Watch me build my autonomous solar 20-footer...

[amaurer]

I've been asking questions in the design forum, but since no one yelled at me for not using Okume plywood, I figured I must be in the wrong place :)

I've been posting high speed videos of the entirety of each step of my build. This is my first time with stitch-and-glue, or building a boat at all, so be gentle - I'm narrating for the non-boat-building crowd so be nice and don't call me on my BS. :D Anyway, thats all at www.lonelybot.com

Here is the plan:
http://www.lonelybot.com/images/composite.jpg

And the progress:
http://www.lonelybot.com/images/9_26.jpg

[Rick Willoughby]

Great stuff Andrew. My only request is to balance the music and voice volume on the clips.

I personally think building a strong back is easier than the full stitch you have done. That is what you do with the leftovers from the bulkheads. Use them as formers to hold the panels together.

After Hawaii send it to Melbourne so I can have a closer look.

The legality of sending a craft of this size unmanned into the ocean might be worth a thought - have you looked into this? I expect with your background you could develop some very interesting collision avoidance systems. Might have commercial potential for light handed sailors. I for one do not cope well with broken sleep. It would be nice to sleep in peace knowing there was something taking care of your welfare.

You could set up a lonelybot friends network for meet and greet sessions in far away ports. Over a few years it would be a huge network.

Rick

[amaurer]

You're a sharp cookie Rick, you're always reading my mind!

Collision detection is a stinker, it'll probably wind up being my largest control system challenge, as the sensors are more complex that a GPS or a compass, say. The plan is to have a laser sensor for keeping from running into things, and a pair of side-facing optical sensors (cameras) for avoiding things running into me.

And your meetup idea is exactly what I had in mind. I actually envisioned a contest, perhaps, something like Where In the World Is LonelyBot? - some sort of daily prize for intrepid souls who can manage to position themselves in front of the craft's camera at a specified time of day. Vandalism concerns exist, obviously, but this project wouldn't be possible if I wasn't willing to lose the money I'm putting into it...

For the nerds among us I intend to dump all the telemetry directly to website for public use. Performance data, sea conditions, etc, will all be monitored and uploaded.

[amaurer]

Oh, and FANTASTIC idea re the plywood - now I can build a cradle without pulling out the trailer to cut more wood!! THANKS!

[amaurer]

Ok, question-time:

Im about to glass the inside of the hull, but taping the seams really taught me a lesson about not having bumps underneath the glass. I'm going to fillet the edges of my butt joints, then, and sand down the one or two errant lumps of thickened epoxy laying about, but what about the edges of the fiberglass tape? Should I sand them down? That might be tough today without damaging the wood...

[Rick Willoughby]

Smooth them down or bog then up so they are smooth. Have you got micro-bubble filler? Actually I was going to tell you that you would need long tapers on your backing boards. The easy way to join the ply is to make a shallow skive and tape over the butt joint with some decent weight triaxial tape. But that was a few clips ago now.


Also do you know about amine blush:
http://www.epoxyproducts.com/blush4u.html
Something you need to look out for if you are glassing over cured epoxy.


Rick W

[bntii]

Is it just me or does this look a little top heavy/tippy?
Panels are heavy, etc..
Are you using outriggers?

[Petros]

Andrew,

looks like a fun experiment. I am also an engineer with lot of Aerospace experience in my past, and love building things (nine boats so far, old cars and lots of other fun stuff). I can not help but notice that Long EZ model on your garage wall. Some day I want to build my own homebuilt aircraft as well, one of my own design, just for fun (I do not even like flying that much, I just building things to experiment with ideas).

A couple of considerations for your boat-bot. As bntii pointed out, it looks tippy to me, unless you have a lot of ballast low in the hull. You might see if there is a navel architect or knowledgeable amateur on this site that will help you with developing a CG location that will be self righting. Sooner or later some condition might come along that will capsize the craft, if it was inherently self-righting you still have a chance to have it recover without a rescue. I probably would have started with a hull with more beam, but with the CG low enough, or perhaps a weighted keel, it can be made to stay upright.

You also do not want it to have a stable inverted condition either, out riggers might make it more stable, but if inverted it can not right itself either. All the equipment should of course be weather tight and able to operate inverted as well.

You might also consider altering your solar panel arrays so it is recessed or built-in to the upper deck. The size and height of the panels show would catch wind or waves and make it hard to control, or worse, keep flipping it over. Even if self-righting, you will not make any progress if rollers, or a strong quartering wind keep flipping it over. It will founder around for days until the seas calm.

