Model Boat (Unconventional)

Right ive posted around a bit on these forums already.
This post will encompass the whole model boat project from start to race.
It will include pictures from the building of the boat design ideas and hopefully you guys can help with the design and making of the boat.
Obviously im going to be the one making it in the end but I would very much like to hear from anyone who thinks they can "give" something to the project.

This is what I have so far.

I am from a school in scotland. We are entering a model rc sailing boat race next year. (about march/april)
We have 3 teams from our school.

One is a junior team who are building a standard bermuda rig boat based on reliability and ease of construction.

The other older team is making A more complicated design but following the conventional sailing rigs. They might opt for an aft mast rig but it is looking like a well built bermuda rig at the moment.

My team have decided to go down the more unconventional route which is where i need your help. I have decided to have a wing sail. In two sections. This has been discussed in this thread. http://www.boatdesign.net/forums/showthread.php?p=97447#post97447
Thanks to frosh for helping me with that.

Now the main things i want to focus on in this forum are the keel the rudder(s) and the hull.
Also any advice on wingsails and finalising the design for the rig are important.

I only have 2 servos. One for the sail or rig control and one for rudder control.

These are the rough rules for the race
max height = 90cm
max length = 70cm
min weight = 2kg
max weight in keel = 55% of total

That is pretty much it. Apart from we are not allowed to
develop electronic systems for control. And no moving balast

I am thinking about having the solid sail interchangeable with a standard rig maybe a swing rig for days where there is less wind.

As of yet i have done very little research into hulls.
I quite like the idea of rudders fore and aft. This could be controlled by one servo through mechanical linkage.

Anything you can tell me which will help the project is greatly appreciated thanks

Alex.

 

Hi AlexG

I’m impressed by the originality of the designs you’re coming up with. OK the conventional ways of doing things often have good reasons for them. But if you try something radical, you might not win so much but you’ll learn more... Go for it!

The hull you’ve drawn is narrow and fine-sterned (like no 70) but short on the waterline. Conventional wisdom is that when overall length is restricted you make the waterline length more or less equal to it. In other words, the reduction in wetted area from shortening the waterline isn’t reckoned to be worth it. Yet no 18, the 2002 SMYC winner and still quite fast this year, is as short on the WL as your hull.... Just a couple of criticisms though. Because model yachts sail nose-down in a breeze you need higher freeboard at the bow and less at the stern than in a full-size boat. Also you don’t get as much advantage from the stern overhang.

Your fin position will normally be centred at mid-WL or a little further aft. If you put it further forward it’ll be harder to balance the rig, also the weight of the bulb, further aft than the fin, may tend to twist it (Michael Scharmer has done that deliberately, but he knows what he’s doing). If the fin’s further aft the top of it will be working in more turbulent water coming from the hull. With the fin around central the rigs you’ve drawn ought to be a little further forward, I think, but you need to make the mast positions movable to find out. Really in an experimental boat like this, with the sort of ideas you’ve got, you need to be able to put masts almost anywhere till you find out what works. But that raises constructional problems.......you might find these easier to solve if the masts are stepped on deck, with the extra rigging that requires, instead of down to the keel. Not so easy with wing masts of course. Stepping the mast on deck worked OK on no 70 except that the optimum mast position turned out to be right on top of the main hatch... oops. Hatches on each side to leave the centreline clear might help.

 

Nice to see you on these forums.

Ive read some of your website it was very helpful. I have alreasy sussed that construction of this boat is going to be hard to say the least. I have asked norman from the invictus C class team about the sail design and he thought the wing was deffinately better tappered with the flaps also decreasing in %chord further up the mast. To create less tipping force.

As for the hull I have a few designs. I will try and upload pictures tommorow.
For boat hulls do simmilar rules apply as in aircraft.
That the flow of the water should be distrubed in as little way as possible. If so would it not be best to design a hull around an airfoil section. With a wider front section and a thin rear which tapers to be almost flat. So that the water is seperated and then joined rather than just been seperated.
It would meen quite a wierd hull shape with lots of displacement at the front and very little at the rear But since there arn't alot of forces acting on the rear surely this wouldnt be much of an issue.

I have done a quick design based around my idea in freeShip along with some others id like to show you.

Thanks
Alex

 

Heres some pictures one is based on an airfoil shape and one is a kind of flat bottom hull. How do you get boats to plane?

