Using Marine Tex to repair a motor mount

Discussion in 'Materials' started by David Grimes, Jul 30, 2022.

  1. David Grimes
    Joined: Jul 2022
    Posts: 10
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    Location: Clarinda, IA. 51632

    David Grimes Love The Lake

    I have a 1991 Mariah 1900-z that the port side motor mount has wood rot and the lag bolts loosened causing the motor coupling splinesto strip out. The mounts in the Mariah are solid wood 10 inches from inside floor deck to top of mount. The two lag bolts for the motor mounts are 1/2"x 3-1/2" long.
    It would be very hard to remove these solid mounts and replace them. My idea is the use a large drill bit and bore out the existing mount holes straight dow and then at angles to make the bottom of the holes larger in diameter than the top. Then using "Marine-Tex" fill the two holes and before it sets up put the motor mount back in place and insert a longer lag bolt 1/2"x 5" into the marine Tex and allow it to set up using heavy weights to hold them in place. I attached pictures of the mounts and the longer lag bolts I want to use. Plus one of my old coupler damage.

    I have used the Marine-Tex in place of welding on cast iron Exhaust/water manifolds as well as cracks in lower units with great success.

    If anyone out there has any advice or better ideas on this process please let me know!
     

    Attached Files:

  2. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Not really a great plan.

    Sorry, but the assessment of the wood rot is needed.

    Typically, you would just overbore the hole and fill it with thickened resin for a minor issue. But, sometimes, thickened resins in high cylinders get hot, so the caution would be say a 3/4" hole heating up and cracking..so, it gets done over several hours or with a plug.

    Anyhow, start by overboring and determine the extent of the rot.

    All rot must be removed or it will continue like cancer.
     
  3. David Grimes
    Joined: Jul 2022
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    Location: Clarinda, IA. 51632

    David Grimes Love The Lake

    My concern is the way that the motor mounts are made from the bottom of the boat and also wrap up and over the stringer. It looks like nearly impossible to take out the old and replace the entire mount. But this is my first attempt at doing a motor mount repair and I was trying to find a easier but yet strong solution. Here are 4 more pics that show how the mounts seem to be built into the stringer. On the inside of motor well the heigth is 8.5 inches and the screwdriver is 7.75" and it went all the way down with two fingers (TOTALLY ROTTED). So would it be better to drill a couple of say 2" holes into the inside face of the mounts for inspection as well as removing the bad wood. Then what w0uld my best option be for doing the repairs??????

    P1130912.JPG P1130913.JPG P1130916.JPG P1130919.JPG
     
  4. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    Sorry, but you need to cut the cap off that box David.

    Get a Harbor Freight oscillating tool. On Amazon, buy a couple of carbide universal cutting tools for it and cut the top off that box.. wear a organic respirator, breathing that burning crap is awful or at least run a fan

    You may need to build a cardboard and then plywood drilling template for it before you cut. So you can add the mounts later. Use the other stringer mounts for the best templating. Take a lot of measurements; they help when repairing..

    You may be a little ill when you do it because the stringers are probably shot.

    After you assess the extent of the rot, you can then plan. But your idea is basically to hope that fillers hold the engine. It is simply going to cause other problems with alignment, etc.

    cut that cap off, start hogging out rot
     
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  5. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    If you discover the rot is limited to the mount area, you can epoxy a plug back in. But first, assess. Then plan.

    Rotten wood is basically mush.. you can't build to it

    If you discover the rot extends through the stringer(s), replacement of the stringers is possible, or you may be able to take steps to use the old stringer as a form, but the motor mounts are non-negotiable and need to be solid.
     
  6. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    The motor mount is just a block of wood wich was carved to fit the stringer and glassed over. It's unclear if the stringer was glassed before or not, so we don't know if the stringer is affected by rot. As fallguy said, cut the fiberglass along the top perimeter to expose the wood underneath. You will probably be able to scrape the rotten wood out and assess the stringer. If you are lucky they glassed the stringer before glueing the motor mount on, in wich case you only have to replace the mount. That can be done two ways, depending on preference. One is to keep the existing glass, sand the inside and outside, then slip in new wood with thickened epoxy and glass over the top (round the edges first). The wood does not have to be all in one piece, you can do a sandwich. The other way is to cut the existing glass away, carve a new mount, glue it to the stringer and glass over it. If the stringer is affected by rot the second method is mandatory since you have to cut away part of the stringer and splice in a new piece of core.
    After this is done you drill oversize holes, fill them with thickened epoxy then redrill for the mounting bolts.
     
