Short hull trimaran

Discussion in 'Multihulls' started by midlifecrisis, May 12, 2019.

  1. midlifecrisis
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    midlifecrisis Junior Member

    I was pondering sizing catamarans that were beam constrained and the following trimaran came into my mind.

    ~~~~~ / \~~~~~
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    | |~~~| |~~~| |
    | |~~~\ /~~~| |
    | |~~~~~~~~~| |
    \/~~~~~~~~~~~\/


    Now since the only thing I really know about hydrodynamics is bow wakes the goal is to make certain that all wakes miss the hulls while allowing for a somewhat longer length of boat.

    Surely this has been tried and found wanting, can anyone explain what is the flaws in this arrangement?

    Thanks
     
  2. redreuben
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    redreuben redreuben

    How much multi sailing have you done ?
     
  3. JamesG123
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    JamesG123 Senior Member

    Probably the complication of the "staggered" hulls for the limited benefit of trying to avoid wake interference. Without specific examples and without designer background and intent, its really hard to speak in generalities. I'm not even sure what YOUR goals are here.
     
  4. W17 designer
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    W17 designer Senior Member

    First thing you need to clarify is whether this is a proposed sailing trimaran or a motor tri. You also mention 'pondering catamarans' in your opening request that could use some clarification. I would also need to know what size and type of craft are you considering and then we might be able to answer you in a more helpful manner.
    As in most cases, 'One hat does not fit all' ;)
     
  5. midlifecrisis
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    midlifecrisis Junior Member

    Since people want to know my motivation, I am just playing with ideals and try to increase my understanding.

    I have been looking at youtube video of people doing the great loop and so that gave me a beam restriction, but since I am not currently working a plan with a budget, asking myself how could get a bigger boat with a restricted beam was interesting. I am attracted to Cat's because I am looking at solar, although propulsion level power isn't here yet a hotel load, solar system seem quite feasible, so a design that maximizes topside surface area for given displacement seems good.

    So this a motorboat with a low speed requirement, about a 16 ft beam, and as much living room as practical, because it would be a live aboard in many ways. Probably outboard engines since the hulls are likely to be too narrow.

    The notion that marine engineer who wanted to build such a vessel, instead became civil engineers and build bridges, did occur to me, as why the design space might not have been explored much. And since I am neither marine or a structural engineer, I am outside my depth. But for a design that would work for a catamaran, should be comparable to a trimaran design of the same beam it seems to me.
     
  6. Dejay
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    Dejay Senior Newbie

    Check out "INTERFERENCE PHENOMENON IN DESIGN OF TRIMARAN SHIP"
    https://www.researchgate.net/public...FERENCE_PHENOMENON_IN_DESIGN_OF_TRIMARAN_SHIP
    "The HALSS model test results have demonstrated that a Trimaran can be designed such that favourable hydrodynamic interactions offset almost all of the side hull drag over a practical range of speeds. For example at the 32 to 34 knot speed, the resistance of the Trimaran, Experiment 5 is equal to the resistance of the center hull, Exp 3. In the Trimaran configuration there was 18% more displacement yet the drag was the same as that of the center hull."​

    I haven't fully read the paper nor would it be easy to understand or reproduce! But the goal seems to be to negate the bow waves and recover the energy, at least for specific speeds ranges. Maybe this doesn't apply to smaller trimarans though.

    I'm also interested in a solar powered boat and I'm currently researching this. But I have zero experience. The trimaran seems to be the most power efficient type compared to a catamaran, at least if you don't need that much space. Something like the ILAN voyager but with a big roof surface. I'm currently trying to learn and sketch some ideas. I picture something like a 40-50' trimaran with a 3m wide cabin on top and additional flaps for something like 40-60m² solar panel roof area. You need long and very lightweight and lots of solar roof area. Windage is going to be a design problem, but I guess less so if you only want to go in sheltered water ways.
    From my very preliminary estimates this could work fairly well in the summer for something like 60 nautical miles range at 8 knots per day in the Mediterranean sea.
    Maybe the amas can be made to slide in and out below the cabin to go from 5m to 8m beam over all. I also would like 5m max beam to go through the french canals from the channel to the Mediterranean.
     
