The Ship Model Forum

First off, I have to say that I have never built an R/C model, but I'm gearing myself up to it.

One thing I dislike about many model ships is how they seem too buoyant. Despite being ballasted down to the correct waterline, they still frequently look like they're bobbing about on top of the water instead of appearing to sit "in" it. Now I know all about the inverse square law and the inverse cube law, surface tension, momentum and such, and I have a (very) sketchy understanding of fluid dynamics, so I fully recognise that a 1:72 ship floating in 1:1 water is never going to behave exactly to scale. But I have wondered if there might be a way to offset the "bobbing about" effect.

Please excuse the non-technical language, as I'm nothing more than an enthusiastic amateur...

Let's say the ship encounters a wave bow-first. Because the bow is buoyant, the wave will tend to lift the ship's bow and the stern, sitting in water at a lower level, will tend to be forced further down. The ship pitches bow-up, effectively rotating around a point approximately mid-way along its length. At this point the bow-up turning moment is at its maximum.

As the wave travels along the ship, its buoyant influence on the bow decreases until it reaches the mid-point of the hull. The turning moment reduces to zero, and the ship assumes (briefly) an even keel.

As the wave continues to travel sternwards, the turning moment it exerts upon the ship increases again, this time making the ship pitch bow-down. Eventually the wave passes and the ship once again achieves an even keel.

Now all but the smallest R/C ships require some extra ballasting to bring them up to scale weight. If this extra ballast were to be concentrated in the nose and the stern of the ship, might this not tend to increase the inertia of the ends of the ship and reduce its liability to pitch? It seems to me that by making the bow "less buoyant" it would have greater inertia and take longer to respond to the turning moment caused by the wave, to the point where the wave would have passed further along the ship by the time the bow had begun to substantially respond to its influence. The same effect of "reduced buoyancy" would apply to the stern as the wave approached and passed it: the ship would effectively have been "damped" in its tendence to rotate or pitch, and it would appear to be ploughing through the wave rather than riding on top of it.

Does anyone have a more qualified view on this theory? Has anyone ever experimented with the distribution of ballast in a model ship? Would the difference it made be noticeable? Am I chasing ghosts?

With R/C ships it is all down to how you build the model, I have been saying for years and showing in my builds that you have to build the superstructure as light as you can this stops roll and pitch, allways put your ballest near to the centre of the model and trim out bow and sturn. from there you should have a stable ship. ron h

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Simple but effective.

Hello there Admiral. I've been reading and re-reading your builds avidly, and you've provided me with lots of ideas for when I do finally decide to get out of my armchair and into my workshop. Thank you for all your contributions over the years.

I certainly understand the principle of keeping topweight down to a minimum. It's just that concentrating the bulk of the ballast in the centre of the ship seems rather counter-intuitive to me. I know that the battery is usually the heaviest item in the hull, and that there's usually only one of them, but it just strikes me (a mere apprentice) that the remaining ballast should be distributed as far as possible towards the ends of the model rather than at the centre.

Essentially, this is correct. Not only the mass but also the mass distribution has an effect on ship motions (mass by virtue of draft). If the mass would centered in a point, it would be most easy for the ship to pitch. When we perform a test in seaway, we always make sure this distribution is correct on a swiveling table, pitching and rolling the model and redistributing the mass until it is correct. When you change the mass distribution you change the so-called the moments of inertia of the ship and you change the response. (Mass is the resistance against changing speed of an object, moment of inertia is the resistance against changing its rotation)

Generally speaking, ships do pitch upwards when encountering a wave head-on, but there is always some phase lag between the wave and ship motion. It is possible to have a ship move exactly opposite to the wave motion.

My guess is that one can reduce the pitching by increasing the pitch moment. This would be achieved by concentrating ballast closer to the ends of the ship. Real water wave propagation is faster than the scale wave action should be, and our moment indicies reduced by a huge factor. Ballasting too low will however typically cause a very large righting moment and accelerate any bobbing, especially in roll. It might however be safer!

