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Underengineered and underbuilt foils are far too common.

There is some information on this here.

 

I have enclosed a section that works real well. You can scale thickness a bit to fit your dagger board case. The first coordinate is cord and starts at the trailing edge (1) with zero at the leading edge.

 

Recommended for rudders too.

 

/Martin

prulsY.txt

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But first, I must repair the crushing around the dagger well.  I intend to lay 4 layers of glass on the sole, and then carbon core foam horizontally across the sole to tie into the opposite hull.  Then overlay four layers of cloth to make a horizontal column (and tripping hazard!).

 

Got any better ideas?  Clearly there is a ton of force in this area, and I think there a design or build issue in not having it supported from the back side.  this may have been in the interest of achieving headroom in the cabin....

 

6bNIoZs.jpg

 

6XXE3Ys.jpg

 

zahkfXa.jpg

 

 

KE4LPQx.jpg

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Having wake up coffee, so hope I make sense here.

 

Is it crushing or tension cracking? When going upwind, the base of the leeward board will be pushed against the inside of the hull, so your planned extra support will be a tension member?

Only if you are loading up the windward board upwind will in add significant support?

 

I would think filling in the other side of the case, against the hull, with more support might achieve more?

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Four layers of glass? What kind of glass and why?

 

Here is what we did 30 years ago instead of following Tennant's instructions. This has worked but I (now) understand there are even better ways of reinforcing this area of the hulls.

 

Some years ago there was a very long thread on the renovation of a tri called Timberwolf. Unfortunately all photos are missing now.

 

Having longer boards with the upper support further up in the dagger board case also helps -- less crow-bar like. My dagger boards reach deck even when fully down.

 

/Martin

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Having wake up coffee, so hope I make sense here.

 

Is it crushing or tension cracking? When going upwind, the base of the leeward board will be pushed against the inside of the hull, so your planned extra support will be a tension member?

Only if you are loading up the windward board upwind will in add significant support?

 

I would think filling in the other side of the case, against the hull, with more support might achieve more?

I think it is crush damage, as the damage starts in the middle of the case and radiates out. There is no tension connection between the middle of the board and the case

If it was tension, I would have expected crack to form at the ends and work in.

 

What is key here is understanding what the failure mechanism was:

  • too much flex in the bottom of the hull causing the fibre grass to break?
  • Opening in the base of the hull too large causing point loading of the board onto a part of the case?
  • localised crushing of the core behind the board (unlikely with ply, unless it has got wet and a bit spongy) ?
  • Loading of the lip causing a local shear failure of the edge taping?

Looking at the wear pattern on the antifoul, it looks like the board is loading up the very bottom most point. It might be worth building up with come high density filler above the lip to try and direct the load into the core of the hull, instead of into outermost skin...

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Nice carpet.

 

IMO crushed by the windward board flying a hull, on the angle of death, over waves, doing close to twenty knots.

 

Pull the board up more in these situations. Will also get more positive helm and able to come upwind in gusts rather than the boat veering of downwind and down the mine. Need to reinforce the case with a second shelf half way up or else the case will break next.

 

I do like the idea to lengthen the boards similar to Martin from Sweden and Hooters.

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Thanks Guys.  Below is the drawing showing the shelf etc, and it is built to plan.   The shelf is 12mm ply and supports the upper part of the board.   It has many strands of uni "gunstock" rope around it as well to tie it into the shelf and the side of the hull.   That part seems intact.

 

My intention now is to remove the sole roughly 3cm around the opening, grind out any fractured glass from the cracking and lay in many layers of triax and db.   No core is compromised in this area as it is all glass.  I will then replace the floor and let it make a horizontal column to further distribute the sideways load of the board.

 

The back side (inner side of the case) appears to be well bonded to the hull at this time, so no changes there.

 

Then I will also attack it in similar fashion from the outside, removing any fractured glass I find and rebuilding.  Glass and more resin are on order!

 

Thanks all for your input.  Martin - that is beautiful work on that trunk.  I'm surprised you have no sole in that area (or was it added later?)   I think the sole would be critical in distributing point loads from the dagger case....

 

o3r4O5u.jpg

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Nice carpet.

 

 

 

Wise Guy!  Yeah, being a finish carpenter by trade, the builder did a few things very nicely.  That "carpet" is that mouse fur stuff - very light, but more inviting for guests I suppose.  Terrapin is a daysailor first and (hopefully) a racer second.

