Hobie 18 Tri

Discussion in 'Boatbuilding' started by Bigfork, Apr 1, 2020.

  1. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    It's going well, the comment that things will become obvious just before you finish is a double edged sword I found .. ha , keeping it light is a really good idea. Perhaps a light dagger board and rudder could add some zip, they will want to bob up underway and the[longer..?] centre board will need holding down,..further down the track but food for thought.
     
  2. upchurchmr
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    Location: Ft. Worth, Tx, USA

    upchurchmr Senior Member

    I can't imagine a worse place to make something too light than the dagger board and rudder.
    These things are highly loaded and a failure ruins your whole day.

    Well the mast would probably be #1 worst part.
     
  3. Bigfork
    Joined: Dec 2009
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    Location: Montana, USA

    Bigfork Junior Member

    It's all vaporware and smoke at this point but...
    If she has enough horse power and large enough floats to drive hard, I could envision a longer centerboard. Using the hobie dagger profile and lengthening it by a foot or so, same with rudder.

    Speaking of which, I know the rudder/transom is a danger spot. The added horsepower and weight may stress the rudder assembly and transom more than usual. Hobie rudder assemblies are a dime a dozen so I'm just leaning that way for cost and ease...and the kick-up feature of the factory rudder. While the back of the hull is opened up, I may laminate an aluminum plate to the inside of the transom. then Glass it in or tab it. Does this sound legit? What thickness of plate ? With the age of the hull, the slight gelcoat cracks around the gudgeon mounting holes, and the added stress factor, it seems like a good preventative move...no? With the interior access to the transom, I'll have the ability to thru-bolt the gudgeons thru the al plate.

    thanks. I'll keep trickling pictures as progress moves. (the virus has sure opened up some garage time eh? :))
     
  4. trip the light fandango
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Carbon fibre is expensive but some strips for rudders and a dagger board plus foam and normal glass isn't too pricey, ..good practice.. Just add an extra layer or 2 of matt[inside ?] rather than glassing in ally if your worried about the transom but I doubt it needs it. Remember H18 's have been photographed flying off the back of 10ft waves, they really are strong, especially the early ones. Depending on the beam width you settle on , do you reckon you'll really get more grunt than a standard hobie? the beams are the main stress point I think, nwguy, revintage have solved issues you may bump into..you could always go on a diet..ha, nah movable ballast could come in handy. Ther's an older post where a guy designed a tri using H18 amas, it was suggested he glue on a 2ft block of polyurethane to the transom, shape it and glass over, that foam is fun to shape. I don't know if he finished it..
     
  5. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    If the dagger board is light it could even add some float, it will need a down haul, that was pressure sensitive ideally.. A long one would be taking some serious load, then there's hitting stuff.. the boom.. etc ..ha
     
  6. Bigfork
    Joined: Dec 2009
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    Location: Montana, USA

    Bigfork Junior Member

    Yea, you're right trip. I've watched the viddy many times..."Sharing the Wind", jumping surf waves and getting what looks like 15' of air under the cat! Great video of a bygone golden age of beach cats. I know that without vacuum bagging and carbon I'm not going to achieve good old fashion H18 horsepower; too heavy try as i may to stay lite... :) I'm aiming for slightly less performance (top speeds) but dryer, more expedition worthy, easier to solo with confidence of staying on her feet.

    I still have a spreadered 34' mast and a matching Solcat mainsail. A Hobie 18 mast is 28' long (SX version is 29.5'). A Hobie 18 rig (or something similar) seems the best route eh? A H16 mast is 26.5'. I even thought about a Hobie 16 rig with a Qwirlwind squaretop main for 800$...

    I guess the size of the engine is pure speculation until I have a better guess of finish dry weight.

    Pic shows near completion of rear receiver bulkhead. There still some tabbing to do. The presented surface is 4" wide by 24" long, leveled, square, and otherwise true to boat. The front version will be same length, but 5" wide and at the same elevation relative to the rear. (seems most designs calc for more load on the forward beam so I went 5" wide instead of 4"). Then whatever beams I use can mate to the top of those two relative planes.

    Question:
    In the pic you can see the blue tape layout for an aft hatch. I need to have some sort of access there so I can fillet and tab the rear most BH to the existing hull, reinforcing the gudgeons from the inside, and to just have an aft dry(ish) locker. After lots of googling, I'm confident I can build my own semi-dry hatch from 3-4 mill...nice curving lid, scuppers, cotton rope gasket, etc. The hull flange provides a lovely bungee cord point to secure hatch lid. I will do the same thing forward of the fore beam location again to gain access to the fillet and tabbing and for a fore/bow storage.

