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  #1  
Old 04-06-2017, 02:54 PM
ArsenalFC ArsenalFC is offline
 
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Need Help for School Project

Hello boatdesign community. I'm in middle school and I have to create a sailboat for a project and race it against other classmates. Not too many requirements but here they are
- Width: 8.5 cm
- Length: 40 cm
- Must have: Hull, Mast, Ballast, and Sail

Also here are the restrictions
- Water depth - 1.5 cm
- No Motors/Propellers

Race Guidelines
- not exactly sure but around a 1.6-1.7m long gutter
- 40m width of the gutter
- One boat at a time
- Boat is propelled by a household electric fan on low speed
- Fastest speed wins

So far I have done some research and decided to make a catamaran with one sail. I would like the boat to be as light as possible to give me the best chances of winning (unless you guys think otherwise). I made a prototype out of popsicle sticks and it worked out alright. Basically, I came here for help/suggestions/feedback. I will post a picture soon of the prototype.

I have four days to work on it and I'm open to creative
Also, try not to use advanced boat vocabulary as I do not know much about boats.
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  #2  
Old 04-06-2017, 03:36 PM
upchurchmr upchurchmr is offline
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If you are going to use a catamaran, use the smallest ballast you can legally get by with.
Actually you don't need any.

Your biggest issue will be keeping the boat straight so it doesn't bump against the sides slowing it down.

The sail should probably be a "square sail" - do you know what that is?
Do you have a size limit on the sail? Normally the biggest you can keep upright will go fastest.

Certainly the lightest boat will be best, but the sail can't pull the boat over.

Actually either cub scouts or boy scouts have these competitions yearly.
Have you tried to see what has won there?

I have one resource and will ask him about what won in his son's competition.
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  #3  
Old 04-06-2017, 03:43 PM
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hoytedow hoytedow is offline
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Welcome. All the boats in the contest will rub against the sides of the little canal. Make the sides of the boat as slick as possible, maybe even a thin coat of paint with a thin layer of lubricating oil on it as that is where you will lose speed.

If the rules require ballast, just lay some pennies with maybe a dab of glue inside the hull on the floor. Put them in the middle of each hull.

I agree a square sail set at right angle to the direction of travel will work best. Make it as large as practical to catch the most wind but don't make it too large.
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  #4  
Old 04-06-2017, 05:07 PM
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PAR PAR is offline
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Make the catamaran hulls as slender as you can (shoot for a 6:1 width, length ratio or better with 7:1 or more being better), with the biggest sail they can carry. Make the sides of the hulls vertical and maximum allowable width. I wouldn't bother with slick stuff, though might consider little wheels/rollers on toothpicks, sticking out at each corner, so it'll roll along if it does hit the gutter sides. The catamaran form will be the most stable with your dimensional requirements, so a single penny or maybe just a dime in each hull, to meet the ballast requirement. If you can get away without ballast, don't add any, the catamaran doesn't need them. Lastly, if the wind will comes from behind, a big square sail is the choice. In fact, make it twice as tall as it is wide. If the wind will come over the side of the boat, again a twice as tall as wide square sail, but round over the top a little, with an ellipse shape and angle the top of the ellipse forward a bit.

Attached is a 40 cm long, by 8.5 cm wide catamaran. It's way longer than it needs to be and could easily be half this (20 x 8.5 cm), but you should get the idea. Long, skinny hulls, flat bottom, plumb bow and stern, etc. Tie the hulls together with popsicle sticks, make the mast from a small dowel, the sail from light paper, maybe glue in an angular support to help hold up the mast. Just think light and simple and you'll do fine. In fact the boat that wills will have two things of note, first it'll be the luckiest, in terms of hitting the sides and second, it'll probably be the lightest and simplest of them all.

The top side view shows a stylized hull, that's a little harder to make, the lowest side view shows a simple square tube, tapered from middle to each end. I'd make them from some foam, carving them with a knife or hacksaw blade.
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  #5  
Old 04-06-2017, 06:27 PM
upchurchmr upchurchmr is offline
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As the guys said, wheel set on their side might be best, if the boat can't really go straight. Perhaps from a cub scout pine wood derby car. Use graphite to lube the axles (as in graphite lube for locks).
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Old 04-07-2017, 06:37 AM
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hoytedow hoytedow is offline
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PAR, your draftsmanship excels.