When working on the hull, if you raise it up on saw horses you will not have to bend over so much to work on it. It will save a lot wear and tear on you back. Just watching your video clips made my back hurt! Trust me, you will be able to work for more hours with less discomfort if you can raise the hull up to hand level. It is also light enough so you can roll it on its side, or flip it, to give you the best access as you work your way around the hull.

Also, why sand the fillets if it will not be occupied by humans? So what if it is not very attractive on the inside, it just needs to be strong, not pretty. Save the sanding and nice finish effort for the outside.

You might consider a short and lightweight mast with a beacon on it so other ships can avoid it, especially at night. The mast can also carry a radar reflector so larger ships will see it and avoid it (better that than have it run down by a tanker). Check the regulations for lighting requirements, I suspect if you meet that you would be okay for an unmanned boat-bot/RPV. You would not be considered a navigation hazard if it is lighted at night and has a radar reflector.

you may have started a new hobby that could lead to remote maritime combat! Way better than War-craft, requires both skill and building knowledge.

Good luck with the project. Keep us posted.

[Rick Willoughby]

For those who have not seen the development threads for the design:

http://www.boatdesign.net/forums/boat-design/10-foot-swath-28081.html

http://www.boatdesign.net/forums/props/trolling-motor-prop-rpm-29446.html

http://www.boatdesign.net/forums/boat-design/my-solar-drone-project-29447.html

Rick W

[bntii]

Thanks Rick

[amaurer]

I'm still trying to figure out exactly when the forum will notify me of new posts via email and when it wont... sorry I've missed these.

Anyway, yes, I realize it looks tippy - I made it tall on purpose because the solar panels need to be kept cool. Most specifications require them to be mounted 3" off the mounting surface to allow ample airflow beneath them. I figured it was good to get them into free air and may out of any spray.

However, looks can be deceiving, as there will be lots of empty space in the hull, so the density is very nonuniform. The ~700lbs of batteries along the keel help quite a bit.

Still, it remains a concern - I'm not a boat builder, so I don't have a feel for stability margins are acceptable. Silicon Valley isn't exactly the boat building capital of the world, so you guys are functioning as my naval architect consultants :) Tell me what you think:

I wrote a script that simulates the hull with voxels (volumetric "pixels") and rolls it through various angles while calculating the resulting waterline. The deck is shown in red voxels and the keel in blue. Green voxels fill the area that lies beneath the waterline:

http://www.lonelybot.com/images/30degroll.jpg
http://www.lonelybot.com/images/30degroll_2.jpg

The script, then, gives me the height of the CG over the waterline and intuitively then we can say that the craft will roll to a local minima.

http://www.lonelybot.com/images/30deg_3.jpg

That is essentially a potential energy function, so the derivative of it can be converted to a force, or in this case a moment:

http://www.lonelybot.com/images/restoringmoment.jpg

So, reading that, by 5 degrees of roll I have 100ftlbs of restoring moment... which to my untrained eye seems like a lot... but what do I know. Moreover since all the moments are positive, the design is self righting from any angle.

[rwatson]

Hmmm - 20 ft unmanned boat with losts of batteries on board. Will probably go close to some financially disadavantaged sea travelling folk in its travels.

What is the current price for scrap lead and low usage solar panels these days ? :-)

[Ad Hoc]

amaurer
Do you not have access to a stability program?

Anyway, just fill the hull with non-hygroscopic foam (so you have always have positive buoyancy) and ensure you have a positive resorting moment by carefully placing your weights, in the vertical plane.

[amaurer]

amaurer
Do you not have access to a stability program?

Anyway, just fill the hull with non-hygroscopic foam (so you have always have positive buoyancy) and ensure you have a positive resorting moment by carefully placing your weights, in the vertical plane.

I have Freeship, but I don't know how to use it [well]. I played around by dragging nodes until I have something that looked like my hull, and ran the stability job - I see it gives me the metacenter but I'm not adept enough with the definition to actually use that info. :( If I recall correctly, the KM value was something like 1.4' - my CG is ~9" up from the bottom, so the separation between the two is about 8".

Its clear to me that thats "stable" in the technical sense, but its still not clear if that means its going to roll at the slightest breeze or if roll excitation by wave action is going to outweigh the damping available... thus my attempt to find restoring moment.

