A lot of the questions in regards to cost sharing are really an outgrowth of the same regulations
that the FAA put into place years ago. While there is a relatively sparse amount of information from the USCG, it is reasonable to look at what the FAA has done, and the way they look at cost sharing. Particularly in light of the same aim of the policy (the USCG actually cited the FAA policy when changing their regulations).
Basically the rule
should be interpreted as saying...
If there is any profit motive by the owner, then it will be considered commercial
. This means that direct costs of operation for the duration of that trip can be shared (fuel, oil
changes, berthing) but not the general cost of ownership
(insurance, depreciation, rebuilding allowance for the engines).
Basically divide the costs into two pots.
1) the cost of ownership
... These must be born by the owner.
2) the marginal cost of the trip... These can be shared
Note that the regulations
require sharing. This means that the owner must pay a pro rata share of the marginal costs as well as the costs of ownership.