Apologies for hitting again what is well travelled road on this forum, but advice is needed and this looks like the best source there is....
I'm in the process of building a small mini-cruiser; it's less than 4 metres long (12' actually), but quite beamy and heavy. It is intended to have water ballast, but I am substituting lead-acid batteries (a couple of 12V 100 Amp-hr)to bring the sailing weight in at about 300Kg (650 lb). You can get an idea what it is like here, if it helps: http://www.youtube.com/watch?v=wRZNWAkdkXM
As much as any, the reason for the change of ballast is to be able to use an electric outboard as auxiliary power to get me off the dock/beach, back home when in a hurry and so on (nearly exclusively sea use, rather than fresh-water).
If they were still available, I would have used a Torqeedo 801, but of course they are not and the replacements, the 503 or 1003 cannot be used with external batteries (I discount using an inverter to produce AC to trickle-charge the built-in lithium-ion cells on grounds of safety and wastefulness of power)and they are pretty expensive for what they are.
Looking around at alternatives, the only real option looks like a Minn-Kota Riptide (at least in terms of UK availability). They produce 55lb, 80 and 101lb thrust versions. My question is therefore whether any of these will do what I want, as I have heard that trolling motors are pretty inefficient and poorly propped for the sort of application that I have in mind (although of course displacement hull speed will be a limiting factor of around 4-4.5 kts); can anyone who has experience of these motors please advise?
I suspect that I am going to have to butcher which ever one that I use because of the shaft length (I don't think I 'll be doing any fishing...) and cut the shaft down to a more appropriate size.
Looking on the Torqeedo web-site ( http://www.torqeedo.com/images/kataloge/TQ_Katalog_2012_EN.pdf), they suggest (footnote at the bottom of the page)and working through the implications, that the static thrust quoted for 80lb thrust from a trolling motor broadly equates to the equivalent of abouta 2HP ic engine, is that about right and therefore the size that I need? Or have I got that wrong? Can I get away with the smaller (cheaper!) size? One difference is that the smaller one is 12V, whereas the larger is 24V, which I guess will have lower losses (everything else being equal); as I have two batteries, the different voltages can easily be accommodated.
Would a change of prop help my application? For example, http://kipawapropellers.com/index.html.
If I could get a suitable outrunner motor, gearbox and controller, I would'nt mind building my own using a Torqeedo prop, but that doesn't necessarily look very straightforward, if there is a suitable alternative.
As I said above, my apologies for covering old ground, but I haven't managed to find much here in the UK (most of the Electric Boat Association craft are somewhat different)
Thanks,
Nigel
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