Order of importance for hull efficiency

Discussion in 'Multihulls' started by Jeff in Boston, Sep 15, 2020.

  1. Jeff in Boston
    Joined: Sep 2020
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    Jeff in Boston Junior Member

    I want to see if I am understanding basic hull design correctly. I want to build a 20' to 22' cat for electric propulsion. I want to make sure I am understanding the order of importance for efficient design, but I want to make this a VERY easy to build cat.

    1) Make the hulls long and light - length /displacement.
    2) Make the hulls narrow - At least a beam to length of 1:8, 1:10 or 1:12 is better.
    3) Make the bow sharp to minimize bow wave. (The stern can be more square)
    4) Keep the hulls wide apart to minimize bow wave interaction.
    5) Make the bottom round to minimize surface drag.

    Is this about right?

    And for seaworthyness add some rocker and make the top of the hull wider than the bottom. To make this an easier built I'm thinking to skip the round bottom and making the top of the hull wider than the bottom and just go with a square cross section for speed of build. I am thinking to build around a high PSI XPS core for flotation and stiffness.

    While I really like the Richards woods designs, I'm looking for something faster to make. I am very time constrained.


  2. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Some random thoughts and questions -
    What speed do you need to achieve with this cat?
    Re 'seaworthiness', what is the worst sea condition that you anticipate you will be using the boat in? Will this be in Boston harbour?
    How much load will this cat have to be able to carry?
    What will she be used for? Fishing, general pottering about, passenger ferry...... ??
    Re your time constraint, how soon do you need to have this boat completed? It sounds like it was probably last week :)
    You can approximate a round bottom with a multi-chine construction.
    This will take longer to build than a square midship section with a single chine, but not much longer relatively, and the efficiency will be better.
  3. Jeff in Boston
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    Jeff in Boston Junior Member

    I want 6 MPH minimum. 10 MPH would be nice.

    Buzzards Bay or Boston harbour would be the worst conditions.

    For load 8 adults but usually 4. I figure for a 2' x 2' cross section 15 feet long (to account for taper) and a draft of 8" I'm good for 2500 lbs displacement so that seems possible.

    Mostly used for puttering around. Maybe some fishing. May camp on it. I need space for my wife to lie down and be comfortable which is why I'm aiming towards "efficient party boat".

    The time constraint isn't when it is done, but how many hours required.

    And yes, I'm looking at multi chine as an option. I will do a 1/2 or 1/3 scale of one of the hulls first to see what I am getting into no matter what I do.

    I'm also watching Craigslist for a hobie 21 or similar.
  4. Jeff in Boston
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    Jeff in Boston Junior Member

    Hmmmm... with a 5 sided hull (vertical sides, 45 degrees, flat bottom) I could build that pretty easily and could pre-calculate the angles.
  5. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    Constraints and relations between paramaters can change the order.

    Functional requirments including required space for occupants and equipment can set a minimum displacement.
    Length is frequently constrained.
    Decreasing displacement usually decreases resistance.
    Depending on the desired speed the optimum length may or may not be the maximum length allowed.
    The beam and length directly relate to waterplane area, and decreasing beam for a fixed length decreases waterplane area..
    Waterplane area is significant in several ways. General trends:
    Decreasing waterplane area increases sensitivity to loading.
    Decreasing waterplane area decreases stability.
    Decreasing waterplane area increases draft which may or may not be of concern.
    Frequently there is a beam limitation or strong incentive to keep beam below a certain amount (trailering limits, launch ramp/facility limits, slip width).
    Generally true if ability to plane is not needed.
    Round bottom vs bottom with chines may or may not affect construction costs dependingon construction method.
    Scuff likes this.
  6. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    Re electric propulsion, are you looking at single or twin O/B motors, or (probably a lot more expensive) inboard electric engines with shaft drives?

    For a maximum load of 8 passengers, that is approx 640 kg or 1,400 lbs - this would be way too much for a Hobie 21 or similar.

    Would something like an open version of the Eco 55 be suitable?
    ECO 55 Power Cat Plans https://www.duckworks.com/product-p/bk-eco55p-id.htm

    The Eco 55 might not be too keen on carrying 8 passengers though.

