Fast Model Boat

[Howkeh]

I want to win this race at my school this year (some teachers are terribly competitive) and I want to milk someone's brain about hydrodynamics for very small scale sail boats.

Here is the situation:
Boat max. Length 35cm
Max width: 12cm
Power: Wind from a window box fan (downwind race only)
Length of race track around 4 meters
Max. sail surface: 256 square cm
No min. weight

First question:
What sort of sail has the best shape? Round, rectangular, triangular? Flat or curved?

Second question:
How do I treat the hull surfaces for minimum resistance at this scale?

I am currently thinking to build a catamaran for maximum stability. Really narrow hulls with circular cross section for those parts that are in the water. Not sure at all about the sails though.

Can't wait to hear your comments and ideas!

[tom28571]

My son and I did a boat for this kind of race in the local swimming pool long ago. Only difference was the size rules and length of the pool. We made a catamaran out of foam and I suggest the same. Sail was square with fixed sheets to the stern. Mast was straight, angled aft a bit and mounted well forward. One major item was the large rudders to maintain a straight course. Boat should be as light as possible and the hulls should be clean and of min cross section. Hulls should not be too fine forward to prevent plowing which is also the reason for the aft angled mast to get a bit of lift.

Did not try a monohull but one might work well also, especially with the narrow beam which should minimize wetted surface. Did not make any great effort on hull surface other than making it smooth.

Ours was the only boat to complete the course without intervention since it could sail straight and was much faster anyway.

[messabout]

Tom has it right on about the sail if it is for downwind only. The cat is probably a best layout for the sake of practicality. A monohull could have less wetted surface but the waterline would be fatter and it could get into an unfavorable wave making regime. The only thing I would add is to be very fussy about the smoothness of the wetted surfaces. including the rudders. A course length of 4M is too short for a real test of speed potential. SO your emphasis would be on lightness on account of acceleration potential. Consider Newtons famous equation...F = Ma For a given force, less mass equates to more acceleration.

Incidentally, Tom is a shrewd dude and he and his son were cheating because he knows so much more than his opponents about what makes a boat go. Cheating with smarts is good, go for it.

[Petros]

I would think you want a very curved sail surface. I would use a symmetrical spinnaker type of sail, kind of a fat triangle with a lot of "belly" (search for pictures of what a spinnaker sail looks like). As tall as practical (high aspect ratio, height to width ratio). Single mast in the front third of the boat length, no boom or cross beam. Sail attaches at peak and than a line from each lower corner to each side of the boat.

The catamaran hull is a good idea, use as small a rudders as you need to keep it in a straight line. As smooth as you can make it, if you use paper or wood, paint it with several coats of gloss paint. And as mentioned, as light as possible. But make sure the mast and any stays on it are strong enough, that is what drives the boat forward.

Good luck. You might post pictures for more input.

[rwatson]

It would be great to test mono versus single hull for drag.

The lightness of a multihull might be the winner in a strictly downwind race, but I would be inclined to try to get get directional control from the hull shape, and leave the rudders off.

[Leo Lazauskas]

Originally Posted by Howkeh

Second question:
How do I treat the hull surfaces for minimum resistance at this scale?

At the Reynolds numbers you will most likely face, I would use a very highly-polished surface.
I doubt that trying to promote turbulence will help much.

Maybe there is some form of hydrophobic coating that you can apply to reduce
the extent of the meniscus and therby reduce skin-friction, hopefully without
adding too much weight.

[Howkeh]

Thank you for the interesting answers so far.

The catamaran I am working on has hulls that are as long as allowed (which turns out to be 30.5cm rather than 35cm) and are about 10mm wide each. Sort of shaped like a sculling hull. No rocker, vertical bow and stern for maximum waterline length. They are made from two 5mm layers of blue insulation foam with a layer of carbon fiber between to get more stiffness and a stronger bow and stern. Foam alone is hard to sand to a really fine edge that will stay fine in the heat of the competition. As suggested, I will not use a rudder. I hope to align the hulls precisely and the boat will run straight for the 4m distance. The hulls will be coated in epoxy (no fibers) so I can sand them really smooth. Wet-sanding with grit 400 should do I think. A coat of wax should repel water.

