Waller 1480 vs Dix 470 vs some other ideas / semi swath

Discussion in 'Boat Design' started by Daniel Mazurkiewicz, Feb 2, 2022.

  1. Daniel Mazurkiewicz
    Joined: Jan 2022
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    Daniel Mazurkiewicz Junior Member

    Hey,

    Previously been looking for plans for aluminum cat, but after buying study plans realized that for a boat of 13-15m size will need 12000-16000 kg of aluminum, and just that would eat my entire budget for boat (current prices of Al sheets 8.5-11USD/kg). Then talked to a nav architect claiming that cat of that size would be more like 4000-6000kg of Al. So that is my first confusion. Does Bruce Roberts plans use thicker than necessary Al sheets??? Or his design isn't the smartest one?

    But in the mean time started to look for some alternatives, and looks like plywood is way cheaper than aluminum. At least based on this material list: Dix 470 material list https://www.dixdesign.com/dix470mat.htm. Then again, mentioned nav architect claimed that plywood materials would cost more less same as Al materials. ???

    Then we come to design plans, found couple, but only these were within a my budget for plans and looking reasonable:
    1800$ - https://www.mikewalleryachts.com/waller-1480-catamaran
    5500$ - Dix 470 Plywood Cruising catamaran https://dixdesign.com/47cat.htm

    Do you know any other plans that I should consider? And if to pick from above, which one you would pick and why? (keep in mind - I want to make solar powered cat, no sails or so)

    And lastly, been thinking also of my own design and came with the idea of semi swath, here is a quick sketch of it: https://ibb.co/QDGK9bx

    I call it semi swath because hulls wouldn't be fully immersed and would act as normal hulls most of the time only higher waves of the water could flow over it. Each of hull when 14m long should have displacement of 5000-7000kg with none of other dimensions exceeding 1m. That should make it way easier to assemble. And the deck part could be assembled on the floor level which should be easier than construction at height. Also feels to me (but feel free to correct me) that entire construction could be lighter as the deck part doesn't have to withstand same water forces as hulls and only hulls would have to be made from thicker materials.

    What do you think of it?
     
  2. Stefano Dilena
    Joined: May 2021
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    Stefano Dilena Junior Member

    A Mumby 48 catamaran only has 4.3 tonnes of aluminium, and is a performance cruiser/ liveaboard cat.
    From my modest research, BR cats are heavy and more of motor - sailers than pure sailers
     
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  3. Flotation
    Joined: Jan 2020
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    Flotation Junior Member

    I your previous posts you explained you want to build an ocean going solar-electric catamaran. With all respect, material costs of the hulls for such a project will make up such a small percentage of the total final costs it is irrelevant to focus that much on them in this part of the design stage.
     
  4. Daniel Mazurkiewicz
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    Daniel Mazurkiewicz Junior Member

    Well, there is a difference when you're to spent 40 thousand dollars on materials or 140 thousand dollars on materials for similar size boat. At least to me. And agree, when we're close to 40k then all I'm left is to worry about design. And that seem the case of plywood cats I've mentioned above.
     
  5. Flotation
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    Flotation Junior Member

    If i had to build an ocean going solar powered boat on a tight budget i'd be looking at second hand boats with rig/engine damage or old ocean racers without an interior, engine and with worn out sails. And determine if the boats i'm looking at realistically have the displacement needed to carry the panels/engine/batteries.
     
  6. Daniel Mazurkiewicz
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    Daniel Mazurkiewicz Junior Member

    Oh, that leads me to a question I didn't think of before - what should be a safe margin for displacement?
     
  7. TANSL
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    TANSL Senior Member

    Actually, the variable studied is the minimum freeboard of the boat, which, for a catamaran of about 14 m, could be over half a meter, although this is a figure that must be qualified.
    Probably that boat will not be required to comply with the International Load Lines regulations, so that figure would have to be determined after studying the conditions that, in terms of stability, the boat must meet.
     
  8. waterbear
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    waterbear Junior Member

    Have you heard of the Sun21 catamaran? It's a 14m catamaran that was first to cross the Atlantic under solar power 15 years ago. Early on they estimated they would average 10-12 km/h, but I think in reality they only averaged 4km/h from Spain to Martinique. Sun21 is probably similar in displacement/drag to the boats you mentioned, so I would imagine you would see similar performance with a similar sized solar array.

    sketch_09.jpg
     
    Last edited: Feb 9, 2022
  9. Daniel Mazurkiewicz
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    Daniel Mazurkiewicz Junior Member

    Found some real pics:
    TRANSATLANTIC SUN 21 FIRST SOLAR POWERED ATLANTIC CROSSING ATTEMPT OCTOBER 2006 http://www.solarnavigator.net/transatlantic_21.htm

    Thanks, that is a good reference. That boat looks pretty lightweight. I have some ideas how to gain more space for solar panels. But even with twice as power from it I expect it won't be performing much better than sun21 and that is fine with me.
     
