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Thread: What happens if you change the bow/stem shape and transom rake?

  1. #1
    Join Date
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    Default What happens if you change the bow/stem shape and transom rake?

    Don't do it - I know. I totally agree with 'stick to the plans'! I would not and will not.


    (but) - hypothetically - purely for education - just a thought experiment ....


    Imagine there was this boat that ticks all your boxes - right size, right weight, right sail plan, proven capabilities etc ...
    but it just isn't quite the boat you had envisioned - purely subjective, nothing wrong with the boat at all. It's just ... say, it has
    a plum stem and almost vertical transom and you have just seen too many Herreshoffs and you (just for fun - secretly -
    you will never tell - nobody will ever know, that you even had these thoughts) ....
    ... you take the lines and the plans and you play around with them - just a little.
    You change the stem from plum to a little sweeping overhang and you rake the transom a bit more to maybe 40 degrees.
    And you look at it and it actually happens to look really nice. So - just for fun - you look at the lines and the sheer and how
    they change and how they fair with the stations - just a 10% change to the top of the first station to accommodate for the extra
    length of the stem.

    All the shape under water and at least half way up the sides stay EXACTLY the same, only slight adjustments at the sheer height.

    So you build a 8:1 model - and you like it - and you know you will never do that, change plans, tinker with someones design - NEVER

    (but) - you do wonder - if someone would do that (some evil twisted disrespectful rogue) - how would it actually affect the boat?

    It'd be longer and heavier, potentially out of trim?
    The raked transom would end up with the rudder pivoting on a different axis.
    If there's a mizzen, the structure to support that would need to change.
    And of course the tiller and deck and coaming and so on ...

    But would it sail or handle MUCH differently? Given that the shape in the water basically is the same unless it heels until the
    gunwales are in the water?

    8/

  2. #2
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    You need to do this and report back! Getting the look right might be harder than you suspect.
    -Dave

  3. #3
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    Hey Dave,

    I'm sure if I tried it would end in tears and the conclusion that it looked much nicer as it was!

    It's fun to play with the models though.

    Comparison copy.jpg

  4. #4
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    It looks good either way.
    Ermmm, I have done what you are talking about to a Bolger Micro. Sacrilege, I know.. Leaned the sides out to the lower rubrail, by 3" a side, then returned them to the deck level. This altered the forward lines, now extended by a foot or so to a raked bow. The transom is raked back to, with the top raked forward, a bit like the Drascombes. Raised the cabin by 4", after another builder thought it was a bit low and I saw a photo of an OZ one with the mod.
    Rig is original and FF has just sent a nice set of sails from Holland. Mast bench is set up and doug fir ready to shape. Just not had much time to do anything recently, otherwise she would be in the water.
    I seem to be an idiot when photos are concerned here, but will wrap a wet towel round my head and read the 'how too' yet again.
    A2

  5. #5
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    Be careful of just changing a few frames.
    What looks good on a small model may not have nice lines in full size.
    When trying something similar in another field ( aircraft) it was inevitable that a close look at the lines showed some nasty transitions between original and modified sections.
    Resulting in the need to go a lot farther back to get the lines to fair - actually back to the max width. From either end.
    Which means you changed the whole aircraft.

    Why not pick a similar boat from Herreshoff with the features you like?

    Of course if it really is a thought experiment I withdraw the suggestion - and have nothing useful to say about the sailing.

    Have fun.

  6. #6
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    What happens if you lean the bow and stern outwards?
    Well, round here you end up with more tax to use the boat, as it's a bigger block area..

    More seriously your rudder area is not in the same place and may affect the centre of effort V the Centre of resistance positioning. This may affect the handling of the boat..
    A more tilted rudder is a less efficient one, more of your effort is used to tilt the boat when the rudder is off centre, rather than to change course.
    Just an amateur bodging away..

  7. #7
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    The rudder, at least, seems somewhat correctable: you can put a rudder post that reverses the rake of the transom. If you raked it by moving the bottom of the transom _forward_, that'd result in the rudder being in the same place. If you raked it by moving the top of the transom back, then the rudder will now be further to the rear, which will change things of course, but it'll still be vertical
    Daniel

    Building a Campion Apple 16.

  8. #8
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    You may well find yourself with an odd looking boat.The convention of using lines on a plane to describe a complex shape is well established and altering just one or two may flatten some areas or increase curvature locally on a hull that is otherwise a blend of harmonious shapes.Experimenting with models will show the sort of thing that might result.Doing something similar with Freeship or Delftship may be faster.

  9. #9
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    This all touches a deep question: Why are rudder posts so raked in so many traditional double-ended boats? And in many keel boats as well? Many designers and builders seem to think it is a good thing, but why??

  10. #10
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    Re, why raked rudder posts for transom-hung rudders: there are people on this list far more qualified to answer this, but I can think of a couple of reasons:

    1. Reduces wetted surface on full or cutaway keel underbodies.
    2. Possibly less damage in a grounding, as impact will be glancing rather than perpendicular to direction of force.
    3. A raked rudder may (hope I haven't gotten this wrong) reduce weather helm when boat is heeled, as the force of water over the slanted rudder tends to pull stern down.

    Jack - Seattle

  11. #11
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    Quote Originally Posted by Jack Loudon View Post
    Re, why raked rudder posts for transom-hung rudders: there are people on this list far more qualified to answer this, but I can think of a couple of reasons:

    3. A raked rudder may (hope I haven't gotten this wrong) reduce weather helm when boat is heeled, as the force of water over the slanted rudder tends to pull stern down.

    Jack - Seattle
    but but?? if the boat has a weather helm one would be pulling on the tiller from the high side, forcing the rudder down, lifting the stern, am-I-right ??

    Ken

  12. #12
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    "but but?? if the boat has a weather helm one would be pulling on the tiller from the high side, forcing the rudder down, lifting the stern, am-I-right ??

    Ken"

    I *think* a raked rudder is slanted in such a way that water would be forced over its upper surface, pulling the stern down. A reverse-rake rudder would have water forced over its lower surface, causing the stern to rise. It hurts my brain trying to think this through, and I could be wrong

    Jack

  13. #13
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    perhaps we'll both wait for clearer minds... :-)

  14. #14
    Join Date
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    Default Re: What happens if you change the bow/stem shape and transom rake?

    I believe this is exactly what Joel White did with an Asa Thompson skiff/tender to come up with the Yankee Tender, the plans for which are available from our host's Wooden Boat Store. I committed the additional heresy of building mine using the epoxy-glued ply construction method, rather than the "traditional" clinker method using all solid woods and clench nails as called for in the plans. It's a pretty nice little boat... ;-)
    58780ED5-884F-48B5-8729-94E4F21AD81E.jpg

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