Is this thread still alive? The original question presents a very interesting challenge. The real issues have more to do with getting the prime mover/movers operating at their thermal efficiency 'sweet spot' and aligning this all at the speed you will most often be running.
Lets go through the knowns:
Simplistically a single shaft will always present the most efficient, largest diameter, slowest speed and minimal drive line losses.
Hybrids sound nice trouble is you will never have enough radiant area to get the solar
surface area you need to run at cruise
and sometimes you want to run at night!! If your hotel
loads approximate your drive power requirements then the equation changes, unlikely in a 50ft!! Works for large luxury ocean cruisers.
Oh were life that simple.
In designing 10/10/10 I started out with a single shaft, when you consider the power speed equation which really goes exponential at about 8knt it means a single engine is never operating at its thermal sweet spot in the 7knt area where most time in practice is spent. In effect any drive line efficiency is rapidly offset by this thermal efficiency loss apart from the long term impact of under loading the engine.
For a given HP in the water
is it possible to design a single propeller
as the optimal in this class of boats? On the surface - excuse the pun - it appears so. Maybe not. In 10/10/10 to get the propeller
efficiency, with the single shaft HP required artificially drives up the draft
, with this wetted surface area, therefore more HP per knt, pushes the engine to operate at poor thermal efficiency, during the majority of applications, means the engine sits in the middle of the boat above the sole line and presents internal layout inefficiency (a real advantage of hybrids). Additionally the shaft angle is higher, affecting propeller efficiency, there is limited redundancy and no shaft steering
In effect when I ran the numbers for 10/10/10 it was more efficient for all speeds to run twin screws/engines outboard
of the draft
centre line' allowing a horizontal shaft, with a concomitant reduction in draft. A double reduction is required to get a total reduction of 6:1, 2:1 from a curvilinear belt drop shaft to the gearboxes which are 3:1. All this forgets the drive line complexity of actual building and twin shafts come at more than twice the price!!
Shaft speed and propeller diameter (efficiency) dominate the whole equation. Getting 80%+ compared with the average 55% propeller efficiency allows true gestalt in the whole design equation.
Now back to the question. Being able to run in cruise
mode turning both shafts from a single engine would provide the absolute best drive line. When considering this in the design, which I rejected, the issue was not the cross connection complexity, modern curvilinear belts can easily handle, it was the fact that to make it work an ability to declutch the non working engine from the drive line was required, and this was the killer in the equation.
With the benefit of hindsight should I have gone with a single offset engine driving two shafts through two separate gearboxes? Small modern, not yet available for marine
application, common rail direct injection probably provide a broader 'sweet spot'. Setting aside the loss of redundancy, the clincher in my decision was the need to fit the engines, as low as possible, and the reduction pulley/belt between the frames which are 600mm. The GFC provided at very reasonable price
two three cylinder engines combined with the stub shaft they have 20mm clearance between frames. Would it be the perfect world I probably would have gone with a modern CRDI single engine driving both shafts.
Just some thoughts!!