    Does that plan sound silly? I have to gain access in there. Should I just go with a 5.5" round twist hatch? Lighter but very limiting as far as creating access and storage. Just had a thought...The square hatch could butt up against the rear BH and serve the same "working as a brace" duty as the two triangular braces going down the cockpit gunnel....get rid of the ugly asym beam pocket just aft...hmmm.

    thanks guys!
    cockpit 4.jpg
     
  7. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    The Hobie 16 +sail. sounds about right, except you have to buy it. The longer masts are just out of my ability to lift and drop the mast, you'll need 2 reefing points of course, maybe 3 for the bigger mast.
    Your hatch idea sounds good, I will be doing similar, using the flange is a good idea I hadn't thought of, maybe cam locking devices for battening down?. Bungee is fine and practical to use but perhaps not enough if conditions ramp up. It could just be tying them off with line. I still think extending the stern would really help, but then I do have a bit of a problem with thinking that for my boat and it is distracting, I may just be trying to coerce you into doing the same..? .ha ,I don't really know.
    One reason I will probably do it is that I can't drop the mast..go under a bridge without sinking the transom as it is now, another I can carry slightly more, it will be faster, feel a little more secure...but it's not absolutely necessary.

    .. The square corners are potential weak points and will need re enforcing, a hatch is far more practical than twist hatch, cheaper and lots more work,.. hinges unless you make them are also pricey. A piece of bungee secured will stop the hatch from getting lost. The old saddle may have a useful purpose ,water bottle? and could be made symetric...?
     
    Last edited: Apr 28, 2020
  8. Bigfork
    Joined: Dec 2009
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    Location: Montana, USA

    Bigfork Junior Member

    At this point, I'm hesitant to extend the stern... longer waterline would have it's bonuses but also open up other engineering hurdles such as rudder assembly.

    I have a Hobie 16 so i could always let it sit for a summer and try it's rig on my current dink around. I've also got a spare H16 sail. Old H16s can be picked up for 400-600$ usually. Might be worth it to source a mast (as well as other bits and bobs). I know my Solcat mast is larger spar in all directions... luff track included.

    Pic is bow locker before bulkheads get installed. Haven't cut deck hatch opening because I'm not quite sure how it will relate to the BH...whether it butts up against or stands off a little. There's 1.7 cubic feet in there! Old H18s can rip their bows off at the crossbeam location (they got more reinforcement in the mid 80's). The blow locker will serve to structurally support the area as well. Made from 1" foam laminated to door skin, all glassed, encapsulated, filleted, tabbed (yet to do).

    So i might want to sprinkle some carbon in some places...I know enough about this game to understand where some high stress areas might be. What is a generic carbon fabric (weight and weave) that anyone can recommend? Just a yard or two to provide a little extra oomph here and there... mostly in the tabbing (I think) so perhaps a carbon tape? Perhaps a roll of CF Tow for some "hold down power" here and there?

    About that tabbing and fg thing. I was given 5 yards of fabric that I think is a woven roving bi-directional 'twill' of a medium weight...(?) It will deform diagonally but not in the 'x' or 'y' direction and the strands go under 2 and over 2 so I think it's called 2x2. I've been chewing my way through this using it for flat panels and tabbing...everything. It loves to fray up and loose strands when i cut it for tape. The edges get feathery then there's more fairing involved because of the wiggly edges that end up swimming about during glassing. What tabbing tape (weight and weave) should I be using?

    Thanks :)

    fore hatch 1.jpg
     
  9. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Tape is much cheaper than it was, post cost is often free here ,both 2 inch and 4 inch [lay both for bulkheads ] would do for most at 6 or 8 ounce. The lighter stuff [<4ounce so times 2] is easy to saturate without extra resin waste but can fold up and disappear[glasses required] more readily. I've been using about 12 ounce? [450] bi ax 45, 6 inches wide [heavier potential stress] ,it was cheap but it is a little inflexible,often needs filling between layers so works best laying a few at a time working air bubbles out. Carbon[retail outlets normally describe fit for purpose] from what I can pick up is only useful for really rigid spots, chainplates dagger boards and cases using epoxy or at least vinylester,[ no hang on polyester has less flex, should be fine] I'm no professional though, so others can fill you in, sanding where you are laying on older glass lightly but thoroughly is an important factor. Tape doesn't go far though, yards get eaten up fast especially doubled up. You may tape your beam seams..? that would take some serious yards.
     
    Last edited: May 1, 2020
  10. Bigfork
    Joined: Dec 2009
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    Location: Montana, USA

    Bigfork Junior Member

    Revival time!
    As the last threads of summer fade into fall, I'm back to daydreaming about my diy trimaran. During the summer I'd rather sail than build:) And I've had a change of plans/ideas. My initial plan was to build my floats from a known catamaran plan (Woods 16 or something similar) and build box beams like the Water's W17. Then I had the idea of picking up a thrashed Hobie 16 (400-600$ will get you an old H16 in the Northwest) and use its floats, rig, etc.

