Weight limits for outboards?

Discussion in 'Boat Design' started by milo12, Sep 5, 2015.

  1. milo12
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    milo12 Junior Member

    I have been searching trying to find recommended maximum boat weight for an outboard. Something like x pounds per Hp would be great.

    All I can find are references to capacity plates and safe weight limits for a certain size boat etc.

    For example, lets assume dual 300Hp Evinrude G2 outboards on a 12 tonne power catamaran, long thin hulls at WL-very low drag. Is that feasible? Is it stupid and inefficient? What problems do you see? I'm not concerned about the gas vs diesel debate.

    I ran some checks on a prop sizing page and given the rpm and gear reduction and desired speed the outboard props are OK. Is that all I need to check?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Define long and thin, and you may get some meaningful answers.
     
  3. milo12
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    milo12 Junior Member

    L/b = 12
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    What waterline length ?
     
  5. philSweet
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    philSweet Senior Member

    Weight is the wrong metric to use. There isn't a maximum weight. Well there sort of is, for practical reasons to do with the way mfgs have chosen to set them up - It works out that less than about 2 hp per ton will cause real problems for OBs because you can't match a prop to the power head and reduction ratio. Even for the commercial, deep ratio setups, 5 hp per ton seems to be about as low as the OB mfgs want to provide efficient designs for.

    Typical snorkle tour cats of 50', are fitted with twin 60-90hp OBs.

    The hull shape determines what the resistance of the boat will be at 12 tons. You look at the resistance as a function of speed and decide where you want to be able to operate. Then you add fudge factors for sea conditions, prop fouling, hull fouling, etc.

    If you want a 12 ton hull that can use twin 300s, a 12:1 cat is not what comes to mind.

    If you are trying to push the limits, either on the high speed or low power end, you need to get professional help for powering and setup. You need to have an actual boat, or several pages of plans. And you need a very good statement of your requirements and operating environment.

    A 12:1 12 ton cat isn't going to plane, but it can be forced to a pretty good speed. Still, I doubt that anything over twin 100's would be of any use. Those 300s can only get the power down at speeds well above what you will be going.

    So, are you looking for a boat to match a pair of 300s, or are you looking to power a skinny 12 ton cat?
     
  6. gonzo
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    gonzo Senior Member

    You could have a rather flat bottom and almost parallel sides on a long skinny hull to make it plane. 300HP will be about 40lb per HP, so it is probably adequate for planing.
     
  7. milo12
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    milo12 Junior Member

    Planing not really a factor. I just want top speed of 20 knots.

    Sounds like the concept is not impossible and worth further research.
     
  8. Ike
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    Ike Senior Member

    If you are talking about planing boats there is a relationship between weight and horsepower but there are lots of other factors that are just as important such as length, beam, wetted area, whether it is a flat bottom or a vee hull and how much of a vee, and so on. What you are getting at is how much HP it takes to move the boat beyond displacement hull speed and overcome resistance so it will lift and plane. But as stated by others there is no direct HP to weight ratio that is the magic number.
     
  9. Ilan Voyager
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    Ilan Voyager Senior Member

    20 knots is a speed very easy to get on a multi. And you do net need 600 HP except for a very big boat. A good motor trimaran 21 meters 8 tons full displacement gets 26 knots with 240 HP. A F40 of 1987 length 12.18 meters weight in cruising configuration about 2.6 tons, more than 18 knots with 2X75 HP.
    You have not great advantage to go over a ratio 10/1 for a fast displacement mode catamaran.
    To slim is an inconvenient to get the volume of displacement without having too much draft and wetted surface. You get also a bad shape with the water climbing on the sides of the hulls creating a big drag by suction mainly at the stern. The other inconvenient of too slim hulls is they have a tendency of horse hobbing and with too much surface of hull to get enough volume.
    A common recipe for a correct motor catamaran hull is 8 to 10/1 ratio, a CP of 0.65-0.68, Center of immersed volume at 65-68% of the length, max flotation width at 55% of the length, width of transom= 80-85 % of the max width, angle of entry 17 degrees (34 degrees at total) that's rather "pointy", 90 degrees of angle of keel at 30 % of the LWL, 135-145 at 55%, 165-180 at the stern. That gives a hull with very little rocker. And a good chine to seat the hull on his stern without immersing when accelerating, and for keeping the spray out of the boat all long of the hull. All these ciphers are crude and purely indicative, just for educational purposes.
    In common words a fine entry, and a big big "cul" with enough volumes over the water to get a decent damping of the pitching (by similarity with the planes is called the phugoid, the good one is when a disturbance is damped is about 3 cycles with minimal vertical accelerations).
    Such a hull, when correctly designed, is a bit wet, has little variation of pitch angle, does not sink at the stern at speed because of the spray chine, pass well the waves without being a submarine. It hates to be overloaded. The draft must be kept at about 50-60% of the flotation width of the hull.
    It's not the best a very low speed, nor at high speed but the hump is very smooth and the global efficiency excellent.
     
