Big Cat, alt CB's & sail rigs

Discussion in 'Boat Design' started by brian eiland, Oct 1, 2003.

  1. brian eiland
    Joined: Jun 2002
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    Location: St Augustine Fl, Thailand

    brian eiland Senior Member

    Alternative Centerboards & Sail Rigs Aboard Big Cat

    BE: I've posted here a copy of a letter I recently wrote to Mr. Rodwell as it touches on some thoughts about centerboards and alternative sailing rigs that might be interesting to some multihull sailors on this forum. For reference B Rodwell had originally posted a message under the subject heading "sock it to me"

    Hello Brian Rodwell,
    You may recall I contacted you about a year ago upon viewing your computer illustration on I had wanted to see if you might be interested in my mast-aft sailing rig concept, and also if you might be interested in assisting me with the computer illustrations & Rhino applications to better present the ideas I put forward on my website, <> . (My computer skills still need a lot of work).

    Just recently I received an eMail copy from N.A. Eric Spondberg that referenced your inquiry with him concerning free-standing rig concepts you are considering aboard your 70 cat design, and his subsequent answer to you that included a reference to my rig design, and his acknowledgement that he was planning to help me with my rig design development. I subsequently visited the
    you had referenced to him and decided to write a note directly to you (and copied toEric).

    First, I would observe that you must have been disappointed with the feedback you received to your 'preliminary design for a cruising catamaran' posting on the BoatDesign forum....not very helpful responses. I think part of the problem may have been the time of the year you posted the message...doesn't appear as though the summer season, and particularly late summer season, is a good posting time. I guess all of the sailors are out sailing. But that's right, it's our summer and your winter.

    One item of your thought processes caught my attention in particular; your desire for a single centerboard, and real shallow draft capabilities. I'll certainly second that motion, that shallow draft idea. One of the greatest attributes of multihulls is their capability to really go exploring ALL the water areas including those tributaries, lagoons, reefs, etc. That's why I had kick-up CB's in each hull of my design.

    BUT, what you may not have noticed was my alternative to the CB's in each hull. Look at the attached drawing, (or the very bottom profile drwg that denotes "asymmetrical CB's, nacelle mounted". 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 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 offin 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.

    And how about the maintenance factor, particularly in remote cruising areas. No need to haul-out the vessel to repair kick-up CB problems, or even bottom painting problems. Everything, including the cables, bearings, and boards is all above the load waterline. The initial building cost should be less by eliminating the trunks in two hulls, and the watertight integrity is much
    better. The twin boards might have to be made a little bit longer as they operate with a 'free-surface' end, but then they are asymmetric so they can be correspondingly shorter. I would further suggest that surplus helicopter blades are prime candidate sources for both CB blades and rudder blades....high tech, extremely strong carbon fiber fabrications that have a
    prescribed limited life span aboard aircraft, but are perfectly happy for our use.

    For rudder designs I would give a cassette system such as the Vara rudder at <> a close look in lieu of a kick-up system. Or maybe even Tony Smith's Gemini cat system. Kick-up rudder systems can get pretty complicated, plus they usually don't steer the boat very well at all when they are kicked up in shallow water. (LOTS of weather helm).

    If I were looking to use my auxiliary engine in a strictly aux manner, rather than in a motor/sailing demand, I would seriously consider a single engine installation. This engine would be conveniently mounted in an enclosure on the cockpit deck and would belt drive a steerable out-drive leg that would be incorporated into the rear portion of the central nacelle structure. Maybe this rear nacelle might appear as on "Earthling's pod" (attached photo and/or <>). This saves the cost and weight of the second engine, trans, shafting, prop, etc, and opens up the rears of the hulls
    for a nice master bath, or whatever.

    Sorry Brian R., I wandered off a bit. Let's return to the sailing rig subject you originally ask of Eric Spondberg.. I think what Eric points out about free standing rigs in multihull vessels parallels my thinking. The multihull doesn't heel over to relieve a big burst of wind, and thus requires a bigger support structure and/or 'bury' of the free standing rig to be strong enough for all the potential ocean conditions. I was pleased to see him recommend my rig as a very real possibility.

    I had chosen to contact him about my rig design with some trepidation initially. While I felt he had some of the most experience with engineering free standing rigs and unusual mast strength's, I was concerned he might reject my ideas right up front, as my design emphasizes headsails and substantial amounts of staying which is quite the opposite of his extensive
    free-standing work.

    Here's a bit of my introductory ltr to him:
    "Dear Eric, I just finished re-reading your 'unstayed rig' article from Professional Boatbuilder magazine issue # 55 for what must be at least the sixth time over a period of several years. I agree with most all of your conclusions, and the viability of free-standing rigs. Now, I know you are not a fan of stayed rigs, and therefore big headsails that require headstays. So it seems rather strange, even for myself, to come before you with a
    presentation of a stayed-rig, with big headsails, and particularly one without a mainsail nor a rotating mast. But, here goes."....

    My feeling was that he had gotten into the engineering of sailing rigs further than most other designers are forced to, as there are 'old standards' out there for use by those designers when they are specifying a Bermudan rig for a sailing design. They don't bother to dig deeper into either the aerodynamics of the situation, nor the finer details of rigging analysis when they are just sticking with the normal rig. I look forward to working with him on my project.

    In all of this preparation I am currently involved with some preliminary studies that should prove extremely interesting and helpful with the final design(s). A young engineer from India has shown a tremendous interest in helping me with a structural analysis, and possible some FEA analysis, of a number of rigging variations of my mast-aft design. We are going to look at 1)
    double-spreader, 2) single-spreader, and 3) no-spreader variations on a single, mast-aft shaft configuration. And we are going to look at two wishbone/bi-pod mast shaft alternatives that appear real interesting when adapted to my mast-aft design. I have some sketches of these plans I can send you, but I would ask that you keep them to yourself for now, as I am not ready to publish them just yet.

    And a final little note for now, I think you would find that most all
    textbooks, etc, would implicate the traditional mainsail as the primary leeway inducer. I believe my mainsail-less rig will thus require less leeway combating forces from my single CB arrangement.

    Look forward to your reply on these subjects.

    Profile,Nacelle CB
    Earthling stern
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