Tonnage tax exclusion limits deductions and loss set offs for shipping income, preserving tonnage income computation under the scheme. Section 230 prescribes that, for a tonnage tax company, tonnage income computation treats specified losses, allowances and deductions as already given full effect; disallows carry forward or set off of certain losses relating to qualifying ships while under the scheme; excludes Chapter VIII deductions for qualifying shipping profits; and requires depreciation to be computed as if depreciation deductions for the relevant years were claimed and allowed. Pre option losses attributable to the tonnage business are to be set off against relevant shipping income while under the scheme and cannot be set off against other income after the company exercises its option, with apportionment made on a reasonable basis.