I believe that the usual consensus is Auto for novices & passengers but Manual for anyone likely to have to brave breaking waves ( because of the risk of it going off accidentally when freeing a jammed sheet forward and being drenched while doing so ).
Far more important (IMHO) to have the type with a built in harness.
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Hammar has to be submerged for a few seconds to go off
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...and to a dept. That is why, when we bought our Baltics last year, Baltic did not make a hammar. You have to be half drowned before they go off! (small exaggeration!)
I would suggest that Auto is safer (how do you operate a manual one if you have been knocked out and overboard?)
As regards the Auto operating systems the following is an extract from the Crewsaver website
"Available on Crewfit 275N, 150N and Junior
This inflation option is available on most Crewsaver Air Only Lifejackets and provides an alternative to the Hammar Mechanism. Inflation occurs automatically when the paper retaining collar dissolves on contact with water.
Capsule fires within 5 seconds of entry into water.
Mechanism pierces canister, inflating Lifejackets.
Can also be inflated as manual version or orally.
Hammar Automatic
Available on Crewfit 275N, 150N and Freedom
The Hammar Inflator gives exellent protection from random activation caused by moisture, rain or wave splash. This is achieved by protecting the water sensitive element with a hydrostatic valve, which operates when submerged in over 10cmof water and, depending upon the Lifejacket style, held there for between 2 to 3 seconds.
Hammar works on water pressure, only operating when submerged in 10cm of salt or fresh water.
No accidentalk inflation, even in extreme conditions.
Cylinder is mounted on the inside of the Lifejacket inflation chamber.
Re-arming instructions and key provided with each Re-arming Pack.
Can also be inflated as manual version or orally."
I would also have an integral harness version and crutch straps fitted.