I tried an empirical experiment for my trip down to Hayling and back from Birmingham.  165 miles.
On the way down I trailed with the top cover on.  6.33 miles per litre.  fast trip.
On the way back without top cover.  Lots of traffic.  7.33 miles /litre

This is a significant variation.
Is it due to the sppeed or to do with the cover.  I would intuitively think that the cover would deflect the wind (clearly apparent by pressing down the cover at the stern)and thereby smooth out the flow and therefore less fuel required.  Maybe cloth drag?  On the other hand, the turbulence created without cover in the transom area would surely raise consumption.

Whats the Merlin Rocket techincal gurus view?
steve

So now we have 3 Merlin designs required!  River, sea/open water and road.  Seriously though, I am sure when towing that the wind direction will have  a huge input.  Was this a factor?  One of the biggest aids to my consumption when towing from Aberdeen down south has been cruise control.  Braking and acceleration are probably the biggest factor in consumption and I wonder was there more of this stop/start driving on the slow rather than the fast journey?  I have a diesel Rover 75 estate and overall I am getting about 40mpg sticking to the speed limit pretty well but of course a lot of the journey is on motorway. My boat isn't light weight by any stretch of the imagination.  Just a few thoughts.

Is it possible that the multiple circuits we saw you doing of the roundabout just off the island affected your mileage and therefore your petrol consumption?

I find speed is a major factor in fuel consumption when towing.

Without bothering to think about it, drag (therefore energy required to maintain a speed) will be a squared function of speed.

How fast was fast?

What was the wind on the way there and down?

A steady 5 mph northerly for the whole weekend would make a 10 mph difference to the drag between down and up trips.

The roundabout incident.  Ah! Crew at helm.  Quite upsetting. Somewhat agressive driver cutting across from middle lane as we sought the motorway in outer lane.  They stopped and complained that there was a child in the back. Tell me, is it reasonable to drive differently if there isn't? Presumably worse. I am always puzzled by this.  

The apparent wind always seemed to be ahead so causing us to beat into the wind at all times.  Otherwise, as you may recall the natural wind was negligible.  Resistance is indeed proportional to area and square of speed.  Hence, reducing solid area at back of boat with cover should reduce apparent area?

Cruise control is very good at fuel consumption.  It would have been dangerous to use on the way back due to stop start traffic.  Not to mention looking for it in my current car, as it doesnt have one.

Fast was speed limit as far as I could tell, and I would not tell otherwise. Lower speed in traffic jams, even stationary.  I would expect this to increase consumption as per extra urban figures.

There must science to this somewhere!  To cover or not to cover, that is the question. Or to suffer the slings and arrows of outrageous consumption.

I'm not surprised they complained if you had a child in the back of the boat, very dangerous practice. I guess if you have the top cover on they wont see the child next time and you will be fine to go around roundabouts as many times as you like without having 'the great unwashed' stop you.

Towing a double stacker at greater than the speed limit results in about twice the fuel consumption. Not exaclty (merlin) rocket science i know, but thought I would put my tuppence worth in.

I say dont cover up, but drive sensibly and save the fuel tokens and exchange them for beer tokens.

I always tow with the top cover on. this is so my boat stays cleaner.. Just look at the decks after you have towed with no cover and its rained. also if you leave sails in the boat like i do they are not on show if you stop at a service station.

you would find rather a lot of sick in the back of your boat if you went the round the roundabout with a child in the back (cover on or not).

I always tow with the cover on. Because when I get home the boat is all packed up and I can leave it til the next trip.

Stop start uses lots more energy than steady.  So every change in speed at modest speeds uses a huge amount of juice compared with the energy required to maintain that speed due to drag.

Doing stop-start - say 0-40 mph at low speeds acceleration is far more significant than drag in terms of energy used.

At higher speeds the drag is more significant than acceleration as the changes in speed are low - say alternating between 50 and 60 mph.  However drag is much higher - perhaps four times more drag for double the speed or something of that order.

I think the forumula is E=½mv² where v is the change in speed and E is the energy required to get you to it.

In other words, try the test again with cruise control in no traffic and you'll get far more meaningful results.

my figures suggest that stop start with no cover is more fuel efficient. Doesnt add up to me.

The late Les Ward who older Merlin Rocket Sailors will remember with his Bronze Coloured SS100 car which he'd kept running all through WW2 (When petrol was very hard to come by!)Les knew a thing or two about cars, driving and fuel consumption he was adament; smoothness in driving (Imagine a glass of water on the dashboard), spark gap, coast (depress clutch) down hill, keep in as high a gear as you can, (Low throttle opening.) and avoid losing built up speed. I used 6 gallons driving to Cheddar on the Saturday and Les (In the same car.) having adusted the spark gaps by sight on Sunday drove back using only 2! All of which may or may not have any bearing on today's automatic and semi automatic computer controlled fuel injected cars but it might just.

The late Les Ward who older Merlin Rocket Sailors will remember with his Bronze Coloured SS100 car which he'd kept running all through WW2 (When petrol was very hard to come by!)Les knew a thing or two about cars, driving and fuel consumption he was adament; smoothness in driving (Imagine a glass of water on the dashboard), spark gap, coast (depress clutch) down hill, keep in as high a gear as you can, (Low throttle opening.) and avoid losing built up speed. I used 6 gallons driving to Cheddar on the Saturday and Les (In the same car.) having adusted the spark gaps by sight on Sunday drove back using only 2! All of which may or may not have any bearing on today's automatic and semi automatic computer controlled fuel injected cars but it might just.