The Great Bottle Boat

Hello folks!

I did a search on the site but couldn’t find exactly what I was looking for. Please don’t take offense to my post, as this appears to be a great resource for serious boaters. I’m looking to you for help with a boat design, one made from scratch, and with specific limitations.

There is a contest coming up in just over 2 weeks where the contestants must build a boat out of nothing but empty water bottles and duct tape. So far we haven’t gotten word back on if we can modify the bottles or not, e.g. cut them in half, so we’re currently at a standstill. The bottles are standard 10”x3” 1-liter size.

The rules are simple:

1. Only empty water bottles and duct tape can be used to make the boat.
2. Maximum length of the boat is 6’.
3. Boat crew will be 2 people.
4. Only oars made of bottles and tape may be used for propulsion.

My questions are:

1. What is the best design for a fast boat using these specifications?
2. What is the best design for the oars/paddles?
3. How many bottles would you estimate this to take? We were thinking it would take around 200 for our current design.
4. The boat will need to support approximately 400 lbs. Can anyone calculate how many bottles would be required to do that?

My teammate and I are currently working on a flat-bottom design, but I’m afraid that will give us too much drag. The other option is a V-shaped hull, with the bottom being completely sealed with tape to reduce drag. We also have a request in with a leading manufacturer of duct tape, hoping they will donate a bunch of rolls to our cause.

So, any advice as to basic speed boat design, with an emphasis on plastic bottle construction, would be greatly appreciated.

 

Curious how old your team is....

 

We are 2 full-grown adult men, deployed overseas

 

Its easy to find out how much a litre of air will lift, so it'll be a challenge to do the rest....

 

One litre bottles will displace 1 kilo each. Count on 200 bottles if you need to support 400 lbs (180 kg). Add extras for reserve bouyancy so call it 300 at a guess.

For a boat that is six feet long and carrying 400 lbs and is made out of these materials I'd suggest you can forget about fast shapes. There probably isn't going to be one. You'll need all the stability you can get. Good luck with oars and paddles. I'd forget about those and just use hand held scoops made from bottles cut in half lengthways and doubled or tripled for strength.

 

So would you lean more toward the flat-bottom design? What about stabilizers on one or both sides?

Is the V-shaped design out of the question due to stability issues?

As for the oars, they must be separate entities from our hands/arms. So I'm worried that a scoop design might be considered illegal. Our initial thought was to make a solid stick out of cut up and rolled bottles, then attach about 5-6 layers of cut-in-half bottles to the end, so that's similar to what you're saying. Our prototype handles are very rigid.

 

Well stabilisers, if by that you mean outrigger floats, are extra weight and wetted surface anyway. Then you have to engineer the crossbeam. Now mind you if it is possible to make crossbeams that will do that job at a reasonable weight then maybe a catamaran might be worth trying.

The thing with this "boat" is that you have a massively high displacement to length ratio (up around 900) and a very low hull speed of a bit over three knots. Wave drag is going to be the killer for what could amusingly be termed high performance. So if you can make it all hang together a cat may be the best option.

Oh and I'm surprised about the paddle shafts but you've obviously put more thought into them than I have. Stick with paddles as there is less bending force on the shaft compared to oars.

 

Sorry about the terminology, I'm doing my best to describe my thoughts. Neither of us has any real boating experience

I think our basic choices are a single hull design, possibly with these outrigger floats that you speak of, which would be 6ft long, and just wide enough for us to sit or squat on.

Option 2 is a catamaran-type boat, with 2 connected hulls, each narrower and shorter than the single hull design. Then we could each lie down on our respective side and use a short-handled paddle for power. How does that sound?

I think option 2 would be more stable, but more complicated to build, although it could be done.

Thanks for all the ideas so far!

 

Don't make them shorter if you go for twin hulls. You only have six feet to play with and you'll want all of that. If you get enough bottles to make two tubes 14" in diameter and six feet long each that would be about right for a basic catamaran. Minimum wetted surface and would float about half way up. Maybe beams could be done by nesting whole bottles that had their bottoms cut off.

Of course another option might be to get really sneaky and just use cut up bottles as plating for an open boat hull. It'd be a real patchwork though and may not hold together to well.

 

Assuming we go with the cat, about how far apart should the hulls be, given they are 6ft long? As for securing them together, we are considering using the paddle handle design, but with about 5 of them lashed together, creating something not unlike a 2x4 board, only round. 2 or 3 of those oughta hold it fast.

 

Are you allowed to melt the bottles into a sheet?

http://www.homedepot.com/Tools-Hard...logId=10&storeId=10051&catalogId=10&langId=-1

 

Believe it or not, we considered that (we've had quite the brainstorming session tonight). But I'm pretty sure that would go against the spirit of the contest. We could essentially make a smaller version of a "real" boat, which wouldn't really be fair.

 

Fair? You want fair? This is serious business. I once helped someone make a "paper boat" for a paper boat contest. The rules said only an A4 sheet of paper plus glue and/or tape was allowed. So I asked her if they had specified what sort of glue was permissible. They hadn't. Epoxy is glue. So I drew up a sharpie that used every scrap of an A4 sheet and we soaked cartridge paper in epoxy and laid it up in a female mould I had made from aircraft ply. The boat was only made of glue and paper. It won.

Anyway back on topic, spacing should probably be fairly close to keep structural problems under control. There are formulae for hull spacing in power cats that take into account wave cancellation between the hulls. Since wave drag will be what stops this thing that could be worth looking into. I've never used such formulae but I know they exist. Someone here will know all about them.

 

If it makes any difference, the race will be held in our outdoor swimming pool (Olympic size). I'm not sure if we will have to maneuver at all, or just go from one end to the other, but I thought it was worth mentioning. In other words, there shouldn't be any waves except the one we're making.

 

Those are the waves I'm talking about. Moving any hull through the water will generate waves. Power cats are often built with the hulls spaced specifically to cancel out waves between the hulls. This reduces drag and lets them go faster.