My Design - Trifoglio 25' - Suggestions & Opinions

Discussion in 'Sailboats' started by DVV, Nov 20, 2020.

  1. DVV
    Joined: Nov 2020
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    Location: Italy

    DVV Junior Member

    This evening I tried to increase the height of the bow, but it did not work, I'll have to try better.
    I rised the bottom part of the bow and adapted the curve of the hull starting from midship, but I think I over did it and it become a little odd looking.
    When I say that I'm going to loose displacement I mean that if I want to keep the same depth and no trim, I will have to compensate for the part of the hull that I moved up, otherwise the trim wont be zero anymore. CG and Center of Buoyancy would not be in line anymore, the latter moving backward. I guess my terminology - and translation as I'm Italian - is not correct - may be also some concepts - keep in mind that I'm just an amateur designer trying to design my own boat.

    The waterline is 7cm (2.7 inch) below the bow (1,107mt depth). How much do you think is should be? I was thinking about trying 12-14cm (5inch), but it just a guess as I do not have examples to follow.

    By the way, when I say 'odd looking' I dont mean I dont like it. It is more something like I would not fell safe in that kind of hull. As I said in one of the first posts, this is the first time I design something, and I find it difficult to design something that is not in some way all right to my eye. I guess similar from what I'm used to see.
    Probably the scow idea is in this aspect not a great choice, but that's how it all started.

    DVV
     
  2. fallguy
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    fallguy Senior Member

    Have you modeled the scow woth the boat heeled over? Do you have a drawing of the boat at full heel from the front? How is 2.7" not underwater then? Something seems missed to me...
     
  3. DVV
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    DVV Junior Member

    Yes, I use Polycad, which gives stability calculation and the render of the hull at all angles.
    No, the bow never goes underwater, even if heeled. The transom does when heeled, but not the bow.
    In Boats with an Open Mind, Bolger commenting his Scow Schooner design, says that in a scow you should try to avoid putting the bow in the water.
    He did not even bother to give any raking the front panel, as he said that with the bow in the water the hull would stop.
    Other scows, though had some ranking to bow, and I used it.
    Anyway, I still think that I could increase a bit the height of the bow. The initial design had a lower displacement so bow and transom were higher from the water.
    I will transfer the design to Policad with all the last changes, I want to check if I'm not mistaking.
    I did those the calculations with the fist drawing I posted, but then I made some changes. May be I changed it more than I thought.
     
  4. DVV
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    DVV Junior Member

    In these images the boat is not heeled, but the waterline is correct.

    Schermata 2020-12-08 alle 01.52.26.png Schermata 2020-12-08 alle 01.52.01.png
     
  5. fallguy
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    fallguy Senior Member

    And how exactly are you interpreting the last sentence?
     
  6. DVV
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    DVV Junior Member

    I dont think it needs to be interpreted. I just try to avoid the bow to go underwater. Aren't we talking exactly about this?! ;)

    This is the hull heeled at 20 degrees (max stability angle). Do you think the bow is too low on the water?

    Schermata 2020-12-08 alle 09.02.48.png Schermata 2020-12-08 alle 09.01.27.png
     
    Last edited: Dec 8, 2020
  7. Will Gilmore
    Joined: Aug 2017
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    Will Gilmore Senior Member

    Have you calculated displacement with an estimated sailing rig and cargo? She seems light, just looking at the drawing. Keep in mind also, the keel and skin resistance in the water against the hight and force of your center of effort will pitch the bow down and lift the stern up. There may also be added displacement from the driving force. Then there is the bow wake. The size of the wake is determined by displacement and cross sectional width and depth, how much water has to be moved how far to get out of the way of the boat. This will bring the waterline up in the front and the stern.

    I don't know the calculations, it just looks like you might want a little more cant and a little more overhang in the bow.

    -Will (Dragonfly)
     
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  8. fallguy
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    fallguy Senior Member

    The water in the drawing is flat; don't forget. Water never behaves that way.

    I am just offering a gutcheck and my gut tells me you will encounter braking forces the way it is drawn.

    What I think you ought to do from a design perspective is answer the question how high can an oncoming chop be before it acts as a braking force on my boat. The lower chine of the scow does not ride above the waves.

