Outboard motor. Too much stress on the transom?

[pfwhitworth]

I have a 1974 Irwin 28 with a Volvo Penta MB10A gas inboard which was on its way-out last season.
I want to install an outboard on the transom for the coming season. The transom is pretty much vertical. I have done some research and found several pictures of Irwin 28's with outboard engines mounted off the transom so it's not an unusual mod.
My concern is the weight. The bracket itself weighs around 30 lbs (rated up to 260 lbs motor) and the outboard I have in mind (Suzuki 9.9 HP with electric start and remote controls) weighs in at 125 lbs. So, a combined weight of 155 lbs hanging off the transom!
To reinforce the transom, I planned to use a 12" x 12" x 1/4" aluminum sheet on the inside. The recommended bolt sizes are 5/16".
My concern is that this kind of weight is going to be too much for the transom regardless of the reinforcement.
Any advice much appreciated!

 

[Simon Sexton]

@pfwhitworth ,

Good afternoon! Depending on the thickness of your transom, it shouldn't be too heavy of a burden. The aluminum plate is a good idea, just make sure your through-hull fasteners are equipped with proper washers and load capabilities, as well as ample waterproofing. On smaller boats (22-25 ft. range), I've seen some sailors lay down more fiberglass on the exterior transom to maximize strength. This also may reduce the chances of getting any crazing/cracks in the gelcoat from the stress. It all depends on how thorough you want to be! I modified my Mercury 8hp. 2-stroke outboard mount with a thick brace made from Starboard that goes between the mount and the transom wall. It works great for me, so you may want to check out similar ideas.

 

[JoeWhite]

What is the horse power of the inboard engine?
You may consider matching the HP.
Does the outboard have alternator to charge the battery?

 

[pfwhitworth]

@pfwhitworth ,

Good afternoon! Depending on the thickness of your transom, it shouldn't be too heavy of a burden. The aluminum plate is a good idea, just make sure your through-hull fasteners are equipped with proper washers and load capabilities, as well as ample waterproofing. On smaller boats (22-25 ft. range), I've seen some sailors lay down more fiberglass on the exterior transom to maximize strength. This also may reduce the chances of getting any crazing/cracks in the gelcoat from the stress. It all depends on how thorough you want to be! I modified my Mercury 8hp. 2-stroke outboard mount with a thick brace made from Starboard that goes between the mount and the transom wall. It works great for me, so you may want to check out similar ideas.

Hi Simon,
Thank you for easing my mind about the weight. I'll make sure I source fasteners that can handle the load and good-sized washers. I like your suggestion about a layer of Starboard between the bracket and the transom. I tend to over-engineer things so I may put a sheet of starboard on the inside of the transom, then my aluminum sheet on top of that.
Thanks again for the great advice!

 

[pfwhitworth]

What is the horse power of the inboard engine?
You may consider matching the HP.
Does the outboard have alternator to charge the battery?

Hi Joe,
Thank you for replying to my post. The horse-power of the original inboard engine is supposed to be 13, but it is nearly 50 years old, so I doubt that's the case now. From my research I believe a new 9.9 hp should be man enough for the job. I sure hope so because the next step up seems to be 15 hp. Much more expensive and heavier still.
The outboard I'm looking at does have an alternator and charging capability.

 

[Benny17441]

I would be more concerned about the vertical acceleration forces that would be imparted when the stern falls off a wave which can multiply the load imposed by the engine static weight by 2X or 3X. In reality it should be fine as long as the transom is properly sandwiched between a backing plate and the motor mount. 2 Things, get a long shaft model for the engine to help keep the prop in the water and get an engine bracket that will facilitate being able to lift or tilt the leg and prop out of the water when not in use. Outboard thieves is another concern, should not be a problem if you are in a good marina or private dock.

 

[pfwhitworth]

I would be more concerned about the vertical acceleration forces that would be imparted when the stern falls off a wave which can multiply the load imposed by the engine static weight by 2X or 3X. In reality it should be fine as long as the transom is properly sandwiched between a backing plate and the motor mount. 2 Things, get a long shaft model for the engine to help keep the prop in the water and get an engine bracket that will facilitate being able to lift or tilt the leg and prop out of the water when not in use. Outboard thieves is another concern, should not be a problem if you are in a good marina or private dock.

The outboard I'm looking at is long shaft 25". The bracket has a long travel of 16" and the motor has a tilt capability so hopefully that combination will keep the prop in the water when needed and out of the water when sailing.
I never thought of outboard thieves! I'm in a private club marina but even so, I will make sure to install a lock or some kind of security.
Thank you!