Hi ...
As a marine GSR installer, I'd like to point out that the responses so far have failed to mention the need for an essential flue component for the MORCO - an adapter collar that sits on top of the draught diverter and provides a correctly dimensioned fit for the inner flue pipe (see image below).
If you are using a 'universal' flue - ie: one with an aluminium inner and a white enamelled outer, the inner (hot) tube goes up inside roof mounted terminal (tight fit) - see images below:
Note that the white enamelled tube is simply there to ensure the inner (hot) flue tube DOES NOT come into contact withe the cabin roof lining of the craft. This outer tube need to be trimmed to about 20mm above the external roof line - if it's longer the roof terminal shroud will not fit over it. The inner tube should be trimmed to the same length as the roof terminal shroud (see picture).
If you have followed me so far there's one more REALLY important issue to address - the flue pipe MUST BE VERTICAL ! If your roof is heavily cambered, the skirt on the roof terminal shroud cannot be massaged to fit - you must have an adaptor collar manufactured (see picture below) to ensure the flue is VERTICAL - if it isn't, the inner tube wont sit squarely on the heater's draught diverter and flue gasses WILL leak out into the gap between the inner and outer tubes - the flue will not operate efficiently and you may be at risk from CO poisoning ...
If you are fitting a flue to a MORCO, please seek the help of a competent (registered) installer.
PS: If you are a live aboard, your boat is in-scope of UK Gas Regulations. If you are not Gas Safe Registered, you CANNOT do ANY LPG work on your craft. So, if you are contemplating fitting a 'standard' MORCO IWH to your boat (ie not replacing one that has died) such an installation is ILLEGAL and you won't find a GSR installer willing to do the work. If it is a new fit on your boat, the IWH MUST have a balanced flue (ie: a room-sealed appliance which draws its combustion air from outside NOT from the cabin). The ONLY boat compatible appliance is the MORCO F11 (which needs 240v to power the flue fan).