Strut Keel Design and Engineering Grounding Forces

Thank you TANSL for the beautifully writen brief summary of procedure; Yes I do totally agree with you.

 

I'm glad to think that it could have been useful, at least confirming what you already knew.

 

An interesting video on the Volvo IPS drives. They are designed to shear off under impact without causing water intrusion into the boat. A brief comment about design parameters within the front of the pod to ensure
that the pod will break at a particular location and I suspect that the trailing edge area within the pod would have attracted some design considerations to allow more deformation (trailing area) at impact so as not to drive the pods into the bottom of the
hull which might/would cause highly localized loads/stresses to the hull.

While not exactly what you are trying to do but it is the aspect of incorporating pre-designed stress concentration/failure areas into the front of the pod/keel to ensure that the pod breaks at a specific location and into the rear of the pod/keel to allow a controlled deformation at the rear of the pod to reduce point/concentrated loads at the back of the pod is interesting .

As compared to creating a rigid pod/keel and relying on ONLY expecting loads to be co planar with the hull.

 

Hi TanSL I do not know if I should pride myself to posess the precisely same type of knowledge to be honest I am more doing dozens of FEA simulations & integrating the correct materrials properties into the loadcases
But I sincerely promise I will also do like you wrote with the ISO 12215-5
The point is when you mount the keel in a recess and you have the grounding event the formost tip of the keel attachement surface as well as the sternwards tip of the keel attachement are the areas that are most under charge
If the keel is set in a recess the recess is acting as a longitudinal flange this stiiening the hull in the longitudinal
But then if the tip of the keel base is being forced into the tip of the recess I think it wont be good . The edges of the recess need to be bevel about '( degrees at least in order to avoid that I think most modern fiberglass boat keels are designed to take very very light grounding if any - the ISO 12215-9 value for the grounding for grounding charge is very light;
Actually the keel stub in traditional wood or epoxy wood yachts was bonded to the outside of the hull ; no recess at all ; while in the case of aluminium yachts most the bonding attachments are welding wich wont let the attached structures move from where they are welded

 

HI Barry yes thank you for your feedback I think this is very interesting
Actually it is quite similar with what I am working on.
Yes I do confirm " incorporating pre-designed stress concentration/failure areas into the front of the pod/keel to ensure that the pod breaks at a specific location"
If only these folks from Autodesk Fusion would just let me do about 10 event simulations ........

 

Hi @an2reir , as I said before, I am not an expert in keel design / calculation and therefore my opinions, which are only opinions, only have the value that you want to give them.
I suppose that all that network of floors, recesses, bolts, the only thing they want is to make sure that those areas of concentration of tension of which you refer do not occur. The entire shoe rotates in solidarity and there are no areas with much more load than others.
I also don't know why ISO gives such a low value to that F3. Unfortunately, the "fathers" of the ISO have tried to ensure that all its formulas and procedures are not an example of transparency.
Sorry, I can't give any more, I think I've even stepped out of the comfort and safety zone in which every calculator should work.

 

Hi TaSL I do sincerely appreciate your feedback an I truly took note and I am giving thought to your methodology. Thats why this great forum is here for us to discuss all sort of boat related topics

 

Andrei
As I noted (over on page 3), if you have established the forces YOU are happy with then those are the values to use. Do not blindly follow a rule like ISO, if you feel the load provided is too low. So what if it is too low, just increase it to one you feel more comfortable with. It is YOUR design, not an ISO design, in that sense. Thus can you justify the Force value you will apply to the grounding?

If so.. then you just design the structure to accommodate those load scenarios... that's it, no magic.

That is all you can do.... otherwise you'll be chasing your own tail in a paranoid state for fear of thinking it wont work.
There comes a time where you have to be confident in your ability to design the feature at hand and you can justify the methodology and hence, the loads and structure to any independent/person/body if required.
If you can - job done. Don't over think this...

 

Hi Ad Hoc

Thanks forthe feedback

I am very confident my design is going to work I have no doubts whatsoever because I did test the structures with two different FEA software in doing so I did cross check them in dozens of loadcases applying various types of loads and as well I did calculate the grounding forces by four different groun,ding criteria the ISO 12215-9, Eric Sponberg criteria based on ABS ORY rules and as well the crieria that have been explained and shared here . As well I did check and compare my design against this catamaran makers keel atached here look how they did it simple and good

 

Let us not discover America now because, as I have heard out there, it is already discovered.
A designer must apply the loads she deems appropriate, but when the boat must meet some standards (which is quite frequent), the designer has no choice but to ensure that his design meets those standards. Those are the minimum values, from there upwards, everything is possible (although not desirable). It is always necessary to demonstrate that a boat complies with ..... You have to comply with something, and in the case of a boat built or repaired in Europe, that "something" is the iSO standards for small boats. So, I am afraid, advising one to forget ISO standards is not good advice. You need to take them into account (which is a great help) and fulfill them.
Even making a direct calculation, as sophisticated as desired, the designer is obliged to adopt certain minimum allowable stresses. Not those that he wants but those that the Regulations to comply with establish. From there down, everything is possible (although not desirable).

 

Hello TANSL thank you for your feedback. America has been dicovered from Alicante and in my opinion it is not a bat attitude to keep dicovering the Americas . But yes of course I do fully agree with you. (But the ISO 12215-9 formula for calculating the grounding forces I still think it is low)

 

Hello an2reir, of course, the Americas need always be discovered, but only those that are not already discovered.
It is not uncommon for ISO 12215-5 to set low value design pressures. For example, when I calculate the minimum thickness of a deck, I always have to adopt a deck load greater than the minimum prescribed by the standard. But, in my opinion, that's what the designer is for, to design.

 

Thank you all who were answering my topics ; yes the calculations as per the methodology givem by the ISO 12215-9 standards being integrated ; I think the paragraphs regarding the calculation of forces from the grounding and forces from the keel from Yacht Design Principles by Larsson Eliasson is a very good complementary methodology to integrate ;
Very interesting I do find especially the Larsson Eliasson method of calculating the impact force on the keel. Bor a boat speed of 8 knots that is 4.11 m:s they give a stopping time of 0.25 s and a stopping distance of 0.5m and they calculate retardation = Vs/Ts= 16.44m/s2. For a Displacement of 8120 kg they calculate Impact force to be Displ x Retardation= 133493 N
Following their procedure result impact force is about 1.5 times that resulted from the calculation per the ISO Standard 12215-9

 

Hi Andrei

Thanks for the update.

You need to consider are you:
1...designing the appendage to NOT fail? under any circumstances..
or
2..designing the appendage to fail/breakaway not create damage to the shaft/prop... but the appendage needs to be replaced.

The 2 load scenarios are very different.

 

Hi Ad Hoc thank you for your feddback Yes I did think of this aspect. On the second scenario I think the Larsson Eliasson method does apply on the part of the appendage system that is designed not to fail.