# model RC racing yacht uni project beguinners help

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

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### TcubedBoat Designer

Scale windspeed will be proportional to F^-0.5
where F is the scaling coefficient.

So suppose F = 1/16 (model one sixteenth the linear dimensions as original) then a given wind will appear to be 4 times faster. ie. a 10 knot wind on the model will be the same as a 40 knot wind on the full size boat.

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### ivor BittleJunior Member

Rc yacht

Scale wind.

What will you do if the boat is scaled up and not down? The idea is a nonsense.

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### squires500Junior Member

well i would assume that it is proportional to the inverse of that function.
however that isnt what i thought it would be, i assume that it only applies if you scale a boat down in size it will not act as initally intended but instead will act as an increase in wind.
but from that if the boat is designed to be on 500mm long then it should have no issue with windspeeds? but then why would a small boat have such an unproportional keel? im very confused by this.

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### ivor BittleJunior Member

Rc yacht

The starting point in designing a model is to accept that you are designing a miniature sailing boat and not a scale model. Scaling full-sized boats is not a way to go. Do not even look at them.

You are trying to design a mechanical device and you should look to mechanical engineering and not ship design. Ship design is based on wholly empirical methods that are very difficult to follow and, assuming that you have some acquaintance with science, it will be very off-putting.

The boat you are trying to design is way too small to be the subject of ordinary science and you must turn to existing designs that have the testing built into them. You say that only 5% of the marks are for design (I would have allocated 50% had I been your supervisor.) so you can afford to throw that away and get on with the construction that appears to carry the other 95%.

Go on to the net and look for the Fiesta and try to track it down. It shows what has been found to work at this size. Then re-draw it to suit yourself and get on with the job in hand.

It took me 10 years from a very secure base of engineering and science to understand how a decent sized boat works so you might find it difficult in the time that you have. I did write it all up on my web site but these days university students do not seem to be very literate and not to understand anything but mathematics.

Ivor Bittle

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### TcubedBoat Designer

I do not see why the formula i posted should confuse anyone.

Suppose F = 4 ( scaling up four times ) then it will sail like the wind has halved in wind speed.

This is why very large sailing ships suffer so in light winds despite carrying huge spreads of canvas. And scaling laws also show how in large sailing ships there is no need for outside ballast.

At the other extreme sailing models need extremely high righting moments to satisfactorily sail in any kind of wind with a decent sail plan. So if they are literal scale models they will behave in very light winds like the full sized boat in a storm. This is why scaling never produces a good boat.

In the case of multihulls you can't very well go oversquare so your oly option is to make a smaller rig. I do think you should get a performance advantage with a catamaran despite your 300 mM width restriction.
The idea is to maximize the ratio righting moment / resistance. As resistance is closely linked to displacement it follows that making it lighter without reducing righting moment will be faster. The cat will suffer from a larger amount of wetted surface relative to its displacement, though.

And of course, a cat will not recover from a gust that you cannot react to quickly enough. The monohull will always recover from any gust, provided nothing breaks.

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### FlyingFishJunior Member

Why do renaults number important in determining scale effect?

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### TcubedBoat Designer

Reynolds number is crucial to determining the flow characteristics.

See ; http://en.wikipedia.org/wiki/Reynolds_number

<<<<<<<<<<Reynolds number sets the smallest scales of turbulent motion

In a turbulent flow, there is a range of scales of the time-varying fluid motion. The size of the largest scales of fluid motion (sometime called eddies) are set by the overall geometry of the flow. For instance, in an industrial smoke stack, the largest scales of fluid motion are as big as the diameter of the stack itself. The size of the smallest scales is set by the Reynolds number. As the Reynolds number increases, smaller and smaller scales of the flow are visible. In a smoke stack, the smoke may appear to have many very small velocity perturbations or eddies, in addition to large bulky eddies. In this sense, the Reynolds number is an indicator of the range of scales in the flow. The higher the Reynolds number, the greater the range of scales. The largest eddies will always be the same size; the smallest eddies are determined by the Reynolds number.

