Hobie 18 Tri

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

  1. BlueBell
    Joined: May 2017
    Posts: 1,268
    Likes: 246, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    1. Beam overall: Yes, beam is fine, I like the 18/14 ratio.
    2. Beam placement: You're going to have to show some comparative sketches before I can comment but you can likely trust your intuition.
     
  2. Bigfork
    Joined: Dec 2009
    Posts: 59
    Likes: 4, Points: 8, Legacy Rep: 17
    Location: Montana, USA

    Bigfork Junior Member

    upload_2020-10-5_18-44-43.jpeg upload_2020-10-5_18-45-37.jpeg

    Solcat is jigged up and in the garage.

    I've got some serious design elements to brew on now...
    Overall Dihedral: ideally, one float just touching while parked up or maybe slight airborne. As all three hulls are the same length, some dihedral sounds logical to help turning(?) Related to this is the rocker of the Solcat vs H18. The latter having much more rocker...So the relativity of the boyancy of the Solcat floats vs the H18 main hull... hmmm.

    Float Cant: canting the floats 'just so' to facilitate 'plumb' when heeled at moderate speed... hmmm. Also related to Overall Dihedral....

    Natural Water Line of Solcat vs H18: How will the floats relate to the main hull, how do they 'sit' naturally, bow to stern... side view

    While I'm brewing, I can build the central box beam from hing point to hing point. Thinking of 7'8" in length to be slightly under the 8' wide ideal trailer limit. I can also build the 'universal' level mounting platform on the floats. The curving beam end will get the cant angle relative to plumb.

    I've been timberframing for 20 years. I've been essentially treating these floats like round logs, ie: using laser to establish X and Y and Z. I'll snap lines on the concrete to establish a layout grid when the time comes to mate floats to beams...just like coping logs.

    Any suggestions, ideas, or otherwise is welcome. At some point I'm just gonna go for it! If my float to rear beam connection is adjustable or in some way, I would have some adaptability in the elements mentioned above. Thats the hard part... without any real calculated fluid forms, it's really all guess work :) The result might be degrees off or inches in the worst case...

    thanks folks
     
  3. BlueBell
    Joined: May 2017
    Posts: 1,268
    Likes: 246, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Looks okay to me.
    What is the question?
     
  4. Bigfork
    Joined: Dec 2009
    Posts: 59
    Likes: 4, Points: 8, Legacy Rep: 17
    Location: Montana, USA

    Bigfork Junior Member

    Yes, more like statements than questions...
    A real question: Am I right to induce some dihedral in order to facilitate turning? As all three hulls are basically the same length, a little dihedral would help turn (among other things like limiting drag)?
    While the hulls are jigged up on the level related to roll, I can use a laser projected on the side to take an educated guess at a level line than balances buoyancy fore and aft. Doing the aforementioned to the vacca (H18, more rocker) and the Solcat (amas, less rocker) should give me an approximate "balance" between the three (??) It's sort of guess work but that sounds logical eh?
    And, while requiring more work, am I right to cant the ama(s) so they are at plumb when boat is healed or should I just build straight across?

    thanks BlueBell and others:)
     
  5. BlueBell
    Joined: May 2017
    Posts: 1,268
    Likes: 246, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Bigfork,

    I would go with cant, yes, but not much.
    Edit: I don't see any dihedral... perhaps CoB between hulls?

    Guess work it is, but trust your intuition.

    I believe the objective here is to achieve the designed water line on the two immersed hulls at target speed. This will be very tricky to guess so it had best be adjustable.
     
    Last edited: Oct 13, 2020
  6. BlueBell
    Joined: May 2017
    Posts: 1,268
    Likes: 246, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Okay, I gotta ask: if you're 6' 7" what do you weigh?
    PM me if you find this an inappropriate question.
    However, it is part of the equation.
     
  7. Bigfork
    Joined: Dec 2009
    Posts: 59
    Likes: 4, Points: 8, Legacy Rep: 17
    Location: Montana, USA

    Bigfork Junior Member

    Hi BlueBell,
    yes, 6'7" but a 190lbs. I'm a bit of a broom pole but ideal for basketball and catamaran trapeze. My first driver's license has me at 5'11" and 115 lbs so I've managed to gain some beer weight....

    Overall weight and relative buoyancy of the finished craft is a deserved concern. That said, I've been on a Hobie 18 before with 4 adults (two on the wire and two on the rail) in a real blow, flying the windward and barely keeping it together. In bigger wind this tri will be a wet ride, I'm expecting it. A beach trimaran comprised of beach cats is still a wet ride.

    Canting the floats 10 degrees seems easy. I can figure that out.
    Relative buoyancy of the H18 vs the Solcat is a head-scratcher tho...

    Factory profile pics of the H18 have the beam pockets on the level (as a way to balance fore and aft)
    Factory profile pics of Solcat have the transom plumb (again, suggesting balance fore and aft) Beam pockets are vastly different elevations, nothing else looks level or plumb.

    I realize you are right BlueBell... it's guess work. I can site under them while they are side-by-side and see the rocker of the H18 a couple of inches below the belly of the Solcat. The profiles then reverse nearer to the bow and transom as the H18 curves up leaving the Solcat lower fore and aft... I even used a laser to establish a relative line on each, then measured from bottom of hull to laser at three consistent locations (my established beam pockets, and 2' back from bow) to compare hull form at those locations (knowing this doesn't take into effect the cross section/profile, just the distance from laser to keel. The Solcat is generally more of a V in profile while the H18 more of a U). If I accept a certain number at the forward beam pocket (basically the middle), then balance the diff between the aft beam pocket and the 2' off the bow location, I have found a starting point to achieve balance between the two... I think:). Think balancing a smile against a flat line; trying to have equal distance from smile to flat line at either end.

    As it is now, The numbers I cultivated with my laser tell me to push the bow down on the Solcat... but...with the bow rocker of the H18, they already appear to be 'bow down'...
    Surely I'll do a float test with mocked up weight before the lot gets fixed.

    Looking closely at W17 (amazing web site) beam construction pics to cypher how to build the vacca box beam. That's my next move.
    cheers.
     
  8. BlueBell
    Joined: May 2017
    Posts: 1,268
    Likes: 246, Points: 63
    Location: Victoria BC Canada

    BlueBell Ahhhhh...

    Is this a potential Race to Alaska (R2AK) boat?

    Do you have a completion date or a target event?
    I ask because you've had these hulls for some time with a plan to do something like this, again, for a long time.
    What you don't know, you must try.
    Stop questioning and second guessing and strap it together and try it out.
    Take video.
    Good luck.
     

  9. Bigfork
    Joined: Dec 2009
    Posts: 59
    Likes: 4, Points: 8, Legacy Rep: 17
    Location: Montana, USA

    Bigfork Junior Member

    Ha! yes...
    I'm in no hurry, no target date, might float next summer, might float in 2 years, might end up in the land fill. I do daydream about R2AK. I also sail hard all summer, ski hard all winter so I have limited time to mess around with stuff like this during the shoulder seasons. I'll post pics once I have made serious progress :)
    cheers.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.