Model Boat Mayhem

Please login or register.

Login with username, password and session length.
Pages: [1]   Go Down

Author Topic: Twin props to achieve scale speed  (Read 606 times)

DBS88

  • Shipmate
  • *
  • Offline Offline
  • Posts: 5
  • Model Boat Mayhem is Great!
Twin props to achieve scale speed
« on: March 11, 2020, 07:12:03 PM »

Hi I have done some calculations to find out the propellers rpm to reach the scale speed for my black swan sloop. The figures for a 50 mm steam prop  to reach 1.44m/s assuming 75 percent efficiency is about 1500. So my query is if one prop at 1500 rpm achieves the scale speed how fast must two 50mm props turn to achieve the scale speed? You advice and help are appreciated
Logged

SailorGreg

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 884
  • Money talks - it says goodbye
  • Location: Hayling Island, Hants
Re: Twin props to achieve scale speed
« Reply #1 on: March 11, 2020, 10:16:37 PM »

I must emphasise that what follows has no basis in practical experience - I have never run a twin screw boat.

If you imagine a propeller screwing itself through the water, the faster it turns the quicker it moves forwards (ignoring drag, slip and all the other annoying factors of real life).  So for a given number of revs (1500 in your case), a particular pitch of propeller will give a certain speed.  Now consider twin props of the same pitch.  To get the same speed, they are going to have to screw themselves along in exactly the same way as a single prop.  But as they share the work, each motor will draw less current (or need less power from whatever source it comes).  So purely theoretically, your two props will need to turn at the same revs as a single prop to deliver the same speed.  However, having the work shared means each prop will operate a little more efficiently, leading to an increase in speed for the same revs, or reduced revs to achieve the same speed.  But exactly how much is beyond me I'm afraid.

Hope that helps a little.

Greg

BrianB6

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 880
  • I'm not sure if I can get up from here
  • Location: Melbourne
Re: Twin props to achieve scale speed
« Reply #2 on: March 12, 2020, 04:46:38 AM »

It's a "can of worms"   There are so many variables from prop design, hull design and motor specs etc. that everyone seems to have a different opinion.
My thought is to use whatever prop suits the motors and use a speed controller to get the scale speed.   It is always advisable to have excess speed in reserve to get out of trouble.
Scale speed does not always look right and if it is too slow causes steering problems.
I have 2 twin prop scale boats and have found a "suck it and see" method to eventually get the right size prop.  I have accumulated a few cheap plastic props over the years for trial purposes before lashing out on brass ones.
Logged

DBS88

  • Shipmate
  • *
  • Offline Offline
  • Posts: 5
  • Model Boat Mayhem is Great!
Re: Twin props to achieve scale speed
« Reply #3 on: March 12, 2020, 07:38:45 AM »

My thoughts are that if one prop needs to turn at 1500 rpm to achieve a speed then two props will need to spin at 1500. I suspect the speed will be reached more quickly and that the engines will share the load so not work so hard as a single engine/propeller, also with independent control of each prop improved low speed turning will be available
Logged

malcolmfrary

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 5,619
  • Location: Blackpool, Lancs, UK
Re: Twin props to achieve scale speed
« Reply #4 on: March 12, 2020, 09:40:27 AM »

A dispacement hull moves forward by its own length in a given length of time because the prop or props have moved at least the displaced volume of water backward by at least the hull length in that same time.
If one prop achieves the right speed, two props will achieve exactly the same speed, but, if there is enough power, will get up to that speed more quickly.
Assuming the same power into the props, to shift the same volume of water as 1 50mm prop, 2 35mm props would do the same job, but would need the same pitch as the 50mm one to get the distance turning at the same speed.  Of course, a smaller prop will allow the motor driving it to spin faster........
Logged
"With the right tool, you can break anything" - Garfield

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 10,475
  • Location: SW Surrey, UK
Re: Twin props to achieve scale speed
« Reply #5 on: March 12, 2020, 10:18:54 AM »

You might find the extract below, taken from an old magazine of interest. Also have a read of this:

http://www.modelboats.co.uk/news/article/but-i-don-t-understand-electronics/18054

As BrianB6 says, the best option is usually to take an empirical view. If it is a scale boat then fit scale standard type props, then fit a battery with sufficient voltage to overpower the model. Scale speed can be a bit feeble in anything over calm pond conditions and the extra power is frequently needed to maintain control of the model and offset wind etc. Then just use your speed controller to run the boat at whatever speed is appropriate to the conditions. A 'realistic' looking speed on the pond is usually rather greater than 'true' scale speed.

