# model RC racing yacht uni project beguinners help

Discussion in 'Boat Design' started by squires500, Feb 25, 2009.

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### squires500Junior Member

i have recently been given a design assignment for a class at university, in which i am required to design and manufacture a model RC sail boat,

a small percentage of the assignment 5% is dedicated to successful design, the rest of the assignment is based around my manufacturing of the boat. and the materials selected ect.

so what i was wondering is if anyone can give me any hints on racing sail boat design?

thanks heaps

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### TcubedBoat Designer

You may already know this but just in case;

The most important thing to know when designing a monohull sailing model is scaling laws. This means that the keel must be very deep and the ballast ratios very high in order for it to sail in any kind of wind.

In other words if you want performance it must absolutely not resemble a full sized boat.

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### squires500Junior Member

ah well i guess that makes sense, i had no idea about this, do you have an experiance or guidelines as to how deep or how heavy?
i presume that in very exesisive wind conditions the boats will not be tested but it must perform very well in moderate wind.

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### TcubedBoat Designer

This is how it works.

Righting moment is
Displacement * Righting arm. (dim. analysis; L^3 * L = L^4)

Heeling moment is
Sail area * Heeling moment. (dim. analysis; L^2 * L = L^3)

So if all proportions are kept the same boats lose relative righting capacity linearly with linear dimensions. To compensate for this you would have to scale down wind speed by W^0.5 (because dynamic wind pressure is proportional to W^2) . Eventually your model will only sail in very light winds. Or put another way a moderate wind for a scale model represents a storm for the full sized version.

Furthermore we have engineering scaling laws which favorise smallness.

As well as flow regime (Reynolds numbers) and surface tension effects.

Then there is also moments analysis which scale to higher powers yet but i won't go into that right now.

***

What does this mean in practice?

My fastest non RC model was 30 cM loa, 15 cM draft, approx. 8 cM beam, approx. 128 g disp of which almost 120 g (if i remember right) was the lead bulb, sail area was fractional rig with double spreader bendy mast and looked about like what you would expect on something like, say a J27. To achieve these very low weights i used a sanded down (to get the taper) bamboo skewer for the mast, plastic vegetable bags for sails, thin fishing line for standing rigging, matchsticks (sanded down to almost paper thin) and yellow soft rigid insulation foam, hollowed out to about 2 to 3 mM thickness everywhere apart from important reinforcement areas and covered in a finely sanded layer of epoxy resin.

The total center of gravity was about 2 cM above the bulb. (that i'm sure about) The tip of the mast was approx. 31 cM above the water.

This boat would withstand up to a bit beyond 20 knots wind and would often plane upwind. Eventually the mast would be so bent that the sail would be "inside out" and the heel so great that at that point performance would drop off to uninteresting levels, but i guess the best speeds would be found at something over 15 knots which is fairly typical for larger boats and thus i would say i got the proportions about right for it.

As yours will be RC and the equipment is heavy (batteries) aim for 100 cM loa for a fair minimum to get good performance. At 100 cM i would say 35 to 40 cM draft and ballast ratio of 80 to 85 % should allow to put up a good spread of sail. Avoid the temptation of excessive rig aspect ratio, if you do not have surface area restrictions imposed.

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### squires500Junior Member

we have some fairly devistating resitrictions which have not yet been finalised, unfortunately our LOA length is max 500mm, and we are only allowed 2 servos in the boat.
this means we will have permenant rigging setup.
other downsides include only having one servo for sail control so best case we may be able to set the servo up to run both the jib and the mainsail if we are lucky.

i see what you mean by the scaled proportions.

i already have plans to use a vacumn molded hull and light weight either carbon fiber or aluminium structural componants, and im in the process of convincing the lecturer to allow high strength micro servos which would give us a great weight advantage,

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### TcubedBoat Designer

Yes micro servo would be great.

One servo for all sails is standard.

50 cM is not much for getting good performance with the extra weight of the RC gear so you're going to have to get creative to get an edge.

It is remarkable how fragile you can make these boats. I found that they almost never broke whilst sailing, instead damage would happen by human handling.

Hint- Never let anyone touch the boat.

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### squires500Junior Member

hahahaha thats not such a bad idea keep the boat in its own little humidi krib.

does anyone have any hints on where i should start with the hull design, types of hulls or the basic objective of hulls?

i have ordered a couple of books on yatch building and design but they wont arrive from the states for about 2 weeks.

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### PortTackerJunior Member

AMYA.ORG to start.

There's TONS of info on building, designs, etc out there.

50cm is Very small, and not much fun to sail as that small a boat just bobs about like a cork. You really need to get to about 76 cm to begin to have an RC boat 'feel' like a real boat, and one meter is much more lifelike...

9. ### Doug LordGuest

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### bhnautikaSenior Member

Squires500 as this is a design assignment for a sail boat not just a mono hull think about a multi hull which will work better in the size required, there’s no need for ballast, you can put a simple but effective sail board type rig on it with one servo for steering and one for sail trim. The hulls can be made of foam to keep it light, with a beam to length ratio of one I would think it would move along quite well.

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### squires500Junior Member

i wish our constraints allowed us to make a larger hull it would be much more convenient, unfortunately i think they have restricted the hull size as to get our own designs as it doesnt fit into any normal model yatch design.

doug lord thanks for the links they look excellent i will be taking a better look at them tonight
bhnautika you are right in saying that a monohull is not required i instantly for some reason withdrew the ideas of a multihull i guess because i assumed they would be slow and cumbersome. though the points of no keel greatly increased stabillity and light weight may very well outweight the incresed friction due to hull size.

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### FlyingFishJunior Member

Hi all,

This is my first post. I have not read much on threads but considering this forum might helped my interest in boats.
Squires500 a lipo batt in your r/c system may helped save weight. Only I could imagine that such model-size might look like floating in a sticky syrup. Good luck!

Phil

Last edited: Feb 27, 2009
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### rww76Technician

Hi!
Are you in solent?
The rule is the same as last year.
You should start by doing a spreadsheet and play with it.
It depends on which material you ll use to build it.
If you have some carbon fibre (which the uni won't give you) you can decrease the mean length.
The general weight is 2.2 kg with a ballast of 1.4 kg.
The sail area is also decrease with the length. That's why , you have to play with the rule to comply with what you can do.
I had a 70cm LOA, 67.5 LWL , SA: 3898cm² , Weight = 2.16 kg, slenderness =5.2.
It is the one that has the less hull weight to sail area ratio in the rule.

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### yachty4000Junior Member

There are quite a few forums on the web with detailed threads on how to build radio sailing yachts explaining how to make foam blank moulds etc.

Just remember the golden rule of radio sailing "the primary purpose of hull is to keep the electronic dry"

Have Fun

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### squires500Junior Member

rww76 im not in solent. i presume that is a place sorry i have never heard of it. i am currrently studying at the anu, in canberra.
any hull that i do build i will probubly build from a vacumn molded poly carbonate, as this i think will be the lightest material being only 10ths of a mm think. i dont know how to make fiberglass or carbon fiber that thin while still retaining the structure of the material.

do you have the rules you used to put into your spreadsheet?
thanks again everyone

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