Motor mount stringer cracks - how to repair

Discussion in 'Boat Design' started by rpdwyer, Dec 29, 2013.

  1. rpdwyer
    Joined: Dec 2013
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    rpdwyer Junior Member

    Hello all.

    I have a 1997 Larson Cabrio 310 with twin 5.7 Mercs. Under each motor mount on the stringer are cracks which have been there since I've owned the boat... 4 years. While they are not getting larger or increasing, I would like advice on sealing them up for good. Below are images of both cracks. Can they be ground opened with a dremel and new glass fibre applied wrapping the whole base so as to create a new outer layer of glass... or will this simply be cosmetic and break over time?

    Thanks for any info.

    [​IMG]

    [​IMG]
     
  2. rpdwyer
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    rpdwyer Junior Member

    wow... looking into how to resize those images now....
     
  3. keysdisease
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    keysdisease Senior Member

    There is probably wood in there that you would rather not get wet. The cracks indicate stress and movement of the stringers. That arrangement of cutting down the stringer just in front of the mount is taking away a lot of stiffness and strength allowing the stringer to move causing the cracks. There is no reason to think this isn't going to get worse before it gets better.

    If you plan to address you may want to consider an atwarthship stiffener that would tie the two stringers together giving the stringers some serious additional stiffness and strength creating a "box section" with the transom making the back member. This should reduce the chances of this happening again and add some serious strength to the engine bed.

    A piece of 2 x from the top to the bottom between the stringers fastened and seriously glassed. For additional strength you could carry all the way down to the hull bottom and glass into the bottom too, however that may be overkill.

    just glassing the cracks is only going to give you a "feel good" fix.

    Steve
     
  4. Skua
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    Skua Senior Member

    Looks to me as though they are already wet and split open from swelling. You need to take a core sample with a drill. If they are wet, cutting them open, and repalceing the core is your only real option.
     
  5. PAR
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    PAR Yacht Designer/Builder

    Skua is likely correct, though the only way to tell is to pull one of the mounts and check the fastener holes or for further investigation, a core sample, say with a 1" hole saw.
     
  6. keysdisease
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    keysdisease Senior Member

    With all due respect, I see no sign of water in the core. There is no brown water stain coming out the bottom of the cracks and they are high enough to be out of the wet bilge area. Also wet wood does not expand to create a crack like the ones pictured, those are where I would expect to see stress cracks from the stringer moving.

    ;)
     
  7. JSL
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    JSL Senior Member

    What I see here is a prim example of a stress riser. There should be a fillet between the two levels of the stringer. Patching it up as is, the crack will be back!
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    With cracks like this, first thing is to establish why it cracked. Otherwise any repair will suffer the same fate.

    It is not so easy to tell from the pix, however, I tend to agree with JSL. The shear path from the webs of the blocks/mounts is sudden. This is a classic stress concentration. When you factor in fatigue, since the engine vibrates and the low modulus of composites, the flexing on such stress raisers could well be the root cause. Thus this should be addressed in any repair scheme you end up doing.
     
  9. PAR
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    PAR Yacht Designer/Builder

    Agreed with most of the above, but I've seen this type of damage before and it looks like the cap tabbing has broken away (for whatever reason). The sides of the stringer were tapped first and caped with (usually) with a hunk of roving over the top of the stringer. It's likely a combination of core expansion and stress riser, from the poor mount design. In any case the laminate will need to be ground back for a look see.
     
  10. rpdwyer
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    rpdwyer Junior Member

    Thanks to all who replied.

    Who would you recommend I have look at it to find out exactly what has to be done? A fiberglass guy... or a marine surveyor?

    If it can be repaired and strengthened without pulling the motors... well that would be ideal. Pulling the motors to do this and I have a feeling I won't be boating this upcoming year.

    Thanks,
    --Rick
     
  11. PAR
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    PAR Yacht Designer/Builder

    Yanking the engines is the way to go and not especially difficult. A "fiberglass guy" may be good at laminating, but not much else, so get the surveyor. In either case, the area needs to be ground down, so the cracked tabbing can be renewed, much of which can be done with the engine in place. I'd consider cutting the end of the laminate off, where it "steps" down, just forward of the mount. I'd use a diamond blade in an angle grinder, and slice off the laminate, so I could see the core. Of course, this is after I've "thumped" the area good to see how bad it is, may a few core samples too. The way I see it is, you'll need some new tabbing, so these minor, intentionally damaged areas, will just be part of the repair.
     
  12. Skua
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    Skua Senior Member

    Mine cracked exactly like this. Water in the core froze over the years and split them. No brown water, or anything leaking out. It's common around here.
     
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  13. CDK
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    CDK retired engineer

    Keep the pictures for future reference. Next year December the stringers won't look much different; after all the cracks were there when you bought the boat, so there is no emergency.

    Engine removal and complete stringer replacement is the only option, but like you wrote, that interferes with your boating plans. And what good does it to have a '97 boat with '97 engines and 2014 stringers?

    Boat builders use an ugly piece of timber (probably fir) and hide it under a lot of laminate. Usually there is a small gap in the middle to allow bilge water to flow to the lowest point, but it also provides an entry point to the unprotected side of the wood.
     
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  14. rpdwyer
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    rpdwyer Junior Member

    Thanks all.
    I have the marina's glass guy who has done lots of stringer work coming down to look at it.
    Will post when I know more.

    Thanks,
    --Rick
     

  15. rpdwyer
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    rpdwyer Junior Member

    Hello all... looking for some feedback on my plans for the season:

    Update.... the motor mount tabs and the stringers they rest on are definitely wet. The moisture came from either the limber holes or from the top of the mount holes as there was no sealant to prevent moisture from working it's way down around the lag screw holes. The good news is the moisture ends several inches on the stringer before the transom and the transom itself shows no indication of being wet.

    So it comes down to whether I skip the season and have the stringers done in the spring... or wait until fall to do the stringers and use the boat this season.

    My feeling is that as long as I can have relative confidence that I can make it through the season without the engine moving around I will wait until the fall and use the boat this season.


    So what I've done in preparation of that was I put a wrench on the lags and attempted to tighten them. They did turn with only a moderate amount of resistance telling me the wood they are grabbing is no longer doing the job. So I removed them and replaced them with 8" galvanized lags of the same size. Once I got down past the 3" mark the really bit into the wood quite well. I was able to tighten them down with a considerable amount of torque. I confident there will be vertical movement with these in.

    Now, for horizontal movement, I intend to do what Keysdisease recommended by tying the two stringers together with a atwarthship stiffener. This is what I envisioned for stiffener: 2 2X8's cut so they fit tight between the two tabs...one near the front of the tab and the other at the back. To secure, I was hoping to avoid glassing and just use some heavy duty galvanized or stainless L brackets to attach the 2X's to tabs. Then a piece of 5/8 plywood over the top of the 2 2X's screwed down to the two pieces. This creates a box that I believe will prevent any further movement side to side.

    Combined with the longer lags, does the above plan seem structurally sound to allow me to use the boat this upcoming season (May to October)?

    Thanks for any input.

    --Rick
     
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