Hi Namir,

Your solution is no longer a dagger board, it's a centerboard. I have a boat with a centerboard and it does not automatically pick up whent it hits an obstacle. You'll have to add a release and lifting mechanism for that, something like this.

Even with that, hitting an underwater obstacle broadside will not cause the board to lift.

And the reason you are having the problem with the areas and centers is that you are trying to redesign a daggerboard boat to be a centerboard boat. That's basically a new boat type.

Maybe you should be considering a different boat?

Good luck,

Laszlo

Thank you for your answer, Laszlo.

Will this solution be ineffective if the trunk will be wider by 1-2mm to reduce the friction force of the centerboard?
I would not like to look for another boat. Dory has a great capacity-to-weight ratio, a flat bottom (necessary for sailing on the river), is beautiful and similar to traditional, wooden boats sailing in the same area. I can redesign the details, but if it is not really necessary, I don't want to change the hull.

What about the Southwester Dory? It has the same hull shape as the NE Dory and it already has a centerboard instead of a daggerboard.

   I didnt't know SW Dory uses centerboard. This boat is too big and too heavy for me, but i'll try redesign NW with solution from SW. Thank you!

Consider leaving the daggerboard trunk alone and fitting one or two leeboards.

That's also a redesign because leeboards go at the point of max beam. On the ND that's right about at the middle oarlocks. Since the sail's CE is well forward of that (at the middle thwart) that'd result in a wicked lee helm, so the sail plan would have to be reworked.

Then there's the whole effort of designing a leeboard and support structure that is robust enough to handle the condition the OP describes. My vote is still to check out other boat designs. Even better, get in touch with CLC support and ask their advice.

Laszlo

Before you try to redesign the NE Dory with a ceterboard, you may want to look at this picture, where John Harris shows why the NE Dory has a daggerboard:

http://www.clcboats.com/images/photos/boats/dory/Why_the_Northeaster_Dory_Has_a_Daggerboard.jpg

Well, there's a picture worth a couple thousand words.  Brilliant find, DanS.  Sums up the trouble nicely.

To add to the problem, even you could work out a functional centerboard--perhaps off-center to clear space for the oarsman, if you're a serious Phil Bolger fan, a pair of pivoting bilgeboards either side, a pair of tandem centerboards fore and aft--I don't know that this solves the issue.  While a pivoting centerboard may be pushed up into the case without damage if the boat passes gently over a shallow bottom or not-too-gnarly obstruction, sailing upwind in a serious breeze with some wave getting up is a whole nother thing.  In that circumstance, the blow is likely to be struck somewhat sidewise and with considerable force, possibly resulting in damage to board, case, or both.  With a never-ending river current also factored into the mix, this sort of lateral impact on the centerboard is even more likely.

This is not purely theoritical.  A couple years ago, we sheered off the centerboard of our Menger 19 Catboat going upwind in 15+ knots with 2-3 waves by a close encounter of the submarine deadhead kind (waterlogged tree trunks floating just beneath the surface) just E of Kelleys Island on Lake Erie.  She was going along nicely, wriggling off the top each wave with a satisfying thump (don't you just love the way a boat moves over waves going upwind?), when one of the thumps came as a decided THWACK!, followed shortly by a regular banging against the hull beneath the cockpit sole.  This, we later realized, was the bulk of the now freed centerboard bumping the hull as it dangled along from the pendant.  Fortunately, the centerboard case didn't fracture and we did not sink.

Anyway, an experience like that will remind you that a moving boat in moving water is a bit more complicated than we might imagine.

.....Michael

I wonder which fails first on the the dory, the daggerboard or its trunk. Hope I never find out, and if I do, fingers crossed it's the board.

Thanks for answers. I think even if the centerboard is not the perfect solution, it is still safer on the river than the daggerboard. I see two possible ways to use it on NE Dory.  
 https://ibb.co/ei7sYH

1:

Advantages:
Is very safe when hit an obstacles
All seats stay on correct positions
No leaks, if trunk with be closed. The rope to lift the board will pass through the tube mounted on top of the trunk.
CLR okay

Disadvantages:
Bottom part of boat is dissected in half on big length
Two rungs cutted in half
Mast needs different method of connecting to floor

2:
CLR OK
Seats moved to not perfect positions
Structular frame not in perfect positions too
Only two seats

Both solutions are not perfect. I think second one is better. What do you think?

I have one more idea. I think it's better than the previous ones:
https://image.ibb.co/mG77Lx/centerboards.png
It's just an concept drawing, all dimensions will be changed. The shape of the centerboard can also change.
With this solution, all bulkheads and seats stays in correct places, CLR and board surface is the same as designed.

Namir:

Phil Bolger would likely have loved it; he seemed to love unorthodox things in general, as long as there was some justfication for it.  Tandem boards are not without precedent from some other designers, as well.  A cat-schooner-rigged, blunt-bowed boat Howard Chapelle shows in his Yacht Designing and Planning (I think--going on memory for that) was equipped with tandem centerboards, though we are talking about a much larger boat than the NE Dory, of course.

I expect she won't be as weatherly vs. the higher-aspect-ratio daggerboard, but at least you'll be able to go upwind in shallower water without having the boards bumping along on the bottom.  She may also resist tacking unless you fiddle with the boards, maybe by raising the aftmost one.  Being able to adjust the boards for optimal tracking on long legs might be an advantage, hard to say.

The problem of lateral stress as the boat slips sidewise over a shoal or obstruction is not eliminated, but the shorter boards will give the bottom less leverage to work through and you'll be in shallower depth before contact is made--more margin of error, so to speak.  I do think you'll want to work in some substantial half-frames, knees, etc. to spread the stress out--you can't just park the centerboard cases in the middle of the bottom without extra support--but I expect you've already thought of that.  It's going to add significant weight (plus the boards will need ballast to sink reliably, unlike a daggerboard), but that's only an issue if you mean to hand-carry the boat.

