Advice on 20ft beachcat design and build

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

  1. Philippe Lapere
    Joined: Aug 2018
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    Location: South Africa

    Philippe Lapere Junior Member

    Hi everyone,

    I’m designing and building a prototype sailing beach catamaran and would like to get your opinions and recommendations: its going to be 20 ft with 8’6” beam, beachable, capsizeable and relatively light, with very good load capacity for cruising with up to 6 people, yet is still fun and relatively fast to sail (particularly when loaded a bit lighter and pushed). It would be somewhere between a supersized Hobie Getaway and a lighter, less powerful Hobie 21 Sports cruiser.

    I’d also like to be able to launch through fairly large surf (I live in Port Elizabeth in South Africa and plan to do coastal camping trips where sea conditions are frequently challenging).

    Some functionalities that I’m planning or considering to incorporate into the design:

    · Mounting for an outboard motor, maybe between 4-9 hp

    · Self-tacking, roller-furling jib, easily reefable mainsail, and a furling gennaker sail

    · Foreward netting area (for sitting / lying on in lighter conditions)

    · Potentials: windvane autosteering, rowing oars (doubling as righting pole and mast step pole), flip-out wheels, (nothing is off the table:)

    On the topic of design: I’m not a naval architect (I’m a Mechanical/Industrial engineer with manufacturing experience), and I’m not under any illusions about the gravity of the challenge of designing and manufacturing a sailing boat, hence this post for advice. I’d like to design something that’s unique and different so I’m not keen to purchase plans, or modify existing an boat.

    I’ve done lots of research and have seen a fair share of red flags and failed projects by being over-ambitious or misguided, so I’d like to avoid such pitfalls by sticking within the realms of existing designs, and building on solid principles and practices.

    I’ve therefore finalized the design by benchmarking extensively, and sticking relatively close to existing cat designs, but maximizing the necessary parameters (most notably hull volume).

    Out of interest, I’ve calculated the prismatic coefficient using Excel by modelling the rocker, waterline and cross-sections mathematically and integrating over the length to calculate volume.

    I’ve designed the hulls in Autodesk Inventor (using the mathematical formulas for the rocker etc as a guide to create the shape). See attached CAD screenshot, as well as pics of a 7:1 scale model of the boat, created by printing out the bulkhead sections and covering with paper and superglue.

    Some questions I have that I’m hoping can be answered:

    - A concern is the hulls causing too much windage, at their widest point each hull is 420mm wide and 660mm tall (front is around 600mm tall and rear 400mm) – this is 25% additional windage than a Getaway, 40% more than a Nacra 5.5 (!). What effects can I expect from this? Thinking of slightly raking the bows back to reduce windage at the bows (and for the cool looks lol), worth it?

    - The Prototype manufacturing method I’m using is to CNC cut 13 pre-laminated bulkheads out of 16mm 60kg/m^3 density PVC sheet based on the CAD drawing, then mount the bulkheads onto a strongback, then create the hull by “strip planking” using strips of the same spec PVC (pre-laminated on the inside), then laminate the outside (3x layers 160g woven fiberglass and epoxy), sand, fill, flip, same strip planking on top, and voila! Any thoughts on the laminating schedule, is there a recommendation for how many layers of which spec fiberglass to use for a beachcat of this size?

    - For leeway prevention, I plan to mount pivoting leeboards on the inside of the hulls (so on the side of the hull, under where the trampoline joins the hull). Do you think that there’ll be considerable interference between the hull wave and the leeboard (so between the board and the hull)? I’d like to avoid the complexity of the centreboard boxes, and hoping it’ll be worth the performance compromise.

    - For the original protoype I’m going to use the mast, sails, rudders etc from my hobie 16 to test – hopefully soon therafter add a furling genoa for some additional power - any thoughts on the performance I can expect, particularly when loaded with 6 people? I know it’ll be underpowered compared to the 20ft sport catamarans that are out there, but that’s not my goal – I’d like to make a boat that’s pleasurable to sail on that has a high load capacity, without completely tanking sportiness (like a Tiki 21 would)

    - Any other thoughts or recommendations? Thanks!

