Hi, first post, wanna build a monstrosity....

Discussion in 'Boat Design' started by parkland, Jul 15, 2012.

  1. parkland
    Joined: Jul 2012
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    parkland Senior Member

    I got my aluminum quote back from a friend,

    All 1/4 inch

    Plate is 5052-h 32 and are 4' by 10'
    Those are 280.80 a sheet

    1/4" by 3" inch flats are 1.81/ foot
    Those are 6061

    Does this seem like the right material for a boat this size?

    I can find thinner and thicker, I have to assumme this would be considered kinda beefy for a low speed cruiser.
  2. parkland
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    parkland Senior Member

    Does it make better sense to weld the hull first, then weld the frame inside it, or weld the frame up, then weld the hull over it?

    I also read that minimal welding should be done between the frame and hull, as it causes bad waviness in the hull.
  3. Tad
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    Tad Boat Designer

    Monstrosity eh?.....we can do that.......:eek:

    Extrusions should be 6061, for plate the 5052 is useable but 5083 or 5086 is better, though harder to come by. That's an attractive price on plate, just about $2 / lb, typically 5086 is close to $3/lb out here.

    Real boatbuilding would include widely spaced transverse web frames with light flat-bar longitudinals notched through them, typically the skin is skip welded only to the longitudinals.

    It sounds like you are building a big box? I'd build it right-side up. Lay out your bottom plates and tack them together. Then tack down the longitudinals, something like 1.5" by 1/4" FB, spaced 16" max. Then lay down the bottom transverse webs, roughly 5" by 2" by 3/8" angle and notch over the longitudinals. Just tack everything until the box is all together. Webs should be spaced 36" to 48", no more or less, if some are bulkheads all the better.

    Set up your side webs (can be smaller than the bottoms, say 4" by 2") vertical and tack the longitudinals in, then hang the side plates.

    Then build the ends and the cabin sole/deck. Then start the continuous welding of the skin first working from midships out to the ends and up from centerline. After all skin welding is done go back and weld the longitudinals to the plate after breaking the tacks to the webs. Finally weld the webs to the longitudinals and perhaps to the plate.

    good luck.......
  4. Frosty

    Frosty Previous Member

    If I was building a steel boat I would consider the above to be extremely useful information.
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    FAST FRED Senior Member

    Build a barge of ply or aluminum.

    Use a used trailer camper as living space , yank most of the frame and road gear to lower weight.

    About 30hp will move the boat at 5-7K so a used 50 hp outboard should have a good service life.

    Inland it should be great fun and Quick to build , could weigh less than you think!
  6. parkland
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    parkland Senior Member

    Wow, Thank you so much Tad.
    And frosty, I assume you mean "steel OR aluminum" ?

    Fast fred; I had tossed around ideas involving hacking up a camper into a boat, but there were many small ande large issues that came up.
    I also considered building a gaint fold out catamaran barge with a ramp on the front, to winch a camper on to.
    It could work, but a lot of BS stands in the way of going for a drive without multiple guys helping getting everything going.
  7. parkland
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    parkland Senior Member

    Also, I went and looked at a few boats, and I found one that is 1/4" aluminum hull, and around 8' wide, and it didn't have barely any frame to it at all, and was rock solid.
    Although it had a nice rounded shape to it, so I imagine thats where a lot of it's strength came from.

    After looking at it and kicking it, any doubts I had about strength are gone. 1/4" aluminum is a lot stronger than I thought.
  8. parkland
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    parkland Senior Member

    Transmssions I'm thinking might work?:


    Engines i'm thinking of ? :

    Are marine transmissions designed to take the forces from the props, or is that done with thrust bearings in the shaft tube?
    Speaking of that, are all propellor shafts and tubes custom made, or are there some generic ones too?
    I can't find anything to do with propellor shafts.

    Also, how much of a benefit is there to running a slower, bigger prop rather than a small faster one? fuel exonomy?

    How does one find out what type of prop to use?
  9. parkland
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    parkland Senior Member

  10. parkland
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    parkland Senior Member

    Data Input
    Waterline length in feet: 36 feet
    Beam at the waterline in feet: 8.5 feet
    Hull draft in feet (excluding keel): .5 feet
    Vessel weight in pounds: 12000 lbs
    Engine Horsepower: 60 HP
    Number of engines: 2
    Total Engine Horsepower: 120 HP

    Engine R.P.M. (max): 2600 RPM
    Gear Ratio: 2:1
    Shaft R.P.M. (max): 1300 RPM

    Number of shaft bearings (per shaft): 2
    Desired speed in Knots: 15 knots
    Horsepower Calculations
    This will calculate the maximum horsepower and torque available at the prop(s).

    Total available horsepower at the engine(s): 120 HP
    Total available torque ft/lbs at the engine(s): 242 ft/lbs
    Horsepower loss of 3% per gearbox: - 3.6 HP
    Horsepower loss of 1.5% per shaft bearing: - 3.6 HP

    Total horsepower available at the propeller(s): 112.8 HP
    Total torque ft/lbs available at the propeller(s): 456 ft/lbs
    Speed & Power Calculations
    Basic displacement speed and horsepower required
    Displacement hull speed (1.34 X sqrt of waterline length): 8.04 Knots
    Minimum horsepower required at propeller(s) for Hull speed: 23.8 HP

    Calculations based on desired speed and available HP
    HP required at propeller(s) for desired 15 knots speed: 155 HP
    Estimated speed with existing 120 horsepower:
    This is the speed we will use for the propeller size. 13.46 Knots

    At this point it is important to note that all of the calculations above are based on full RPM and HP. Most engines are rated to run at a percentage of thier full RPM. This is what will determine your maximum cruising speed. The propeller sizing calculations below are based on 90% of full RPM, which allows the engine to develop it's maximum power without overloading. The chart below shows typical engine ratings, you can find this information in your engine specifications.

