Advice on 20ft beachcat design and build

Discussion in 'Multihulls' started by Philippe Lapere, Apr 29, 2021.

  1. Robert Biegler
    Joined: Jun 2017
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    Robert Biegler Junior Member

    I agree. I was thinking of end plates on low aspect ratio keels. Perhaps that would give Philippe the performance he wants, with less complication than lee boards. Even the weight might not be much different, despite a metal shoe, if there are structural advantages.
     
  2. upchurchmr
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    upchurchmr Senior Member

    As far as I know, endplates on fixed keels have to have have some serious hydrodynamics, design, and manufacturing for them to actually work.
    Most just add drag.
    And the trim of the boat needs to be constant so they work as designed.
    Constant trim on a boat - not often.
     
  3. garydierking
    Joined: Sep 2004
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    garydierking Senior Member

    Use the rudder from Michael Storer's Goat Island Skiff with an endplate and kicking up is not a problem.
     
  4. SolGato
    Joined: May 2019
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    SolGato Junior Member

    You can always do what I did for camping and mount a camper shell on your Cat!

    Seriously tho, if you planned ahead you could set your hulls up to accept the mounting of a standard truck shell. Of course you wouldn’t be able to sail, but with a little motor on the back you’d have a fun and functional camping rig.

    I used a high-top long-bed version on my Getaway during the winters to keep dry from the rain. People can’t give them away around here, and being fiberglass they hold up well on the water.

    I positioned mine backward so the rear hatch opened up to the front trampoline. Even custom fabricated a stainless rack to hold gear and boards.

    The only problem was sometimes people at first glance thought someone had driven their truck into the river or bay.
     
  5. Robert Biegler
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    Robert Biegler Junior Member

    I have read that, too. But I also read this:
    There is a drawing going along with this.

    I have a vague memory of a Tornado owner having replaced the centreboards with low aspect ratio keels with end plates, stuck into the centreboard cases, to sail in shallow water. I can't find the report now, though. If I remember correctly, that worked well enough. I don't remember this coming with information on whether the rudders were modified for shallower draft.

    Wharrams, with their fine end, have a reputation for pitching more than average. End plates seem to have worked well on three of them anyway, and van Denen mentioned "favourable reports from another member of our Catamaran and Trimaran Club", so there might be another boat. Bernd Koehler puts end plates on some of his catamarans. I have no idea how much modelling or experimentation went into that. And I don't know whether the heeling of a monohull makes fine details of the design more critical.

    Philippe could glue end plates to the outsides of the hulls of his Hobie 16, and see what that does. He can compare to otherwise identical boats.
     
  6. upchurchmr
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    upchurchmr Senior Member

    I believe that endplates on a fixed "keel" have a chance of doing well (not having seen the reports myself).
    The devil is in the details.
    Or even directly to the hull, but I wonder if it is actually better than a centerboard or is it just good compared to no board.

    I'll have to look at the end plate on the Goat Island Skiff rudder.

    If you have pitching problems as on a Wharram then end plates on the rudder probably address that directly, but it doesn't show the rudder works better for turning.

    All of these things can be done. But what works best?
     
  7. Philippe Lapere
    Joined: Aug 2018
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    Philippe Lapere Junior Member

    Hi Rumars,
    Thanks for the feedback, appreciated it.
    I understand that the inside fibreglass layer should be continuous, but I was hoping that it might be an acceptable compromise to reduce inside strength and add additional outside strength if necessary?
    The (pre-laminated on the inside) strips will be glued together with thickened epoxy or bonding paste, could that possibly be sufficient?
    I believe you're right that having 13 bulkheads is excessive, but I was hoping to keep things simple, rather than needing to laminate the inside too after doing the outside.
    Thanks for leading me onto the Kelsall KSS system, some potential methods I could adopt from there.

    If I were to adopt your method of laminating the inside, I'm thinking I could use (many more) temporary bulkheads CNC'ed out of hardboard or some cheaper material, then strip plank over them (with unlaminated PVC), then laminate the inside, then fit the 4-5 permanent bulkheads therafter. How would I then shape the upper half of the hull, would I have to create 2 halves and bond them together afterwards?

    I have been wrestling with this general concern of the PVC strip planking for a while, my primary concern at the time was planking pre-laminated (and therefore flat) PVC strips onto the curved bulkheads, so the strips will need to be quite narrow around the sharper curves, then therafter needing to sand the outside shape round - any thoughts? Thanks
     
  8. Philippe Lapere
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    Philippe Lapere Junior Member

    Thanks for the advice regarding leeboards and endplates, I think I'm going to keep it simple and skip the additional complexity that endplates pose, the idea of using leeboards is to simplify.

    Regarding performance, I'm hoping to keep things fairly sporty, but accept that there will be performance losses when trying to maximise load capacity and simplify the rigging, leeway prevention etc.

