Jack:

I served on two snorkel boats, RAZORBACK was a Guppy IIA. BLUEGILL was a former SSK with a big SQS-4 sonar in the bow, Both were 3-engine boats.

We could snorkel at 7-8 knots. Anything faster than that and the masts and periscopes would would've vibrated so badly they'd probably have broken in the "up" position (not a desirable event.) We could only snorkel on two engines anyhow -- three would have pulled such a vacuum in the boat that we'd have shut down anytime the head valve shut. We used standard aircaft altimeters to keep visual track of what "altitude" we were at. It's been a long time since i did any snorkeling, but as I remember it, on two engines at 80/90 we'd "fly" at 1,800 feet. When the head valve shut we'd "climb" faster than an F-4.It was quite a sight watching the altimeter hands go around. Not to mention having your eardrums popping (if you were lucky) and your sinuses sucked dry.

The engines would shut down automatically at 6,500 feet.

One of the things that snorkeling forced was an increase in the lobe-to-lobe clearance in the engine scavenging blowers to compensate for their having to run in a partial vacuum. They'd heat up and could wipe (thereby ruining your whole patrol.)

I can't think of any good reason for having to snorkel at high speeds anyway. You're noisy as hell when you snorkel at any speed. You essentially render yourself deaf passive sonar-wise. In an environmemt where other, unfriendly submarines might lurk, that's not a good way to be,

Hope this helps,

Fritz