and her loading condition ? things like what's the distribution of her weight & buoyancy during that trial ? and what is the "fair" margin ?
Is this based on calculations ?
Indian Military News, Reports, Data, etc.
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and her loading condition ? things like what's the distribution of her weight & buoyancy during that trial ? and what is the "fair" margin ?
Is this based on calculations ?
by78
General
It's clear that you don't know what you're talking about. It's as simple as that.
Do you know what you're talking about in other hand ?
Like, i have my share in calculating speed-power requirement. This is a simple spreadsheet i developed some years ago. It's simple but it does give some reasonable result. I will never judge what i see without asking and researching more about it. Just like Radar Cross Section estimates, the devil is in detail and these details have NUMBERS which should be calculated.
Back to spreadsheet tho..
Above depicts my estimates on speed-power requirement on Vikrant. Let's use your 90% for the waterline length and 40m of waterline breadth. as you can see 38 knots requires about 91602.3 SHP While Vikrant is afaik have 110000 SHP of propulsive power. I would say such speed are perfectly reasonable although not necessarily efficient and maybe uncomfortable for crews but clearly there is a margin for Vikrant there which apparently being tested.
40 Knots however are unfeasible as it needs 137000 SHP. but 35-38 Knot and particularly with lightly loaded trial condition should be feasible. Does Vikrant will operate at that speed at all times ? No as the hull does have optimization on sustaining certain speed (e.g 25-30 Knot) and for fuel and engine reliability reason plus potential growth in the future.
Now in respect on "alarming" and sensationalism you show so far.. in the previous page we have news that the carrier can safely launch planes and landing. meaning that the phenomenon you keep clinging into only happens in certain moment. I already stated my guesses. Still i will not simply raise alarm or "Indian engineering failures" etc WITHOUT knowing properly what are the story behind it.
by78
General
Strange, why have you assigned 1 (as in yes) to the bulbous bow when Vikrant clearly doesn't have a bulbous bow? Do you have Vikrant's blueprint? Have you fabricated a scale model of the hull and used it to conduct a towing tank test? If not, how then did you arrive at your estimates for the appendage drag fraction, its wind resistance, total hull resistance, propeller losses, so on and on?
Surely, for someone who knows what he's talking about, he or she would know whether Vikrant has a bulbous bow or not, am I right?
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Well i call that honest mistake as i developed that sheet originally not for Vikrant but for my own Frigate design. It can be corrected
and now you have a correction. Should i also include the propulsive efficiency ?
And no, no need for Blueprint or various fancy things you mention but i am sure you never do nor willing to do. I am using a generalized values one can find in research paper and shipbuilding textbooks, so does the appendages resistance which usually adding 20% extra.
and i guess you are now running out of point eh ?
a proper discussion requires 2 way sharing which you dont seem to do and instead relying on sensationalism. Numbers should be responded with numbers too. That will foster a productive discussion.
Now tell me, do you do your homework and can actually provide something better than my excel spreadsheet ? It would be a shame if you cant and just rely on technoblabber on how i should do tow tank testing.
What's the history behind HAL and ADA?
I've seen some Indians who think HAL is good and it's actually the Indian government refusing to award orders that's causing problems
by78
General
So let me get this right. You think it's acceptable to derive estimates for Vikrant's total hull resistance and effective horsepower from a completely unrelated design, but my serious question arising from very clear footage of severe heaving of Vikrant's hull is sensationalism. Am I getting this right?
And missing such a basic fact as Vikrant's lack of bulbous bow is no trifle matter, since a bulbous bow or lack thereof can have a large impact on a hull's performance, a fact your own spreadsheet confirms.
By the way, about your spreadsheet, didn't you say it has reasonable predicative power? Please elaborate. What is its predicative track record compared to published values on specific ships? How and why does it obviate the need for such 'fancy things' as Vikrant's blueprint and conducting tests using scaled models to arrive at its total resistance, EHP, and propulsive efficiency? How have you tried to avoid the problem of 'garbage in, garbage out'?
I think these are valid questions.
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The problem with your question is you lack the intent to elaborate. You dont seem to do anything at all to understand how that happens, what is the condition of the test, is that always happens, when that happens what is the ship's condition. Yet you call it "serious" and such like sensationalist. Just like some people claiming Chinese carrier have runaway damage based on some unclear grainy Satellite picture.
The real serious thing would be if the ship capsized on port or launch... Then you can call it serious because clearly someone has messed the metacentric calculations during design.
While that heaving.. is that always happen ? When that happens ? What's the ship's condition when it happens ?
My method ? Well it's acceptable for a quick back in envelope. and the only mistake i did so far is not changing the Bulbous bow switch. There will always be error but at this level of discussion i dont think one should be try to be super accurate in the first place. methods are more important.
What ships ? and what published value ? and most importantly what kind of error do you consider acceptable...
Because you wont likely to provide anything comparable. Should i really waste my time on something like Holtrop-mennen or something more complicated like ANSYS when you dont seem to be able to provide any comparable measure ?
Show me why should i go into more complex detail when you dont seem to willing to go to the same length ?
Well i believe reference books are enough. and of course i am open to correction.. a proper correction. In shape of comparable spreadsheet. Or if you actually do Tow tank testing etc. If you dont, then i wonder if you really intend to have a proper 2 way discussion.
Estimating the propeller diameter as 0.395*T + 1.3 yields 4.6m. From pictures it might be closer to 3.4m
The wake-thrust fraction calculations yield nH of 120.9% or 122.4%.
Let's presume nG is 99%, nM is 99.5% and nR is 101%
We can generously calculate nO based on cTh as 79.3%.
Vikrant has 88 MW of power so hitting 38 kn with that power would require resistance to be only 1135 - 1149 kN. That's clearly not possible.
The expected design speed for 88 MW based on these efficiencies is about 28.5 kn.
Pe = Sw / sqrt(Disp. * Lwl) * Disp. ^(2/3) * V^3
In fact, using Holtrop-Mennen and presuming a normal stern, Rt should be on the order of 8330 kN, 8500 kN for a U-shaped stern. Going 38 kn with that resistance would require an absurd amount of power closer to 380 MW. If we drop the speed until the power predicted by Holtrop-Mennen is roughly 88 MW, we get about 28.87 kn.
Coincidentally, this is pretty close to the design speed reported by various media (28 kn) when Vikrant was commissioned.
CNN -
Defence News Updates -
IndianExpress -
Hindustan Times -
Two different estimation methods for hull efficiency / resistance as well as public news yield similar answers which are totally divorced from your answer of 38 kn. Your resistance calculations follow the power-speed cubic law almost too exactly and are way too low. I'm pretty sure your resistance calculations have significant issues.
The thing that tipped me off was seeing your calculations say a 40 kt ship can go 50 kn with only 188 MW of power. That's crazy.
Even the Essex carriers needed 110 MW of power at 27-30 kt to reach a design speed of 33 kn. Yet your calculations suggest a much heavier ship is 55% faster with only 71% more power. That itself breaks the cubic law and unlike your calculations, we actually know the speed of historical ships.
vincent: restored this post because it contained thoughtful calculations.
Please no more discussion on this until new info become available
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