Basic numbers 

4T boat doing 8kts has a drag of 390N so requires 1600W at the water. Allowing overall efficiency of 70% gives a power demand of 2285W.  

Panels 

Operating in low latitudes guarantees solar insolation average better than 250W.  

With collection efficiency of 16% gives harvested average daily power of 40W/sq.m.  Solar panel area is therefore 60sq,m.  Based on power area density of 150W/sq.m that is 9kW of panels costing AUD15k for semi-flexible weighing 300kg.  Rigid panels are lower cost but heavier.  Their use would require assessment of self-righting ability.

Batteries

Recommended battery capacity of 24 hours at design cruise, giving 50kWh.  Totals USD22.5k at USD450/kWh for LiFEPO4 large format cells.   Recommend two separate banks of 500Ah cells. Weight installed 600kg.

Hull configuration would be stabilised monohull similar in proportions to this model:

https://www.youtube.com/watch?v=N8NYTwLfZxk  

Central hull overall length 18m for 4T boat with design speed of 8kts.

Depending on how adventurous you were with the electrics - at the easy to do but expensive end are Krautler drives:

An ACR8 would be a reasonable starting point.  There are other makes.

At the low cost end but more involved effort - I have been very pleased with 4QD controller made in the UK coupled with ME1003 brushed DC motor capable of 10kW continuous and over 15kW burst on a 50V system.  It needs a reduction box to swing a large diameter prop to get high efficiency.  This motor will achieve 92% efficiency at 1600W.  Brush life stated as 4000 hours but likely longer at low power level.  Brush replacement USD95 and takes 20 minutes.

With a solar boat you plan to sail along a route offering sunny days in calm water - the best place to relax on the water. There are challenges but they are not white-knuckled nor death defying.  Battery capacity anxiety is easily overcome with a small generator that is only ever run to check that it works.

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