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[TurboTyne]

 I carried out the calculations of hull volume/displacement as suggested by Plastic, although I used a different calculation method. I made a scanned image of the M-class body-plan (i.e. cross-sections) taken from the Admiralty lines drawing. (The NMM does not have lines for the R-class but Admiralty M and R-class ships had basically the same hulls). This image was imported into my CAD software. After checking that the dimensions of the image were accurate I traced over the lines to create line drawings that the CAD programme could analyse. So I ended up with copies of 22 half-sections with each showing a standard water line.


The CAD package is just 2-dimensional  (Solid Edge Free 2D) so calculation of volume used an approximation. It was simple to get the programme to calculate the area of each cross-section up to the water line. Next, I calculated the hull volume between each of the 22 sections by multiplying the distance between each pair by the average of the areas of the two sections. Extra estimates were made of the volumes at each end of the hull. Adding up all these volumes and multiplying by the density of water gave a displacement of between 22lbs  for freshwater at 20 degC and 22.6 lbs for sea-water at 4 degC .


The admiralty M-class drawings say the displacement is “about 960 tons”. The trial displacements varied from 890 tons (one of the M-class ships) to 1186 tons (the heaviest R-class ship). I do not understand if these values were calculated from weights of everything on board or from measurement of the depth of hulls in the water. Also, I wonder if the differences between ships of the same class were due to differenct loadings or to differences in basic structure.


Using the rule that displacement of a model can be estimated by dividing by the scale factor cubed, a 944 tons displacement at 1:48 scale is  19 lbs and a 1186 ton displacement is 24 lbs. So I was pleased that the value of 22 lb from calculations on the drawings seems to be a reasonably sensible result, falling in just the right range.


Apparently M-class ships were designated as 13 tons per inch, meaning that every 13 tons additional weight caused an alteration in draught of 1 inch. So another way of thinking about this is that between the 890 and 1186 ton trial displacements, the draught would have changed by about 23 ins. At 1:48 scale this is 0.48 ins. However, it seems things were not as simple as this because different yards built hulls with slightly different finished sizes and this would have affected the draught for a given weight. 


For me, this was an interesting exercise that has taught me some useful things.

Thanks for all the inputs.
Regards    Mike
 

[dreadnought72]

I think you're virtually on the money there!

Don't forget that "averaging the areas and multiplying by the distance between those areas" is not (quite) the same as finding the volume of a hull "chunk" - with the hull curving inwards from "wider" to "narrower" areas, the volume enclosed will be somewhat higher than the simple calculation.

...But near enough your 22lbs!

And where (roughly) is the centre of buoyancy along the long axis? (Enquiring minds want to know!)

Andy

[TurboTyne]

Hi Andy
Thanks for prompting me to do yet more calculations  .

This is another aspect I had not thought about. Well, I estimate the centre of buoyancy is about 34.5 ins from the bow (at water line) in a hull with a length of 66 ins to stern post and 67.75 ins overall.    i.e. it is  about mid-ship. 

But what does this mean in practice and is it typical of most ships? e.g. are battleships the same?

I guess I need to arrange for the centre of gravity of the final model to be at the same point?

On the actual ship, this point was just aft of the last (third) boiler and between boilers and the turbines which, I guess, could well coincide with the c of g.

Regards  Mike

[roycv]

Hi does not the meta-centric height come in here? 
Very low for battle ships so they have a slow roll for firing the guns and I have read of just 2 feet for a full size battle ship.  More for a destroyer to give stability but a some what jerky ride.
regards Roy

[dreadnought72]

I'm sure metacentric height will be next on the list! 

Thing is, my Dreadnought's relatively insane beam and little topworks will allow me to model a realistic MH: I'm going to be aiming for a scale 2cm and I'll still get form stability to 66 degrees of list. Meanwhile that skinny destroyer with plenty "up top" (given we tend to build superstructures heavy in order that we can handle them) is always going to be that bit more "delicate". 

