AeroCat power cats, opinions?

Discussion in 'Powerboats' started by Krauthammer, Dec 6, 2011.

  1. Krauthammer
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    Krauthammer Junior Member


    Please elaborate. The F1 tunnels seem to perform at the highest levels of efficiency with singles in the tunnel.

    A Morelli and Melvin designed 22' cat, still unsurpassed in its qualities, did extremely well with a single OB before market trends motivated Corsair Marine to build it in a twin configuration. In the process the boat lost efficiency while adding cost.
     
  2. Alik
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    Alik Senior Member

    The difference between F1 (split hull) and average cruising cat (usually with symmetrical hulls) is huge and principles are different. We designed and built both symmetrical and asymmetrical cats; for the first one wash in tunnel is significant and any protrusion (such as engine leg) will effect performance. For asymmetrical, there is no spray or waves in tunnel so engine in CL will work, but split hull is intended for higher speeds.

    Hmm... I never heard about that 'unsurpassed' cat :)
     
  3. Krauthammer
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    Krauthammer Junior Member

    Here is a transom photo of it.


    Corsair.JPG
     
  4. Mr Efficiency
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    Mr Efficiency Senior Member

    As Alik says, the environment inside a 'split hull' tunnel is very different to a symmetrical-sponson cat, with the amount of disturbed, aerated water in the latter necessitating a more deeply immersed drive leg, to avoid ventilation. In conditions where the boat is inclined to leave the water periodically, unless the prop is running deep you will get cavitation/ventilation drama. That said, there are examples of acceptably successful singles, usually with specialist props, but the concensus seems to be it is be it doesn't work as good across the full range of conditions.
     
  5. Alik
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    Alik Senior Member

    This is split hull; as I said it is suitable for single outboard, but there are disadvantages of use of such shape on small cat. Stability is lower compared with symmetrical; driving is more complicated.

    I don't see anything special in design here; we launched few cats of similar shapes.
     
  6. Krauthammer
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    Krauthammer Junior Member


    Interesting. If I understand correctly the twins behind a symmetrical sponson cat need to run deeper while a single center mounted leg on a split hull tunnel can run higher?

    If so the efficiencies of the single should be obvious, single and lesser frontal surface appendage and possibly the ability to run surfacing propellers. From all this one could deduce that a split hull tunnel with a single OB mounted on a jack plate would be the most economical and efficient multihull. Or ... ?
     
  7. Alik
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    We tested this layout on RC model and later on full size prototypes. Yes, outboard is higher and it catches air especially on following wave. By moving engine down advantage of reduced appendage drag is lost. Consider that engine leg also intersects free surface and generates wave/spray resistance as additional to common outboard resistance components.

    Another problem with higher engine position installed at CL is directional control and turning ability. The boat tends to broach at following waves due to reduced appendage area. The possibility to turn boat by reversing one engine is also lost. Also at sharp turns aeration of propeller occurs.
     
  8. Krauthammer
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    Krauthammer Junior Member

    It is still one appendage vs. two.

    This would be countered by lowering the jack plate and possibly further enhanced with various trim devices.

    No doubt that single screw craft require operator skill and there is no free lunch anywhere but the cost benefits in rigging and operating a single OB are very hard to ignore. No?

    Are you dismissing split hull tunnels in general even though it was stated above that they have the advantage of allowing clean water conditions in the tunnel which results in the single OB ability?
     
  9. Alik
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    Alik Senior Member

    In this case, the resistance of ONE appendage could be higher compare with TWO appendages. If engine leg is deeper and intersects free surface, the resistance is higher...

    Split hulls are optimum for at higher speeds. Most of cruising cat users tend to cruise at slower speeds that FnV<4.0, this is just more fuel efficient and provides comfortable ride. So symmetrical shapes are the way to go...
     
  10. Mr Efficiency
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    Mr Efficiency Senior Member

    The adoption rate of split-hull tunnels (trailerable size, anyway) among hard-nosed offshore users is low, I can only deduce there are problems that can't be made to go away. I'm guessing unpredictable handling traits in stern quartering situations might be part of it. As for the symmetrical sponson vessel with a central single engine, they seem to have been refined to the point of tolerability, but the worst of the problems with aeration or cavitation arrive at the most inconvenient time, when things have got a bit rough. If you google Shark Cat 500 ( later called Noosacat 1800), you can read about user experiences. Use of 5-bladed props was one recommendation that seemed to help. Also google Shark Cat 700 diesel sterndrive to learn about how a specially extended Volvo Penta Duoprop leg to plant the prop deep in clean water was a solution of sorts, albeit a slow one (cruise around 20 knots). Presumably using a higher powered petrol engine mated to that leg would have encountered a serious loss of performance at higher speeds with all that extra leg drag.
     
  11. powerabout
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    powerabout Senior Member

    I agree, a single engine cat is a dog
    just works on small light boat in a lake
    or an F1 powerboat ( small light boat on a lake)
     
  12. Mr Efficiency
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    Cats tend to get a bit more airborne than monos when heading in to waves offshore, to the point where aerating the props on a twin engined boat is not too difficult, so how much easier for that to occur with a single rig mounted half way down the depth of the tunnel. The fuel efficiency and cost attractiveness of a single is what keeps people trying to make it work, seemingly with limited success.
     
  13. Krauthammer
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    Krauthammer Junior Member

    The UIM Class I offshore racing cats are split hulls.

    It appears that as long the design is correct the split hulls work fine in any shape and size and any kind of conditions while poorly designed twin hull cats rigged with twins -e.g. the Glacier Bay 22'- suffer badly in quartering stern seas:


    http://www.hookedonfishingboats.com/glacierbay22.html


    It looks like it is all reduced to qualified design and poorly designed boats will not perform regardless of the basic design category that they emulate.
     
  14. Alik
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    Alik Senior Member

    So show us 'qualified design' :D

    Split hull cats are asymmetrical by nature that effects their behaviour. Qualified design is approach to choose right hull for right purpose and speed, not to justify any hull shape for single engine installation.
     

  15. Krauthammer
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    Krauthammer Junior Member


    Just being polite; "qualified" as in seriously educated naval architects with a full portfolio of proven successful designs.

    Having researched the field extensively I was surprised to see how few of the currently marketed cats in the US come from a "qualified" drawing board. For instance the company of the OP draws its designs from an individual without any credentials whatsoever.

    Anybody can choose, Larry Graff of Glacier Bay "chose" the design of his 22 ft and as we see in post 43 the results were disastrous.
     
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