Effects of racing rules on boats

Discussion in 'Boat Design' started by rugludallur, Sep 17, 2010.

  1. rugludallur
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    rugludallur Rugludallur

    This is something that has been on my mind for a while but I thought I would ask your opinion.

    It's clear looking at sailboats for the last couple of decades that artificial racing rules have affected which forms are built at any given time, people see fast racing boats and assume that a production boat with similar features will be fast.

    Some of the features that come to my mind are straight (vertical) bows, sugar scoop sterns and open sterns.

    Which features do you think serve no other purpose than to imitate boats built to racing rules?
     
  2. MikeJohns
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    MikeJohns Senior Member

    Have you read Marchaj "seaworthiness the forgotten factor" ?
     
  3. rugludallur
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    rugludallur Rugludallur

    Hi Mike,

    I have not read it but I definitely will, too bad it's not available as an ebook.

    Thanks for the tip.
     
  4. DCockey
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    DCockey Senior Member

    My recollection is the almost universal reverse slope on transoms originated in large part to gain an advantage by reducing the deck length in one or more rating rules and obtain a lower rating than if the transom had been upright or sloping back. The argument has been made that the reverse slope also reduced mass at the end of the boat and the pitching moment of inertia. True, but I believe the rating rule advantage was the prime motivation.
     
  5. MikeJohns
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    MikeJohns Senior Member

    Also the reverse transom lowers the CoG in a light displacement craft this is significant. Less deck and topside weight represented in your total displacement. Aft deck area isn't missed on a racing boat.

    The longitudinal gyradius is affected by the transom type but it's not so significant once you get above a moderate DL ratio. Pitching acceleration zero point in wave encounter is not around midships but well aft so weight fwd and in the rig extents has much more affect on this. It's also impossible to generalize without considering the whole RAO (vessel wave encounter response curves) and areas of operation.

    In some cases with for example windward in short coastal seas you can get a lower resistance with higher inertia cutting through the chop rather than having the vessel pitching close to resonance and nearly stopping. But then that leads to the shape of the hull, and it's RAO again. Nothing can be considered in isolation it's all a matter of juggling and compromising factors for the best use outcome.
     
  6. Paul B

    Paul B Previous Member

    I think what you are seeing is similar to the reason so many automobile designs look so similar. The design tools are now sophisticated enough that we know what works best, and form follows function. It has little to do with racing rules of the past 20 years.

    In the case of the near plumb bows it is a function of having the most boat for an overall length, based on the very expensive mooring fees owners are charged. Why have a 25 foot waterline in the 30 foot slip you pay for, when you can have a 30 foot waterline? The longer waterline provides more speed (greater cruising range in a limited time) and more accomodations per overall length.


    There has never been any rating advantage in any rule I know of for a "sugar scoop" stern. It does allow for a nice swim platform and dinghy dock.


    There has never been any rating rule advantage in any rule I know of for an open stern. Open cockpits drain very quickly. They are very nice to have in stern-to mooring and for access to the swim platform.
     
  7. rugludallur
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    rugludallur Rugludallur

    Thanks for you input, I'm not sure those arguments are enough to persuade me of your observation but I think your on to something.

    When ever I go down this path I end up thinking that most of the cost associated with a boat is proportional with the sail area, the sail area again is proportional to the LWL and wetted area. I end up thinking that perhaps it's not such a bad deal to get a 36' boat for the same price as a 33' assuming they have the same waterline. You make a valid point regarding berths but that depends heavily on location.

    I should probably have used the term "reverse transoms" instead of sugar scoop or open cockpit, again my thinking here is that since the cost is mostly proportional with the LWL a more traditional stern is an easy way to get extra interior space cheaply. Your observation regarding berthing fees applies equally here.

    Correct me if I'm wrong but I think the crux of your observation is that people are trying to get the longest waterline possible for a given total length.
     
  8. Paul B

    Paul B Previous Member

    So if you tack on an extra 3 feet of bow overhang to a 33 foot boat what does that get you? Bragging rights? It is pretty useless space, except maybe to have a chain locker.

