You heard it here first: Low Profile "Hidden" outboards coming soon.

Discussion in 'Boat Design' started by Squidly-Diddly, Jun 25, 2013.

  1. 805gregg
    Joined: Apr 2010
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    805gregg Junior Member

    I know where you can stick that outboard
  2. tom kane
    Joined: Nov 2003
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    Location: Hamilton.New Zealand.

    tom kane Senior Member

    One of the very first outboard motors had its cylinder head under the water for cooling and nothing much more above water and secured over the transom. Got a picture somewhere.
  3. whitepointer23

    whitepointer23 Previous Member

    outboards are by far the most efficient marine engine available and the new versions are that quiet you don't realize they are running at idle sometimes. a genset and pod drive might okay for very low speeds .
  4. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    Points taken, but all the objections also hold just as well for an IO plus mechanical complexity that can be serious$$. True that a custom prop used on a shaft drive where diameter is not such a constraint is more propulsive efficient. How many shaft drive props have you seen that were designed for maximum efficiency without constraint in diameter? I have only seen one or two and they were home made. However, a shaft drive inboard carries a lot of baggage in other obvious areas that an outboard avoids.

    I wonder if a reduction gear could be included in the outboard powerhead where size would not create such drag as the lower unit gearbox does. I suppose the loading on the lower unit drive would be higher but prop diameter and RPM would be as open as you please. I use the high thrust versions which are better but do have larger lower gearboxes that make more drag than their fast turning counterparts.
  5. Jeremy Harris
    Joined: Jun 2009
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    Jeremy Harris Senior Member

    Unfortunately that's just not correct at all. Outboards are a compromise, and some of the trade-offs make them less efficient than other forms of propulsion. For example, as I've already mentioned, the need to keep the lower leg profile slim, to minimise drag, results in the reduction ratio being a bit on the low (numerically) side. The result is that the majority of outboards use smaller diameter multi blade props, spinning at high rpm, with wide chord blades to enable them, to absorb the power. This results in poorer efficiency, in many cases well below 50%.

    They also compromise in terms of fuel economy, as to get the power needed from a relatively light weight engine they end up increasing the rpm, which both increases frictional losses and also decreases engine SFC.

    You can get more efficient outboards, for example the ones with the larger lower leg, increased reduction ratio and larger diameter, narrower chord, props can get pretty close to the efficiency of a decent inboard.

    I'm not knocking outboards here at all, just pointing out that one of the compromises we accept for having a self-contained, boat-independent, lightweight and powerful combined propulsion and steering unit is poorer overall efficiency.
  6. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    JH you would be right if your talking displacement speeds but outboards are not designed for that hence wrong configuration for that
  7. Jeremy Harris
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    Jeremy Harris Senior Member

    Prop inefficiency is prop inefficiency, regardless of boat speed. The fact is that many (most?) outboards have props that are of too small a diameter, turning too fast and with blades that have too wide a chord for best efficiency. It's a compromise forced on outboards because of the need to keep the lower unit cross section as slim as possible, to reduce drag at high speeds.
  8. DCockey
    Joined: Oct 2009
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    DCockey Senior Member

    The constraint on lower case size may be the diameter of the bevel gears, which in turn would be governed by the output torque to the prop shaft. Even with a gear reduction in the powerhead the output torque to the prop shaft would be the same for a given overall ratio between engine and prop. Hence the gear reduction in the powerhead would not reduce the size of the output side bevel gear.
  9. powerabout
    Joined: Nov 2007
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    powerabout Senior Member

    yes you want nothing in the water but large torque at the propshaft, hard to do.
    Towing a skier an outboard and an inboard does the same job what would the most efficient prop look like for the inboard if you could ( you can) choose the gearbox ratio?
    When you say efficient do you mean fuel economy or bollard pull?
  10. Jeremy Harris
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    Jeremy Harris Senior Member

    In my book efficiency is the ratio of input power to the prop to output power from the prop. In the case of a boat prop that's the ratio of the shaft input power (rpm x torque) to the prop propulsive power that's moving the boat.

    Propulsive efficiency will be zero at zero boat speed (there is no power output from the prop at zero speed, just a force, because no useful work is being done), will increase as speed increases to the props most efficient blade loading condition, then will decrease at speeds above the most efficient working point.
  11. daiquiri
    Joined: May 2004
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    Location: Italy (Garda Lake) and Croatia (Istria)

    daiquiri Engineering and Design

    Now that I think of it - I have never, ever seen an outboard engine manufacturer publicize a data about the efficiency of their props.
    Which makes me think that there is a good reason for that... ;)
  12. Squidly-Diddly
    Joined: Sep 2007
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    Squidly-Diddly Senior Member

    Manie B, I think "light" and "cheap" are relative. Compared to 30yrs ago, sure.

    But I see debates over the wisdom of putting large outboards on MacGregor and similar sailboats because it means carrying so much mass up high, and it is standard safety advice that a runabout with decent sized outboard has stability issues I/O boats don't.

    Serviceable boat hulls are 10 cents a ton all day long where I live in SF bay area, but it is cost of even a used motor that keeps people off the water. But I'm not expecting my low design to be any cheaper, probably be 50% more expensive.

    Imagine how much nicer this MacGregor would be if instead of what you see the motor unit (as seen from cockpit or when boarding) was just an extension approx flush with cockpit floor sticking back about 18" and about 24" wide, with ladder rungs on the sides.
  13. tom28571
    Joined: Dec 2001
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    tom28571 Senior Member

    I'm not an expert on gearing of props but I don't think that is all true. If the lower unit ratio is 1:1, the diameter of the gear case should be no larger than that necessary to take the torque on the gear teeth. Granted, that torque would be higher than the input shaft of a normal outboard gearcase. Is that wrong?
  14. Manie B
    Joined: Sep 2006
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    Manie B Senior Member

    Interesting points made here.
    The set up that I would love to experiment with is the "generator and electric motor" setup.
    Big ships use pods and generators.
    I would love to see how efficient this could be made when very small and light - my quest would be the under 10hp range. Torqueedos are beyond my budget so I would stick to the biggest Minnkota and a small boat. Speeds below 6 knots.

    The reason for this setup is this - there is nothing like a misty morning on the water with no wind and the water is like glass. That first cup of coffee at 2 knots is a joy that very few people will experience. I have done that with my 30 lbs. Minnkota for about 30/45 minutes at a time, but that is not enough. My 30 lbs. Minnkota is a toy. My 2x 105A batteries are not enough and I don't have a generator. By the time the sun is up the batteries are down enough anyway.

    A generator can be made super silent and placed anywhere in the boat that pleases you. A Torqueedo electric drive is very light on the transom.

    So that's my 2 cents where I would like to see the future ;)

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    Agree in spades, although there are a few areas yet to be penetrated by CAD let alone CAD/CAM. Use of CAD/CAM doesn't guarantee the product will work, but, used competently it ensures that it can be assembled, and it saves time and money. Because if its complexity and non-cartesian nature my last project before retiring would have required several prototypes without CAD/CAM. It went together first time and worked as intended, ten tonnes of servo-controlled robot with dozens of actuators of every type, electric, hydraulic and pneumatic with humans within the work envelope.
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