(heavy) overloaded Trimarans... where are the limits ?

Discussion in 'Boat Design' started by Skip JayR, Oct 1, 2015.

  1. Skip JayR
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    Skip JayR Tri Enthusiast

    This thematic "overload" is a common topic within the world of multihulls. I suppose it has been discussed tremendously here and there in this forum.

    [​IMG]

    I am very aware that cruising catamarans over time can become heavy overloaded with all that "memories" the cruisers bring on board during their stop-overs. Quickly come together 4-500 kg per year.... and the speed is gone, sucks the stern heavily.

    The potentials (speed + safetyness) which are given by the designer of multihulls are based recently on light displacement building methods (compared to heavily keel-weighted monohulls).

    More sensitivly than cats react the three hull compagnons. Trimarans loose their potential speeds quickly if the boat builder is not taking care the materials he is using or and the owner is storing lots of equipment (e.g. heavy diesel engine, big tanks etc. ...).

    As I am in midth the process to look for a Trimaran (living + working) I ask herewith:

    Does exist a kind of "table" to get an orientation about what the maximum weight/displacement shall be ?

    E.g. by experience I can say that 40 Foot trimarans are built round about 3.5-4 tons. A 50 footer estimated at 5-6 tons.

    Is there a (more scientifically proofen) matrix, given by four main parameters to define the max. weight / displacement ? (A kind of table would be helpfully)

    - length (water line)
    - width (beam)
    - mast (length)
    - sails area (main sheet + jib/genua)

    To give an example: actually I have a sales offer for a 36 foot (LWL) Trimaran (LOA: 37', Beam: 23') with a newer 40' Aluminium mast 20 years after it was built. The mast was designed by one of the most renowned multihull specialists: Lock Crowther.

    The Trimaran was built professionally (cold molded) by a boat builder and launched 35 years ago who had built different Trimarans, and some bigger ones in the range of 40-50 feet.

    The boat is well proofen (practically) and has an excellent survey... which gives the amas a 180% reserve buoyancy.

    It was sailed two handed long distances: e.g. from San Francisco to New Zealand/Australia, from Alaska to Hawaii (and back) etc. ...

    It seems a very robust boat. Maybe too robust loosing its potentials of speed ??? - The official papers list a "net tonnage" of 10,000 kg. The discplacement is ~6,000 kg.

    It is not to expect, that the boat needs any further equippment as it is fully equipped: from 2x autopilots to 2x anchors incl. parachute anchor, 2x roller furling (Jib, Genua) beside 3 reef main, watermaker / fresh water tank with 2 water pumps, icecube maker/frigerator, 4 burner stove (with oven), head with sink and vaccuum toilet, solar + wind generator, fully security equipment (life jackets/life belts, automatic Halon fire extinguisher in the engine compoartment of an 18 hp saildrive and others), Radar/Epirb, all navigation electronics (speed/log, VHF, GPS, Deep sounder, compass) and 2.85' inflatable dinghy with 6 hp outborder.

    Is such a heavy boat a risk ? Or is it just slow ?

    As I dont have urgent need to make a quick decision and will proof some different other trimarans, it would be helpfully to have a kind of "table" available. Kind of "BMI" :)

    Tks in advance...
     
  2. Richard Woods
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    Richard Woods Woods Designs

    Don't forget that the official tonnage probably relates to interior volume and nothing to do with actual vessel weight

    How much trimaran sailing experience do you actually have?? Many of the boats you have discussed were designed for racing by some of the worlds most experienced, toughest racers. and even so some couldn't cope. And certainly all had help getting in and out of port and usually had a full time repairman with a container of spares/tools that followed the boat

    RW
     
  3. Skip JayR
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    Skip JayR Tri Enthusiast

    Think big... small steps...

    Tks for your concerns, RW. But no need to worry. I know very well how to narrow new boat types as I sailed many different during my professional skipper job. If you get overhanded boats for deliveries you must handle such situations quickly and efficiently. And I did this in the range of 80 footers.... and worked as helms man on 30-40 meter Tall ships. Much more complicated than "normal yachts".

    I have skippered two seasons professionally for a private owner a 15 meter Alu catamaran (single built) we sailed >20 knots... little bit more easy with two mashines to manoever in the harbour. But not too easy under heavily wind pressure to handle 12 tons weight. Never had an accident from my side...
    [​IMG]

    So no need to worry about... I was educated extremly conservatively in the spirit of traditionally seamanship (strict educational programmes in the 70th/80th). It has burnt into my genes. I am not a mad risk taker.