    On the other hand, if you look at it the opposite way and are constrained by LOA (due to mooring cost) I think you would want the extra 3 feet of waterline.


    If you look at the popular modern designs (Beneteau, J Boats, etc) you will see little in the way of a "reverse transom". Transoms have been getting closer to plumb over the years since the IOR rule influence departed in the mid-1980s.


    That is the explanation given by some of the biggest builders in the world, based on their market surveys.
     
  9. terhohalme
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    terhohalme BEng Boat Technology

    36 feet waterline sails upwind 0.3 knots faster than 33 feet waterline...
     
  10. MikeJohns
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    MikeJohns Senior Member


    Yes in racing boats, but in heavier cruising boats I think there are definite advantages.



    It depends of course on the other factors in the hull design but generally a raked stem provides flare which can have advantages in pitch damping, reserve buoyancy, and in shifting the waterplane area forward along with the CoB as the vessel pitches. Flare can also add quite a lot of positive area under the stability curve and give a better trim with extreme heel. You also get a drier boat, more spray is deflected and less sea comes over the bow.

    A raked stem is also a good anchor handling platform and it also stops the forefoot over-running the anchor chain when the vessel sails around its anchor in strong winds.
    Another advantage is a reduction in bowsprit length for boats that use one, and that extra deck space is useful and a wider deck fwd can give you a better spread for the rig stays especially in a twin masted vessel.
    Plumb stems are poor in collisions where a raked stem lets the vessel ride up.

    It depends on just what you design the vessel for. Moderate overhangs are quite acceptable and the operational waterline is a dynamic condition.
     
  11. Paul B

    Paul B Previous Member

    So now the game has changed. Your reply is not referencing plumb vs raked bow, it is referencing the advantages of Flare. I guess in your world plumb bows cannot have flare?

    I'm attaching a jpeg of the modification done to an old IOR 45 footer. The change to the plumb bow INCREASED the reserve bouyancy forward. In fact, it is better compared to the original in almost every area you highlighted in your comment.


    Extending anchor rollers seem to handle this "problem" adequately. Note the Beneteau 46 photo below.


    A plumb bow does not have to result in a narrower deck at the bow. I have no idea why you would have this idea. The Beneteau 46 looks to have just as wide a foredeck as a raked stem design would have.


    Wow. That's how hard you have to grasp at straws?



    The thing that is really amusing is people like Johns will sing the praises of the old gingerbread looking designs like the BCC (lines attached), complete with plumb bow. But put the plumb bow on a modern boat and somehow it won't work!

    Just because a modern boat does not look like the boats you had on your PJs when you were 6 does not mean they are not designed correctly.
     

    Attached Files:

  12. jimbo2010
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    jimbo2010 Junior Member

    As an owner of a Ben 36.7 designed by Bruce Farr they sail great.

    Several sister ships have crossed the Atlantic, one won the Newport to Bermuda Race, it was one of the smallest boats in the fleet.

    It was designed to the rules.
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    True, they can't. Except for a pram ...

    Yes it does. Except on a WW2 aircraft carrier.

    If sheers are flared at the stem to increase buoyancy and/or deck area, surely the stem cannot be plumb. If it can, show me (I'm from Missouri for this one)

    Marina fees and racing rules aside, the bow shape should reflect the purpose of the boat.
     
  14. Paul B

    Paul B Previous Member

    I understand you have little experience with sail boats, so maybe you should not make such declarations until you learn more...

    These two plates are from a book sitting here on my desk. I can provide more examples in other sizes if you still have doubt.
     

    Attached Files:


  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    The flare of the sheers is indicated by the spacing between the waterlines in the plan view. On both boats the waterlines meet each other at the stem, therefore there is little or no flare in the sheers at the stem. There is flare elsewhere, but my comments - and Mike's - were explicitly about flare at the bow. If there is flare at the stem the waterlines necessarily do not meet and the stem must be raked.

    Stem geometry is not restricted to sailboats, I admit I've only built one of those. However I do design and build canoes. Solid geometry does not distinguish between boat types.
     
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