Strut Keel Design and Engineering Grounding Forces

Sound plan, good stuff.

Quite possibly.
BUT... if you wish to account for such a load case, the strength of the keel/skeg will be so large, it will be draggy and also affect flow into the prop.
Whilst this is a "nice" to do... there is only so much you can account for.

I had a similar problem designing P-brackets on our patrol boat. I just took a load case if one blade is damaged and then the prop spinning unbalanced and eccentrically, the forces it creates became the load case.
I designed the palm to suit - and touch wood, hasn't broken yet after many years of service.

Try not to over think this...

 

Hi Ad Hoc thank you for your very friendly and useful feedback. I will absolutely follow your advice. I am thinking of the situation a friend of mine senior engineer Bob Bradfield in the Royal National Lifeboat Institute marine Design and Engineering Department told me one day about a mission one of the RNLI boats had to go try save some kinds that were stranded in a cave somewhere on Dorset coast. The lifeboat could not go too near that cave. I think I saw a video of a canadian Coast Guard boat attempting to do the same namely approach people stranded on rocks near the shore on stormy weather and powerful swells. That situation may be considered somehow close to this video of coast guard assisting

The idea is to keep the manoeuvering capability even when hitting rock. I myself have ran aground twice on sailing yacht but was able to tow myself off the rock. I think the ideal will be if the boat keel can take the chrages of hitting rock without distorting in such way as to block propeller, as well protect propeller - but keep power to motor off ; and in the case of a massive load situation break clean so does not block propeller. Very likely the solution seems to be that of bolts that are designed to be the weak link

 

Hello there and thank you for your very useful feedback. I did advance in designing a motor boat keel that will take grounding and will protect the propeller and rudder in the event of a light grounding on a beach of sand or pebbles and will at the same time not generate an amount of stresses in the hull that would damage the hull. I am now working to pre-set more precisely the amount of grounding force the keel is to take and to triple check against the ISO standard 12215-9.
I do have a french version of the ISO Standard 12215-9 that gives the equation for the grounding horizontal force and flexion moment on the keel like this




Yoyur feedback will be very useful thank you

 

Andrie

Many thanks for the feedback.
How did the value compare to the values I suggested, as a rough guide?

 

Hello Ad Hoc I wish you a happy deconfinement ! Thank you for your friendly and useful feedback. I think your method is good and valid . However I think it is a must to check as well against the ISO 12215-9. In my research I came upon these sacrificial keels of catamarans they designed like this they are bolted via horizontal bolts to a very solid fiberglass keel stub. I am not sure : but I think the keel stub seems to be attached by adhesion/ glued inside a cutout that has been made in the hull bottom , and also bolted on the hull .

 

Ditto... i hope you're enjoy the decompression?

Only if you feel you need a measure of compliance to demsotrate to a client....or.. as a double check against the rough method suggested.
Remember, rules, any rules, are just a measure of compliance, and, must always be treated as the minimum standard of compliance too, not the absolute, as many appear to do.

Ultimately, you have to satisfy yourself, it'll be ok!

 

Hi Adhoc


The ISO standard 12215-9 gives this formula for grounding force that makes about 1,2 x Mass of the Boat if the motorboat keel is the designed lightest. We added a safety factor of x2 This amount of force is about the same as than the one calculated by the ORY ABS with a factor of 2.37

 

Good to see you got some validation.

 

In my opinion though a good way to design appendages these days is simulate the collision with the EVENT simulation capability of Autodesk Fusion

 

If you are designing for how much energy a structure can absorb, the correct value to look for is toughness. The rate of the deformation is also critical. Think about metal stamping, or explosive forming.

 

Thank you Gonzo. Yes good point rate of deformation. The catamaran makers did the sacrificial keels like this bolted to this massive keel stub made in solid FRP . I wonder how is this keel attached inside their hulls

 

The image shows a recess, thus I suspect there is a seating of some sort internally above the recess and the base of the appendage is bolted directly into that seating structure.

 

Hi Ad Hoc yes I think they did the attachement like you wrote before ,
I however did not clearly decide is it a recess they did in the hull bottom in order to fit in the keel laminate so that the laminate is flush with the hull ? Or may that be an allway through the hull bottom laminate cutout they did in the hull bottom . Because is appears the keel stub has been fitted from the inside of the hull and glued with adhesive and also bolted with bolts thst can be seen in the image filled with putty ?
The reason why I am pondering on this is I personally wold not dare to practice a cutout in the hull bottom laminate but would rather do so that I heep the hull bottom laminate integral. It apreas though the engineerrs at Leopard catamarans have opted for more courageous solution?

 

I understand what you're saying...but,..this is common practice for fitting appendages.

Here is one of ours, for a P-Bracket.... a recess is made and it is through bolted into the hull. The structure inside is stiffened up to support the P-Bracket and the loads it is expected to experience.



I accept this is an aluminium hull, not composite, but it is same principal....as we have done this on composite boats too.

 

Hello Ad Hoc greetings from Bordeaux France and thank you for your feedback. Yes I do understand recess made in the hull thats is done by making an insert in he hull mold. That practice is well known and accepted. What I meant is that the photos above appear to show no recess but a cutout in the hull bottom . The reason why I am pondering if that may be the case is their edges instead of being a little rounded -like when you have a recess in a fiberglass stricture - in the above photos the edges are sharp - like when you practice a cutout in the laminate. I made now a 3D model to explain what I mean. I wonder if the Leopard engineers did like my sketch drawing here. Recess is safe beacuse it preserves the structral integrity of hull bottom laintae. A cutout in the hull bottom would be a more daring procedure even if the hull is reinforced on the inside and it would be interesting to know if that was a practice of catamaran makers