Best way to attach a keel

bbg

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For the kind of keel the OP is talking about I like the idea of a recessed "female" moulding in the hull, with the keel going up into that and some bolts securing it horizontally. The head of the keel is supported by the structure and the bolts that secure it are up out of any bilge water.
 

William_H

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Attaching a keel. Much depends on keel material. Lead may require additives to make it stronger in itself or a frame inside the lead to take the fastenings and make it stronger.
In attaching the base size is important. Both in fore and aft direction (not usually so difficult) and athwartships a wider base gives geometric strength. So bolts should be as wide apart as possible and also at the front of the keel base. A cast iron keel we would think would have strength in itself so a narrow base width might be OK but then this gives greater load on the bolts and greater pressure on the hull itself under the bolts.
Number of bolts will also aid in attachment strength.
I tend to agree that ballast inside a GRP keel as part of the hull has much to be said for it. However I think this becomes the least efficeint keel in terms of foil efficiency and wind ward sailing. More suitable for long keel design. I believe also that the most efficient keel does not have the huge widening base that is dictated by strength of attachment concerns. ie a keel the shape of a racing dinghy dagger board when in place is more efficent. But then so is a deep narrow chord keel which is just not practical on a larger boat.
We are getting newer boats into our keel boat fleet some of which have over 2metre draft and they are suddenly finding the shallow parts of our river /estuary. So it is all a compromise.
 

30boat

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Glue isn't strong enough.
Not true actually.Andrew Simpsons boat has a glued on deep fin.The hull is cedar strip and the keel is composed of an outer GRP shell holding a lead inner part .The bottom of the boat has a GRP stub flanged on top that is glued on from within the hull .To that the GRP/lead keel is glued in a male female fit.The glue was a special high performance epoxy with an astronomical shear strength,can't really remember which make.It was all in PBO at the time.
 

fisherman

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As a 'build it twice the size and then some' sort of boat builder/repairer: if it's the usual dish shaped hull I would want to spread the load over most of the underwater sections. At the hull moulding stage I would mould a keel box inside the hull with lateral quarter bulkhead stiffening. One problem would be the keel would have to be made a tight fit and this I haven't quite got a grip on, given you may need to remove it sometime. Otherwise a fin keel would have to have a very large flange to meet a suitable flat section on the underside of the hull, extending fore aft and sideways, with fixings at the furthest extremities, through to frames extending across the underwater internal area. Such a flange would ideally sit in a moulded rebate in the hull to reduce water flow problems.
A local builder retro-fitted bilge keels to a 19ft FV hull. He moulded the keels, 8ft long and 18in deep 100mm thick, then simply cut a slot in the hull, then made 'T' section stiffeners which were inside the keel and across the inside of the hull. (New moulding, so good chemical bond). Considering this boat was rolled over the bilge keel and onto its side on the beach with no structural damage it has proved sound.
 

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Big problem of that approach is that a grounding may cause a delamination between the frame and the GRP hull. Typically galvanized steel has a poor bonding with GRP.
Not saying it's a bad solution but it has it's issues as well.

The galvanised steel frames of X boats and Arconas are not bonded to the GRP. The keel is bolted to the frame that included the spreaders the hull is bolted by being the middle of this sandwich.
 

Tranona

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Not true actually.Andrew Simpsons boat has a glued on deep fin.The hull is cedar strip and the keel is composed of an outer GRP shell holding a lead inner part .The bottom of the boat has a GRP stub flanged on top that is glued on from within the hull .To that the GRP/lead keel is glued in a male female fit.The glue was a special high performance epoxy with an astronomical shear strength,can't really remember which make.It was all in PBO at the time.

See post #5. I watched the last one being built, but missed the actual gluing on of keel.
 

Sailfree

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I would like to make it clear I do not own a X boat or Arcona.

Modern boats are fin keels. These are used for sailing performance.

Encapsulated keels are not good for grounding in different ways as GRP is a relative soft material compared to rocks. Yes it may not fall off but it requires a lift out and GRP repair.

I answered the Q as a structural engineer but like all problems there are various solutions with no one being perfect. Having designed bridges you soon learn that over strength is easy by going heavier but then it causes its own problems in needing to be supported. Lightness and efficient structures have enabled greater spans but they will never last as long as a Roman masonry structure!

Keels are obviously like car makes - the best one is the one you have!
 

pmagowan

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This is lots of great information and plenty to think about. The layup of the boat is going to be a cold moulded structure consisting of (from outside to inside); epoxy/glass fibre (with possible other exotic fibres), 3 layers of wood veneer epoxied at different angles, cedar stip planking and a internal epoxy/glass fibre.

The layup should be very strong and monocoque, being essentially a complex plywood structure in the shape of a boat. The mast will be keel stepped. There is potential for a reinforcing epoxy/fibre ring around the mast/keel section. Internal stiffening will be provided by structural bulkheads and built in structural berths/units.

The purpose of the boat is for normal cruising but with occasional long distance cruising and , in particlar I have a craving for high lattitudes. I like to be able to touch bottom without risk of damage (I am used to long keelers). I do, however, want to nod towards more modern performance hence the 'modified long keel', similar to a Rustler. This means there should be plenty of surface area to make the join. The keel will contain lead as appropriate.

Of course I am interested in keels and boat design in general so keep all the ideas coming even if they are not strictly relevant to the above. You never know, some things might change.
 

geem

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But this will never be a performance boat just a traditional heavy slow cruiser.