What is the explanation for this phenomenon? A large Reynolds number indicates that viscous forces are not important at large scales of the flow. With a strong predominance of inertial forces over viscous forces, the largest scales of fluid motion are undamped -- there is not enough viscosity to dissipate their motions. The kinetic energy must "cascade" from these large scales to progressively smaller scales until a level is reached for which the scale is small enough for viscosity to become important (that is, viscous forces become of the order of inertial ones). It is at these small scales where the dissipation of energy by viscous action finally takes place. The Reynolds number indicates at what scale this viscous dissipation occurs. Therefore, since the largest eddies are dictated by the flow geometry and the smallest scales are dictated by the viscosity, the Reynolds number can be understood as the ratio of the largest scales of the turbulent motion to the smallest scales.

Example of the importance of the Reynolds number

If an airplane wing needs testing, one can make a scaled down model of the wing and test it in a wind tunnel using the same Reynolds number that the actual airplane is subjected to. If for example the scale model has linear dimensions one quarter of full size, the flow velocity would have to be increased four times to obtain similar flow behaviour.

Alternatively, tests could be conducted in a water tank instead of in air (provided the compressibility effects of air are not significant). As the kinematic viscosity of water is around 13 times less than that of air at 15 °C, in this case the scale model would need to be about one thirteenth the size in all dimensions to maintain the same Reynolds number, assuming the full-scale flow velocity was used.

The results of the laboratory model will be similar to those of the actual plane wing results. Thus there is no need to bring a full scale plane into the lab and actually test it. This is an example of "dynamic similarity".

Reynolds number is important in the calculation of a body's drag characteristics. A notable example is that of the flow around a cylinder. Above roughly 3×106 Re the drag coefficient drops considerably. This is important when calculating the optimal cruise speeds for low drag (and therefore long range) profiles for airplanes.>>>>>>>>

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### ivor BittleJunior Member

Rc yacht

In reply to the question on Reynolds' number it might serve your purpose to read Chapter 7 of my fluids book on my website www.ivorbittle.co.uk in its entirety and the section on Lord Rayleigh in particular.

You might also find the article on making sense of aerofoils useful.

Ivor Bittle

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### FlyingFishJunior Member

Well done and interesting.

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### hbrJunior Member

Ivor what do you mean the fiesta ???

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### hbrJunior Member

stability

What aboyt the hull stability ?
wondering what the best hul curve would have to be selected in order to cope with these wind ?

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### TcubedBoat Designer

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### hbrJunior Member

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### squires500Junior Member

sorry to confuuse you ivor the marks for design are 70% 15% is in stiffness 10%quality of manufacture and 5% in regatta performance.
we are not allowed to copy a yacht and just remake it the project requires us to research and design a yacht unfortunately.

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### ivor BittleJunior Member

Rc yacht see attachment

Squires

I am not at all sure of the ethics of helping project students but, when I help I expect some input from the student so this is not step-by-step.

You need a hull design that can be run on your machining centre. It needs to be a simple design that looks the part.

This, in outline, is what I would do. I will work in inches you can translate to mm.

The first thing that you need is a deck plan and side elevation of the hull. I shall work freehand with a fountain pen. You will have to put up with it.

In the sketch I show a deck plan that is simply two arcs of circles set out so that they look the part and can be cropped off to give a transom. The widest part is say 5² and is more than half the length of the boat from the stem. This looks attractive.

I would make both stem and transom vertical but plan to fit a shaped bumper as the stem of say 0.5² thickness.

Now you need a side elevation. Plan to use a maximum depth at the point of maximum beam. Draw another arc or, if you fancy, one arc from the max beam position to the stem and one from the max beam position to the transom.

Now we must deal with the front and back separately. This is how I would proceed. In the second sketch I have drawn typical shapes for the forward hull at the stations shown in red.

These lines can be arcs of circles with the one at the point of maximum beam a semi circle.

The arcs must change in radius from station to station something like I show in the sketch. The radius needs to increase slowly near to the point of maximum beam a more quickly near the nose. You need a mathematical relationship.

This is it. The radius at the point of maximum beam is 2.5². So we need a graph relating radius and station as I have drawn it. That graph has the form

Set the equation up in your mathematics package and find some sensible values for and . Now you have to try them to see what they look like in 3d.

Either draw them or use your maths package. I would draw them and cut them out in card and set them up on a card deck and see what they look like. It is very quickly done using cyanoacrylate.

That is the front end. The back end is very easy because I would make them all ellipses.

This gives a hull design that lends itself to programming for machining and it could be used without making allowance for the cutter by using a ball-ended milling cutter of about 6 mm diameter. It is not the precise shape that is important only that the curves should be smooth.

It is really just trigonometry from here.

Ivor Bittle

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