Wave splitter on catamarans

[Mark Peiffer]

Hi,
I have been following the forums for some time and was reviewing a posting about the catamaran Galactic Cat. To be short it included a pod that functioned as a stifening member and a wave breaker to following seas. I have seen a similar pod on the Prout 46 that extends forward. I was wondering with all the effort to lower slamming, why is a wave splitter seldom incorporated in most common sailing cat designs. And why are these pods that may stiffen the structure, so seldom seen ? Is there a real draw back to incorporating a wave splitter on a pod such as on the prout 46 ?

Thanks

[brian eiland]

As I conducted a search (search function above)I happened upon your posting that I had not seen. So I copied a portion of another posting I had made back about a year ago.

First, imagine a flat plate, on edge, mounted down the centerline on the underside of the bridge deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel, a somewhat weaker characteristic in a catamaran structure vs. a keeled monohull. If a tow bundle (rope, etc) of carbon fiber (kevlar, PBO, etc) was laid along the bottom edge of this flat plate, the rigidity could be even greater (sort of akin to a bottom truss structure, or a flange of an 'I' beam).

Now on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle, and rotate on oversize (possibly 1-foot) diameter bearings. The flat fit & big bearings would together supply a great big surface for the large bending moments to bear against. Only one board at a time would be lowered. In fact the two could be linked together such that the act of lifting one automatically lowers (& powers) the other down. And they both could be rigged to 'kick up' upon hitting any solid object and/or for shallow cruising. The control lines (cables) could be routed right up to the cabin top and back to the cockpit.

There are several advantages to an asymmetrical shaped centerboard. First, it requires less total board area to develop a leeway reducing force....so the board size is reduced. Secondly, since it is asymmetrical, it does not require an angle of attack (does not require the boat itself to be sailed at a skewed angle) to develop the 'board's lift' (leeway reducing force). This actually
may result in the vessel making less leeway. Plus the drag forces associated with the CB lift forces are on the centerline of the vessel, rather than off in one hull that produces turning moments about the center of the vessel.

This centerline mounting may also improve the tacking capabilities of the vessel as it allows the 'clean' hulls to slip a little while pivoting about the central board.

The front of this nacelle/plate could be configured to act as a wave splitter to actually attack, up front, the formation of those peaky waves under the tramp areas that eventually slap at our bridge deck underside. We kind of slice those waves down a bit. A lightweight fairing might also be added to this 'flat plate nacelle' so it appears outwardly much more esthetically pleasing, as well as more curvature to shed those peaky waves

[oldsailor7]

As I conducted a search (search function above)I happened upon your posting that I had not seen. So I copied a portion of another posting I had made back about a year ago.

First, imagine a flat plate, on edge, mounted down the centerline on the underside of the bridge deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel, a somewhat weaker characteristic in a catamaran structure vs. a keeled monohull. If a tow bundle (rope, etc) of carbon fiber (kevlar, PBO, etc) was laid along the bottom edge of this flat plate, the rigidity could be even greater (sort of akin to a bottom truss structure, or a flange of an 'I' beam).

Now on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle, and rotate on oversize (possibly 1-foot) diameter bearings. The flat fit & big bearings would together supply a great big surface for the large bending moments to bear against. Only one board at a time would be lowered. In fact the two could be linked together such that the act of lifting one automatically lowers (& powers) the other down. And they both could be rigged to 'kick up' upon hitting any solid object and/or for shallow cruising. The control lines (cables) could be routed right up to the cabin top and back to the cockpit.

There are several advantages to an asymmetrical shaped centerboard. First, it requires less total board area to develop a leeway reducing force....so the board size is reduced. Secondly, since it is asymmetrical, it does not require an angle of attack (does not require the boat itself to be sailed at a skewed angle) to develop the 'board's lift' (leeway reducing force). This actually
may result in the vessel making less leeway. Plus the drag forces associated with the CB lift forces are on the centerline of the vessel, rather than off in one hull that produces turning moments about the center of the vessel.

This centerline mounting may also improve the tacking capabilities of the vessel as it allows the 'clean' hulls to slip a little while pivoting about the central board.

