Fin Keels

[KINGSCRUISER]

Is it not the case that if the wind forces against the mast and sails and the balancing forces against the keel are equal and the mast is stayed off with shrouds and fore & aft stays then is it not inevitable that the keel will eventually give way if it is only attached to the glass fibre hull with a few keel bolts ?
It seems that no matter how clever the designers are historically this problem keeps recurring.

 

[sailorman]

Is it not the case that if the wind forces against the mast and sails and the balancing forces against the keel are equal and the mast is stayed off with shrouds and fore & aft stays then is it not inevitable that the keel will eventually give way if it is only attached to the glass fibre hull with a few keel bolts ?
It seems that no matter how clever the designers are historically this problem keeps recurring.

No problem with the 3 tons of lead hanging from my Holman & Pye yacht

 

[RichardS]

I don't think so.

It's just a matter of making sure that the fastening of the keel to the hull is stronger than the wind force needed to blow the boat flat.

Making the keel fastening stronger than an event such as a grounding or a collision is a whole different ball game as these are much less predictable events.

Richard

 

[KINGSCRUISER]

Yet ?

 

[sailorman]

Built like a brick outhouse

 

[Tranona]

then is it not inevitable that the keel will eventually give way if it is only attached to the glass fibre hull with a few keel bolts ?
It seems that no matter how clever the designers are historically this problem keeps recurring.

Afraid that is a load of nonsense. There are literally 10's of thousands of boats in service with keels bolted on to the hull. Apart from a brief period when encapsulated keels were common the method has been used universally since external keels started to replace internal ballast.

Where keels have become detached there is almost always a sound explanation for the failure, and that is rarely poor design or manufacture - although it is not unknown, particularly in boats designed for racing and exploring the limits.

 

[KINGSCRUISER]

Maybe nonsense but if I was crossing the North Atlantic in a yacht I think I would be happier with a long keel or an encapsulated keel.

 

[sailorman]

Maybe nonsense but if I was crossing the North Atlantic in a yacht I think I would be happier with a long keel or an encapsulated keel.

So be it, its your decision, encapsulated is lay-up dependent & that long keeler will have a bolted on keel too & possibly those bolts might be wasted where you cant see them. Fair winds

 

[Tranona]

Maybe nonsense but if I was crossing the North Atlantic in a yacht I think I would be happier with a long keel or an encapsulated keel.

Your choice - but you will be in a tiny minority. Thousands of boats with bolted on keels cross the Atlantic each year without a moments trouble.

The encapsulated keel was a method that was a suitable solution to a problem related to specific designs and methods of construction. As others have said, not necessarily trouble free and quickly abandoned when better solutions were developed.

 

[rob2]

...and quickly abandoned when better solutions were developed.

As far as I can see all methods of attaching keels have their problems and it's likely that the current disfavour of encapsulated keels has more to do with the price of lead! Iron stampins used to be used a lot, but should a problem occur, the expansion of the rusting material turn a small leak into a disaster. My own keel is encapsulated lead and the only real disadvantage is that I can't use a drying mooring long-term as it would eventually lead to wearing through the GRP and reapirs would be slow and expensive.

Most problems with losing keels happen with either racing boats or cruisers based on old race designs - I'm open to argument though. A flat bottomed hull with a keel bolted on is inherently weak as the hull shape offers no support or rigidity to the joint. Should the boat suffer a grounding or should the fastenings come lose, the keel will be open to wrenching itself off - quite possibly taking a large chunk of the hull with it. The practice of fitting a hollow spacer between the keel and the hull only exacerbates the problem as it will crush in failure mode, probably at a lower load than would be the case with a solid keel.

Interestingly (to me, at least) in the class of boat that I sail the original keel specification was for a bolted on lead keel (2% antimony). As the hull has something of a wineglass shape, it is a strong construction. Some boats opted for a (presumably cheaper) iron keel, but to maintain the CofG it is deeper so has greater drag and leverage, so is potentially weaker. However, in forty years, none of the boats has lost a keel and at least a couple of them have crossed the Atlantic.

Rob.

 

[dunedin]

Lots of bolted on keels are lead. So that is not a factor.

And plenty of (particualrly wooden) deep keelers have been lost over time - for various reasons, including planking failures opening up leaks and/or collisions with objects or animals - which could have happened here to start the problem.

Could just as well say that airliners should go back to having stayed wing biplanes, as these narrow bolted or glued on wings are bound to fail. Goodness me they even hang the engines off them which must put lots of strain.

 

[JumbleDuck]

So be it, its your decision, encapsulated is lay-up dependent & that long keeler will have a bolted on keel too ...

The ton and a half of lead in my long keel is all encapsulated.

 

[KINGSCRUISER]

Have not heard of too many wings coming off. Maybe they have cracked (!) the problem

 

[knuterikt]

Have not heard of too many wings coming off. Maybe they have cracked (!) the problem

Airplanes has a different maintenance / inspection schedule than most boats...

 

[Tranona]

As far as I can see all methods of attaching keels have their problems and it's likely that the current disfavour of encapsulated keels has more to do with the price of lead! Iron stampins used to be used a lot, but should a problem occur, the expansion of the rusting material turn a small leak into a disaster. My own keel is encapsulated lead and the only real disadvantage is that I can't use a drying mooring long-term as it would eventually lead to wearing through the GRP and reapirs would be slow and expensive.

Encapsulated keels came about in that transition period when "wood shaped" boats were built of GRP and it made sense just to create a GRP envelope the same shape as a wood boat then fill the bottom with ballast. Many of the designs also had tumblehome in the topsides so were moulded in two halves, joined together along the keel and filled with ballast.

