Barge efficiency-displacement speed

Discussion in 'Hydrodynamics and Aerodynamics' started by jbo_c, Mar 30, 2020.

  1. jbo_c
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    jbo_c Junior Member

    I’m looking to build a small river cruiser for two. 20’x7’(ish). The plan is basically to do a barge with a small inside space for sleeping only.

    Essentially, my question is planing hull or not. I really don’t plan to ever use above hull speed, but obviously, sometimes plans aren’t as permanent as we expect.

    So, more specifically, if I do a planing hull, how much inefficiency will I really see at hull speeds? I’m not talking about trying to get up to 8-10 knots, but at 4-5 knots. The idea I’m noodling is that I can always push a planer at hull speed, but I can never push a displacement boat to plane.

    This boat will likely have a 10-15hp motor, so planing won’t even be an option if I wanted to. I realize probably 3-5 would hit hull speed, but I want remote start, so I think 10-15 is as low as I can go.

    Thanks.

    Jbo
     
  2. jbo_c
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    jbo_c Junior Member

    OK. Reading another thread and realized the term “barge” caused some consternation as it has a specific connotation and I was using it in a more general sense.

    Please consider the word “barge” in my title/initial post to mean platform.

    Jbo
     
  3. fallguy
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    fallguy Senior Member

    You must define the planned use of the barge and total payloads engines, fuel, passengers, gear, superstructure.

    Most planing hulls are horribly inefficient below the hump. So, an ideal hull design would be semi-displacement if you want some speed.

    Once you define the payloads; you can get an idea of the hull needs.

    The length of the craft is more determinate of the hull speed. So, you are a bit out of order defining the length. You need to define payloads thoroughly and then define desired speeds and this will dictate the hull; not the other way around.
     
  4. jbo_c
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    jbo_c Junior Member

    Hmm. Thanks for trying to help. I must be a terrible communicator. Felt like I answered essentially all of that in my first post.

    My question, though, is not about speed and payload as you seem to think. I understand hull speed. My question is, on a boat this size How much efficiency loss will I realistically see?

    This is for recreation, not business, so if I spend 5% more in fuel to push a planing form at hull speed than I would if it were designed for displacement, maybe that means I use 7 gallons of fuel instead of 6 over a weekend trip. - Not a lot of difference in the overall scheme of things.

    But to try to be more specific to the questions you asked since you’re trying to help.
    1) Use - river cruising/sightseeing overnights at low(displacement) speeds
    2) Payload - 2 adults and weekend gear call it 700 pounds, likely less
    3) Superstructure - low height “cabin” 7 feet long x 4.5 feet tall
    4) Cruising weight including boat/motor <2000 pounds
    5) Power - outboard, likely, but not necessarily 10-15 horse(and, yes, I realize that won’t push the boat to plane, that sort of goes to the original question)

    Thanks, again for the reply.

    Jbo
     
  5. fallguy
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    fallguy Senior Member

  6. jbo_c
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    jbo_c Junior Member

    Yes. Although that is much more boat even than I’m thinking. (And I desire a “boater” looking boat) If I can get a picture to post, I’m thinking more on these lines, but for power only no sail.

    hesperia boat article - Google Search


    The question remains though, since I’m building it now and may later decide I’d like to go faster, is it worth building it as a planing hull and living with the inefficiency in my planned use?

    Thanks immensely for taking the time to thoughtfully respond.

    Jbo
     
  7. jbo_c
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    jbo_c Junior Member

  8. fallguy
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    fallguy Senior Member

    read the payloads and realize the cabin is optional
     
  9. fallguy
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    fallguy Senior Member

    To your question. No.

    You will not be pleased with the way the hull performs below hump speed. It isn't just about fuel economy, but also handling ease. The last thing you want is to fight the tiller nonstop cruising slowly down the river. Steerage, for example, is also more difficult running planing hulls at low speeds. In fact, it can be a real pita.

    A planing hull sits low in the stern when not on plane. This is known as the hole. This is because the boat is not level inside and riding bows high. So, you would also have unpleasant riding beneath hump speeds.
     
  10. fallguy
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    fallguy Senior Member

    Here is a prime example of a planing hull deep in the hole running slow. He might have actually just hit the throttle which drives the bow even higher than just going slow, but the point remains.

    You won't experience this with all hulls; mind you, but certain hulls perform very poorly slow.
    29E64138-3E93-48F0-8DD7-7B17834F6C33.jpeg

    Other hulls do better at slower speeds; pontoons, for example.
     
  11. fallguy
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    fallguy Senior Member

    The Hesperia is a modified garvey styled hull.

    Quite a great blog on it, but a bit small for the Inside Passage for me.
     
  12. fallguy
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    fallguy Senior Member

    Here is an 18' Garvey.

    I believe the designer is Evan Gatehouse, but it could be Jacques Mertens.

    It might be possible to build her 10% up and just enough for a sleeping house forward; although that changes the navigation station, so perhaps a wheel and aft cabins would be better.. Any cabin would be built in foam; ultralight to avoid the hull getting dangerous. You could run with a 25 quite nicely.

    Garvey 18 (GF18) - Study Plans https://bateau.com/studyplans/GF18_study.php?prod=GF18.
     
  13. fallguy
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    fallguy Senior Member

    It must be noted the Hesperia certainly has a modifed garvey hull with some vee somewhere or he would probably not be taking it to sea. Garvey hulls are generally not seagoing vessels.
     
  14. jbo_c
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    jbo_c Junior Member

    Thanks for all the time you’ve spent formulating responses. I meant to ask about steerage as I knew that might be a challenge with a planing hull at slower speeds too.

    I am enamored with Hesperia and would surely build it if plans were available. I’d prefer the slight v of the Garvey style hull, but I think in practice for my use, it will make little if any difference.

    I think you’ve convinced me to stick with a displacement hull as my intent is truly to go slow.

    Thanks again for your time.

    Jbo
     

  15. DCockey
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    DCockey Senior Member

    The behavior fallguy discussed above is typical of planing hulls with high deadrise at the transom. Planing hulls with small deadrise at the transom usually behave considerably differently. They tend to have considerably lower drag at non-planing speeds than hulls with higher deadrise. The transistion with the low deadrise hulls tends to be much less severe with little or no "hole". The tradeoff is lower deadrise hulls will have a rougher ride at higher speeds.

    The Bluejacket designs by Tom Lathrop are examples of boats with gentle transition from displacement to planing speeds. Bluejacket Boats – at mildred's cove boatshop http://bluejacketboats.com/
     
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