Having purchased Paul's Henley 18 designs in Study form, I felt the overall look was far too stubby for the sort of thing I was after, and although Paul indicated that it was suitable for outboard installation, that would have meant a huge, unsightly lump in the rear deck.  I guess the original idea for the outboard position came from this boat at Val Wyatt marina:

 

 

which was moored just a few steps away from the launch offered for sale that I'd been admiring at the time:

 

 

Thoughts of sailing boats (many of which have a forward mounted mast & rear rudder), Spitfires and Tiger Moths (which are obviously driven from the front), coupled with a few commuting hours with pencil and paper, gave rise to the following idea (pencil lines sketched over the SF 18' Henley slipper):

 

 

note the outboard's new position, which would also mean moving the screen much further back, more in line with my ideal proportions a la Temple Maiden.

 

I can already hear you thinking "never work", "You sure?" and Hmmm..." etc.  It happened to me too. Still, whenever I encountered a negative reaction, I posed the question "....Why?".  No one was able to sufficiently convince me through their own practical experience that it would NOT work. Yes, it might mean providing extra airflow through the engine compartment for adequate carburation, but a suitably mounted car radiator fan could do that. If I find an outboard with thro-prop exhaust, even that might not be necessary.

 

So we move into the realms of efficiency - after all, an outboard has to be at the back of the boat to push it properly, right? Absolutely. Consider, though, that I'm legally limited to 4kts on the Thames, and if the stream gets above 3kts, they tend to put the yellow flags out anyway. So, if we can reach 7kts, we should make perfectly adequate headway on the river in either direction in any conditions we're likely to be out in.

 

Various calcs using Froude numbers, LOA, beam, displacement & God knows what else indicated that for this type of hull with a displacement of 2000lbs, I shoud need a max of 7.6 hp to reach 8kts. What about power losses? Fair play, I'll stick a 10hp lump in - 30% extra should do it!

 

The shortlist for short shaft engines so far is:

   Johnson 9.9 High Thrust,

   Honda 9 or 10,

   Yamaha 9.9.

Whatever I finally go for will ideally be a remote option, electric start, prop-exhausted four-stroke.

 

Oh yes, and whilst I'm hoping to use the outboard for propulsion, I'll still be using a rear mounted rudder for steering (but the idea of linking up the outboard in a geared manner to use as a kind of bow-thuster may have fleetingly crossed my mind ;-))

 

Having thus convinced myself (if not entirely everyone else) that this approach could work, I set about making some initial estimates & measurements from the types of outboards I might be looking for. This showed that I could approximate a 21" overall draft, which is more than acceptable for Thames cruising, as long as the principle works (see the model) then I'm happy to give it a try in full scale.

 

As far as I can see, the worst case scenario here is that I waste a couple of months cutting a hole for the outboard, then patching it up when I'm proved wrong, before going with the traditional (and more costly) inboard installation.

 

Back to the Selway design: The stem is also too vertical to my eye, the screen not raked back enough, and the overall air-draft slightly too high. Practicality also means she has a far wider beam than my aesthetic ideal based on a 4:1 L:B ratio.

 

My tweaked design from the Carlson software was finalised as below:

 

 

 

To get this overall profile, and yet still have a 7 foot long cockpit area meant going up to at least 18ft long. Fortunately, I had some avaliable space behind the garage into which I could extend if I knocked out the back wall and put up a temporary roof (see August 08 for details).

 

Between this aesthetic requirement and measuring my scarfed panels, I realised that I could actually go up to a 19' basic hull, giving me a little more beam to play with & a better displacement safety margin. Maybe I'd have been better starting with the SF 22' design & scaling down, but I'm happy with the way things have gone.