[Ad Hoc]

If you can 'roughly' model the hull, that you think looks about right, print off a set of hydrostatics. Check this against your own little macro, the two should be very close...if not, there is an error. Keep doing until the two values are very close.

Once you ahve done that, use metric!!...sorry don't understand imperial units.

The CoG...is that a guess or a calculation?

Once you ahve the CoG, plug it into the program, and run from 0 to 90 degrees. Then post the output on here.

One step at a time.

[gonzo]

Get some lead blocks, install them in the model where the batteries should go, put panels of proportional weight to the solar panels, and float it in your bathtub. You'll see really fast if it is self righting or not.

[Rick Willoughby]

....
I wrote a script that simulates the hull with voxels (volumetric "pixels") and rolls it through various angles while calculating the resulting waterline. The deck is shown in red voxels and the keel in blue. Green voxels fill the area that lies beneath the waterline:


So, reading that, by 5 degrees of roll I have 100ftlbs of restoring moment... which to my untrained eye seems like a lot... but what do I know. Moreover since all the moments are positive, the design is self righting from any angle.

Andrew
Your little script is producing a reasonable result. The only aspect I question is the kick up in righting moment between 160 and 180 degrees. The moment should be steadily reducing from the peak to 180 degrees.

The boat is very stable. So long as it has positive moment up to about 160 degrees it will be self-righting. It will have very strong tendency to roll upright from the inverted position.

Any boat this small on the open ocean will get rolled around and surely turn turtle in the worst conditions. The likely worst thing will be that you rip off a solar panel. Could be serious if you lose all of them of course or the wiring is damaged.

As far as Freeship goes you should be able to produce the cross curve for design displacement similar to what I have attached. This goes a long way toward determining the righting moment. You will note the similarity to what you have already produced.

Rick W

[amaurer]

If you can 'roughly' model the hull, that you think looks about right, print off a set of hydrostatics. Check this against your own little macro, the two should be very close...if not, there is an error. Keep doing until the two values are very close.

Once you ahve done that, use metric!!...sorry don't understand imperial units.

The CoG...is that a guess or a calculation?

Once you ahve the CoG, plug it into the program, and run from 0 to 90 degrees. Then post the output on here.

One step at a time.

The CoG I posted was just a guess, but I do have realistic numbers from my CAD model: 13.5" to the CG (from the lowest point), 840lbs total.

I doodled up the model into Delftship and ran the hydrostatic report - it turns out my version can't do the neat crosscurves function that Rick's can, I guess thats only for Pros - and got transverse metacentric height = 1.51ft. Is there another package that can plot the roll data?

[As for my units, I prefer metric as well, but everything from my tape measure to the ShopBot use imperial. C'est la vie, until I own my own... everything... sigh.]

Your little script is producing a reasonable result. The only aspect I question is the kick up in righting moment between 160 and 180 degrees. The moment should be steadily reducing from the peak to 180 degrees.

The boat is very stable. So long as it has positive moment up to about 160 degrees it will be self-righting. It will have very strong tendency to roll upright from the inverted position.

Any boat this small on the open ocean will get rolled around and surely turn turtle in the worst conditions. The likely worst thing will be that you rip off a solar panel. Could be serious if you lose all of them of course or the wiring is damaged.

As far as Freeship goes you should be able to produce the cross curve for design displacement similar to what I have attached. This goes a long way toward determining the righting moment. You will note the similarity to what you have already produced.

Please just ignore the data near/above 170 deg of roll in my plots, my data processing is to blame. Generating the plot of restoring moment means taking a numerical derivative of the CG height - since my step size is still fairly large, differentiation is "noisy", and I use a smoothing function to keep the moment readable, but obviously this has trouble near the ends of the dataset. I'll run it again with a finer step size near the edges of the set and it should help.

I'm certainly not trying to avoid turtling all together, I think thats unavoidable, but I do want to be stable enough that the damping provided by viscous friction is enough to keep roll oscillation from building up in "normal" seas. I'm sure you guys have a word for this (stability margin, perhaps?) but I don't know it. :)

The panels will always be a weak link if the weather is bad enough to risk turtling. Even assuming my mounts are robust enough to stand the forces on them when getting hammered, the cover glass will always be at risk. It'll be interesting to see how they hold up... how exciting!

