The Crouch formula is nonsense applied to a displacement hull. I can get it to give 16.8MPH with a 9900lb hull using 40HP -absolute nonsense. It is meaningless. It can produce any figure you want depending on the Hull Factor chosen but then does not produce a cube relationship for a given Hull Factor that is a fundamental where viscous drag dominates.
A shaft drive diesel or a sail-drive diesel has design challenges in adapting to a proa. Diesel outboards would not work well in a seaway. Two long-tails will be incredibly cumbersome.
Lithium battery pulled at 2 hour rate and charged at 5 hour rate gives 95% efficiency not 90%.
As a matter of interest the 18m proa uses a single DC generator directly coupled to the electric drives. There is no controller. Prop speed essentially follows diesel rpm. Cables are selected to have losses less than 2%. The controllers are not in circuit but if they were, losses are less than 3% throughout their operating range.
A decoupled diesel charging batteries can be used at optimum power setting while charging. Realistically a generator for this purpose should be selected to charge at 4 to 5 hour rate on the batteries. Larger if the boat is intended to be motored while charging.
The bollard pull and traction is what counts in heavy weather. Diesel electric tugs demonstrate that the electric transmission is highly desirable for high bollard pull applications. Outboard drives usually suffer traction problems. A submersed electric pod drive can be placed to maintain traction in any weather conditions. The endurance in heavy weather will be a function of the stored energy. Bashing into heavy weather for long duration under power is not consistent with the harryproa concept. If that is the aim then a steel boat able to carry a 100 tonnes of fuel to power twin 1000kW diesels might be a better choice.
On 20/03/2015, at 8:49 AM, "taladorwood@yahoo.com.au [harryproa]" <harryproa@yahoogroups.com.au> wrote:
Rick,
I am not naysaying electrical propulsion, or I think disputing anything you have said.The numbers Crouch's formula produce are relatively close to the numbers you have produced. And I have verified them a number of times on different hull forms, true they are for a planing hull, but the Proa is almost a planing hull, better actually.About efficiency though, the generator may be 90% efficient and the batteries may be 90% efficient (I have 800 amp hours of LiPo batteries on my boat by the way, I have thought about cooking breakfast on them occasionally) but you are still dealing with DC power and the attendant line losses, bearings and shafts and controllers etc. And did I mention the diesel motor itself isn't that efficient? Every extra step in the process adds 'friction'.Using a diesel engine to drive the boat through a generator, controller, and electrical motor is never going to be as efficient as using the the diesel motor driving the boat through a shaft.But I agree with you, there are some big advantages to electrical propulsion. primarily the reduction in drag while sailing, less total weight (especially in fuel), hybrid systems are proven tech, the boats electrical needs get taken care of by default, prop regeneration and better fuel economy. Electricity doesn't need to be oversold.The only 'downside' is that the boat doesn't have the power or mass to bash against a big storm or travel long distances against the wind and waves. In my mind those aren't really negatives though, schedules break boats and kill people.
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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