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Author Topic: prop revs  (Read 5999 times)

boatmadman

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prop revs
« on: October 02, 2013, 10:13:29 PM »

I am re furbishing a 40" riva and intend to fit two x 45mm 3 blade props.


I think I need to be looking for about 5000 rpm on these props to give a reasonably scale speed.


Does this sound about right?


Ian
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Shipmate60

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Re: prop revs
« Reply #1 on: October 03, 2013, 12:57:18 AM »

No, closer to 10,000 plus.


Bob
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deadwood

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Re: prop revs
« Reply #2 on: March 19, 2015, 03:37:27 PM »

Though this is quite a stale thread I would like to add that if the propeller revolutions of the original ship are known,
and if geometrically similar props are used (what is implied by the words "scale speed") then the laws of similitude could be applied,
as is in self-propulsion model tests in scientific and industrial ship model basins.

There Froude similitude is obeyed, so that Froude Nos. for model scale (index m) and 1:1 scale (index s) should be equal.

FNm = FNs
Vm/sqrt(g*Lm) = Vs/sqrt(g*Ls)

solving for Vm with Lambda being the scale factor

Vm = Vs * sqrt(Lm/Ls)     or    Vm/Vs = Lambda^(-1/2)


Applying this similitude to velocities in the propeller plane the advance ratios for model and ship should be equal too.


Jm = Js
Vam/(Nm*Dm) = Vas/(Ns*Ds)        # with N being Nos. of revs. and D prop. diameters

again solving for number of revs of model

Nm = Ds/Dm * Vam/Vas * Ns

substituting the scale factor Lambda

Nm = Lambda * Lambda^(-1/2) * Ns

we get

Nm = Lambda^(1/2) * Ns    or   Nm = sqrt(Lambda) * Ns


So for example, if the original prop rotates @ 120 min^(-1) or 2 Hz and the scale factor of the model is 100
then the model prop should rotate @ 20 Hz or 1200 min^(-1)
which is way below the number mentioned and, given the usual rpms of model elctric motors, would almost always necessitate a reduction gear.



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Shipmate60

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Re: prop revs
« Reply #3 on: March 19, 2015, 04:45:58 PM »

Despite all your calculations you do forget several factors.
The water does not scale down.
The efficiency of model props is notoriously bad.
BUT
Despite this there is one over riding factor.
Actual testing.
At the revs you consider correct the model would hardly move and not have any power to counteract the non scale wind.
Yes I am familiar with Froude as an ex Chief Engineer so can categorically say you are not only wrong, but so wrong to be embarrassing.


Bob
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deadwood

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Re: prop revs
« Reply #4 on: March 20, 2015, 10:29:41 AM »

Despite all your calculations you do forget several factors.
I didn't calculate anything.

Quote
The water does not scale down.
The efficiency of model props is notoriously bad.
I am well aware of the so called scale effects.
But because model tests can only be performed at Froude's scaling law, and not at Reynolds' scaling law (to mention just the most important two) at the same time and scale (other than 1:1)
there are introduced a whole bunch of correction factors and correlation coefficients during the test procedures that I left under the carpet not to totally confuse readers here.

Quote
BUT
Despite this there is one over riding factor.
Actual testing.

I only was referring to actual testing, and this is nothing that I made up.
If you really are interested in the model testing procedures and the corrections that are applied for the inevitable negligence of scaling laws other than Froude's
then I recommend you have a glance at the 1978 ITTC Power Prediction Method which was a recommended procedure for all towing tanks and ship model testing facilities world wide to be followed to arrive at comparable data and predictions based on that.

You may find a list of all recommended procedures issued by the ITTC here.

I also fond a more tangebile outline of the model testing procedure in e.g. this thesis.

Quote
Yes I am familiar with Froude as an ex Chief Engineer so can categorically say you are not only wrong, but so wrong to be embarrassing.
Beg your pardon, I cannot see what I should feel embarrassed about.
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inertia

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Re: prop revs
« Reply #5 on: March 20, 2015, 12:20:52 PM »