[Ad Hoc]

amaurer
"..but I do have realistic numbers from my CAD model.."
With respect, this is also a guess. A guess by the computer. What I should have said is, have you done a proper weight and centres estimate based upon the calculated or known weights to obtain a KG (CoG) of your boat?...i think your reply is a no :(

Ricks plot....no idea what hull it is and the KN values are very different yours too, a difference of some 25~30%, not minor!

"...got transverse metacentric height = 1.51ft..."
Sorry but where did this come from?...looks like a guess again :(

as for:
"..but I do want to be stable enough that the damping provided by viscous friction is enough to keep roll oscillation from building up in "normal" seas.."

You're barking up the wrong tree. No need to consider such things. This is also a 'different' branch (keeping the tree pun going :) ) of stability. You're referring to dynamic stability which is a result of the radius of gyration, added mass, inertia of waterplane area etc etc...so, just stick to the simple basic stuff that you need for now.

As i said before, if you can post the output from your program, we can see how much data is right and wrong and take it from there.

However, the Q i ahve is, from your comments above, are you just interested in how far your boat can heel before she capsizes?..and when she does, can she self right?...if so, as noted above, ignore all the fluff about viscous damping.....woof woof :P (pun intended again :))

Finally, the ability to use a program (and produce fancy plots) in no way implies one being a pro!

[Rick Willoughby]

....

I doodled up the model into Delftship and ran the hydrostatic report - it turns out my version can't do the neat crosscurves function that Rick's can, I guess thats only for Pros - and got transverse metacentric height = 1.51ft. Is there another package that can plot the roll data?

...!

Andrew
The cross curve function is in Freeship. It is a high cost option for Delftship. There are different version of Freeship still around the web. You should be able to produce a Freeship compatible file from Delftship. There are some odd versions around though that are not compatible.

Your RM curve looks just about right apart from the kick up as noted previously. If you look through your script you will probably find a simple error.

Your hull will not capsize easily. The hard chine improves roll damping over a rounded chine. You can add a keel to further increase damping. It could be long with a slight taper from say midship to stern to provide protection for the shaft and prop. Set it lower than the prop so any floating trash is guided below the prop. It will add a little more drag but nothing like a plastic bag wrapped around the prop.

I cannot remember if I pointed out that Kelly Controls has a nice little 800W PMSM for USD149. They have compatible controllers with current limit and reversing for USD99. I think you will need this sort of capability to have any hope of freeing trash from the prop.

Rick W

[amaurer]

amaurer
"..but I do have realistic numbers from my CAD model.."
With respect, this is also a guess. A guess by the computer. What I should have said is, have you done a proper weight and centres estimate based upon the calculated or known weights to obtain a KG (CoG) of your boat?...i think your reply is a no :(

I'm not following you here. The estimate comes from the computer, yes, but from the solid (CATIA) models that I used to design the structure itself, not from a secondary analysis program. The assembly models contain actual weights and volumes for the materials and components (batteries, panels) that I'm using. I don't have any other way of producing estimates at this point.

Ricks plot....no idea what hull it is and the KN values are very different yours too, a difference of some 25~30%, not minor!

"...got transverse metacentric height = 1.51ft..."
Sorry but where did this come from?...looks like a guess again :(

As I said, thats what DelftShip's "Design Hydrostatics" function produced for me. I'm not doing model testing, so if you consider all computer output to be "just guesses" then yes, its a guess. The full report is attached. I did not use tremendous care setting the waterline and hull shape, however, so you'll notice a 60lb discrepancy in displacement.

as for:
"..but I do want to be stable enough that the damping provided by viscous friction is enough to keep roll oscillation from building up in "normal" seas.."

You're barking up the wrong tree. No need to consider such things. This is also a 'different' branch (keeping the tree pun going :) ) of stability. You're referring to dynamic stability which is a result of the radius of gyration, added mass, inertia of waterplane area etc etc...so, just stick to the simple basic stuff that you need for now.

As i said before, if you can post the output from your program, we can see how much data is right and wrong and take it from there.

However, the Q i ahve is, from your comments above, are you just interested in how far your boat can heel before she capsizes?..and when she does, can she self right?...if so, as noted above, ignore all the fluff about viscous damping.....woof woof :P (pun intended again :))


I'm primarily interested in static stability, but if its marginal I'll want to worry more about dynamic stability... so both.... maybe :D

[amaurer]

Andrew
The cross curve function is in Freeship. It is a high cost option for Delftship. There are different version of Freeship still around the web. You should be able to produce a Freeship compatible file from Delftship. There are some odd versions around though that are not compatible.

