Short hull trimaran

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

  1. gonzo
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    gonzo Senior Member

    The short hulls will have more drag than longer ones. When waves are on the beam or astern, the front hull is likely do dig in and make the boat broach. Notice the successful trimarans with shorter amas have the stern of the three even or the main hull somewhat longer.
     
  2. W17 designer
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    W17 designer Senior Member

    Oh geez Ad Hoc … now you’re really challenging my old brain ;( .... but let me see what I can remember.

    It would be easy to just say this is an historical empirical formula, but it actually has its base in mathematics and science.

    I quoted this from my ‘little black book’ that I was urged to start while doing an apprenticeship 65 years ago under ‘KCB’ … the noted naval architect K.C .Barnaby (descendent of a line of ship designers going back to Charles ll in the UK!). In the 1950's KCB wrote a book called ’Basic Naval Architecture’ that you may be aware of, that dealt more with propellers and powering than ship forms.

    In the UK, horse power (prior to metric conversion) was mechanically defined as 550 ft.lbs/sec of work, which means 33,000 ft lbs per minute.

    See here: What is horsepower (hp)? - Definition from WhatIs.com https://whatis.techtarget.com/definition/horsepower-hp

    So Effective HP = ft lbs/min OR resistance (lbs) x v (ft/min) / 33,000 and this represents ‘the actual work done’.
    To convert ‘v’ to knots, you have to multiple by 6080/60 .... (6080ft in a nautical mile & 60 min per hr)
    So when you combine the 6080/(60 x 33000) you end up with the 1/326 in the formula I gave you, but with the speed now in KNOTS.

    The ‘real push power' to get the target speed, requires that you divide the ‘theoretical ehp’ by the Propulsion Coefficient (generally in a 0.5 to 0.8 range) .. but that’s a whole other story.

    Hope that’s enough for you ;)
    I actually left all this ‘ship stuff’ behind about 25 yrs ago but it’s still useful, as by scaling things relative to the Speed/Length ratio, much of it is still applicable to small boats as long as they are not planing or on foils etc. After all, ship models as tested are about the same length as dinghies ... typically 12-20ft. .Hope I'm not boring too many.

    mike/
    www.smalltridesign.com
     
    Last edited: May 14, 2019
  3. midlifecrisis
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    midlifecrisis Junior Member

    Correct.
    Well that is definitely a hand wavy, all of things being held constant, zero order approximation. Of what happens as the scale of the boat changes, power being collected goes up faster than power needed to push the hull. As a counter example, I would say that a solar power hovercraft isn't going to be come more feasible by changing it's size.
     
  4. W17 designer
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    W17 designer Senior Member

    Gonzo: "The short hulls will have more drag than longer ones"

    I think Dejay made a good first stab at this ..
    Once hulls get to a L/b of about 13, their resistance generally does not increase and for a motorized tri, there's little reason to go longer as you're just adding excessive wetted surface. But I would also recommend to move the amas back, as when the ama buoyancy is close to the mainhull LCB and LCF, the boat will experience considerable pitching.

    I would also be concerned with the extended sides at the bow, as these will crash into wave tops ... effectively hitting them square-on. Of course, this will depend on where the boat is used ...but lowering those sides farther aft would be my recommendation. Adding 2 or 3 longitudinal Vee'd ribs to the underside surface can help to soften the impact and this can be done with fiberglass covered foam.
    As the overall beam is wider, you might be able to add a few more solarpanels at a 30 degree angle to prevent wind getting under the ones you have, and you'll need to think about how to get to the bow for mooring/anchoring too ;)
    m/
     
    Last edited: May 14, 2019
  5. Dejay
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    Dejay Senior Newbie

    Thanks W17, I think Gonzo was referring to the original ascii sketch from midlifecrisis. Twice as much buoyancy in the back could make the front ama dig in.

    But my whole approach is naive of course and will need to be engineered, ideally as outlined in that adastra article. That really gives a good roadmap of how to do this well. You'd probably need to increase the height of the hull and the bridgedeck clearance and add flare or a large "fillet" to cushion against large waves.

    Or maybe have more of an "aircraft carrier" bow, with the whole cabin front angling forward. But it's hard to find a design that maximizes roof surface and doesn't look ugly haha. I kind of like the star trek shuttle cockpit.

    Anyways I just wanted to show midlifecrisis that it might be possible to use a trimaran design for a more efficient and hopefully easier to build boat that still has enough solar roof area.
     
  6. midlifecrisis
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    midlifecrisis Junior Member

    In my sketch I figured that having engines, and such in the back hulls would even out the weight distribution, would that lead to other complications?
    For a boat doing the great loop, line handling while going though locks is a issue with a trimaran that would at least need some thinking about.
     
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  7. W17 designer
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    W17 designer Senior Member

    Just to clarify.
    When I mentioned to move the amas aft, I did not intend to mean, totally so. You want an offset between their buoyancy and that of the main hull to dampen the pitching. But if you go too far aft, you can end up like a 3-wheeler doing wheelies .. with the bow lifting up in the air ... not something you want! ..... something like 70% aft should work fine.
    I'd also go for near-vertical-sides for all 3 hulls, so keeping wave-resistance to a minimum and again, lowering pitching forces ... all as per previous articles I've written re my W17.
    I would also experiment having some space on the inboard side of the amas, to allow wave tops to come up and escape, rather than slamming the underside of the wings. Nets or slatted deck planks would do that.
    mike/
     
    Last edited: May 14, 2019
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  8. Ad Hoc
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    Ad Hoc Naval Architect

    That's what I assumed. It may have elements of real science and maths in it, but it is ostensibly empirically based.

