My solar drone project...

[amaurer]

A while back I argued with the BoatDesign braintrust about SWATH hulls, and was slowly convinced that a long monohull was probably a winner for a solar electric endurance craft like I was proposing.

So fast forward three months, I've started building and... well I have concerns about what I've built, but thats my style, its more fun to learn by doing.

Anyway, here is the [hull] design I whipped up:



Its ~23' long, 3' max just below the waterline, tapering out to 4' at the deck. Tall sides, I realize, will catch lots of wind, but I thought it was more important to have the panels up out of the spray and in the cool breeze.

Here is the progress to date:



Now... my concerns:

The hull is 1/4" ply, which, on paper, I calculated to be "strong enough". (By strong enough, I mean able to support the weight of the craft when supported solely in the center (I guess this is hogging?) and also when supported at the ends (sagging?). However estimating torsional rigidity involved a lot more handwaving for me, so I planned to put bulkheads every 2 feet down the length of the hull and "surely" it'd be adequately stiff........

So, the question is, I have my hull stitched together (but not filletted, or fiberglassed, or the bulkheads installed) and it is frighteningly floppy. Can someone reassure me that the gued seams, full layers of 10oz fiberglass inside and out, and the set of 10 bulkheads are going to stiffen this thing up? (Not to mention the full deck, which I won't be installing for a while)

I *think* I can blame all the floppiness on the play in my merely-stitched seams, but I need to hear some kind words...

[Guest625101138]

I think this is more pratical than a SWATH type craft as I previously noted. It should propel easily.

By the drawing the hull will be fully decked to make it completely sealed. In this case you will have a very strong structure for torsional loads. Enclosing it makes it capably of resolving torsional loads

The immediate problem I see is more with stability. It does not show any keel to make it self righting. I guess you will have batteries as ballast but I doubt these would provide much righting when inverted.

You could consider a mast with radar reflector and aerodynamic float so that it cannot fully invert. With a mast about 2m high a float on top would not block any significant sun from the panels. The float would not need to be very big. Just a few litres in volume.

Looks nice and simple so far.

Rick W

[Guest625101138]

I should add that any holes you place in the deck to access the drive and batteries will need to be strengthened otherwise the edges of the opening will see high local stress.

Rick W

[amaurer]

Both good points.

A lot of people look at it and say it looks tippy, so I'm listening. As you guessed the batteries will be as low as I can get them; that, and the solar panels are the CG drivers, so I've been booking (reasonably, I think) a CG that is 15" above the keel.

Adding some numbers, the panels are 39" over the keel, and the batteries will probably average 8" above it. Weight estimates are close to ~600# of batteries and ~170# of panels... so 15" for the CG seems about right.

I've got some scripts written for this issue, here is a plot of the distance from the waterline to the CG location for roll angles of 0 to 180 (fully inverted):



I haven't bothered to convert this to a measure of righting moment directly, but the quantities are proportional - I read then, that she'll be self righting to 140deg, although the restoring moment is... less than dramatic... between 0 and 45deg or so. Any thoughts?

As for the deck openings, the plan was to make two decks, essentially - a deck which gets bonded and glassed in place, but with large rounded openings between each bulkhead, and a solid deck to which the solar panels attach, and which attaches to the boat with screws and captive nuts around the entire 23' perimeter. I'm on the fence as to the quality of that idea, but I need a way to feed batteries in and out of this thing with relative ease, and hatches don't really fit the bill...

[Guest625101138]

One thing to consider is a centrally located drive leg with a protective keel that has a good taper so it will shed flotsam. It will also protect the propeller and maybe rudder or another skeg in front of the rudder.

This would provided added roll damping and could be made with a bit of weight to help stability.

The holes in the deck need to be reinforced to keep it stiff.

You could test what I mean by temporarily stitching a full deck in place before you cut the access ports. You will need some form of hatch coaming to stop water getting into the hull anyhow.

Rick W

[drbobbob@aol.co]

I love your project. The hull that you are working with is rather similar to the one we are using. I am working with a group from Santa Monica HS to build and race a solar boat. Our design objectives are obviously very different, But if you can share any insight as to how to best choose a propeller it would be greatly appreciated. I recently posted what we are working with on this website.
Many Thanks
Dr. Bob
Thanks
Dr. Bob

[Submarine Tom]

Dr. Bob & Dr. Bob,

What's with the double double??

