Mashup Trimaran (continued)

Discussion in 'Multihulls' started by Bigfork, Feb 17, 2021.

  1. trip the light fandango
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    trip the light fandango Senior Member

    Fill the hulls with water , measure incoming or outgoing, there are many other calculations that are not in my skillset, so comparing other craft that are similar is the approach I use.
    lateral thinking is also useful. Big forks parts are commercial and have specs.
     
    Last edited: Feb 25, 2021
  2. gonzo
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    gonzo Senior Member

    I can't make any sense of this. Why would he fill the hulls with water? What big forks?
     
  3. upchurchmr
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    upchurchmr Senior Member

    Gonzo,
    Thanks for saying that.
    You think the same as I time after time.
    :D
    ( I think that was a compliment - at least that was how I meant it!)
     
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  4. trip the light fandango
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    trip the light fandango Senior Member

    I must have missed something,.. measuring the volume of a container.
    I wouldn't do it Big fork, in a lake maybe.. but why bother.
    Modern cats and tri's move the sail centre of effort aft .. keeping your ends light and weight central is the go. Moving the amas so their bows are slightly forward of the centre hull is an option, and a bit of toe in helps apparently.
     
    Last edited: Feb 25, 2021
  5. gonzo
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    gonzo Senior Member

    I think you missed a few things. The weight of the boat, light and loaded, have to be calculated to know where the waterline is. I don't know what the reference to "Big forks parts are commercial and have specs" means though. As far as lateral thinking, or any other, it has to give useful results and not simply be used to disagree with proven technology.
     
  6. Bigfork
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    Bigfork Junior Member

    Hey folks,
    Getting more feedback now and perhaps stirring it up by-proxy...I appreciate all the feedback! :)

    Beam
    : I think I'm gonna stay with the current plan for beam...approx 13' outside-outside. I'll rotate my design of the box-beam and make it a little smaller, knowing I can add water stays if needed.
    Displacement/buoyancy: As Gonzo stated, If empty boat targets near 600lbs, I should have 9 cubic ft of displacement. This is challenging my brain for a few reasons. It's really hard to know the displacement of a single Hobie 18 hull by simply looking at pictures of it wet; movement fore and aft drastically changes the waterline and the diff between 1 person-3 person weight also drastically effects displacement. The H18 also has much more rocker than the relatively flat bottom of the Solcat. The way things are now, with the deepest part of the H18's hull's curve hypothetically pushed 8" deep, the Solcat hull (x2) will be about 3" deep at the same point. The H18 displacement "rocker" then tapers toward the transom and bow while the Solcat would maintain the approx 3" of dis for most of it's length (much less overall rocker). Dihedral is the variable that is changing between the main and the floats which also affects displacement....(going down a rabbit hole). I could physically map the cubic in of the deepest part and extrapolate infos at "x" locations along it's length to further estimate displacement...

    The Cross 18 pictures I've looked show a similar-ness yet some large differences when compared to my 'design brief'. All three hulls of the Cross 18 are much larger volume for one, the main hull in particular. In all the Cross 18 viddys, there's not a one showing any daylight under the main hull (or even close). It is obviously not designed to approach those loads (2x4 beams). I'm not aiming the fly the main hull, but I envision it skimming in 12-15k with potential to do the wild thing beyond that... I'm not sure what canvas the Cross flies but most of the pics show something smaller than a H18 main and jib. I have access to a H18 mast and sail plan so I think I'm going that way. So my tri will have less weight, less displacement, but more sail area...

