Torqeedo outboard

[Kevin McCready 83]

"fit a prop that suits being driven rather than being designed to drive"

I imagined just putting it in backwards.

Low drag design? LOL. Energy ain't free.

[wheels 543]

Drag and Resistance are two related but very different things. Water Resistance is what causes the force on the blade to make it turn. Drag is the resistance to the water flowing over the blades surface. So drag is a sum of the surface area. The ultimate goal for a prop designer, is to create the best Force with the least Drag to get the required power.
 ex TL is correct in his assumptions. A bigger diameter equals a greater turning force. Pitch equates to turning speed. Size of Blade area and number of blades equates to better efficiency, but yes there is a lose due to drag from the blade surface moving through the water. The Blade itself is a wing. The shape of the wing, as in it's shape, is what determines the ability of the wing to "grab" the water.
The faster a Prop spins, the less slip through the water. So the Prop effort goes more into a turning force rather than dragging through the water. This can result in greater Drag being experienced by the boat as it tries to pull the prop along. But then the faster the boat is going usually means there is more wind, so the drag should not be that noticeable.
Yes you could have a Propeller designed to have greater efficiency at being towed rather that propel, but in this case, I wouldn't worry.

[tuffyluffy 76]

Why don't you turn it upside down, put bigger blades on it and call it a wind turbine?

[Kevin McCready 83]

Thanks Wheels. Interesting also to note that fluid dynamics is ultimately an NP-hard problem so no ideal solution can ever be known (in our current understanding).

[erice 732]

another way to look at it

 

propellers used to power planes - small + fast reving

 

are not the same as the ones used for wind gen - big + slow

 

[ex TL systems 63]

yes I am correct in my assumptions is very nice to hear, its not much different to the fixed props on sail boats spinning undersail, and I think that at 6 knots the yammy will be spinning at around the equivalent of 3000 revs and at 10 knots close to 5000 [ these are complete guesses but also totally accurate] because at 9 knots even pushing a dinghy with that prop the motor would be at max revs, and yes a Wharram can sail at 10 knots plus. And the drag from the motor would not slow it down that much, I am not sure how much resistance would be felt when turning the prop if the pistons were removed but I would say that that big prop will turn it without a lot of slipping at 6 knots , but I will try to test it first without the plugs to see if it will turn the flywheel.

[ex TL systems 63]

Of course if I was really clever with tools I would remove the motor completely and fit a 25 or 50 amp alternator to the top of the driveshaft with gearing if necessary to get the right revs, and it would all be inside the engine cover out of the sea spray

[DrWatson 375]

You'll need the turbine propellor to generate quite a lot of torque to overcome the 90° gearbox in the outboard, and if you gear it to make the alternator spin faster, you'll need more torque again.

 

All the towing generators are based on direct drive without any gearing losses, I think.

 

I really want to see how it works out with the converted outboard, though. Do it, do it, do it

[ex TL systems 63]

Didn,t some cruising boats use the spinning shaft with a fixed prop and the gearbox in neutral to turn a generator or alternator with pulleys and a belt.  They would have bigger props though with more power.  I am on the boat next week so may be able to try the plugs out test on a different outboard motor to see if it will turn.

[wheels 543]

Yes do it and let us know.
There will most certainly be losses. You get a loss of energy, driving gears and bearings etc due to friction. Then there is the effort with producing the current itself. The more current, the more power required to produce it. 750W (748W to be more exact) is equal to 1Hp. So that is 1Hp worth of drag (plus some losses) trying to slow you forward effort down. Watts (power) is the sum of Voltage x Current. So 12V x 8A = 96W. 750W would equate to 62A of current. These will be subject to the voltage produced, which will be regulated and I would expect should be regulated at 13.8V. But voltage below and up to the regulator coming in will be subject to RPM and RPM will be subject to boat speed and gearing. Remember that a prop os normally spinning much faster when in propulsion due to loss and loss varies depending on Boat speed.
I know of a guy that has a Generator connected via a belt drive to his prop shaft. It's a big boat 60ft? (how big is Cpt K's boat KM???)

 

[wheels 543]

And if it looks like it could work but you need more oomph, you could cut the tail piece of the lift platform of the outboard and fit a much larger prop.
I would also look at the prop shape. You will note that the trailing tips tend to be shaped such that the angle of attack can be less than the leading part of the blade. So a tweek to add more pitch at that point may help making it more efficient.

[rigger 47]

http://justalittlefurther.com/the-blue-view/the-blue-view-prop-shaft-generator-pt-1/

 

Lots of stuff out there

[too_tall 15]

Wheels, your missing one little point in your calculations - Alternators are not overly efficient. Is it not a rule of thumb that you need around 1HP per 25 amps? thats 750w input power to get out about 345w assuming 13.8v.

 

I have some Balmar literature here which states their 70a alternator has a power draw of 2.8hp. 100a alternator 4hp and 120a 4.8hp.

 

Assuming mounting a 25a alternator on the prop shaft on an inboard..

 

You would need an impeller which could produce the sort of work output at a speed which could be nicely geared to get the alternator spinning at the correct speed. There will be further power losses in the gearing mechanism and also the bearing/glands and where the shaft enters the transmission your going to get power losses even in neutral. 

