G'day salty sea dogs!

You might look at the title of this thread and think WTF?
Let me explain and ask what maybe a stoopid question.

Let's say I was looking to buy a 40ft+ yacht and one of the items on the wish list was that it had to have a deep draft - say at least 2.0m+, is it possible to make a deeper draft vessel out of a shallow draft??
I'm obviously talking about bolt on's and I prefer encapsulated keels, but the choices of yachts in that area seem limited.

I don't really wan't to discuss the pro's and con's of deeper keels vs shallow - it's been done before.

Cheers!!

Hi

An interesting question as most people would probably be wanting to go the other way to make it easier to access shallow water

If it was a bolt on keel I suppose it wouldn't be difficult but you would need to make sure that the mounting point on the hull and the hull structure was up to the job as I assume that additional load might be placed on them
I guess the first point of call would be the boat designer and get there thoughts on the modification

Regards Don

Just a question of the engineering.
As to keel depth, just remember that there is big difference between 2.3m and 2.6.

we did this years ago on a race boat

the first and most critical issue is how well is the keel attached to the boat ? most modern yachts have 'bolt-on' keels and then you need to be super careful that the structure is going to stand the steep increase in load involved.

even if it is an older boat with an encapsulated keel there is still likely to be issues

as regards attaching : we ground the bottom of the keel flat and used epoxy and coach bolts to add about 300mm / 150kg. transformed the boat !

finally, don't overlook the change in waterline due to the extra weight. this can be compensated by carrying less 'stuff' but maybe you want all this 'stuff' ? if you just want the additional draft to make less leeway rather than add righting moment, perhaps you don't need to add lead...eg make the extra depth timber. this would reduce the load significantly !

as a designer, david lyons (if he is still around) is better than most at this sort of calculation (nb : insurance is certain to require professional sign-off on such a mod)

good luck !

cheers,

David is indeed still around and is working at UNSW
but definitely the man for the job.

Interesting topic.
I read some Keel replacement posts from Michael Storer a few years ago, that may be of interest

www.storerboatplans.com/boat-design/the-fallen-keel-an-adventure-in-keel-design/

www.storerboatplans.com/boat-design/slug-into-a-quick-swan-the-orange-boat-changes-into-racing-yacht/

Saw a Little Harbour 50 foot for sale, 1.6 m standard draft, but 3 m with the centre board down!
best of both worlds as long as the centre board slides up and down easily!

Extending a keel is quite the effort. firstly you need to make sure the structure / attachment is going to take it.
You can extend a keel by casting on an extension and cutting in / casting in rod, bolts and a plate but in all honesty it's just not worth the effort.
I just recently removed 270mm from my fin keel and that came to a shade under 500kg of lead.

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NowandZen said..
G'day salty sea dogs!

You might look at the title of this thread and think WTF?
Let me explain and ask what maybe a stoopid question.

Let's say I was looking to buy a 40ft+ yacht and one of the items on the wish list was that it had to have a deep draft - say at least 2.0m+, is it possible to make a deeper draft vessel out of a shallow draft??
I'm obviously talking about bolt on's and I prefer encapsulated keels, but the choices of yachts in that area seem limited.

I don't really wan't to discuss the pro's and con's of deeper keels vs shallow - it's been done before.

Cheers!!

I should say that there is a much easier way to do it.
One kg out the rig is about 5 kg added to the keel and the boat gets lighter not heavier.
By example, a carbon rig on a Sydney 38 is the equiivent of putting about 500kg on the bulb but the boat gets almost 100 kg lighter.
Much more economical to take weight out of the rig and deck gear than to put more in the keel and increase draft.

I guess the question that should be asked is what are you trying to achieve

Regards Don

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Donk107 said..
I guess the question that should be asked is what are you trying to achieve

Regards Don

If you just want to go faster it is far more efficient and cheaper to take the weight out of the top including deck gear.
A kilo off the deck is a kilo on the bulb.

My Hanse 575 is a shoal draft and draws 2.2m and the standard keel version draws 2.8m. Both boats have the same righting moment. To achieve this my shoal draft boat is carrying 1 ton of extra weight in the keel which would technically make my boat slower as it is carrying 5% more weight. I don't believe you are going to achieve very much by lengthening the keel. You may improve the righting moment a bit but you are going to be heavier and slower. It is more of a matter of moving the existing weight further down and you can more than likely be able to reduce weight. I know that there were a number of Young 11s that had new torpedo deeper keels fitted which were lighter than their original keels. This deeper keel gave them a better righting moment and lighter over all weight. At the end of the day I don't believe you are going to achieve very much by adding additional weight or length to an existing keel unless the boat was a poor design in the first place.

I'm an engineer (a civil engineer, not a marine engineer), so here's an engineer's response.

If it were about righting moment (the leverage the ballast has), then a deeper keel could be lighter. The righting moment is force (weight) times distance. So, you could reduce the weight by the same proportion as you increase the distance. Actually, you should reduce the weight so that you don't overload the joint where the keel is fixed to the boat.

Keels are also fins, so you need to see the same area, looking beam-on. Longer and thinner can give you the same area as shorter and fatter. The sideways force on the keel when sailing upwind creates a shear force on the joint between the keel and the hull, because fin-keels are designed to resist being pushed sideways through the water.

One thing to watch is to keep the same area of contact in the joint between the keel and hull, because that is where the righting moment and shear forces are transmitted between keel and hull. You certainly should not reduce the number of fixing bolts. Good systems have pairs of bolts oriented beam-ways on a boat (for better load distribution), but retrofitting a system like this (to make up for lost bolts) could require you to rebuild the bilge to get the additional anchor points, at which point it will cost more than the boat is worth.

