Motor mount construction - which wood and orientation is best

[oldshore]

My question is regarding rebuilding wood motor mounts.
Option 1) 3/4 ply on vertical on end, stacked next to each other.
Option 2) 3/4 ply stacked flat
Option 3) Solid piece of wood.
I realize working with plywood is easier to shape. Each piece cut separately then laminated.

Which will hold the lag bolts the best? Are all acceptable?
What wood would you use for option 3

I have a 1979 23' Chris Craft Lancer project boat. I believe the stringers are structural fiberglass, with no coring, about 3/8" thick. The deck is fiberglass. The motor mounts are wood cored and the wood is rotten.

Thanks very much, I have recently been lurking here, reading and watching most of Friscoboater's videos and learning a lot.

Jay

 

[Bomber Goober]

Re: Motor mount construction - which wood and orientation is best

Option 2 would be my vote and how I have seen most guys do it.

 

[GT1000000]

Re: Motor mount construction - which wood and orientation is best

+1 to Option 2, Laminate using any of the excellent quality waterproof glues like Titebond III, Gorilla Glue, etc...

 

[GA_Boater]

Re: Motor mount construction - which wood and orientation is best

Another + on option 2. It's the strongest method.

 

[oldshore]

Re: Motor mount construction - which wood and orientation is best

Thanks for your suggestions. Flat stack it will be.

 

[oldshore]

Securing motor mounts

Securing motor mounts

Boat is a 1979 Chris Craft Lancer with structural fiberglass stringers, going from about 3" wide to 6" wide for a 2 foot run where motor mounts are located. I removed the top of the fiberglass over the rotted wood motor mounts so the vertical sides remain. The replacement wood will probably be approx. 3" X 3" X 2 feet.
My question: when replacing the wood and glassing it in I assume, just as with the stringers, foam spacers about 1/4" high must be placed beneath it to keep it from punching through the hull. Am I correct?

 

[jbcurt00]

Re: Motor mount construction - which wood and orientation is best

If you are dropping the new motor mounts down into the slot left behind when you cut the tops off the motor mounts & removed all the wood, you want to use spacers & bedding to have a zero void area below the motor mount. <<< One of the reasons leaving the sides intact isn't ideal. It makes bedding the motor mounts & stringers a bit more difficult.....

It's the bedding that prevents the hard edge of a stringer from unevenly point loading the hull... The spacers are only to allow you to fill in below the stringer.

Make sure the verticals are still 100% and crack/defect free. They weren't intended as stand alone structure, and w/out the wood core (particularly during the wood removal) they can be cracked along the hull easier then you'd think.

What are you bedding the motor mounts in/with?

Are you coating the interior faces of the verticals as well to adhere them to the new wood?

 

[Lwarden]

Re: Motor mount construction - which wood and orientation is best

My question is regarding rebuilding wood motor mounts.
Option 1) 3/4 ply on vertical on end, stacked next to each other.
Option 2) 3/4 ply stacked flat
Option 3) Solid piece of wood.
I realize working with plywood is easier to shape. Each piece cut separately then laminated.

Which will hold the lag bolts the best? Are all acceptable?
What wood would you use for option 3

I have a 1979 23' Chris Craft Lancer project boat. I believe the stringers are structural fiberglass, with no coring, about 3/8" thick. The deck is fiberglass. The motor mounts are wood cored and the wood is rotten.

Thanks very much, I have recently been lurking here, reading and watching most of Friscoboater's videos and learning a lot.

Jay

I just did the exact same thing and used Lam beam material cut and glued on the flat!

 

[oldshore]

Re: Motor mount construction - which wood and orientation is best

Stringers are about 4" thick and widen to about 6" in area of motor mounts. They are at least 5" high. There is NO filler of any kind in any of the stringers anywhere in boat, except for a block of wood about 30" long 3" thick and 4" high. The base is cut at an angle to conform with the hull so the outboard side is about 2" thick. The wood only comes in contact with fiberglass on it's top and inboard side (side closest to the midline). After 44 years the wood (mahogany) was free floating - not attached anywhere. Half of it was rotted. This is not typical construction as far as I have seen on this website.
I have decided to replace the wood with a solid piece of wood shaped to fit as best as possible, saturated in epoxy and bedded in an epoxy peanut butter. The top would then be reglassed. Plywood and lam wood are both options but I can't see either of these being superior to a solid block of hardwood. Anyone disagree? Lag bolt holding strength comparing ply/lam/hardwood?