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  7. David Grimes
    Joined: Jul 2022
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    Location: Clarinda, IA. 51632

    David Grimes Love The Lake

    Thank you fallguy and Rumars for your help.
    I already have the tools to do what you both suguested. Just need a good mask or maybe my paint resperator will work with my face shield. I sure hope the stringers are still good. This will be my first time at this and at fiberglass too! :)
     
  8. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Drill lots of exploratory holes, they don’t have to be huge, you’ll know if it’s rotten, when the bit just plunges in once it’s through the glass.
    Marine Tex is very expensive epoxy with filler and color added.
     
  9. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    A shop vac and chisel is a handy tool here.

    to be clear on the cardboard, you take a good size piece to the compartment and set it across all the mounts, and then push down on the cardboard, and it'll make marks where the bolts are. Then you can cut those out and get all the center to center dimensions, then you only need measurements to the bolts from fixed locations. I'd sharpie some marks and write how far to each bolt, etc..

    Don worry if you damage the box a bit..it may or may not be all that thick..

    You do need a vertical reference or two as well. Just get in there with a level and get measurements that way as well. The more you write down now, the better chance you'll get the repair right.

    And you are past using marine tex, by the way..
     
  10. David Grimes
    Joined: Jul 2022
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    Location: Clarinda, IA. 51632

    David Grimes Love The Lake

    Great advice. My Dad was a carpenter, Cabinet maker and home builder and he alwalys said measure twice and cut once!! I am 72.6 years young but the old back does'nt last long. I pulled to motor by myself, which was a pain (Literally). So I am resting for a day or two. In that time I will measure and make sketchs. Used to be a draftsman, then into industrial maintenance. As I get into the mounts I will post some updates from this first timer. LOL
     
  11. David Grimes
    Joined: Jul 2022
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    Location: Clarinda, IA. 51632

    David Grimes Love The Lake

    UPDATE: I drilled 1/4 " test holes just thru the fiberglass on the stringers and then took a 4.5" HD wood screw and my drill driver and screwed it down into the stringer. Port side is rotted at least 4 foot from Transom forward but the Transom & Starboard side are good and solid!
    I really want to keep this boat since it is in great condition other than the port stringer & motor mount. Is there info on YouTube that would help me figure out how to repair just the bad part of the stringer and I guess I will have to make a new motor mount. One picture shows a ball peen hammer on the right which is as far as I have found bad wood and is 48: from transom. In one of the close ups you can see some one's attemp to fix God only know's what? The last pic is the bulkhead rlywood judt in front of the gas tank and it is rotten. I crawled under the hull to look for a holes or cracks. There are a few (4 or 5) small chips that are just thru the gell coat they are about 3/8" in Diameter. No cracks but I could not see what was uder the 4 bunks. Anyway I am at a loss as to how to repair the 4 + foot of port stringer and motor mount.
    P1130923-001.JPG P1130923-002.JPG P1130925-001.JPG P1130926-002.JPG P1130929.JPG
     
  12. kapnD
    Joined: Jan 2003
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    Location: hawaii, usa

    kapnD Senior Member

    Stringers need to be continuous structural members, so patching in can be tricky. Long scarfs will help marry old to new.
    Sistering alongside the existing stringers without removing the rotten parts can be done, but the motor mount issue complicates that.
    I think you’re in for some major demolition, and likely removal of the fuel tank too.
    I’ve had some success with just removing the glass from one side of a rotten stringer, allowing removal of all rotten wood.
    That would work for your motor mounts too.
     
    Barry likes this.
  13. David Grimes
    Joined: Jul 2022
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    Location: Clarinda, IA. 51632

    David Grimes Love The Lake

    Thank You for your information. I have never seen or owned such a nice boat everywhere else have just one side rot out. Plus the fact I have never refurbished one yet. If they are in very bad condition I part them out and the hull goes to the landfill. But this 91 Mariah looks like new inside. I just can't scrap it so I will take my time and try to do it right. Thanks to all who contributed your coments. a few last pics before I tear the interior out. :-( 1a.jpg 2.jpg 3.jpg 7.jpg 36.jpg 41.jpg
     
  14. Anthony Garifo
    Joined: Aug 2022
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    Location: Freeport, New York

    Anthony Garifo New Member

    We have all been there. The first goal in repairing or replacing rotted or fractured material is too fully identify the full extent of the damage. You should never assume an isolated area of damage or rot is quarantined from the surrounding structural tie-ins. Unfortunately, you really need to go hunting until you find good wood. It's not difficult to replace rotten stringers. Put the marine tex back on the shelf and keep it there. Good luck, I enjoyed your post.
     

  15. Anthony Garifo
    Joined: Aug 2022
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    Location: Freeport, New York

    Anthony Garifo New Member

    If you need some help or guidance with stringer fabrication and installation, I would be happy to make some suggestions regarding the best, and least painful method of making the repair. In for a penny, in for a pound. There are zero shorts cuts in structural repair; only those that folks make up in their dreams. Lol
     
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