Keels are Falling Off

From Scuttlebutt:
Keels Are Falling Off >> Scuttlebutt Sailing News http://www.sailingscuttlebutt.com/2018/03/29/keels-falling-off/

 

Additional article on the same subject by Geoff Van Gorkom, Van Gorkom Yacht Design: http://cdn.sailingscuttlebutt.com/wp-content/uploads/2018/03/Keels-Falling-Off-9-5-13.pdf

 

I don't think this problem will go away until the production community goes back to encapsulated keels. They withstand running aground much better because they distribute the load. Until then, I concur with the notion of a "mandatory keel system inspection". I would suspect some insurance companies require it for offshore coverage. I know they require a full rigging inspection. Until then, it's up to owners to inspect their own hulls. It's a roll of the dice to skip that critical step. Ideally this should be done once a year and before an offshore passage that is out of range for the coast guard. I personally feel an encapsulated design like the one below is perfect. Granted it may not be as fast as a fin or bulb keel, but it is a robust and safe design. That's what matters most.

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Good list of others to compare to here: Choosing a Blue Water Yacht - Keel Type | Grabau International http://www.grabauinternational.com/news/choosing-a-blue-water-yacht-keel-type/

 

Thanks for this interesting synthesis on a crucial issue, which includes a kind mention of the RM yachts offering bilge twin keels option : effectively Fora Marine (the builder) has developed a quite original construction mode including plywood hull and a galvanised steel skeleton for the keel fastening, a different one for either mono, twin or lifting keel. The skeleton is itself fastened to the plywood hull through glass epoxy stratification (+ some bolts ?). Illustrations here attached, first photo for twin keels, second photo for mono keel.
Exclusive concept - RM Yachts http://www.rm-yachts.com/en/the-concept
Another similar arrangement is the one adopted by Bob Perry on Sliver 62 "Francis Lee", here attached.
These arrangements seems very clean and robust, easing the check and the maintenance, are there lot of other builders adopting this kind of interface with either GRP or wooden hull boat ?

 

There were many thousands of boats built without encapsulated keels that were sturdy and built to designs that had no issues with keel loss. Bolt on keels were in common use from about 1970 and there were almost no keels lost until about 1987. During this period an enormous number of boats were built, so there is an enormous amount of experience that indicates that bolt-on keels can be perfectly seaworthy and durable.

Who is to pay for the mandatory annual keel inspection? Who is to certify those who can provide the inspection? Where are the certifiers going to find insurance? Why should those who bought a design that may have 400-600 examples afloat and over 40 years without a single keel loss have to pay?

How many people will be driven out of offshore sailing is keel inspections and similar inspections are made mandatory? What other costs will they skimp on to pay for these inspections?

What is the actual risk involved in offshore sailing? I once ran some rough calculations and it appeared that even counting flimsy keels, there was NO greater chance of dying in an offshore race in Australia, for example, than of dying if you just stayed at home or went to work instead. The main risk for sailors is from things like cardiac disease and obesity.

 

Dolfiman these are excellent references. The RM yachts concept shows a well thought out set of keel transverse & longitudinal frames. The pics show easily accessible keel bolts. The notion of burying the keel bolts in a blind area, in my opinion, is a ticking time bomb. Who is to know when the keel bolts are beginning to fail? Periodic inspections are the only answer until robust keel designs are re-established as the norm.

 

Hi CT249, some good points. Dying offshore really depends on IF a) you have a life raft and b) whether or not you can get in it if your bloody keel snaps off!

I agree some bolt on keels are robust designs, but SOME of them are frankly just cheap. They can't take running aground or bumping a reef without fracturing. That's the riddle that needs to be solved. Much of the riddle, in my opinion, could be solved with a robust design. I would rather focus on keel quality issues than delve into insurance companies & legal issues. Looking over the higher keel design standards for larger ships it's clear to see shortcuts have been made on smaller scale sailboats. Performance requirements aside, robust keels should have the following basic structural features:

1. First and foremost, robust lateral & longitudinal frames designed to absorb a keel strike at 5 knots* in the sand or mud from fore/aft/port/starboard. *This is just an initial benchmark and the speed of 5 knots is just general cruise speed number to establish a keel rating system (similar to 5mph bumper collision test for automobiles). This is a general load test that can also be initially calculated using CAE software, but ideally should also be tested on at least one test vessel.
2. All keel bolts should be visible for inspection (Translation: inspection panels needed above & below the waterline)
3. Corrosion resistant materials for mounting hardware including a corrosion anode if needed.
4. Waterproof keel housing.
5. Inspection procedure listed in the owners manual.
6. Other requirements???? --->>>Please add here

There are different types of keels obviously and each may bring it's own unique requirements, but all in my opinion should be able to pass a 5 knot grounding in the sand or mud.

In my opinion, it would be good to focus on what should be the essential elements of a robust sailboat keel design. This is one area on a sailboat that should NOT take shortcuts. The investigators, lawyers & insurance companies are compiling a list of suspect boat models. These actions are a knee-jerk response and resulting after lives have already been lost. I don't blame them for suing the manufacturers with lousy designs. It really shouldn't have to come to that point.

Ok I threw this out there have at it.

 

All keels build to current EU/ISO standards should easily pass that test. Which boat would not?

5 knots is a slow speed. Even on a beat or motoring 6-7 knots is more typical and on a reach clearly more.