    Here is a nice article about the Eco 55 -
    Eco 5 Power Cat - Small Boats Magazine https://smallboatsmonthly.com/article/eco-cat/

    The Skoota 18 has a very simple hull form -
    Skoota 18 Power Cat Plans https://www.duckworks.com/product-p/woods-skoota18-id.htm

    Sailing Catamarans - Skoota 18 trailable 8ft wide daycruising or fishing http://sailingcatamarans.com/index.php/designs-2/6-powercats/489-skoota-18

    But they mention that the load displacement is 1,500 lbs, and the light displacement is 700 lbs, so 800 lbs carrying capacity, hence a maximum of 4 people plus equipment.
    An open version of the Skoota 20 would probably be better able to cope with occasionally having 8 people on board.
    Skoota 20 Power Cat Plans https://www.duckworks.com/product-p/woods-skoota20-id.htm

    Sailing Catamarans - Skoota 20 trailable with small cuddy http://sailingcatamarans.com/index.php/designs-2/6-powercats/262-skoota-20
    Jeff in Boston likes this.
  7. Russell Brown
    Joined: Jul 2012
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    Russell Brown Senior Member

    While it seems like you understand hull shapes well, I would think a set of beach cat hulls would be the best place to start. Tornado hulls are my favorite and maybe you can find a set.
    Jeff in Boston likes this.
  8. philSweet
    Joined: May 2008
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    philSweet Senior Member

    Jeff, I hate to say this, but you have everything either wrong or backwards.

    1. Electric is a very expensive, premium, heavy, and complicated way to propel a boat. So in order to mitigate those issues, an electric boat hull must have a very highly developed form to even barely succeed. For an 8 passenger vessel, it will cost more than your house, and will take a few pros the better part of a year to design and build. So forget about electric beyond trolling motors if you want to build a simple hull. For a cheap simple hull, you want a cheap, simple outboard motor.

    2. XPS foam rates somewhere between varnished paper mache and waxed corrugated cardboard as a building material. It might last more than one use, but probably not much more. Marine foams are pricey, somewhat difficult to work with, require a good bit of expertise, have high waste factors, and tend to be a slow way to build. For simple and cheap in this size, go with painted marine grade plywood.

    3. In this size, catamarans are heavier and far more complicated than monohulls. They will cost a lot more to build, take much longer to build, and never be as versatile as a monohull. For a given load, the monohull will be faster or easier to push in the size you are looking at. The catamaran will have a larger deck space only if it is much, much heavier that the mono because the boat has to be stable with all the people anywhere on the deck. This means small, light cats have to have very small decks.

    4. For a basic 8-pax monohull, you are looking at about a 1000 pound hull - which means buying about 1200 pounds of materials at around $15 - per pound if done on the cheap. And you definitely want to build to a set of plans, otherwise the completed boat that you spent $18k in materials on will be worth $0 when completed. Build a 22' plywood skiff. It is the most efficient boat for the size and speed and cost and time constrains you described.

    5. For comparison, a 22 foot pontoon deck boat with with a 1600 pound pax capacity listed a 3000 pound "dry weight" and a 4959 pound "average package weight". You would probably have a hard time duplicating it under 6000 pounds. That's about $90,000 in materials, or twice what you can buy the factory boat for. A Duffy 22' electric launch sells for $55k and has a top speed of 6.5 mph. Duffy has been doing this for a long time and you would have difficulty duplicating the Duffy for under $80k. This is why we love little skiffs with outboards. I purchased a 21 footer - boat, motor and trailer, for $100. It took a whole afternoon to get the trailer fixed up and the outboard running, then I was good to go. I ended up with about $400 into it.
    fpjeepy05, jehardiman and bajansailor like this.
  9. Jeff in Boston
    Joined: Sep 2020
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    Jeff in Boston Junior Member

  10. Jeff in Boston
    Joined: Sep 2020
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    Location: Boston

    Jeff in Boston Junior Member

    https://www.epropulsion.com/ prices aren't that high. And every summer they are likely to be even lower.

    That said, I've gotten enough pushback from more experienced boaters on this that I am looking other options, like a shroud around a 5 or 10 hp outboard engine to quiet it down.

    I was thinking that I would use XPS only in compression, like on a roof deck which it was designed for, not in a standard glass / foam / glass sandwich that depends on shear resistance.

    Painted marine grade plywood is likely to be my main material.

    But doesn't an open skiff have the same problem of stability? Or does it just have so much more buoyancy that it doesn't matter? Of course I could add small outriggers to a skiff.

    Here, what you are saying makes sense. A plywood skiff would be slower for the same motor..... but it would be faster to make, cheaper, easier to trailer, easier to buy used, and easier to sell used. And fit under the house without disassembly.

    I'm setting up craigslist searches now for a skiff. I may still build it myself, but I think you are right. And a cheaper boat can always get a more expensive motor later on.

  11. bajansailor
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    bajansailor Marine Surveyor

    Re complexity of build, I doubt that you would find another power cat of similar size that is easier to build than a Skoota 20' or 24' - I think that Richard has worked very hard to try to keep these boats as simple as possible.

    They are designed to achieve a faster speed than what you are intending to do, but that is no bad thing really, as there is then always the possibility of going faster later on with different engine(s).

    Edit - sorry, for some reason your last post above only 'showed up' after I posted this note.

  12. redreuben
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    redreuben redreuben

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