Tom

There is always going to be a problem with 1:1 scale water and wind acting on a model. It's one of the reasons the old movie models used to be fixed to a track on the bottom of the tank in which they are being filmed. They could be solidly fixed as the water moves around them (and the frame rate was increased to slow down the overall action).

Because you can't change the frame-rate of reality, the issue of ballasting is key. I honestly don't think you can place ballast too low in a hull. I use sheets of roofing lead placed flat in each compartment, evenly throughout length of the hull (assuming ballast is needed). These days we can't 100% rely on the battery, with NiMh or even LiPo packs the ballast will often outweigh the battery.

If you're building in 1/72nd the issue of "bobbing" is greatly reduced, indeed for any model over 4 foot or so I think this is the case. I build "stand-off" scale in order to add 20mm or so to the draught, this helps the ballast sit as low as possible; model's CoG is always higher than the real thing.

However, the most important thing to watch in the case of any RC boat is the way it is operated. Far too many people have only a binary throttle and rudder action (on or off!). Smooth operation on the water, taking the conditions into account can make for a reasonably realistic sight on the water regardless of the model's scale.
One of the reasons I build in 1/144th.

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On the Slips:
1/144th Stand-off scale:
IJN Nisshin
HMAS Albatross

Keels laid: IJN Tone

Frames laid 1/25: DDR KleineSchnellBoot

http://www.edinburghmodelboatclub.org.uk
https://www.facebook.com/pages/Edinburg ... 2565540179

Just food for thought but is there any reason that most don't consider doing some sort of external keel much like the RC sailbots do for those competition boats? I know it isn't scale but it would go a long way to introducing stability to a model while underway and wouldn't be seeen once in the water. A 10lb lead bulb keel sitting 12" below the bottom of the hull would do remarkable things. Just my take mind you. I have never built a scale RC model of any sort other than planes and stability is a totally differnt bird there (pun intended).

John

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John Martin
Tega Cay, SC

Building:
1:48 USS Laffey (Sumner class DD-724)
1:24 HMAS Bundaberg ACPB 91 (Armidale Class Patrol Boat)

I have found that a bigger model will behave more realistic then a smaller model.
I think this has something to do with the weight of the model.
As a slightly larger model will need much more weight you can quickly reach a satisfying point.
To me it appears that my Well Enhancer behaves pretty much like the real ship would do in waves corresponding with larger scale of the real ship.
I put most ballast in the bow and stern, though not as far forward as possible, actually more in the engine rooms which are pretty much the heaviest places on the real ship, and some ballast midships.
Having the ballast at the ends of the ships (bow/ stern and PS and SB) will slow the movement down as it gives a large momentum.
Having the ballast in the centre does not stabilise the hull.
As the amount of ballast increased you should build the superstructure heavier in order to have the centre of gravity as close to the point where the real ship has it.
This should also improve the scale-like behaviour.
In the end it's pretty much a matter of testing.
I would suggest to make room for multiple times the required ballast and try different places.
Then see what suits you best.
This is not particularly hard when one just tests quite a lot and then one can learn a lot about the ships behaviour.

My next model will have ballast tanks in the bow and in the stern, just aft of the 1/3 points of the ship with 2 heavy batteries just aft of midship.
Water ballast might give a loaded cargo vessel a more realistic behaviour too as cargo vessels often take on ballast when sailing empty through rough seas.

Greetings Josse

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Making a complete new Well Enhancer, again scale 1:75.

See my thread at RC Groups , about building "ballast tanks":

http://www.rcgroups.com/forums/showthread.php?t=1769388

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Pat Matthews
Get your boats wet!
Blog: [redacted]
Shapeways Shop: [redacted]
MWS Gallery: http://www.modelshipgallery.com/gallery/users/Pat-Matthews/user-index.html

Armstrong 440

I think just about everyone's comments are on the right lines.

I have a little bit of experience under my belt having built some long slim shallow drafted working models.

