stab re-calcs for pimped Trimarans

Discussion in 'Stability' started by Skip JayR, Jul 12, 2019.

  1. Skip JayR
    Joined: Sep 2015
    Posts: 367
    Likes: 2, Points: 18, Legacy Rep: 25
    Location: https://trienthusiasts.wordpress.com/

    Skip JayR Tri Enthusiast

    Dear fellows ! Hope to get some help from you. - As a multihull enthusiast I am in the process to buy a Trimaran (cruising-racing type). - It will be one of the elder designs (around 1985-1990), e.g. drawn by naval architects like Norman Cross, Lock Crowther, Philippe Briand, Hedley Nicol, Phil Herting and Mick Price or Dick Newick. The size I am looking for to buy is around 40 ft.
    [​IMG]
    Different times (and for one concrete sales proposal) I experienced, that such Tris have been pimped by the owner (or pre-owner). E.g. stretched in length (e.g. from 39 to 42 ft), higher mast (e.g. from 16.5 to 18m), longer boom (for resizing the main sail), newly fitted with retractable or fixed mounted bow sprit (for asymmetric Spi/Reacher). Displacements have increased from original size (~4,000 kg) to remarkable heavier weights (~5.5-7.5 tons), while the cross beam size (in most cases) is the original size (e.g. Boat length 16.5 meters, total width 9.9 meters).

    Actually I only can judge by watching photos or videos if such re-designed Tris (with retractable central daggerboard) are seaworthy (and trustworth). Like this one show of a Tri at 25 knots speed. "Walk through" vids like following can give an orientation of the all over conditions the boat is in, only.

    (Rec.: As I live in Northern Europe, where such Trimarans are not available, I have to look overseas. Before I take the plane, I like to proof the datas as much as possible.)

    Mostly video clips of Tris are done in light winds. By watching I don't get the feeling for such pimped boats how their behaviour is in rough seas/higher waves and if there is a bigger risk of nose diving (pitch poling). Such observations under perfect conditions might be impressive on first moment, but are not very trustworthy in my understanding for the abilities to go offshore / long distance cruising.

    Cruising Multihulls of good performance can become dangerous as they move in higher speed ranges (16-22 knots), and I like to have a more detailed proof. Two central questions for me:
    1. Do the Amas (outrigger) have enough buoyancy / uprighting moment with the new sails plan and/or extended length of main hull (vaka) ?
    2. Can the Cross beams (aka) bear the bigger loads (e.g. by increasing displacement, higher mast and bigger main / head sails) ?
    I am a professional sailor (having started with sailing more than 40 years ago), but neither I am a boat builder nor a naval architect. My skills in stability calculations are only "very bascis" (which I had to study when graduated with captain diploma for commercial sailing vessels (monohulls) up to 500 tons at Dutch Nautical Colleague in Noord-Holland).

    Does exist an excel sheet or any freeware program with templates for Trimarans ? - Tks in advance giving attention. Any advice is very welcome.
     
  2. Ad Hoc
    Joined: Oct 2008
    Posts: 6,079
    Likes: 345, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    That's 2 questions in one and while they appear similar are in fact different.
    The outriggers provide the stability in terms of righting moment.
    In terms of buoyancy, is partially answered by the above, but, this is more about the arrangement of the hulls/sail area etc. and the stability book should provide you with that.
    If the vessel you are keen on doesn't have one - go find another. Or...if your heart is set on the vessel, employ a naval architect to investigate this for you. Since like buying a car, you need the "paper work" to fully answer your questions.

    They certainly should, otherwise the beams would have failed by now - most likely! But of course it depends upon their manufacture i.e. the quality in build, as well as has the vessel gone beyond sea states what it is designed for?
    Again, unless the owner can provide you with the "paper work" you seek, employ a naval architect.

    Since these questions, on paper, are straight forward enough to answer, but whether the vessel owner has this data available that you seek, is another matter.
    Just like buying a car without the paperwork of provenance and service history. Doesn't mean the car wont work - you just don't know how well it will work...or fail...in the future.
     
  3. Skip JayR
    Joined: Sep 2015
    Posts: 367
    Likes: 2, Points: 18, Legacy Rep: 25
    Location: https://trienthusiasts.wordpress.com/

    Skip JayR Tri Enthusiast

    Hi, AdH. - Tks for your feedback. Most Tris, which had been stretched and are now in 2nd hand ownership, don't have such stability calculations as you mention. I always request it from the owners. But till today, I never got one. - E.g. actually I am in contact with an owner of a 41 ft Norman Cross Tr, built mid the 90s (by a professional boat builder in Europe). He want sell the boat already since 5 years. The photos I got are of 2014.
    (Rec.: This Tr was stretched from regular 38 ft to now 41 ft.)
     
  4. TANSL
    Joined: Sep 2011
    Posts: 5,654
    Likes: 170, Points: 73, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Do you have the body lines plan of the boat?. With that information it would be possible to calculate what you need.
     

  5. Ad Hoc
    Joined: Oct 2008
    Posts: 6,079
    Likes: 345, Points: 83, Legacy Rep: 2488
    Location: Japan

    Ad Hoc Naval Architect

    If you have concerns and still unable to get the information you seek, then i would strongly advise you to engage the services of a naval architect to provide the information you seek, based upon the vessel in question.
    If the owner is unable to provide said data, for whatever reason, then you must satisfy yourself first and foremost you're getting what you want.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.