I’ve read its often best to upgrade direct to 500T but I recently took my 100T at the REC and am applying for the 200T because it seems I can self certify my time as owner of an uninspected 82 foot catamaran. I’m not sure what the foreign manufacturer put a tonnage on it but since it is not documented and has a modified deckhouse I calculated the GRT per the formula and its 106T.

Do I simply declare the GRT when submitting my time or should I include some backup?

Do I understand correctly that I need only 180 days since over 101T? I have more than 1000 days overall.

Discussions here suggest the only additional exam to the 100T is stability. Is this true? I’m looking for an upgrade study guide for my testing at the REC.

I’m in agreement with others that you likely overcalculated the GRT on your catamaran. It seems very high for the length (and I assume this is a sailing vessel, correct?). But the rules don’t actually say you have to apply for a license under 200 GRT to self-certify your time. You can still self-certify with time accumulated on a vessel under 200 GRT when applying for a larger license.

I realize the difference. What I was trying to get the OP to see is that catamarans use a formula where each hull is calculated separately. And private vessels certainly qualify for USCG documentation.

To get his calculated 106T, I think he used the overall beam of the catamaran as the width. If you work the formula backwards using the OPs result and the given Length, you end up with W x D = 193, so the extreme beam must have been used, giving a very incorrect result.

The OP could use that formula for each hull separately, then add the two for a final result that would be a good bit closer.