Take a leaf out of the RC model aircraft builders hand books and "build in the lightness". Reduce the models weight "all over" by pruning every last ounce off the overall weight, paying particular attention to deck, deck structures funnels and masts. Reject all white metal fitting unless you absolutely have to.

This gives you the opportunity to then place the internal weight/ballast where it needs to be, low down on the hull floor (forward and aft if possible). Assuming your motive power is electric, go for 2 lead acid gel cell batteries laid on their side, one forward and one aft. This kills two birds with one stone. Instantly you have most, if not all the ballast, in the right place and lots of motive power. Check the dimensions of the batteries before you buy them and choose the biggest that you can fit in

If I can get a 1/48th scale HMS Velox (1904) to sail without bouncing up and down, I'm sure you can manage a larger vessel at 1/72nd scale. By they way you should not need to resort to either external keels or exaggerating hull depth unless you plan on building something very exotic or an aircraft carrier!

Good luck and keep us posted on your progress.

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PICKETBOAT

www.chyldshallmodelshipyard.com

I agree, when it comes to a realistic look on the water, size does matter! To insure that, I built my RC CB-1 Alaska 2 .10"=1'. Thus I ended up an 81" model. I also made the Hull flat bottom. The result was a model that plows through waves rather than bob like a cork. It id not bother by wind much either.
Also,to get real looking bow wave, mid-ship truogh, and wake, the model needs to be or heavy. IMO: I think the viscosity of water is too high for smaller models look to look real.

Any pics of your ship showing how it looks in the water? And how far below waterline did you put the bottom? I had considered a flat bottom for my 1:48 Laffey and wondered how it would work. It wouldn't need nearly the ballast of a full hull and would ride a lot better through the water. When not in the water I was going to build a display board to sit it in much like a diorama uses to replicate sitting in the water.
John

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John Martin
Tega Cay, SC

Building:
1:48 USS Laffey (Sumner class DD-724)
1:24 HMAS Bundaberg ACPB 91 (Armidale Class Patrol Boat)

Here is a movie of my big vessel, made several years when I was building the previous version:
https://www.youtube.com/watch?v=4NBjZzGX19A
And here f my earlier model, weighing about a quarter of the model in the movie above, though only being some 40cm shorter:
https://www.youtube.com/watch?v=FzmF95m8Tz4
Never sailed the smaller vessel in as heavy weather as in the first model, never had the guts with this model...

Greetings Josse

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Making a complete new Well Enhancer, again scale 1:75.

Part of enhancing realism is the choice and quality of waters where one chooses to sail. I despise swimming pools and anything with walls as the wave action just gets amplified and unrealistic, especially with multiple boats present. Try to sail on calm, windless days on ponds and lakes and a 1/350 battleship, properly ballasted, will look great with a nice wave pattern and "trough" in the water left by the hull.

I had the same thought regarding scale models being too buoyant, even rather cork like. Isn't this likely because while the scale appearance is perfect, the wight is not?

If I recall, the weight of a scaled object is the cube root. i.e. if you build a 1/4 scale model of something that weights 2000 pounds in real life, the scale model should weigh 31.25 pounds, the cubed root being 1/64 (4 x 4 x 4 = 64)

And while water ballast may add weight, it is not fixed, so any angle the boat can yaw, the water mass (if centered) will not restrain or dampen the motion as it would if it were a rigid mass. The water would be a gimballed action.

The New Jersey, BB-62, is 888 feet long and approximately 50,000 tons. At 1/200th scale, it would be 4.4 feet long and should weigh 12.5 pounds. Most similar r/c models I have seen weigh much less. The less they weigh, the more quickly they will react to wave action. Battleships should not jiggle, but they will if lightly constructed. This is the same reason that a heavy car feels so much more solid going down the road than a light one. Each has their place.

I'd be very interested in your comments and/or criticisms if I have missed the mark.

Most important reason for smaller models behaving less realistic is that though we scale down the ship the water is not scaled down. That means that the weight of water pushing against our boats is far heavier relative to our boats toen compared to the real ship.

Greetings Josse

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Making a complete new Well Enhancer, again scale 1:75.