The front of this nacelle/plate could be configured to act as a wave splitter to actually attack, up front, the formation of those peaky waves under the tramp areas that eventually slap at our bridge deck underside. We kind of slice those waves down a bit. A lightweight fairing might also be added to this 'flat plate nacelle' so it appears outwardly much more esthetically pleasing, as well as more curvature to shed those peaky waves

This idea seems to be a possibility, but we tried just that in 1964, and it really is not worth the effort. Two asymmetric boards are unnecessary complication, and used individually are not as efficient as one symmetrical board. Also surface piercing boards are much less efficient due to wave action and air entrainment problems.

[rick.hayjpn]

Oldsailor, I was about to post a reply to warn about the drag and reduced efficiency of surface piercing boards when I came across yours. Rick

[Doug Lord]

Surface piercing centerboards can work satisfactorily if boards in the hull aren't used for some reason. I'd prefer a single board just for simplicities sake and I might consider a gybing board as well. Don't discard surface piercing foils for lateral resistance in every case:

Pictures(click on image),L to R : 1) Kona Kat with one pivoting centerboard-worked well, 2) "Happy Feet"-unique catamaran with bi-foiler foil arrangement-lateral resistance from surface piercing daggerboard and rudder, 3) Hydroptere.ch with surface piercing center daggerboard-used when conditions are not suitable for her surface piercing lifting foils, 4) The fastest sailboat on the planet, the 60' Hydroptere which develops all lateral resistance from two surface piercing main foils with some contribution from a surface piercing rudder, 5) Moth foiler with surface piercing daggerboard and rudder: upwind with "veel heel" the hydrofoil unloads the daggerboard(and rudder):

[brian eiland]

Thanks Doug, for that reinforcement.

Certainly I realize that the surface piercing centerboards I have suggested here aren't necessarily the most efficient form...BUT look what other positives I've arrived at.

I chose the asymmetrical ones because I could get a really nice big flat surface to bear against the nacelle plate, and a BIG bearing;
"Now on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle, and rotate on oversize (possibly 1-foot) diameter bearings. The flat fit & big bearings would together supply a great big surface for the large bending moments to bear against."

I'm very familar with the Stiletto 27 cat's use of a center mounted foil and the difficulties supporting its surface piercing end. I wanted to avoid that, and yet still be able to not have to build two holes in the hulls. Besides lets make double duty of the wave splitter form.

[Doug Lord]

Thanks Doug, for that reinforcement.

Certainly I realize that the surface piercing centerboards I have suggested here aren't necessarily the most efficient form...BUT look what other positives I've arrived at.

I chose the asymmetrical ones because I could get a really nice big flat surface to bear against the nacelle plate, and a BIG bearing;
"Now on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle, and rotate on oversize (possibly 1-foot) diameter bearings. The flat fit & big bearings would together supply a great big surface for the large bending moments to bear against."

I'm very familar with the Stiletto 27 cat's use of a center mounted foil and the difficulties supporting it surface piercing end. I wanted to avoid that, and yet still be able to not have to build two holes in the hulls. Besides lets make double duty of the wave splitter form.

================
Brian, the pictures of Happy Feet and Hydroptere.ch both show a sort of nacelle running fore and aft about in the center. The one on Happy Feet also moves athwartship(!). Prout catamarans, a long time ago, had such a thing but I don't believe the board was in the center.
In certain situations there is no question that a center mounted surface piercing foil can make sense.

[oldsailor7]

Doug said:- "In certain situations there is no question that a center mounted surface piercing foil can make sense."

That is absolutely true.
But it is still less efficient than a foil which has, in effect, a flat plate at the top, below the water surface. Which in most cases is the hull of the boat.

In the Piver "Nugget" which I built in 1963/4, the daggerboard case was not installed as we wanted to utilise the space for a double berth, formed by folding out the bench seat bottom. Instead the flat plate support under the wings at the hinge line, was made double, with a pivotting, surface piercing C/B, coming out from between, into the water. It was a pain to use and not very effective and was replaced the following year by a Norm Cross LAR keel under the main hull, which transformed the performance of the boat.

My subsequent Tri, a B24, had a transom hung rudder and we solved the problem of air entrainment at high speeds, by adding small boundary layer fences in three places down the rudder. Worked like a charm. :cool: