Can you tow a propped tender?

Yes, absolutely. Have a nice big stainless custom towing eye with backing plates installed and tilt the bravo drives up and tow away. The Corsairs are fantastic boats but they don't make good tenders as their a bit to luxuriously fragile interior and cockpit wise to take the constant wear and tear of scrubbing off the carbon and exhaust soot after each tow. The sophistication of systems just don't take well to the pounding and salt water incursion that towed tenders encounter The enclosed bow or foredeck make hooking and getting crew on or off any type of sea is down right dangerous. Beautiful boats, lousy tenders.

I towed a Monte Carlo Offshorer for a year and it was a beautiful boat but a beast to handle close in with the big boat. It took a minimum of three crew to keep it from eating the stern of the big boats swim platform during payout on the towing hawser and to haul in on the stern winches and unhook to bring alongside to the whips. After one season the interior bolsters and cockpit panels had to be replaced from the constant cleanings . More than a couple of times I had damage to both boats and a couple of crew injured from a 32ft boat plus I had to have one crewperson dedicated fulltime to that tender to keep it clean, perform mechanical maint. and keep it in shape for guests. It was great to have on certain occasions but tenders of these dimensions belong in garages with overhead gantry cranes .

What about towing a SeaDoo Sportster...I picked one up with the idea of using it as a dinghy.

My concern is about how far it should be behind the boat while under tow.

Has anyone ever do this ?

On a 34 I owned long ago, I used a long moor whip pointing aft.
A painter was tied to my stern cleat, to the whips tip and then 4-5 feet to the wet-bike's tow eye.
Underway or at anchor the Sea-Doo never hit my swim platform and towed well.

On a larger or faster boat, some more scope and adjustments may be needed.

100' I would think as it has no drag to speak of. You will need to install water shut off valves since it is jet powered and towing it with any speed at all will flood the motors on it and fill them up with water. You can create drag by towing 10' of decent sized anchor chain in the water behind it with 10' of rope to a stern cleat. This will help it track straight.

i towed a 13' whaler with 30hp outboard for 20 years. 10 knots cruise on my trawler-no problem

I tow my 17' Whaler from my 50 Hatteras at 28-30kts regularly. I tilt the engine so the skeg is in the water so it traks straignt & about 100' out. The problem is when the weather goes downhill and you will have to reduce speed so the tender isn't taking a pounding. Hah, I just remembered a funny lol. We were towing our 6 or 8' Rib along one day from our older Post. My 4 year old son at the time looked back and said "dad, where's the engine". LOL!! The bouncing made it work it's way off because it had the hand crank down mount vs. bolted.

Off the original subject.

I was told by Mercury outboards to never tow with the engines down per the request of the insurance company, the thrust bearing are not made for the much reverse torque over long periods of time.

If you are towing smaller whaler and the such, but a line thru the aft center drain and run about 20' of line out with a loop, this will make it tow nice and straight. We learned the hard way with an 11' behind a 28' Bertram, the thing flipped right over! Oops!

I've towed several boats (not as same time) but with motors up and over.