I have soldered a few fittings of that size before we went over to using crimping tools. The thing with soldering is that it takes more skills than the crimping tool. Basically, the trick is to first quickly prime the tip of the wire with solder tin (using a torch), and then do the same to the inside of the terminal. Then the wire is fed into the (hot) terminal and finally some more tin through the hole in the terminal is added. It is essential to work fast, or the tin will creep up along the wire without getting good tin saturation inside the terminal.
Therefore, one has to make a number of trials first to learn it before one moves to the real wires. The correct heat setting of the torch is critical.

Arne

I'm probably teaching you to suck eggs, but just in case....  or maybe I'll learn something or someone else might find the discussion interesting.

I'm not sure what the European standard is, but ABYC recommends a mechanical connection, especially for high current connections.  You can solder as well if you like, but they recommend against solder alone.  The reason is that you can get the connector hot enough in use to melt the solder and then have wires the come loose and short.  Loose ring terminals or a dodgy copy of a genuine connector are two potential sources of heat creating this kind of failure.

100A is enough current to lead to an interesting situation, especially since it is also part of your backup propulsion system.  Around here many places that will sell you wire will also do crimp for you at little or no cost.  I've used a dimple crimper for large wires for some time, the larger connections all test well mechanically (e.g. 400lb tensile for 4 gauge wire) and I've tested the smaller gauge wires for voltage drop and they also test well above spec.  I don't have a means of electrically testing the larger gauge wires, but mechanical strength is a reasonable proxy (the rafters of my garage groaned when I tested the 00 samples :-).  Given that you live in an area that sensibly uses metric cable and lugs, there are probably many more affordable options for crimpers.

I'm not a fan of splices if they can be avoided, why not get an extra Anderson connector and avoid the splices (with their additional resistance and potential failure points).  I'm admittedly a wiring weenie, years of designing and flying electric RC planes before it was popular or easy has left me loath to leave any extra electrons lying around.   Check you Anderson connector size, I linked to a 175A connector.  However the only link I found for Haswing and Anderson was for 50A connectors, but that must be for a smaller motor?

My thoughts on solar were formed largely on our 12V system, your 24V system changes things a bit.  However, it is possible to get boost charge controllers so that it isn't necessary to have the panel voltage above the battery voltage.  My thought was that with the location of your panel, shading from the battens/sail is likely to be a problem and could knock out the output from the entire panel.  Two skinny panels parallel to the centre-line, each with a controller, would make it less likely for most of the solar to be lost with shading.  As usual, a set of compromises.

The oceanvolt looks marvellous, although the price must be shocking.  I was going to go considerably lower tech.  My take on hydro or wind is that they both don't produce much power at low velocity, so you might as well just optimise them for higher velocity.  Low tech hydro would be a towed version similar to a Hamilton Ferris or Ampair.  For coastal cruising it would seem like something like a WattandSea might be more useful in that it would be easier to kick up and put down.  Perhaps an old trolling motor could be made to do the job.  The Haswing wouldn't have the ideal prop, but maybe it would be possible to get power out of it if you were to turn it backwards (to make the prop more efficient).  It probably has three wires running from the brushless motor to the controller.  If you had a way to disconnect those wires from the controller, and connect them to a rectifier, then you just need a charge controller to the batteries (maybe a cheap solar pwm controller).  Make a neat control box and you could have three modes for the motor 1)Power, 2)out of the water for low drag sailing 3)turned backwards and switched for generating when you're sailing in brisk winds.  It ought to be possible to experiment with a cheap second-hand trolling motor before experimenting with the Haswing.  Of course all of that isn't at a necessary for the first season, or maybe ever.

I'm not sure about the wire calculator you used.  I usually use the Blue Sea systems Chart for sizing wires and they give a different result.  For 100A and a 7.4m round trip (7.6M on the table) run they would recommend 1 gauge for a 3% drop.  It would be easy enough to measure voltage drop during use and then only change things if it annoyed you.  3% would only be three extra amps lost at full throttle and things would be much better at the lower power settings you plan on using most of the time.  It's easy to be a voltage weenie, on the other hand you paid extra for a brushless motor, which is only about 10% more efficient than a brushed one.  Remember you're also likely to have few percent loss due to connections, resistance of the connectors, resistance of the thermal breaker etc.  By the time you get over 5% loss, it might be enough loss of range and max power to consider a wiring upgrade?

David, I'm pretty sure the PV panel gives a certain number of amps, the watts is determined by the battery voltage. I'd be confident that a horizontally mounted panel in British waters isn't going to exceed the controller's rating. Some panels have built in by-pass diodes so they can keep working if some cells are in partial shading, you might be lucky. 

The wire sizings look fine to me, I'm intrigued to see if 30 to 100 amps through a 25sq mm cable circuit does funny things to your compass! 

Also, when I've worked out how to clone myself to have time for all the projects I'd like to have a go at making a hydro generator, I've built a wind turbine from scratch so have a few ideas to throw at a DIY outboard genny.