View Full Version : Bolting through a sandwich floor

tonybroad - n/a
23-Mar-06, 10:59 AM
Hello friends..

I'm going to be bolting my engine mounts to a foam sandwich floor (glass top and bottom with 1" sheet foam between)the idea is to have a plate under the craft with two studs/bolts going through the sandwich, then bolt the mounts directly onto the hull floor via the studs/bolts

I'm conscious of potentially 'crushing' the sandwich but if i use spacers the mounts may not have sufficient clamping force or may loosen over time even though tightly bolted together

Has anyone any experience of this or have suggestions, i can't make my mind up on this one

Thanks in advance


andycollins - n/a
23-Mar-06, 11:43 AM
Hi Tony,

How about putting sleeves over the studs? These could be a couple of mill shorter than the thickness of the hull floor. Then, when you clamp down onto the studs the floor won't be crushed.

The drawback is that you'll not get the stability in the mounting that you would get be crushing it all together, after all that's how bolts work. To counter that use plates to sread the load inside and out and stick them to the hull with Sikaflex or similar.

Have I explained that well enough? if not I'll have another go.




tonybroad - n/a
23-Mar-06, 11:58 AM
Hi Andy

your thoughts are the same as mine, i mentioned spacers which will be the same as your tubes if i go with the idea but my fear is the same that even if the mounts are tightly clamped on the tube, the mount be secure enough

my other idea was to bond in a sunken 'cup' through the foam to the underneath layer then i could bolt through glass only and the sunken cup will give good rigidity and no fear of squashing the foam

thanks again keep them coming


Nick Long - n/a
23-Mar-06, 11:59 AM
Hi Tony,

You want to transfer the load to both faces of the sandwich, and you want to keep the clamping force that holds the engine to the mounts off the sandwich.

If I understand your dilemma, it is because you are trying to connect to the sandwich by clamping around it. I agree that this may well loosen with time.

To get round this, why not have a top and bottom plate that are each bonded to the sandwich and connect the studs to both plates? You can do this by passing a rod through the assembly with a spacer in between, bolting up with a nut and then having a stud protruding to mount the engine on. The spacer needs to be large enough to prevent rocking, which will destroy the sandwich.

Or you can fix the studs, bolts, mounts, whatever to the top plate only and have separate bolts and spacers between the top and bottom plates.

Possibly the problem comes from combining two tasks, making a hard point on a composite structure and mounting the engine. First treat the issues separately and then, if possible, devise a common solution.


Nick Long - n/a
23-Mar-06, 12:05 PM

Andy's and my reply came in together, bu there's another thought.

What is the nature of your engine mounting? Are the mounting bolts held fixed relative to each other, in which case you only have to consider axial and shear loads at each point? Or are they flexible, in which case there is also possible bending at each mounting point?


Hovertrekker - n/a
23-Mar-06, 12:07 PM
Put extra layers of glass in the areas of the bolt holes to thicken it up more, then use large washers on both sides like fender washers, and use 3M 101 marine sealant all around the bolts on the bottom to seal. There will be some slight crushing if you over tighten, but it should work fine.

Nick Long - n/a
23-Mar-06, 12:10 PM
my other idea was to bond in a sunken 'cup' through the foam to the underneath layer then i could bolt through glass only and the sunken cup will give good rigidity and no fear of squashing the foam

Not a bad idea, but you may need to ensure there are no bending forces on the individual mounts. Why not do this from the underneath, so that the bolt heads or nuts are recessed and don't sc**pe the ground or scuff your trailer? A top side cup would also fill up with dirt.


nickyd - n/a
23-Mar-06, 01:13 PM

I think it may be a bit late now you have done the laying up, but one way to do it is set an insert into the foam core and then lay up over the top of it. You can then drill through and when you clamp to it it clamps the glass to the insert and therefore eliminates the foam squashing issue....


I think this is how they do it on race cars and the like.

Even if you have done the lay up you may be able to do this still by cutting a hole in the glass and foam, putting the insert in and then glassing back over the top.


jar2 - n/a
23-Mar-06, 03:22 PM

The above reply details what the Sevtec designs do.

They use filler (microballon or lightweight body filler) as a non-crushable insert in the foam core. You can insert them after the skins are glassed by using a small hole cutter (20-30mm would do), drilling a plug out of the skin and foam and stopping at the second skin. Fill the hole as above and put a couple of layers of glass as a patch over the filler while it's still wet and one extra layer over the other skin (the patch only needs to be about 50mm square - all it's doing is connecting the filler to the skin). Because the fill material is in compression and is attached to the outer skins it's very strong. The engine frames and almost all of the hull hardware (including the towing points) are attached this way on most Sevtec designs (and the skins are only one layer of 6oz woven).

Sevtec also embed bolts into the filler so they don't stick out on one side - the rudder pivots are made this way. The disadvantage of this is if you damage the thread/stud it's a pain to have to cut it out of the hull.

tonybroad - n/a
23-Mar-06, 04:15 PM
Sunken cups from underneath it's going to be i reckon, thanks for the advice


Mart366 - n/a
25-Mar-06, 08:17 PM
you could try looking for druz fasteners, if i recall they do a range of large headed fasteners, possibly suitable for what you want.

When we were building lifeboats, the method we used, was to lay the

metal plate on the pu foam and mark round it, we then would cut the shape of the plate out of the pu foam and sink it flush, we then layed up over the plate effectivly retaining it, we then drilled and tapped it as required.

it was an approved method by Lloyds, and met the saftey req's for survival craft.

we used to retain the saftey harness,s in this way