How to make a daggerboard breakoff in case of a collision?

I am in the process of fitting a dagger board to the 11ft dinghy I am finishing. I made a dagger board for a old beach cat that I don't own anymore, which happened to be the foil and fore-aft size as in the dinghy design. It is just a little longer than the designer specified but that can be sorted by cutting the top off a little. However, I plan to sail in areas with sandbars and some garbage in the river floor and was thinking of making the dagger board breakable so that, in case of collision with something, it can break off instead of damaging the dagger board cassette.

The dagger board is 20mm thick about 5cm aft of the leading edge which is the thickest part. About 300mm fore-aft size. It is made of construction quality pine that was laminated with alternating grain of 1x4 in boards, planed down and then glassed.

Is it worth the headache to make it breakable and if so any good strategies? I added a crude image of my ideas - feel free to shoot them down - I was thinking of using a router and weakening the board along one of the colored lines. If I weaken the board with a routed groove along fore and aft and then glass over that area, it should have reasonable strength for side to side flex but still break if I hit something.

Workable idea? Goofy idea?

Thanks all

 

Cool idea but not as drawn.
This could lead to unwanted failure during hard pointing when the daggerboard is subjected to elevated side pressure.

If designing for a forward collision with a rock, sea-container, deadhead, etc,
I would only dimple the leading edge where you want the fracture to start,
at the hull line. Bring the dimple around the sides tapering it away to nothing
within 3 or 4 cm.
You're basically creating a surface flaw to locate a failure point in the event of a collision.

Now, the big question: How big a surface flaw?

Are the numbers in your statement below correct?

Normally the thickest part ( 20mm ) is 1/3 of the way aft on the wing, not 1/6 as in your case ( 5cm ).

 

Very important topic with increasing world of foilers . Once I watched some Russian yt movie about Volga hydrofoil and one comment was important to me : "nice till first driftwood " not exatcly driftwood but soaked lumber .

 

@Thule more communicative drawings ?

 

Brain fart. I measured the handle part. Foil part is indeed 10cm aft of the leading edge

I will do some more digging about the surface flaw and see if I can find something
Thanks

 

Hi Montero
Sorry about cruiser drawings. What info would you like to see? I can add that

 

I would say it's not worth the headache. Once broken the situation becomes a mess to get home and clean up.
If the sailing grounds are that bad, consider a leeboard on a pivot and use the daggerboard and case when waters run a bit deeper.
They could be one and the same board with a drilled hole in the board and bolt-through mounting to the side of the boat.
On my boats with the daggerboards, they are retracted either partially or fully near shore.
I had one with a leeboard and pivoting rudder and grounding was a non-issue.

 

To be fair, the sailing area is not that bad and mostly things like partly submerged crab pots, sand bars and occasional log. Nothing that should really stop the dinghy to a dead halt. So, perhaps, only using half of the daggerboard until further out in the water should be fine too.

 

I am a mechanical engineer, a commercial mariner, and an avid sailor.
I would not introduce a surface flaw as a designed "point of failure" in this case.

If you choose to, how will you determine the size and proportions?
This is critical and very hard to estimate without destructive testing... $$$.

 

I have no engineering background, any more than what can be gotten from wikipedia and forums. So, I don't even know where to start on how to estimate a surface flaw as a means to start breaking off there.

I am perhaps overthinking this issue. I recall another post that said, they added a radius to the bottom end and coated the top of the board with plastic to make it super slippery and said it always kicked back up when they hit something.

I think I will go on a less windy day and scout out the area I sail and see if I hit any sandbars - they do shift but typically in about the same culprit areas. I can do repeat passes with more daggerboard in the water each run.

Thanks all

 

What I did with a high speed sailboat is taper the daggerboard wing profile to square where it meets the hull.
This leaves square edges in the trunk above the hull bottom.
The daggerboard pivots up into the hull on impact
and is returned immediately by an adjustable bungee cord arrangement.
The wing profile transition to square provides a point of failure under extreme loading.

I don't believe any of this necessary in your case, although the square transition makes a more secure fit in the trunk.

 

Thanks Bluebell. I think i will give it a try as it is and see. I should try to find a very slippery plastic coating another post mentions. That may help the boat to lift up easily

 

I've had boats where the daggerboard transitioned to square inside the boat and they worked well with the board fully extended.they were less successful when the board was partly raised as there was nothing to keep the board aligned with the direction of travel.
I have a feeling that the flaw with the whole concept under discussion is that the weak point will have no sense of the direction of travel and will break with a much lower loading applied transversely as the thickness is so much less than the chord.Any weak point that is introduced is automatically going to be a stress raiser and will have no respect for the direction that a sudden large force comes from.Which might be an unhappy discovery in the event of a capsize.

 

I agree. To make the board break in the direction of travel would make it too weak to function as a foil.

 

I couldn't disagree more Gonzo.
The majority of failures begin at a surface flaw.
Destructive testing would be needed to effectively design an intensional flaw.

The failure would not be initiated by side forces which the centreboard is subject to and designed for,
but rather from a front impact with an immovable, or near near immovable object.
The frontal forces are not excessive when sailing compared to the force resulting from an impact.