# operational envelope

Discussion in 'Class Societies' started by navalarchitect, Mar 12, 2013.

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### navalarchitectJunior Member

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You need to use formula in Pt.3 Ch.1 Sec.2 B204.

You alter the speed and Hs to maintain the max design vertical acceleration. From this you'll then get a speed v wave height result. You can calculate this using the formula, but whatever values you get, whether the same or not, it is issued by DNV and relates to the principal design values you supply to them; which is how the design vertical acceleration is calculated. A typical one looks like this, from a previous boat:

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### navalarchitectJunior Member

If I use the formula in Pt.3 Ch.1 Sec.2 B204, what should be the value of Vs and Hs to get the max design vertical acceleration? Is the Vs the service or design speed? And Hs is 0.50m if the vessel will operate in Sea State 2?

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It is what ever you want it to be!

If you want to run at say, 45 knots in say Hs=3.0m seas, the vertical acceleration that you calculate using the formula in B204, will yield a totally different result, if the is Hs= 1.0m rather than 3.0m.

Once you calculate the Acg in B204, you use that value to calculate your slam pressures. That slam pressure will yield the section modulus required based upon your stiffener spacing and frame spacing.

So using a higher Acg will mean, yup...a higher modulus, which means, yup..increase in weight..which means, yup..a slower boat.

So, the Operational envelope is base upon what YOU the designer has said the boat will do. If you say the boat can run at 45knots in Hs=3.0m....then that is what YOU are designing the structure to satisfy, which yields a calculated Acg, and then that is reflected in the speed v wave height table.

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### navalarchitectJunior Member

Thanks. It is clear now.

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### jcamilleriJunior Member

on the subject,

the formula given by the DNV is it for the 1/10th accelerations? I am using Savitsky and Brown's formula and the DNV to cross-check the results. I can translate the Savitsky formula from average acceleration to 1/10 using his proposed equation.

one more question, is the equation given by Savitsky and Koelbel
a_rms = a_(1/10)/6
accurate?
i want to measure the vertical accelerations of a boat and make sure that it is within acceptable criteria. I intend to use the Nordsfolk criteria

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### navalarchitectJunior Member

using dnv formula I got this:

What do you get in Savitsky using the same value I used?

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Well....firstly you need to review that criteria and see what the limits are and if it is applicable to your design..if not..why not.

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### jcamilleriJunior Member

The average acceleration at CG is 1.68g (16.48m/s^2) and the average 1/10th is 5.55g (54.45m/s) which is quite different from your results. However the speed to length ratio 50knots/Root(41 ft) is 7.81 which is outside the range of applicability of savitsky's formula. That is one of my main reasons why i want to use also the DNV formula since it just specifies that the speed-to-length ratio has to be greater than 3.

Adhoc can you elaborate a bit more please. I briefly looked at the criteria. Bascially what i understood is that criteria can be based on safety, comfort and workability, structural loading and response, and machinery and propulsion loading response.
With regards to structural loading and response i intend to look at the problem the other way round, that is, instead of limiting the speed and sea state i fix those two paramters and design according to the resulting vertical acceleration. I want to look mainly on the safety, comfort and workability criteria. I know about the ISO 2361-1 standard but can't get the hang of it.

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### jcamilleriJunior Member

To help you understand better my problem i attached an excel sheet. I calculated the vertical accelerations for my entire speed range using the Savitsky and Brown and the DNV formula. I converted the 1/10 highest impact acceleration into rms values. Then based on the safety, comfort and workability criteria i check whether the craft can operate at all speeds. The problems i have is that I don't know if the acceleration values given by the DNV formula are the 1/10th so that i can convert them to rms values.

#### Attached Files:

• ###### vertical accelerations.xlsx
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Well, I think in some ways you’re looking at this too deeply, or slightly the wrong way around.

If you have a design…lets say hull “A”, what is the purpose of that hull? The purpose…is really what we call the SOR..or statement of requirements. What it simply means is, just that..what is the boat for, where does it go..what speeds etc etc.

So hull “A”, if this hull is to run at 50knots…in what sea conditions? Since the accelerations at 50knots in 0.10m sea will be different to that of 1.0m sea and again of 3.0m sea, all at 50knots.

This brings about the main issue, fit for purpose. Would you want to go at 50knots in Hs=3.0m seas? Well, if hull “A” is just 5.0m in length…then no, no way Jose!!...but if the length of hull is now 100m…well, sure…reason?..the different responses of the hulls at 50knots in Hs=3.0m. Which again comes down to the SOR and the fit-for-purpose.

So, what Hs for the length of hull A, would you feel happy to run at? Lets say, Hs=1.0m for the sake of argument.

So, once this ahs been “fixed”, this determines everything about the boat. Since your structure must be “fit for purpose”. Thus why use an arbitrary accelerating value, from a Savitskys or others papers?...all these papers are doing is trying to arrive at a suitable or some guidance to what accelerations a boat could experience; given that the hull is within the limits of their study/findings.

BUT, if you want a structure that is fit for purpose, why use an arbitrary vertical acceleration value, as how will you arrive at the slamming pressures and modulus etc?..again needs more research and validation too.

Thus, once you have fixed your SOR, in this case of 50knots in Hs=1.0m..these are the parameters to plug into DNV rules. Whether the results are the same, better or worse than Savitsky’s et al, so what?....you want/need a fit for purpose structure based upon the limits you have set your hull/design.

DNV rules are based upon many different formulae, coupled with their own in-house database and in service feedback. Since they must stand by their rules, they must therefore be “fit for purpose”.

Does this help?

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### jcamilleriJunior Member

I understand what you are saying. But what about the acceleration levels experienced by the craft for the persons on board? Lets say that the hull is 6m and designed (structurally) to operate in sea conditions were the significant wave height is 0.5m. Ok. But what is good for the hull structure does not mean that is good for the crew no?

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Correct. Since you want the crew to "fail" first, not your hull structure!!

The structure is in variably much more robust than the crew, or should be. Since extreme/peak slamming loads and the rogue wave etc will be accounted for in a 'fit for purpose' designed structure.

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### jcamilleriJunior Member

Thanks very much Adhoc for clearing that out!!

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### navalarchitectJunior Member

I can't see in the rules where to use Acg for calculating frp structures: http://www.boatdesign.net/forums/classification/frp-structural-calcs-46734.html

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