 

All told, your probably going to be losing upwards of 3hp to gain 25 - 30 amps. How much speed would you lose if you had a 3hp outboard fighting the sails? Probably a quite noticeable amount. 

 

With an outboard modified, you need to overcome the power losses in the 90 degree drive, and any bearing etc drag. Probably a similar amount of loss?

 

The specifically designed units are going to be considerably more efficient I believe. Be they something like a rope towed impeller spinning a purpose built generation device on the transom or a "outboard leg" type which you can fold down. 

[Black Panther 1,584]

Yes you would loose some speed, but if it pumped enough amps it wouldn't need be in the water all the time, couple of hours a day on a long passage?? That's why the outboard conversion makes sense, pop it up when fully charged.

[Island Time 1,235]

It's an interesting idea, and units like the watt and sea do work well on fast sailboats.

 

For most of us though, with modern solar and wind generators readily available, it seems to me like it is no longer such a difficult thing to get the power you need.

 

If it is windy enough to sail dragging a 3hp brake, then a wind gen would be working as well. There is, of course, drag on a wind gen too, - the power has to come from somewhere! I'd expect the wind gen to be more efficient - no gears, no shaft angle change etc, but it would be interesting to see some real data.

 

If there is no wind, there is either not sufficient boat speed for either system, or you are motoring. Hopefully the sun is out so the solar is working!

[Fish 0]

You can get an electric outboard for $200 or $300 dollars.

One of these would have the electric motor directly driven off the prop, meaning you wouldn't get all the gearbox and 90 bend losses.

The total drag and losses would be substantially less. You'd just need to bastardise the top part (controls etc) and wire up appropriate wiring and regulator etc.

Given the possible cost of bits and pieces for modifying the old outboard, getting a $250 electric outboard to test the theory could be worth it.

 

I would think the success of the project is all about the losses, efficiency and ability to not substantially slow the yacht down. Just the surface area of an old outboard and prop looks like it will induce just a tad too much drag for the power gain. Could be worth looking at.

[erice 732]

if you could get some good pics and measurements of a watt + sea prop

 

and found the blade pitch was similar to a plastic outboard prop

 

you could probably cut the blade area down 

[Kevin McCready 83]

Just on batteries. My techo mate in Oz says about his PV home bank "Currently on my 9th year of Sonnenschein gel cells. Discharging them 20% per night. Charging them at 80A. Rated for 16 years at that depth. Would not ever go back to flooded cells, due to the intense labour and supervision, and inefficiency. Even if my gel cells died 2 years ago, I would not buy flooded again. They are that bad." Solarbloc (A600). 6000 cycles to 30% DOD.

[wheels 543]

Wheels, your missing one little point in your calculations - Alternators are not overly efficient. Is it not a rule of thumb that you need around 1HP per 25 amps? thats 750w input power to get out about 345w assuming 13.8v.

 

On purpose, simply because I don't want to complicate a project that is more a bit of fun and suck and see rather than mission critical. You will note I did say plus losses or inefficiencies or something.

 

 

Sonnenschein gel cells.

 

That's the make I have Kevin, except mine are their FLA units.

Mine are discharged to 50% and often far more. They are 10yrs old now and still working just fine.

     I have no idea what you mean by intense labour and supervision. I have topped them up with water twice in that 10yrs. They have the water saver caps on them. I charge mine with 140A

     What do you mean inefficiency? There is no difference between efficiency between FLA, Gel or AGM. But there is a difference in plate voltage, with Gel being ruffly a Volt higher than FLA. When talking efficiencies, you have to factor in all aspects. As I said, you can get the current back into FLA much faster and thus less charging time and thus less cost when using a Generator or Alternator.

      You have to be very careful not to charge Gel too fast, as that can cause gas bubbles, which cause the Gel to lose surface contact with the plates.

 

I think using batteries helps their life.

Sadly no. All batteries have a cycle life. If you can keep a battery fully charged (that does not mean left in float only) and the Electrolyte stirred up (equalisation charging ruffly once per month) then they will last a very long time.

Batteries work by a Chemical reaction taking place. As the battery is charged again, the reaction is reversed, but each recharge does not result in a full restoration of the chemical state. There is a small lose of plate material to Sulphation, with each discharge/recharge cycle. The depth of discharge and the time the battery is discharged for, all factor in how much the plate degrades. So hence the variance of estimated cycles, of which the rule of thumb is 300 to 600 cycles. Where as LiPo can be 1000 to even 2000 cycles. The down side of LiPo is that they degrade at a constant rate right from the day they were made. So to get the greatest efficiency from LiPo, it is best to use them as often as possible. Leaving them stored with no use does not increase their life.

Gel is often used in Standby power applications like say Exit lights, UPS systems, Alarms etc, because they don't need equalisation charges to stir up electrolyte. They are not subject to Plate Surface Charge like FLA. This is where an FLA can actually loose it's Current capacity if left in Float state for a long period. The internal portion of the plate actually discharges. FLA needs to be put under load once every few months and then recharged again, so as the Charge state is maintained through the depth of the plate.

 

[Kevin McCready 83]

Just my friend's take on the Sonnenschein gel v FLA. I can't speak to his reasoning and just threw it in for info.