Hi James and group,
Rules of thumb are OK in the absence of real world test data and facts, but first surely we need consensus or rather a set of standard test conditions (STCs) to compare drive systems accurately, prior to boat installation. As you've pointed out, rules of thumb vary from installation to installation in terms of comparing overall system efficiencies and are subject to the individual requirements of an owner. There is no point in lambasting an owner, by saying they are foolish in choosing a system below a certain kW/tonne. Maybe they have a high pointing, fast sail boat that can punch waves to get off a lee shore and the motor is just an auxiliary. Recommend a figure per tonne by all means, but don't presume a sailor's ability or personal choice?
In my case I'm comfortable with a 1hp/long ton motor based on a continuous 1hr rating and max motor temp of say 90C (note not prop shp). My goal was 2/3rds hull speed cruising and 3/4 hull speed max. But then again I'm interested in cost effectiveness and overall cost. I'm using a double row magnet PM Lynch brushed DC motor with direct drive, being mindful of forces when surfing above hull speed when using a 12 x 14 prop 3 blade prop on a long keel 26ft sailing boat, in both freewheel and prop spin modes at varying voltages. At rest it takes 20 Watts to overcome drive friction. Prop loads and amps/voltage clearly also vary when using the regen braking function of the controller at power inputs between neutral throttle all the way through to full throttle. Apologies for the digression, yet does this not indicate some of the variables and considerations when building a system and comparing one to another?
Which brings me back to standards. Note the use of both imperial and metric units above. In that case we first need to standardise on SI units - and use English, not American spellings ;) I'll agree to use 'z' in spelling words like standardize and bow to the masses, if we can all agree to use SI units?
So what would be our STC? I propose a bollard pull rig tank test with a given standard prop (tbc) such as Jame's CNC machined 12.5 x 14 prop from Acme, with a given water salinity, water temp and ambient air temp. Say 15C water and 25C air? We'd need to also standardize on the batteries and cables supplying the controller and motor. Say AGMs, 2 x 8D for 24V peak Systems, 4 x 8D for 48V peak etc., giving the quoted battery RC at 25C. Agree a shaft down angle, what continuous and intermittent use means in terms of time, agree acceleration, braking throttle times, flat, calm, no waves, etc. This would cater for most drive systems and baseline situations in sailing boats up to 10m?
Why do it this way?
Using a strain gauge or spring balance to measure the drag of a hull at varying boat speeds is the same as knowing how much thrust we need for a given boat speed. As 'weight' or a thrust of 1kg is a force of 9.98 Newtons and Power is force x speed then 1 Watt = 1 Newton x Metre / Second, the working backwards, if we know what current and voltage we have at the battery and at the motor/controller we know how 'good' our overall system is in terms of power conversion and thrust. This to me seems to be as close as we can get to a set of STCs as each hull shape will have a varying drag even if the boats have the same LWL/beam/displacement. Should we include the size and type of charger and the input power too? Wind/Solar/Nuclear/Coal electricity? This just to get an idea of true power creation to propulsive thrust conversion and how kind we are being to the planet compared to less efficient IC engines?
I welcome your comments, because like you I am frustrated by the no load, on load, continuous,intermittent, in boat, out of boat max power and current figures, max motor/controller/peak currents, stall currents etc. Without a standard, a potential customer is simply left confused in choosing a marketed drive system.
I also suggest prop and ancillary component manufacturers for electric propulsion, may even be tempted to sponsor such a test rig in a University test tank environment. To me this would be a good way for customers to compare the differing systems on offer and encourage the manufacturers to beat the competition and come up with the most efficient system.
Over to you and the group James, to add and formalise (formalize) a set of STC criteria?
http://John.Rushworth.com
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