  7. midlifecrisis
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    midlifecrisis Junior Member

    I've look at web items from that study or set of studies. It seems to be working on the fact that at certain speeds the bow wave destructively interact to cancel each other out. It not clear to me how useful that would be in the real world because that could be a very narrow speed range, and at other speed ranges the wave would build each other up and make the drag worse. but it is what made me ask if a short center hull could be an answer.

    I am not sure why you say a trimaran is the most power efficient type, I would say that a long narrow boat is power efficient boat and multihull designs allow you to get to a layout where you are not sacrificing stability with a long narrow hull.

    Solar power and catamaran are an interesting combination because I know my power available goes up with the square of the length, where as the power required is a function of the square root of the length, so I am guaranteed a solution as to how fast my boat can go for a given size, and if that isn't fast enough I just need to increase my size. Your ILAN trimaran relies on stored power (fuel) which gives it different requirements and tradeoff from a strictly solar powered vessel.
     
  8. Dejay
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    Dejay Senior Newbie

    I'd love to know a rule of thumb for power required dependent on length, taking weight increase into account. Weight increase might eat up at least some of your power gain and increases costs. A trimaran needs less heavy structural elements and should be lighter for the same length.

    I've run a 12m long 4t catamaran in prelimina.com and got values of 2.84kW for 6 knots and 7.3kW at 8 knots with an assumed 50% efficiency from input electrical power to propulsive power. For a 15m long 4t trimaran (as a monohull for now) that was about 30% better. Which kind of makes sense to me because your overall wetted surface is lower (two semi-circles vs one slightly larger semi circle) and your hull is longer for higher "hump" speed. Ignoring the "training wheel" outriggers that would only bear like 5% of the displacement and are hopefully negated. With a power boat a specific fixed cruising speed wouldn't be that problematic. But I don't really understand that study myself.
    I have to do those calculations properly with a better hull shape and compare that to michlet. I hope propeller efficiency can be better too.

    You could fit something like 42-60m² of solar panels on a trimaran (420kg glass PV with 20% efficiency) and 20-30kWh LiFePO4 battery (200kg) could work quite well for island / harbor hopping.

    On a catamaran you could fit more solar panels and could have easily twice as much living space. A trimaran would have enough space for 2 people and be cheaper and probably easier to build too.

    Lots of assumptions of course! Again, I have zero experience :)
     
    Last edited: May 13, 2019
  9. W17 designer
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    W17 designer Senior Member

    I am sure you have noticed this by now, but sailing catamarans typically have a range of B/L of 0.4 to 0.6, whereas sailing trimarans are typically in the range of 0.7 to 0.9. Part of the reason is that both sail on just 2 hulls, so a trimaran with a catamaran beam would clearly not be as stable.

    But a motor trimaran is a different fish as there is no heeling from sailing loads. This means that both amas will be in the water (even if lightly) all the time. Tank tests have shown that amas quite far aft work well for a motor trimaran and as they do not need the same stability as a sailboat, can also be smaller and closer-in, giving less overall beam.

    The ideal ratios of displacement and ama location for large motor-tris are frequently determined by expensive model tests but the results are proprietary and seldom published. For level sailing motor-tris, the amas are often moved fore and aft to actually benefit from the wave of the main hull so as to achieve the least overall resistance. One of the latest designs to be developed using tank tests is Adastra by the notable designer John Shuttleworth, so I’d suggest she’s worth having a good look at ;-)

    One possible scenario is to get the bows of the amas to ride on the bow wave of the main hull, the wave that‘s generated at the chosen cruise speed. Another might be to negate the ama bow wave. A 3rd might be to avoid any wave interaction .... as seemingly is the situation for Adastra. The lines of both hulls plus the spacing of the amas and their location fore and aft are all interrelated, but only controlled model tank tests can pin down the right combination for the speed selected. As always though, light weight will permit hulls to be slimmer, displace less and therefore have less resistance.

