Mercruiser engine instalation?

C-NUMB · 17 June 2014, 12:04

I am about to install a 3.0 L Mercruiser( +alha) and need to build the engine bed for it.
In fact it’s not a bed as it has only one front mount in addition to transom. Previous engine had different mount configuration (one on each side), so have to start from scratch. But the question is, what is the best way to build a solid platform for the engine front mount in a GRP hull?

Have been considering two different configurations:
1 building up the needed thickness with epoxy glued plywood, on top a 5 mm thick flat iron( bolted to the wood). The whole piece then covered /laminated with epoxy and suitable fiberglass fabric . On top of this will fasten the front mount, drilling through the flat iron and making suitable threads.

2 building up the needed thickness with epoxy glued plywood, the whole piece then covered /laminated with epoxy and suitable fiberglass fabric.
Through this will drill horizontally and bolt a 5mm thick angle iron. The engine mount will also be attached to the angle iron.

3 How would a pro do it....?

A very rough “drawing” attached….

Thanks for any comments and views

C-NUMB · 17 June 2014, 12:11

picture...

beamishken · 17 June 2014, 13:41

its more usual to just bolt down the mount with coach screws as in the left diagram rather than use steel glassed in.
Or do it like your drawing showing the angle bar with through bolts but better to use ally angle rather than steel either method would be acceptable

two stroke mick · 17 June 2014, 13:58

From an engineering point of view the first system looks better. The second induces a rotational effect into the strip of timber which can be engineered out

beamishken · 17 June 2014, 16:49

Quote:

Originally Posted by two stroke mick

From an engineering point of view the first system looks better. The second induces a rotational effect into the strip of timber which can be engineered out

I have 2x 315 6lp yanmars fitted exactly like the right picture and my previous boat had 2x 300 hp merc diesels the same hadnt been a problem just go with whats easiest to install its not like its a heavy engine

C-NUMB · 17 June 2014, 18:39

Thanks for comments and advice so far, all useful points. I am not totally comfortable using coach screws ,its a planning hull and the short chop over here can be stressful on the mounts so rather do a bolt trough solution of some kind.

How about this, i make the beam hollow(thicker GRP), flat alli inside and then bolt through.
Access to the nut through a hole big enough to put a fixed wrench inside(with the nut inside with tape)?

The holes is maybe weakening the structure a little, but don't think the impact is significant to the overall strength.

beamishken · 17 June 2014, 18:57

a bit complicated when there are much larger engines than yours sat on coach screws
the transom mounts do most of the work in your application

captnjack · 17 June 2014, 19:22

Quote:

Originally Posted by C-NUMB

Thanks for comments and advice so far, all useful points. I am not totally comfortable using coach screws ,its a planning hull and the short chop over here can be stressful on the mounts so rather do a bolt trough solution of some kind.

Skip all these internal steel reinforcements, they are just going to rust out and change the mount shape/integrity.

Epoxy the coach screws in and not have all the holes for water to get into and rot your wood.

C-NUMB · 17 June 2014, 19:26

Quote:

Originally Posted by beamishken

a bit complicated when there are much larger engines than yours sat on coach screws
the transom mounts do most of the work in your application

I agree, edit 3 is maybe little overkill

Is there a "marine grade" coach screws available,
i only ever used cold galvanized ones for some wood construction stuff in the past.

A simple solution would be nice, guess it can be tough to get the mount placement 100% correct before having a "dry" test with the engine in place. My plan was to preliminary test the position, then lift the engine and do the final laminating to the hull before final installation.

beamishken · 17 June 2014, 20:48

You can get stainless ones from a decent chandlery or screwfix do a range of a4 stainless coachscrews

kubcat · 17 June 2014, 21:03

My 38ft Cigarette with a pair of petrol 8.1L big blocks weighing 500kg each has stainless coach screws holding the engine mounts in. It is a boat designed to do 80mph through the ocean so I am thinking coach screws may work for you.

yam man · 17 June 2014, 22:44

Are you on about the triangle
Front mount , bond wood to hull an glass in an bond 10mm think steel plate to tap in to.

C-NUMB · 17 June 2014, 23:06

Quote:

Originally Posted by yam man

Are you on about the triangle
Front mount , bond wood to hull an glass in an bond 10mm think steel plate to tap in to.

Yes, the triangle. The way You describe it was close to my original idea, but most over here recommends the coach screw route. Guess both ways works equally well, a 12 mm coach screw will bite pretty well in epoxied plywood.

Here a picture of the ones supporting the original 4.3L, they have ally or steel inside.
The main stringers has no wood and are really heavy duty built. In the picture engine room bulkhead, not transom. I am not sure do i need to remove the original supports or not( i rather not)the 3.0,L and 4.3 are pretty much same size.