# Strut Keel Design and Engineering Grounding Forces

Discussion in 'Boat Design' started by an2reir, Apr 21, 2020.

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### TANSLSenior Member

I think ISO 12215-9 gives various solutions that could be suitable for your boat: figure C.3, figure D.1, figure D.4, and attached picure.

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### an2reirfifty boat designs

Hello TanSL thanks for feedback ;
I do have the ISO 12215-9 french version ; but I do not have the figure C.3, figure D.1, figure D.4,
I am quite advanced with my motorboat keel solution and I am now pondering over the detail of the motorboat keel to be fitted on a recess I need to practice in the hull or to just fit
my keel onto the hull surface no recess at all and attach by adhesive and bolts
For example here is a strut keel of a trawler yacht maker .
The strut keel is supposed to take the load of grounding and collisions
That means is has to be able take without distorting or breaking a horizontal collision force of
F=1.2 x g x (mLDC- mKeel) Newtons
In the case of this below trawler yacht the weight mLDC= 24000 KG
That makes a force of about 282240 N= 28780 kg= 28.8 tons
I multiply by a factor of 2 that makes 57 tons
To me it seems low for a grounding

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### TANSLSenior Member

Very nice boat.
Don't you have the appendices of ISO 12215-9? I attach the mentioned figures, in case they help you.
<If figure is from ISO, it cannot be attached here or it could violate their strict copyright>

Note the units of the variables in the ISO formula:
F3 = g * (mLDG-mKeel) Newtons​

where:
g is 9.81 m / sec2
mLDC in Kg
mKeel in kg​

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### an2reirfifty boat designs

Hi ! Ies I do confirm I did not have the appendices of ISO 1225-9 . Until now when you were really very helpful , gracias TANSL !

Now the decisions are:

1. to fit in a recess made in the hull or to fit on the hull surface no recess.

2. my keel is designed on an aluminium frameing. I was thinking it is not good to fit metal directly on the hull surface as their mechanical properties are dissimilar ;
therefore I did design a keel stub made of fiberglass to be fitted to the hull surface with adhesive and my keel frame ( wich is metal) is to be bonded with bolts to the keel stub .here are some trawler yacht stub keels. In this case the drawings from the ISO 12215 dont work as they are because the motor boats keels are positioned near the hull chine and not central on the boat axis as the sail yacht ballast keel

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### TANSLSenior Member

1.- I would fit the keel in a recess made in the hull (but I am not an authority on keel design).
2.- The iSO 12215-9, as far as I know, also works for the side keels, with or without a bulb. Could you show us an outline of the solution you have thought of.

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### an2reirfifty boat designs

Thank you Tansl

Unfortunately I cannot show my keel design but thanks for feedback.
But I can say I am inclined to choose the option of having the keel mounted in a recess as well. The reason I was pondering over the recess is I do have this kee; stub wich is 2.5 meters long and I was trying to save weigh because if the keel stub is made of solid fiberglass and is 2.5 m long by 30 cm wide by 30 cm high it will be quite heavy

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This is how I see it:

It is in a recess...and there is simply a 'fill-in' fairing laminate added over it once fitted.

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### TANSLSenior Member

Why not foam lined with GRP?. I hope I'm not asking an absurd thing.

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### an2reirfifty boat designs

Hi Tansl thank you for your feedback

I did design the keel stub with an internal web type structure in fiberglass. I think the keel stub needs be very solid in order to take compression 56 tons. But I as well have weight constraints ; my entire keel should be 30 kg !!!! Regarsding mounting the keel in a recess the question is Whyin a recess? what is the purpose of the recess? What happens if you do not mont in a recess but on the hull bottom? The positioning of the appendage on the boat bottom makes the geometry of a recess very different when the appendages are near the boat chine the boat bottom is angled therefore a recess is to take more interior volume . Have a great week!

Last edited: Jul 20, 2020
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The easiest way to think of this is image the foam as just "air".
So, remove the foam or air what is left... the skins....and thus, so long as there is sufficient shear area and material thickness/area for compression in the skins, that is all that matters.
Whether there is foam - or air - in between wont really effect the final result. If the skin thickness is inadequate to absorb the load it is subjected to, it matters little if there is or is not foam.

So once you run the calc's of how much thickness of materiel is required, that will dictate if foam can be added to save weight or not.

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### an2reirfifty boat designs

My case is I have this constraint of the keel to weigh 30 kg

In order for the weight to be minimal I made my keel aluminium

I need a high strenght but weldable aluminium that will be AL 7050

The metal in my opinion shoud not make direct contact with the hull therefore I designed a fiberglass keel stub: I made the inside of the keel stub reinforced with a web like steucture like a fish bones

I will fit this keel stub in a recess; however TANSL showed me his spanish images from the ISI 122159 and one image is of a keel that is being fitted dire tly with no recess

What are the advantages of having a recess made in the hull?

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### TANSLSenior Member

Ad Hoc, as you know perfectly well, or you should know, foam has the sole mission of shaping the element. It does not serve, or serves very little, as a resistant element, and, if there was a way to eliminate it, once the element was formed, the foan would be eliminated to save as much as possible in weight.
It is possible to make two molds for the keel, two symmetrical halves, laminate both parts with their respective mold, unmold, and glue them, but in general it is preferred to laminate on the foam base, use the same precedent as on surfboards or, what is more normal, in the omega profiles of the reinforcements of an FRP structure.

Maybe avoid breaking the laminar flow in that area?

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### an2reirfifty boat designs

Hi TanSL Thank you for your feedback greetings from Bordeaux France there is no doubts AdHoc knows all the above and more he was olny writing some thoughts that are always welcome to me: I made now the interior reinforcement for the keel stub that is to be made in two molds and monoaxial fiberglass epoxy; the point is keelstub is to withstand the 56 tons compression Is it any good? :

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### TANSLSenior Member

Of course that's the question.
I am not an expert in keels but using the direct calculation of beams, it would suppose that the 58 tons will be distributed among the floors that support the keel and each floor must have the resistant modulus and the section necessary to resist the efforts to which it is subjected.
Each floor is a beam, embedded at its ends, subjected to a uniform pressure derived from the distribution of the 58 tons. Calculate the stresses on that beam.
If you have defined the laminate of the floors, ISO 12215 will give you the maximum allowable stresses for that material, depending on the composition (% fiber by weight) of that laminate. This will allow you to check whether each item is sufficient or not to support the loads on it.
The shear stresses in each zone will allow you to size the bolts (I would put two bolts on each side of each floor).
I hope you will forgive me if all this is very obvious to you.

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