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  #1  
Old 04-26-2011, 07:10 PM
CatBuilder CatBuilder is offline
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Single Dagger Board vs Double Boards

As if the other thread on the keels vs the daggers wasn't enough, I am wondering what people think about doing a single board vs a double.

My boat is designed for a dagger board in each hull.

However, this really gets in the way of the galley. So, I'd like to do like so many other boats do these days and do a single board in the hull without the galley.

What are the pros and cons?

What changes to the board would need to be made (size? trunk structure?)

What would you do, or what did you do in your single board boat for layups of the board and trunk?
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  #2  
Old 04-26-2011, 08:59 PM
redreuben redreuben is offline
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Shuttleworth is the "man" on this subject, my concerns would be the redundancy aspect, lose the board and your screwed, and also not having a choice in which to deploy especially in beam seas in a seaway where given the choice you would retract the leeward board but I don't have much sea time in offshore multis so other opinions would be more valid.
I would expect with a single board it would need to be considerably larger and built accordingly usual method is foam or strip plank core with layers of double bias glass and tapering strips of Uni down each side at the thickest part of the chord.
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Old 04-26-2011, 09:04 PM
redreuben redreuben is offline
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Sorry, didn't say what I would do. If I was building a new boat or considering a single board boat I would be looking at this closely, as you are. But given you have a fully functional twin board boat I would leave well enough alone.
"If it works don't fix it"
RR
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Old 04-26-2011, 09:22 PM
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Dirteater Dirteater is offline
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I am currently considering these same options on what is probably a smaller scale. I'm curious if you dagger-boards are at an angle (say 30 degrees) or are they 90 degrees to the hull? There seems to be stability issues (which doesn't really make sense to me). and in my situation another concern is the possibility of debris getting caught between the two dagger-boards, this probably doesn't apply to your build, but perhaps worth mentioning. We are actually a little opposite here , as I am trying to create space via dual dagger-boards where you are finding they take up space. Another thought that Redreuban touches on is if you have two daggers, can they afford to be shorter? also the posibilty of redundancy?

Interesting concerns for sure. what interests me at this point is the effect on over-all stability/performance. I'm curious CAT as to the beam of your craft?
I apologise if I'm missing a thread here? motor or sail/both?

thanks
DE
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Old 04-26-2011, 09:34 PM
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oldsailor7 oldsailor7 is offline
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In the late 1970s I test sailed the Macgregor 36 for Multihulls Magazine.
Forgive me if my memory is failing me ---but I think it had only one big dagger in the port hull.
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  #6  
Old 04-27-2011, 12:42 AM
Alan.M Alan.M is offline
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Quote:
Originally Posted by redreuben View Post
Shuttleworth is the "man" on this subject, my concerns would be the redundancy aspect, lose the board and your screwed.....
Screwed? Not really - the boat will still sail, even to windward. There'll be a fair bit more leeway, that's all. But it's sensible to build your board with sacrificial section on the bottom 1/2 metre - hit bottom and you only lose part of the board.

Pro's - cheaper and easier to build, lighter, more internal space...

Con's - some people find the assymetery looks odd.

I don't notice any performance difference from one tack to the other.
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Old 04-27-2011, 01:20 AM
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rayaldridge rayaldridge is offline
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I don't know how much relevance such a tiny boat would have to this discussion, but my 16' cat has only one big deep daggerboard. It takes little internal room, because the case lies just inside the topside planking. This means that the board is "heeled" a bit but you actually lose very little depth.

My theory on single daggerboards is that they are more efficient for much the same reasons that a sloop is more efficient than a ketch, given the same area.
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Old 04-27-2011, 03:45 AM
redreuben redreuben is offline
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Dont understand your thinking there Ray, a dagger in each hull is far removed from one mast behind the other.
RR
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  #9  
Old 04-27-2011, 03:54 AM
redreuben redreuben is offline
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Alan M.
Snap of tips are a given as are crash boxes in cases. There are many ways to lose the use of a board, it is a moving system so string can beak, pulleys can seize, hardware can pull out/ fail, the board can jam in it's case up or down and any where in between usually at the most inopportune moment !
RR
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  #10  
Old 04-27-2011, 04:21 AM
catsketcher catsketcher is offline
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Catbuilder

Like Ray I have experience with using a single board in a trailer sailer. I did it for exactly the same reason as you - to get better accommodations. I like the arrangement and can find no difference between it and my more normal two board bigger cat. Its a fine idea but there may be some problems.

As far as I can recall your boat is a big one and this is the real issue. I like to be able to remove my boards and one big board will be a bit much to fiddle with.

There may some other options. My boards lie just off the inside of the hull in the big boat. Board cases certainly do not have to take up valuable room. Push them out to the outer side of the hull and make them canted. Designers who put vertical boards in accommodation layouts must not live in these boats a lot. A couple of things to look at - Don't cant the boards so much that when raised they are the widest part of the boat. Don't change the laminate or the longitudinal position.

I was looking over a nice Crowther cat that had very canted boards. A friend was test sailing it and I was thumping on the top of the board (located outboard of the gunwale) to help it down. All of a sudden it went down and I went overboard. Thankfully into the warm lake. Keep the tops of the boards inside the lifelines or keep your crew off the board tops.

cheers Phil
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Old 04-27-2011, 04:30 AM
catsketcher catsketcher is offline
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Where is Tom Speer

Red

I think what Ray is saying is that drag is often proportional to span or chord and that doubling the area for one big boards may not involve a doubling of the induced drag. Certainly skin friction drag would double. Racers would never go this way because they fly the windward hull.

So if Tom is listening he may have some wisdom on the merits of a double sized board and its drag.

cheers

Phil
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Old 04-27-2011, 05:03 AM
redreuben redreuben is offline
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Hey Phil,
Thats funny !!!!
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  #13  
Old 04-27-2011, 05:09 AM
redreuben redreuben is offline
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Ok so which is the best side to place the boards outboard canted in or inboard canting out ??? He He He !
RR
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  #14  
Old 04-27-2011, 05:17 AM
catsketcher catsketcher is offline
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On some Tennant cats he had them canted out (top in further than the bottom - cases on the inside of the hulls). I guess this was to make the leeward one more vertical when the cat heeled. Crowther and most others went the other way (that's why I got wet)

If you talk to Seawind 24 sailors (Seawinds also have very heavily canted in boards) they say that if you got hit by a large gust the boat would go sideways and the action of it doing this would cause the leeward hull to lift and the windward one to bite into the water - all making the boat safer.

Maybe Lock was doing this too. He was clever.

cheers

Phil
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  #15  
Old 04-27-2011, 05:34 AM
jamez jamez is offline
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Quote:
Originally Posted by catsketcher View Post
On some Tennant cats he had them canted out (top in further than the bottom - cases on the inside of the hulls). I guess this was to make the leeward one more vertical when the cat heeled. Crowther and most others went the other way (that's why I got wet)

If you talk to Seawind 24 sailors (Seawinds also have very heavily canted in boards) they say that if you got hit by a large gust the boat would go sideways and the action of it doing this would cause the leeward hull to lift and the windward one to bite into the water - all making the boat safer.

Maybe Lock was doing this too. He was clever.

cheers

Phil
Malcolm once said in an article that the reason he placed the boards towards the inside of his cat hulls was to make them easier and safer to set, not for any hydrodynamic reason.

My understanding is that there may be some lift gained by having the board canted in (top out) although I'm not aware its ever been quantified. To me the top out configuration just looks like an accident waiting to happen, if a board jams or it needs to be pulled up in rough weather.
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