 

Here is a shot facing aft from the head in the starboard hull:  

 

UmeNgG7.jpg

 

He put that green KIWI grip on the floor.   I had to sand it off on the other side, and wow that stuff is tough!

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My intention now is to remove the sole roughly 3cm around the opening, grind out any fractured glass from the cracking and lay in many layers of triax and db.   No core is compromised in this area as it is all glass.  I will then replace the floor and let it make a horizontal column to further distribute the sideways load of the board.

 

 

Then I will also attack it in similar fashion from the outside, removing any fractured glass I find and rebuilding.  Glass and more resin are on order!

 

Thanks all for your input.  Martin - that is beautiful work on that trunk.  I'm surprised you have no sole in that area (or was it added later?)   I think the sole would be critical in distributing point loads from the dagger case....

No sole in my boat. None specified and my boat is small with some 1.3 m head room so adding a sole would not really add to comfort.

 

I see Tennant stuck to the same design we rejected back in 1985. There were a number of things we change in the structural engineering. The only things that have broken are things we did not change such as the cross beams.

 

Yes, the sole helps distribute the load but only from one side unless you wrap fibres around the dagger board case and spread them onto the sole.

 

Are you in a hurry? If not, why not spend some time on a bit of proper engineering before slapping on more material?

 

/Martin

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No sole in my boat. None specified and my boat is small with some 1.3 m head room so adding a sole would not really add to comfort.

 

I see Tennant stuck to the same design we rejected back in 1985. There were a number of things we change in the structural engineering. The only things that have broken are things we did not change such as the cross beams.

 

Yes, the sole helps distribute the load but only from one side unless you wrap fibres around the dagger board case and spread them onto the sole.

 

Are you in a hurry? If not, why not spend some time on a bit of proper engineering before slapping on more material?

 

/Martin

 

Not in a hurry - I'm out of the water for at least 4 months for winter   I certainly appreciate your Tennant experience and insights.  Winter will limit my glassing days though as the boat is outside.  So there is some sense of urgency.

 

The sole isn't much - 6mm exterior ply.  I have some 18mm carbon core (similar to Coosa) with which I might replace the ply.  Laying in uni to tie in the sole sounds like a good approach as well.

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So I couldn't help myself and started modelling the board-hull interaction:

Wildfire_hull_section_in_the_making_1.png

Very much work in progress and I can't guarantee the usefullness of the results but the idea is to load it up with some of those finite elephants and hopefully understand better where stresses go.

 

/Martin

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No FEA results yet but I have created an illustration to show the importance of having a healthy separation between upper and lower (boat bottom) supports for the dagger board:

DaggerBoardCaseSupportLoads.png

Figures are from my boat and the support separation in the drawings we bought is roughly 20 cm! The Wildfire design looks better in this respect but I think I would prefer even more separation in the interest of reducing material stress.

 

/Martin

 

PS Only using free and open software this time: LibreOffice on a Linux system.

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Interesting.   I see your point about having a longer daggerboard to potentially take advantage of a higher shelf.

 

Wildfire's shelf is 500mm above the hull exit.

 

I'm thinking some extraordinary event occurred to cause both the dagger to break AND the inner edge of the hull exit to crush.   A hard sideways impact of some sort.

 

I removed the floor in that area and layered up the inner side of the hull over the weekend.  it won;t crush again, at least not in THAT area!  I expect to start grinding on the outside over the next week.   The weather is down to 0 C this morning.   I'm sure that will slow progress for a while....

 

BTW, whether it was an intentional or accidental typo, "finite elephants" is funny either way....

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Fascinating!   I like your style, Brother!

 

The last slide is the most telling for me.  Without the floor, the left image shows a tremendous amount of stress on the hull bottom adjacent.  

 

The right image tells a lot too:   It "feels" like a significant wrap of "Gunstock" (Tennant's term for Uni) running between the hull and the dagger case and then back around and splayed out along the hull/floor would go a long way toward spreading that stress.  Are my non-technical instincts correct?

 

To take that further, it seems like the forces at the leading edge and trailing edge are huge.   The board seems to be trying to tear the boat apart in that area.   And in fact I did have a lengthwise crack both fore and aft in those areas.  The core was not wet, at least not yet.   I DO plan to put further reinforcement in that area.

 

Those cracks are in evidence here:

6bNIoZs.jpg

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