Mind you, the thing could be a complete failure.  However, if that's a risk you are willing to run, it would be a worthy experiment.  Personally, I'd live with the dagerboard as originally designed and make a shorter one to use in shallow water, as I've done with our Passagemaker Dinghy.  (John Harris kindly supplied me with a shorter, non-tapered board blanks when I mentioned the idea too him--he'd aready done that for his own purposes.)  That's worked out well for me; she gets upwind over shoally stuff pretty well with the stubby daggerboard.  In really shallow water...well, heck, it's a rowboat, right?  <;-)

Please tell us some more about your location, what river you mean to sail, and also some more about the traditional wooden boats sailing in your area to which you referred in your earlier post.  Sounds fascinating.

.....Michael

Thank you for advices, Michael :) It is important to me and i will think about it. Last days I tried to find solution for push down centerboard. This moment, this is the best idea, but i'm still thinking how to improve it:

https://image.ibb.co/nEKK2n/centr.jpg

I want to sail on Vistula river in Poland:
https://www.youtube.com/watch?v=-ewp9lLc2Jg

Riverbed of Vistula wasn't regulated for decades, so is a lot of obstacles and shallows there. If you sail there, you should watch your way all the time, because depth of water changes in seconds. I love it, but it is the reason, why daggerboard is not good idea there.
 
Traditional boats sailing at the mouth of the Vistula are called "Pomeranka". You can see them here:
https://www.youtube.com/watch?v=bNb8wa_FqKE

Fascinating!  Beautiful boats, and a most splendid, historic river.  I should love to see it sometime, but, alas, that is very unlikely.

What part to you plan to sail?  Do you live somethere near the river?  I hope you will share some photos of your adventures.

Never did much river sailing, myself.  Something about that constant, one-way current give me pause.  I live not far from the Ohio River, and might like to try that sometime...in our catboat with her fine, strong, reliable Honda auxiliary motor, that is, heading first upstream so that "home" will always be downstream.  I have done some sailing on Kentucky Lake, a long impoundment on the Tennessee River.  Not much current, but there is much commercial traffic with towboats pushing barges up and down the Tennessee and Cumberland rivers from the Ohio river, and to and from the Gulf of Mexico via the Tennessee-Tombige waterway.  It was fun talking to the river towboat pilots on Channel 13 as we passed.  The need to be careful to avoid them made for a great adventure.  It also made me start to feel like Huckleberry Finn.  Are you familiar with Mark Twain's novel, and his Life on the Mississippi?  Riverboatin'!  Gets the adventure juices flowing, it does!

I wish you all joy of good sailing on your splendid Vistula River!

.....Michael

   I forget to answer, but i finish my boat one year ago. I found other solution and decided to share, because maybe someone need this. I made two centerboards and they working quite good. Solution looks like this:
https://ibb.co/NZrkmwF
https://ibb.co/NyVwdZd
Here is solution to keeping board on up or down position:
https://ibb.co/9pH6CnJ
Gray rope is for pullig board up
Red rope is elastic and pullboard down. If board hit obstacles, rope stretch little bit, and board is covered in hull.

Michael, here you can find some photos of our one month trip: ;)
https://photos.app.goo.gl/7Gxm5jAaC7gmXh2d6
Sadly i dont make too many photos, because i afraid about my phone battery. We sailed almost whole river. From Cracow to sea. Last week wind was really strong, and going from north (very bad for us). We tried to sail upwind, but waves pushes us back. On one of photos you can see end of river ;) We tried to sail little bit by the sea. But then again waves push us on the land and we crashed on beach. It was really dangerous. Next day we waked up early in the morning, and escaped from there before waves grow bigger ;)

   Of course you give up 5-6 inches of draft, and I have no idea how well it might sail without testing it, but a very simple solution would be to remove the skeg and run a 5 - 6 inch deep keel the whole length of the boat - of course establishing the leading and trailing curves to match boat geometry (bow and stern sweep anlges).  You could run a brass or satinless steel or epoxy/rope wear strip along the bottom edge of the keel.  The drawbacks of course include the increase in fixed draft and (probably) less than ideal lateral resistance generated as compared to a dagger or centerboard, and perhaps reduced turning maeuverability, but the benefits would inlcude zero chance of leakage and nearly bullet proof strength, especially if an interior strengthening plank (1/2 inch thick by 3 or 4 inches wide ?) was added down the centerline of the boat atop the keel.  If you do ground while side-slipping, expect no damage as might happen to a board/trunk arrangement, you'll just roll a bit to the side until (perhaps) the hull also grounds.  With plenty of glue, long screws, an inner keel cap and a big filet, I doubt you could break a 6 inch keel off even if you tried kicking it sideways.

Namir:

Oh, what joy!  Looks like a splendid trip.  Thank you for sharing the photos.  The video was quite stiring!

You seem quite adept at sailing with your toes.  I once thought I should have to give up sailing if I ever got really old and lost my teeth, as I find I must often hold a sheet or some such in my teeth briefly while both hands are otherwise engaged.  You know how it can be: mainsheet in one hand, jib sheet in the other, tiller in the other, scratch the nose with the other...oh, wait...we're already a couple of hands short, and we now need a drink from the water jug.  Perhaps I should rethink my aversion to sailing barefoot.

Your tandem centerboards look very functional  I hope the boat is living up to your expectations.  It certainly looks as if "a good time was had by all."

.....Michael