12' sailboat design opinions needed

Discussion in 'Boat Design' started by PhilippeCE, Apr 7, 2022.

  1. Rumars
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    Rumars Senior Member

    For ballast go to the scrap yard. Ideally you find some thick plate to make the keel from (as a sandwich), but if not simply buy a few cast iron engine blocks, smash them with a sledgehammer, put the pieces in a plywood box and fill with concrete. The more iron you can pack in, the better since iron is three times the density of concrete.
     
  2. Will Gilmore
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    Will Gilmore Senior Member

    A dense steel cage of clean and painted rebar will make the concrete stronger and heavier. You wouldn't want to just pour the concrete without some steel/iron reinforcing armature, anyhow. Keep it as rust free as possible before your pour.

    -Will
     
  3. PhilippeCE
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    PhilippeCE Junior Member

    @Will Gilmore 900lbs just boat no supplies or crew. yes id at least put some large rebar in the keel to i can put a few bolt thru to the floor i intend to fit, so its not just plywood and epoxy that hold the weight of keel in place.

    @Rumars I think iron would be a real pain if i had to melt it...

    @Dolfiman If I could find lead for cheap that would be my pick!

    @Kayakmarathon I was in the process of building one out of mdf to check the distribution of weight of the ballast, I think there might a bit too much of it in the aft section, not too sure.

    @gonzo I added up the weight of the plywood panels and the glass per square foot with 7 oz cloth plus mast and sails. I know the 12' Scamp is 420lbs so i should be able to make the boat minus the ballast to be 450, at least roughly.

    I approximated the center of mass to be between the center of mass of the keel and the hull, since they are both 450 lbs, that ends up being just above the keel or just below the waterline @ 2' 3" from the bottom of the keel.

    Light displacement, yes. All submerged area were about 15 cubic foot so 900 lbs where the waterline stands, including the volume of the keel.
     
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  4. gonzo
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    gonzo Senior Member

    You need to calculate the longitudinal position of the center of gravity for the boat to float on the drawn lines.
     
  5. PhilippeCE
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    PhilippeCE Junior Member

    @gonzo does the center of gravity have to be exactly in the middle, or do I have to take into account that the force exerted by the sail will force the bow into the water, or other any considerations?
     
  6. Will Gilmore
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    Will Gilmore Senior Member

    She sits on her lines when no forces act upon her except gravity.

    Other considerations are separate issues.

    Where are her lines when rigged and loaded?
     
  7. gonzo
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    gonzo Senior Member

    The center of gravity will align vertically with the center of buoyancy if no external forces are applied. However, for the boat to float on her lines. The centers must be where you decide they should be to align properly. Otherwise, she will float bow up or down depending. It means you have to calculate the weights of everything and its longitudinal location.
     
  8. PhilippeCE
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    PhilippeCE Junior Member

    @gonzo yes I agree. but I've read in some places the LCB is generally forward of the LCG. not sure in what proportion.

    @Will Gilmore the unloaded waterline is at 2' 3" from the bottom of the keel. that's is where the lower chine is, where the bow starts to curve toward the back or where the transom ends and the keel starts. I reuploaded the plan again with the water line drawn, and some minor modif
     

    Attached Files:

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  9. gonzo
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    gonzo Senior Member

    That is not correct. The CB and CG always align vertically. It is like hanging a weight from a string. It will always be vertical. You need to calculate all the weights to make it work.
     
  10. PhilippeCE
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    PhilippeCE Junior Member

    @gonzo OK. But dynamically under sail, would the wind on the sail act to push the bow more than the stern into the water therefore requiring to make it somewhat unbalanced on paper?
     
  11. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Can you maybe edit post #23 and attach the drawing full size, rather than as a thumbnail please?
    That will make it much easier for everybody to see it.

    Just concentrate first on doing your weight calculations to try to ensure that the centre of buoyancy and the centre of gravity are in the same vertical line at your intended waterline. A spreadsheet is good for tabulating these weights - calculate the weight of every item you can find on the boat, including the different parts of the hull structure, and take moments about the keel and the transom to find the vertical and longitudinal position of the centre of gravity.
    It is tedious, but a necessary evil in boat design, even on 'small' boats like this.

    When you go sailing, it all gets a bit more complicated - your centre of lateral resistance is an elusive beast, and tends to wander about, depending on your angle of heel (and the mood of the boat and her crew :) ). Hence why you will very often find you have a lot of weather helm if you are over pressed, and the boat wants to round up.
    But you are off to a good start if you can get the CB and the CG in line first.
    You could then calculate where the centroid of the immersed side profile area is below the waterline.
    And where the centroid is of the gaff mainsail as shown.
    You generally want to have the centroid of the mainsail area a bit further forward than the centroid of the immersed side profile area. This is referred to as the 'lead', and it is a bit of a black art. It is not helped by these both moving about when you heel, and / or adjust the mainsheet. But it is a start.
    If you do not have any lead at all (this is not good), then you might have to add a wee bowsprit for a jib, or reduce the length of the boom.
     
  12. PhilippeCE
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    PhilippeCE Junior Member

    @bajansailor Hi. For the picture, it takes some time to get approved and I was advised to put it there in that way as it doesn't have to go through approval. you can download it and it is the full resolution.

    You have a few really good points. I was aware of most of them and took them into consideration, mostly the lead. my CE is a bit forward of the CLR of the keel, although not too sure how if the CLR of a long keeled boat is as important as in a smaller high aspect-ratio keel. The sail is also maybe 20% too big, but that would be for very light wind at its full size, and I took into consideration that the CE would move forward as you reef. I wouldn't want to introduce lee helm when the CE is even farther forward when the conditions get rough on the last reef. => what is your opinion about where the CE is on that design?

    I made some calculation regarding where I should distribute the weight longitudinally, separating the length of the boat in 3 equal parts and seeing how I can roughly match the buoyancy of the section with the keel shape underwater. I'll put the picture of that, I changed the thickness of the keel and I think it make sense. I mostly shifted some keel thickness forward. not sure how much sense the drawing will make though... the part at the right is the rudder and the numbers are cubic feet of concrete for each small section. nothing some trim can fix once its build?
     

    Attached Files:

  13. Rumars
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    Rumars Senior Member

    Did I say anything about melting iron? Go find a piece of 1 inch thick steel plate, and cut your keel from it. A 9ft long 2ft wide piece will do. Or smash an engine block and put the pieces into wet concrete to make it more dense (the concrete is there to prevent the pieces from moving).

    At your size all weight calculations have to be done at loaded displacement, a standard person plus some gear and stores will make over 25% of lightship. Simply moving your weight forward will affect trim and therefore CLR. Look up how a traditional catboat handles, your boat will behave the same.
     
  14. PhilippeCE
    Joined: Apr 2022
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    PhilippeCE Junior Member

    @Rumars Yes that would work, but there would surely be some welding involved and I'm not geared for that at all. I'm talking about the 1" steel plate. As it stands, the concrete keel isn't much of a compromise. if the steel plate was only 1" wide I don't know how I would make it stable and stopping it from wanting to lean from side to side under stress. Something I could easily bolt to the underside would be ideal, 2" wide maybe. not sure how common that is, I'll look into it see what I could get. Thanks.
     

  15. Will Gilmore
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    Will Gilmore Senior Member

    How were you planning to do it with concrete? You should be able to drill through the top edge of the plate and marry it to your garboard with some sort of shoulder blocks. Cutting, of course is required, but no welding.
     
    Last edited: Apr 10, 2022
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