Hi Alison,
I think you would have to mount the dinghy very high on the stern to clear the top of the wind vane, and still higher to give it clear airflow - especially in lighter breezes. Another reason it would need to be mounted high is to prevent the dinghy from digging in to the water when the boat is healed over upwind, or rolling heavily. I saw a very bent dinghy davit on a heavy 40 footer once, where the dinghy caught a wave and put a massive pull on the davit...
I have seen a couple of dinghies mounted high on davits at Shilshole Bay Marina here in Seattle. These have been on bigger boats - like a big 46 or 49 foot Hylas - for example.
My opinion is that on a Tartan of 40 feet, with a narrow stern, and medium light displacement, trying to mount a dinghy on stern davits might not be a good idea for an ocean crossing. That being said, I know there is a Tartan 3800 in Anacortes with davits, and these would be useful for coastal cruising here where the waters are protected, and the dinghy (with 10 HP engine) goes in the water every evening to go explore the shore.
We have been able to tow our dinghy most of the time around Puget Sound and into B.C. However, sometimes the engine comes off then dinghy gets lashed to the foredeck to cross one of the straits if the weather is lumpy.
We have used an Avon Sport boat 310 with wooden floor boards and transom for 30+ years. It was great to roll up and store in the garage for 50 weeks a year. It was OK to store in the quarter berth of our C&C34 on occasion when we would have a long transit before going exploring. Just now I have bought a 10 foot aluminum bottom RIB, on which we will put beach wheels on the transom to ease the weight when pulling up the beaches above the rising tides of our cruising grounds. (Wheels are important for landing on the exposed beaches of Mexico too.) This will be heavy to hoist aboard with a halyard, and cumbersome to deflate, turn upside down, and strap down to the foredeck where it will significantly block movement on the foredeck and access to the anchor. (The baby stay shortens the available deck space on the Tartan 3800.) Oh well. I promised my wife we will not sell the lighter more compact Avon till we see how the new RIB works out for us... But that old Avon is getting really worn at the seams after all these years.
Chris
Wind Dancer
Tartan 3800 number 9