22' Planing Catamaran Design

Discussion in 'Boat Design' started by Alexander Peter Bromley, May 15, 2020.

  1. Alik
    Joined: Jul 2003
    Posts: 2,800
    Likes: 184, Points: 73, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Important is: the bow should not dive so the outboard engine is not getting aerated. This can case loss of control at following seas. This effect is pronounced on smaller cats, thus we need volume at bow.
    We did a lot of tests on 6 and 8m cats, also with outboard engines height. The outcome is: with narrow bows, engine should sit lower.

    Here one can see bottom shape of our 6m design
    [​IMG]
     
    brendan gardam likes this.
  2. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,377
    Likes: 450, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Are you thinking of the engine on the high side, when stern quartering ?
     
  3. Alik
    Joined: Jul 2003
    Posts: 2,800
    Likes: 184, Points: 73, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    This can be avoided by steering. But in general, if engines are high, and bow volume is small, the boat tends to broach.
     
  4. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,377
    Likes: 450, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    There's not much freedom of engine height, regardless, but cats that are not lightweight, still don't broach, there is just too much boat in the water aft (x2) for them to do that. The matter of weight is important, too light and they are a dynamically unstable horror show, especially if powered for speed, and worse still if the lightness is combined with "fat" hulls.
     
    brendan gardam and Ad Hoc like this.
  5. Alexander Peter Bromley
    Joined: Apr 2020
    Posts: 16
    Likes: 2, Points: 3
    Location: South Africa

    Alexander Peter Bromley Junior Member

    Hi Guys thanks for the really good insights, so I have carefully read the comments and watched the shark cat video which was really interesting.

    On the topic of full bows I tend to agree with Bruce Harris and you guys on the full bow being good for running with a following sea. But my hulls were still a little too full so have done some shaping just to reduce the volume just a little bit and turned the angle to add a bit of bow flare.

    I have been on a friends cat, 5.4m and it rode really nicely all except for running with a following sea. It felt as if there wasn't enough lift or buoyancy up front to keep the bow proud so it felt like it was digging into the back of the swells, although it showed no sign of broaching.

    I'm still quite surprised as the topic of LWL/BWL ratio doesn't pop up neither much on tunnel height, and further ratios between the chinebeam of a demihull to tunnel width ratio. Or maybe it doesn't come up much because its the designers secret to success.
     
  6. Alexander Peter Bromley
    Joined: Apr 2020
    Posts: 16
    Likes: 2, Points: 3
    Location: South Africa

    Alexander Peter Bromley Junior Member

    Interesting handling from the test model. These were the only with broaches the rest the model handled well. What do you guys think?



     
  7. brendan gardam
    Joined: Feb 2020
    Posts: 70
    Likes: 3, Points: 8
    Location: east gippsland australia

    brendan gardam Junior Member

    . cats can do this too.
     
  8. Alexander Peter Bromley
    Joined: Apr 2020
    Posts: 16
    Likes: 2, Points: 3
    Location: South Africa

    Alexander Peter Bromley Junior Member


    Hahahah yes that's actually just up the coast from where I live!
     
  9. brendan gardam
    Joined: Feb 2020
    Posts: 70
    Likes: 3, Points: 8
    Location: east gippsland australia

    brendan gardam Junior Member

    certainly a different way to go fishing, i bet they go through a few water pump impellers.
     
  10. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,377
    Likes: 450, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

  11. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,377
    Likes: 450, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

  12. Jimboat
    Joined: Feb 2002
    Posts: 247
    Likes: 15, Points: 18, Legacy Rep: 130
    Location: Canada

    Jimboat Senior Member

    Alexander, check here: Aeromarine Research
     
  13. Alik
    Joined: Jul 2003
    Posts: 2,800
    Likes: 184, Points: 73, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    These pool tests are useless, because You evidently pull the bow up.
     
  14. Mr Efficiency
    Joined: Oct 2010
    Posts: 8,377
    Likes: 450, Points: 83, Legacy Rep: 702
    Location: Australia

    Mr Efficiency Senior Member

    Not quite the line of thrust, unless it has been lassoed by a UFO :eek:
     
    Alik likes this.

  15. Alexander Peter Bromley
    Joined: Apr 2020
    Posts: 16
    Likes: 2, Points: 3
    Location: South Africa

    Alexander Peter Bromley Junior Member

    To me I was hoping the videos would show some vague resemblance of how it will run, unfortunately line of thrust is definitely not ideal, I was hoping it could resemble the motors trimming the bow up, which is generally how planing cats like to run.

    Thanks Mr E, those dimensions from the storm cat help a million, lets hope it was a good riding boat!
    Cobra cat.jpg

    Something similar to this, a South African style cat proven to handle surf launching.
     
    bajansailor likes this.
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.