Froude’s Rules

[BarzoParticipant @barzo]

Does any one have any experience of relating Froude’s
scaling rules to model boats. I am a bit sceptical of the power scaling
factors.

The Fairy Huntsman looks to be powered by two engines, giving approx. 480 shp giving a max. planing speed of approx. 30 knots.

For a model at 34 inches (scale approx. 11:1)  would give a scale speed of 9.95 knots with .11 hp – 82 watts.

A boat weighing 8.26 lbs., with a motor running with 12 volts (no losses) would consume 6.82 amps.

The watts and hence the amps with 12 volts do not seem enough – where am I going wrong?

Cheers Barzo

[BarzoParticipant @barzo]

[Richard SimpsonModerator @richardsimpson88330]

There was an extreemly good article in the Model Boats Magazine Commemorative issue last year on the application of scale.  I’ll dig out my copy and see if it has anything to offer in it.

[TelstarParticipant @telstar]

Hi Simplified scale factors I use are  

   length of model :   length of craft /scale factor   ie 11 in your case

   area   of model :    area    of craft / scale factor squared    ie 121

  volume or force   on model : force of craft / scale factor cubed   ie 1331

 

Applying this to 480 hp   gives 0.36 hp or approx  260 watt , about 22A on 12 v

A Waverider ( Vernon Kit) I use for ‘playing’ with motors planes nicely on 12v 10A with three different brushed motors fitted, and has very similar performance on 12v 8A with a brushless motor fitted. Waverider is 24ins long

 

merry christmas Tom

[Dave MilbournParticipant @davemilbourn48782]

And my 24″ Fairey Swordsman used a standard cheapo 540 motor on 7.2v and planed beautifully. I doubt if it was pulling any more than 10A. Assuming maybe 50% efficiency of the combined motor and drive train, that’s putting 36W into the water (or 1/20HP).

The best maxim about applying full-size theories to models is not to bother. I never have!

Dave M

[Paul TParticipant @pault84577]

Like Dave (how is life up on Olympus)I never bother about trying to apply full-size theories to model building one reason being that the fluid dynamics calculations dont scale down in a pro-rata rate to match the build scale.

The second reason is that I just cant be bothered as the resulting ‘best mix’ of engine and batteries usually wont fit within the hull.

 

I end up sticking in a combination of 1, 2 or 3 of my favourite Johnstone motors and batteries go wherever I can fit them. 

The model always works out and just requires a little ballast adjustment for trim during the first sailing.

 

Heath Robinson

[ashley needhamParticipant @ashleyneedham69188]

Mr Robinson. i full agree with this, not that I actually KNOW any formula to apply in any case.

 

Once the hull is built I generally have an idea of what I want to power it and then I float it in the bath and bung in such mechanical devices and batteries to see where the waterline might be, how much ballast I might need and of course where to put said items for trim.

 

For my “Specials” even this is not enough and certainly in one case some drastic surgery was called for. Aircraft just dont fit in my bath…

 

My experience is the location of said devices is normally governed by access arrangements and placement of bulkheads and the like, especially in the case of warships  (I never use a removable deck, only superstructure, for watertightness)

 

Mr S Law

[Dave MilbournParticipant @davemilbourn48782]

(how is life up on Olympus)

 

Mighty fine, thanks, Paul! I can see your bald spot from up here……………………

DM

[BarzoParticipant @barzo]

Happy New Year to every one,

The various replies have certainly given me a lot to think about.

WRT ‘telstar’, power is a function of force and speed. This gives a factor which is the product of ‘linear scale’^3 x ‘linear scale’^.5. Using my scale of 1:11 the power scaling factor is – 4332.

As far as scaling the power. I have assumed that the losses in the engine, propshaft, propellor train of the actual boat is also in the gross hp calculated for the model.

My scepticism  of the scale power calc. is probably due to the unconscious notion of what the scale speed should be. it is clear that when one actually calculates the scale speed, it is much lower than the speeds achieved when using IC engines in a model. I do not think it unreasonable to say that actual model speeds of 20 – 30 knots, ( full size 66 – 99 knots), is massively over scale.

So I guess that if one wants to be a slave to scale thats fine – but on the other hand a model going like weasel poo off a telflon shovel with the accompanying elevated heart rate there is nothing to beat it – at least not at my age!

Cheers and thanks again to all Barzo

[ashley needhamParticipant @ashleyneedham69188]

I think that there is no such thing as “scale power”…only the use of “scale throttle”.

 

Always best to over power a model, for fun and also to have “something in reserve” for non-scale emergency situations. How fast you go and not   how fast you CAN go is what ESC`s were made for.

 

Your own opinion of what looks right is what matters I always think.

 

Ashley.  ….    rumored  to have had ducks surfing in the wake of his battlecruiser.

 

 

[BarzoParticipant @barzo]

For any one still following this post, I think I have the definitive answer!

In a Serendipitous purchase of a remaindered ‘model Boats commemorative special 1950-2010’, there is a reprint of an article from the Model Boats, February 1983 issue, by Ron Warring – Scale Questions and Answers.

To refer only to the Scale torque and horsepower section.

The calculation of scale power using the function  – scale^3 x scale^.5 – produces a horsepower figure that is too low and in referring to two examples quoted in the text, Ron Warring makes the following point:

“Neither calculation is consistent with practical results! Why? Mainly because it is impossible to achieve scale weights, so models have more displacement and wetted area – and surface tension ‘drag’ is quite different. This is also the reason why model tests for hull drag measurement can be widely inaccurate unless the model is quite large. What it does seem to show is that models need about five times ‘scale’ power!”.

Q.E.D.- out.

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