Sailboat scale-up questions

Discussion in 'Boat Design' started by sdantonio, Dec 12, 2021.

  1. sdantonio
    Joined: Dec 2021
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    Location: Massachusetts USA

    sdantonio New Member


    I found a set of free plans on the internet for a 14ft x 4ft "box" called a scow 420. I'm thinking that I would like to scale it up to 16 X 4 to get a little more stability and room for sailing protected bays. The idea is to use this primarily as a 2 person dive boat and thinking that the flat front (kind of a plumb bow) may be a great place to hang a ladder and store dive gear.

    The keel runs about half the length of the bottom and the mast looks to be quite close to the front. What I'm hoping that the scale-up may be a simple matter of just lengthening the keel and moving the mast proportionally.

    Since the mast in the plans looks to be approximately 3 ft from the bow, this would move it back to about 41 inches for the 16ft scale-up (The numbers I'm kicking out here are just approximate from eyeballing the pictures).

    i.e. 36 * (16/14) = 41.14

    Will a simple scale-up like this work or am I oversimplifying it to much?

    Thank you

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  2. bajansailor
    Joined: Oct 2007
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    Location: Barbados

    bajansailor Marine Surveyor

    Welcome to the Forum Steve.
    That is interesting how Duckworks have the plans 'free' -
    Scow 420 Free Plans

    Yet they are selling plans for the Cruiser version for $55.
    Scow 420 "Cruiser" Plans PDF

    Don't be lured by the word 'free' - even if the plans cost $150, this would still be a small fraction of the total cost to build the boat.
    Hence you need to make sure that you are building the best boat for your 'Statement of Requirements' - ie you need to make a list of what you want this boat to be capable of doing.

    You say that "The idea is to use this primarily as a 2 person dive boat" - but do you really want to have a sailing rig as well?
    Would you have a pair of oars, or a small auxiliary outboard motor in addition to the sails?
    Where do you intend to go diving, and what distance would you have to typically sail (or motor) to reach your dive sites?
    What are the typical wind conditions like in this area?

    If you scale up from 14' x 4' to 16' x 4' you will get more longitudinal stability, and in theory you might get a bit more transverse stability (depending on how full the hull is), but I wouldn't count on it - you are effectively making the boat even more skinny (and she is already quite skinny, at only 4' wide for 14' long).

    If you are more keen on diving rather than sailing it might be better to build a boat that is more suitable for use as a dive boat?
  3. TANSL
    Joined: Sep 2011
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    Location: Spain

    TANSL Senior Member

    I think so, that you oversimplify things. The total weight and the distribution of weights, and therefore its center of gravity, will change, the position of the center of thrust will also change, as will the center of gravity of the drift area. Greater weight will require a greater sail area ....., that is, it would be convenient to study the thing in a little more detail. And I agree with @bajansailor in that the boat does not seem the most suitable for diving.
  4. clmanges
    Joined: Jul 2008
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    Location: Ohio

    clmanges Senior Member

    Mast placement will depend on where the sail's center of effort is, and where the boat's center of lateral resistance is.
    I'm not thrilled by those plans; it should have leeboards instead of that keel, and a kickup rudder. Also, it's a waste of plywood to deck the whole thing for your purposes, but I think they did that to save on internal supports.

    Do you really want a sailboat for diving? Also, you might be able to do with a shorter boat than a longer one. Look at the plans for the Oz Goose; I think those would work with a small motor, and they're only 12' long.
    bajansailor likes this.
  5. sdantonio
    Joined: Dec 2021
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    Location: Massachusetts USA

    sdantonio New Member

    Hi All

    thank you for the comments. Actually I like the idea of a sail boat for being a sail boat, I could spend all day on it with or without the diving. The diving aspect is just an added attraction to get the girlfriend into it. She is a dive instructor, but doesn't have the interest in sailing I do. Additionally I really dislike the idea of putting a motor on a boat. Something about that just rubs me the wrong way.

    It totally slipped my mind that in most states trail-able boat with is 8ft, not 4 so there is no reason not to scale the width proportionally.

    The kick up rudder is something I would have done anyway,

    I will definitely check out the Oz Goose. Previously i also have really liked the blobster by Jim Michalak

    Cof Lateral resistance and Cof Effort, those are the things was worried about, it's good to know my concern was well founded in this case.

    I think I will re-look at the Blobster and check out the Oz Goose.


  6. Tops
    Joined: Aug 2021
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    Location: Minnesota

    Tops Senior Member

    Hi Steven,
    The pain-to-climb-back-into is the rub on any boat purposed for diving. Next is a layout that works for tanks and gear.
    Diving off a normal v-hull is miserable, a purpose-fitted pontoon is a joy.
    I have gotten to ride in a couple 'birdwatchers' at the Messabout here in MN on Lake Pepin, fun designs.
    Arcebus is an impressive boat, still sailing 17 years after its online build log. I have been onboard and underway a couple times in this boat.
    Camp cruising adventures in home-designed, home-made sailboat | Arcebus
    The step-through bow with a swim ladder on the Blobster might be interesting for diving.
    Unless the girlfriend is of an 'understanding' type, she may not like your attentions split between her and (building) the boat.
    ExileMoon and bajansailor like this.
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