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  #31  
Old 05-14-2015, 07:37 AM
Boatguy30 Boatguy30 is offline
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So any ideas on how much to taper the hulls?
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  #32  
Old 05-14-2015, 04:10 PM
Manfred.pech Manfred.pech is offline
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Originally Posted by Boatguy30 View Post
So any ideas on how much to taper the hulls?
Why taper the hulls if you want a motor for the catamaran ??? If you absolutely want to taper, google for the puddlecat 8 of Roger Mann. http://rogermann.org/puddlecat/designs/puddlecat-89/



https://www.youtube.com/watch?v=Pa21...E1E5D8&index=2

Better and simpler to construct without rocker or taper:

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  #33  
Old 05-14-2015, 04:49 PM
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waikikin waikikin is offline
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So any ideas on how much to taper the hulls?
I built an 8' alu cat dinghy, I tapered/gave some rocker aft for rowing..... regreted it as compromised motoring performance/squat & still rowed like a pig/barge with load.... good exercise- let the transoms suck a little or put the extra cat material into a longer mono, maybe nesting with a center pram that stands/works alone & bow & stern extensions...

Jeff.
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  #34  
Old 05-14-2015, 07:12 PM
Boatguy30 Boatguy30 is offline
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don't laugh!

Here is a pic of my test model I started late this afternoon. I went with a good bit of taper aft. transoms are about 75% as wide as max beam. my only intention is to test this a few laps in my canal. If it floats!
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Catamaran dinghy 10-11' long-win_20150512_210544.jpg  
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  #35  
Old 05-14-2015, 07:19 PM
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waikikin waikikin is offline
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Quote:
Originally Posted by Boatguy30 View Post
Here is a pic of my test model I started late this afternoon. I went with a good bit of taper aft. transoms are about 75% as wide as max beam. my only intention is to test this a few laps in my canal. If it floats!
Cool stuff BG,

Just thinking, have you put some thwart into your sitting/rowing posi/ergonomic.

Jeff
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  #36  
Old 05-14-2015, 07:28 PM
Boatguy30 Boatguy30 is offline
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I was thinking a flybridge like all the new sailing cats!

But seriously, I don't intend to row it but will carry some paddles. The actual dinghy will have quite a bit more freeboard and maybe no be proportionally as wide overall if not needed for stability. Maybe will use some swivel seats common in small fishing boats.
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  #37  
Old 05-14-2015, 10:39 PM
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waikikin waikikin is offline
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A flybridge is a great idea, you get some shade underneath & an easy step over from dinghy to deck of the cat from up top, you could incorperate a buoyant roof structure to restrict inversion also, and solar panels, electric drive, maybe a bbq, rod holders, esky........
J
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  #38  
Old 05-15-2015, 01:16 AM
Manfred.pech Manfred.pech is offline
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Originally Posted by Boatguy30 View Post
Here is a pic of my test model I started late this afternoon. I went with a good bit of taper aft. transoms are about 75% as wide as max beam. my only intention is to test this a few laps in my canal. If it floats!
Even if it is only a model, please check up the displacement of the hulls before you clap the sheets of ply together and paint them. Great boat in background!
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  #39  
Old 05-15-2015, 04:43 AM
Mr Efficiency Mr Efficiency is offline
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I don't know whether this link will work for you, but it's a 3.8m cat (12.5 ').......and 40hp, I doubt anything less than 25hp would work too well. A planing boat, of course.

http://www.gumtree.com.au/s-ad/warrn...ary/1061207481
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  #40  
Old 05-15-2015, 05:42 AM
Boatguy30 Boatguy30 is offline
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I don't have any intention of painting it. If it last a few hours deter launch I'll be happy. Full old displacement I meant to be only about 100 kg. The sea water canal will do the calculating.
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  #41  
Old 05-15-2015, 08:04 PM
Boatguy30 Boatguy30 is offline
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she floats

Ok, her she is 100kg loaded. This scales to about exactly 300kg at 11' Floating just above my guesstimate waterline black marker waterline. I did make it just a bit deeper for a 7" loaded draft on the 11' model. Will try and check immersion rate at waterline tomorrow.

I get 5.5 knots so if that scales I would get 7.9 knots at 11'? That is the question. 5.5 knots is pretty fast for a 7.8' waterline. Is more possible?
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Catamaran dinghy 10-11' long-win_20150513_200632.jpg  
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  #42  
Old 05-15-2015, 09:32 PM
Boatguy30 Boatguy30 is offline
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stabil

And she is pretty stable. I can stand in one hull with no problem. I would think the bigger version will be 2x or more stable. I may do a bit more testing and try to look at wave shape between the hulls, would be good to make the full size 11' version a bit narrower. 5.5' seemed ok, but with the hulls so narrow it makes for a big bridgedeck.
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  #43  
Old 05-16-2015, 06:16 AM
Manfred.pech Manfred.pech is offline
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Originally Posted by Boatguy30 View Post
And she is pretty stable. I can stand in one hull with no problem. I would think the bigger version will be 2x or more stable. I may do a bit more testing and try to look at wave shape between the hulls, would be good to make the full size 11' version a bit narrower. 5.5' seemed ok, but with the hulls so narrow it makes for a big bridgedeck.
Congratulations! What a great little testbed for your project. I really donīt know how to scale it up - linear or exponential and how much to get the final stability. But the little cat looks stimulating.
I think it might be interesting to test the waterflow around the hulls and between at different speeds. How do the bows raise and how the sterns are sucked down? May be it will encourage you to modify the underwater area.
Your son will love the small catamaran when you don`t need it for further testing and he will give it some paint because it is worth while.

Here is another small catamaran of 12ft with dory hulls: http://www.duckworksbbs.com/plans/gu...at12/index.htm and here one of 8ft: http://www.duckworksbbs.com/plans/gu...cat8/index.htm
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  #44  
Old 05-16-2015, 04:40 PM
Manfred.pech Manfred.pech is offline
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Scaling up ...

..from a model: Testing a model
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  #45  
Old 05-16-2015, 07:54 PM
Mr Efficiency Mr Efficiency is offline
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Quote:
Originally Posted by Boatguy30 View Post
Ok, her she is 100kg loaded. This scales to about exactly 300kg at 11' Floating just above my guesstimate waterline black marker waterline. I did make it just a bit deeper for a 7" loaded draft on the 11' model. Will try and check immersion rate at waterline tomorrow.

I get 5.5 knots so if that scales I would get 7.9 knots at 11'? That is the question. 5.5 knots is pretty fast for a 7.8' waterline. Is more possible?
With the same 5 hp outboard, you expect to go faster with a scaled-up version of this ? Won't be happening.
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