I had the ubiquitous 15 h.p. OMC Zephyr 50/1 oil mix saildrive on my old O'day 28. It was, according to OMC long after they were no longer available, a detuned 25 h.p. commercial powerhead with lower unit. It was mounted on a cast plate and had no rubber dam/gland/or whatever it is. There was no soft "hole" in the boat. Outside was a fiberglass molded area thru which came the lower unit. Nothing soft. As a power supply it worked flawlessly for the years I had the boat including motor sailing the 300 miles from Seattle around the outside to the Columbia River.

I confess I was surprised to find the sort of outdrive mentality type soft "gland" in the newer saildrive units. It seemed counter safe to me. I've seen several examples of outdrive soft "glands" that have failed over the years. Perhaps those may have had a large UV factor in their demise?

As I said, it doesn't measure up to two cents worth but a curiosity to me.