An OUPV License lets you work on sailboats without a Sailing Endorsement. It's valid for uninspected vessels, including sailboats.
Yes. Your six pack USCG License is only valid on uninspected vessels regardless of what type of vessel it is. It is valid on a sailboat, motor boat or even a drift boat with oars.

So if you have an OUPV (Six Pack) and are doing work for hire on an uninspected vessel, you do not need the Aux Sail endorsement.
OUPV (Six Pack) licenses are only valid on uninspected vessels and they can be any boat in the US that is not inspected by the USCG and doesn't have a COI (Certificate of Inspection).
If you are working on a vessel that is inspected by the USCG, then it has a COI (Certificate of Inspection) and a licensed master is required to operate the vessel when carrying passengers or cargo per the safe manning document on the vessel.
If that vessel is a sailboat, then a licensed master with Aux sail of appropriate tonnage would be required per the safe manning document.
The Sailing Endorsement can only be attached onto a Master or Mate level license. You CANNOT add it onto an OUPV license since the OUPV/Six-Pack license already covers uninspected sailing vessels.
To add the Sailing Endorsement onto a Masters Inland License you must provide documentation for at least 180 days on a sail or auxiliary sailing vessel. To add the Sailing Endorsement onto a Mate Near Coastal License you must provide documentation for at least 180 days on a sail or auxiliary sailing vessel with 90 days on Near Coastal waters.
To add the Sailing Endorsement onto a Masters Near Coastal License you must provide documentation for at least 360 days on a sail or auxiliary sailing vessel with 180 days on Near Coastal waters.

We hope this made your life a little easier and if you have other questions the MM-SEAS team is always here to help!
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Nate Gilman is President and Co-Founder of MM-SEAS. He hawsepiped from the deck to a 3rd Mate Unlimited license with full STCW compliance over more than 15 years at sea, and is a veteran of the NOAA Commissioned Corps. He works directly with the National Maritime Center, USCG-approved training schools, and vessel operators on credentialing policy, and writes on sea service documentation, evaluation standards, and how mariners move up the license ladder.