model solar boats - hull design

[SeaSpark]

Think the river boats bhnautika mentioned are know as longtail boats.

This is a good hull shape for low speeds and planing when you make them not too wide. A width length : length ratio from 1 : 4 is a absolute minimum to make this shape efficient. Some may say 1 : 5 or more.

The flat bottom makes it plane easy and becouse it is not to wide it will be quite efficient when it does not plane.

This shape is not difficult to make with a flat bottom and sides. There is only one hull to build and no cross beams. They also solved the shaft trough hull problem (please see picture, the bow ends in a very high point, would like an explanation for this but think students can omid it).

When rechargeable batteries are not permitted it is very unlikely the solar panels alone will provide the power needed to plane.

Very curious about results of race between all mentioned options.

[SeaSpark]

Tried to combine all suggestions into something workable:

Please forgive missing prop, this is just a very basic scetch, hope proportions or sort of right.

[woodyp]

seaspark,

why do you say our hulls won't plane. they do i've seen them race. in fact it takes my son's boat about 2 metres to climb out of the water but then it and other boats of a similar design out paced all other hull types at last years' australian international model solar challenge.

our problem is we don't know why they are so fast or if there is anything we can do to make them faster.

i'm starting a project club at my school and rather than say to my students this hull is fast and have them copy it, i want to be able to present them with some design principles and get them to design it from there.

i'm looking for statements such as:
~a longer hull is faster for the same motor or
~a three blade propellor is faster than a four blade propellor in these circumstances.

[woodyp]

seaspark

those designs are great and the suggestions about longtail boats

[woodyp]

its been suggested to me for instance that narrowing the bow a la wallypower 118 will also work but i still can't visualise the full hull and wally designs are very generous in their views of their boats hulls

[SeaSpark]

Great to see you have this experience already, sorry if my remarks about hulls not planing offended you. It was just a guess that the solar panels would not provide the energy needed to plane. Its good too hear they do.

Theory behind planing hulls is very complicated and i am not an expert in it.

One of the problems is that you need to get to planing speed so the hull also must be efficient in non planing mode. It seems this is why your sons hull takes so long to come out of the water. As mentioned before wheight is a very important factor here, the less weight to lift out of the water, the easier it planes. Planing boots need an angle of 90 degrees between bottom and transom, with a sharp edge so the water "does not stick to the hull".

The hulls in my picture are very very crude. The propellor in the pictures is tilted very much this also is not very efficient, i'd place the engine as low as possible to make it more efficient, also good for balance.

Perhaps you can post a new thread asking for advice on a book on powerboat design that explains the basics. I cannot think of any rules of the thumb here. All the books i have are on sailboat design.

As the race track is in a straight line it is a very good idea to tilt the solar panels towards the sun. Perhaps you can make them adjustable to compensate for the sun angle at the time the race is run.

[bhnautika]

Woodyp your son’s boat sounds like the centre of gravity is to far forward.
No matter what the style of boat you need a shape entry.
Two books that come to mind about powerboats are” All about powerboats” by Roger Marshall and “Guild to powerboats” by Sorensen, (I think there’s a “Boat Books” shop in Brisbane).
How big are the solar panels?

[Dill]

My son is also wanting to build a model solar boat but is uncertain on hull design, motor size and propellor. The size of the solar panel is 350 sq cm. Can anybody tell me the final outcome of the initial thread or provide any further information on the above.

[bhnautika]

Dill the main decision that needs to be made is, do you want to go fast or slow and the available power you have. A lightweight catamaran will move along with much effort where as planning hulls need more power. You can get ready made motor and prop combinations for both. There are a number of web sites but this one gives you a good over view of what’s available, (www.rchobby.co.uk.)

[woodyp]

dill, is your son involved in the same competition as my son and students are? are you in australia and are we talking about the Australian INternational MOdel Solar Challenge or one of its state precursors?

[Dill]

Woodyp. It is just a school project to teach them about solar. First term was making a solar oven and this term is about solar power to move things. Were going to make a car but teacher thought a boat would be more fun. Am from Perth Western Australia.

[woodyp]

dill, if you're in australia there is the opportunity for your son to enter solar challenge if you wish. my thyoungest son is in year 10 and has entered the challenge for the past 2 years. his team won the junior boat division in 2005 and made the finals last year.

his teams boats have all been mono-hulls with a very flat bottom. they start slow but once they're going its them and daylight. the other aspect of their design that might be relevant is that their boats have all had a vee shaped stern rather than squared. that might be relevant if you've read the response about longtail boats. we've seen multi-hulled boats and while theory says they should be fast, it has been the monos that have blitzed the field.

i run a boat programme with primary students and we've tried all sorts of designs without much success. the reason for this is i think the dedication of the students to fine tune their hulls, getting them as smooth as possible, care with panel placement, angle of the shaft etc.

our competition rules restrict us to motors of no greater value than $20. if you aren't resticted in this, rere earth motors are best but the cost runs to $100+.

props can have any number of blades but my observations are that two blades are optimal. a brass prop might be useful, because the leading edges of the prop can be filed to a knife edge to improve efficiency.

try the internet. scorpio technology in melbourne and jaycar are useful. i think western power sponsor wa teams and events.

[Dill]

Woodyp. Thanks for the feedback. It does appear that there is a bit of a competition here in Perth. I have passed it onto his Grade 7 teacher. What sort of testing regime do you encourage your students to undertake? Is there any particular order. I am trying to encourage test all his ideas so there is some basis for what he is doing. What length boats has your son made and what would be the weight of these/ Thanks again for your feedback.

[mettmac]

Two considerations -
Amount of hull in the water. The less the better. Low flat is good as you will not be going very fast
Amount of superstructure above water - too much will cause drag due to wind etc.Small model boats don't go very fast