    Recomended RPM for continuous operation
    Type of engine % of max RPM
    Light-duty gasoline and diesel automotive conversions 70 - 80%
    Light-duty or high output marine diesels 80 - 85%
    Intermittent-duty marine diesels 88 - 92%
    Continuous-duty heavy marine diesels 98 - 100%
    Propeller Size
    Number of blades Diameter (inches) Pitch (inches)
    2 Blade 20.2 X 17.8
    3 Blade 19.2 X 17.6
    4 Blade 18.0 X 17.3

    The propeller sizes shown above do not contain calculations for cavitation or blade loading.
    If you find that the recommended propeller is too large to fit your vessel, you can try increasing the shaft speed. Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch.
  11. Luckless
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    Luckless Senior Member

    I strongly suggest stepping back from the project and taking a look at what you are actually trying to do.

    Before working on prices and materials, start working on a detailed list of what you actually Want out of the project. Diving in with such a vague goal is a surefire way to produce a product that is neither cost effective, nor one that you will actually enjoy at the end of the day.

    So, go back and write yourself an intended usage document. Basically write yourself a short little story about a weekend of using the boat. How does it handle, what tasks does it achieve.

    After you have that not only firmly in your head, but clearly on paper, you can begin to pick it apart for design elements.
  12. parkland
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    parkland Senior Member

    I have thought about this general size and design for a long time now, although a lot of the technical fabrication I am not so skilled with.
    The goal of the boat is to have something of a good size, still trailer-able, tough enough to not have to baby it, easy maintenance, and not cost a million dollars.

    I have some hull shapes made from styrofoam, which I should upload tomorrow, but basically the hull is 8'6" wide, and 4 ft tall, perfectly square and cubed the whole length, except for the bow and stern.
    My plan for the hull was to use roughly a 90 deg V shape, angle down at about 45 deg, then about 2 ft down, increase the angle much flatter so the entire bow should ride into and onto the waves without cutting much into them. Basically, the bow shape would much resemble a V nose front seater fishing boat type shape, but with angles instead of curves.

    The cabin will extend 48" higher, giving a cross section dimension of roughly 8' x 8' at the cabin. Considering the bulk of the weight will be in the bottom of the hull, I expect that in the event the boat ever tipped over, it might be inclined to go back right side up, provided not too much water has entered the cabin, aft deck, etc. (I planned on covering the front deck in.)

    I'm thinking I want roughly 36 ft of length, with about 22 ft of cabin room. Exact placement, I haven't decided yet, because I still need to fully decide on the engine placement. I hope to have engines placed immediately behind the cabin, on the rear deck, with safety cages installed to prevent injury. In this area, batteries, fuel, and engine equipment would all be located.. protected from the elements somewhat, but not enclosed.
    The water and sewage tanks will go in front of the cabin, in the enclosed bow.
    I am aware that there is a type of marine grade door that you can get that is designed to hold back some water? This is what I want for a door exiting the cabin to the rear deck. The bow will be blocked off with a "bulkhead" style divider, so if it gets ruptured, the water won't fill the rest of the boat. I also want that fancy door, so that if the middle gets ruptured, the rear won't fill with water. ..... and 3 electric bilge pumps.

    Non powered hydraulic rudders.....

    2 4cyl diesel engines, 60 - 100 hp

    10 - 15 knots wanted

    I expect this thing would drive much like a barge, yet not build up so much water in front of the bow like barges do with those single angle hulls.
    I imagine it will not have much hull under the water line, because of the flat bottom design. I think it might really blow around with some decent wind, I'm ready to accept that, it is a trade off for having so much floor space in a small boat.

    I realise engine noise might be louder than some people would want, oh well.
    I am hoping it will cruise around without burning much fuel, but for the size and shape of it, again some of the economy is traded for a brick shaped boat with a lot of usable room.

    I will try to sketch up some drawings, and take pictures of the styrofoam hull, but I don't THINK theres anything anyone can say to talk me out of it, but I will be all ears. :p
  13. Red Dwarf
    Joined: Jun 2012
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    Red Dwarf Senior Member

    One thing you need to include is the trailer. It won't be cheap, small or light. A trailer that can carry a 12,000 lb boat will weigh almost as much as the boat. How are you going to tow 23,000 lbs? (12,000+10,000+1,000)
  14. BobBill
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    BobBill Senior Member

    Seems the idea is the safety of others, not getting by...but it appears you understand that.

    Big project. Seems a better idea to find a used boat, or small ship and refit.

  15. Frosty

    Frosty Previous Member

    I know Americans dont do kilos or meters but don't you do tons either.

    12,000 lbs whats that in ounces.

    After lbs is a stone and then cwt and tons., Its a smaller number but weighs the same.

    Come on just for me,--- do kilos please its easy 1000 kilos is a metric tone.

    If you can get the the moon you can do kilos.
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