there is no real load from the seal
Essentially the load is just
your applied pressure on the bellows, when you set it, per PYI instructions.
There is a very small bellows compression [spring] when you forward thrust your boat.
Your engine mounts take the thrust load.
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Note: If those rubber bellows took the 50+ horse power thrust to move you boat, there would be no gap between the ribs on the bellow.
Tip: Measure the rib to rib gap when it is brand new, no engine on. Log that. In say 6 months, check the gap again. Log it. Repeat that seasonally for sure. Change in gap, change in seal effectiveness over time. Adjust if necessary.
Jim...