ISO 12215-5 Speed above 50 kn

Hi to everyone,

ISO 12215-5 states, that it is only applicable to boats that can reach velocities of up to 50 knots. I couldn't find any rules for scantling above this limit. What scantlings are to be used? My question regards scantling for CE-Certification of motor boats. Additionally, does anyone know what is required for CE, as far as scantling is concerned?

Best Regards,
Martin

 

Guess I found an answer. But it is a (probably intentionally left open) loophole. The max. velocity of 50 kn is defined only for max gross weight (m_LDC). So I guess, that the designer can specify payloads, at which higher speeds are allowed. Is that assumption right?

 

You can still use it above 50kts, all formulas will work.

 

There is a risk of bow dive above 50 kn and forward 1/3 of sides and deck should be assesed as some more concervative way like as sides without height correction at least or even as bottom! As well global loads are worth of to be calculated through but keeping in mind true accelerations (this is not inclued in ISO12215-5). ISO 12215-5 does not give right anwers concerning true accelerations. There are easily over 7 g's accelerations when driving fast over 50 kn (recorded in 20 hz).

 

ISO 12215-5, in Chapter 1, has 2 notes, from which I extract the contents:

• Note 1: ... this rule may not be suitable for racing boats.
• Note 2: ISO 1221 is based on the assumption that the scantlings are governed solely by local loads.

You can only deduce that : the formulas of this standard are NOT VALID for speed, fully loaded ship or lightweight, over 50 knots.This is my humble opinion.

 

Topic starter and me are not talking about racing boats.

Design loads in ISO12215-5 derive from design accelerations. If their formulas predict design accelerations (and they do!), they also give proper design pressures.

Also note that for 6m boat and 24m boat the speed of 50kts means completely different Froude numbers FnV. Say, if for 6m boat the ISO12215-5 method can be used at FnV=8 (50kts), why it can not be used for 24m boat above FnV=4.2 (also 50kts)?

We have tried few times ISO12215-5 above 50kts (say, 55-60 kts) and I see no problem in the method though I understand this is extrapolation and beyond legal limits of rule. This is just kind of legal disclaimer for speed rather then physical limitation or limitation of method itself. If it was physical limitation, they would have used FnV instead of absolute speed.

Regarding general strength I see no point at all. For small craft (LH<24m) never presents a problem excerpt some craft with unusually low hull depth. General strength is not really related to speed...

 

OK Alik, your explanations on the number of Froud seem very clear.
All I'm saying is that when you try to get a certification of your boat, based on the ISO (eg CE), you can not get it if your boat is out of the limits set by such a standard.
You can calculate as you want but if you use a Regulation, you must stick to the limits the same traces.
I do not know what you mean by the term "general strength." Respect for that term, actually there is nothing in ISO 1215-5. If we talk about "longitudinal strength", ISO 12215-6 says a few things about and, of course, any boat with open structure (undecked) should be examined in accordance with this standard.
Best Regards

 

Gooday 'F-M-S' - hay bloke - geat a multihull sail boat. They can do it. Enjoy your quest.

What's required is - money - wheel barrows full & all big notes.

Ciao, james

 

Thanks for your answers so far. I think Alik may be right reagrding extrapolatory applicability of the standard. However as far as CE certification is concerned I think TANSL is right. Certification bodies will probably not accept scantlings that are done with ISO 12215-5 if the boat can reach speeds of more than 50 kn.

I would like to know if anybody has experience with certification procedures for higher design speeds.

I could certaily do everything it takes to get a fast boat certified. I am gradute engineering student and I am proficient in strength calculation and also FEA. But I need to know where to start.
Since there are CE consultants who do certifications in retrospect also for fast boats (e.g. for boats which are imported without proper certification) there has to be a simple way. Especially in the light of the fact that they do it for aroud 1500 Euros. So it can't be that hard, can it?

Best regards,
Martin

 

Hi Martin,
As you probably know, there are companies approved to extend the CE certification. I am convinced that you can negotiate with any of them to allow you to escantiling your boat by the Rules of any Classification Society. Talking to the technicians you can always negotiate a compromise.
B.R.
Ignacio López

 

High Speed Craft rules of classification societies have speed limits as well. Everything above the limit is 'special consideration'. The same 'special consideration' can be made with ISO12215-5 then...

I would be careful to use rules of classification societies for pleasure small craft; most of them would produce heavy and costly structures with features such as watertight bulkheads, thick outside skin, shear ties, tested laminates, etc. Just to mention, the design stresses are usually 0.3 of ultimate while at ISO12215-5 they are 0.5.

 

Alik, I think (maybe I'm wrong) you now defend the opposite of what you said in post # 3, and align yourself, in part, by my criteria
Martin, the better for you to forget this unproductive discussion and talk to one of the companies certified that surely will give you the solution. Whatever you have to do, should be with the approval of one of them.
Classification Societies indeed lead to higher ISO 12215.5 scantlings but maybe they are the solution.

NOTE : ".... the design stresses are usually 0.3 of ultimate while at ISO12215-5 they are 0.5."
The design stresses depend on the characteristics of the boat, navigation area, the design category, and various other things while the Utimate strength is a specific property of each material. Therefore, establishing a relationship between the two is not possible

 

Completely wrong.

Maximum permissible design stresses depend on safety factors applied by different rules. For ISO12215-5 they are about 0.5 and in rest of rules about 0.3 of ultimate stresses, for composites. Look at ISO12215-5:2008 Table 10 — Design stresses for FRP sandwich plating; this is the exact term used in the standard.

What You are talking about is called design loads or design pressures and yes, they depend on category, hull area, boat particulars, etc. But design stresses generally do not.

 

The design stresses are those that are and there are no maximum or minimum permissible values.
The maximum admisible stresses (characteristic of the material used and that has nothing to do with the ship) it does depend on many things and also, as you say, on the safety factor that each regulation want to adopt. The same standard, depending on whether you follow their formulas or you decide to do a direct calculation, may accept a higher or lower allowable stress.
And this is so, and I do not need to read any paragraph of ISO 12215-5.
It should, especially when we give advice, be very strict with what we say and not to mix the concepts.
I will not continue this discussion, which is absurd.
With all my affection
Ignacio López

 

Next time when You want to lecture someone, look at the rules Yourself first. And it is also recommended to look at opponent's profile to find out if You are in position to lecture him or not

See how it is called in standard and what it depends of: