Fast Model Boat

[jim lee]

Auto pilot? Toy helicopters use gyros and they work really well.

-jim lee

[keith66]

Simple vane gear hangs over the stern on a short extension, balanced rudder has a 1" dia plastic cogwheel on top of the rudder stock & a smaller one with vane mounted revolves on a fixed pin behind it, bit of fine elastic to just hold the rudder centralised at rest. You just aim the boat where you want it to go & set the vane to the anticipated apparent wind direction.
Boats using these tend to win but then again one i built a scale model of Capt Cook's Endeavour bark has also done very well on several occasions. Work that one out!

[Howkeh]

So, I got a pretty fast boat now. I will post a picture after the race. I would like to know one more thing:
We are racing downwind. Is it better to have a full sail (shaped like a hammock) or a flat sail (shaped like a board)? Which one catches more wind if the sail opening and shape of the outline are the same? I have the feeling that it may not make any difference and a flat sail is easier to make. Quick answer would be great - race is tomorrow.

Thanks!

[kerosene]

You should read about the faster than the wind directly downwind threads. You would have a turbine propeller in water spinning and air prop (propulsion). Thanks to relative speed differences (water/craft vs. Air/craft) you can - with right props - turn resistance of the turbine into bigger pushing force.

Be prepared for lengthy theoretical discussions with fellow teachers.

[Howkeh]

Originally Posted by kerosene

You should read about the faster than the wind directly downwind threads. You would have a turbine propeller in water spinning and air prop (propulsion). Thanks to relative speed differences (water/craft vs. Air/craft) you can - with right props - turn resistance of the turbine into bigger pushing force.

Be prepared for lengthy theoretical discussions with fellow teachers.

That would not be allowed. Needs to be sail powered.

[Petros]

the more curvature of the sail the faster it will go. There is outdated terminology used to describe curves sails as "catching" more wind, but thrust from any fluid (or moving gas like air) only happens when you accelerate the flow path, or to "curve" the flow of air.

So the more curvature of the sail, the more thrust you will get out of it. Consider what the large spinnaker sails look like, very large curved surfaces, which are designed to get the most thrust from low wind conditions.

Although it is more work to make a balloon shaped sail, it would provide the most thrust for the same surface area. Use several panels and scotch tape to make a nice even curved shaped. You can just eye-ball the shape, no need to use the fancy sail design programs.

[tom28571]

Just looked in on the thread again. Some questionable advice coming out. You do want a rudder and you do want it as far aft as allowed. Stability, as mentioned by another poster is a problem with such small light boats and you do not want the sail/mast up very high. A horizontal yardarm with loose footed square sail works well. The sheets should not go further aft than necessary. That stability thing again. The loose foot of the sail will add curvature to the sail and provide some lift to the bow.

These are very small boats and the course is very short. Stability and sailing a straight course will trump all kinds of fancy ideas.

[kerosene]

does it define that the sail needs to be static...

[Howkeh]

Originally Posted by tom28571

Just looked in on the thread again. Some questionable advice coming out. You do want a rudder and you do want it as far aft as allowed. Stability, as mentioned by another poster is a problem with such small light boats and you do not want the sail/mast up very high. A horizontal yardarm with loose footed square sail works well. The sheets should not go further aft than necessary. That stability thing again. The loose foot of the sail will add curvature to the sail and provide some lift to the bow.

These are very small boats and the course is very short. Stability and sailing a straight course will trump all kinds of fancy ideas.

No worry, I am not making it too complicated. I rather try many simple variations and test tun the boat 200 times -than spending too much time on a few high-tech solutions. The sail is relatively low, but when competing with other light boats on a very short course one cannot forgo that strong initial push that the fans deliver about 6 inches above ater level and rely on end speed. There is little moving air further down the course close to the water. I am using a trapecoid sail with the narrow part up. Plenty of accelleration but does not topple the boat over. No rudder though. The boat runs straight by itself. No competitive boat has the mast in the front half on the boat for this race. There is just too much of a violent wind at the beginning of the course to have a sail anywhere near the bow unless you create a bow with lots of buoyancy which seems to prohibit a hull shape that can reach high speeds or be really light. We are talking about boats that need to weigh less than 25g and cover the 4 meters in about 4-5 seconds to be competitive.

[Howkeh]

Originally Posted by Petros

the more curvature of the sail the faster it will go.

Good to know. I will try installing a food wrap sail and compare it with my current waxpaper sail. Waxpaper is very light and has nice stable edges. I tried making a foodwrap sail but it ended up weighing almost twice as much as waxpaper since the edges needed to be reinforced. I have some more thoughts though.

The race is now on Monday since we canceled school today due to a freezing rain situation.

[Howkeh]

Well, it was close but mass mattered. My boat (red) had a mass of 26g and was second fastest that day. The winning boat (the other one) weighed only 10g. Next steps for next time. Lighter hulls, better sail. I did not have time to get a full sail shape on the boat.

BTW, both boats have the sails aft.

Thank you folks for all the help!

[Leo Lazauskas]

Originally Posted by Howkeh

Well, it was close but mass mattered. My boat (red) had a mass of 26g and was second fastest that day. The winning boat (the other one) weighed only 10g. Next steps for next time. Lighter hulls, better sail. I did not have time to get a full sail shape on the boat.

BTW, both boats have the sails aft.

Thank you folks for all the help!

Good on you!
With boats that light maybe we should consider an insect-like contraption that
can take advantage of surface tension
http://www.youtube.com/watch?v=DvKQMV_j46M