Shoal Draft v. Full Depth Keel

[Polux]

Originally Posted by El Pinguino

Same way I did/do I guess.

Jim, thats not a high aspect ratio fin on your boat..... this is.....

According to Polux... all boats are designed with ones like this these days.....

I never said that so... can you please re-post the relevant post...or retract yourself :-(

[Steadman Uhlich]

I recently came across a keel I had never seen before.

Bilge Keels with a bulb that looks like a torpedo.

[Polux]

If you like them, look at this Lerouge design, a fast boat:



Lerouge has a good paper about low draft keels. Sorry about the English but the guy is French:

"Technical answers (lead bulb at the end of a deep fin or even canting keels) are easy to find to reach the ultimate performance.
However, the real design challenge is to achieve the best possible efficiency within the cruising requirements such as shoal-draft, safe drying out, reliability or low maintenance.

A lifting keel provides stability and high lift. A possible choice for a small trailerable boat. Unfortunately, it becomes complicate to built, difficult to handle, expensive and hard to maintain when the size increases while the casing takes room in the accommodation. It is impossible to raise the keel under sail which restricts the access to the shoalest moorings.

A centreboarder with internal ballast shows a nice picture with the bow touching a white sand beach close to the coconuts. Unfortunately, its casing takes lots of room inside and its maintenance is sometimes difficult. Its biggest drawback is the need to increase ballast weight, hence displacement, to comply with the large angle stability required for offshore sailing or by the CEE rules.

The twin rudder centreboarder with an outside ballast keel is a happy compromise, which worked well on our designs. It is still more work to built than fixed keel solutions and a crane is needed to scrub the board.

Shoal fixed keels are the simplest solution but draft will never be shallow enough and in the best cases beaching legs will still be required. The poor aspect ratio of the single keel greatly compromises its windward ability.

Twin keels have many advantages:
-They are simple to built.
-Only one rudder is needed.
-No board casing to clutter the interior.
-Low centre of gravity of the ballast.
-Fool proof drying out.
-No need of a crane to maintain a board or its casing.

Bilge keels have unfortunately often been fitted to poor hull shapes, which spoiled their reputation. However a VPP study proved that this solution promised a good potential provided that the keels had decent aspect-ratio and that their tip were optimized to reduce induced drag and interference.
For safe drying out, the rudder must be protected by a skeg.

After designing several bilge keelers with bulbs whose handling was quite good, the opportunity to go further arised.
One of our clients commissioned us to convert his 36' ULDB with over 7' draft into a cruiser able to dry out without losing her sparkling performance!
After many different appendages tested in a VPP, twin winged keels obtained the best result, the LEROUGE Twin Keel System was born.

Ballast weight was slightly increased to keep the same stability although draft was reduced by 2' 6" and a small increase of wetted surface area had to be supported.
Performance prediction was similar windward performance in a breeze and less speed of the wind in light weather, which was easily cured by an increase of the fully battened mainsail.
Test sails proved the calculations. Steering seemed even improve in strong winds with the reduced heeling arm of the keels.
Drying out was obviously very stable.

Delighted by these results on a boat, which was never designed for such a small draft, we decided to continue our work.
The results are even more interesting on our hull shapes designed from the start for shoal draft with good low angle stability.
Our VPP simulations showed that there is a limit of draft under which performance are rapidly lost.
These same VPP even proves that a Twin Keel System is faster that a single keeler of similar draft!
A centreboarder is faster off the wind in light weather when it benefits from reduced wetted surface board raised. The Twin Keel System benefits from its greater stability upwind in strong winds. The increase of wetted surface is more than offset by an increase of sail area allowed by the better stability.

Our own and our client's voyaging experience proved that the most important is the ability to dry out easily without the need of legs or been alongside a quay and without risking to damage the hull, rudder or propeller. One must be able to scrub the boat without the need of a crane or marine hoist. The appendages must have the minimum of moving parts for total reliability....

We are convinced than the LEROUGE Twin Keel System answers to the program requirements of most cruising people. Therefore we propose it as standard on our holiday and voyaging designs.

We now have developed the LEROUGE Twin Keel System for ULDB which require high stability to achieve good windward performance.
The solution combines a bulb with the winglets to concentrate the ballast at the chosen draft.
The high-aspect fin is in composite.
The first sails demonstrated a good potential."

http://www.lerouge-yachts.com/mono_TKS.htm

[Steadman Uhlich]

Polux,

Thanks for posting the text with the Lerouge boat pic. Good to read that perspective.

And that boat looks striking! Cool!

[Jim Cate]

Originally Posted by Polux

I call a modified fin keel what they call a modified fin keel:
Gozzard Yachts Brokerage - Gozzard Yachts Brokerage.


Being bulbed has nothing to do with that (that is the same thing being bulbed or not), being just a very narrow racing foil with a torpedo bulb can have more problems. Most boats with a fin keel can stand on the keel for some time even if it is a lot more safe to take also hull support to prevent any hull deformation on the long run.

But one thing is to put the boat carefully over its keel, other is let him dry out against a pontoon over a somewhat irregular bottom and in what regards that a modified fin keel or a full keel give a lot more confidence and support.

The Gozzard blurb is a very nice description of how their keels developed, and I suspect that most designer/builders have a similar history over those years. I think that all "full keel or else" fans should read it, although I doubt if it would change their viewpoints much. Really, the only thing different about Gozzard's evolution is that it seems to have stopped sooner than many other designers.

But, I don't see how it relates to my keel or its ability to support the vessel on the hard without fore/aft balance issues. I dunno if it meets your "modified fiin" definition or not, and truthfully, I don't care. It does what it does... and of course I know that a very high aspect fin with or without bulb is a different design issue. You do seem to take delight in making those pronouncements that demonstrate our ignorance...