    Indeed, one of many books we also used as students and i have on the bookshelf behind me.

    Yes, but i was hoping for the actual original of the empirical formula.

    My old boss and I would always test each other with "old" facts and formula, to try and catch each other out, whilst attempting to find out where such "rules of thumb" came from., along with nautical terminology.
    The origins of all these "rules/terms" is very interesting and often important historically too.
     
  9. Corley
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    Corley epoxy coated

    Noosa Marine did a rather odd looking little power trimaran that was designed by Tony Grainger and powered with Torqeedo electric outboards. I've not heard anything more about it since the prototype was built. It appeared to use a full length main hull with rear set floats.
     
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  10. W17 designer
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    W17 designer Senior Member

    I personally would not call that 'an empirical formula' ....especially if we base such criteria on this physics definition.
    Quote "equations are considered empirical in the sense that they were selected only because they fit experimental data and are without theoretical justification".
    The one I gave you certainly has theoretical justification.
    EHP - WorkDone in lbs/ft/min ..ie: Pushing the hulls resistance (in lbs) through water at a certain speed (ft/min).
    Hope you can feel better about this now ;-)

    Thanks Corley for posting the Grainger boat. Can I guess there may be cylindrical hulls below the water like a Swath ? Would make sense anyway. I wonder why she was built so tall? Could be unstable in side waves with the VCG so high.
     
  11. Ad Hoc
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    Ad Hoc Naval Architect

    I think that is where we differ.
    Empirical is simply based on observation or other cases on 'experience' rather than any exact theory.

    It is indeed an odd looking Trimaran. But I doubt there would be torpedo like tubes, like a Swath. Given the simplistic nature of the look/shape, it would be at variance with the raisons d'etre.
     
  12. Dejay
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    Dejay Senior Newbie

    So what is the practical speed limit on the great loop? If you only need like 4 knots then solar power becomes much easier than what I am thinking of. Or if you only need a boat that is more like a camper instead of a liveaboard cruiser then you can save a lot of weight.

    You probably saw the trimaran "RA" that completed the great loop. I've attached a list of solar powered cruising yachts I've found so far, unfortunately it's a rather short and colorful list, and info is rather hard to find. Let me know if you know any other not listed (only boats you can live on, there are a lot of experimental sport or fun vessels of course).

    Do you even really want a solar powered boat or did I misunderstand you?
     

    Attached Files:

  13. W17 designer
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    W17 designer Senior Member

    Ad Hoc
    I think that is where we differ.
    Empirical is simply based on observation or other cases on 'experience' rather than any exact theory.

    No great difference there with the physics dictionary version I gave you. So we're in agreement.
    They both confirm that the formula I gave does not really qualify as Empirical.
    But call it as you see it .... its science-based enough for me and used as such for decades.
    Not really an issue worth wasting time on imho.
    cheers
     
  14. midlifecrisis
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    midlifecrisis Junior Member

    Neat list, i've not been a diligent as you in my web searches.
    I started out thinking about solar for propulsion, but when I looked at it I realized that while possible it was not useful for most situations, the power density of the sun is just too low and it is a fundamental limit. When you look at household load, the situation is somewhat better, although most people like their creature comforts, and the amount of energy people are using is going up. If you turn off the AC it's not a crisis, it just feels like one in some places. So solar is still a driver in my thoughts, and a lot of my research have been household systems related.

    In terms of speed limits, yes lots of the loop has no wake speed limits, but parts of it have river currents, so higher speeds would be good to be able to use. I find it fascinating that from the mouth of the Mississippi it is possible to boat to place as far inland as Sioux city, Minneapolis, Pittsburg, Knoxville. If I had the magic combination of time and money, I would love to try to visit them all.
     

  15. Dejay
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    Dejay Senior Newbie

    Yeah if you want to continually travel then solar is more problematic, or when it's not the sunny half of the year.

    If you stop one or two days to recharge from the sun then you just need big enough batteries. That is how the "RA" did it, lots of waits and going very slow. BTW lithium batteries like NCR chemistry used by Tesla is quite cheap now, ~270€/kWh. LiFePO4 is at ~400€/kWh. Cheaper (much cheaper?) than lead acid when you factor in actual capacity and number of recharge cycles.

    Or build an incredibly long and narrow and lightweight trimaran, but that has it's own challenges. Like not being able to just buy plans for it haha. But I'm sure the concept works, at least for island / harbor hopping at slow speeds. Plus while anchoring you'd be able to have the AC, water maker and the washer dryer on while cooking with an electric stove :) Well actually I still have to make a budget for household electricity.

    For me the equivalent to the great loop would be visiting England, Ireland, then going through the french canals into the Mediterranean sea and hopping along the coasts of Italy, Greece and Spain and all the other places I've never been to.
     
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