In the 1840s I think they were still using British pounds for currency - other than things like the holey dollar etc., or rum.
The pay rate of 2d. per diem means two pence per day and there were 240 pence to the pound.
Presumably they got more when on shore leave because they had to pay for their accommodation (a bit like holiday pay these days??

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His 'gratuity of 12 months pay' would thus have been at least 2d x 365 = 730 pence or 3 pounds and tenpence. But more if they paid him at the shore leave rate.
How much did a voyage Australia to UK cost in 1840s? Well, the bounty immigrants from Uk to Australia were paid for at the rate of 15 pounds per adult which presumably gave the agents reimbursement for their passage plus some profit. I have an idea passage was somewhere around 5 pounds but stand to be corrected by someone more knowledgeable

Dawn M