motor mounnt stringers

[Indymike]

Hello all.
I am replacing the motor mont stringers on a Wellcraft Nova 21 with a 260 mercruiser. From the factory these were construced of clear dimensional lumber species unknown. Question is, should I stay with the dimensional lumber method or go to a laminated ply approach.

Example

What material is best for the lag bolts? Stainless steel?
Should the pilot holes for the lag bolts be drilled oversize... say 3 times the bolt size...and filled with peanut butter, then pilot drill the cured epoxy mix or just pilot drill the stringer and wet out the hole with resin?

[PAR]

The stringers are SYP or Douglas fur. Fur is orange looking amber and the SYP (Southern Yellow Pine) is pale yellow with less pronounced grain lines.

Stick with the solid lumber, plywood doesn't accept fasteners through the end grain very well, likely splitting the veneers.

It's always a good idea to bond your fasteners (oversize holes filled with goo) which can increase the pullout strength by several times. You only need about a 30% increase in diameter for the bolt (lag) hole, so a 1/2" lag would need a 5/8" hole (a little bigger wouldn't hurt, but don't get carried away). Wet out the hole with unthickened goo, then add thickened epoxy, then insert the fastener. I would expect the fasteners on your boat are all stainless so use that.

[marshmat]

If you want to go one better than the current setup, consider through-bolting some hefty aluminum L-angles to the stringers, and bolting the motor mounts to those. A bit more costly than just lagging the mounts directly into the stringer tops, but I've heard quite a few times that the L-angle setup is a lot stronger and more reliable.
Don't try to screw the lag bolts into cured epoxy; it'll crack. If you do lag bolt the mounts directly to the stringer tops, do as PAR says and fasten while the epoxy is still soft.

[Indymike]

Thanks for the replies. I'll check the lumberyard for a couple of pieces of solid lumber. Probably fir or spruce.

Mike

[stevel]

I think the only thing that should be screwed on a boat is (sexual reference goes here).

I can't tell you how many "dancing" engines I have seen because of the lag-bolt intallation method. Even if the mount is such that the lag-bolts are at 90 degrees to each other, you are still inviting the engine vibration, torque,...etc. to create a breach in your encapsulation that will admit water, and cause rot.

I have done several of the aluminum angle motor beds, and the are not that hard to do. Just bolt everything up to the new stringers once for fabrication purposes, then take it apart to relieve the sharp edges and send it off to the powder-coat shop.

When it comes back, bed it down well with Marine-Tex (or your own favorite concoction), and it will be good to go.

It also looks REALLY COOL!

[trisailing]

See my post here. engine beds for Beta JD3

[Indymike]

Is this a reasonable rendition of the angle aluminum approach?

Drawing of starboard stringer looking aft

[stevel]

That should work, but I have always seen and done it by having the vertical part bolted to the inside edge of the stringer with the horizontal part facing away from the stringer, toward the other rail. That also makes it easier to get at the fasteners, and, if so inclined, to attach strengthening members between the two rails.

[Indymike]

Look at this one, I think its closer. This would require the stringers to be moved outboard.

port stringer looking aft

[PAR]

Aluminum isn't much stronger then good hardwood so think thick stuff with big flanges. Also don't use spruce for the stringers. Use the heaviest (densest) piece of fur you can find, literally pick up each board in the stack and use the heaviest one, which will also have the tightest grain.

[TerryKing]

Originally Posted by Indymike

Look at this one, I think its closer. This would require the stringers to be moved outboard.
port stringer looking aft

What I did, which placed the motor mounts atop the stringer, was to have the "L" of the angle go up the inner side of the stringer and OVER the flat top of the stringer. I rounded the edge of the stringer so the angle stock (steel in my case) sat flush on both the side and top of the stringer.

Then I thru-bolted the angle as shown. I drilled and tapped the top surface of the angle (it is 3/8" thick) for the motor mounts. I bedded the angle on the epoxied stringer of course.

That way the actual weight of the engine or Vee Drive etc. is right on the top of the stringer. Even if the thru-bolt failed the engine would not shift downward. I used metal stock about a foot long, to distribute the forces more widely.

You COULD place bolts up thru the angle, and weld them on the backside, or tap the holes and screw them in from the bottom before installation. This would give you 'studs' sticking up. You'd need small 'divots' or recesses in the top of the stringer for the bolt heads.

I'd send a photo but the boat is 6000 miles West of here...