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Trim Tabs to aluminum hull

Discussion in 'Chris Craft Roamer Yacht' started by CVS, Jan 15, 2014.

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  1. CVS

    CVS New Member

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    I have a 37’ Roamer Riviera express in aluminum. Purchased her in the water in VA, have had her out of water in TN fixing things. I have a problem I would like help with, concerning the aluminum hull.

    A previous owner installed trim tabs. He used 16 threaded 1/4" bolts to attach each trim tab at the bottom of the transom. The tabs are straight edged where hinged, and to accommodate the difference between the straight tabs, and the curved surface of the transom, the installer filled the difference with 36” fillers 1.5” thick, curved concave on the side facing the stern, and straight where receiving the tab hinge. Each tab has a filler with 16 holes, and the transom has 32 holes in it, where the bolts are threaded through, and there is not so much as a nut on the bolts inside. 32 holes below the water line filled with screws merely threaded through the hull!! Now get this: the fillers are made not of aluminum; not of steel, but of pressure treated wooden 2X4!

    I could not believe this. Making matters worse, I twisted out the bottommost bolt and after ½ turn, collected rainwater in the bilge began draining out! One half turn of a bolt (1 of 32 bolts) away from perhaps sinking the boat. Loosening several screws, it also became apparent that the filler was improperly shaped (too thin), and thus tightening the screws was pushing the straight edge of the steel tab, making the tab act as a reverse spring working to loosen the screws!

    My first question is, what is the best way to remount these tabs and repair the through-hull holes? All 32 of them, plus about eight more for the other end of the cylinders, mounted to the transom the same way. I really do not want ANY through hull penetrations for this type of thing. I am thinking using the wood fillers (with correction of spring action as to hinge) as mold for an aluminum replacement filler to be welded to the hull, with threaded bar welded to the new filler to match the hinge. Any other thoughts or ideas?

    Attached Files:

  2. K1W1

    K1W1 Senior Member

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    Hi,

    If you were to make your spacer out of thick material could you drill and tap the hinge directly onto this?

    For the holes behind the rams get some alu bar of say 1" dia x 2" long, drill out each hole and put these alu bar sections in, weld around around the outside and then re drill and tap for the mounting bolts.

    The holes I am suggesting you drill would be blind holes and not penetrate the interior of the vessel, you would need a set of taps to tap the threads if you want to get full depth thread in a blind hole.
  3. CVS

    CVS New Member

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    Thick stock

    Thanks K1W1 for your help. I was thinking along those lines but thought I might be overreacting to the wood spacer and all of the through-holes. Some of the things one finds on these boats, you just cannot make up. I was thinking the previous owner may have used steel for the spacer and so I was concerned with dissimilar metals as well. But to find soft, wood, and 32 holes... !
    Cary
  4. K1W1

    K1W1 Senior Member

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    Hi,

    Another thing to remember is when screwing into Alu some type of anti seize should be used such as Tef Gel or Lanolin if you can get it.

    Even though it is not so crucial in FW it is good practrice and will be rewarded should you ever want to take them off again.
  5. Capt J

    Capt J Senior Member

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    I agree that Kiwi's method is spot on and probably the best way to do it. Weld Aluminum bar stock to the hull blocking all holes penetrating the hull, and drill and tap the bar stock with blind holes, screw in with Tefgel and you have a safe, long term solution without spending a ton of money either......If you do the work yourself, the largest expense will be paying a certified welder to weld the barstock to the hull (this I wouldn't do yourself).
  6. Capt Ralph

    Capt Ralph Senior Member

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    AND, fasten a nice bonding cable from the tab to a dry hull location. Get a nice braided monel or stainless cable from an outboard shop.
  7. CR CRUISER

    CR CRUISER Member

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    You may have a bigger issue than you think. If that is pressure treated wood it may have reacted with the aluminum in the transom and created a corroded mess. Pressure treated wood has copper in it and aluminum does not get along well with copper.

    If you are lucky and there is no signifigant corrosion of the aluminum, then this is how I would rectify the tab mounts. Weld up all of the holes through the transom. The aluminum has to be pristine clean to be welded. A competant aluminum welder will know what has to be done before welding.

    I would fabricate mounting plates out of 2" x 1-1/2" x 3/16" channel. Trim it to fit the curvature of the transom leaving about 1/4" gap at its narrowest point. Have it welded on then drill and tap for the mounting fasteners. You may want to drill and tap it before welding it on just in case you break off a tap. Use a good antisieze such as Tuf-gel on the fasteners.

