Wooden Engine Bed Modifications

Good day members. I have a 21ft displacement hull (matelot 20) which had an old 2.2 BMC engine in. As the engine was unserviceable, I have removed it and acquired an excellent little 15hp Lister TS2 with Hurth HBW100 Gearbox. In order to get the new engine far enough down to line up correctly, I would have to notch out the engine beds in the boat by maybe 2 inches to allow for the feet and engine cowelling to go down further.

The engine beds are fibre glassed wood and run from the stern almost right up to the bow. My question is, if I were to notch these out then re-glass over them, would it compromise the strength of the boat? as im guessing that these beds may have something to do with the lateral strength of the hull.

Thank you very much for any help.

Regards, Chris.

What are you doing. You have a 2 page thread on an outboard conversion now you want to fit a lister ?.

I am exhausting all options before I go ahead and get rid of the engine and fit the outboard.. Literally the only way it would go in and allow me to keep that pretty stern is to notch out the engine beds, but im worried it will compromise lateral strength of the boat. If its a no go, then I cut out the stern.

It was the comments on how good the boat looks that have made me give one last effort to trying to get the inboard in.

It is a bit hard without a drawing or picture to 'see' if 2" or so will make a difference. If say the engine beds were 12" deep where the engine is, well it won't make much difference!. If the beds are 3" thick obviously it will make a large difference. But also be prepared to think sideways too - literally. As in you may get away with adding a side part relatively locally to the existing, but cut down bed to keep good enough integrity.

Those Listers are well known for long life and it may be a good option. Remember to try and avoid hard sharp notches when cutting out as they will create stress raisers. Also rounding the ends will make glassing over much easier and allow more integrity.

It's all about the depth of the current engine beds. How deep are the engine beds now (I'll guess about 6")? 2" is fine, though as mentioned you'll want some local reinforcement to gain back lost stiffness. This can be done with metal, wood or a combination of things, such as wood covered with 'glass, like you have now. I like metal for this type of thing, because it makes things easier, but any of the options will do.

I agree with what PAR said. If the engine bed is so low profile because the engine needs to sit low, we use a steel cradle. The engine bolts to the cradle and the cradle is bolted to the stringers. The steel gives it the stiffness the wooden bed/stringers cannot give.

In higher speed craft, it's common to use a metal bridge both fore and aft of the engine/trans assembly, if only to keep the beds from spreading under load. I use the cradle/tray system frequently, often incorporating the engine/trans mounts right in it, so the whole thing is dropped in place as a homogenous unit. The cradle is usually and simply some deep sided angle stock, to fit over the stringers/beds and through bolted through their sides, rather than lagged down from above, plus a few cross braces. This places the fasteners in sheer and also helps prevent moisture from climbing down the threads, from above.

Engine beds for 339 Intrepid converted to inboards

I had to rework a poorly made extension for a 339 Intrepid converted to inboard. Attached are pictures of the old "stringer/engine bed" and the new engine beds made with:

Foam shaper
2 layers CSM
1 layer 0/90 Biax with CSM base
2 layers +-45 Biax with CSM base layers +-45 Biax with CSM base
1 layer 0/90 Biax with CSM base
2 layers +-45 Biax with CSM base layers +-45 Biax with CSM base
1" x 3" Aluminum flatbar entire length of bed, scored and shaped. Embedded with epoxy and milled glass
1 layer 0/90 Biax with CSM base
2 layers +-45 Biax with CSM base layers +-45 Biax with CSM base
1 layer 0/90 Biax with CSM base
2 layers +-45 Biax with CSM base layers +-45 Biax with CSM base

Drilled and embedded 3/4" KeenSerts with 5/8" inner thread

After every Every 3 layers belt sanded to keep flat top shape.