Your RM curve looks just about right apart from the kick up as noted previously. If you look through your script you will probably find a simple error.

Your hull will not capsize easily. The hard chine improves roll damping over a rounded chine. You can add a keel to further increase damping. It could be long with a slight taper from say midship to stern to provide protection for the shaft and prop. Set it lower than the prop so any floating trash is guided below the prop. It will add a little more drag but nothing like a plastic bag wrapped around the prop.

I cannot remember if I pointed out that Kelly Controls has a nice little 800W PMSM for USD149. They have compatible controllers with current limit and reversing for USD99. I think you will need this sort of capability to have any hope of freeing trash from the prop.

Rick W

Rick yes it appears my code has an error when Roll = 180deg, which then my smoothing functions propogate into the nearby data. I'm fairly certain this is because the trig functions in my code flip signs when they hit 180deg.

I've been speaking to Castle Creations for motor controllers - they've tentatively agreed to sponsor my project's controller needs, and yes I've asked for units with reverse.

[Rick Willoughby]

Rick yes it appears my code has an error when Roll = 180deg, which then my smoothing functions propogate into the nearby data. I'm fairly certain this is because the trig functions in my code flip signs when they hit 180deg.

I've been speaking to Castle Creations for motor controllers - they've tentatively agreed to sponsor my project's controller needs, and yes I've asked for units with reverse.

Castle make nice RC stuff and I think offer Neumotors, which are also well regarded. However I believe most of this stuff is made for model plane applications where power to weight is the main consideration. This is not the case with your boat. Durability will be important.

If you use a PMSM to avoid the wear of brushes then the motor really needs field tracking to get good break out torque. This would be the situation if you were trying to free weed. If the motors have field tracking they develop rated torque at zero speed similar to a DC motor.

This shows the Kelly 800W PMSM:
http://www.newkellycontroller.com/product_info.php?cPath=21_63&products_id=509
Compare that with the RC stuff. Compare bearings and size for rating.

Also I have had great success with a folding prop to shed weed. There are folding saildrive props that might be worth a look for you. These are highly developed and quite reliable. They will shed rubbish much easier than fixed blades.

Rick W

[Ad Hoc]

amaurer
"..The estimate comes from the computer, yes, but from the solid (CATIA) models that I used to design the structure itself, not from a secondary analysis program..."
Have you done any hand calculations to verify the output from CATIA is correct?..if not, how do you know what is what?..that is the point I'm making. You need to validate your 'computer' model. Since how does the program know it is correct, it doesn't. That is where the designer, ie you, must review the output.

"..The assembly models contain actual weights and volumes for the materials and components (batteries, panels) that I'm using..."
Sounds like you have some kind of breakdown of items then?...where did these values and list of items come from, a guess or known weights.
have you added a margin too?..you'll need one, around 10% at the final estimate.

"..I don't have any other way of producing estimates at this point..."
This is much easier than you can imagine.
All you do is list everything that is on board. Then guess, calculate or establish exactly, the weight of each item. Then from your GA, you locate the midships, or transom, or a datum you are happy with, then do a simple weight and centres, ie a summation of all the moments to obtain the overall CoG, or KG, vertical, or LCG, horizontally.

If you are unsure what will go in the boat, then either
a) make a guess and add some nominal weight
or
b) if you don't know, who else does?

"...I'm not doing model testing, so if you consider all computer output to be "just guesses" then yes, its a guess..."
That is not what i am saying.
As noted above, how do you know the output is real??...you must validate it by an independent method, ie hand calculations!...not the whole lot but just a few quick rough and ready calc's...if the difference is say 5~10% then it is probably ok. But if the difference is say 150~200% then clearly there is a mistake somewhere.

So...have you or can you produce a list of the items, all, including the structure and then obtain the levers for the KG and LCG?..once you have done that, we can address the stability.

[amaurer]

Castle make nice RC stuff and I think offer Neumotors, which are also well regarded. However I believe most of this stuff is made for model plane applications where power to weight is the main consideration. This is not the case with your boat. Durability will be important.

If you use a PMSM to avoid the wear of brushes then the motor really needs field tracking to get good break out torque. This would be the situation if you were trying to free weed. If the motors have field tracking they develop rated torque at zero speed similar to a DC motor.

