what would be the design cost?

Trying to add daggerboards to the 34 foot catamaran design.
Can't put them on the inboard hull side as that is where the beds are.
Can't put them vertical in the hull as the Hoyt booms run close to hull tops, takes up internal space, and the boards become too long.
Don't want to use one central board from the bridgedeck as design loads are high.

This partial outboard recess seems to the best but of course there is the free surface effect.
How much additional board length is needed to offset the free surface effect? Just one width (approx 40cm), less, or more?

 

If the enclosure is open to the sea, there is no effect due to free surfaces.

 

Difficult to see but the opening is 12 cm above DWL.
It could be set under but marine growth would be a problem.

 

do not think I understand in what compartment and why you think that the effect of free surfaces will occur.
From what you see in the pictures, the enclosure that houses the daggeboards will not experience this effect.

 

Sorry, could be my misunderstanding. Here is close up of the water surface.
Normally the daggerboard exits the hull under the water surface and if the case exit is a tight fit around the board there is no air entrainment (which I understand would cause some loss of foil efficiency.
This arrangement shown would seem to permit air entrainment. If it is a minor problem that's good to hear.

 

It seems to me that you are not talking about what is normally understood as the effect of free surfaces, for example on tanks, which produce a decrease in stability. It seems to me that you are talking about something other than me, I am sorry, I do not understand. Excuse my intrusion.

 

No problem, thanks for the replies. I am not sure what to call the effect. Free surface effect was what came to mind.
I have seen videos of foils suffering ventilation to some depth when the foil is surface piercing - i.e. not sealed by the hull. But the angles of attack were high as were the speeds apparently.

 

Why can't you fully incase the board with a traditional slot (obviously angled) on the garboard or bottom panels?

 

If the board is angled sufficiently to emerge from the hull underwater then either the upper end is too far outboard for comfort when the board is lifted OR if the board is more traditional and vertical it becomes too long etc.
I suppose the boards could be removed completely in port which would eliminate the outboard lean problem.
Having the upper end of boards half way down the outer hulls (when in use) is not exactly service friendly however

 

Maybe look at putting a constant radius curve in the board to allow a snugger fit against the hull side, also, you are looking at the hull in a static unhealed mode, when underway it may be heeled enough to cover your slot.

 

Thanks for the comment. Not really happy with the amount of effort/work needed to modify the side of the hull to inset the board (straight or curved).
A simpler alternative is to use a single lifting center board. Will have to check clearances

 

If you haven't seen it already;

Bridgedeck centreboard why don't they work??? https://www.boatdesign.net/threads/bridgedeck-centreboard-why-dont-they-work.57051/page-24#post-798306

 

Google NG yacht design, he has quite a few rather nice cats that have actually been built that have the board exiting the topside well above the waterline. Maybe if you contact him you could find out if it is any kind of issue in real life rather than theory and speculation from those of us without actual experience with this exact settup. All transom hung rudders on many very fast cats operate in this way and in practice its a non issue, sure underhung may be better in theory but sometimes practicality trumps a small efficiency advantage.

Steve.

 

Steve W, thanks for the heads up. Nicolas Gruet seems to be the designer and his Ekolo'kat 34' seems to be very close.
Designed for 20 knots, 2500kg light ship weight, 10m LOA 5 mBOA
All surfaces are developable.
Lots to think of.
edit: boat is actually doing 19 knots in the photo - on a 34 foot hull, looks like they are not surfing big waves so the hull must have gone from displacement to planning mode?

 

Seems this would only be an issue if moored hard against a piled wharf with the whaler close to the face of the piles. Boards are often let down some for close quarters maneuvering. Daggers set leaning out & with cases set though seem to be the most simple of installations. What are the circumstances of mooring in port that drive your concern?
Jeff.