Build spec for a centerboard?

Discussion in 'Fiberglass and Composite Boat Building' started by bntii, Jul 7, 2011.

  1. bntii
    Joined: Jun 2006
    Posts: 731
    Likes: 96, Points: 0, Legacy Rep: 1324
    Location: MD

    bntii Senior Member

    I have got to build a new board for a ~ 40' mono hull sailboat.
    The existing board broke during a grounding & on getting it off the boat I don't like what I see from factory so would like to improve on the construction with the replacement board.

    What I've got-

    The board is 8' 6" long 35" at top and tapers to 23" at bottom. Six feet extends from the trunk. The board is 2.5" thick in trunk and tapers to 1.5" at lower end. It carries the 2.5" to the highest stress area at trunk bottom. The existing board has a solid core which looks like wadded up glass strand wet out with resin. It is very hard but filled with voids, pure resin pockets, is brittle and shattered across the board during the grounding. The board is skinned with ~1/8" laminate.....:confused:

    I don't like the brittle core or the thin skin laminate on the factory board.

    What I am thinking is a coosa core shaved out to the correct profile and skinned with ~5/16" biax. I would like to see some four 2" tows of CF on each side buried in the layup. I would put a G10 bushing around the pivot bolt (2.5" on this board) and G10 around the pennant anchor. I will be casting some lead disks for the bottom edge.

    I haven't looked into the coosa lineup but figure on using the highest density board they offer...

    What say ye?

    Is this spec sounding about correct for this board?
    I believe I am in the ballpark but and wondering just a bit about the coosa board density.. high enough for this core?

    This is a quick and dirty sketch of the board with none of the taper or foil drawn in:

    board.png

    Thanks all
     
  2. Saildude
    Joined: Jun 2011
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    Location: Seattle, Washington, USA

    Saildude Junior Member

    The design revision is above my pay-grade.

    However you should be cautious about making the new board too strong. One of the boats across the fairway from mine a few years ago hit something with their centerboard. The centerboard did not break and the trunk broke, tore the bottom out of his boat and he spent 3 or so months in the boatyard getting the bottom of his main hull rebuilt (it was a trimaran). He was not a happy camper and really wanted the board to break.
     
  3. Stumble
    Joined: Oct 2008
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    Location: New Orleans

    Stumble Senior Member

    I would also add that coosa is pretty lightweight, and floats. It is great stuff but why would you want to reduce righting moment? My thoughts would be to cast it out of steel, or lead then add in a sheer point that is designed to fail before it rips the centerboard trunk out of the boat.
     

  4. bntii
    Joined: Jun 2006
    Posts: 731
    Likes: 96, Points: 0, Legacy Rep: 1324
    Location: MD

    bntii Senior Member

    Thanks Stumble, saildude.

    This is a lightweight, slightly negative buoyant board as built by the manufacturer. I will build to the same weight as the original which had no bearing on righting moment.

    Though it seems sensible to build to as a sacrificial part, the board as supplied is designed to kick up in the trunk in the advent of a grounding or hitting a submerged object. The failure on this one occurred as the boat spun while being taken off a shoal by tow. The vessel is a fairly heavy cruiser type with a trunk build to withstand the loads placed by the board. I don't believe any composite built board will endanger the boat.

    My primary concern is just if the coosa product is sufficiently dense to for this application from a engineered beam standpoint...

    -I am interested in consulting if there is a NA or engineer working in the Annapolis area who would care to spec this project.-
     
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