What are the calculations I have to make to buy the right motor and solar panel for a small boat?

Question

The boat (8 feet X 4 Feet X 1 Feet approximately) will move around 50-100lb, speed = 6 miles/hr (one, two or three propelled motors of 12 V, but how many watts?, how many mA?, what about torque? I mean the load that the motors will have to move?, I want that the batterias get their charge from a solar panel, that will have to provide me how many volts?, how many watts?, how many mA?, is this project possible?, or would I too much solar cells?, please help with the calculations? Thanks

Answer 1

Normally a boat like that would be fitted with up to a 4hp outboard.

1hp = 750w (approx.), so you're looking for up to 3kw of power.

3kw from a 12v battery will draw a maximum of 250A.

After that you need to decide how many hours a day you need to use the boat for in order to decide your battery capacity; they'll need to be deep-cycle batteries which you can discharge heavily and the battery supplier can tell you the charging rates suited to your chosen batteries which in turn determines the size and type of solar panels which you use. In any case, your project is only likely to be fully solar-powered if you live in a sunny climate and have a large solar canopy on your boat. Or you could be inventive and tow the solar panels behind on a pontoon to minimise drag and keep the boat from being top-heavy. If the pontoon also carried some or all of the batteries then your most expensive parts will be on a separate module which can be safely stored away from the boat and you'll also be able to experiment with different battery packs and solar panel combos. It also allows you to have one power pontoon on charge (possibly even a mains top-up) whilst the other is in use.

When you calculate the daily running time you'll need to guesstimate the percentage of the time you'll be running at full-throttle. If it's close to 100% then it's a good idea to add 50% to your running time: this not only gives you a safety margin but it also prolongs battery life and compensates for the fall-off in charge capacity which the batteries will suffer over time. If you're only at full throttle 25% of the time and around 50% for the rest then you don't really need to add a margin.