Adding 2nd pump to alde

I think the flow might be quite hindered if the system returns back down the boat (particularly if these are 90 degree elbow joints and not 'slow bends'/curled pipe). So I can certainly see the benefit that an extra pump may give offering a more uniform heat - you might also just need to use it like a 'boost' function until the temperature is up and then just switch back to the impeller to save ont ta amps.

The jabasco/waeco inline pumps might be the ticket - bloody pricey though! Looks like it would work but if it were me, I would try and run it by an engineery type first.

Side note: I have a certain affinity to this boiler - got us through this winter on the cut after, I kid you not, we actually remembered we had gas central heating! Fired-up first time after 5yrs of not being used (yes, I was careful).

Side note 2:
Think you can convert it to have the option to run on electricity - maybe useful if in a marina.

Please let us know how you get on fitting a pump if you end up doing this - would be interested in doing this if it works (could be yet 'another' job on our list for winter!).

Yes thinking the same, its a long way, uphill, around sharp 90 degree bends and up two rads for such a little pump.

I think if i do try it i will go for the cheap pump first, its placement though is another question, just after the boiler, up at the bow to boosts along?

To do the job properly, calculate the resistance of the pipework, valves, joints, bends, radiators and boiler and compare it to the performance of the pump chart.
Also calculate the heat carrying capacity of your pipework to the radiator. This will help deciding whether you need another pump.
It still won't answer why your radiator is hot at the bottom but not at the top. When bleeding the radiator, close off the flow valve and bleed it. If you get water, then close off the return valve, open the flow valve and see if you get water. With both valves open, the system will bleed through the easiest route, and if there is an air lock it will never show unless that route is closed.