I used a Yanmar 27 as the auxillary on my boat when circumnavigating in 90's. I used it several times for various reasons, totalling more than 3500 miles if I remember correctly - with two occassions of more than 1,000 miles each....the Indian Ocean between Diego Garcia and the Seychelles when my oil cooler failed and between Hawaii and the Line Islands to conserve fuel because...well it's a long story better told in a pub!
I would typically be very heavy with fuel (300-800 gallons during an ocean crossing), almost always in a following or beam sea, and the motor would push me along at a reliable 3-4 knots in almost any condition. Once I raised an emergency sail while using the Yanmar to see if it would increease speed - although it was fun to look at and the rigging was a welcome handhold on deck it only added about .5-1 knot without (no lateral resistance or efficient rudder). I wanted to power the entire voyage so never used it for any distance...just wanted to see if it worked in case everything else failed. A bigger mast/sail would obviously have helped. I've attached a picture that shows how the motor sat on the transom (mounted on sail tracks with an overhead block and tackle), and a couple images taken from a sailboat that was shooting documentary footage at the time I had the sail up.
the motor was really amazing and easy to work on and the engineering is a pleasure to see. I would recommend it to anyone who has the $ (mine was sponsored by Yanmar), space, can tolerate the weight, and has the need for a very reliable motor that will get you out of trouble when it counts the most.