    Beachcat views.png
    beachcat scale model 2.jpg beachcat scale model 1.jpg
     
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  2. Barry
    Joined: Mar 2002
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    Barry Senior Member



    This is a Nacra 5.7, scary fast and there are a few shots in the video which shows the plan view of the hulls. There are other Nacra 5.7 you tube videos which show the profile as well.

    We owned one for a short while, found it much to fast and skittish for windy days. Cheaper to buy but you lose the satisfaction of the build
     
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  3. upchurchmr
    Joined: Feb 2011
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    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    6 people is too much. 3 on a Tornado was not manageable without lots of conflict while tacking.
    You might do 3 or 4 since you plan a fwd tramp, but in certain conditions that area is not useable due to needing the weight aft, unless you are willing to reduce the performance significantly.
    Why do you need so much height of the hulls? If you are hoping to reduce wetness while sailing, that's a pipedream. Beach cats are wet. Somewhat less if you go slow, but not much.
    If you want all the gadgets like a motor then make the boat longer, move the rear beam farther fwd. These boats are dependent upon the crew weight moving fwd or aft to sail right and prevent pitchpoling.
    Simplify and you will have a much better boat for your camping/ expeditions.
    Don't show a bare boat, show us where the sails will be sheated, especially the big roller furling sail. All that gear typically lands right where people want to be seated.
    Consider sailing benches like some of the Hobie 18's had. That gets you more righting moment which you lost by going to 8'6", and gives a more comfortable place to sit for a long trip. If you really want 6 people, extend them farther forward than the center beam.
    Raise the fwd beam height, that is going to collect waves and stop fwd motion when you hit one.
    You better have really good leeboards, and bigger than a normal centerboard. How will you control their position? They will potentially require a slot in the trampoline right where you need it to be strong for crew weight.

    What cats have you sailed?

    Sounds like a fun project in what I assume is a great place to sail.
    I was in Cape Town once but never had a chance to sail.
    Good luck.
     
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  4. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    See what you can find on the caper cat 18 and the windrush 6000.

    The capercat had large volume hulls with huge lockers in them including one insulated for ice. Most were the 14' version but a few 18s were built.

    CC14s have been used to sail from Brisbane to cape York. I think it's 2000 kms or something.

    The windrush 600 or 6000 can't remember is a 20' cat with minimal cabins in the hulls. 8' wide, there was a 10' version I think. They used leeboards. I know one has won the bay to bay race on handicap several times. The right crew can really make them go.
     
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  5. SolGato
    Joined: May 2019
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    Location: Kauai

    SolGato Junior Member

    I really like what Hobie was able to accomplish with the rotomold construction of the Getaway and Wave. The hulls have a lot of volume above the waterline and bulge out to float a lot of weight at cruising speeds, and then when powered up at high speed, the volume is lifted out of the water and the built-in keels remain and do a great job of keeping the boat on point considering they don’t have boards and have pretty small rudders.

    The other thing I love is how the plastic hulls (especially on the Wave) act as a shock absorber when sailing in chop and swell. Well at least my butt appreciates it!

    I only mention all this because you have 8 person capacity on your list and talked about camping, while also desiring good performance.

    Now the rotomolded boats might not be the fastest, but they do strike a nice balance between those two needs which isn’t an easy feat. Usually you sacrifice one for the other, and I think the hull design with regard to volume and the built in keels have something to do with that, so they might be worth studying more closely.
     
  6. Philippe Lapere
    Joined: Aug 2018
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    Location: South Africa

    Philippe Lapere Junior Member

    Hi Upchurchmr,
    Thanks for the advice and comments
    - hull height is for additional load capacity, pitchpole resistance, and hopefully some reduced wetness, but I agree that it won't be a dry ride
    - good point on the sails and sheets, will look into that. Forgot to mention that the wing seats were also on the cards, as you suggested.
    - regarding leeboards, why do you say they should be bigger than normal centreboards? I'm planning to position them in the correct spot, then just pivot from horizontal to vertical using pulleys, hopefully avoiding the slot in the tramp.
    - I haven't sailed many beachcats, mostly hobie 14 and 16, see clip below starting at 9 minutes, some "show us your cat" action from PE
     
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  7. Philippe Lapere
    Joined: Aug 2018
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    Location: South Africa

    Philippe Lapere Junior Member

    Will do, thanks!
     