 

Your left-hand hull is what used to be called ‘cod’s head and mackerel tail’ shape. It was popular through much of the 19th century for reasons rather like yours, although not expressed in these words. Boats do share lots of principles with aircraft and birds. But at speed they also make waves, and that doesn’t happen with anything that flies. A short bow (or a short stern) means a short wave formation that gobbles up power when the boat is moving fast.
The best compromise for reducing wavemaking resistance at speed is to put the centre of buoyancy around the middle, so that the bow and stern are about the same length. Doesn’t mean they have to be the same shape. In fact your very asymmetrical right-hand hull, with fine bow and long, flat run out to the stern, is a good shape for reducing wavemaking resistance at high but non-planing, speeds.
Planing is different territory again. A planing powerboat is usually designed to have the transom immersed at rest, so that the water will break cleanly away from it and leave a groove behind it in the water that extends the wavemaking length of the boat, and lifts the hull by reaction of forces as the water is pushed down. But you can’t immerse the transom of a sailing boat, because the drag will be horrible at low speeds in light winds. A hull like your no 2 gets round that problem because the stern tucks down at speed until the transom is immersed far enough for the boat to get onto the plane.
It used to be accepted that keelboats didn’t plane, not in smooth water anyway, because without crew weight as movable ballast they couldn’t carry enough sail to give them the high power-to-weight ratio needed. Some light modern keelboats break that rule. Model yachts are of course keelboats, and they have the extra problem that the forward drive of the rig forces the bow down much more than in a full-size boat. (one of a range of scaling factors that make model yacht design different from full-size). So it’s hard to get the stern-down trim needed for planing even when enough power is available. Despite that the longest and lightest models, the current Marbleheads and 10-raters, plane pretty well downwind.
So far as I know, nobody yet has got a Strathclyde 70 to plane. They just nose-dive instead, often spectacularly. So there’s a challenge for you!
The S70’s length-to-weight ratio is similar in the International One-metre class, and some of the wider IOMs like the TS2 design will plane after a fashion. You might want to look at some of Olivier Quinot’s designs for an alternative way to go. But just remember that most races are won on the windward leg, not downwind.

 

Update On Boat

The baot is coming along nicely.
We are making the hull and sails out of foam which will then be coated in kevlar / fibreglass.
The sail is a 85 cm tall wing sail with a flap hinged at the trailing edge and supported by aluminium spars.
The mast will be placed just in front of the widest point at the bottom of the mast.
The mast will rotate in a drill head which has been slavaged from a drill which will then be fixed into the hull.
The keel bulb has been made by laminating lead.
We are currently making the final hull design and attatching the controls to the wing mast.
The hull is very narrow in my oppinion.
the L/B ratio is 5-1
The sail is being prepared to be sanded down to create the final shape and then the flaps will be attatched.

Also we have officially named the Quantum.

 

model boat

Alex, why aluminum spars? And a drill head sounds heavy? There have been numerous 36/600's built in the US with length/beam of 9. If you could contact Lester Gilbert(google his site), Graham Bantock(Sails etc) or Chris Jackson (of Marine Modelling Int.) you might be able to get them to donate carbon spars and bearings that would be much lighter than a drill head.
Can't wait to see your wing rig- sounds very interesting! Good Luck!
PS- if you can't find contact info e-mail me and I'll give you their e-mail addresses..

 

Thx Doug ill contact the sailetc guy tonight. Could you PM me Lester and Chris' contact info.
The drill head is pretty hefty, havnt actually wiehged it yet but id estimate about 300g.
TBH the sail is heavier than i would have liked and it would be really great if we could get our hands on some carbon fibre.
But the drill is pretty smooth and im not sure if my construction skills are upto making a bearing system myself.
When we measured the sail up against the hull it is almost as wide as the hull at the foot of the sail.
Ill try and get some pictures on my mobile so i can post them here.

Do you know if the foam will be disolved by the epoxy or watever solvent used to fix the CF/fibreglass?

Have you ever shaped the whole hull from foam and then just covered it with carbon firbre?

If that works then we might just do that instead of making a mould but we were worried that the surface would be too rough.

Thanks for the Help

 

Update:

Dont know if the race is still going ahead but the wing sail didnt work out it seems far too heavy and would just capsize if we put it on the boat. We are going to see if we can make it lighter after we have at least one boat finished. The hull is looking good so far and we are almost ready to have it vacuum formed. I have decided to go for a high aspect ratio bermuda rig as the hull is very narrow and might incorporate a square top style sail in the jib as well as the main having a small gaff extension.

Alex