In many areas hitting a rock is more likely than sand or mud. With bad luck the boat will bounce back with zero stopping distance. That's a real test for the boat and most will fail, even the early 60/70's GRP boats, if there is enough speed.

 

I agree with much of what you said. Where we differ is that bolt-on fins built prior to '85 did not fall off, and I don't think people should have to pay to prove it each year. The long term consequences of making this a sport even more for the rich could lead to even greater death tolls.

I've long said that the current keel loss rate is horrendous, and where I come from people have been found guilty of manslaughter and jailed because of it. I'm actually fairly happy with that - pros should not take shortcuts that kill people. But that doesn't mean we should make sailing less affordable for those who own boats with strong bolt-on keels.

 

CT249, yes a bit faster might be a more realistic test. The density/firmness of the sand & mud will also be a factor.

I'm sure I'm not the only one that noticed, but perhaps there has been a trend to remove the forward keel taper to towards the bow. Many older bolt-on keels have this. The encapsulated keels like the one mentioned in my previous thread have a natural tapered angle.

Why was this critical design benefit tossed out? Granted a perpendicular keel offers a bit less resistance, but the long term safety trade off is huge. Keels with a forward graduated taper are less risky because they:

1. Deflect the load and distribute it over a much broader area.
2. Push the hull up, which acts as an inclined brake.

By contrast, the leading edges of many keels today are nearly perpendicular to the hull. There is no taper. Thus, any time you have a strike on the keel all the load is applied directly on those keel bolts. No question the keels on racing yachts have influenced this trend. Cruising yachts really should opt for the forward taper in my opinion. Scroll through this list of bluewater hulls and you'll see the safety trend with taper is clearly advantageous.

Sailboat Reviews of Offshore Cruising Yachts : Bluewaterboats.org http://bluewaterboats.org/

 

In "Principles of yacht design" (second edition), authors Larson and Eliasson proposes this reference scenario for design purpose : grounding on a rock at 8 Knots (4,11 m/s) leading to a full stop in 0,25 s. That means for the boat CdG a decceleration of V/ t = 16,44 m/s2 (so ~ 1,68 g) and a stop distance of ( by 1/2 acc t^2) about 0,5 m for the boat CdG (actually CdG tangential distance resulting from the pivoting move of the boat around the keel tip impact point). With this assumption, the equivalent horizontal static force to introduce at the keel tip for design purpose is simply 1,68 x weight of the boat.

 

Very good info Dolphinan. That’s a good benchmark...8 knots is moving along pretty good. No wimpy keels allowed in that test! That load could be simulated with CAE. A good consulting team could do some reverse engineering & load checks of suspect hulls & keels with the goal of checking for vulnerable vessels.

 

That already exists in ISO 12215-9 and is required for CE through it. Although it does have different coefficients than PYD. E.g. FIrst 40.7 was found afterwards to comply with it except for some details: http://www.sailing.org.au/wp-content/uploads/2015/04/AnnexesToMAIBInvReport08-2015_CheekiRafiki.pdf

Here is a document before the standard was final including some boats calculated: https://www.google.fi/url?sa=t&rct=...y-pdf.37626/&usg=AOvVaw2zRfP3NgXGWFgwb4q2KCns

I don't have access to the standard. Someone here certainly has and can comment how the formulas differ at the final stage. Is there still two different options for grounding forces like in that document? Or was just one of them chosen?

 

Thanks for this document, in page 9 is mentioned the longitudinal impact force, called “F4- Enhanced” = 0,8 Mg (Lwl/10)^0,5 (from ABS ORY guide for 20 years) >>>
0,8 g decceleration factor for a 10 m length boat, 1,1 g for a 20 m length

This is a lot less than the 1,68 g from PYD approach (8 knots and full stop in 0,25s), although it is said usually high to comply with. So they proposed a dual grounding load cases : the “Enhanced” one as above with 0,8 and the “Normal” one with 0,4 (50%) for recreational boat less exposed to such grounding.

They are very reluctant to relate that case to a speed. The reason is given in the final sentences of chapter 3.4 page 10 : in case of a damage, “ no doubts the ISO 12215-9 will be cited by the various parties. This is perhaps why an explicit speed-load relationship would be dangerous as well as scientifically invalid”.

I can understand the first argument (dangerous, within a lawsuit context) but the second one (scientifically invalid) is a bit frustrating. I propose here attached a simple hydrostatic approach that can give a relation between speed V and impact force Fi in case of a rigid rock grounding : the boat is pivoting up to when buoyancy forces moment (which moves forward) balance the moment due to this impact force (assuming the boat a rigid body is consistent as the hydrostatic forces offer a lot more flexibility that the internal structure to absorb the shock). The set of equations and the computation loop are proposed attached, and can be easily solved as long as we can compute the LCB forward move with the rotation (i.e. with the nose-down trim). When I have time, with Gene-Hull I can propose a numerical study to illustrate that approach.

 

I can understand well that there is no clear way to calculate maximum forces from speed and boat geometry. It's a very complicated task, if one is not making a lot of (invalid) assumptions.
The keel will deform, the ground will deform, the force on the keel is not constant, a rather big part of the mass is in the keel and thus not causing force on the keel joint when stopped, also water moves with the boat changing the mass needed for F=m*a etc.

It's rather easy to calculate an order of magnitude, but saying that this keel structure will tolerate grounding at 8 knots is another thing even when a stopping time or distance is specified.