham Quote  Post ReplyReply Direct Link To This Post Posted: 01 Mar 05 at 3:45pm

(This is really not doing my degree much good, but then again these posts are probably the most use I am going to get out of it! )

Lamina Flow...

Lamina flow in the real world is rare, but in principle does occure everywhere that a fluid passes over a surface.  From a dinghy sailing point of view there is little that can be done other than keeping your hull smoothe & fair. 

Lamina flow begins with a uniform flow, that is a stream of fluid flowing with every molecule moving in a straight line (or an object moving through completely still water).  In reality this is impossible if nothing else due to the rotation in water linked with waves.  However the principle is very powerfull when it comes to the mathematics involved in calculating fluid flows. 

As water flows past a surface (eg hull, rudder) friction with that surface slows the flow, to the point where the water directly in contact with the surface does not move in relation to it.  The speed of the water (in relation to the surface) increses as you move away from the surface back up to the speed of the free stream (ie: before contact with the surface).  Because the speed of the water at any given distance from the surface, is different to the speed of the water next to it, both closer to, and further from the surface, there is a shear force produced between these 'layers'.  This shear force disrupts the liniarity of the flow and causes the water near to the surface to become turbulent. 

As the flow contiues over the surface, the fluid flow close to the surface actually smoothes out and forms a very thin layer of laminar flow.  This is where having a smoothe hull is important.  If the imperfections in the hull surface do not stick out above this new layer of flow (called the 'laminar sub-boundry layer'), then the surface can be considered as hydrodynamicly smoothe.  Both mathematicly and in the real world this has great benefits in terms of efficiency through the water. 

The transitions between these types of flow are dictated by a quantity called 'Reynolds' number' (Rn).  It is a function of the density of the fluid, its velocity.  As you move along the hull from bow to stern the value of 'Rn' increases and when a certain value is reached (dependent, in part, on the roughness of the surface) flow changes from being laminar to turbulent. 

Lamina flow is better in terms of drag, but the tranistions between types of flow on the surface create more drag.  So in reality getting the lamina sub-boundry layer established early on is the best option.  At a boat speed of around 5 knots lamina flow will only continue over the first two feet of the hull and that is assuming that the water is completely still.  In the real world the best you will probably get is about six inches of lamina flow on the bow of a hull.  This means having a hull which is smoote enough to be hydrodynamicly smoothe is the important thing to worry about.  If you are on the plane the mechanics change completely and lamina flow does not really enter the 'equation'. 

I think most of this is right, (I'm doing it from memory because a friend is borrowing the best two books I have on the subject).  I will read up and may update this tomorrow, for now I think this covers most things.  If you have any questions do ask, I'll see what I can drag up. 

Enjoy. 

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Post Options Post Options   Quote hydrographer20 Quote  Post ReplyReply Direct Link To This Post Posted: 01 Mar 05 at 10:44am
no im wondering what will happen to the new boat laser have juist released,  it looks quite good the xeron or somtihing.  i wpnder if it will last?
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Post Options Post Options   Quote Wave Rider Quote  Post ReplyReply Direct Link To This Post Posted: 01 Mar 05 at 9:17am

Ok thanks that makes it really clear !

That was a brilliant explanation !

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