Speed Bumps

At the heart of fluid mechanics/dynamics is boundary layer theory.One of the very first principles of boundary layer theory is the "no slip condition" which uses an assumption that the water/or anyother fluid at the surface ( a sail or hull in this case) has zero relative velocity.

The differences in drag due to surface imperfections is due to the boundary layer being increased in thickness. which means that you drag along more water with you. Also the surface irregularities cause turbulence which can trip the boundary layer into a turbulent flow. Turbulent flow is generally a lower drag state as the velocity profile has greater speed at lower levels when compared to laminar theories. As a result the smoothness of your hull and foils are much more critical when going slow.

53 SKIN FRICTION
A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer.  !!!!!!!

So my aged boat would be illegal if I'd scratched it on purpose, but OK since I did it by accident???

Only one thing to say to you lot.  Trip turbulators.

right as the other speed bump topic seems to have gone more technical i thought i might try and sway this one towards another type of speed bump which you seem to find down at the sailing club.... the ones in the car park!!! WHY oh WHY do sailing clubs all have speed bumps?! Do they have a secret link with boat repairers, manufacturers and MOT men? When i am trailing my boat i take that as reason enough to drive slowly, i do not need some idiot to put in some mini mountains to shake apart my beloved piece of kit as it is brutalised by the trailer!

Originally posted by Stefan Lloyd Originally posted by Ian29937 Reducing drag is much less important with sails which work on lift generated by pressure difference Creating lift also always creates drag. Prandtl's theorem says so. Impress your friends by dropping that into the conversation.

Obviously true, but it is the lift which propels the boat.

As I said the drag is 'less important' than creating the optimum sail shape to generate the maximum lift, to go faster.  If speed creases do this, and they clearly do for many classes in certain conditions, then this is the way to go. 

Originally posted by jeffers 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

The sharksin was on Conners 12 at the Freo Amcup. It only worked when very carefully placed and over a very narrow speed range. Fortunately 12 metres, unlike dinghies, only really work within a pretty narrow speed range.

Such things are illegal now under the racing rules:-

53 SKIN FRICTION A boat shall not eject or release a substance, such as a polymer, or have specially textured surfaces that could improve the character of the flow of water inside the boundary layer.

So if you see someneputting dimples on or whatever you can protest them!