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Old 18-05-2020, 17:46   #1
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Options for keeping extremely low draft on catamaran

Am in the process of designing a catamaran and would like to keep the draft as small as possible while still maintaining good rudder control. No appendages on the hulls, like a wharram or hobie 16. My concern is with the rudders. I'd like them not protrude deeper than the hulls, meaning about 9 or 10". From what I gather, low aspect ratio rudders are not very efficient, for the same reason that low aspect ratio sails aren't. Then I get to thinking about maybe having 4 smaller rudders, but that sounds like too many moving parts. I could have breakaway rudders that are deeper, but that brings increased complexity which I hope to avoid.



Any ideas, suggestions, thoughts?
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Old 18-05-2020, 18:31   #2
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pirate Re: Options for keeping extremely low draft on catamaran

Well the Wharram ones on the Tiki 21 work just fine..
Just got to shape em right.. mind, they do have a skeg to the same depth as the keel.
Maybe a hint on the hull shape would help, round or vee.
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Old 18-05-2020, 18:43   #3
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Re: Options for keeping extremely low draft on catamaran

Of issue with rudders that do not protrude deep is that they can be dewatered by wave action and lose influence. And yes that leaves very little torque surface so not a very responsive boat and also very little in the way of leeway resistance, especially if you don't have much of a keel to aid in that regard.

It is simple to rig a kick up rudder that runs deep and just swivels aft upon impacting the bottom. They are not breakaway rudders. Reference loads of images of designs:

https://www.google.com/search?source...w=1920&bih=842
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Old 19-05-2020, 13:21   #4
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Re: Options for keeping extremely low draft on catamaran

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Originally Posted by boatman61 View Post
Well the Wharram ones on the Tiki 21 work just fine..
Just got to shape em right.. mind, they do have a skeg to the same depth as the keel.
Maybe a hint on the hull shape would help, round or vee.
It would be a 60 degree vee hull.
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Old 19-05-2020, 13:24   #5
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Re: Options for keeping extremely low draft on catamaran

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Of issue with rudders that do not protrude deep is that they can be dewatered by wave action and lose influence. And yes that leaves very little torque surface so not a very responsive boat and also very little in the way of leeway resistance, especially if you don't have much of a keel to aid in that regard.

It is simple to rig a kick up rudder that runs deep and just swivels aft upon impacting the bottom. They are not breakaway rudders. Reference loads of images of designs:

https://www.google.com/search?source...w=1920&bih=842
Good points. I had meant kick up rudders. Will look more into them.
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Old 19-05-2020, 14:33   #6
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Re: Options for keeping extremely low draft on catamaran

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Good points. I had meant kick up rudders. Will look more into them.
Both of my smallest and my middle sized sailboats have swing keels and swing rudders. I keep the pin out of the heavy iron swing keel when it is down and I am in shallowish waters that way its 6 foot depth becomes a depth sounder of sorts; if it tags the bottom it just shifts aft and slants upward on its hinge axle and then I just raise it with the winch in the salon at the companion way to let the boat become unstuck. The rudder goes to a depth of about 4 feet and it too will raise up if it is struck and I haven't secured it in it down position. When they are both up I only draft 21 inches, great for beaching and for ease of loading and unloading on a trailer. Both the swing rudder and the swing keel are of high aspect ratio and NACA shaped for providing lift. The smallest of my three boats is a Hobie Tandem Island Adventurer Sailyak, think a kayak with twin amas to form a trimaran. The swing keel / and the rudder both are simply levered down with handle on the keel and a strong cord on the keel. Neither of those are NACA shaped, just simple straight high profile ratio daggers.
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Old 19-05-2020, 16:32   #7
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Re: Options for keeping extremely low draft on catamaran

It would help if we knew the size and intended use of cat the OP is thinking about designing. The parameters for a Hobie-like day sailer are very different to those of arger cat with accommodation facilities of any sort..
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