On a different subject, I find the Lerouge twin keel design to be very interesting indeed. It is wonderful what a client with open eyes and deep pockets can induce a designer to create. I wonder how expensive (relative to a conventional single keel) it would be to build? And, in the real cruising world, I wonder what happens to such a design if it encounters a bit of net or big kelp on ONE keel when at speed? The sudden drag on one side would seem to be a destabilizing factor that could be "interesting" in the wrong situation, like surfing down a wave face. Regardless, that sort of design would be entrancing to a sportive (to use your phrase) sailor who lives in a drying out environment. I bet that we will see more of these before long.

Jim

[Randyonr3]

Originally Posted by Steady Hand

I recently came across a keel I had never seen before.

Bilge Keels with a bulb that looks like a torpedo.

Those are torpedos.. used for Jet Ski s and wakeboard boats

[Polux]

Originally Posted by Jim Cate

The Gozzard blurb is a very nice description of how their keels developed, and I suspect that most designer/builders have a similar history over those years. I think that all "full keel or else" fans should read it, although I doubt if it would change their viewpoints much. Really, the only thing different about Gozzard's evolution is that it seems to have stopped sooner than many other designers.

But, I don't see how it relates to my keel or its ability to support the vessel on the hard without fore/aft balance issues. I dunno if it meets your "modified fiin" definition or not, and truthfully, I don't care. It does what it does... and of course I know that a very high aspect fin with or without bulb is a different design issue. You do seem to take delight in making those pronouncements that demonstrate our ignorance...
...
Jim

Jim, I did not invented the term modified fin keel and I posted about the Gozzard because they describe what is a modified fin keel.

Your Keel is not a modified fin keel. It is a non ballasted fin keel with a design that looks from the mid 80's. At the time that would be a very modern keel. Jeanneau and Beneteau used and popularized them.

[Jim Cate]

Originally Posted by Polux

Jim, I did not invented the term modified fin keel and I posted about the Gozzard because they describe what is a modified fin keel.

Your Keel is not a modified fin keel. It is a non ballasted fin keel with a design that looks from the mid 80's. At the time that would be a very modern keel. Jeanneau and Beneteau used and popularized them.

Non-ballasted??? What are the four tonnes of lead that are cast into the lower ~10 inches of the shell? Not quite the same stuff as a four tonne bulb? I will agree that the foil shape is not nearly as hydronamically clean or lift providing as a strut t ype foil can be, but I don't see what the real difference in having the four tonnes outside or inside of the keel shape at the same depth is in terms of ballast.

But to me, that's not the issue. We were discussing whether a fin keel is a good design for long term cruising in the sense of potentially standing the boat upon it for some sort of service. I reckon that my type is better than a strut and bulb type in that sense. There are some intermediate types, ie a hefty fin blade with a flattened bulb that offers some chance of supporting the vessel on the hard. I have no personal experience with them, but they appear to be a useful compromise.

Again, I think that our differences of opinion on this subject lie in our different definition of cruising. When you are never out for years at a time, and never very far from a travel lift equipped yard, the game changes dramatically. El Ping and others who post here have to deal with remote areas, and often for longer periods of time than your short seasonal cruises in European venues. This sort of exposure changes one's outlook on life and boat design parameters.

Jim

[Polux]

Originally Posted by Jim Cate

,,
Again, I think that our differences of opinion on this subject lie in our different definition of cruising. When you are never out for years at a time, and never very far from a travel lift equipped yard, the game changes dramatically. El Ping and others who post here have to deal with remote areas, and often for longer periods of time than your short seasonal cruises in European venues. This sort of exposure changes one's outlook on life and boat design parameters.
Jim

Sorry, a mistake. Not the what I meant to say, I meant bulbed not obviously ballasted.

Most of the time I take my boat out of the water with a crane not a travelling lift. Cranes that can support the weight of a boat like yours or Pinguin's boat are very widely available. A movable one on a truck will do the job.

A modified fin keel is much better than a fin keel, bulbed or not to support the weight of the boat on the circumstances like the ones Pinguin posted. Not only the surface of contact with the ground is much bigger, given more stability to the boat as the weight of the boat is better distributed by a longer area. If you have to do that with a fin modern cruising keel you can do it and in most cases it will not have any problem with that but is far from the ideal.

[Tafika2]

Keel Swap – Standard vs. Shoal draft Keel

We have a standard 6.988’ (2,13 m) deep draft keel on 2008 Jeanneau SO42DS. Has anyone on this site ever changed out the deep keel with the shoal draft 5.25’ (1,60 m) keel? There is a weight different between the shoal at 6,131pounds (2781 kg) vs the standard keel at 5,628 pounds (2553 kg). Since after I retire, most of my sailing will be in the Bahamas & Caribbean, I was “thinking” about changing keels. What might I expect from the performance if I made the change?

[robert sailor]

Originally Posted by Tafika2

Keel Swap – Standard vs. Shoal draft Keel

We have a standard 6.988’ (2,13 m) deep draft keel on 2008 Jeanneau SO42DS. Has anyone on this site ever changed out the deep keel with the shoal draft 5.25’ (1,60 m) keel? There is a weight different between the shoal at 6,131pounds (2781 kg) vs the standard keel at 5,628 pounds (2553 kg). Since after I retire, most of my sailing will be in the Bahamas & Caribbean, I was “thinking” about changing keels. What might I expect from the performance if I made the change?

Don't know your boat or the performance change but I do know that you will not point as high as you did in a deep draft boat. That said you may not miss this unless you are doing lots of upwind sailing.