Andy

[roycv]

Hi I have an HMS Dreadnought built from Model Boat plans.  I am sure that it is inaccurate but she sails well and is quite manoevreable (not sure I have the spelling right!  I checked against a spell checker and nonredeemable was the nearest!)  with 2 props and rudders.
It is just 34 inches loa (860 mms).  Very stable though!

I used to do the calculations using a computer in the late 1960's more in the form of printed look up tables but most useful.  I was more towards prismatic co-efficients useful for yachts etc.

regards Roy

[TurboTyne]

Prompted by the posts by Roy and Andy concerning metacentric height, I realised that I can quite easily use the CAD software to calculate the centre of buoyancy in the vertical direction - is that what you were thinking of when you first raised the topic Andy?  I've attached a picture showing the result of this calculation for section 10 which is approx mid-ship, although I guess I would need to repeat this for all the sections and average them together in some way. It is about 1 inch below the water line, but it seems to me that this would only be really useful if I knew the centre of gravity - which I have no way of predicting.

I do not know the metacentric height of the M or R-class ships, but apparently the stability range of the M-class destroyers was 86 degrees deep loading and 76 degrees light. The period of roll from one extreme to the opposite was just 2-3 seconds!  (I guess that is one aspect of why destroyer crew were paid hard lying).

Apparently these were very stable ships because, at that time, the machinery was heavy and armament light. As for superstructure, they basically didn't have much at all. The bridge was the main structure - see photo which shows an R-class bridge. This was larger than the M-class bridge, but still very light - it needed a splinter matress to give some degree of protection. Apparently this all changed by WW2 when machinery was lighter, armaments heavier and superstructure greatly increased, so the range of stability had become smaller (70 - 61 degrees) and roll times were 8-9 seconds.

Regards  Mike

[dreadnought72]

I'd be requesting an immediate transfer to a land job if I was on a destroyer rolling to near-horizontal.




Andy

[warspite]

Think that this 86° was all inclusive left to right - not all one sided i.e. to port - you would have to now call that the Concordia effect at 86° - what - too soon

[Plastic - RIP]

My feeling about this thread are that the OP is over-thinking the problem.

Total finished weight - approximately known
finished dimensions - known

weight of equipment - not known
weight/materials of hull - unknown
weight/materials of superstructure - unknown
weight/material of fittings - unknown

Surely trying to calculate centres of buoyancy, centre of gravity, roll centres and anything else is pointless?

I would have thought that this project is at the stage of 'just build it and see'.

If it's built big enough to have loads of buoyancy, then lead ballast very low in the hull will fix any of the problems of rolling, waterline, stability etc.

If it is built small scale, then the unknowns can become an insurmountable problem!

.

[TurboTyne]

Hi Plastic

I agree entirely with your general point and accept that it may not be very useful with regard to getting a model built. 

But I'd like to explain that I always assumed that Andy had his tongue in cheek when he prompted me about calculating centre of buoyancy. Also, I may be a bit sad  but I have enjoyed going through the motions of analysing  stability etc. since, for me,  it has been a very effective learning exercise about ship stability in general, about features of the R-class destroyers in particular and about what I can achieve in this regard with SolidEdge 2D.    Also, it gives a useful feel for the weight and positioning of the machinery etc that I plan to design and make. One other thing, since I enjoy doing this sort of stuff, I will find it interesting to use the CAD software, to estimate the weights of hull and superstructure (from surface area, thickness and density of materials to be used) and I certainly plan to weigh the components of the steam machinery as I build it - to keep it within the total displacement, if nothing else.

Regards  Mike

[TurboTyne]

Think that this 86° was all inclusive left to right - not all one sided i.e. to port - you would have to now call that the Concordia effect at 86° - what - too soon

Yes, Warspite, you are correct. I double checked in the book "British Destroyers" by E.J. March and it definitely says this value is the stabilty range. Also, of course, this does not mean the ships often (ever) rolled that far, but this was the range, that if exceeded would result in a capsize.  So, as I understand things, a range of 86 degrees means the ship could tilt so the decks (if flat rather than curved) would tip to 43 degrees from horizontal and still return to an even keel.

