My Small Bluewater Trimaran

Discussion in 'Boat Design' started by Prismatic, Jan 26, 2014.

  1. groper
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    groper Senior Member

    As noted above, this is but 1 problem with your design - it has extremely high wetted area for a given displacement. Also, it shares little in common with the grainger concept youve likened it to. Unless of course, it is in fact very different to what it *appears* to look like...

    I also beleive stability will be a serious problem aswell... have you done a stability analysis?
     
  2. Prismatic
    Joined: Mar 2013
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    Prismatic Junior Member

    It would be hard to compare it to a monohull of equal displacement as this boat is unconventional in all areas.
    At 3.5' draft this vessel would displace about 1250 lbs with an underwater volume of roughly 20 cu. feet with a usable deck space of 60 square feet.Hypothetically, if you had a flat bottomed (rectangle) boat of equal volume you would have a 10x2x1 submerged volume with 1' draft and 42 square feet of wetted surface area and 20-30 square feet of usable deck space.
    My design has the same WSA but has 2-3 times the usable space and is much more stable.
     
  3. Prismatic
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    Prismatic Junior Member

    Please elaborate on your high wetted area comment...

    I have done my own stability analysis...tested models etc...once fitted and weighted, she will be very stable with a moderately quick roll period. That is one thing I anxious to note and correct on her maiden voyage as I would like to slow the righting action down if it is jerky.
     
  4. groper
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    groper Senior Member

    Well if you used 3 hemispherical caps as the legs of your tripod, you would be down to circa 17.7sq feet WSA... Same configuration, same displacement...

    Also, this design you have 3 transoms with 3.5' immersion, all with negative wave interference between them...
     
  5. Prismatic
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    Prismatic Junior Member

    True...but I don't think I would get depth I desire....those 'legs' will be weighted at the points to lower the COG and also be fitted with small strakes/fins beginning at water line. How much resistance do you think the neg wave interference will cause? Thanks! (maybe the waves will cancel out a la 'double slit experiment')
     
  6. groper
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    groper Senior Member

    Well im still unsure of exactly what you have... but it seems as tho you have 3 upside down triangular pyramids as your 3 hulls?

    If so, then you also have 3 hulls with 60deg entry angle - which is quite blunt and will create a large bow wave from each. Then you have 3 large and very deep transoms, which will create a large stern wave. All 3 hulls are in very close proximity, so will likely combine in a negative way. The total negative interference drag will likely be very significant. If it is, and its hard to say with such an unconventional design, it will dominate WSA considerations youve made.

    Slow speed resistance is dominated by wave making drag, WSA only becomes important at higher speeds...
     
  7. Prismatic
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    Prismatic Junior Member

    Yes 3 inverted triangular pyramids or tetrahedra. 60 degree entry angle, 0-60 degree deadrise, and ~ 60 bow angle from vertical at waterline. All will be weighted to act as a keel which makes the horizontal COG at the midpoint of the equilateral triangle the points form. Vertical COG will be slightly higher.
     
  8. Mr Efficiency
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    Mr Efficiency Senior Member

    How is it proposed that this vessel will be propelled ? And how much power do you anticipate will be required ? I think Groper has summed it up fairly well, the thing will have plenty of resistance, especially if there are ambitions to attain speeds above walking pace.
     
  9. Prismatic
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    Prismatic Junior Member

    I have a 60hp outboard/rudder I had planned to mount amidship but I have since changed to a hybrid electric pod with a large prop. Yes, I am fully aware of the lack of 'clean' water to the prop. This was not designed to be an efficient setup, but to be a light weight, deep draft, small offshore boat. I simply want it to be more efficient than the larger boats that get 2-3 mpg.
     
  10. Dave T
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    Location: Anamosa Iowa and North Buena Vista on the Mississi

    Dave T Senior Member

    Definitely an interesting concept. It's hard to tell much from the pictures it's different than any boat design I've ever seen. You said that you have tested a model of the design. Do you have any pictures of the model? I hope you do actually finish it, it's going to be real interesting to see what happens on maiden voyage. You said that it's 10' long at the water line and 12' wide is this correct? I won't make any comments about whether I think it will work or not I just want to see what happens when you put it in the water. It looks like you've already put a lot of work and money in the project and I hope you get it finished. A lot of people told me my boat would never work on the Mississippi when I was building it but I proved them wrong. Good luck with the project and definitely keep us informed.

    Dave T :)
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    It is almost inconceivable that something 10 feet long could be anything other than slow and uncomfortable off shore. And then there is the small matter of safety !
     
  12. Prismatic
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    Prismatic Junior Member

    Thank you so much! You, sir, are a breath of fresh air! I guess I will call it a platform since only I see the boat :p At her full displacement capability she will have a LWL of 10' (12' measured along hull) and 12' BWL. (See attachment) Sorry, I don't have pictures of the model or tests.
     

    Attached Files:

  13. Mr Efficiency
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    Mr Efficiency Senior Member

    It should have been a simple exercise to make a wood model out of this, with its flat-faceted surfaces, and then test it in the bath ! There you can compare it to the performance of a rubber duck, my money is on the duck ! :D
     
  14. Petros
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    Petros Senior Member

    you were asked, but I have not seen an answer. Exactly what are you trying to accomplish with such a design? what are your design objectives?

    it appears more like you had an idea for a hull, and are looking for reasons to justify it, rather than starting with a design objective.

    There may be good reasons why no one, after 6000 years of documented boat construction, that no one has ever built a hull like yours. OTOH, if you are trying to accomplish something that has not been done with a floating craft, that would be a good reason to try an unconventional design. If so, you have not stated it.

    If you hung a deep weight off the tips of the three apexes, than you could have a stable platform for a stationary floating deck, but I see no benefit to it as a boat. i.e. something designed to be propelled through the water. So if you have such a goal, you have not stated it.

    You might consider altering it by making the hull sections three, double diamonds, rather than a simple equilateral triangle on each. the water flow would than close on a sharp trailing edge, as well as separate over the sharp front. the back of a bluff body has so much drag that this form makes no sense if it is to move at all.

    but what are you trying to accomplish with such a design? tell us please.
     

  15. Prismatic
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    Prismatic Junior Member

    I did just that and weighted it down and let my kids create a tsunami in the bathtub. We couldn't get it to capsize or pitch pole...perfectly balanced and the waves had no surfaces to push or 'grab'. The pyramids kept piercing and biting downward into the water even on crests. The only flaw I observed as mentioned earlier was a quick roll period. I've read that Mr. Gerr recommends a roll period of 1-1.1 seconds x beam in meters (~4m) or 4 seconds in my case. My roll period is much shorter than this which if excessive may be uncomfortable.
     
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