Put that together, and you might have a upside-down shark-fin or trapezoidal keel with the same length (bow to stern) as the original at the hull, a thin mid-section and a thin low section. It would have a longer leading edge, and there would be a trade-off between drag on the leading edge and drag down the sides of the keel. Judging from the dagger-like profiles of racing boat keels or centre-boards, long thin keels have less drag than short fat keels.

That's the theory, but you will need to get a professional or qualified opinion. Before you modify your keel, make sure your insurer will not walk out on you.

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Donk107 said..
I guess the question that should be asked is what are you trying to achieve

Regards Don

I think the general consensus, is that a deeper keel would give you better upwind performance by allowing you to point higher.
I also believe it would help stabilise (but not stop) an uncomfortable night at rolly anchorages.

Last part not really right.
Deeper keel alone will not let you point higher, that is rig issue but you will make less leeway if area is the same and you may have more righting moment.
Deeper keel will not make the boat more comfortable at anchor but may make it worse as the period of roll will be faster if C of G gets lower.
So you roll less but quicker.

Keep the keel and chase down a secondhand carbon fibre mast.

If the boat is stiffer due to keel modifications it will impact the strength of the rig. Stiff boats require stronger shrouds etc as forces are higher. Less stiff boats by definition place less force on masts and rigs.

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lydia said..
Last part not really right.
Deeper keel alone will not let you point higher, that is rig issue but you will make less leeway if area is the same and you may have more righting moment.
Deeper keel will not make the boat more comfortable at anchor but may make it worse as the period of roll will be faster if C of G gets lower.
So you roll less but quicker.

Hey Lydia
So like sliding the weight down on a Metronome stick thingy the swing at the top is shorter but faster ?

Exactly the same physics.
Why the original Sonata 8s were one of the most seaworthy little ocean boats.
Most of the ballast was inside the boat.
Slow pitching moment.

Just go IOR style. Put all the lead in the bilge and do a wooden keel, then you can go as deep as you want!

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lydia said..
Exactly the same physics.
Why the original Sonata 8s were one of the most seaworthy little ocean boats.
Most of the ballast was inside the boat.
Slow pitching moment.

Sounds like the Seaway 25! They had 600kg internal ballast.

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Jethrow said..
Just go IOR style. Put all the lead in the bilge and do a wooden keel, then you can go as deep as you want!

It was not just IOR
We took an well known IMS boat to Hobart in the late 1990s that had a wooden keel tip and internal ballast.

The big question is why modify?
Rather look for a boat that already has a a deep draft, and other less critical design details that you can change to suit your needs.

Note the recent example of a keel which fell off after some "after market" mods.
The original designer disowned the whole design after that incident, and I totally agree. Once you make structural changes to a design you own total responsibility for the whole design. In addition, the value might be much less when you need to sell.

Incidentally, the drift of this thread indicates to me that what you need is one of those designs with a ballasted shallow keel and a deep swing centreboard.

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lydia said..
Exactly the same physics.
Why the original Sonata 8s were one of the most seaworthy little ocean boats.
Most of the ballast was inside the boat.
Slow pitching moment.

agree : internal ballast has it's uses but definitely less righting moment than sticking it on the keel

i actually had one of these sonata 8s from brand new. nice little boats and certainly seaworthy, but the reason for the internal ballast was that the boat was designed and initially built as a centerboard trailer-able. only later was the fixed keel version introduced, and these outsailed the centerboard version easily

internal ballast was used a lot in IOR boats as the rule favoured high initial stability. unfortunately this meant taking the weight out of the keel, which led to low ultimate stability !

cheers,

We used to day 200 mile ocean racers in those little boats.
Imagine that now!
We learnt lot

Resolute Salmon designed by Britton Chance was probably the epitome of the internally ballasted centreboarders - winning the 1976 one ton cup in Marseilles. See here;

rbsailing.blogspot.com/2013/05/resolute-salmon-chance-one-tonner.html#:~:text=The%20winner%20that%20year%20was,in%20the%20previous%20four%20races.

A real handful downwind in a breeze...............look at that photo of her on the slips showing underwater profile and cross sections - fugly as..........it was designed to the IOR rule at the time and got the conditions to win...........never heard of again.

Of course Chance designed the 12m Mariner with the underwater transom, which didn't work...........old mate Ted Turner pointing out to him that "even tu_rds are tapered at both ends............"

www.nytimes.com/1974/06/30/archives/mariners-designer-aims-to-add-speed.html

Chance was part of the design group of Stars and Stripes 87 which won in Fremantle, along with Bruce Nelson and David Pedrick.

en.wikipedia.org/wiki/Stars_%26_Stripes_87

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Madmouse said..
If the boat is stiffer due to keel modifications it will impact the strength of the rig. Stiff boats require stronger shrouds etc as forces are higher. Less stiff boats by definition place less force on masts and rigs.

This!
Know of some boats that were built too stiff and actually stretched the toggles on the bottom of the turnbuckles ??

Yes but that's due to undersized standing rigging / turnbuckles / toggles not the boat being too stiff. The standing rigging and all components need to be sized based on the righting moment (resulting from the boat's designed stiffness) and rig geometry (chainplate width, number of spreaders etc).

Actually it wasn't!!
The company I work for built the rig and rigging to spec.
The boat was home built and more lead was put in the keel than the design called for.
It was also the first of a new design and I believe even after weight was taken off the keel the boats were stiffer than the designer calculated.

It is always about the engineering

We were meaning that if you increase the stiffness you may need to beef up the rig.