Past experience has shown that it is possible to make the boat too light. The fans are right behind the boats at the start and really light boats just get blown over. The plowing is an issue that I forgot about. If needed I will experiment with a short third hull in the front of the boat that prevents the plowing but will lift out of the water further down the race course.

Sail, hmm. Higher is better? Interesting, but it results in a lot of leverage at the start. Two of you have suggested putting the mast further forward. Would that not result in more load on the bow and create a bigger bow wave? The winds further down the course are rather mild. I cannot imagine that I could get lift really.

Also, why should I NOT use a boom or cross beam? Why? I am limited to a maximum sail size and fear that not forcing it open for maximum exposure to the wind will be a disadvantage. Thoughts?

[BATAAN]

Square sail on a catamaran.

[BATAAN]

Make the hulls of styrofoam, the sail of plastic bag. Sail far enough forward it only goes downwind.

[messabout]

If you are concerned with pitching at the start, you can add a wing between the hulls forward.

I once had a Hobie 12, yes there was such a thing. It had a fatal flaw in that it would try to spit me off the boat by burying the lee bow. By chance I had an old, but well shaped, daggerboard that just fit between the bows of the hulls. I cranked in a bit of incidence and fixed it firmly in place just a little above the waterline when in normal trim. Worked like a champ. No more burying the lee bow. The "wing" took care of that problem quite nicely. It allowed me to sail with reckless abandon.

(Aside comment; I think Doug would have approved of this foil like thingy)

[Howkeh]

Originally Posted by BATAAN

Make the hulls of styrofoam, the sail of plastic bag. Sail far enough forward it only goes downwind.

Why the sail forward?

[Howkeh]

Originally Posted by messabout

If you are concerned with pitching at the start, you can add a wing between the hulls forward.

Interesting idea, I will try that if needed.

[rwatson]

If you have the sail raked to the bow, and as Bataan suggested, a long way forward, the sail will pull the boat in the direction of the wind. Its when the sail is on or behind the 'lateral centre' that it 'pushes' the hulls, and then they tend to steer themselves.

You can observe the effect by getting a straight stick, and standing a good size feather at one end - the wind will cause the feather to pull the stick straight downwind.

You might like to experiment with a slight rising hull at the stern ( keep a 'fin' for steerage of course ). The fine exit seems to make the hull move easier on sailing models form the few I have tried.

You probably ought to build a few hulls, and race them to select the best one.

[Howkeh]

Originally Posted by rwatson

If you have the sail raked to the bow, and as Bataan suggested, a long way forward, the sail will pull the boat in the direction of the wind. Its when the sail is on or behind the 'lateral centre' that it 'pushes' the hulls, and then they tend to steer themselves.

You can observe the effect by getting a straight stick, and standing a good size feather at one end - the wind will cause the feather to pull the stick straight downwind.

You might like to experiment with a slight rising hull at the stern ( keep a 'fin' for steerage of course ). The fine exit seems to make the hull move easier on sailing models form the few I have tried.

You probably ought to build a few hulls, and race them to select the best one.

That makes sense. The boat may want to turn around eventually so the wind pulls the hulls rather than pushes them. I will try both and see what happens on those 4 meters.

Yeah, I *should* build several hulls and test them. It is so annoying to have a full-time job when those competitions happen.

[keith66]

We have an annual race in February at Benfleet yacht club to raise money for the RNLI, The Paper boat race has been run for about 40 years & is a race across the creek from south to north, distance is about 250 yards. Rules are hull 12" max & hull & deck must be made of paper You can use paint, epoxy or whatever to waterproof the boat. , rudders bowsprits etc as long as you like, No radio control either!
In the early days we used to get over 300 entries but today its normally about 30 to 50 boats. As you can imagine just about every permutation of boat has been tried, gaff cutters, fat ones, thin ones, cats, tris, proas, monohulls, square riggers, bermudan the lot.
In general if its a run a type of cat tends to do well, its evolved into a wharram style cat, the hulls have heavy rocker & a spinnaker type sail tacked to each bow, long rudder hanging out the back.
If its a reach its anybodys guess, if its windward work the minature yacht style tends to win, the best of these are similar to a modern marblehead but with more rounded ends, deep fin keel with lead bulb & spade rudder, virtually all boats use vane steering gear. The race is never cancelled & has been run in a howling gale many times. Luck takes a big part as well!
This year its on the 20th Feb.