Query carbon rudder box layup and repair

Discussion in 'Multihulls' started by Gary Curtis, Apr 5, 2021.

  1. Gary Curtis
    Joined: Apr 2021
    Posts: 2
    Likes: 0, Points: 1
    Location: Singapore

    Gary Curtis New Member

    I just found this forum after a lot of searching on carbon construction. I did not find specific advice, so apologies for my first post being a query for help.

    We broke the rudder off our Corsair Tri in a storm 14 months ago. We were doing about 17 knots wind to forward beam after bearing away on the start line and the wind jumped through 35knots. I bore away trying to take pressure off the boat and reef (wrong) the rudder cracked on a big gust, broke off and flew away on the next.

    After considering limited options we ordered a new carbon long spade, cassette and tiller from a well known builder. We got it fit third quarter of last year after easing of movement restrictions and returned to sailing. About 5 light air races and one fifteen knot start, the cassette on left in photo exploded. From the damage, bottom of cassette blew out first then overloaded the bottom pintle which shattered, tore the cassette again and then tore the top pintle in about half. Blade is fine. My foot is there for scale and to hide logo.

    The manufacturer agreed to replace it and the new one is on the right. It weights about twice as much and has been significantly redesigned. We are waiting to test it but have a few fitting issues to resolve.

    Query:

    The manufacturer suggested we locally repair the old one, and the instructions were to grind off damage, replace pintels and laminate with 9-13 plies of I think 200 gram uni incorporating a pintle wrap ever 4 layers.

    On the exploded cassette, I have shown the direction I think they want the uni to lie, perpendicular to that I have drawn what I think is the failure mechanism. To me the uni layup thinks the biggest stress will come from tiller to blade is horizontal direction, where as the failure was bending moment caused by the blade. So aside from upsizing pintels or pin enclosure, I am wondering if to strengthen it one of the options might be appropriate:

    1) Adding bias ply to layup

    2) Changing uni ply layup to radiating pattern from pintels into cassette body

    3) Adding some type of core to base blank and then building up with a combination of 1-2 from above.

    Other than using a lot of carbon in windsurfing and limited boat parts I am new to construction and repair. Any thoughts or suggestions would be well appreciated.


    Gary
    Corsair 28
    Miss V
    Singapore
     

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  2. Tony.Ellen
    Joined: Nov 2019
    Posts: 10
    Likes: 3, Points: 3
    Location: Sweden

    Tony.Ellen Junior Member

  3. Ganzi
    Joined: Apr 2021
    Posts: 1
    Likes: 0, Points: 1
    Location: USA East Coast

    Ganzi New Member

    Hello Gary,
    I have the same cassette rudder for my Countour34 tri.
    I am very amateur at laminate but in case this helps:
    My cassette suffered multiple injuries and failures since the 4 years I have it, although not the same as yours. So, first, when I see the new cassette design they shipped, I feel ours was very inadequate. I have to be ready for the worst, I guess.

    The weakest point of mine was initially the tiller bracket. It seems that yours (original) had a flat surface at the top moining both brackets, mine didnt had that. The layout was uni, first I tried to repair identical, it failed again after a few months, next I used biax, and added horizontal lips at the top, and 50 % more layers. So far this hold.

    My second source fo failure was the blade trying to pop out the cassette, as I drove the boat over a rock (slow speed) (obviously cant blame the vendor for that one...). The rudder blade ended up about 45 degrees tilted to the back. I had replaced the series of bolts that hold the « leech » of the casette by nylon ones, except the top and middle one, and those 2 original metal bolts prevented the cassettte to fully float away (which of course I would have preferred).
    The damage on the cassette was not terminal, only fractures cracks around the pintle casings (sorry, my technical english is amateur too). I repaired with bi axial as well, following mostly the original design, with a bit more overlapping onto the cassette body. After one season I see small cracks appear again and try to persuade myslef that this is just superficial.

    In short, my point is, I agree with your reading of the efforts, and using biax instead of uni seems the easy answer.

    As for myslef, I am still wondering how the blade « exit scenario » is supposed to work. Nylon bolts are breaking constantly, last season I changed for a bigger diameter, and made a plate that make them sit square to the blade (did you notice how crude is the original assembly ? the bolts are at 10-20 degrees angle from the cassette cheeks). Thos broke again one by one. But using the original aluminium bolts seems absurd to me: In addition to corrosion, I dont think they would play thei - sacrifice and open the cassette leech. And lastly, if they do, unless the balde slides neatly and promply out back, I feel the cassette will explode.
     
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