    All Trimarans I am seriously dealing, you haven't seen yet by picture en detail... as I wont ruin my chances to make a good deal. :) But that does not mean I am not interested in fascinating racing boats.

    The boat I target at "as starter" wont need a professional "pit crew" and container workshop following my route... we talk about something in the size and range similarly like this 35-40 Footer... to give you an orientation. Nothing to scare.

    [​IMG]

    I know where to start, and i know where I want to go.

    As mentioned different times, I need a boat for living + working. Yes, it shall be faster than monohulls... but I wont select a LendingClub 2 where the crew gets worried when the log falls down under 25 knots. :) (*joking but true see vid*)
    www.youtube.com/watch?v=TTazugpcuCg
     
  4. Skip JayR
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    Skip JayR Tri Enthusiast

    If you put "volume" in relationship to "weight"... shouldnt it give one an orientation related to the length and width of the boat ?

    I never dealt with boat building details/parameters of trimarans. So that's why I am asking and delivered the upper known values.
     
  5. TANSL
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    TANSL Senior Member

    Richard Woods is very right, the gross ton (official tonnage) has little to do with weight. A paper boat (if possible build) could have a huge gross tonnage (large internal volume), but its weight will be a few grams.

    There is no <<a kind of "table" to get an orientation about what the maximum weight/displacement shall be.>>. There are calculations for determining the maximum weight and maximum height of c. of g. a ship may have to comply with all the rules she must follow.
    In any ship can be traced, for example, the so called limiting KG curve, indicating for each displacement the maximum height of its center of gravity allowing to comply with the stability criteria applicable to the ship.
     
  6. Skip JayR
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    Skip JayR Tri Enthusiast

    Ah... tks remembering me. Yes, KG as indicator for stability...
    [​IMG]

    Must take out my old books :) I suppose the designers do not publish these parameters of their Trimarans, right ?
     
  7. TANSL
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    TANSL Senior Member

    That is a calculation among many more calculations to do when designing a ship. It is so "secret" as colud be the tables with hydrostatic values. If you have the body lines plan of your ship, and some other information, you can get that curve. It is a laborious but not difficult calculation.
     
  8. Skip JayR
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    Skip JayR Tri Enthusiast

    Yacht manuals...

    In my understanding, every "boat handbook" (yacht manual) should contain the documentation of the structural engineering calculations.

    E.g. taking trimarans at the age of 20-30 years it is very difficult to get the datas (e.g. the designer has demised or even lost the plans on his own as already experienced) and its a challenge to get a real understanding about the boat performance, seaworthyness and safetyness.

    It seems that (too) many owners of Trimarans are very careless about such a documentation not keeping the yacht manual up-to-date. E.g. Sails plan, Polar diagrammes would be helpfully, too.

    I was educated in sailing and skipper licences to have such a yacht manual always on board, I took care for owner's yachts I skippered professionally, e.g. looks similarly like this:
    http://sailkerkyra.com/media/Kerkyra_Yacht_Manual.pdf

    The big yacht companies which produce bigger numers in series, deliver to the owners yacht manuals, e.g. Bavaria is doing
    http://www.sailingsicily.com/public/file/schede_barche/Bavaria_Cruiser_33_manual_en.pdf

    In the world of "big boats" its even a service by yacht management companies to deliver such manuals, e.g. system manuals and operation manuals.
    http://www.yachtwork.com/operating-manuals.html

    Such a manual for a single built Trimaran should same contain the relevant design parameters. - Isnt ?
     

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  9. FAST FRED
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    FAST FRED Senior Member

    When heeled slightly if the weather ama does not clear the water , it will be slow going indeed.

    The overloaded fear is the boat will attempt to go thru waves , rather than float on top.

    Most trys are not strong enough if they do not float on top.

    45FT Hedly Nichol Voyager with a decade of experience.
     

  10. TANSL
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    TANSL Senior Member

    In general, to my knowledge, the boat's user manual is not required to include engineering data on the boat.
    In workboats it is required that Captain have several methods to find out, in each loading condition, if the ship meets the stability requirements applicable to it. One such method is the limiting KG curve. The designer of the ship, once made the inclining test, should make the stability booklet which, among other data, will including this curve.
    I do not think this is mandatory for pleasure craft. In any case, if it were, the User Manual should make some reference to it.
    In short, I do not think you can find it among the documents of the ship. You must calculate it yourself or commission someone to do it.
     
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