Interesting idea.  Building not necessarily for efficiency for ease of management.

I suppose the same factor results in the Survey ships staying out longer, using up more supply, rather than coming home to be overhauled?  Ease of management?

Actually the idea was that they would be able to go out farther from their primary base and still get a worthwhile bit of survey done.  If it takes three months to reach the current border or explored space and then they start surveying, they would already have gone through over half of what 1 engineering space can carry.  It also allows them to be used in an emergency to supply combat units.

I have noted that with the power boost these ships tend to be almost twice as fast as the warships are.

Brian

Missed the point that supply could be stored in cargo holds, can you draw supplies from your own cargo holds to resupply yourself I wonder?

have been trying out some survey vessels with 5 engineering spaces which allows them to stay out until they need an overhaul after 2-3 years so getting more productively out of them, less time spent moving to pick up supplies and more time on station.

The 50hs survey ships that I have been trying out seem to work quite nicely.  They have 4 engines, and 3 survey instruments.  Depending on the level of armor that has been researched they have anywhere from 2 to 7 engineering units so they can stay out a long time.  
I also have been using power boosted engines on non combat ships.  It can make a major difference in their speed once the power boost is over 10%.

Survey 50 class Cruiser    2500 tons     255 Crew     831 BP      Signature 50-288
5760 km/s     Armour 1     Shields 0-0     Sensors 0/0/6/0     Damage Control 0-0
Supply 1400  
Ion Engine PB-20 AR-0 (4)    Armour 0    Exp 20%

Advanced Gravitational Sensors (3)   6 Survey Points

The Jump ships for survey operations tend to be larger.  Sized for the smallest jump engine (usually a 60, sometimes a 75).  They also have one survey instrument so they can be given the same default order of surveying locations without constantly going back to change the default after splitting the fleet.

Compared to the costs of researching the 4-ship or 100,000 km capability, the cost of researching the jump drive itself is fairly low.

And for the size ship involved it is the difference between a size 42 jump engine that costs 441 BP and a size 32 jump engine that costs 256 BP.

And the research project cost for the small jump engine is 1200 RP.

Hmmm.  Running the numbers, I found something weird:
Comparing the size 15, size 20 and size 30 jump engines, the ratio of Sorium to Duranium was all over the map.

The prototype Jump Engine lists at 50 tons Sorium, 50 tons Duranium.  However, with any researched version of the jump engine, the ratio is 4 to 1, Sorium to Duranium, or 80 tons Sorium, 20 tons Duranium.

Also checking the costs, I find that Jump Engines below size 20 actually make up only a small amount of a Jump Cruiser's cost.

Another question:
How do you organize your survey fleets?  Go for concentrated survey efforts with all the survey ships in one fleet?

Lots of small survey ships that can disperse, go to the waypoints, and survey, or larger, more capable survey ships that can complete each waypoint faster?

I am at a loss as to how to do a mathematical comparison of the relative advantages.

By having a large number of small survey ships the ratio of survey cost to jump engine cost is good, but there are other considerations.  Smaller ships spend more on armor.  With the same payload/engine ratio, smaller ships will therefore be slower.

Trying to figure out how to quantify the problem:
Assuming a certain ratio of engines/survey instruments, the amount of time spent travelling from waypoint to waypoint will be the same whether dealing with one ship or multiple ships.

So what kind of ratio of time spent on waypoints to travelling should one shoot for?  Do multiple ships spend a larger percentage of time travelling?

Compare, I dunno, one ship with 24 survey instruments and three with 8.
One ship flies to a waypoint, spends a third of the time the others do, then flies to the next.  The three survey group gets three times the flight time covered at once, for the same amount of idle survey instrument time.

Of course, with more than three survey ships the total distance travel does increase.

Sheer wild-ass guessing, two survey ships are definitely better than one, three is somewhat better, and beyond six it gets more inefficient.

I only designed one type of jmp drive to start with so it got fitted to all ships.
it may not be using both types of sensors at once but it will be using at least one most of the time. My normal useage is enter new system start some on mineral survey and some on jump point survey then move which ever finshes first on to the other, repeat on next system etc.