Regards  MIke

[dreadnought72]

I always assumed that Andy had his tongue in cheek when he prompted me about calculating centre of buoyancy...

Not at all.

The thing is, I've always heard that destroyer hulls are "difficult": long, narrow, not much righting moment. So I think it is a worthwhile endeavour to do at least some calculations before you cut any wood. After all, what's the point of a beautiful, scale, steam-powered model destroyer if it's unstable once built, and best sat on a shelf?

Better save yourself the effort of building (or realise some tweaking's required) before you even start, I think.

Andy

[Plastic - RIP]

I still think you're worrying too much - after all, you're not going to be sailing the North Atlantic in a hurricane - it's probably only going to sail on a mill-pond smooth lake. 

[tonyH]

Isn't it all a bit of a compromise anyway, whatever the intentions?

If you're dealing with steam as the main motive power, in model form each of the main items, boiler, gas tank, engine etc. is a 'big lump' rather than a series of smaller items which could be distributed to a greater extent in the full size vessel. Rarely did the real ship have a single boiler, for example. Capital ships could have 20.

Isn't it more a case of the usual rules of keeping the weight down as low as poss and the stuff in the right places that it floats level, especially if you don't have much spare displacement to adjust with ballast?

I reckon you'll have more fun sorting out the speed and the steering

Great project and good luck,

Tony

[TurboTyne]

Isn't it all a bit of a compromise anyway, whatever the intentions?

If you're dealing with steam as the main motive power, in model form each of the main items, boiler, gas tank, engine etc. is a 'big lump' rather than a series of smaller items which could be distributed to a greater extent in the full size vessel. Rarely did the real ship have a single boiler, for example. Capital ships could have 20.

Isn't it more a case of the usual rules of keeping the weight down as low as poss and the stuff in the right places that it floats level, especially if you don't have much spare displacement to adjust with ballast?

I can't disagree with any of that Tony, but doing the calculations has certainly made me think about these aspects more seriously. With regard to boilers, the R-class destroyers had 3 one behind another. I plan to use one monotube boiler - long and low which will pretty much resemble the overall shape of the three separate units and, like them, will be just forward of midship. Also, regarding fuel, the actual ships had fuel oil in war tanks in the very bottom of the hull beneath the holds. (Also peace tanks higher up on each side). I have always planned to use an electrically-pumped parafin burner (a lighter version of the one I described in another thread:  http://www.modelboatmayhem.co.uk/forum/index.php/topic,47453.0.html). Thinking about buoyancy etc makes me realise I should position my fuel tanks in the same places as the war tanks in the prototype - this hopefully being one of the several advantages of using paraffin.  The engine will (in theory) be in a similar location as in the prototype - just aft of midship.

I cannot contribute usefully to the debate about how useful buoyancy analysis might be since I have absolutely no experience of building model boats. Also it is clear that one can locate the items I mentioned above without undertaking detailed calculations of centre of gravity etc. But, I find this all interesting stuff and I hope to attempt more of the analyses just out of curiosity - especially since I am further encouraged, knowing that Andy's tongue was not in his cheek after all.   

Regards  Mike
 

[dodes]

You may be interested to know that all the real ships were subjected to a incline stability test when new and after refits when alterations etc are made, so even with real ships there is a belt and braces attitude. By the way the principle of the longitudinal stability and trim is BMl = I to power of 2 over 12d, where Ml is is the longitudinal metacentre  of a ship, I = length of ship  and d is her draft.

[TurboTyne]

You may be interested to know that all the real ships were subjected to a incline stability test when new and after refits when alterations etc are made, so even with real ships there is a belt and braces attitude. By the way the principle of the longitudinal stability and trim is BMl = I to power of 2 over 12d, where Ml is is the longitudinal metacentre  of a ship, I = length of ship  and d is her draft.

 Thanks Dodes, I did not know any of that – or about merchant ship displacements that you mentioned previously. 