This shows the Kelly 800W PMSM:
http://www.newkellycontroller.com/product_info.php?cPath=21_63&products_id=509
Compare that with the RC stuff. Compare bearings and size for rating.

Also I have had great success with a folding prop to shed weed. There are folding saildrive props that might be worth a look for you. These are highly developed and quite reliable. They will shed rubbish much easier than fixed blades.

Rick W

Oh, I was referring to motor controllers only (sorry, missed the "PMSM" acronym). Castle only distributes the Neumotors (they tell me) so they can't help me there.

For motors I'm going with a toothed belt drive, on the order of 6:1 reduction. This lets me use a hotter motor to help with the fieldless starting issue. However, I don't trust any of these motor's bearings, especially under radial loading from a belt, so the gearbox (belt box?) will support every shaft with its own bearings - the motor bearings will be loaded as lightly as possible this way.

I figure a spring loaded idler and low belt tension will spare the belts if the prop gets knocked. I've read your (very helpful) posts on belt efficiency and I have plenty of power budgeted for those loses.

All that said, I used a Castle Controller once upon a time on a research project with NASA (thats the source of the zero-gravity photo on my site). I was using a low kv (about 60) sensorless motor driving a shaft with a double oring gland seal, it probably took 20ft-lbs to start spinning. Adding a some high-power throttle pulses to the firmware whenever the motor was to start up proved to be a miracle solution, nothing could stop that motor from starting.

[amaurer]

amaurer
"..The estimate comes from the computer, yes, but from the solid (CATIA) models that I used to design the structure itself, not from a secondary analysis program..."
Have you done any hand calculations to verify the output from CATIA is correct?..if not, how do you know what is what?..that is the point I'm making. You need to validate your 'computer' model. Since how does the program know it is correct, it doesn't. That is where the designer, ie you, must review the output.

"..The assembly models contain actual weights and volumes for the materials and components (batteries, panels) that I'm using..."
Sounds like you have some kind of breakdown of items then?...where did these values and list of items come from, a guess or known weights.
have you added a margin too?..you'll need one, around 10% at the final estimate.

"..I don't have any other way of producing estimates at this point..."
This is much easier than you can imagine.
All you do is list everything that is on board. Then guess, calculate or establish exactly, the weight of each item. Then from your GA, you locate the midships, or transom, or a datum you are happy with, then do a simple weight and centres, ie a summation of all the moments to obtain the overall CoG, or KG, vertical, or LCG, horizontally.

If you are unsure what will go in the boat, then either
a) make a guess and add some nominal weight
or
b) if you don't know, who else does?

"...I'm not doing model testing, so if you consider all computer output to be "just guesses" then yes, its a guess..."
That is not what i am saying.
As noted above, how do you know the output is real??...you must validate it by an independent method, ie hand calculations!...not the whole lot but just a few quick rough and ready calc's...if the difference is say 5~10% then it is probably ok. But if the difference is say 150~200% then clearly there is a mistake somewhere.

So...have you or can you produce a list of the items, all, including the structure and then obtain the levers for the KG and LCG?..once you have done that, we can address the stability.

I assumed you were speaking of some obscure and mystical naval architecture technique. The CATIA output is reliable - I weighed a sheet of my ply to ensure my densities were right, the battery data comes right from the mfg, etc.

[Rick Willoughby]

The shaft gearing will get you into a shaft speed enabling a decent prop. You could certainly look for a small folding prop.

If you do not use a folding prop then consider a full shield or guard for the prop. It will also be a safety precaution for approach of people.

My folding prop is a very simply affair but achieves high efficiency because it uses small chord, high aspect blades. The saildrive props are not as efficient but are considerably more robust and durable.

You can use JavaProp to determine your the efficiency for various props. Have you tried it yet?

Rick W

[Ad Hoc]

"..The CATIA output is reliable."

If you say so...!

[amaurer]

The shaft gearing will get you into a shaft speed enabling a decent prop. You could certainly look for a small folding prop.

If you do not use a folding prop then consider a full shield or guard for the prop. It will also be a safety precaution for approach of people.

My folding prop is a very simply affair but achieves high efficiency because it uses small chord, high aspect blades. The saildrive props are not as efficient but are considerably more robust and durable.

You can use JavaProp to determine your the efficiency for various props. Have you tried it yet?

