nmisis

Hi there,
I am a university student with no previous boat building experience and have been given an assignment to design and build a electric speed boat that will race against my class mates for grades. The restrictions are that we must use the materials supplied and on a limited 'budget'. We build them out of Toughened Polystyrene, which is basicly plastic moulding with a max size of 40cm * 70cm. I am calling on you knowledge and experience for the fastest electric boat for a square coarse (needs pretty good handling also). Is two motors better than one? or will it weigh it down? is it better to have a small prop and high rpm or large prop and low rpm. We will use normal batteries but have a max. voltage of 7.5v. Thanks.
Kyle

pompebled

Hi Kyle,

What you design, depends largely on the waterconditions this race is run on.

If it's a pond with flat water, go for a rigger, the fastest electric driven design available, tricky to set up and needs a bit of experience to run.

If the conditions are choppy, go for a V-hull, less fast, but drivable in choppy conditions.

If the conditions are in between, a catamaran might be an option.

The restriction to 7,5V (7,5? = 5 nonrechargeable batteries?) needs some careful prop & motorchoice and battery selection (C-cells, D-cells, penlights or what?)

What's the thickness and weight of your building material?

Please give us more information so we can give better advice.

Regards, Jan.

Neil McGrath

The type of boat you select will depend mainly on the size of course you need to steer. ECO boats turn on a dime, have very fast acceleration and are best suited to a tight circuit (less than 100ft square). Surface drive boats are faster in a straight line but have a much larger turning circle. The fastest hull form for this type of boat is the outrigger hydro. It is also the hardest to get running correctly.

How long is the race going to be? This will influence your choice of motor and battery pack.

Neil

nmisis

Hi again,
The voltage restriction means that we can only have 5 aa batteries in series, but are allowed as many as we want in parallel to supply more current. The race will be held on a pond/lake. The material is about 2mm thick, fairly strong and very light. I have considered keeping the polystyrene mold in it for strengthening. So what tends to go wrong with the outrigger? it seems like a good design. long sleek lines with a wide stabilising system. I have also seem miss budweiser and it also looks like a good all round boat. Any comments? thanks
Kyle

Neil McGrath

For such a small boat I would resist going for a hydro and would play safe using a submerged drive mono-hull design. AA batteries can have huge variations in performance and most are not really suited to discharge currents above 2 Amps. I have personaly found that GP 1300 mAH nimh cells are better than anything else I have tried and are good for up to 8 amps. The motor I suggest is the Graupner 4.8v Speed 400 linked to a graupner 26mm carbon fibre prop. Make everything as light as possible. Futaba receivers are generally the lightest of the standard 2 channel variety and a micro servo will also save some weight. Have a look at some of the boats on this site for more ideas (some of mine are shown here) http://www.floppyracing.com/

Neil McGrath
http://homepage.ntlworld.com/fast.electrics

Fluid

The best AA cells I've seen are the GP2000s (17.5mm by 43.5mm). I watched Ed Hughey break a SAW record with one of his FE boats using them, then stay on the course to break a two-lap oval record. These cells will put out 40+ amps at high voltage...if legal they give you a great advantage. http://www.battlepack.com/loosecells.asp However, it sounds like you have to use alkaline cells of 1.5v each....that changes everything.

I believe that a small catamaran would be the best considering the limited power and turning requirements. It gives a small wetted area and sharp turning. What motors are allowed? With the limited power available out of the cells.....hmmm.

nmisis

We are only allowed to use standard aa batteries so need plenty to draw enough current. This will in turn weigh down the boat alot more. We are going to build two a spec model eg. hydro of some type? and a safe design like a shallow v hull. If the pontoons are quite wide on an outrigger and the rudder is massive what would happen?

pompebled

On a rigger you'll want the wetted area to be as small as possible to keep the drag down, this goes for the rudder too, on fast boats you don't need a big ruddersurface.

Regards, Jan.

Neil McGrath

Looking at the specs for AA disposable batteries, it looks like Duracell Ultra cells are your best bet.

Duracell Ultra (MX1500) has a quoted internal resistance of 81 mOhm.
Standard Druracells (PC1500 and MN1500) internal resistance is 120 mOhm.
Energizer cells are 146mOhm.

If you draw 5 amp from your 5 cell pack the voltage available to your motor will be as follows:
Duracell Ultra 5.475v (27.375w)
Duracell 4.5v (22.5w)
Energiser 3.85v (19.25w)

If the manufacturers specs can be believed the Duracell Ultras should give almost 50% more power than the Energisers.

I suggest that you try one pack of cells with the 6v Speed 400 or two packs with the 4.8v version.