Does anyone sell these boats new anymore. I do say sell not offering to build at an uneconomical price!

Not true. We have a lead encapsulated keel on a 44 ft boat. We are 2.2m draft. The ballast is 6000kg. Consequently we have a large righting moment and can carry large sail area. We are fast in light winds. We are close winded and capable of extreme heavy weather. Sailing around the Caribbean for the last 6 months we are quicker than most other cruisers of a similar legnth(anything less than 50ft). If you are interested I might even tell you what it is.... Ps weighed 19 tonnes on the scales when we lifted out
 

ip485

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Island Packet is ineed the "modern" in production example. :)

I plan on better than 7 knots in any reasonable wind. The long keel and weight confer other advantages upwind and down.
 
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Not really wanting to drift the thread but will comment on encapsulated fin keel, full forefoot type hull performance. Most of these yachts are sensitive to the right sail for performance which the ubiquitous roller reefing head sail does not provide. My Rival with a lightweight ghoster will scoot along at a few knots but come to a standstill if the roller reefing is fully out. Similar in heavy winds, the working jib, will outperform the roller reefed headsail hands down. Having said that, a modern hull shape and associated weight with roller reefing sails will outperform my tub hands down. The point though is that these older hull styles do benefit from individually sized headsails (and a good roach on the main).
 

lw395

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.......... The layup of the boat is going to be a cold moulded structure consisting of (from outside to inside); epoxy/glass fibre (with possible other exotic fibres), 3 layers of wood veneer epoxied at different angles, cedar stip planking and a internal epoxy/glass fibre.

The layup should be very strong and monocoque, being essentially a complex plywood structure in the shape of a boat. The mast will be keel stepped. There is potential for a reinforcing epoxy/fibre ring around the mast/keel section. Internal stiffening will be provided by structural bulkheads and built in structural berths/units.........

however the keel is actually connected to the hull, it does not change the bigger picture of how the loads from keel and rig are fed into your monocoque shell.
The shorter you want the chord of the keel, and the thinner its section, the bigger the forces at the keel root.
That's what you need for performance, but it's not really compatible with being indestructible from grounding at speed. The heavier the hull gets, the worse the problem.
 

prv

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OK, so imagine you are building a boat from scratch. What is the best way to attach a keel

The way the designer specified :p

Or are you planning to design your own boat as well as build it? Personally I have some ideas of the type of boat I'd want in this situation, and a million and one ideas for details, but for the actual hull and rig design I'd be approaching an experienced naval architect with a track record of similar types rather than investing all my money and ten years of my life building my own unproven first attempt at a design.

Pete
 

doug748

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Not true. We have a lead encapsulated keel on a 44 ft boat. We are 2.2m draft. The ballast is 6000kg. Consequently we have a large righting moment and can carry large sail area. We are fast in light winds. We are close winded and capable of extreme heavy weather. Sailing around the Caribbean for the last 6 months we are quicker than most other cruisers of a similar legnth(anything less than 50ft). If you are interested I might even tell you what it is.... Ps weighed 19 tonnes on the scales when we lifted out

Yes.
And a lot of people confuse the method of construction - Encapsulation, with a particular design - Full Long Keel.
For clarity here is a UFO 34 with encapsulated lead keel:

View attachment 52087
courtesy Yacht Focus

Not that this helps the OP much but there we are.
 
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Tranona

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Yes.
And a lot of people confuse the method of construction - Encapsulation, with a particular design - Full Long Keel.
For clarity here is a UFO 34 with encapsulated lead keel:

View attachment 52087
courtesy Yacht Focus

Not that this helps the OP much but there we are.

However, it was not like that because it was necessarily "better" - just that the tumblehome meant that it need to be moulded in two halves and it is easier to mould the keel shape in this way, join the two halves together and fill the hole woth ballast, rather than build a structural framework inside the hull to take a bolt on keel.

As ever design and construction is a compromise and the keel is the consequence of construction which is a consequence of design. As soon as design (and rules) changed the shape of hulls, construction methods and keel designs changed.
 

Tranona

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The way the designer specified :p

Or are you planning to design your own boat as well as build it? Personally I have some ideas of the type of boat I'd want in this situation, and a million and one ideas for details, but for the actual hull and rig design I'd be approaching an experienced naval architect with a track record of similar types rather than investing all my money and ten years of my life building my own unproven first attempt at a design.

Pete

That is a valid point. although the boat the OP is designing does not necessarily have to comply with the RCD if he does not sell it within 5 years of completion, it would make sense for him to design it so that it does comply (as Andrew Simpson referred to earlier did with the boat he built for himself). He will then find most of his questions answered by following the standards required for design and construction to get the CE mark.

With the design as he describes it, he seems to avoid the type of problems that concern builders of composite GRP hulls with deep narrow keels, so following the guidelines in the ISO (formerly ABS) standards will ensure that his keel is well attached. There is nothing magic about the standard - basically big bolts or studs spaced correctly and going through structural members with nuts backed by big washers. How "big" being determined by the size, weight and shape of the keel. His challenge is going to be to design the supporting structure, but again the RCD includes scantling rules for different methods of construction. If (as one hopes) he is sufficiently technically minded to follow the rules he can design it himself and then get it checked by a third party as required to obtain the CE mark.
 
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