    I've used this proceedure on hundreds of aluminum hulls and have had no problems.
  8. K1W1

    K1W1 Senior Member

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    Hi,

    Do you think 3/16" is enough meat to give a viable thread for the tab to be screwed to?
  9. CR CRUISER

    CR CRUISER Member

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    I've done it on 3/16" many times and even 1/8" without problems. I usually use 10-24 X 1/2" SS machine screws.
  10. Capt J

    Capt J Senior Member

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    Ok, but explain the math to me though. If the trim tab is 1/8" thick or even a 1/4" thick, 4/16"+ 3/16"= 7/16" and 1/2= 8/16" so you're still puncturing the hull plate and defeating the purpose.

    I'd have 1" aluminum bar stock welded to the hull, use 5/8" screws, or even 1/2" with tefgel and you're good to go.....but make sure to use a drill bit stop so you can control the depth when drilling the holes.
  11. CVS

    CVS New Member

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    Bar stock or channel?

    Thanks to all for your help and time in responding. I am thinking that if I use solid bar stock, it may serve well to block the existing holes but would be harder to shape to the curve of the transom. (If I go with solid bar, should I have the holes welded closed first, or just cover them up with the bar and then weld,? Or both??)

    As CR Cruiser suggests if I am reading his comments rightly, using channel instead of bar, (and welding the existing holes closed) would make the shaping of the spacer easier as (I think) I would just be shaping the two flanks of the channel and not the solid bar. I would think that tapping the channel would be easier than the blind holes in the bar stock. And CR has a good deal of practical experience on this method.

    As to the penetration of hull, I think CR is suggesting making the very narrowest portion of the shaped channel at ¼ more than the thickness of the channel itself (so the "gap" CR refers to would be the space between the interior of the channel and the hull), so with the 3/16 channel and the thickness of the tab, a ½ inch screw would not touch or penetrate the hull (1/4 gap between inside of channel and the hull; 3/16 channel itself; plus 1/8 tab thickness should be more than ½, so the ½” screw would not reach the hull (will peek out of its threaded hole in the channel about 3/16, leaving 1/16” clearance in the 1//4” gap to hull if my math is right.) And there would be much more clearance on the ends of each tab where the channel spacer is much farther from the hull. CR, let me know if I have this messed up. I think I understand...!

    I had a thought or two about the copper in the CCA treated wood as a problem area. Thanks for a heads up on that and I will check that out.

    Thanks to all for your help. Will send pics of the new attachment of the tabs.

    Cary
  12. davidopie

    davidopie New Member

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    The way I did it ....

    You cannot avoid the need to form a flat surface for the piano hinges. I offer an outline of the method I used for mounting the trim tabs. This may not be the best, but I did the following and it seems to work fine. Basically, I made wedges that fit between the trim tabs and the transom. For my boat, I used wedges that went from near the middle of the trim tab out to the end. The middle of the piano hinge is supported directly on the transom.

    This is done by making a mold with plywood. The mold is placed on the transom so that transom forms one side of the mold box.

    I used only two molds: one for the left side of the trim tabs and the other for the right side of the trim tab. I supported the mold by stacking the wooden blocks that are used for blocking the keel of the boat. Use wedges, screws (only into the wood!) to get a stable platform for the mold, one that holds it tightly to the transom. I lined the mold and the transom with thin plastic sheeting (to act as a mold release). Then, I mixed up the epoxy with chopped glass and stuffed it into the mold. It is ok if the thick epoxy/glass mix hangs out of the top of the mold and squeezes out at the bottom because, after it cures, just use your table saw to trim it to the right size and clean up the edges.

    The bottom of the wedges and hinges are aligned with the bottom of the hull. It seems that the upward force on the hinge and wedges could be quite high. Therefore, making the wedges a bit taller than the piano hinge seems prudent, so as to form a solid base.

    Sand, prime and paint the wedges for a bit of protection.

    When you are done, you have 4 wedges - two inboard and two outboard. I used the 3M 5200 to fix the wedges to the hull. Once that was set, the holes were drilled for the bolts. Each hole was filled with 5200 and the bolt was slathered in 5200 for pushing through the hole. Avoid voids in this structure that can fill with water. The person in the bilge needs to quickly clean the excess off of the bolt, then apply the washers and nuts.

    It worked. More than 1 year after, there are no leaks and the tabs still work. Nobody enjoys drilling a couple dozen holes in the bottom of my 37' 1965 Al Roamer. But after some serious contemplation, this is the path I chose.

    If you want a more detailed description, let me know.

    Good luck.