Reducing a Featherwind / Nutmeg

Discussion in 'Boat Design' started by renatogui, May 23, 2013.

  1. renatogui
    Joined: Jun 2006
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    renatogui Junior Member

    I bought Nutmeg sailboat plans. This boat is a simplifyed version of Bolger's Featherwind that is 15.5' x 4.5'.
    The hull panels are 16' long cut from 2 but-joined 8' x 4' plywood sheets.
    Unfortunately, I live in Brazil, where the standard plywood is 220cm long (about 7' 2-5/8").

    In order to avoid a second but joint in the panels I must reduce 10% the boat length. I can have this in one of 2 ways:

    The first is to reduce the distance between stations 10% keeping all other dimensions.

    The second is to cut the bow near station 2 adding a bow transom that could have the same angle of original stem. The hull has a big forward overhang (aft too) and this cut is supposed not to change WL, CLR, CE etc.

    I would like to use the first solution as I don't like the apearance of a bow transom but I have read so many warnings about scaling plans that I am affraid it could not be a good idea.

    What do you think about it?
    Thanks,

    Renato
     
  2. DCockey
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    DCockey Senior Member

    Do the plans show the shape of the sides and bottom before they are cut?

    If so then reducing the stations by 10% but keeping the breadth the same will result in a different shape boat.
     
  3. JRD
    Joined: May 2010
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    JRD Senior Member

    I would have thought that scaling the design would take a lot of additional work compared with making one more join, and reducing the length will make it slower (as well as tubbier as David has suggested above).
    If you are worried about the strength of a butt-join you should scarf if instead. There are plenty of guides to do this on the net. Search on boat design, there are alot of discussions on scarfing.

    Good luck
     
  4. renatogui
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    renatogui Junior Member

    Yes, the plans show the developed sides. I did not realised that only 10% longitudinal shortening could be so harmful.

    The bottom has a very smooth out of the water slope forward and backward so that reduction of the boat lenght should not produce a substantial change in the length of WL. You can see this in the enclosed profile picture.

    The WL shown is calculated for 700lbs. The weight of my girlfriend and me is about 330lbs, less than half the design displacement.
    I draw a strong line in proposed cut section. The remaining forward overhang is about the same size of the aftward for 700lbs displacement.

    How about this cutting alternate solution?
     

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  5. messabout
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    messabout Senior Member

    You will enjoy the boat, built according to original plan, long after the inconvenience of an extra scarf has been forgotten. Bolger put all that forward rocker in the bottom for his own good reason.

    If the intent is to make this a row boat to be used only in calm water then go ahead and clip it. If it is to be sailed then obey the plans. When heeled, the whole length of the bottom will be immersed thereby lengthening and narrowing the WL while reducing wetted surface. A devilishly clever ploy that is well proven by International 110 type boats as well as thoughtfully designed flatties.
     
  6. renatogui
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    renatogui Junior Member

    OK messabout. You win.
    Thanks.
     
  7. ancient kayaker
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    ancient kayaker aka Terry Haines

    If you are building stitch and glue using the 2D developed plank shapes provided by the designer, you could reduce the plank dimensions in both directions by 10%. You will end up with a boat that is 10% smaller in length, width and height, and the displacement and capacity of the boat will be reduced by (almost 2/3 incorrect) (should have said 1/4). That may not be suitable for your purpose.

    Reducing just the length requires either building a mold of the finished hull shape (less skin thickness) so you can form the plank over it or importing the lines into a hull design program like FreeShip that can provide plank developments. There is a manual drawing method but it requires advanced drafting skills . . .
     
    Last edited: May 25, 2013
  8. PAR
    Joined: Nov 2003
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    PAR Yacht Designer/Builder

    A Payson butt joint is just as strong as a scarf and can be made stronger, though this is just wasting materials. Look up the "Payson butt joint" with the search tool. Much has be posted, previously about this joint.
     
  9. renatogui
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    renatogui Junior Member

    Thanks ancient kayaker. I did not imagine the reduction in displacement could be so dramatic. Going to numbers: The passengers weight 85 + 65 = 150kg that means 330lbs. The original boat weights 130lbs according to Bolger.

    Total weight without reducing the hull weight to consider the shortening: 460lbs.
    This is around 2/3 of the design displacement.
    The expected reduction in displacement keeping the same relative WL should be 0,9³ or about 3/4 of design displacement.
    I think this would be ok for the pretended displacement.
    I am really worried about the loosing of stability due to the reduced beam. That is why I was planning to shorten only the lenght of the hull.
    That means to redraw the planks. I can do this with a hull design program like you said.

