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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #1  
Old 02-22-2002
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Shortening Keels

I have heard on various email lists and other areas of owners shortening keels, attaching bulbs on the bottom, etc ... to stiffen boats and lessen draft.

My question to this group.... Is this a dangerous practice? Does it affect the seaworthiness of a boat by going outside the original design parameters, etc ...?

MH
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Old 02-22-2002
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Shortening Keels

Obviously. it really varies with the particular boat and the amount of the keel being altered. In theory if you are only shortening the keel a little and the bulb is designed to maintain so that the keel maintains the same center of gravity, then it should not affect the structure of the boat or its seaworthiness. It just won''t go to windward as well.

The problem is in the details. First of all a boat is generally designed with a certain amount of keel area that contributes to lift. When you cut a keel down you are not only reducing the area of the keel by what you cut off but also by the area of the keel covered by the bulb. You are also adding the additional drag of the bulb. All of that is bad medicine for upwind or downwind performance.

Then there are construction issues. Modern bolt on keels are constructed with ''L'' hooks in the end of the bolts. These are cast into the lead and are ''nested'' at different depths in the lead so that they do not interfere with each other''s bond to the lead. If you cut too much of the keel off, or accidently drill through one of these bolts when you drill to attach the bulb, you have reduced the strength of the connection.

With encapsulated keels the problem gets worse as you are cutting through the bottom of the encapsulation which is what holds in the ballast and then depending on secondary bonds to support your keel. You also potentially disturb the bond between the ballast keel and keel encapsulation which is quite difficult to repair.

In terms of seaworthiness, even if the boat ended up with the same stability, it would roll at a faster rate (smaller moment of inertia) and through a wider angle (because it looses the dampening affect of a deeper depth foil) with the keel depth reduced .

One minor point, asked if adding a bulb will make a boat too stiff. Stiffness generally refers to form stability rather than ballast stability and so adding a bulb may make a boat more stabile but not more stiff.

Regards
Jeff
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Old 02-24-2002
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Shortening Keels

Ahoy Mikehoyt, Yes and yes etc( what Jeff said), Next Such an expensive and risky change of a vessels performance would only be considered if the vessel were a tub to begin with or the owner had read to many articles and asked to many questions trying to talk himself into a nightmare. OK Big Red 56 the Pirate of Pine Island
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Old 05-01-2002
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Shortening Keels

In a recent sailing magazine I saw an article on this very subject. There''s a nice photo of the boat''s owner who has just sawed the bottom off his lead keel with a circular saw.

The keel then gets 2 big chunks of lead bolted to the bottom part, the void areas are filled in and faired and the boat now has a shallower draft and possibly a bit more weight if more weight was added than was sawed off. The boat''s now a bit stiffer and the keel is now shaped like a wing giving it better upwind performance.

A very nice retrofit.
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Old 05-01-2002
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Shortening Keels

One mimor point, while it is possible to add enough weight to end up with a similar stability, and wings can help some with lost performance, it is not really possible to add wings and end up with the upwind performance of the original keel. Upwind performance is a real balancing act between creating too much drag to overcome the minimal drive of being close to the wind and generating enough lift in the keel to prevent leeway. Wing keels generally generate more lift that a shoal keel but they also generate a lot more drag. So while they reduce leeway, the combination of lower lift and more drag means that the shortened keel had to give up some windward performance over the original keel. That said all the windward performance in the world does you no good if you can''t get into your slip because you are too deep.

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Jeff
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Old 05-02-2002
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Shortening Keels

Oh, lord, didn''t even know that people would try to jury rig a bulb keel from their original design!!!!

Work in a boat supply store now, so should be able to say I''ve heard everything to cut costs, but not this!! Are people really doing this?

Do people just not get it, or is it the want to have a certain boat and cut the price and make it something it''s not made for?

Sorry, but this is pitiful.

Mary
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Old 05-03-2002
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Shortening Keels

I think that keel swapping or cutting is not that uncommon. These days there are relatively few keel centerboard boats on the market and for people who live in a shallow area, the normal fin keel depths may be too much for them. Companies, like Mars Metals in Canada, make replacement keels and they make semi- stock bulbs and winglets that can be bolted through the bottom of a shortened keel. It is not that hard to do assuming you only want to take a little off of your draft and don''t mind giving up a little performance. Doing a keel cut is less expensive than paying a brokerage fee to sell your boat and many people prefer owning ''the devil that they already know''.

Jeff
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Old 09-11-2009
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Jeff, I read with great appreciation, your response about keel shortening. I understand that this modification has been done when converting sailboats to tall rigs in order to maintain the required C of G without compromising draft. If the original boat designer is involved in the conversion, would this be the recommended approach or do boat designers get coerced into keel shortening against their interest, in order to simply satisfy a customer?
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Old 09-11-2009
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If you are in an area where you are likely to run aground, a bulb is not a good idea - more surface area to get unstuck and a bigger lever on the bottom of your keel to loosen your keel bolts as you wiggle out of the muck.
Leave keel design to the naval architect (or backyard wantabee sketch artist) that designed your boat.
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Old 09-11-2009
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Jon:
There are designers who for a nominal fee will review proposed modifications to one of their prior designs and make recommendations as to the feasibility and modifications required to successfully alter the design. Not all designers will do this and not all boats that list a particular designer were actually engineered by that designer. It is one of those 'your results at home may vary' sort of a thing.

There are a whole range of issues in fitting a bulb, such as will the boat trim properly with the added weight placed where it is, was the rig designed for the increased loads implicit when you increase stability and adding a bulb to an unshortened keel will increase the load on the keel bolts, but more to the point will increase the loads on your boat's internal structural system and so proper engineering is important.


John Shasteen,

At least from the research that I have encountered, conversations with designers, and from conversations with towboat operators, while wings may complicate getting a boat unstuck, bulbs at the bottom of keels apparently tend to make getting unstuck easier than a simple fin and should not add to the stress on the keel bolts 'as you wiggle out of the muck'. As noted above, bulbs do add to the sailing loads to the bolts, rig and surrounding supporting structure.

Respectfully,
Jeff
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Last edited by Jeff_H; 09-11-2009 at 01:48 PM.
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