It is certainly interesting and, prompted by your post I  Iooked up about  incline testing.  I was picturing a ship being tipped until it almost capsized, but gather instead that, according to current International Maritime Organsation rules, it involves moving calibrated weights across the ship to create inclinations of just 2 to 4 degrees which are accurately measured to enable calculation of the location of its centre of gravity.  Is that right and, if so, do you know if the admiralty requires (or at least did require in 1916) similar tests?
Hmmm, I shall have to make a more determined effort though, if I am going to understand about BM1.
Regards  Mike

 
 

[dodes]

I know that all the major vessels between the wars where inclined and most probably the ww1 vessels, ref centre of buoyancy that is where the force of buoyancy acts upwards and depends on the underwater form of the vessel and is the same for transverse and fore and aft metacentric height calculations etc.

[ballastanksian]

Hi Mike.

I am in the process of building a 1:96th scale model of HMS Ready (qv) and i chose this one beacause her albeit a bit dark and small photo in janes 1919 shows her to have a ram bow. Have you come across any suggestions in your readings that any of the class were initially built with ram bows? Ready was a Thorneycroft boat and assume she was originally built as such before having the bow rebuilt as a clipper bow (Am I using the correct term?)

Regards your concerns over only creating white smoke, could you spray some parafin into the boiler at the top to carbonise and create some black smoke like tanks and real life ships did to create screens??

Good luck with your build.

[TurboTyne]

Hi Ballastanksian
Thanks for the suggestion about making smoke - that is definitely something I shall keep at back of my mind for when I eventually have a model ship nearing completion.

I've had a look though the books I have and cannot find any mention of a ram bow on HMS Ready or any other M-class boat. I have made copies of parts of the book "British Destroyers" by E. March. He summarises the Emergency war programme M-class boats as follows: "In all, 79 destroyers were built to Admiralty design, 7 to Yarrow and 4 to Thorneycroft designs.  In certain boats the length was increased by 3 to 4 inches, and the beam by up to 2 inches, some of the later boats had an L.O.A. of 275 ft 10 in. Many did not have geared cruising turbines ......." I would have thought that if any ship was designed with a ram bow it would have been mentioned here, but I suppose that lack of any mention does not rule it out.  In the book on British destroyers by Manning there is again no mention of ram bows but there is a good largish photo of Ready with a clear view of the bow. Also, March's book has a good photo of HMS Patrician - another Thoneycroft special built in the same order as Ready. Please let me know if you'd like a copy of  either/both of these and I'll scan them and send it (direct to you rather than posting it on this thread because of copyright).

Sorry not to have any positive info on this, regards

Mike

[ballastanksian]

Thanks Mike. I would like to see these so I can see if they match the postage stamp sized image I have! Sods law says that there was a dreadnaught or armoured cruiser parked just behind her when the image was taken with the ram bow protruding at just the right point to make it look like it belonged to the Ready!

[Bob K]

In terms of actual model stability for a ship of this type, I have a Deans 1/96 HMS Amazon (1906), 900mm long.
As you can see from this movie clip despite being only 89mm beam she is a good sea boat with little roll even on tight turns.

https://www.youtube.com/watch?v=7Cl_HNu06WY

[TurboTyne]

Thanks Mike. I would like to see these so I can see if they match the postage stamp sized image I have! Sods law says that there was a dreadnaught or armoured cruiser parked just behind her when the image was taken with the ram bow protruding at just the right point to make it look like it belonged to the Ready!

I've just sent you a PM

Mike

[TurboTyne]

In terms of actual model stability for a ship of this type, I have a Deans 1/96 HMS Amazon (1906), 900mm long.
As you can see from this movie clip despite being only 89mm beam she is a good sea boat with little roll even on tight turns.

https://www.youtube.com/watch?v=7Cl_HNu06WY

Hi Bob K

Thanks for that link. I very much like your boat and it is interesting to see a model of these destroyers in motion. It is certainly reassuring to see that is so stable - I was fearing these pencil shaped and shallow draft boats would be a bit delicate. I'd be very interested to know what motors you used in it. Also, did you need much ballast? If so, where abouts is it placed - I read in other posts that placing the ballast up the sides of a hull has a different effect to placing it in the bottom, so I'm wondering how this might affect these destroyers.

Regards  Mike