  10. Daniel Mazurkiewicz
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    Daniel Mazurkiewicz Junior Member

    Checked that convention and it says:
    So I guess we're good :)
     
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  11. waterbear
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    waterbear Junior Member

    Hi Daniel, the Sun21 is actually not very lightweight, I think I saw somewhere it is listed at 12T. In addition to the weight, you can also see the hulls are very wide, which means there is a lot of surface area which is the primary cause of drag at these low speeds.

    If you want better performance, you need keep the underwater surface area of the hulls down. The easiest way to do this is to keep the boat very light: a lighter boat displaces less water and has less underwater surface area. A 24ft rowing scull (racing rowboat) with only one person rowing can go 20km/h because the boat is very light, the hull is very fine (narrow) and the wetted surface area is kept to a minimum.

    You don't want to build a semi swath boat because the low buoyancy of the hulls makes the boat less stable at sea and less safe. The swath hulls also have more surface area relative to displacement and will be slower. Even a semi swath hull has more surface area as you are piercing a wave. If swath was efficient it would be used in sailboat and rowboat racing.

    Plywood is a good material. It's cheap, it's easy to work with, and a well-designed plywood boat can be almost as light as an expensive composite boat at a small fraction of the cost. This is assuming you are using good quality gaboon/okoume marine plywood, not cheap exterior ply which weighs about 50% more.

    With all this in mind, I recommend you choose a lightweight plywood sailboat as the basis for your project. Richard Woods, who is a talented multihull designer, has two designs called Meander and Rhea that are 12M and only weigh 3.5T. Something like this will be 1/3 to 1/2 the weight of the Sun21 and as a side benefit will cost much less to build because you only need to buy a fraction of the material. You will want to build in plywood with the round strip planked bottom for minimum wetted surface. Richard may also have recommendations on further reducing weight, such as reducing the hull thickness.

    Meander 40' performance cruising catamaran by Woods Designs (sailingcatamarans.com)
    Rhea 40 cruiser with central cuddy by Woods Designs (sailingcatamarans.com)

    Richard woods no longer lists designs that are longer than 40' presumably because he thinks life is too short to build boats that large. He will also advise you against building a solar boat. The links above are from his old website. He will probably still sell you the plans, I think they were 1300-1500GBP.

    You will also need some help designing the drivetrain. You will need a belt reduction for your motors, and you will need to make a custom propeller (not hard to do). Normal props are only 40%-50% efficient and are not optimized for efficiency. You can build a prop that is more like 100% efficient, but it will have some drawbacks like poor weed shedding.

    Please note I am not a boat designer so some of my advice may be wrong! Also, as you suspected, you should avoid designs by Bruce Roberts.
     
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  12. waterbear
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    waterbear Junior Member

    And here are the specs for the Sun21

    LOA 14m
    BOA 6.60m
    Solar peak output 10 kW (that's 36 280W panels)
    Solar area 65 m²
    Lightship 11t or 12t (depending on source of information)
    Loaded displacement 14t or 15t
    Engines 2 x 8 kW (2 x 10hp)
    Battery 50 kWh lead acid

    So again, Sun21 is a pretty heavy boat. They would have had about 1.1t of lead acid batteries, plus maybe another 0.8t of of solar panels on the roof, plus motors, which means roughly 2t of propulsion equipment. Had they chosen to build something like Woods' Meander they would have cut their weight in half (3.5t + 2t = 5.5t with panels and batteries vs 11-12t), which would have had a very strong effect on performance.

    Again, reducing weight not only allows you to have less wetted surface area, it also allows a more slender hull which will have lower wave making drag. As a guess, I would say cutting the weight in half could result in a 50% higher cruising speed, although someone more knowledgeable may have a better estimate.
     
    Last edited: Feb 3, 2022
  13. Daniel Mazurkiewicz
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    Daniel Mazurkiewicz Junior Member

    Thanks for these, adding them to my considerations.
    Dunno how durable are these motors (they claim 10k hours), but I would go for something like that:
    Maytech - AliExpress


    Unfortunately these are exotic here, will need to rely on local plywood sources so pine is probably one of not many options I will have. But as was looking for materials found something interesting, check it out:
    Plywood with aluminium insert – SKLEJKA-MULTI S.A. https://www.sklejka.pl/en/products/plywood-with-aluminium-insert/


    Really liked the idea of simplified build - hulls separately and everything within a reach of hand, main deck separately and on the floor. And it should be way simpler/cheaper to design. Hard to find something similar, but would take this as an example ;-) https://seawings.ae/storage/app/uploads/public/5cb/bd3/32e/5cbbd332e880d203065856.jpg

    And technically wanted it to act as swath only in really rough conditions, main deck then could also act as a 3rd hull. So performance in rough conditions wouldn't matter that much actually.

    My estimates were also to reserve in displacement about 1900kg for solar panels, batteries, electric motors, fridges, laundry machine, oven, and induction panel. So that is starting point for displacement for me. Then I'm a bit torn between installing an electricity generator or some vertical windmills. Getting windmills would let me definitely save on fuel weight for generator, provide additional power even during the nights, but it will add some air drag

    Thanks for sharing your thoughts! Definitely not going for Bruce Roberts, out of my budget.
     
