A Photo & Discussion Forum for Wharram Design Enthusiasts
I have 2 x 15hp Yamaha Enduro two stroke motors on Dragon (T38). They are light, cheap to run and reliable. The T38 sails so well that motoring is a low priority. I probably used 100 litres of fuel from Cape Town to Auckland.
Electric is always a good topic to re-visit from time to time. But it has to be light, cheap and easy before it beats the outboard system.
My twopence worth: it depends where you will be sailing and your personal risk assessment. For me the engine has 2 primary purposes: to get me out of trouble when the wind fails or I have gear failuer and I am in shipping lanes or being swept in to danger and to enable me to manouvre in harbours (where the wind is often fickle). Of these the former is key (I sail in the UK where there are strong tides and a lot of shipping). If you are working it is also good to know that you can get back on time!
Your engine selection needs to be up to it, and if you have to motor off a leeshore in a gale you will need more than half an hour battery power and plenty of hp.
Be very careful with information given by salesmen!!!
I do not know what size batteries you propose but it sounds a lot like 4 x 100 = 400
2 x 5000w = 10,000w at 12v current draw 10,000 / 12 approx 800 amps. This is a truly staggering current .
It sounds like your salesman has divided this 800 by 400 = 1/2 hour or 30 min.
This is not true. For various reasons batteries only supply about 45% of their rated amp/hours at the voltages you need and without damage - and this measured at a discharge rate of only 25 amps.
On paper these batteries would give power for 13 minutes but in reality they probably simply could not supply an output of 200 amps each.
This is in fact the sort of discharge that truck batteries ARE designed for but without more boring math try imagining ripping the sparking cables off your truck and holding the starter button down for 10 continuous minutes - that is the sort of job your salesman is suggesting !!!
A second thing here is that I cannot see 2 x 5kw as equal to 2 x 15hp. My experience is only with small trolling motors but I would only see a 5kw as equal to a 7 or 8hp. Also the anglers who use electric trolling motors here do not use deep cycle batteries as the current draw is at least as well suited to an auto battery and at a fraction of the price and they seem to last quite well provided you only draw off the "top"of the battery perhaps 25 or 30 out of a 100amp/hour battery.
The safe and green compromise might be as simple as two good but used motors - and then do your very best to never use them ???
One might also question whether 30 hp is enough motor for a 63'catamaran. It would seem about 1/2 the minimum in my book. If all you want is low speed maneuvering in a harbor then my thought would be to put the hp in the tender motor and make the tender that much more useful and tie the tender to the big boat when needed. This is how the skipjacks did it.