Question: Is the mast step made with three popsicle sticks instead of two so they will rest level on the gunwales?
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Old 04-07-2017, 07:47 AM
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hoytedow hoytedow is offline
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Two would work if you attach them using hot melt glue to bridge the gaps.
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Old 04-07-2017, 12:06 PM
portacruise portacruise is offline
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Keeping the sail pointed so it catches the most wind may be the biggest factor, unless the fan is pointed so the boat always hugs/rides one side of the gutter? You wrote 40m on gutter width, but even if that is 40cm, it might mean the boat tumbles or turns the sail sideways so you sometimes lose propulsion from the fan. So a sphere shaped sail would work from any direction- maybe an inflated balloon tied to the mast? The smallest circular hull would roll against either side of the gutter with such a low wind speed and tumbling, streamlining might not matter?

PC
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Old 04-07-2017, 04:26 PM
messabout messabout is offline
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Arsenal you have found the right place to get good advice from experienced people. The guys are being generous and correct with their advice.

Some of us are actually RC sailboat racers.

I think that it would be a good idea to mount the mast a little bit behind the center of the boat. The reason is that the very pointy hulls of the catamaran do not have much capacity to resist the push of the sail. Think of the sail as if it were a lever. The pressure on the sail caused by the wind will tend to "pry" the front ends of the boat down. Compensate for that by moving the mast aft of the middle a little bit.

We could easily get involved in an engineering conversation with this. A bit of math would be involved and some simple physics too. Maybe you have already had some classwork about such things.

We will be rooting for you. Please tell us about the outcome, win or lose we are on your side.
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Old 04-07-2017, 07:24 PM
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PAR PAR is offline
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On my hull profiles, I kept it simple, but to compensate for bow dive, I'd rocker the forward portion of the bottom, with the exact same curve as used on the sides of the hull in the same area. This would offer some lift, decrease her volume forward, yet would quickly gain volume as it gets immersed under a press and cut down on pressure bleed off, at the speeds I suspect these will move. I think the real key to this "competition" is side wall friction with the gutter it'll sail down. They'll use a fan for air, which is going to offer a sprialing mass of wind to the sail. Without a rudder or testing in the sprialing air flow, it'll just dive to one side and bounce along the gutter sides, so the idea of a few rollers or wheels, to reduce this friction, makes sense.
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Old 04-08-2017, 03:32 AM
Poida Poida is offline
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As I know nothing about yacht design.

The channel is 40 cm wide and the boat is 40cm long. High likelihood of a cat getting jammed sideways in the channel.

How about a single hull with out riggers set back from the bow.
I think the sail should loose with a hole in the centre. Shaped something like a parachute, with the air passing through the hole keeping it in a straight line.

Problem with a cat is the two hulls have to be precisely correct or it will veer to one side.

The outriggers should only just touch the water.

Good luck
Poida
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Old 04-08-2017, 11:39 AM
portacruise portacruise is offline
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It's not clear if the 8.5 x 40 cm boat dimensions are just MAXIMUM or must be the SAME for all competition. Also is the 1.5 cm DRAFT or gutter DEPTH?

You asked for creative- can be tested quickly by attaching a helium mylar balloon to a foam ball or egg about the size of a tennis ball in your gutter tank, assuming it is allowed by the rules. The weight and drag of this egg boat can be controlled by the balloon size.

If it is allowed and you keep the cat shape, you can keep the flat sail pointed into the wind by dragging very small sections of beaded lamp chain behind each pontoon. Experiment with beaded chain sections lengths so maybe the cat rocker and travel runs straight. You lose a little speed to the dragging chain, but can adjust the boat travel straight, so the sail is almost always catching wind.

PC
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  #13  
Old 04-08-2017, 02:29 PM
Barry Barry is offline
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Quote:
Originally Posted by Poida View Post

How about a single hull with out riggers set back from the bow.
I think the sail should loose with a hole in the centre. Shaped something like a parachute, with the air passing through the hole keeping it in a straight line.


Good luck
Poida
If you put a hole in the middle of a sail, the windward face will not develop full pressure and the leeward side will not develop max low pressure.
I would leave the hole concept off the table


if the wind from the fan is set at any angle except from the rear, the cat will reduce heel, reduce ballast, provide the least form resistance if the hulls are narrow.
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  #14  
Old 04-08-2017, 08:06 PM
Poida Poida is offline
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Thanks Barry

The reason the old parachutes had a hole in the centre was because they had little control by the user and the hole stops it fluttering like a leaf.

With a solid sail once the craft gets only slightly off course, there will be more wind pressure on one side of the sail, making it veer off course more until it is sideways.

The hole will ensure the wind spillage is in the centre.

The fan will have to be at the end unless you would expect the contestants to include a rudder, keel and have the sail set at the correct angle.

Also, I feel that a fan will not supply the same consistency of wind down the gutter and it is better to have less pressure on the sail and more control of the craft.

Poida
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