Do you know what the assumed efficiency is for this formula? It would be nice to be able to calculate what changes in propeller efficiency would do. Maybe even have a factor for propeller diameter.;-) Another thing thats interesting is that for my boat (31 lwl @ 15000 lbs) a 1 kw generator would "cost" .2 knots at hull speed.
--- In electricboats@
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> Here's the excel formula for speed in mph for a displacement hull as a function of propshaft hp (B12); waterline length in feet (B3); and weight in pounds (B4).
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> =10.665/(POWER(
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> This isn't a physical law, like F=MA, it's an empirical curve fitted to many data points by many respected naval architects from observations of many 'typical' boats.
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> Hull speed in knots 1.34 times the square root of LWL in feet. This is also expressed as an S/L ratio of 1.34. This is the speed that the bow is at the crest of the first wave and the stern at the crest of the second wave. Any faster and the boat starts going up the back of the first wave. That increases the power requirement greatly.
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> Another thing is going on, as IIRC, someone posted here recently (but maybe it was the Woodenboat Forum). The underwater portion of an efficient displacement boat is shaped somewhat like a spoon. As the spoon in the kitchen faucet experiment shows the spoon is sucked toward the moving stream of water. A displacement boat is sucked downward by the flowing water. As early 19th century speed attempts proved, it was possible to put enough power into a displacement hull to pull the stern under water.
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> Displacement boats can go faster than hull speed without huge power increases or sinking. Boats that are narrow for their length and light weight for their length create proportionally smaller waves and so have an easier time climbing up the back of the bow wave a bit. That's why sailing catamarans go so much faster than monohulls. Much lighter as they don't need to carry ballast to resist heeling forces of the sails and two really narrow hulls instead of one fat one.
>
> As far as the "free lunch" aspect of extracting energy from the sails by dragging a generator propeller goes, I think this is a neat logical theory that is mostly wrong. As the apparent wind gets up in the free lunch area of 15 - 20 kts range where most cruising monohulls are in the S/L = 1.3 to 1.5 area you are going to want to reduce sail to lessen the heeling and make it easier (or even possible) to steer. Slowing the boat down by adding drag wouldn't generate very much power and reduces the already marginal effectiveness of the rudder.
>
> My $0.02
>
> Denny Wolfe
>
>
> ----- Original Message -----
> From: constancedraper
> To: electricboats@
> Sent: Wednesday, June 17, 2009 9:14 AM
> Subject: [Electric Boats] Re: Regeneration
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> Thanks, I think I get it now. The car mileage analogy helps; don't need the physics to be convinced.
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> md
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> --- In electricboats@
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> > If you use f=ma then the concept of hull speed is non existant. Drag increases as the square of speed. More drag = more power required. Look at cruising speed requirements. Relatively small increments in speed at the top end require relatively large increases in power requirements. Look at posts in this forum on battery life at higher speeds. Universally you will see that dropping back the cruising speed drastically increases cruising distance (i.e. power usage). Or you could look at automotive MPG ratings at differant speeds. 70 mph uses more gas than 55 mph. I'll see if I can find the physics for you.
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> > -----Original Message-----
> > From: constancedraper <drmark.draper@
> > To: electricboats@
> > Sent: Wed, Jun 17, 2009 8:39 am
> > Subject: [Electric Boats] Re: Regeneration
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> > I iguess I need a review of college physics. I can't figure out how power required increases as a square of speed. I thought it was a linear function along the lines of F=ma.
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> > What am I missing?
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> > md
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> > --- In electricboats@
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> > > Regen at hull speed is a free lunch. But as you are approaching hull speed it is more like a happy meal. Power required increases as a square of speed, so the faster you go the more power you need. Or, the less speed you lose to regen.
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> > > -----Original Message-----
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> > > From: constancedraper <drmark.draper@
> >
> > > To: electricboats@
> >
> > > Sent: Tue, Jun 16, 2009 10:02 pm
> >
> > > Subject: [Electric Boats] Re: Regeneration
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> > > So, it seems that if you're at hull speed, there is a free lunch, because you can use the surplus motive force to generate electricity instead of piling up a bigger wake.
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> > > I'm just a learner here, but I'm not convinced that the 3HP outboard facing the wrong way is a good analogy. First, such a motor only produces an effective 1.5 hp in reverse when you are really hauling. The resistance to forward momentum is, I assume, proportional to forward speed.
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> > > Second, I don't think anyone thinks a kilowatt of regen is a realistic design goal unless you're a clipper ship. If you could do an average of 100 watts 24/7 while under sail you'd be producing two or three times as much power as an $800 solar panel. And rather than overcoming a 3 HP outboard motor, you'd only be fighting less than a 1/3 HP motor.
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> > > But I admit I'm speaking with the impunity of ignorant inexperience.
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> > > --- In electricboats@
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> > > > --- In electricboats@
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> > > > > Regen does not use the straight volts/rpm ratio...
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> > > > From one point of view: generating electrical energy requires force. If your sails provide that force and you drag a propeller through the water to generate, that is going to slow the boat down unless you're already at hull speed, and there is a surplus of motive force.
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> > > > Rule of thumb is 750 watts/horse power. So, generating 1000 watts (1KW) requires at least 1.5HP of force. I think of it this way: how would you like to drag behind your boat a running 3HP outboard facing the wrong (reverse) direction? Same is more or less true for wind gen's while sailing. You can't get something for nothing. What a DRAG!
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> > > > A case for solar, and multi-fuel generator, not withstanding a prop configured to be driven not drive and a separate generator.
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