Very quickly though designers discovered ways of making keels into foils once they were free of the structural requirements from wood that necessitated a full length keel. You can see the evolution from looking at say the S&S designs of the late 1960's when keels became separate entities from the hull and it was no longer possible to build using the encapsulated method. Variations included moulded stubs and bolt on ballast which was very popular from late 1970's to around 2000. However the benefits of flatter bottom hulls, lower ballast ratios and separate foils for keel and rudders has led to the almost universal use of bolt on foil shaped ballast keels - most of which in production boats stay very firmly attached to the hull and the internal framework they are bolted to.

As has been noted many times, encapsulated keels are not problem free. Firstly they restrict keel shape and limit performance as well as being mostly used on hulls with narrow beam and restricted interior space. Many are very badly built, either through the use of steel punchings and concrete for ballast or by poor quality GRP moulding, particularly in deep narrow keels. Some builders even built GRP tanks integral with ballast keel and you can imagine the problems that creates if there is any leakage. The best builders usually did a good job, but there are some real horrors around, even in boats that have an otherwise good reputation. The defects are usually hidden and only come to light for example after a grounding or even after blasting the hull in preparation for refinishing.

 

[grumpy_o_g]

Airplanes has a different maintenance / inspection schedule than most boats...

Mainspars and attachment points on some aircraft probably get inspected less often than on yachts - you're talking a total strip down to get at them and then NDT to see inside a complex piece of engineering. I reckon the analogy was a little false for a couple of reasons though. First is the hostile environment - immersed in salt water but very well oxygenated and actual in the open air sometimes, The second is that the wing on just about all aircraft actual handles a very large proportion of all the forces as a discrete structure with the fuselage simply hanging off of it.

I'm not too worried about keels but I frequently look up at the mast and rigging and wonder why it hasn't crumpled under the loads.

 

[prv]

I'm not too worried about keels but I frequently look up at the mast and rigging and wonder why it hasn't crumpled under the loads.

+1

I was happy with Kindred Spirit's stubby tree-trunk and multiple shrouds. Sky-scraping aluminium masts with a bare minimum of rigging require a bit of a leap of faith from me

Pete

 

[William_H]

OP 's statement is quite wrong in it's generality. A fin keel obviously must have a wide base to provide the geometry for supporting the keel when the boat is laid down mast touching the water. Most of the time the weight of the keel provides stability through form stability where it is the mass of the boat pressing down against the buoyancy of the chines that keep it upright. The design of the keel attachment and the f/g structure under floor all must be designed to take the stresses of the keel and ultimately transfer the loads to the chain plates and mast. Attachment design must have a safety factor to allow for detriation over time. It is tragic that this boat in the Atlantic has apparentlyt lost it's keel. Probably from hitting something like a container. I suppose however that we would have to say there was a failure of the keel attachment that should not have happened. Certainly if one or nore bolts fail then the remaining bolts will rip the f/g when the keel departs.
Regarding the encapsulated keel these tend to have a wide base at the interface with hull. This is somewhat detrimental to keel performance which I think works a lot better as a constant chord fin. So a dinghy witha dagger board emerging from a flat bottomed hull works well to windward compared to a long keel boat. olewill

 

[BruceDanforth]

Performance isn't everything though. I always fancied one of the Bruce Roberts Spray designs if I ever get enough cash together for long term cruising.

Racing boats are usually exciting because they are uncomfortable and have an element of danger.

OP 's statement is quite wrong in it's generality. A fin keel obviously must have a wide base to provide the geometry for supporting the keel when the boat is laid down mast touching the water. Most of the time the weight of the keel provides stability through form stability where it is the mass of the boat pressing down against the buoyancy of the chines that keep it upright. The design of the keel attachment and the f/g structure under floor all must be designed to take the stresses of the keel and ultimately transfer the loads to the chain plates and mast. Attachment design must have a safety factor to allow for detriation over time. It is tragic that this boat in the Atlantic has apparentlyt lost it's keel. Probably from hitting something like a container. I suppose however that we would have to say there was a failure of the keel attachment that should not have happened. Certainly if one or nore bolts fail then the remaining bolts will rip the f/g when the keel departs.
Regarding the encapsulated keel these tend to have a wide base at the interface with hull. This is somewhat detrimental to keel performance which I think works a lot better as a constant chord fin. So a dinghy witha dagger board emerging from a flat bottomed hull works well to windward compared to a long keel boat. olewill

 

[Daydream believer]

Is it not the case that if the wind forces against the mast and sails and the balancing forces against the keel are equal and the mast is stayed off with shrouds and fore & aft stays then is it not inevitable that the keel will eventually give way if it is only attached to the glass fibre hull with a few keel bolts ?
It seems that no matter how clever the designers are historically this problem keeps recurring.

A lot more masts fail before the keel drops off
As others have pointed out form stability comes into play so it is not a simple reaction force

Personally i would not like an encapsulated keel
It is difficult to examine & if it is filled with steel rather than lead & water gets in the the expansion of the rusting process could (note -could) expand the casing & weaken it. On an old boat one might not know if the leading edges had been damaged in a grounding & botched over
There is also the consideration of extent of encapsulation
Does one assume that the encapsulation thickness is sufficient ( in the same way that one has to assume there are sufficient bolts & means of transferring their stresses to the hull). It may be that constructors have not put enough GRP around the keel. failure would (one assumes) be slower but still failure in the end

With a bolted keel one can always stick a spanner on the nuts & give a good twist to see what happens (or better still an air powered nut wrench)- if a bolt has sheared then it should in theory rotate the stud
If that is the case then one immediately knows there is an issue
i am surprised that this is not done more often - I think I would do it with my boat if attempting an offshore passage- (but I would not want to add this comment to the Cheeki rafiti post as i feel it is unfair on all concerned in a sad situation)