  14. Rumars
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    Rumars Senior Member

    Weight depends on several factors, from the NA's skill to the actual enclosed volume of the boat (same lenght does not mean it's the same size boat on the inside). There is also a difference between material you buy and material that goes in the boat. But the conclusion is: only use a designer you trust, otherwise it makes no sense to pay for his work.
    I for example look at Roberts Euro Cat 1300 and see a published displacement of 13585kg. If the designer told me I need 12000kg of Al for the boat, I conclude that he either has very poor material optimization (50% waste), or his displacement figure is wrong. One simple solution is to have the designer provide you with references to actually buildt boats of the same or similar design, then ask the builders/owners about real numbers.

    To be able to compare prices between materials, you need to look at the whole picture and make a price per square meter to finished state. Al construction is not only the Al plate, it begins with the paint on the outside, then skin, stringers, frames, insulation, ceiling (the inside skin wich you see and touch). Then there is cutting, welding, grinding, etc. all cost something. Plywood is similar, only a different materials mix.
    Location is also very important since prices can vary significantly, even for the same brand/manufacturer.

    Plywood construction is full of variables, but it always starts with the ply quality, and that depends on the factory QA. Then we have wood species differences, and that impacts more then weight. Softwood ply needs fiberglass encapsulation, hardwood only epoxy. This impacts epoxy consumption, not only for glassing but for fairing (glassed surfaces need more fairing).
    Your options are limited, import certified hardwood ply (probably expensive), use birch ply with epoxy only (heavy, even more so then sapele), use fiberglassed pine ply (almost as heavy as birch after glass, more epoxy needed). Suitable domestic wood for stringers, beams, etc. are larch, black locust, oak, mulberry, and with certain precautions, ash.

    Other systems for you to consider are sheated strip plank using spruce or fir, cold molding with veneer, cored and solid fiberglass with vinylester/polyester.
    I would take a hard look at sheated strip and cold molding. Wood is so cheap that you can afford the best quality, the price difference to ply will probably cover the increased epoxy consumption, and there will be absolutely no doubts about the quality.
     
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  15. waterbear
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    waterbear Junior Member

    Re: sizing the motor, you first need to answer the million dollar question: how much power is typically available from the solar panels?

    Let's say you have a large 10kW array like Sun21. Using the PVwatts calculator, we can enter the array size and location, and it will calculate the output based on historical weather. As an example, I entered Odessa (Ukraine), 10kW for the size of the array, 0 degrees for the tilt and got these results:

    January 267 kWh
    February 420 kWh
    March 753 kWh
    April 985 kWh
    June 1,445 kWh
    July 1,417 kWh
    August 1,216 kWh
    September 917 kWh
    October 582 kWh
    November 323 kWh
    December 215 kWh

    This is the power that would be produced by sun21 per month if it were docked in Odessa. In June, for example, it would be 48kwH (1,445/30) per day. 48kWh x (1 horsepower/750 watts) = 64 "horsepower hours." That's 64 horsepower for one hour, or 6.4 horsepower for 10 hours (2.4 kW per side).

    In December, the situation is much worse with just 7 kWh per day, or 9.25 horsepower hours, assuming you do not use any lights or appliances. If you have two motors running at 1hp that's just 4 hours of runtime. This means in the winter you will need to use strategies like moving with the tides, staying in one place while charging, charging from a dock, and creeping at low power.

    Now, The Lagoon 42 hybrid uses 72V 8kW continuous motors, which you can see here:
    lagoonmotor.png

    Let's assume Lagoon are smarter than us and 8kW is "enough" motor for a 42 foot catamaran. We can get larger motors, but large motors have lower efficiency at lower power, and this will hurt our range. We could get smaller motors, but that might get us into trouble, say, with a lee shore. So maybe shoot for an 8kW continuous motor with good low speed efficiency.

    With all of this in mind, the $500 Motenergy ME0709 might be a good candidate. Like the lagoon it's 72V and can put out 9kW continuously, and 21kW peak for 1 minute. More importantly, the efficiency is above 80% from 400W to 3700W. In the winter and spring when power is scarce the motor will be very efficient. In the summer when there is excess power you can accept lower efficiency if you want to go faster. These motors are based on the briggs and stratton Etek RT and use technology originally licensed from Lynch. They are well regarded, and replacements should be available for many years.
    motenergy.png

    With the maytech motor we don't know the efficiency below 2520W. More importantly the continuous power rating is not stated, only the peak power rating of 10.5kW. it might be suitable, but the information is not all there.
    maytech.png


    Also, if you are inclined, you can draw your catamaran in freeship, then export it to michlet to calculate the drag and estimate the performance. This way you can see how much the weight of the boat and the fineness of the hulls affect performance. This of course will not tell you anything about windage, which can be a big problem for a low powered boat.
     
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