[portacruise]

Here's something that's self righting under severe conditions, built for another purpose. Hope there's something useful for you in there somewhere...

http://www.youtube.com/watch?v=hoCjcIPZl14

Porta

Originally Posted by amaurer

Both good points.

A lot of people look at it and say it looks tippy, so I'm listening. As you guessed the batteries will be as low as I can get them; that, and the solar panels are the CG drivers, so I've been booking (reasonably, I think) a CG that is 15" above the keel.

Adding some numbers, the panels are 39" over the keel, and the batteries will probably average 8" above it. Weight estimates are close to ~600# of batteries and ~170# of panels... so 15" for the CG seems about right.

I've got some scripts written for this issue, here is a plot of the distance from the waterline to the CG location for roll angles of 0 to 180 (fully inverted):



I haven't bothered to convert this to a measure of righting moment directly, but the quantities are proportional - I read then, that she'll be self righting to 140deg, although the restoring moment is... less than dramatic... between 0 and 45deg or so. Any thoughts?

As for the deck openings, the plan was to make two decks, essentially - a deck which gets bonded and glassed in place, but with large rounded openings between each bulkhead, and a solid deck to which the solar panels attach, and which attaches to the boat with screws and captive nuts around the entire 23' perimeter. I'm on the fence as to the quality of that idea, but I need a way to feed batteries in and out of this thing with relative ease, and hatches don't really fit the bill...

[Guest625101138]

Originally Posted by drbobbob@aol.co

I love your project. The hull that you are working with is rather similar to the one we are using. I am working with a group from Santa Monica HS to build and race a solar boat. Our design objectives are obviously very different, But if you can share any insight as to how to best choose a propeller it would be greatly appreciated. I recently posted what we are working with on this website.
Many Thanks
Dr. Bob
Thanks
Dr. Bob

Bob
If you want to get the best speed for your power on a solar boat there is a more suitable hull shape than this. Andrew's boat is optimised for about 4kts. A hull shape to go faster would be quite different.

Start your own thread setting out the design constraints and get some idea of what the best hull would look like.

Rick

[drbobbob@aol.co]

Thank you Rick
With respect to our hull, we have little choice. The good news is, all of the the HS in this project have to use the same hull.
I did start a thread, but did not get much feed-back. Not sure why, but is maybe I just need to be patient. I responded this thread as it carried a similar theme.
I will take you advice, review my post and revise
Thanks

[drbobbob@aol.co]

Originally Posted by Submarine Tom

Dr. Bob & Dr. Bob,

What's with the double double??

As much as I would like to say it has to do with my fav. So Cal Burger (In & Out), it is the extension of a long standing torture I have endured from m sister who, in her youth called me BobBobBobBobBob and on and on and on and well you can only the pre-adolencent anxiety this resulted in. (at least that is what my wife the shrink, Dr. Lynn tells me)
In my attempts to shed this alliteration nick name I became a doctor thinking the repect would make a difference.
NO it only made me DOCTOR BobBob!!!
I will now go to open another bottle of wine which I will pour over the nearly healed pre-adolescent wounds you have just ripped open.
Seriously, just kidding
Dr. Bob

[amaurer]

Originally Posted by drbobbob@aol.co

As much as I would like to say it has to do with my fav. So Cal Burger (In & Out), it is the extension of a long standing torture I have endured from m sister who, in her youth called me BobBobBobBobBob and on and on and on and well you can only the pre-adolencent anxiety this resulted in. (at least that is what my wife the shrink, Dr. Lynn tells me)
In my attempts to shed this alliteration nick name I became a doctor thinking the repect would make a difference.
NO it only made me DOCTOR BobBob!!!
I will now go to open another bottle of wine which I will pour over the nearly healed pre-adolescent wounds you have just ripped open.
Seriously, just kidding
Dr. Bob

Oh man, thats funny.

To your original question, I'm no expert. To the first order, the most efficient prop would have long thin blades - it'd look more like an aircraft propeller than a boat's. Such a beast is a little trickier to fit under a boat, however, and isn't (perhaps) the best option with respect to debris shedding and durability... so for my boat I'll be using something intermediate.

For your project, you should check out Ricks threads on flexible shafts driving model aircraft propellers. That'd be hard to beat, IMO.