engine mount installation on wooden core: discussing different options

Discussion in 'Fiberglass and Composite Boat Building' started by pistonfields, Feb 9, 2022.

  1. pistonfields
    Joined: Dec 2020
    Posts: 7
    Likes: 1, Points: 3
    Location: Switzerland

    pistonfields Junior Member

    Good day to you all!


    I’ve read through several posts on this topic but I am still unsure what my best option is:

    We are replacing the engine on our 27ft sailboat. We will put in a 2 cylinder Diesel Engine with 16hp, weight approx. 120kg (before that 1 cylinder petrol engine, had 5hp when new 50 years ago, don’t know how many horses escaped).

    We needed to modify and reinforce the previously hollow stringers on which the old engine sat. The structure is now longer and made out of a wooden core (fairly good quality European spruce, the only thing I could get hold of at short notice) that sits in one half of the old stringer (the other half was cut away to put in the core). I will glass over it with epoxy when done. I retained the old structure as a reference. We had to drill out and replace the stern tube and other bits and pieces too and that gave me a good reference to align everything and mount the drilling jig. Anyway…


    I did not want to mount the flexible engine mounts with lagbolts into the wooden core as a) it’s the wrong wood for that kind of job, b) I wanted to prevent rotting, c) the engine shall be removable for maintenance and to get to the fuel and water tanks behind the engine room if later require cleaning (I’m not a fan of removing lagbolts and then reusing the holes as is) and finally d) the rear mounts sit on only 2cm of stringer, no way a screw that short will do anything!

    Therefore, I decided to fix the engine mounts on a solid steel plate (8mm) and mount that plate onto the stringers (not glassing them in as I then have to thread into the plate in situ after proper alignment of the engine and the engine room is way too small to get that job done properly). The initial idea was to mount them on two steel plates welded in an L shape and bolt through the stringers horizontally. I recently discovered clearance issues (everything is rather small and cramped) that would leave me with substantial additional modification to the stringers to make it fit. The two surfaces would need to be parallel and all that seems too much of a hassle for what it is. That’s why I’m considering alternatives:


    A: fix the steel plates with lagbolts to the wooden core: the most common; for me not an option as explained above

    B: fix the steel plate with normal bolts (metric) threaded into epoxy filled holes / threads cut into epoxy inside the wooden core: not sure how deep those epoxy pockets should be and what strength to expect from thread in thickened epoxy. I’ve done that on several other occasions, but always for low stress items and not for engine mounts or similar force/weight.

    C: thread inserts epoxied into the wooden core, apart from that same as B

    D: studs epoxied into the wooden core. In my mind the best option in terms of strength. I would maximize bolt length. Only downside: with options A to C I could remove the bolts / lagbolts and slide the engine with the metal plate on the stringer forward to access the rear for maintenance. I cannot do that with studs in place. But I might be able to live with that and just loosen the rubber feets from the steel plate (until they collide with the studs when sliding forward, movement is again limited substantially) or mill a slot into the steel plate to provide for additional freedom.


    Any other ideas or considerations that help me deciding on course of action?
     
  2. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    It's overkill, but not wrong. Lagbolts into fiberglassed spruce will hold well though. A small engine like yours won't generate a lot of torque. The old engine was probably fine for decades. The usual problem with rotted stringers is that they did not caulk the lagbolts properly.
     
  3. Rumars
    Joined: Mar 2013
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    Location: Germany

    Rumars Senior Member

    Threaded inserts, bronze, knurled and finned, almost as long as the stringers are high (that's to increase bonding area with the epoxy). If your machinist is to busy to make you some, it's easy enough to make them yourself with a hacksaw/file/dremel/etc. the outside geometry does not have to be picture perfect (they don't even have to be round).
    Use bronze for two reasons, first no corrosion, second no galling with A4 stainless bolts.
     

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  4. pistonfields
    Joined: Dec 2020
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    Location: Switzerland

    pistonfields Junior Member

    Thanks for your suggestions. I'm actually liking the bronze insert idea. I can manufacture them myself on the lathe, knurl the outside for a bigger surface area and just epoxy them in. I'd do that after glassing the stringers because I fear I'd fill the threads when the inserts are already in place (even when covering them with tape or whatnot).
     
  5. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    I might be going against the general sentiment here but I would make big efforts to find a way to recess the plates into the new wood and bond the parts to the hull prior to tapping.All it takes is a method of drilling the holes in the correct location and a way to turn a tap.The perfect tool for the tap turning part is a joist brace,if you can find one.I wouldn't recommend just a knurled threaded insert as the possibility of tearing it free from the wood fibres is a very real risk in a soft wood.Its a safer bet to use something like a bighead as you can bond the larger flange as well as adding a screw or two.Which is still less secure than the good size metal plate that I would advocate.

    [​IMG]
     
  6. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    You don't need to bed anything in the wood. Even the inserts can be bedded in an overbore/fill with thickened resin and rebore within primary window. Make sure to use a well waxed bolt when you bury the inserts and remove the bolts after about 12- 16 hours of cure. Or overbore and do it in one step. You might want to make a template to get positions correct. No different than a drilling template for ob motor.

    When you place the knurled nut; you really want to fill the holes with plain epoxy and then force the thickened resin into the overbore before inserting the nut and bolt. Adding the overbore with only thickened resin can result in joint starvation if the wood pulls resins in. And a wetter mix might shrink a bit more, and is messier, but better results and less risk of the entire plug moving.

    I disagree with milling in place. That is what you'd do if something goes wrong.
     
  7. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    Anybody is free to ignore me,I've only been around engines from 6Hp to 1200 Hp on a couple of hundred boats.
     
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  8. pistonfields
    Joined: Dec 2020
    Posts: 7
    Likes: 1, Points: 3
    Location: Switzerland

    pistonfields Junior Member

    Milling / tapping in situ is what I definitely want to avoid. It's a mess to work in the engine room with only little space to fit my big *** in in the first place and I can't access the mounts from top or the sides. I will use the plates as templates for drilling and setting the inserts. That should give me a perfect fit.
    I always use epoxy primer with wood (and more or less anything I want to glass over or add epoxy in any form) and soak it to avoid joint starvation and improve adhesion. I have had good results with that so far. The primer I use is way thinner than the laminating epoxy and wets very well. I'll just mill some bits out of one end of the insert to give it a non-round shape. that should help against tearing it free. Even when I doubt that an insert ~20mm in diameter and ~50-100mm long with a propper knurl will break itself loose that easy. But I might be mistaken.
     
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