I was going to take pix but I stepped on my camera in the boat and crushed it!

 

[Lwarden]

Re: Motor mount construction - which wood and orientation is best

Stringers are about 4" thick and widen to about 6" in area of motor mounts. They are at least 5" high. There is NO filler of any kind in any of the stringers anywhere in boat, except for a block of wood about 30" long 3" thick and 4" high. The base is cut at an angle to conform with the hull so the outboard side is about 2" thick. The wood only comes in contact with fiberglass on it's top and inboard side (side closest to the midline). After 44 years the wood (mahogany) was free floating - not attached anywhere. Half of it was rotted. This is not typical construction as far as I have seen on this website.
I have decided to replace the wood with a solid piece of wood shaped to fit as best as possible, saturated in epoxy and bedded in an epoxy peanut butter. The top would then be reglassed. Plywood and lam wood are both options but I can't see either of these being superior to a solid block of hardwood. Anyone disagree? Lag bolt holding strength comparing ply/lam/hardwood?

I was going to take pix but I stepped on my camera in the boat and crushed it!

From what Ive seen and heard around Iboats lag bolts are common practice! I found that astonishing to be honest! but when ! pulled my 5.7 v8 mercruiser the 4 lag bolts that held her down only 2 were in good wood 1 on each mount. So I assume that they are there as just a hold down and not for torque? if you think about it there isnt much torque in a boat engine like there is in an automobile, spinning a prop doesnt take that much torque. But you'd still think they'd come up with a better way to hold em down on the boat!

 

[oldshore]

Re: Motor mount construction - which wood and orientation is best

Boats are just plastic and wood, and lag bolts hold good enough for the life expectancy of the average boat (10-20 years?)
Ideally I would like an imbeded piece of stainless steel or titanium to attach to. Totally unrealistic. If bolts came through the metal, threads up, that would work out great until the the bolt loosened or the threads got damaged. How would we replace the bolts? We really need a metal framed boat - like a car - where bolts for motor mounts are accessible and replaceable. Now we are getting into a whole different animal. We are stuck with lag bolts. Or an aluminum boat??

 

[jbcurt00]

Re: Motor mount construction - which wood and orientation is best

Boats are just plastic and wood, and lag bolts hold good enough for the life expectancy of the average boat (10-20 years?)
Or an aluminum boat??

Carp, you've just turned on the SC tin mob's equivalent of the Bat-light............. Expect frequent stops from the mob extolling the tin's inherent virtue's over the glass...

Don't fall for the donor boat spiel......

 

[oldshore]

Re: Motor mount construction - which wood and orientation is best

Nothing like the sexy swooping lines of an aluminum boat..... NOT!
Well maybe for a work boat I suppose

 

[Ned L]

Re: Motor mount construction - which wood and orientation is best

Some of the comments here about what the motor mounts in a boat have to do will apply to inboard outboards ( I/O's). They do not apply to inboards. The motor mounts on an inboard have a very different job to do.

 

[oldshore]

Re: Motor mount construction - which wood and orientation is best

My take: (just my two cents)
Inboard - direct torque from prop shaft on the four motor mounts
I/O two motor mounts and transom mount. Torque forces transmitted directly to the transom. Little to no torque on mounts. Just my two cents. Can anyone comment on this?

 

[GT1000000]

Re: Motor mount construction - which wood and orientation is best

Evenly distributed among all mounting points...
Sit in your car, put it in drive, hold the brake pedal down and press the gas...the motor torques the car...
In your boat, remove one of the motor mounts, not really, don't do it, could cause damage, and rev the engine, the torque created by the engine will try to lift it off the mount and if done enough, could create a lot of damage...