Bridgedeck centreboard why don't they work???

that is why I was disappointed to see how far the ventilation extended down the foil in the video - centerboards are fixed foils unlike rudders
This metallic foil is much higher aspect (9:1?) than anything we could use as a home built centerboard.
37s mark https://www.youtube.com/watch?v=0J0jaMIRMR0

 

Yeah its pretty bad. However the point stands this issue will be worse as the board gets shorter. As mentioned previously the speeds (or more accurately froude numbers) here are not comparable to what a CB might encounter. Also does that foil have forward sweep or is it just the camera angle? What is the angle of attack? It says -50 whatever that means? Surely not 50 degrees.

 

interesting reading
http://www.ikarus342000.com/Boardsandrudders.pdf

 

Not sure that it's applicable, but this little cat appears to be doing quite well with one central board, AND one central rudder

https://www.youtube.com/watch?v=pJt1auzlUmo

PS: Some lively discussions been going on while I've been away looking at Thai temples,...have to catch up

 

quite sure its not applicable! Unless you want to foil a 4 ton cat on two appendages

 

believe it reads -5 degrees - thats the symbol for degrees

 

Sponberg noted this in his design of the Saint Barbara rudder and keel (barf barf)

This concave trailing edge is also used in plane wings to aid in vortex shedding;
the idea is to get two vortex streets and prevent oscillation of large vortexes across the single edge and along the length of foil. Probably acts like the physical vortex generators.
Not much on the web. Any one have any other details?

 

The o looked as big as the 5 at first glance but now I look again its slighty smaller.

 

I don't think this is regarding surface piercing foils? I know certain sections exist which are optimised for this task. Foils with the max thickness further back are usually laminar sections which need to be perfectly fair for best results.

I tried to find the ls1 foil and could not locate it to see what it looks like.

 

The Saint Barbara had a lifting keel so not sure if the end plate effect was present. But certainly the rudder was surface piercing as it was a rudder in a barrel.
He does say "modified" so how much is the question.
Not 100% sure but he could mean this NASA LS(1) profile -- as stock it is asymmetrical so he could have simply reflected the top profile to create a symmetrical foil with max thickness at 40%
http://m-selig.ae.illinois.edu/ads/afplots/ls421.gif

 

That is not a symmetrical foil so I can't see how it could be used on a keel? I also do not understand why a dagger rudder in a drum is any more surface piercing than a regular daggerboard in a normal hull mounted case? As far as I understand the bottom of the drum is flush with the hull underwater and the blade extends through a slot?

 

the figures below give a rough idea
as to the rudder in a barrel, the hull bottom rises with sheer -- not flat so the barrel end face projects when the rudder is turned. the seal is never perfect. The usual placement is just above the waterline to minimize water entry into the barrel.

 

Something else to consider.

In High Performance Sailing, Frank Bethwaite describes some testing he did of small foils behind a planing motor boat. The drag was over 6 times higher than it should have been.

They attributed this to it being in the turbulent boundary layer under the 17'/5.4m reasonably smooth hull. At 5 knots boat speed, this turbulent layer was estimated to be ~3"/75mm thick.

A centrally hung foil does not operate in this turbulent layer, so has less drag.

Apologies if the following are in the wrong thread.
Red Reuben,
Kick up foils
If the axis of rotation of the foil is ahead of the foil centre of lift, and the control rod acts aft of it, the foil will stay horizontal when the board kicks up. Or, mount the foil on a shoe which fits over the bottom few centimetres of the rudder and allows it to slide off when the rudder or board kicks up.

Valery,
Your willingness to look outside the box is refreshing. Some other things you might like to consider:
For load carrying and low cost marinas, design a boat with unstressed ends (no forestays, forebeams or travellers) and fold then up.
To lower the cost further, reduce the beam length by folding, scissoring or telescoping http://harryproa.com/?p=1757#more-1757

50% in marinas, 50% at anchor is a good idea, but a small dinghy with 2 hp outboard (all you will be able to take on a 9m cat) will limit the places you can anchor to sheltered bays. A tender with a bit more grunt and size allows you to anchor off in more exposed places and avoid the smells, noise, thieves, fees and crowded anchorages.

Designing a boat to slide down (or across) the Trades is pretty simple. All it really needs is enough payload.
It is far more difficult to design a boat for near shore use. Shallow draft, decent upwind and light air ability, ease of handling, ease of shortening sail, safety, low maintenance and a decent turn of speed under motor will all make it more enjoyable. If you are going to build it as well, choose a build method which is quick, clean and easy and minimises future maintenance.

A 60' harry was built with very blunt bows on one hull. It threw up a lot of spray and was faster and drier when they made them pointed.
A blunt bowed catamaran may be faster if it is kept light so the bow skims the water. This will probably mean a lower payload than a pointy bow of the same length. Not just slow, but uncomfortable, as are bunks in the ends of hulls. To increase payload, look at reducing the rocker and rectangular bilges (which are also lighter and easier to build and fit out than round bilges), but choose a rig and boat type that is easy to tack. Rockerless hulls with flat sections are less likely to pound than rockered ones as the ends are submerged more often.

 

Just ran across another discussion of center mounted boards....am reading thru that now.

http://www.boatdesign.net/forums/multihulls/single-dagger-board-vs-double-boards-37787-3.html

...and this particular posting just noe...

 

Didn't I read 5 degrees in that first test,..."angle of attack=5 degrees"...?
And what is that thing that keeps waving thru the video so fast that I can't catch even on pause??

On the second test on that second video. it says "angle of attack=14 degrees...WOW, and the speed 0-70 feet per second (almost 48mph). And what is that surface piercing foil attached to at its foot,...a ski like plate????

To tell the truth I need more info to be provided to properly interpret either of these videos.