  8. Philippe Lapere
    Joined: Aug 2018
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    Location: South Africa

    Philippe Lapere Junior Member

    Hi Solgato,
    Thanks for the input, yeah I've done some benchmarking on the Getaway - like the low aspect ratio keels but hoping to get better performance with leeboards
    cheers!
     
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  9. Robert Biegler
    Joined: Jun 2017
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    Location: Trondheim

    Robert Biegler Junior Member

    I am guessing that the six people are you and your family, and that makes the number non-negotiable. What is the combined weight, with clothes? Could be 350kg, assuming some are not adults? Camping gear and some food and water for six could be 100 - 150kg. Outboard with fuel 30kg? All that weight will give more righting moment, which increases loads on the rig and beams. Hulls will need to be able to take that load, too. So let's say the boat weighs 250kg. That's a total of 700 - 780kg. I think for good longitudinal stability when flying a hull you want a hull volume at last twice, perhaps up to three times the total displacement, so in the range of 1600 - 2400 litres. From the measurements you have given, and assuming a prismatic coefficient of 0.7 for the whole hull, I get only 930 litres. I expect you have a more precise estimate. What is it?

    Is the length limited by where you store the boat?

    My choice of beachable, trailerable boat that can carry six people through surf would be a double Bruce foiler with medium aspect ratio foils and a hull about 8 metres long. A scaled up version of Gerald Holtom's Foiler 21. Bend the lowest part of the foil down a bit, and the whole thing becomes a little more robust against changes in the height of the centre of effort. The Foiler 21 had a board. Other people relied on the foils alone.
    [​IMG]
    You would need to assemble and disassemble the foils, but I think that should take a lot less time than assembling a cat too wide to trailer, because you are not putting in a trampoline and connecting the rudders.
     
  10. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Centerboards which are well sealed off at the hull keel line avoid wavemaking and air entrainment.
    Lee boards do not, which requires them to be bigger to get equal side force capability. Also there is wave interference issues with the hull - just a little more efficiency loss. The area between the side of the hull and the bottom of the hull is largely inefficient and should not be counted on for area.
    You also need to insure the board is well supported at the hinge point and at its highest point so it won't deflect from a vertical position. If you plan to use both at once the windward board will be trying to pry sideways at the top away from the hull. That means more restraint structure. If you don't have that, it will deflect sideways and try to tear out your pivot point.
    The issue about running rigging issues is similar for leeboard control. Not the seating issue as much, but finding appropriate space for the control lines and how the lines run across the tramp then creating seating issues and issues about moving around the boat.
    Would you want to control both boards from the windward side? Another significant issue.
    Its not really good to have to go to the leeward side to control a board. Can upset your sideways (tipping) control. It can be dealt with by leaving both down until it is safe to change one setting.

    If you sailed Hobies then you have experienced many of the beach cat's issues, good.

    Bruce Foilers have not been known for being good sailing boats. I was once really impressed with the concept, but the few reports of actual performance and safety were not impressive. They have gradually dissappeared after several reinventions with very little discussion of issues. The foiler shown above was inordinately wide. Bruce showed that you "should" be able to dispense with a foil on one of the sides.
     
  11. Rumars
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    Rumars Senior Member

    The building method you propose is not correct. The inside fiberglass layer must be continuous, wich is not possible if you use prelaminated strips. The bulkheads can be prelaminated and then tabbed in, over the interior strips. You can use them as building molds, but they must be removed for interior skin lamination. For stripping with foam you would need additional molds, the bulkheads are to far apart for propper support. 13 permanent bulkheads are to much for such a small boat, 4-5 would be enough. Foam has the big advantage that it can be thermoformed or contour cut, so instead of horizontal strips you can use vertical, much wider ones. This implies a stringered mold, male or female. Or you could look into alternative ways of building like the Kelsall KSS system, if your hull geometry allows.
    Read trough the "Multihull Structure Thoughts" thread to see examples of scantlings. I would use 80kg foam, add some glass on the underside to allow for dragging the boat on the beach, and use a heavier cloth so there is no need for three layers.