Rick W

I'm a little bit torn on the propeller, I'm not convinced a folding prop is a slam-dunk solution. I'm no expert, but I'm concerned about the reliability of the joints and mechanisms - is a folding prop really designed to motor for months on end? Sure, the power level is low, but that might also mean that the joints end up moving more since the blade forces are lower.

And on top of that, it obvious to me that a folding prop is a winner against solid debris, but against tangling/fouling debris, it is really that much better? I think the latter is the bigger risk for open ocean usage. Even if IS a little better, I imagine it will affect my reverse thrust, which might be nice to have for a detangling maneuver.

I dunno, in short.

I have also been considering the Torquedo prop. They're cheap so I was going to by one and see how strong it is - I understand they're composite but would like to get my hands on it to get a feel for the elasticity and strength. It would also be within my abilities to mold and recast it in bronze.

[Rick Willoughby]

It would be worth your while to locate a nice folding prop and take a close look. They last a long time. They require very little power to fully deploy. You are right about poor reverse thrust but they will give reverse thrust if revved up.

Once the blades are deployed they do not suffer wear on the pivot pins. You would need to cycle once in a while just to clear any fouling. Maybe something like twice a day.

Some have geared blades so they are always in balance. This would probably be the best choice.

Suggest you look at guards and folding props so you know the options.

Actually the idea of a flexible prop is not that silly. The Hobie flappers produce reasonable thrust and these use rubber with a spring steel stiffener. You are dealing with very low forces. Rubber may be less prone to fouling with sea life.

You have lots of challenges to get reasonable life without attention on the open sea for months at a time.

Rick W

[bntii]

One of the new anti-fouling coatings might help in the above.
We are seeing two year and greater service on the Prop-Speed product.

They perhaps would desire the advertising provided by your autonomous boat.

[amaurer]

I sat down this weekend with an experienced sailor who's made the SF-Hawaii trip 7 times. His comments were

a) my solar panels are going to get torn off

b) my prop, even a folding one, is going to get fouled. In fact he said on two trips a folding prop got fouled so badly as to require going overboard to free it by hand. Obviously thats not an option.

To solve a), which I'd always worried about, I figure I'll just have to bite the bullet and glass the cells to the deck instead of using commercial panels. This will require a heat exchanger to keep the cells cool, but thats a second order failure mode compared to the threat of the panels getting ripped right off.

To solve b)... well, I dunno. I think I'm going to have to put a screen/basket/cage around the prop. I'm sure this will suck from an efficiency standpoint, but I guess I'm not trying to win any races.

Other ideas are welcome. The propeller is a non-redundant system, hard to escape the problem.

[Rick Willoughby]

a) Aluminium inserts in deck

b) http://www.streamdancer.com/

Rick W

[amaurer]

a) Aluminium inserts in deck

b) http://www.streamdancer.com/

Rick W

a) The concern is that, since I left space beneath the panels for airflow and the curvature of the deck, that when the boat rolls they'll be torn off. The other problem would their unsupported middle getting crushed. My plan was to build steel frames to attach them through the deck right to a bulkhead, but that doesn't really mitigate the second problem.

b) Hmm. Is that you telling me that you think a shroud and screen of some type will be so inefficient as to be infeasible? I can't say I'm excited about developing and testing a screw like that, that'd be a significant unknown...?

[Rick Willoughby]

Aluminium gives good heat transfer. So you can build a shroud around the panels using aluminium to give less obstruction for green water. Alternatively make the deck flush with the panels and epoxy aluminium plates in the deck to imptove heat transfer into the body of the hull.

The auger is a suggestion to beat fouling. It will be low efficiency compared with an unfouled prop but significantly better than a prop fouled.

Flapping foils can be made better than 80% efficiency. It would be shaped like a fish tail with round steel rod supporting a flexible rubber centre. Something similar to a dolphin tail. Needs to have a pivot arm and a driving rod. Have you seen the Hobie flappers. It would be a variation of this. Some development work.

I think a folding prop behind an aft raking skeg would be almost immune to permanent fouling. You can set up a fouling sensor based on water speed, wind speed and motor power. Cycle between forward and reverse a few times until it is freed.

Another method is a tilting outboard leg that allows the prop to be elevated from the water and spun at high speed in air. This tends to get rid of weed.

Fishing line can damage shaft seals when it wraps around the shaft.

I have knife sharp trailing edges on my props so they cut stuff up when I reverse. Possibly a little hazardous.

Rick W