    So I also have an old Solcat 18 that was frankenstien-ed into a 10' beam (a weak attempt at giving Tornado dimensions/performance). I have the whole boat taken to bits and stashed against the garage for a 'someday' project. That someday project was going to be a stitch and glue center hull and the Solcat rig, beams, etc as the donor boat to make a small cabin tri. I'm beginning to realize that I can only have 1 "someday" project; the current project must take precedent.

    So...new plan: use the Solcat 18 hulls paired with the nearly done Hobie 18 main hull. While fixing some issues, it opens up others. That said, a bird in the hand is worth two in the bush eh:)
    The H18 hull and Solcat 18 hulls are basically the same length. "Square" trimarans seem to have fallen out of favor for various reasons. I was thinking of shortening the Solcat hulls to 15-16' by simply whacking off the couple feet of the transom. Solcat 18's are a little weak in bow volume so shortening from the transom will preserve some buoyancy and move the reserve buoyancy forward. The profile of the Solcat 18 tapers from the front beam pocket towards the transom.

    So, for example:
    --nearly done main hull, H18 (see pics earlier in thread)
    --Chopped Solcat 18 Floats, cut to 15'
    --Beam overall, 14' (ish?)
    --maybe a bigger rig, H18...

    The existing beam pockets on the Solcat will not match up with what I've done on the H18 vaca... And would likely be too low relative to what I've done on the H18. Too much dihedral. I might be able to reuse the Solcat beams incorporated into a folding system like the Strike 18 but because I've raised the beam location on the H18 (see earlier pics), the original Solcat beam pockets would cant the float inward, the wrong way contributing too much dihedral.

    I guess before I jump back in I must ask:
    1. What are the disadvantages of a 'square' footprint trimaran? Do they require more dihedral in order to facilitate turning; not dragging 3 forms through a turn?
    2. Is shortening the old Solcat 18 hulls to 15' sound stupid? Seems pretty easy to butter on a new transom.

    Once I commit, I'll post pictures again of progress.
    Thanks forum folks!
    cheers.
     
  11. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Don't chop the solcat transoms off, the extra weight is negligible, I still think that you need extra buoyancy.. add to the H18 transom.
    I'll explain my two cents worth when I get a chance. Search for similar tri's, square is ok, very stable, loads the mast up if you don't reef, beams need good sea stays, weight is an issue, carrying stuff for camping.
     
  12. Bigfork
    Joined: Dec 2009
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    Location: Montana, USA

    Bigfork Junior Member

    Thanks Trip!

    My thoughts in shortening the Solcat were for the following gain(?):
    1. making the floats shorter than the main hull by 2' or so would allow for swifter initiation and ease of turning
    2. I wasn't even considering the weight loss advantage...I agree, negligible

    I'm gonna have to mount my beams in a custom manner anyhow so the location of the original beam pockets is also a mute point

    You have suggested adding length to the main hull before. While doable, this seems like loads of work (mating/rabbiting ply or ceder strip to existing hull shape, fair-ing for true-ness, gel-coat or top paint, reinforcing new transom to take rudder loads, remounting gudgeons/pintles,...) And it seems there's the potential for yet more weight gained, counterbalancing the gains from added flotation.

    I'm entertaining the idea...it just seems like lots more work. What would you suggest I add for length to the H18? Like the Solcat, the H18 tapers a bit towards the transom. The added length would have to follow the trend of the taper.

    Thanks!
     

  13. trip the light fandango
    Joined: Apr 2018
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    Location: Rhyll Phillip Island Victoria Australia

    trip the light fandango Senior Member

    Hi Big Fork, well the further you can get those floats forward the better.
    I'm going to wet the mdf side of melamine veneer, bend it around the centre hull, screw it in place and lay glass[put plastic film over melamine to make sure the form /mould will release] to make a sugar scoop stern[with a watertight step built in] , use the cat rudders on the outer hulls, I'll let you know how it works in about 2 or 3 months...Ha. So I have a bias. keeping the centre of effort of the sail amid ships rather than forward makes going turtle less likely.

    The rudders on a hobie are very good, durable and very easy to kick up, a single larger rudder on the main hull I think is less practical,1 it encroaches on limited seating space,2 having two rudders further forward will create better steering, so 3 good reasons.
    If you were to make all those fittings it would be not worth, but we have donor boats so all that hard work and clever design has been done, why waste it?
     
    Last edited: Sep 21, 2020
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