why the different transom shapes?

Discussion in 'Sailboats' started by Petros, Sep 24, 2009.

  1. Petros
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    Petros Senior Member

    I have been looking for a long time at the various way to close off the end of the hull, and I wondering if there are any specific reasons that there are so many different ways to make a transom.

    Some older more traditional hull designs have the transom raked aft, this looks nice and it would add more deck area, and I suppose it could be used to move the back stay as far back as possible to allow for a larger sail leech. Why else would you do this?

    Sometimes this can be taken to an extreme where the transome is a tiny little surface way above the water. So much that it seems unlikely, even heeled over, that the elongated hull would ever even come in contact with the water.

    Many boats have a vertical transom, easy to build, easy to mount an outboard. Not very impressive looking for the most part.

    But many modern designs have the transom raked forward, which looks really racy and fast, but I can not think of any reason to do this other than looks. Perhaps it reduces the windage of the hull profile some?

    What are the other reasons to shape the stern the way they do? Is there a performace reason, a cost reason, or is it just appearance and tradition?
  2. PAR
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    PAR Yacht Designer/Builder

    Transom shapes viewed from astern are governed by hull shape. Profiles can have many reasons behind the shapes employed.

    Reverse rake (sloped forward) transoms where specifically developed to save weight in the stern of the boat, back in the early 1960's. These shapes have been stylized a bit, with each designer and manufacture employing their own "signature" raked and shaped stern profile.

    A vertical transom suggests a pretty bored designer though the early 70's had quite a few on sailboats. Over hang in the ends of a yacht aren't doing a darn bit of good for a sailboat, so the end of the boat is just terminated when the LWL runs out. The same can be said of bows in this regard as they have become progressively more vertical in recent years. Same reasons apply, reduced weight, maximized LWL usage and styling.

    There are many different types of stern treatments, elliptical, double ended, double transoms, barrel backs, torpedo sterns, I even have one called a turtle back, plus there are many that I haven't mentioned.

    The logic employed is countless and timeless. To answer your questions, there are basic fundamentals behind these boat butt shapes, some weight reduction, others to offer additional deck space, possibly boarding ease, windage, racing rule related, styling, whimsy and every manor of combination.
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    Here's a few design input considerations to add to your list:

    Will the rudder be hung from the transom, through a well or under the stern?
    Cockpit size and shape.
    Accomodation for outboard on transom or in a well.
    Recovery of swimmers over the stern.
    Following seas in blue-water or relatively flat lake cruising.
    Charge-by-the-yard mooring in a marina.
    Net fishing over the stern.
    Displacement or planing type hull.

    There must be dozens of factors to consider; and there's always tradition.

    Afterthought: hi! Paul; your post popped up while I was writing mine. You have trouble sleeping too?
  4. BeauVrolyk
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    BeauVrolyk Sailor

    Another consideration

    One factor that others haven't brought up is reserve buoyancy. It is very nice indeed for the stern (and the bow for that matter) to carry more buoyancy than you normally need far above the water where it isn't dragging along. You see there are times when the seas will wish to submerge either the bow or stern, in those situations the larger volume of an overhanging stern resists submersion.

    In modern boats, which are typically quite light and are often quite fast, there are far fewer times when a wave might submerge the stern, thus they can get away with a reverse or even an entirely open transom safely. The same is true of the bow. As boats have become lighter and more buoyant generally, they have been able to move to more narrow vertical bows. However, in cruising boats, which are typically quite heavy and loaded down with gear, reserve buoyancy above the water line is a very nice thing to have.
  5. alan white
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    alan white Senior Member

    My personal thinking (while completely agreeing with what's already been said) is that a designer often chooses an aesthetic look he likes and then builds a rational case around it. I'm sure Alberg, who designed the lovely Ensign way back, was matching that reverse transom to a reverse sheer for more than practical reasons. The boat just looks right that way.
    A hundred years ago, most sailboats designed to race had long overhangs and tiny sterns. Back then, designers still hadn't escaped the notion of a racing boat as a work of art rather than a purely functional machine.
    There's something to be said for every shape of bow or stern of course, but each feature will always cost something at the other end of the comprimise scale. Too much overhang and the LWL shortens beyond the point where better light air performance is worth it vs strong wind performance.
    Yet there is a place for nearly every type profile in at least some races. A smart designer will design a shape that would win the most races on average in the most places.
  6. Paul Kotzebue

    Paul Kotzebue Previous Member

    I believe most of the stern shapes we see on sailing yachts were developed to take advantage of a rating rule, even a hundred years ago. The modern short ended shapes make sense with a box rule that limits overall length such as the TP52 and Mini Transat rules. The CCA, Universal, and International rules all favored boats with long overhangs. The rule in effect one hundred years ago, the Length Sail Area rule, measured length on the waterline but not overall length. This encouraged very long overhangs. Before that, the Tonnage rule in England favored very deep, narrow, heavy boats with plumb bows.

    I would say the Herreshoff Manufacturing Company and others built quite a few all out racing machines over a hundred years ago.
  7. tkk
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    tkk Junior Member

    Wasn´t the reverse transom inspired by some LOD-based rule as well?
  8. PAR
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    PAR Yacht Designer/Builder

    The reverse transom was an attempt to make fairly competitive boat more so, by eliminating weight in the stern. The boat wasn't too long for it's rule then chopped, it was just a top 5 boat that wanted to be a head of the fleet boat. I'm pretty sure it was Ted Brewer that did it and they literally used a chainsaw the night after a race and competed the following day with better results. The idea caught on and has been used ever since. It think this was in the very early 1960's.
  9. Paul B

    Paul B Previous Member

    I believe Paul is correct.
  10. Paul B

    Paul B Previous Member

    The long sloping "reverse transom" sterns on IOR boats were done not to reduce weight, they were done to move the aft measurement point of "L" forward, reducing the measured length and hence the rating.
  11. Paul Kotzebue

    Paul Kotzebue Previous Member

    That is the effect. The cause is in the applicable rating rule.

    I know the 12 meter Columbia had a reverse transom in 1958, and I'm pretty sure Ted Brewer was not involved in that design. I believe the 12 meter rule doesn't measure anything aft of a waterline located 180 mm above the flotation waterline, so it makes sense to get rid of anything in the back of the boat that does not contribute to sailing length. The 12 meters used in the 1987 America's Cup looked like the reverse transoms went all the way forward to the aft measurement station.

    Paul B is right about the IOR reverse transoms. The designer could locate the optimum position to measure the aft end of "L" with the transom corner.
  12. Ad Hoc
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    Ad Hoc Naval Architect

    It also depends upon the type of boat, sailing, motor yacht, crew boat, work boat, fast passenger boat, container ship etc...these all have their 'own' styles too. But many reasons, apart from the obvious technical ones, can be simply 'fashion' or 'trend'.

  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    Power boats with reversed transoms were all the rage in the 1920's and 30's. There was also the slipper stern or thames launch in which the rear deck sloped down almost meeting the water. Various styles like beavertail were being experimented with at that time and may well have established fashions that were followed by sailboat designers for a while.
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