Colin

From an article by Ron Warring:
Scale Speed Scale speed is another interesting subject, but most boat modellers probably know this one. True scale speed equals the square root of S x fullsize speed, where S is the scale. In other words, for a 1 ⁄12th scale model scale speed is the square root of 1⁄12 = 0.2886 x fullsize speed. (This is where a pocket calculator with a square root function comes in handy). let's apply this first to our original powerboat example and take a fullsize speed of 90mph. Scale speed for a 1⁄12th scale model would be 0.2886 x 90 = 26mph. Back to our 1⁄100th scale model frigate, fullsize speed 30 knots. Scale speed in this case = 1⁄10 x 30 = 3 knots, or around walking pace. Of course, this calculation can be worked the other way round. Speeds of 80mph are claimed for model hydros say, nominally, 1⁄8th the size of a fullsize hydro. The square root of the scale is 1⁄8 = 0.3535. To scale up we divide scale speed by this figure. So equivalent fullsize speed = 80⁄0.3535 = 226mph. Naval architects use a slightly different formula - Froude’s law for dynamic similarity. This states that the speed ratio between model and fullsize is equal to the ratio of the square roots of the respective waterline lengths. This gives exactly the same results as above. It is no accident that all these ‘equivalent’ speed figures are realistic - unlike the strength comparison calculators that were not. at equivalent speeds - i.e. scale speed on a scale model and fullsize speed on the full size craft - both the wave patterns and spray patterns generated are the same, with one slight difference. Because of the relatively greater effect on surface tension on the smaller size hull, spray droplets will be relatively larger but the general nature and direction of the spray is not materially affected. If you are puzzled by the greater effect of surface tension, think of this. You can float a steel needle on water because of surface tension. But the same surface tension would not support a ‘scaled-up’ needle in the form of an iron bar.
Logged

GG

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 214
Re: Twin props to achieve scale speed
« Reply #6 on: March 12, 2020, 10:22:45 AM »

DBS88,
         I recall that many years ago Philip Connolly commented that a twin motor and screw scale model achieved much the same speed whether it driven by one or two propellers.  His reasoning was that in effect at full speed the second propeller could be considered to be virtually "free-wheeling" and adding little to the total thrust.  Similar to your thoughts in your last post.


But, when the model is at rest, two motor/propellers ought to produce double the thrust of a single motor set up and twice the acceleration, sometimes very handy.


My own experience seems to bare this out since twin motor/prop installations have proven to have no obvious speed advantage over equivalent single motor/prop driven models.  This applies to scale type displacement models, but all bets are off with high powered "missiles"!


Glynn Guest
Logged

chas

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 397
  • now in los montesinos Spain.
Re: Twin props to achieve scale speed
« Reply #7 on: March 12, 2020, 11:04:16 AM »

My own experience is the same as GG. The ships with twin screw's are about the same speed with one or two motors
Running. High speed scale models show a little more difference, but not much, especially considering the extra ampage used. As to scale speed, I've never regretted setting up a model so that the speed is what looks right on the water with the throttle at 3\4.  The last bit is to get me out of trouble when I steer badly.
 Charles.
Logged

Allnightin

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 214
Re: Twin props to achieve scale speed
« Reply #8 on: March 12, 2020, 04:49:56 PM »

I am wondering if the reason that most people don't see much difference between 1 shaft or two is because in a typical model there is already ample power being delivered by one shaft to move the model at around the Froude formula for maximum economic speed.  Increasing power by switching on a second shaft would take the model into a part of the shaft power versus speed curve where even a large increase in power produces very little increase in speed.                             

From my experience with operating a 4.5m long model of a Type 42 destroyer (1/32nd) to claim the Guinness record for distance in 24 hours in 2002,  where the power requirement was considerably larger (about 150W per shaft), I would definitely say that when the model ran on only one shaft it was appreciably slower.  In fact we had to abandon the first record attempt about 4 hours into the run when a rope got caught in one of the shafts and caused a reduction gear to break.  Continuing the run on one shaft after clearing the rope would not have sustained the necessary speed to break the record.
Logged

likeomg

  • Shipmate
  • *
  • Offline Offline
  • Posts: 17
  • Model Boat Mayhem is Great!
  • Location: North East
Re: Twin props to achieve scale speed
« Reply #9 on: March 13, 2020, 01:55:00 AM »

I guess when it comes to scale boats it doesn't really matter... but when you start adding the mass and resistance of a actual ship a large difference is noticeable..