    In a two foot chop; that boat is going to be having problems I'd say, but it might be far less than that. Again, just a gut check. The boat doesn't ride on the wavetops...

    I wish I could offer you a more technical answer than it looks like trouble, but that would be wrong of me. I am just a boat lover. Also, I think the line from the book is intended to be a warning and easy to misinterpret. How high above the water was his scow?
     
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  9. DVV
    Joined: Nov 2020
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    DVV Junior Member

    The problem you both refer to - bow being too low on the water - is something I underestimated.
    The bow was 5 cm (2,7) inch above the surface. Today I tried again increasing it, and it seems to me it worked better.
    I increased the height of the bow above the water to 11cm (4.4inch). I like this. I understand waves may be much higher, but its an improvement.
    What you both think?

    Displacement i around 1,8tons. And weight 1,77.For the aluminium free standing mast with junk sail I used 70kg, found on junkrig.org forum. Is this too light (it should be aroud 10m)?
    The framing structure and the internal structure has been all drawn, I set thickness of ply and weight. The weight comes from that (I included no provisions nor crew). If you would be so kind of give me your opinion on that, I can share all the dimension I used, so you can tell me what you think.
    The keel itself its 880kg.

    Will: I dont know the meaning of 'cant' in nautical terms. Does it refer to the curve of the hull at bow level?

    I appreciate this a lot. The reason for which I posted my design here, is because I think I can find opinions of people with much more knowledge than I have - like you. And I'm willing to change the drawing all the times that will be needed to make it a proper one. Sometimes explaining others the reasons of your choices make you more convinced. Other times you realize that you made a wrong assumption or estimation, and you try to fix it. I'm not an expert. As you say, I love boats. I started this project as a game, but now I cant think about anything else!

    Here below the new bow with the waterline on the hull. I dont have a pic with water surface as yesterday one at this moment. Schermata 2020-12-08 alle 16.24.49.png
     
  10. fallguy
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    fallguy Senior Member

  11. DVV
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    DVV Junior Member

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  12. Will Gilmore
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    Will Gilmore Senior Member

    I was referring to the angle of the bow. Probably the wrong term to use, but you might consider giving it a little more angle to meet the waves with less direct opposing force.

    -Will (Dragonfly)
     
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  13. fallguy
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    fallguy Senior Member

    Another thing to note is none of the scow bows in the discussion I linked are hard chine. If you make a large radius there; it would give you less likelihood of stopping completely.

    I think it would be rather easy to strip build a radius section.

    Also, by making the scow narrower; doesn't that also drive it up?

    Just some thoughts on it...

    I could be wrong altogether..
     
  14. DVV
    Joined: Nov 2020
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    DVV Junior Member

    Yes, a radius would be better. But, I saw blogs about scows with radius bow, and I cant really do that. Way to difficult to realize for me! Check if you like the website of Yann Quinet Design (construire un voilier http://www.boat-et-koad.com/). He is circumnavigating on his 4mt scow Balushon (!)
    Yann https://www.facebook.com/100006919687129/videos/2537160923191206/

    All of this building is actually out of my skill. But I'll do it, one way or another. I have to. But at least I try to keep it as simple as possible.

    Today its holiday where I live, so I had time to design a little modification to try the idea suggested here, and in your post also in some way:

    I draw a little modification on the bow, in order to increase the raking (?) of the bow just in the lower part, actually adding a chine on the bow..
    I'm not quite sure about that. I've never seen anything like that, and this generally (always?) means its wrong. But may be there are designs like that and I just dont know.
    The fact is that adding a chine on the bow, doesnt it add a weak point? And increases the difficulty of the work.

    What do you think? Which one would you choose (and why, if you want)?
    I would keep it straight and if needed set a little more higher Schermata 2020-12-08 alle 16.24.49.png Schermata 2020-12-08 alle 18.03.07.png
     
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  15. fallguy
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    fallguy Senior Member

    That would be an improvement. I think it looks about 100 times better as well.

    But I still think you need to evaluate what happens in a chop.

    (the multichine)

    As far as strength, I don't see how it would be weaker if it is sheathed and tabbed on seams.

    Sure would be nice to have the waterline imposed on the above drawing or is it blue area?
     
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