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[RipSlider]

Hello all.

After corresponding with Martin13, what with his swanky new tools and exceptionally forgiving wife, I have decided to stand up to Mrs Steve and demand my building table back when I finally get back to the UK full time. ( I honestly will do this, and I assure you I am in no way scared...)

So, the project on the go is the Earthace boat, which will be very nippy indeed, but it has got me wondering:

What are the factors that limit the maximum speed of a MODEL boat.

This is really a question in two parts:

1) What are the limiting factors to the speed of an object moving through the water
2) Which of these factors are changed by the fact that there is a human in the boat.

As an example:
Boat X can go at 30 mph ( or we could use knots, but I find these scary and confusing so I'll pretend they don't exist ). There will be a number of reasons why Boat X can not go faster than this. some of them will be mechanical, and some of them will have been designed in so as to make the ride in the boat bearable by the human who is driving it.

Hydrofoils for example, especially on passanger boats, are sub-optimal if viewed from a purely speed point of view. However, if they were speed optimised, they would give a horrible ride and passengers would have to endure extreme shaking and much bouncing about. So they are a comprimise.

On a model boat, we don't have to make these comprimises - there are no people we have to stop from downing or to provide a smooth ride for, so we can force ( if we choose to ) the boat to be much closer to the optimal design possibilities.

So, what are the purely mechanical factors which influence speed ( straight forwards speed and turning speed ) of a boat?

Any thoughts greatfully recieved

Steve

<<edit - Oops - didn't see the new section called "Technical question". Mods, please accept my apologies and find it in your heart to move this to it's correct home>    >>

[Shipmate60]

The speed of a displacement hull is defined by the hull form and its ability to displace the water ahead of it.
It will reach its optimum speed then need a huge increase in power to increase speed IE speed increases with the SQUARE of the power required.

Planing hulls have a totally different way as they are not pushing the water they are riding on top of it.

That is why it is easier to overpower a planing hull, see some of the OMRA threads.

Straight line speeds aren't such a problem, but turning is.
As the speed rises the "wetted" area of the hull decreases, this is what gives you stability in turns.
With very little wetted area there is very little "holding" the hull in the water so when you try and turn it will spin out as all the forces are well above the waterline with very little to hold it in the water so it tries to act like an aircraft but in boat form.
 Hydrofoils work in a totally separate way as the wetted area is spread fore and aft and wide at the bow giving better stability.

Bob

[RipSlider]

So, for the two main hull types, is it possible to create a list of influences that affect maximum speed.

For example, I assume that on a displacement hull the factors would include:

- wetted volume
- length of hull
- shape of bow ( a concave hull may have less resistance to a convex one )
- surface friction of hull
- other stuff

I know that the only way to do this properly would be to break out the Navier-stoke equations and do lots of complex sums. However, I'm just thinking more generally than that.

Thanks for the info so far.

Steve

[tigertiger]

For boats that do not plane (Earthracer won't), there is a theorhetical maximum speed related to the length of the waterline. There is a formula but I cannot remember where I have seen it.

Everything after that is subtraction

[Colin Bishop]

Most scale models are "overpowered" compared with their full size counterparts so the actual speed max speed of the model will depend upon how much power you can put into the vessel and still retain a measure of control over it!

[dreadnought72]

Hydrofoils for example, especially on passanger boats, are sub-optimal if viewed from a purely speed point of view. However, if they were speed optimised, they would give a horrible ride and passengers would have to endure extreme shaking and much bouncing about. So they are a comprimise.

I'm not quite sure I'd agree with that. A properly designed hydrofoil minimises the wetted area of the hull, and the wings can be made virtually a constant area even when waves slosh up and down the support struts. Can't get smoother than that. As speed increases you'd simply reduce the lift from the foil (also decreasing drag).

Of course, for the maximum speed of a "boat", you'd want to take the ekranoplan route. Which isn't a boat at all... 

Andy

[tobyker]

I think the formula for displacement hulls is the square root of WL(in feet) divided by 2 = max speed (in Knots). If not I'm sure this will jog somebody's memory. 

[malcolmfrary]

Using the above formula, a 900ft ship (say a US carrier), sqrt of 900=30, divide by 2, 15 knots.  The right formula is out there somewhere, we just need someone who knows it.........
In all probaility, the length/beam ratio will come into it somewhere as well, or that might be one of the subtractions mentioned.

[dreadnought72]

A couple of sites suggest:

Displacement Hull Speed = 1.34 * SQRT(LWL)

Where LWL is in feet and Speed is knots. This is probably not bad for a rough rule of thumb - it makes a small battleship around 30kts, tops.

And it also means that a seven foot scale model travelling at its scale speed is well under the limiting displacement speed - meaning relatively less power than might be thought.

Andy

[Colin Bishop]

Googling "Hull Speed" brings up quite a bit of information.

[Ivor Bittle]

The sailing world use what they call the hull speed to set up a handicap system. They use Hull Speed = 1.4 x square root of length in feet to give a speed in knots. This expression is in fact based on Froude's work in the mid-nineteenth century to predict the resistance to motion of commercial steam boats. It is good enough for most model purposes and, as most models lie in the range of 3 feet to 6 feet in length. the range of hull speeds is about 2.5 knots to 3.5 knots, that is, from fast walking for old men to fast walikng for young men. At the hull speed the boat will produce a significant bow wave.

You might just find my section on the bulbous bow in my web site at www.ivorbittle.co.uk of interest. It includes a discussion of bow waves.

Ivor Bittle

[farrow]

It is a long time ago and my memory is a little vague, but 40 years ago I believe the formulae used then to work out the maximum speed of a convential hull, was length  X beam X block co-efficient of the hull. The bit I cannot remember is the average co-efficient of hulls. As you probaly know every hull has its max speed in theory, but achieving it is very hard and nie on impossible due to the amount of power required.

[tigertiger]

It is a long time ago and my memory is a little vague, but 40 years ago I believe the formulae used then to work out the maximum speed of a convential hull, was length  X beam X block co-efficient of the hull. The bit I cannot remember is the average co-efficient of hulls. As you probaly know every hull has its max speed in theory, but achieving it is very hard and nie on impossible due to the amount of power required.

Hi Rmasmaster
Is this crude formula for calculating displacement volume?

[geoff p]

Hi All,
Not quite the scale speed, perhaps ...
I used to travel on a ferry-converted-from-a-fishing-boat, between Fair Isle and Shetland.

The journey time was generally about 3 hours and 5 minutes at 3/4 throttle.  It was a fairly bumpy ride.
At full throttle the journey shortened by a whole ten minutes!  The ride was now appalling - green around the gills was a normal look for all aboard.  Meanwhile, fuel consumption increased alarmingly.

At lower throttles the boat rode at just about her 'displacement' speed - supported by the bow wave and stern wave, with a blooming big hollow in between - and all the extra power at full throttle just couldn't get her over the hump.
Most displacement boat when 'overpowered' will just tuck down at the stern.

Cheers,
Geoff

[farrow]

Hi tiger Tiger no, as I said it was 40 years ago but it was a simple formulae like that to work out a ships max theoritical speed, used to do it on a back of a fag packet. Last time I used it, was when I settled an argument over the speed of HMS Manxman over the measured mile after she came out of reserve in Malta in about1955. She far exceeded her designed speed and this fellah said it was impossible for a vessel of her size so we calculated it and I won, my father was in charge of the engine room refit. But the formulae was fairly simple like that. To work out a ships displacement, simpson's rules come into play.