The main reason was the fact that Alice (my wife) was finding it difficult to raise and lower the engine ... and also to pull start it. We didn't have self starting gear. We are both in our 70s.
The final 'push' came from an incident on the way back from Holland when the French 'black shirts' came and searched the boat for drugs in Dunkirk.They saw the row of petrol cans I used as spares, in case of a long motoring trip, and asked where the next port was. When I said Dover they let me go.
They wouldn't have allowed me to have that much petrol on my boat unless in a seriously secured tank ... in France.
I got some post retirement consultancy to pay for a new diesel and took the opportunity. But be warned ... since you need everything including the possibility of newly built up skeg to house the propshaft, it could cost you in total a good 6 grand.
Of course the huge advantage is fuel consumption and safety of less inflammable fuel in a proper tank. I have a Lombardini 13 hp diesel fitted and it uses little over 1 litre per hour.
I also managed to keep the well with the plug. (Most havn't done this so they could build up a bigger skeg under the filled in well). This means for me that should anything get round the prop I can clear it from inside the boat.
The main disadvantage of the diesel is that in over 20 years with an outboard I never had to worry about sucking debris into the engine wheras now I am constantly looking over the stern for the tell tale water spout!It can get quite weedy in Wootton Creek and even the Solent.
After a trip through the Kennet and Avon canal (Circumnavigating Southern England) the outboard was never in trouble even through solid duckweed. When we did this trip again in the reverse direction, some years later with the diesel, we had to clear the input filters every hour or so ... or worse. And once got the system completely jammed up.
If you can fit a secure tank for petrol, have self starting, are not concerned about the lifting and don't intend to do long motoring trips, I would stick with the outboard particularly in view of the capital expense. You will never get that back if you tried to sell your boat later.
The Super Seal is a fantastic sailing boat. Fast, safe and so versatile with the very simple lifting keel. Why would I have kept it for almost 30 years!
Chris.