I work on a vessel with 3x 20mw pods (motors), spinning 6.2m props with a top speed of about 25kts, if we loose a pod (techniacl fault... they rarely fall off :) )max we can make is in the 19kts range -


we tend to make around 18-20kts, i remember departing palma and a pod had a issue for a hour or so we ran 2, keeping the same available power we lost a good 2.5kts using 2 pods than 3... obviously the 2 pods also span faster as more power was available... but the ships speed dropped 2.5kts.
Logged

DBS88

  • Shipmate
  • *
  • Offline Offline
  • Posts: 5
  • Model Boat Mayhem is Great!
Re: Twin props to achieve scale speed
« Reply #10 on: March 13, 2020, 08:52:23 AM »

It appears that a hull has a maximum theoretical speed after which a disproportionate amount of power has to be added for little additional speed.
Logged

mrlownotes

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 294
  • Location: A bit NorthWest Cambridgeshire
Re: Twin props to achieve scale speed
« Reply #11 on: March 13, 2020, 09:57:19 AM »


This Youtube has no mention of prop count but explains some simple hull speed considerations.




https://youtu.be/H8hvKZ73S0Q
Logged
Bintur Ellenbach

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 10,475
  • Location: SW Surrey, UK
Re: Twin props to achieve scale speed
« Reply #12 on: March 13, 2020, 01:12:24 PM »

Going back to the original question. If a boat has one prop turning at 1500 rpm and you add another prop doing 1500 rmp then you are pushing twice as much water astern (or very nearly) so basic physics dictates that there will be an increase in speed although it will come nowhere near to doubling and be affected by the hull form, the speed the hull is travelling (water resistance) and the extra weight of the additional motor etc.

As said above, with full size ships, the loss of a propulsor inevitably results in a drop in vessel speed if the other propulsor(s) are running at normal speed. Happens all the time with ferries and cruise ships, often resulting in a truncated itinerary for the latter.

Colin
Logged

DBS88

  • Shipmate
  • *
  • Offline Offline
  • Posts: 5
  • Model Boat Mayhem is Great!
Re: Twin props to achieve scale speed
« Reply #13 on: March 13, 2020, 10:16:50 PM »

So with two props it appears that two smaller engines and or props could be used to achieve the same speed as one 50 mm prop at 1500 rpm - is there a way of working out the size of two props = 1 50mm prop?
Logged

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 10,475
  • Location: SW Surrey, UK
Re: Twin props to achieve scale speed
« Reply #14 on: March 13, 2020, 10:25:03 PM »

Possibly, but life's too short I think! No magic formula I'm afraid.

As I and others have said earlier, fit the scale size props and then adjust the input voltage to get the speed you want. Works for most people.

Colin
Logged

DBS88

  • Shipmate
  • *
  • Offline Offline
  • Posts: 5
  • Model Boat Mayhem is Great!
Re: Twin props to achieve scale speed
« Reply #15 on: March 14, 2020, 07:28:16 AM »

Firstly thankyou for all the helpful and supportive replies, they have helped my understanding. The model. being built is steam powered so your advice is valuable since the option of varied voltage to achieve the desired rpm is not available so the effort to understand and get this right is fundamental to the success of the finished model. Your thoughts are most welcome
Logged

malcolmfrary

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 5,619
  • Location: Blackpool, Lancs, UK
Re: Twin props to achieve scale speed
« Reply #16 on: March 14, 2020, 05:49:33 PM »

Firstly thankyou for all the helpful and supportive replies, they have helped my understanding. The model. being built is steam powered so your advice is valuable since the option of varied voltage to achieve the desired rpm is not available so the effort to understand and get this right is fundamental to the success of the finished model. Your thoughts are most welcome
Still the same thoughts as reply#4.
Logged
"With the right tool, you can break anything" - Garfield

Colin Bishop

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 10,475
  • Location: SW Surrey, UK
Re: Twin props to achieve scale speed
« Reply #17 on: March 14, 2020, 06:04:37 PM »

It would have been helpful and saved some effort if it had been made clear that this was a steam powered model at the outset. Whilst the principles remain the same the practical implications could be somewhat different.

Only a very small proportion of model boats are steam powered so it was a naturall assumption on the part of responders that electrical propulsion was involved.

Colin
Logged

justboatonic

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,394
  • Location: Thornton Cleveleys
Re: Twin props to achieve scale speed
« Reply #18 on: March 14, 2020, 06:26:48 PM »

Id just use the proportional throttle control of the radio and think the OP is just going too deep.

Id honestly say getting a 'scale speed' to look right is virtually impossible with a model boat. You'll either get what you consider looks like a scale speed but you wont get the scale bow wave and wash to go with it. That's because the water we sail on is 1:1 scale while our models are anything but.
Logged

SteamboatPhil

  • Full Mayhemer
  • *****
  • Offline Offline
  • Posts: 1,816
  • Location: Dieppe, France
Re: Twin props to achieve scale speed
« Reply #19 on: March 14, 2020, 08:42:19 PM »

Tis true, you can’t scale water  %)
Logged
Steamed up all the time
Pages: [1]   Go Up