Fastening a heavy piece of equipment

[cruiser_pete]

I have a new piece of equipment (inverter/charger) that I need to install. The unit is about 10"x17" and 7" deep. It weighs 75lbs! . The manufacturer calls for "six 1/4" steel screws". I need to hang it on a piece of marine plywood that is 1" thick. I could add another piece of plywood to increase thickness to 1.5" to accommodate longer lag screws.

Question: Will 6 wood screws be enough to hold this in a marine environment? If so, how long is long enough?

The manufacturer is mumm on the length, they just give the number and thickness of the fasteners. It is a marine unit, so they expect this environment.

Thanks.

[nigel1]

Rather than screw's, could you use through bolts with large backing washers, that would be more secure if you can get to back of the ply.

[cruiser_pete]

Originally Posted by nigel1

Rather than screw's, could you use through bolts with large backing washers, that would be more secure if you can get to back of the ply.

I should have been more clear. I can't use through bolts as the other side of the plywood is exposed in the interior of the boat.

[Sailmonkey]

I've several heavy pieces of equipment mounted with truss head screws thru a bulkhead.


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[Stu Jackson]

Originally Posted by cruiser_pete

I should have been more clear. I can't use through bolts as the other side of the plywood is exposed in the interior of the boat.

Flat washers with finishing washers and acorn nuts. Or put the screw heads on the cabin side and the nuts on the unit side.

We mounted our Freedom 15 on fiberglass underneath our nav table with four 1/4" screws in 1998, it's still there.

[rwidman]

Originally Posted by cruiser_pete

I have a new piece of equipment (inverter/charger) that I need to install. The unit is about 10"x17" and 7" deep. It weighs 75lbs! . The manufacturer calls for "six 1/4" steel screws". I need to hang it on a piece of marine plywood that is 1" thick. I could add another piece of plywood to increase thickness to 1.5" to accommodate longer lag screws.

Question: Will 6 wood screws be enough to hold this in a marine environment? If so, how long is long enough?

The manufacturer is mumm on the length, they just give the number and thickness of the fasteners. It is a marine unit, so they expect this environment.

Thanks.

Can you remove the plywood and then reinstall it? If so, inserting stainless steel "T nuts" from the back side will give you the same effect as through bolting.

If the plywood is vertical, hanging something on it places the stress vertically so it's not like the weight of the equipment is trying to pull the fasteners out, it's trying to pull them sideways through the plywood. The same fasteners will hold more weight this way than if you were hanging the equipment from the ceiling.

Another plan would be to use the "T nut" trick on the second piece of plywood and then screw it to the original with as many screws as you think are necessary. Gluing the second panel as well as screwing it would add security.

[Dsanduril]

In the worst case a single #8 screw with 3/8" thread penetration in plywood has a 150# pullout strength and nearly double that for shear. Four or six screws with 3/4" embedment for a 75# load should be more than adequate.

Plywood Screw Strengths

[onestepcsy37]

Install it properly now so you wont have to do it again while you're bouncing around in rough seas and the wood screws pulled out.

As others have said, use big stainless bolts with washers and locknuts. If needed, add backing plates to thicken the surface. Dont obsess about a few boltheads showing in the cabin....

[cruiser_pete]

Thanks for the replies. These are useful ideas.

I will look into the T-nut suggestion. I have not used them in the past, so I have little knowledge of them. As I will likely add another piece of plywood, this could work well.

The unit will be mounted on a vertical surface.

The six 1/4" screws will certainly be enough to hold the weight. I am only worried about the impact or repeated loads over the course of many years.

One reason I am concerned is that the unit that is being replaced was installed at the factory. This unit only weighed 25lbs. The mounting bracket allowed for 12 screws, and they used 12 #10x1 wood screws.

[model 10]

I think mounting it vertically rules out just wood screws. If it were horizontal that would be completely different. Some units look like they are meant to be mounted any old way and some not so much. Our zantrex for example only has 4 mounting holes and it weighs a bunch. A 75# thing hitting a 4g drop going over a bar somewhere would really put some stress on simple wood screws.
Lag bolts can be mighty strong if done correctly.

[Terra Nova]

Originally Posted by Dsanduril

...six screws with 3/4" embedment for a 75# load should be more than adequate...

This--perfectly fine.

[cruiser_pete]

Originally Posted by Guy

I think mounting it vertically rules out just wood screws. If it were horizontal that would be completely different. Some units look like they are meant to be mounted any old way and some not so much. Our zantrex for example only has 4 mounting holes and it weighs a bunch. A 75# thing hitting a 4g drop going over a bar somewhere would really put some stress on simple wood screws.
Lag bolts can be mighty strong if done correctly.

We are intending lag bolts. What do you mean by "done correctly"

[model 10]

Originally Posted by cruiser_pete

We are intending lag bolts. What do you mean by "done correctly"

Hole diameter and depth for the un-threaded portion. The drill diameter & depth for the lag part and the torque.

[colemj]

Originally Posted by Terra Nova

This--perfectly fine.

I'll third that. No need for lag bolts, through-bolting, T-nuts or the like. You will be fine with the 1/4" screws in the 1" plywood.

Mark

[Terra Nova]

Wherever you must mount heavy equipment to thin bulkheads, and through-fasteners must be avoided, simply screw and epoxy glue a large, thick, plywood doubler, first. This will provide adequate depth for the equipment mounting screws, and also help stiffen the bulkhead.

note: if the equipment produces vibration, the bulkhead can amplify this to a disturbing level.