    The effect of size can be minimized by relating the boat to the same Speed/Length ratio or V/L^0.5

    Adastra - Shuttleworth 2012.jpg
    mike/
    www.smalltridesign.com
     
    Last edited: May 13, 2019
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  10. W17 designer
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    W17 designer Senior Member

    A far more modest electric boat could be created based on this design FLAIR by friend and fellow Canadian, Laurie McGowan, created for the WoodenBoat Sketch Book. I will share this PDF of the article here.

    But it’s important to realize and accept that batteries are very heavy, and the range they offer would only allow a few hours without recharge. Such range could be extended with lithium batteries (at very high cost) and/or slower speed but trips would be very dependent on recharging ability.

    Whether the trimaran format could be justified for a small boat is questionable though, as it would give less internal hull volume.

    mike/
     

    Attached Files:

    Last edited: May 14, 2019
  11. Ad Hoc
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    Ad Hoc Naval Architect

    Ostensibly, that is correct.

    I assume your first statement is based upon the available area exposed to capture solar energy ..otherwise I don't understand your reference.
    Your second statement is not correct. The power(thrust) required to push a hull has many inputs and it not solely sqrt of the length.
     
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  12. W17 designer
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    W17 designer Senior Member

    Ad Hoc: Your second statement is not correct. The power (thrust) required to push a hull has many inputs and it not solely sqrt of the length

    Ad Hoc is totally correct. Sorry to get a little technical but just for starters, ‘thrust required’ relates to items such as Length, Displacement, Speed, Hull form & Windage and then there's the ‘Propulsive Efficiency’ that factors-in shaft and propeller efficiency that includes flow efficiency to the prop. The design Speed-Length ratio will guide the ‘pounds of resistance per ton of displacement’ for different hull forms, and then you can at least get to a basic formula that we used to use for a quick power estimate for displacement-mode ships (and also applicable to most non-planing motorboats):

    Effective HP = Res/Ton x Displacement x Speed/326 (Imperial units with Speed in knots)
    You then divide the EHP by the Propulsive Coeff to get the required SHP (shaft HP).

    In practice, small motorboats are more commonly powered by comparing the performance of similar existing boats and then making minor adjustment for the differences.

    mike/
     
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  13. Ad Hoc
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    Ad Hoc Naval Architect

    Mike

    Where does this formula come from...historical empirical formula?
     
  14. Dejay
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    Dejay Senior Newbie

    Oh thanks for reminding me, I ignored the Adastra earlier because it's a superyacht. John Shuttleworth wrote two awesome articles explaining his design decisions on power trimarans, including power efficiency and seakindliness. Quite a lot of thought going into position and displacement of amas for both efficiency and motion from waves. Even the weight is spread out to increase moment of inertia and reduce motions. I can see why he won an award for it!

    http://www.shuttleworthdesign.com/p...eakindly-Fuel-Efficient-Vessel-Web_Part_1.pdf
    http://www.shuttleworthdesign.com/p...eakindly-Fuel-Efficient-Vessel_Web_Part_2.pdf

    But that boat is just too beautiful for me haha ;)
     

  15. Dejay
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    Dejay Senior Newbie

    This is my rough and probably somewhat naive 3D sketch of a possible solar boat. This would be a 15m trimaran with a 3m wide cabin and everything on one level. I imagine the straight walls to be rather easy to build and to be light using a mix of structural and insulation foam.
    The hulls would be double ended and symmetrical maybe if that makes building easier. A single electrical "outboard" at the stern with an large and efficient propeller that can be lifted for beaching.
    Two rows of additional solar panels on a hinge so they can be angled towards the sun or stowed for less wind resistance.
    There would be beams to connect to the amas that can slide in below the cabin to reduce the beam to 5m. You would add trampolines in the area between the amas and towards the bow to get access. Solar roof surface would be 50m².

    Trimarani v28.jpg
     
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