    I analised the original project planks to evaluate the consequences of using Brazilian plywood. The lateral planks are 45cm (1'6") longer than 2 sheets of plywood and the bottom is only 9cm (3-1/2") longer than 2 sheets. It is a pity to use 3 sheets for so few inches missing.

    PAR - I used Payson but joint in other 2 boats but it is so ugly! In previous boats I manage to hide the joints but the Nutmeg is all open and I will be forced to see not one but 2 ugly joints if I use the original length. Thanks for the sugestion anyway.
     
  10. ancient kayaker
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    ancient kayaker aka Terry Haines

    You're right, I must have hit the wrong button on my calculator!

    BTW, where did you buy the Nutmeg plans? I was looking for an hull drawing image - maybe there's a simple way to reduce length . . .
     
  11. philSweet
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    philSweet Senior Member

    I see nothing wrong with the pram bow idea if that is the easy way out. I'd make it a bit steeper than you show, and scab a stem on the front to recover another six inches of waterline. If you can scrounge up a natural crook about 8" in diameter, you could carve a nice little stem and prod. What ever is easiest, but I wouldn't scale it down because of the loss in capacity. I'd take the hit with the pram bow. You may want to widen the sheer at the bow if you do this. That can help visually, but it means you either have to redevelop the panel expansions or leave that edge wild and finish it to shape once the sheerclamp is sprung.

    ...but they're not butt ugly if you disguise them. Fishing rod holder, paddle holder, oarlock brace, ladder mount, backing plate for stays, throttle mount, antenna mount, radio mount, bimini hardware mount. Make the butts out of nice wood and fashion them into useful mounting pads. In the bow, an anchor bracket, anchor rode bucket, or just one good bomber cleat could make that all disappear. Remember, if you switch to two butt joints, you gain the freedom to position them in different places.;)
     
  12. renatogui
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    renatogui Junior Member

    Hi, PhilSweet,

    I used in the pram bow the same rake of the original stem. Then I adjusted the angle slightly to have a rectangular bow, only to make construction easy.

    I tried also a vertical cut that maximize the WL length. I didn't like it. It is ugly and will stop the waves like a wall.

    I enclosed a 3D view of the 2 solutions obtained with Carlson's hull.

    Maybe the better is to stay between these 2 like you suggest.

    My poor English wasn't enough to understand what you said below.

    Quote:

    If you can scrounge up a natural crook about 8" in diameter, you could carve a nice little stem and prod.

    Well, definitely, I am convinced to not scale the hull down. If I can cut part of the bow to have a more economical construction without compromising the balance and performance, I will certainly do this.
    Otherwise, 2 ugly but joints in the bottom and the lateral planks down.

    Thanks for your opinion.
     

    Attached Files:

  13. Petros
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    Petros Senior Member

    your English is just fine, he is using some obsolete old English terminology from both house and boat building.

    He means for you to go find a curved or naturally bent branch from a suitable tree (or drift wood) with a dia of 8". A naturally curved branch from a tree is called a "crook", the root word for "crooked", and also used for dishonest people as well. In some languages it is called a "bent".

    You carve it into the correct shape to use in the front of the boat where the further most forward vertical frame member (the stem) will curve down to meet the front of the keel. It is more effort to make such a part from a curved branch but it makes a nice pleasing shape that is strong.

    "scrounge" means to search about in scrap piles or in back lots and find what you need. this term is kind slang for salvaging or rummaging through scrap piles. It used to mean to "borrow" without the intent to return it or to obtain without the owner knowing (i.e. to steal it).
     
  14. renatogui
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    renatogui Junior Member

    To ancian kayaker:
    I bought the plans from Thomas Vetromile that have today the rights of selling Nutmeg.
    He had a site (smallboat forum) that is not running anymore but you can contact him directly as I did to buy the plans. He is very kind and honest.
    The plans are complete and very detailed.
    His address:
    Tom Vetromile
    499 Camp Bay Rd
    Sagle, ID 83860-5068
    email: tomvetromile@yahoo.com

    I don't know what you mean with "hull drawing image". About scaling, I like the free software 'hull' by Carlson Design. It is very easy to scale up or down a hull with this small program. You can scale only the lenght or all the dimensions and then have all the new design relations calculated to compare.
    I made a Nutmeg model that can be explored with this program and I can send you in private if you give me your email.

    Renato
     

  15. renatogui
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    renatogui Junior Member

    Thank you Petros for your explanation.
    Here is a place to discuss boat design and building but also to improve our obsolete English. You where very kind to lost your time teaching English to a stranger.
    Nutmeg is a very simple project and the crook will be reserved for the next boat.
     
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