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  #76  
Old 01-25-2017, 10:36 PM
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DennisRB DennisRB is offline
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Quote:
Originally Posted by brian eiland View Post
Were I betting man...I would definitely say ____ .
What do you think, it is pretty obvious.
Its obvious. That was my point.

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Originally Posted by brian eiland View Post
Sure would be nice to access your boards and your drive leg without hauling the vessel...

I conjunction with the 'central nacelle' I had suggested to contain those 2 asymmetric boards, I had also suggested it might incorporated a single drive retractable propulsion leg (at least on smaller vessels). Might appear something like this nacelle on this....
Agreed. Not sure the asymmetrical boards would be worth the effort though. One steerable OB at the back with the board down should give decent manoeuvrability.

Quote:
Yes exactly what i had in mind when I first ask about centerboard on a catamaran. Why don't they work?

I believe that if it is mounted in a pod under the bridgedeck it should work. But this configuration is pretty much close to a trimaran with a really small main hull.
I think it would work. Its 100% proven that a single larger dagger in one hull works very well instead of one in each hull. Shuttleworth and Oram designs are very successful. I know many Oram owners and they all say they can not notice any difference in performance of the stb or port tack. These boats are great performers and smoke most other boats out there including to windward.

But the ventilation issue still exists with the central board. I don't believe this issue rules the concept out. Forward sweep, fences, pods etc. Surface piercing foils with forward sweep are fairly resistant to ventilation. This would work well considering the sweep angle is infinitely variable with a pivoting CB.
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  #77  
Old 01-26-2017, 06:13 AM
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Its obvious. That was my point.
But the ventilation issue still exists with the central board. I don't believe this issue rules the concept out. Forward sweep, fences, pods etc. Surface piercing foils with forward sweep are fairly resistant to ventilation. This would work well considering the sweep angle is infinitely variable with a pivoting CB.
Sweeping forward with an infinitely variable pivoting CB puts incredible twisting stress on the board at the hull junction, which is very dangerous. I had a pivoting centerboard on my Buccaneer 28 trimaran, and it was a wonderful asset. However we would only use it tilted forward when we were hove to, just behind the starting line at the beginning of a race. Ten seconds before the gun we would pull the board back to vertical and sheet in the sails. Best way to be first across the line.
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  #78  
Old 01-26-2017, 07:00 AM
valery gaulin valery gaulin is offline
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Really sweeping a centerboard forward help reduce ventillation? Cool

But this setup will collect weeds(algea) like crazy. But sweeping it backward would release the weeds(algea). Hopefully
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  #79  
Old 01-27-2017, 10:17 AM
Mulkari Mulkari is offline
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If racing is not top priority then single centerboard can work good enough. I have built a 9,5 m cat that have pivoting centerboard hung below mast beam. Here is a video where centerboard is visible at around 1 minute mark. https://vimeo.com/167481774

At speeds above 12 knots there is some vibration and ventilation, but such speeds usually are achiewed when sailing offwind and board is retracted then so not a problem. At typical upwind speeds of 6 to 10 knots board works well. When tacking upwind I can make 80 degree angle between tacks even with my crappy sails. With good set of sails 90 degree angle would be achievable with WMG similar or a bit better than good monohull.

Main reason why I choosed single board is simplicity and safety when hitting something. I have hit rocks several times at speeds that would resulted in broaken board or ruptured hull if it was regular through hull daggerboard. My board just kicks up and nothing breaks. Rudders also kick up so there is little chance of breaking an appendage in case of grounding. With board and rudders raised cat with light load have draft of only 30 centimeters. I can get into places usually accessible only with inflatable boats
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  #80  
Old 01-27-2017, 05:12 PM
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Tom.151 Tom.151 is offline
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Quote:
Originally Posted by Mulkari View Post
If racing is not top priority then single centerboard can work good enough. I have built a 9,5 m cat that have pivoting centerboard hung below mast beam. Here is a video where centerboard is visible at around 1 minute mark. https://vimeo.com/167481774
Brilliant boat! Truly an inspiration of simplification.

Would love to see more pics of the boat - especially out of the water hull shapes, and the rudder arrangements, and more of the centerboard ..

