Robbe top cat

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11 April 2019 at 10:35 #81834
Richard Caughey 1
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@richardcaughey1
I just acquired a couple of hulls for a Robbe toocat, lots of bits missing so I am going to have to get creative….

First issue are the rudders… do they need to tilt up? ( I don’t have the rudders so I am going to have to make some) I see in lots off pics the rddersillt, any particular reason for than?
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11 April 2019 at 10:35 #4577
Richard Caughey 1
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@richardcaughey1
11 April 2019 at 15:43 #81836
Ray Wood 3
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@raywood3
Hi Richard,

The fullsize cats rudder blades pivot up for running up the beach and launching in shallow water, but for sailing and your model they need to down vertical for best effect. I notice from the images of the model it has a central fin and bulb weight to keep it up the rightway

Regards Ray
13 April 2019 at 12:26 #81870
Richard Caughey 1
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@richardcaughey1
Mmm so I need to make a fin and bulb weight too . . . . .
14 April 2019 at 09:57 #81879
Malcolm Frary
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@malcolmfrary95515
Cats get their performance from being light, their stability from being very wide. When going as fast as possible, one of the hulls is usually just out of the water.

On the real thing, this is finely judging the edge of disaster. When it does happen, the on board crew have a brief chance to take action to prevent it. If they don't manage that, they are on hand to right it.

Model cats are not self righting and a descripive quote I heard was "It was going very fast before it fell over". A fin and a fine ballast bulb will probably help stability, but at the cost of performance. There is probably a reason why model cats are not a common sight, my guess is the strong likelyhood of the need for a rescue mission.
14 April 2019 at 11:33 #81881
Tim Rowe
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@timrowe83142
Another feature of fast multihull dynamics is that they are designed to dissipate excess energy of a gust of wind by accelerating. The light mass reduces the inertia . Clearly this is limited at some point by the maximum speed that is possible. Another curiosity is that at the high speeds being obtained, the apparent wind moves forward radically which means that they are never sailed dead downwind but is a series of tacks where the sails are sheeted well in.

I think model mono-hulls are better adapted to being scaled down and multihulls less so. If I were to build a catamaran model from scratch I would probably increase the beam at the expense of some manoeuvrability and use a lever arm type servo running at maximum permittable voltage to get the ability to ease the sheets very quickly. For belt and braces I would probably have a float at the top of the mast to prevent total inversion. I think Ray Wood is going to do that on the cat he is building.

Tim R
14 April 2019 at 11:48 #81883
Ray Wood 3
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@raywood3
Hi All,

The advantage the smaller racing cats have is a mobile crew for ballast, the Topcat looks similar to the Dart 16 which only has a small skeg, in Model form somebody produced a kit for a Musto skiff with a fin and movable weighted crew figure who could be swung out to windward, must have been fast with no bulb on the fin, but pretty tricky to get the boats balance when tacking / gybing, the latest generation of racing cats have dagger board foils.

Regards Ray
14 April 2019 at 12:42 #81884
Richard Caughey 1
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@richardcaughey1
I have been told by several of the clubs more experienced sailors that cats are a waste of time, but I haven’t seen any sail at my club. I like to be different so rather than stick with standard mono hulls I am going to give the cats and trimarans a go, I am happy to modify and tinker to try and improve the stability. Daggerboard foils sounds interesting.
14 April 2019 at 19:15 #81888
Ray Wood 3
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@raywood3
Hi Richard,

Yes that's all the motivation you need proving them wrong 😊 I'm building a cruising cat next a 1970's design Comanche 32 which Will have have twin fins & bulbs and a masthead float, as the full-size had, just in case !!

Regards Ray
8 May 2019 at 18:01 #82224
Richard Caughey 1
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@richardcaughey1
A bit late but, I had all the yacht guys were read for a laugh, and they were all disappointed the cat sailed very well, no falling over, no rescue boat all good.
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