36' x 9' hull design

Discussion in 'Boat Design' started by oytunsan, Aug 25, 2022.

  1. oytunsan
    Joined: Aug 2022
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    oytunsan New Member

    Hi, I am trying to learn about boat design and I did some reading and wanted to ask a theoretical question. I am not going to build this boat tomorrow (or any time soon). I am just trying to check if my expectations from boat design in general is feasible.

    Assume a boat that is 36' x 9' , 12000-16000lbs full load displacement, Cruise speed 12-15 knot with efficient fuel consumption. Max speed is not too important but low 20s are preferable.

    Can a feasible hull be possible for this specs providing acceptable stability and sea-keeping in Mediterranean environment. If the answer is yes which type of hull should I begin learning and experimenting.

    Thanks everyone
     
  2. HJS
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    HJS Member

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  3. TANSL
    Joined: Sep 2011
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    TANSL Senior Member

    Welcome to the forum.
    Since you leave the choice of draft free, you don't specify the necessary power and you give a very wide range of speeds, in principle it is possible to make something like a boat. But, if you don't give more information about what you want to do with it, in what conditions you want to sail, what materials you want to use, etc... no one in this world can reasonably tell you if "that thing" you have thought can be transformed into a real boat.
     
  4. oytunsan
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    oytunsan New Member

    TANSL, thanks for the reply. First step in my mind was to understand feasibility. I will try to give more details.

    I want to be able to cruise with it in Mediterranean conformability in mild to moderate seas, uncomfortably in rough seas until I can go to safety.
    I am ok with a big draft, more space for tanks etc.
    Materials would be plywood, epoxy fiber glass.

    My understanding is a boat designed to go at pure displacement speed at that size wont go more than 8 knots. I want a little bit more than that like 12-15 would be perfect for cruising and 20 max but I can live with a less maximum speed.
    I am ok with a large engine but at 10-12 knot range I want to have decent range and fuel economy like maybe 15-20 liters/hour.

    I am currently looking at sharpies of Tad Robert's designs which I really loved. Yellow cedar 38 has a very good look, kinda near to my desired length beam ratio and does 7.5 knots with 21HP ! But I guess it wont go 12-15 even I put a 200hp engine since it is displacement ?

    Yellow Cedar, 38' Classic Motor Yacht. ~ Power Boat Designs by Tad Roberts https://www.tadroberts.ca/services/new-design/power/yellowcedar38
     
  5. bajansailor
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    bajansailor Marine Surveyor

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  6. fallguy
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    fallguy Senior Member

    Most experienced boaters would not enjoy sea cruising on a vessel with such narrow beam. At 9', you are too big for the road and are probably looking at a slip. Most slips 36' long are wider than 9', so I'd like to better understand the l/b desire. If it is for fuel economy; the tradeoff may not be desirable.

    For cruising comfort, more beam, man!

    The second you get abeam, that boat is gonna roll like a pencil spinning in water. So, it won't be that good in moderate seas.

    Those narrow beam boats are generally hurry to and fro commuters or get to fishing grounds and back boats.

    Need to understand the sor better; if you have some space constraint, etc.

    Also, for cruising, 3' more beam would make a much more spacious boat.
     
  7. TANSL
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    TANSL Senior Member

    Those shapes are suitable for speed, which is always contrary to comfort. Although it has little initial stability, the freeboard is very high, so the stability at large angles can be very good. Being a speed hull, you can't claim to have a lot of headroom either unless you're a cargo or pleasure boat, which doesn't seem to be the case. A cargo boat would be very different from this one and a pleasure boat, to sunbathe, would need more free spaces. It all depends on the ship's SOR. Without it, it is risky to comment.
     
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  8. fallguy
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    fallguy Senior Member

    this is a boat designed to GO TO places, not cruise along at 10 knots with 270 hp....

    not a ding against Martin!

    but it bodes the question on how the sor requires narrow
     
  9. bajansailor
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    bajansailor Marine Surveyor

    If I maybe be allowed to copy @Tad Facebook post on the 21st August, where he said this :

    "Beam is overrated, at least when it comes to seakeeping. Mollymawk is narrow by today's standard, with a beam/length ratio of .25 vs the usual .35 or higher. Yet the boat is not in the least tender, rolling to lower angles and noticeably slower and damping out more quickly than a wider and somewhat lighter similar boat. This is a function of waterplane, hull shape, and centre of gravity. A lot of it will be down to carrying a good load down low, so the centre of gravity is low. She also has a full length external keel to damp out motion, and wider than usual horizontal chine flats. The narrower waterplane just follows surface waves slightly less than a wider and faster reacting (due to lighter weight) hull. So it's not one feature that significantly alters motion, rather a overall concept that bends multiple features toward a single aim. The tricky part is making these seakeeping requirements work with all the other requirements any boat must fulfil. And finding owner's who see value in the undertaking....."

    Tad also noted this on the 15th August :
    "Mollymawk, my 32' x 8' aluminum outboard cruiser is surprising some folks....”Where's the hump?”, there isn't one, at least not anything discernible so far, but I'll try to get some closer numbers later this week. The owner's report she runs flat from idle to full speed, which was the aim of the exercise, so I'm happy."

    And that her usual cruising speed is 'in the upper teens, but she will go a lot faster'.
     
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  10. fallguy
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    fallguy Senior Member

    a great bit, but people right away think I want standing headroom and bait tanks and things can go awry quickly, but a really good add to the thread, a keel certainly changes roll
     
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  11. Alan Cattelliot
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    Alan Cattelliot Senior Member

    Hi oytunsan,

    a motorboat with your basic characteristics would be indeed in the market.
    upload_2022-8-30_11-32-18.png
    An hull of this kind would sail most likely in displacement mode under 16-17 kts, and would be preferably made in GRP or Plywood. The 9' beam seems a little narrow for the boat to get to planning mode without exploding the fuel consumption. Also, accomodations would be kind of spartiate. A regular beam@chine would be more like 12' to 14', giving a wider space.
    A cruising speed of 8kts to 9kts is likely to be achievable for monohulls. If you want a cruising speed higher, I would suggest to evaluate a concurrent multihull design of your craft, regarding your concern about fuel consumption.

    If you're ok to lower your cruising speed, you can still consider this kind of design, although I agree with Fallguy that its use in the short mediteranean seas can be quite rocky.
    https://nantucket-rangeboat.com/range-boat/

    upload_2022-8-30_19-32-4.png
     
    Last edited: Aug 30, 2022
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