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Originally Posted by Mulkari View Post
At typical upwind speeds of 6 to 10 knots board works well. When tacking upwind I can make 80 degree angle between tacks even with my crappy sails. With good set of sails 90 degree angle would be achievable with WMG similar or a bit better than good monohull.
?? Are the 90 degs and the 80 degs accidentally reversed here ??

Anyway, thanks for posting so we could see a bit of a tease of the centerboard details.

Excellent!
Cheers,
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  #81  
Old 01-27-2017, 05:16 PM
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oldsailor7 oldsailor7 is offline
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Of course a surface piercing board will work----but it will be very inefficient .
See the turbulent wake it makes in the video, at the 40 second mark,
Also, as has already been mentioned, the bending stresses on the board and the extra weight of the structure to hold it, must be addressed. The lesson of "Occams razor and the Gordian knot" comes to mind.
I don't know why this subject is dragging on.
Elan Voyager answered quite succintly in page 1, post 12.
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  #82  
Old 01-27-2017, 06:30 PM
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Doug Lord Doug Lord is offline
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Bridgedeck Centerboard

Paddy, I think there are plenty of good reasons to use a bridgedeck centerboard particularly on a cruising boat designed and engineered well specifically for it.
Rob Denney's comments were spot on.(p2, post 20 )
While there are losses from the board being surface piercing I don't think they are as significant as the gains from a well designed and engineered board in the center of a cat.
The foilers that use surface piercing foils are a case in point-the losses can't be too great or the foilers would be slow, unable to point and have other problems that they don't have in practice.(posts 3 & 13, page 1)
I don't think it is a perfect solution in every case but ,in my opinion, it is a viable option with some real advantages when designed and engineered well.
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  #83  
Old 01-27-2017, 07:40 PM
redreuben redreuben is offline
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I don't think there is any doubt that a through the hull board is more efficient, the endplate effect of a hull is hard to beat, however for a cruiser I think a single board between the hulls could work well enough.
My thoughts on this are along the lines of what Kelsall did with his Trail cats and the Surtees boats.

http://s-boat.com.au/center-pod/ that is the board sits in a central pod or demi hull.

I'm thinking pivoting foil shaped dagger case might work better for more support and smaller pod with an endplate at the waterline or a bit below, forward of the board the flare in the pod will act as a dolphin striker for the mast, forward again and you have anchor well and ultimately a pole for the asymmetric.

http://s-boat.com.au/img/moreton_bay_docked.jpg

Behind the board are batteries and outboard mount.
I'm less keen on a single rudder.
I like the idea of less holes in the hulls, more space in the hulls especially in a sub 9m boat and the ability to kick the board forward to lie a-hull.
Yes there will be more structure but with a bit of thought that structure can multitask other work, like forestay tension, mast support, battery, fuel and gas storage, etc, further, lying under the deck instead of in the hulls could allow the extra size and depth required.
Another advantage of course is accessibility for repair and maintenance.
In the cruising context I think it's worth consideration.
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  #84  
Old 01-27-2017, 07:42 PM
redreuben redreuben is offline
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First link didn't work, try this;

http://s-boat.com.au/center-pod/
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  #85  
Old 01-27-2017, 08:37 PM
ImaginaryNumber ImaginaryNumber is offline
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Quote:
Originally Posted by Mulkari View Post
If racing is not top priority then single centerboard can work good enough. I have built a 9,5 m cat that have pivoting centerboard hung below mast beam. Here is a video where centerboard is visible at around 1 minute mark. https://vimeo.com/167481774

At speeds above 12 knots there is some vibration and ventilation, but such speeds usually are achiewed when sailing offwind and board is retracted then so not a problem. At typical upwind speeds of 6 to 10 knots board works well. When tacking upwind I can make 80 degree angle between tacks even with my crappy sails. With good set of sails 90 degree angle would be achievable with WMG similar or a bit better than good monohull.

Main reason why I choosed single board is simplicity and safety when hitting something. I have hit rocks several times at speeds that would resulted in broaken board or ruptured hull if it was regular through hull daggerboard. My board just kicks up and nothing breaks. Rudders also kick up so there is little chance of breaking an appendage in case of grounding. With board and rudders raised cat with light load have draft of only 30 centimeters. I can get into places usually accessible only with inflatable boats
Mulkari,

Would you mind sharing details/sketches of your centerboard?