I have to agree with Bob here, in that all the theory in the world will not accurately predict what a model boat will do in any given situation. It's possible that a steam-powered model with a very large, coarse-pitch prop would perform adequately at 1200 RPM but the model under discussion was a scale Italian speedboat with a planing hull.
I always used to design my model aircraft on the basis that if it looked right then it would probably fly right - maybe with a bit of tweaking. It worked for me - there are thousands of the things out there still happily flying around. My old colleague Rob Millinship used to draw his aerofoil sections "freelance" - by using the edge of his shoe as a template! Not exactly a lot of science was used back then.
The main rule is to trust your own or other peoples' experience. For example, if you are building a twin-engined tug of about 36" long then find one similar that performs well and start with that motor and prop combination. If you want to use it for towing then you'll need more power than you would for a steering-course model etc etc.
I've no idea what the optimum speed of a 45mm prop underwater would be but I'd suggest - purely on the basis of experience and anecdotes concerning 1/16 scale lifeboats - that Ian would have been looking for a no-load (out of the water) speed of maybe 8000 - 10,000RPM. If this were a model with brushed motors I'd have used two 12v Speed 700 Turbos. 35mm dia outrunners of about 800kv would seem to have been a logical place to start with brushless motors. Later tweaking can always be done by changing the prop size and/or motor voltage. I wonder what Ian eventually settled on.
Finally I'm sure that Vic Smeed didn't reach for his log tables when a new design occurred to him, and neither does Glynn Guest. Both could be said to be successful model boat designers - QED.
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deadwood

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Re: prop revs
« Reply #6 on: March 20, 2015, 12:43:46 PM »

Hi Inertia,

I shouldn't have posted the stuff in the first place. And since the thread was quite old Ian's curiosity must have been satisfied a long while ago.

I didn't read carefully either.
So it had escaped my notice that the question dealt about a speedboat yacht with a planing hull where the regime of (model) hull propeller interaction is totally different to that of displacement hulls designed for low Forude Numbers to which the test procedures related.

So I apologize for the noise I spread.

Regards,
Ralph
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inertia

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Re: prop revs
« Reply #7 on: March 20, 2015, 02:55:43 PM »

No problem at all, m'duck. It makes a pleasant change to see someone on this forum actually admitting to being wrong.
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GG

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Re: prop revs
« Reply #8 on: March 20, 2015, 03:03:32 PM »

Dear Mr Inertia,
                  I do in fact try to make a few basic calculations before committing myself to building a new model, if only to avoid trying to defy any well known laws of the universe.
One obvious thing to estimate is a models likely weight, very handy to avoid building an unintentional submarine when you really wanted something else.
As for the subject of "prop revs", when in doubt I'd divide the desired model speed by the propeller pitch to find the rotational speed needed before the propeller can start to make any thrust.
I'll admit to not using log tables, but if mental arithmetic is too difficult, one of those little boxes with numbered buttons seems to work OK.
Glynn Guest
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Shipmate60

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Re: prop revs
« Reply #9 on: March 20, 2015, 04:20:37 PM »

I did find that in model boats initially the prop speeds were so fast.
With a bit of experience on most drive trains a rule of thumb is 3500 rpm (eg Deccaperm) to 18000 max covers most applications from tug to fast electric.


Bob
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inertia

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Re: prop revs
« Reply #10 on: March 20, 2015, 07:30:35 PM »

Dear Mr Inertia,
                  I do in fact try to make a few basic calculations before committing myself to building a new model, if only to avoid trying to defy any well known laws of the universe.
One obvious thing to estimate is a models likely weight, very handy to avoid building an unintentional submarine when you really wanted something else.
As for the subject of "prop revs", when in doubt I'd divide the desired model speed by the propeller pitch to find the rotational speed needed before the propeller can start to make any thrust.
I'll admit to not using log tables, but if mental arithmetic is too difficult, one of those little boxes with numbered buttons seems to work OK.
Glynn Guest

 {-)
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boatmadman

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Re: prop revs
« Reply #11 on: March 22, 2015, 04:36:38 PM »

Interesting discussion.


I settled on two 900kv brushless outrunners on 4s lipo which gives 13000rpm unloaded.


With 40mm x type props it gives a good planing speed very similar to full size.


I will get some pics later.


Ian
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boatmadman

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Re: prop revs
« Reply #12 on: March 22, 2015, 04:41:21 PM »

Pic here
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McGherkin

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Re: prop revs
« Reply #13 on: March 23, 2015, 05:26:45 PM »

Interesting discussion.


I settled on two 900kv brushless outrunners on 4s lipo which gives 13000rpm unloaded.


At full charge (16.8v) they should theoretically be around 15000rpm! My setup is the same and maximum speed logged by the ESCs is at the top end of 14krpm.


I can't see me needing any more for my lifeboat. I think that will actually be too much but power in reserve = extra runtime so I'm not complaining.
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