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-   -   Twin Keel vs Bulb Keel (http://www.sailnet.com/forums/general-discussion-sailing-related/6601-twin-keel-vs-bulb-keel.html)

megman 03-28-2003 07:10 AM

Twin Keel vs Bulb Keel
 
Well I finally got my plans for the ''28 Hartley I am planning to build and it came with a Twin Keel version. Now I know the obvious advantages of a Twin Keel is stability in low tide and ease of storage.

But is there any benefits to actual sailing performance? Need lots of input as I am getting ready to power up the tools.

DelmarRey 03-28-2003 12:34 PM

Twin Keel vs Bulb Keel
 
I own a twin keel that I keep on a trailer for out on the lakes or around the Sound. Performance wise there are no advantages just disadvantages. Before the wind or a broad reach you will you get your best COG. On a close haul you''ll loose so much ground that you''ll be tacking excessively. Unless your keels are as deep as a fin. Then you’re dragging too much hull.
I do like mine for anchoring or close to shore adventures. She''ll sit right on the beach in a low tide.

Del........

Irwin32 03-28-2003 04:56 PM

Twin Keel vs Bulb Keel
 
The last issue of "This Old Boat" has an article by Ted Brewer on twin keel boats - or as he calls them, twin fins. I have not yet read he article, but Brewer would certainly know more than any of us.

Jeff_H 03-28-2003 05:56 PM

Twin Keel vs Bulb Keel
 
Generally bilge keels (the traditional US name for twin keels)have a couple major disadvantages. They give up a lot of performance and they are really a bear to unstick when you run one aground.

In most cases bilge keels have significantly greater wetted surface and frontal area than a fin keel of similar lift characteristics. This means significantly more drag. This negatively affects light air performance where the drag really can hurt performance. It hurts windward ability very dramatically in all wind ranges, and hurts heavy air performance where the greater resistance means that the boat can''t disipate the energy of a gust by accelerating and so heels more and also must carry more sail area to keep going in heavier conditions.

In sailing ratings between identical models except one has a fin and one has a bilge keel, the difference in rating is often as high as 30 seconds a mile which is a very significant difference, (all things being generally equal this is roughly the difference in performance between a 28 and a 35 footer).

Relative motion comfort and seaworthiness is widely debated and frankly depends on the specifics of the design in question. That said, bilge keel boats when of near equal weight generally have a higher VCG which would tend to suggest less stability and a less comfortable ride. This is sometimes offset by greater dampening created by the greater surface area of the bilge keels and by carrying more ballast.

Of course the deal breaker for me is the difficulty in ''unsticking'' one. When I lived in Florida I was a sailing instructor in a fleet of boats that includes both fin keel and bilge keel versions of the otherwise identically same English boat model. The performance differences were quite obvious to those of us who spent time jumping back and forth between the two keel types. But the real issue was what to do when you ran aground. Even though these were pretty small boats, when they ran aground they were planted. You could not heel them out. You could not pivit them or fishtail them out. You were aground and you had to hope this was not high tide.

Jeff

megman 03-28-2003 06:19 PM

Twin Keel vs Bulb Keel
 
I guess I have a lot more to consider than I thought. Was hoping the Twin Keel would be the set-up to go with.

And in all honesty, I''d rather have to mount 2-800lb keels instead of one 1600lb.

DelmarRey 03-30-2003 09:03 PM

Twin Keel vs Bulb Keel
 
Twin keel: They are very common amoung the English and Dutch who have a major tide change. When the tide went out the twin keel would sit in the mud wihout careening. And when the tide came back in they lift up out of the mud and back to normal.

And mounting a 1600 lb keel is really nothing. On my 40 footer, my keel is 6500 lb, and I just use a 10 ton forktruck to lift it up to the hull. If your going to build a vessel your going to need to hire in some equipment anyway to move it around, flip it over or step the mast.

megman 03-31-2003 04:05 PM

Twin Keel vs Bulb Keel
 
Yes, I realize either way some equipment will be involved.

Has anybody actually built their own keel?

I plan on constructing mine out of 1/8" steel, but melting and pouring 1600lbs of lead is going to be a chore. I am a profficient welder and own my own MIG, so that is the easy part.

geohan 03-31-2003 05:36 PM

Twin Keel vs Bulb Keel
 
FWIW,I saw a lead balast keel for a wooden boat being made in a welded steel form (melting pot)set up on concrete blocks and fired with charcoal using the exhaust blast from a vacuum cleaner. The dimensions of the form were about 1'' wide by 1.5''high by 6''long. A slow cool down minimized the shrinkage cavity and the extracted keel looked pretty good.

megman 03-31-2003 06:45 PM

Twin Keel vs Bulb Keel
 
Thanks geohan. Excellent idea.

The ingots I''ll probably be dealing with are 50-60lbs and 18"x4"x4" and this would allow a good continuous pour also.

Not too worried about shrinkage as it will just be filling the steel casing.

Jeff_H 04-01-2003 02:05 AM

Twin Keel vs Bulb Keel
 
Actually, you need to be very worried about shrinkage as the lead will pull away from the steel when it shrinks and will create a cavity that is perfect to allow crevise electrolysis in the cavity between the steel and the lead. Depending on the design of the keel, this cavity can be as large as and inch or more fore and aft, and a 1/4" side to side. Beyond that the lead is free to move within this cavity giving it momentum to shift and do damage in a grounding or shift and make a lot of noise in a rolling rodestead. The more traditional way to do this is to extract the lead after the pour, and to fill the cavity with a semi-soft material like hot tar or concrte (I don''t especially like concrete as it traps moistre against the steel) and then put the lead back in. This fill helps to restrain the lead as well as eliminate the cavity. Frankly it would be easier to extract the lead and not even bother with keeping the steel which will be a maintenance pain in the butt over the life of the boat.

Jeff


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