    I weigh 200lbs. My daydream is 1 up and lots of gear and mostly dry, 2 up and 50lbs of gear and a little wet, or 3 up and a doggy wet ride. I've seen my brother's H18 with 4 adults ripping (flying a hull with 2 in trapeze and 2 on the rail), albeit very wet and on the edge of disaster. I've had my Hobie 16 with 3 adults (2 in trap, one on the rail) and barely been able to hang on...I've pitchpoled many times...even turdled with the wife in lightning and gusts to 3ok.
    I know I won't be as fast as a 2 up good old fashioned H18 with double trap, but I'll be able to carry a small anchor, more dry storage, maybe a little electric prop/battery, sit higher and dryer on the windward float, and maybe fly a second foresail. Hopefully it's a bit more 'expedition worthy' than a H18. I've overnighted on my H16 but the tramp is a pile of drybags and it had to be beached. I want to be able to drop a small anchor and pull a tarp over the boom. I'm building dry lockers into the Solcat floats at the weight cost of maybe 6lbs per.

    As far as adding a sugar scoop to the transom...I'm gonna hold off on those thoughts till it floats. Doing so would add another layer of complexity to the rudder. As it is now, I can use a Hobie factory rudder assembly. In fact, most of the rigging and deck hardware will be easily swapped from H18 bits and bobs.

    If I can get the box beam/hinge done by fall, there will be an unrigged float test. I can add weight to simulate at that point. As soon as it can float as a unit, it's going to. That will tell me a lot...

    As I'm still crewing at the club, sailing my other boats (laser, H16, and US yachts 21), there's really no rush; I still get to sail. Float test might not be till next spring... ha!

    Thanks again for the comments! I'll post pics when there's more progress.
    cheers.
     
  7. trip the light fandango
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    trip the light fandango Senior Member

    If Big fork fills his main hull in a body of water with the weight he expects to carry loaded or unloaded he will find the water line, without doing calculations. I have no interest in disagreeing with other posters .
     
  8. gonzo
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    gonzo Senior Member

    That means that he would build a boat and then probably find out it was a waste of time and money.
     
  9. trip the light fandango
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    trip the light fandango Senior Member

    There are governing factors that dictate whether the craft will be a success or not, if you know how to do the calculations it is a serious advantage.
    The factors have been discussed, being able to adjust where the beams [aka] sit for instance in relation to both centre hull[ vaka] and floats amas.
    So glassing in bulkheads ie secondary ones may be required, don't make the originals too heavy or strong until you know they're in the right spot. The main strength in a H18 hull is in the gunwales so it is easy to move that position. Adjusting the height and therefore amas angle on the rear aka can be blocked or ally plates secured with a range of holes drilled for adjustment. Food for thought anyway Bigfork. cheers.
     
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  10. gonzo
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    gonzo Senior Member

    Are you advising to glass in some bulkheads and if the hull fails cut them off and place them somewhere else? That sounds like a poor plan.
     
  11. trip the light fandango
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    trip the light fandango Senior Member

    As discussed on Bigfork's earlier thread these hulls don't have bulkheads so there is a risk of creating a hard point with no flex that mean the areas next to this are at risk of shear. Big forks water proof box needs to feather the extra strength it creates longitudinally.
     
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  12. gonzo
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    gonzo Senior Member

    I can never make any sense of your posts. For example, what does it mean:
     
  13. upchurchmr
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    upchurchmr Senior Member

    Trip over you own feet.

    If you could write a claim that remotely represented actual engineering issues, I wouldn't be upset about what you say.

    Can you even describe shear without copying a Wikepedia definition?

    Shear in what element of the design?

    Do you favor bulkheads or not? Which ever way you fall, why? What do you think the bulkhead would do?

    Now I don't want to argue with you, I just want to find out if you know what you are talking about.
     
  14. trip the light fandango
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    trip the light fandango Senior Member

    By feathering I mean getting thinner the further away from the join, lessening reinforcement/ fibreglass.. The largest applied first so there is less likelihood of sanding through the matt if it was externally, internally like this it may not be sanded so it doesn't matter. Hopefully big fork can make sense of my posts.
     
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  15. BlueBell
    Joined: May 2017
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    BlueBell "Whatever..."

    Trip, I feel your pain.
    I know what you mean, dispersing the load thereby avoiding breaking points.
    I agree with your approach, this is BigFork's thread,
    you should have no issue with Upchurchmr so long as BF gets it.
    I applaud your efforts.

    BB
     
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