Surely the boom won't lift anything. The topping lift will do the lifting, along with a rope bridle from the boom end to the load? If that is the case the strength of the boom itself is not an issue, as it really only takes a compressive load. The mast will be taking a loading at the gooseneck, and with the boom over the side that load is trying to bend the mast sideways at the gooseneck. It is that load which I would want to calculate.
As said above. However I would think that the friction of the topping lift system would be such as to make it difficult to lift the dinghy etc. I base this on experience trying to hoist a man up a mast on main sail halyard. So assumiung halyard is robust enough I would suggest you might do better witha tackle from the end of the boom to the dinghy with low friction sheaves. good luck olewill
Not sure about 'guarantees'... reduces the risk might be a better phrase?
The real answer is that it all depends on the size of the fittings, boom and topping lift. Not convinced by Olwill's reasoning about friction. Our topping lift on the main boom goes round the same size sheaves etc as the main halyard to a winch (if necessary).
We have a 3.2 m rib with Yamaha 15 hp two stroke engine ( lighter than four stroke) my mizzen topping lift will lift the rib and engine BUT only when the topping lift is taken back to the anchor windlass which has an electric drum winch - the small manual winch on the mizzen isn't enough to hoist the weight of both rib and engine
Your nwxt problem is swinging the load from the water to the deck without taking chunks out of your top sides , we gave up , our engine comes off first using a three sheave manual set of blocks (lifting billy) and then the rib is lifted onto the foredeck using the main halyard and the anchor winch.
When there is any kind of swell rolling in , getting the engine from the yachts transom and into the rib is just too dangerous and like wise trying to take it off the rib transom and back into the yacht is a ball ache too , hence we tried to lift both rib and engine as one load. - wasn't possible for us
I haven't found a good solution and often just row ashore rather than do battle with the engine ....