    Centreboard boxes do not need to be complicated. On a beach catamaran you have the luxury of using full height, open top boxes (like a daggerboard case, only longer), with the board hanging from a yoke or riding on top, and a foam filler piece to close the slot. Much simpler to make structurally sound then using leeboards, but if you want leeboards look up Kholer's arrangement.
     
  12. guzzis3
    Joined: Nov 2009
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    guzzis3 Senior Member

    Sigh.

    In theory lee boards cavitate etc but in practice they work fine. They do need to be big and the attachment fairly robust but remember this is a 20' cabinless catamaran, not an ocean liner.

    I happen to have a set of hobie rudders laying around. I was quite amused to find they are only slightly smaller than the specified foils for several cat designes in the high 20' range. It's obviously true that they are transom hung rudders and do the work of center/dagger boards on the hobie cat, but they serve as an illustration of how effective a pair of big rudders can be especially coupled with similar sized leeboards.

    I do recommend hull lockers though. As a camp cruiser it makes the boat so much more for not too much extra cost. Of course at 20' you have the temptation to go to in hull accommodation. 4 compact single berths and a semi private space for a porta potty takes a boat to a whole new level.

    But then again where do you stop ?

    Good luck. Have fun.
     
  13. Robert Biegler
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    Robert Biegler Junior Member

    I also have found only a few and brief reports on what I think of as the classical design, stabilised by foils and noting much else. They tend to be fairly positive, though, from Windlass over Foiler 21, Mantis III, and John Bull's dory described in AYRS Catalyst 15 (https://www.ayrs.org/catalyst/Catalyst_N15_Jan_2004.pdf). Known quirks are capsizing after falling off sharply and reversing the flow over the lee foil (Foiler 21), and the foil not doing much if the boat loses too much speed in a tack (John Bull's dory). Also, Dalibor and Mantis IV were slower than hoped. If you know of reports that I have not mentioned, and they are online, would you post links, or send them in a DM?

    I don't quite agree. As I see it, there are actually quite a few double Bruce foilers around. The short definition of a Bruce foil is that lift is controlled primarily through coupling to leeway, and then possibly also depth of immersion, washout, and foil curvature along the span. Not counting the L-foils that have become popular on foiling multihulls, because the way they couple lift to leeway is a bit different, that leaves Hydroptere, Icarus, Mayfly, Windrider 10, Figaro 3, and most of the foils on IMOCAs and Minis and some of the foiling monohulls, like Quant 23. The common theme there is that the boats have enough stability not to capsize even when the foil is doing nothing.

    It's what you need if there is nothing much but the foil keeping the boat upright. And one possible reason why that type is uncommon is that you can't go alongside anywhere. That wouldn't matter if you always sail off a beach.

    From what I read, that works fine on fairly flat water, but the foil tends to pop out when going over waves while sailing to windward with the foil upwind. Hagedoorn proposed hinging the foil to deal with that. That seems to work well at model scale: https://www.youtube.com/user/vectorfoilproa Fritz Roth showed pictures of him building a foil for a larger boat, but that was years ago, and I have seen no follow-up. The proa Nixe has such a foil, but I have seen no account of how much it contributes to stability. In theory, the idea can be adapted to a tacking boat. But the double Bruce foiler seems to have an easier way to reduce draft by reducing heel angle. That should be useful when beaching the boat.

    Of course, if total weight is important, a Pacific proa offers another way of building a beachable boat. It needs only one hull that can carry all the weight. But rig and rudders then offer some design challenges.

    My main concern with the proposed beach cat design is whether a cat with those dimensions can provide the hull volume to carry the intended weight without giving up on the desired speed. I don't know how to do it.
     
  14. Robert Biegler
    Joined: Jun 2017
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    Robert Biegler Junior Member

    You could improve their performance by putting end plates on. And if you make the bottom of those end plates out of steel or aluminium, you don't leave microplastics all over the place as you drag the boat over the sand.
     

  15. upchurchmr
    Joined: Feb 2011
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    upchurchmr Senior Member

    Endplates have a serious issue if you try to retract the board while sailing.
    They become an instant brake at any position other than full up or full down.
     
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