hybrid

[Cheesecutter]

What size electric motor would I need to replace a 28hp diesel and if I ran this electric motor from batteries(how many amph) then use a dc diesel genset to replenish the batteries.What size genset would I need either to run staigth to the motor or as a charging unit for the batteries and run the electric motor from the batteries only.This would be on a sailing boat.

[marshmat]

Depends. What's the boat? How fast do you want to go? How far do you want to go under power? You can't just specify a complete electric drive solution based only on the BHP rating of the diesel it replaces.

[Cheesecutter]

The boat is 34` and lets say 5 s knots in good condition.probably only used to come and go to the mooring and motoring in windless conditions.the boat displaces about 5.5 tonne.
How do you come to a solution as regards the size of the motor .
Fred

[marshmat]

OK, then. There's a lot of 'rules of thumb' regarding power/tonne for auxiliary engines. Sooner or later everyone will chime in here with their preference. Let's say for now 2 kW/t as a starting point. (1 hp=0.745 kW). Then we're looking at 11 kW.
Now, there's a lot of talk about 'well, electrics have more torque than reciprocating motors, so a 5hp electric is like 10hp gas' and so on. Ignore that. What you are concerned with is how much energy is lost to waves and friction per second (energy/time=power). A higher-torque motor may be able to swing a bigger propeller, thus making the drive more efficient, but the rate of energy use, that is, the power, depends on the boat and not on the internals of the prime mover.
So you have an 11 kW motor and a matching controller picked out. What voltage? Higher voltage means less power wasted in cables, thus smaller cables. P=I*V, so for P=11 kW, a 110 V main bus would pass 100 A of current. Look up some possible motor choices and compare voltage/current tradeoffs.
Now to get that voltage, you need batteries in series. 120 V = ten 12V batteries in series.
Now for capacity. Say you want 1 hour of running time at full engine. 11 kW motor drawing 110 V at 100 A, in our example. Then that battery bank needs 100 amp-hours (A*h) of useable capacity. Batteries in series add voltage, but the total capacity is the same as each individual unit's total capacity. Ten 12V batteries in series, with 50 A*h each, gives you a 120 V, 50 A*h battery. To increase capacity, duplicate the whole thing in parallel. Need 100 A*h in this example? Twenty 12V 50 A*h batteries, arranged as two parallel strings of ten series-connected batteries, will do that.
So that's the basics. I suggest investing in a good textbook on DC power systems if you intend to build it as there are many more calculations involved, re. wiring gauge, different controller and motor types, capacitive pulse buffering, etc. that would have to be done if it is to work as intended.

[hartley]

I would not wish to pour cold water on anyones ideas re propulsion or other matters ,but I would urge Cheesecutter to to think long and hard ,and study well ,what he is proposing to do .

If we look at the Forklift industry,I believe they are the leaders in Battery Electric propulsion ,that is they have the technology that works ,and is accepted by Industry ,no pie in the sky schemes there.
Now in say,a 2 ton machine ,you will have a 24 volt 10 hp traction motor,motor voltages over the years have come down from 72 volts to 24.
A battery bank to power that motor ,over a 8 hour shift would weigh approx one ton!!.Now you could say the weight is needed as a counterbalance ,but that is only partially true ,you still need the battery capacity to run that motor for that time ,and of course you need to charge these batteries ,anywhere between 4 and 8 hours ,on a dedicated state of the art charger ,what it would take with a engine driven charger ,i have no idea ,but my guess is the batteries would ever be only half charged .

I know you only want to use this type of propulsion for coming to your mooring ,and on windless days etc ,therefore you will not need to run the motor for 8 hours as per the example above ,but the basic principles remain.
In short I dont think the idea would be ever cost effective ,far better and less headaches to stick to you present engine ........cheers Hartley