Is the catamaran design yours or someone else's? (Wood's Skua?) I'm curious what the hull is made of? The other catamaran that you have a video of (https://vimeo.com/104815976) appears to have a very thick hull, maybe of foam? Finally, on the outboard ends of the pole that connects the two tillers there appears to be two horizontal rods, one at each end. (From 0:30 to 0:45) I'm scratching my head trying to figure out what they are for?
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  #86  
Old 01-27-2017, 09:26 PM
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brian eiland brian eiland is offline
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Of course a surface piercing board will work----but it will be very inefficient .

I don't know why this subject is dragging on.
The conversation is dragging on precisely because there really are alternative solutions to this old style myopic thinking that its just not doable

Doesn't appear as though theey had a very difficult time fabricating this board.
http://s-boat.com.au/centerboard-features/

Cheers, Brian
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  #87  
Old 01-27-2017, 09:29 PM
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brian eiland brian eiland is offline
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Quote:
Originally Posted by Mulkari View Post
If racing is not top priority then single centerboard can work good enough. I have built a 9,5 m cat that have pivoting centerboard hung below mast beam. Here is a video where centerboard is visible at around 1 minute mark. https://vimeo.com/167481774

At speeds above 12 knots there is some vibration and ventilation, but such speeds usually are achiewed when sailing offwind and board is retracted then so not a problem. At typical upwind speeds of 6 to 10 knots board works well. When tacking upwind I can make 80 degree angle between tacks even with my crappy sails. With good set of sails 90 degree angle would be achievable with WMG similar or a bit better than good monohull.

Main reason why I choosed single board is simplicity and safety when hitting something. I have hit rocks several times at speeds that would resulted in broaken board or ruptured hull if it was regular through hull daggerboard. My board just kicks up and nothing breaks. Rudders also kick up so there is little chance of breaking an appendage in case of grounding. With board and rudders raised cat with light load have draft of only 30 centimeters. I can get into places usually accessible only with inflatable boats
Thanks for your posting.

I do have a question however. In the video it appears as though there is a forward and aft 'stay' to the blade that attaches below the waterline. Is that so, and if so how does it kick-up??

As the others have mentioned would like to see some 'out-of-water' photos of the arrangement.
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  #88  
Old 01-27-2017, 09:35 PM
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brian eiland brian eiland is offline
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Quote:
Originally Posted by redreuben View Post
... however for a cruiser I think a single board between the hulls could work well enough.
My thoughts on this are along the lines of what Kelsall did with his Trail cats and the Surtees boats.

http://s-boat.com.au/center-pod/ that is the board sits in a central pod or demi hull.

I'm thinking pivoting foil shaped dagger case might work better for more support and smaller pod with an endplate at the waterline or a bit below, forward of the board the flare in the pod will act as a dolphin striker for the mast, forward again and you have anchor well and ultimately a pole for the asymmetric.

http://s-boat.com.au/img/moreton_bay_docked.jpg

Behind the board are batteries and outboard mount.
I'm less keen on a single rudder.
I like the idea of less holes in the hulls, more space in the hulls especially in a sub 9m boat and the ability to kick the board forward to lie a-hull.
Yes there will be more structure but with a bit of thought that structure can multitask other work, like forestay tension, mast support, battery, fuel and gas storage, etc, further, lying under the deck instead of in the hulls could allow the extra size and depth required.
Another advantage of course is accessibility for repair and maintenance.
In the cruising context I think it's worth consideration.
Nice looking arrangement, but I really have to question the sideways stability of the centerboard trunk?
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  #89  
Old 01-27-2017, 10:02 PM
upchurchmr upchurchmr is offline
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This discussion has gone on enough.

Early on it was clearly stated that a single centered board could and has been done.
It was also pointed out that there are problems and potential loss of efficiency.

Nothing since then has done anything to quantify any of that.

Waste of time anymore.
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  #90  
Old 01-27-2017, 10:34 PM
valery gaulin valery gaulin is offline
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@ Mulkari: Nice video thank you
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