model solar boats - hull design

[woodyp]

hey,

i'm a teacher running a programme for students to design and race model solar boats. the boats are made from styrofoam with a hobby motor.

i'd like to find out more about hull shape and propellor shape to help make the boats faster

[bhnautika]

Mono or multi hull

[JonathanCole]

Check out http://www.cyberiad.net/library/mult...lar1/solar.htm

[woodyp]

bhnautika

it doesn't matter mono or multi

[woodyp]

Jonathan,

Thanks for the reference and it's pretty interesting though i don't understand most of it having failed physics at school.
It seems to be saying that planing mono hulls are faster. I sort of already knew that. I interested though in what makes for a fst monohull and for a fast multi.

[SeaSpark]

I'd go for a catamaran design:

1: You need a large horizontal surface to put solar panels on, the space between the beams. A monohull would have to be very wide(much drag) to accomodate these.

2: In a solar boat you never know how much power you have, a planing monohull has high drag with lower speeds, a displacement monohull will never plane. The long and slender hulls of a catamaran are efficient in a much wider speed range.

3. Mount the motor between the hulls under the solor panel, then you wont need a trough hull propellor shaft as with a monohull.

As for hull form, make it long and slender, round bottomed, look at nacra cat (for example plenty of others around) for inspiration on hull form.

For propellor shape, it depends on the engine you plan to use, the weight and hydrodynamic qualities of your boat. perhaps the normal rules change on a small model propellor. You may consult a remote controlled model boat racing club on this?

Good luck with your project!

[woodyp]

seaspark,

thanks for the input. i realise model boats probably differ from full size ones in performance. my question relates to hull shape - step hulls and the like.

[SeaSpark]

Did you have a look at:

http://www.solarsplash.com/

http://www.solarsailor.com.au/
The cat with movable solar wings is amazing!

http://www.solarboats.net/pages/intro/introAng.html

I have seen some planing hull forms in these pages but did not see them plane very convincingly, the question is: Will your hull plane? The engine and solars cell will be relatively heavy on a model scale.

Solarsplash has a width limit in their rules.

The stepped hull also is efficient only at high planing speeds. A round bottom hull has least drag at low speeds. Any appendage good for planing will induce drag at lower speeds.

With all these uncertainties i still think a catamaran is your best bet. It will go from dispacement to planing "mode" with much less fuzz. Make sure to make hulls slender and not longer the neccecary for buoyancy to reduce wetted surface area.

I am more a sailboat man so dont know much about props. Not sure if rules change on small scale. A hobby rc boat club will know where you can get props for a good price for you students.

There should be more electric boats on the water!

[bhnautika]

Woodyp I agree with SeaSpark’s points. The only thing I would add is the possible use of a Proa configuration. This is a long skinny mono main hull, semi circular in section, maybe pointy at both ends, with a small outrigger. This gives you the stability for the solar panels with slightly less drag of the cat.

[SeaSpark]

Some boats in the solarspash sprint race go very fast for a short period of time.

They use power stored in batteries filled from solar panels over a long period of time before the race.

For me this is too far from the real life situation.

[woodyp]

bn nautika,

the proa idea is one i've thought of, though i didn't know its name. my observation so far is that mono planing hulls are the fastest in still water, which mirrors the performance in the full size world. our boats are between 300mm & 600mm long and no wider than 300mm. i'm looking for some simple rules that i can give to young students(10yrs-13yrs) to guide them in hull design.

[woodyp]

seaspark,

storage is not permitted in our competitions.

[JonathanCole]

This will help you. Your students can use it as a general approach. It is accurate and sensible.
http://www.solarnavigator.net/catamaran_hull.htm

[SeaSpark]

"planing hulls are the fastest in still water" may be true but your hulls won't plane! Did not put it this bluntly before.

Some simple rules:

The more water you displace, the more drag.

The more surface your hull has, the more drag.
(that is where the circle shaped mid section comes in, a circle has the largest volume compared to it's surface 10-13 yr old will understand it) when a hull looks different it has been done to increase stability, to increase loading capacity, for a race rule or to make it plane more easy sorry if i forget something.

Now this circular mid section needs a shape to "cut trough the water", here they need to understand a pointy back is just as important as a pointy front, especially at low speeds. I have heard people say "The boat has to leave the hole is has dug for itself". This is not true for planing boats.

The picture of Jonathan Coles's solarnavigator is a very good example of this.

Don't know of more tips to explain hull design to a 10yr old.

[bhnautika]

Woodyp Planing hulls require a lot of energy to get up and planing, ask your self do I have that energy with the equipment I have? If you must try a planning hull, a flattish bottom will be the way to go (like tinnie) with the centre of gravity about 75% aft on the waterline. Something similar to the long skinny river boats in Asia. No matter what hull, make it as smooth as possible and keep it lightweight