Mini keel placement

Discussion in 'Multihulls' started by cookiesa, Jan 12, 2015.

  1. cookiesa
    Joined: Mar 2007
    Posts: 122
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    Location: Launceston, Tasmania

    cookiesa Senior Member

    I Am fitting mini keels instead of the centreboards in my cat. The boat is for cruising only, will live on a mooring and from time to time dry out on sand and small rocky beaches. The coastal cruising we do has previously been done in a 20ft Twin Keel mono with a draft of 800mm, never an issue.

    The design is for centreboards, pivot style, not dagger board) and the pivot point is 25% along the width of the board from the front.

    I am looking at keels approximately 2m long (overall length is 7.5m) and placing them with a point approximately 25% back from the leading edge to keep the resistence point the same (I'd also like to move the rudders underneath and free up the rear step area for boarding/swimming) but the ease of transom hung rudders versus making rudder tubes etc is likely to win :)

    The top will be 27mm ply spaced along the centreline under the four central bulkheads, shaped obviously!

    I'd appreciate any thoughts on the positioning of the keels. I'd rather have a slightly longer keel for balance when beached than have to place "props" under the bow or stern.

    Outboard will be placed in a central nacelle rather than on the rear beam to try and reduce cabitation as well as help with balance.
     
  2. aussiebushman
    Joined: Oct 2009
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    Location: Taralga NSW

    aussiebushman Innovator

    Hi Cookie

    You may have seen on this forum that my Tri has been stripped and burned at the mooring by some local as#hole

    I have managed to salvage the 2 amas, most of the rigging and will also recover the mast (that will be shortened down to about 28' to remove the damage at the foot) so I'm stupid enough to consider building a simple, trailerable 20' catamaran - essentially a small bridge-deck/cockpit permanently fixed to the original amas (not unlike the Jarcat 6 but hopefully a whole lot better looking.

    The amas currently have dagger boards but the trunks have been a pain - always leaking so I'm with you on thinking to replace these with fixed mini-keels, just like the ones on my former Simpson Signwave 30' Cat. The Simpson was not too happy about coming about when close-hauled and we both might find the same if we go the mini-keel route. Also, in my case, keels will also complicate getting the boat on and off the trailer. That is definitely the plan. Keeping the boat out of the water solves many problems (and maintenance costs). Instead, I might consider a SINGLE kick-up dagger- board - in one hull in a reinforced trunk.

    If you do add keels, look at the Simpson study plans for his cats or get back to me because I still have many of the plans from when I built mine, including the keel shapes.

    Alan
     
  3. cookiesa
    Joined: Mar 2007
    Posts: 122
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    Location: Launceston, Tasmania

    cookiesa Senior Member

    Thanks for the post, sorry to hear of the circumstances.

    My design is trailerable, however we have access to cheap moorings here in the river, and in the upper reaches these are in fresh water.

    You make some interesting points, I'd love to hear your thoughts on placement. The trade off in pointing ability is something I am happy to live with (previous mono had twin keels and I believe I should get a similar, if not better pointing ability)

    Mike Waller has released a bigger version of his TC trailerable cat, I have the study plans for both and they look like good plans.

    Are you thinking demountable, or genuine trailerable? (I think the demountable like mine is designed and Seawind 24 etc aren't really practicle unless you have mast up storage somewhere, down here you either fry in around 30mins in the sun or need to get out of the weather if you wantt o be able to use the boat for a reasonable amount of the year lol
     
  4. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    Hi Cookie

    I send you a PM on the Simpson issues. Re your comments in this post, the moorings are cheap at St Georges Basin too, but my recent experience has convinced me of the benefits of trailering, if the boat is only small.

    Like you, I think I will go the twin mini-keel route, because performance is not an issue - I just want to pleasure-sail and the ability to trail to wherever I like is a distinct advantage. That was one of the motivations for building the Tri, but in practice, the setup time was too great to make that worthwhile, which is the reason it was on a mooring. It had folding beams - a far better option than demountable, but is not as easy as it sounds and bloody expensive to buy and/or build.

    There are several reasons for the next boat being a small cat. It will have a beam of exactly 2.5 metres that puts in on the legal trailerable limit. The existing amas have more than enough volume and are plenty long enough to provide a small cockpit and a simple sitting-headroom cabin, with tramps forward. Part of the existing flimsy"decks" of these hulls will be replaced by much stronger ply panels with ply "foot-wells."

    New ply box beams will be bolted on top of the original cross beams for rigidity and to increase the clearance from the waterline. These will provide a flat surface for the cockpit and accommodation. The mast will go though the "cabin" and be stepped on the forward cross beam structure, so the CE of the rig etc will be unchanged.The former F18 rig was very tall, so losing about 2 metres by cutting off the damaged section is no big deal. The existing twin rudders will be links by a tube and tiller, clear of a 4-5 Hp outboard on a lifting bracket.

    All up, this ain't going to be a racing boat, but enough for me and my dog to potter about for pleasure and it makes use of the various bits and pieces left over from decades of boats building (and the wreck of the tri).

    Send me whatever you want to share by way of ideas - I'll do the same as the thinking progresses. Others - please feel free to comment
     

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  5. cookiesa
    Joined: Mar 2007
    Posts: 122
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    Location: Launceston, Tasmania

    cookiesa Senior Member

    I think we are after very similar things.

    I probably would have built a trailerable rather than demountable except I would like to explore some of the islands and coastal hop around Tassie, that's my reason for wanting more beam... Must say I was very keen on building a trailable Cat for the abilities you mention, especially for the west coast and lakes of Tassie, but it came back to the stability and space (I have four kids who also like to come sailing)

    Unfortunately most designers seem determined that the only option is for cat's in this category to be demountable, I found very few options for a bridgedeck version in this size, although going smaller there are several options for trailable Cats with a birdgedeck? (jarcat, waller, kendrick) to name a few
     

  6. aussiebushman
    Joined: Oct 2009
    Posts: 274
    Likes: 25, Points: 28, Legacy Rep: 132
    Location: Taralga NSW

    aussiebushman Innovator

    Concept ideas

    I have just fallen over this one from Spirited designs that has provided much inspiration for my new Cat. I'm working up drawings and specs and will post these after more thought.

    It may not be big enough for your tribe,
     

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