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  10. Mr Efficiency
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    Mr Efficiency Senior Member

    Those big outboard are really optimized for a speed of 30 knots+, they are not really suited to the task of pushing a displacement cat around, which realistically would be best suited to a speed of 20 knots tops. It would be hard to find a large displacement cat with 2x300 hp outboards anywhere, but there would be planing cats with them.
     
  11. milo12
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    milo12 Junior Member

    Yes you are correct.

    I said 300hp because the same block and weight outboard is 200-300hp. I think the 200hp makes more sense and saves $10 grand.

    I can't go below 200hp because no one offers joystick controls with a smaller motor.
     
  12. milo12
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    milo12 Junior Member

    Thank you for the excellent post, lots of stuff to think about.
     
  13. Squidly-Diddly
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    Squidly-Diddly Senior Member

    Mercury "Bigfoot" outboards are purpose designed to push bigger, slower boats, using lower gearing and bigger props, but IIRC the biggest is 60hp.

    Sales pitch seems to be for pontoons and houseboats. I'd think they would be the obvious choice for any sailboat.
     
  14. Ilan Voyager
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    Ilan Voyager Senior Member

    You're welcome. But better is to give the job to a Naval Architect...
    To give a first idea a 40 feet catamaran displacement mode of about 5 metric tons gets 22 to 25 knots with 2X 130 KP. A trimaran 40 feet, about 5 tons fully loaded with an optimized design for low speeds, gets 10-11 knots with 2X27 HP. We are very far from 400 to 500 HP. For a 30 feet 2X90 or 100 HP are largely enough.

    So 2X 200 HP it's for a big boat. On a displacement 20 knots cata needing 2X200 HP to get 20-22 knots, there are far better alternatives than costly high tech outboards, with delicate variable valve timings, and other marketing bells and whistles useless on a 20 knots boat. It's around 14-15 meters boy. That's huge and expensive.

    Displacement cats are killers of outboards hanging on the transom. These poor engines end drowned. The reason is simply that a slim hull with correct hydrodynamics is not made for 300 kg on the transom, so the NA has to go to heavy work to put the outboards on lateral pods, when it's so simple to put the engines well protected inside the hulls, at the right place for the centre of gravity. Add fresh water cooling and you get 10-12% better efficiency, and long term survival of the engine. Add properly sized propellers and you another 10-15% efficiency gain.
    An humble 4.3L 6 cylinders with its simple technology of GM truck gives 220 ponies, it works and it's cheaper to buy and maintain than a big outboard.
    OK it's not as phallic as the "big pack **** stallion" look of a big outboard. it's one of my daughters, industrial designer, who explained that to me, along with the shapes of a lot of sport boats.
    It's the first time in my life I hear that a joystick is the main argument for buying an engine...I'm definitely too old.
    Can you give us your program? sport? cruising? size? budget? zone of navigation? Because I'm afraid that the thread is totally sterile for the moment.
     

  15. Mr Efficiency
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    Mr Efficiency Senior Member

    I don't know what is going on here, if it is a matter of looking for a good home for two 300 hp outboards already at hand, a displacement cat is likely out of the question, although it might have a ghost of a chance if they are 30" shaft, otherwise completely hopeless. But the boat of suitable size would be so expensive, these engines would be small % of the cost.
     
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