    Another idea I had to simplify the sailing experience that I haven't seen on beachcats was to have the mainsheet traveller on a dyneema rope over the tiller extension (like on some dingys), so that you don't need to flip the tiller around the back every time you change tack? Might need to modify way the tillers move when the rudders kick up, so the crossbar is in an acceptable position whether the rudders are up or down (or have an additional bar extend forwards from the centre of the crossbar to slide underneath the rope traveller, to which the tiller extension would be attached) - thoughts?
     
  9. Philippe Lapere
    Joined: Aug 2018
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    Philippe Lapere Junior Member

    I agree, something's got to give, and I'm hoping the design will be a suitable compromise between the two - either going to come up short on load capacity or speed, hoping to find out soon!
     
  10. Rumars
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    Rumars Senior Member

    With foam as a core you need a continuous inside fiberglass layer, there is no way around this. Longitudinal stripping with foam is not an ideal method, it's much better to thermoform or contour cut larger sheets, much less sanding this way. Visit The construction of the first trimaran float http://www.fram.nl/float1.html and look at the pictures, you will understand how it's done (click on the pictures in the text, a gallery will open each time). Male or female form, as well as the exact position of the hull joint, and hand laminating or infusion, are personal preference. Sometimes a method is better then others, for example if you can not get inside a deep narrow hull, it's better to have the joint horizontal near the deck where you can reach it trough deck openings, or make a flange beforehand.
     
  11. Robert Biegler
    Joined: Jun 2017
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    Robert Biegler Junior Member

    You could consider a different building method. Have a look at Rob Denney's intelligent infusion approach: Intelligent Infusion – HARRYPROA http://harryproa.com/?p=1845. Build a female mould out of flat panels, use filler to round the chines, lay in the fabric and whatever foam you need, then infuse the resin. Flat panels work for Rob's designs; they may work for you when considering that many of the fastest Multihulls, like Macif, Banque Populaire XI, etc look like flat panel hulls with rounded chines. If you design a hull that is symmetrical fore and aft except that you insert a temporary frame into the mould to make a transom, or if you make the top half of the hull a mirror image of the bottom half, then either symmetry will let you get all four halves out of a single half hull mould. You put some effort into fairing the mould, but using flat panels reduces that a bit. If you need only one half mould for both hulls, that cuts down on work some more, and you don't need to fair the hull after you pop it out of the mould.

    Here is another proposal that may be simple to build. You mentioned hiking platforms. If you build those anyway, they might give you suitably strong points for attaching leeboards, roughly like this, though probably without the flying:
    [​IMG]

    A-cats used to do that with a wire. I don't know how they got the tiller extension to the other side by swinging it forward, where there is less space. Possibly they had tiller extensions that extended to both sides. There was a beach cat in the 80s that had the rear crossbeam very close to the transom. The rudders were linked by a slightly bent connecting cross bar aft of the rotation axis, which in turn was linked to a tiller in the centre that went under the rear crossbeam. I don't remember how the tiller extension was handled on that boat. I found just one slightly useful picture of the Trac 18, but resolution and contrast are too low to see those details.
    [​IMG]
     
    Dejay likes this.
  12. fallguy
    Joined: Dec 2016
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    fallguy Senior Member

    How is anyone believing a 20' beach cat suited for 6 pax? What is the PPI? Available displacement to WL v dry hull?

    Be much easier to not reinvent the wheel and find a proven design.

    I recommend the 6.4M Janus or the 6.6M Wizard designs by Richard Woods. Both offer around 1000 pounds available and sufficient ppi for more. Proven boats, trailerable, etc.
     
    redreuben likes this.
  13. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Or run a double mainsheet ? They have the added advantage of preventing accidental jibes and banged heads. Cheap too...

    Wharrams don't pitch because of V hulls. They pitch because of fore and aft symmetry. Make the hull more teardrop shaped and it should pitch less.
     
  14. Philippe Lapere
    Joined: Aug 2018
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    Philippe Lapere Junior Member

    I'm confident the cat will be able to take 6 pax, but possibly only for a leisurely cruise in light winds, and not in moderate to strong winds.

    In the calculations below I'm ignoring the heeling moment of the boat and additional volume required in the leeward hull to overcome that - which reminds me that I should reply to Robert Biegler's reply earlier about boat loading..

    The PPI when loaded with 6 pax is 161 - if I'm understanding the calculation correctly, with a load of 450kg (990lb), draft is 268mm (10.55inch). If I increase draft by one inch to 293mm (11.5 inch), volume increases by 161 pounds.
    With draft at 268mm, there is (660 - 268 =) 392mm of dry hull protruding from the water.

    I understand that the Woods designs are proven and solid, but I'm looking to try something sportier and lighter than that (with more load capacity than the standard beachcats available on the market)
     

  15. fallguy
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    fallguy Senior Member

    what is the design draft? Surely not for 6 pax...

    so, let's say you have 400 pound or 3 pax..each added pax lowers the hull by one inch at 161....

    just a conversation...

    dry hull weight is X
    Design displacement is Y
    Y-X is allowable load
    Ppi after that is immersion below design at say 161 per inch..

    so, can she tolerate running 3" low?
     
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