Donal, it's difficult to quantify the effect of leaving the engine down or puting the 'plug' in the hole. The RYA Portsmouth Yardstick scheme allows a further 40 points for a three bladed prop, so with a base number of 976, this suggests a loss of about 4 per cent in speed. When I changed from a standard shaft engine (which allows you to stand the engine on the 'plug' in it's upright position) to the longshaft it became a problem as to where to put the engine when racing. I asked our club if I could race with the engine down and claim the extra 40.
In my experience it doesn't pay off. You not only have the extra drag of the three blades but the open hole causes drag as well.
The longshaft engine has the advantage that the main engine head and cowling is clear of the narrower part of the well and allows the engine to be turned at a much greater angle. This can assist sharp turns, particularly in reverse.
The alternative, if using the longshaft, is to manufacture a new 'plug' which would extend down over the 4 inches or so that sticks out below the hull when the engine is in it's 'parked up' position. In that position the top is just under the tiller.
I've been meaning to do this for years but not got around to it as racing is becoming less and less of a priority these days! Chris.
Chris Turner