Adding a bulb to keel

Keel Design: useful links

http://www.vacantisw.com/keel design.htm

http://www.vacantisw.com/foildesign.htm

http://boatdesign.net/forums/showthread.php?t=2224

http://www.google.com/search?hl=en&ie=UTF-8&q=keel design&btnG=Google Search

 

Thanks everyone for the input. I cannot lengthen the keel because draft is already as deep as my marina will allow. If I proceed with a bulb addition to the keel I will most probably have it designed/cast by Mars Metal and affix it myself. I will fair the bulb to the keel.
I don't think I can agree with the comments about the boat being noticeably slower downwind with a 200 lb. bulb on the keel. That is only the weight of one extra crew member. With 3 crew, fuel and gear aboard the boat weighs at most 5000 lbs. It flies a 275 sq.ft. main and a 650 sq.ft. spinnaker so handling the power and avoiding round-ups is the issue, not speed. The Laser 28 surfs/planes easily.

 

With draft restricted, a bulb is the only way to go, but I'm pretty sure you will go slower downwind, but it's difficult, if not impossible to say how much. There's only one way to find out - build it and sail it!

Good luck and remember to tell us about your results!

 

So you wan'na add a bulb huh?

There are many factors in deciding to add a bulb. I would agree with the previous caller noting that the added weight would be a hindrance downwind. You noted planning conditions.. Actually any extra weight would add to wetted surface area, which makes the boat more sticky, which is slow in low wind conditions and also slows the boat from breaking the water surface tension critical to initiating a plane.

But besides the added weight, a boat has to be able to work together in harmony. Your keel was designed to match the hull configurations, the rig, sail plan etc. changing it, if not done correctly could throw off that configuration and while you might excel in one small band of wind strength, all other areas may suffer making the project an expensive mistake.

If I were you, I would hire a qualified designer to look into all of these factors when planning a new keel.

The placement of the bulb could affect the balance of the boat. To far forward can induce lee helm (not to mention make the bow pound when going through waves) Too far back could induce weather helm. In compensation you may have to pull harder on your tiller creating the barn door effect. Then you have the weeds factor. If you sail in area with lots of kelp, a bulb could become a hook for dredging the lake and cleaning it of kelp.

Besides the over all weight, there is the added surface area to deal with.

Pitching is another factor. When you move the weight away from the center of the boat, you add to power robbing inertia. In waves, this is critical.

I know you don’t like the idea of adding depth but if it were my choice, I would. Its not as simple as bolting on a bulb or section to the bottom of the keel.

Keel performs lift. If its deeper, you don’t need as much thickness (chord). For instance if you added a section to the bottom of a keel (weight aside for now) you would in theory, add to its lift. However it might only work as slower speeds. As you go faster, now you have added drag.

The designer of the boat would have made his calculations for this when designing the whole boat, rig etc.

If you add depth, you have to reduce the chord (and keep the wetted surface the same) This means the entire keel changes.

In 1978 there was a C&C 38 called Peppermint. They did lots of studies on this and after much careful planning, removed the standard keel (which then drew 6.1) and replaced it with a new, daggerboard type keel that drew nearly 9 feet. The result was a way faster C&C 38. (they may have modified the rig as well to compensate – some time ago I forget).

There are also structural issues to look into.

Another factor, winds are not the same as they were 30 years ago. Today its far more radical. It puts more onus on a design to be good in many (or a broader) range of conditions.

There is going to be some expense associated with your proposal. I would suggest, if you are going to spend the time and the money, do it right.

Best,

Phil Nicols
Producer www.theartofsailing.com

 

I've enjoyed following the discussion this topic has produced but no-one has been able to specifically answer my original question so I'll phrase it a little differently.
Does anyone have actual experience of adding a modest bulb (200 lbs added to a 1500 lb keel) to the keel of a high performance cruiser racer, ideally to a Laser 28?

Laser details: Displacement approx. 4250 lbs, 5000 lbs with crew. Boat designed by Bruce Farr. Kevlar construction, SA/D of around 26.5 D/LWL of around 150.

 

....or simply take a f.g. casting off the bottom of the keel,to use for casting a new peice!?

 

Have you thought about water ballast? Instead of playing with the keel install some water thanks on each side of the boat to "replace" the crew. The good think is you can pump the "crew" overboard on downwind courses. Connect the tanks with a big hose with a valve in the middle. Before a tack open the valve so that the water runs down into the tank on the lower side. Close the valve and after the tack the water is where your crew used to be,

karsten

 

Hmmmm.Putting a bulb to the keel sounds to me a good idea.It is going to reduce KG distance (center of gravity) of the boat and provide extra stability. Unfortunately it will cause a slight increase of friction resistance.
Why don't u try putting 2 winglets with similar weight ? This will surely have better hydrodynamic performance.

 

That's a good idea but unfortunately it won't work in my particular situation. The Laser is a very open plan boat, with exceptional cruising capabilities for such a quick design. I don't think there's anywhere I could put tanks without severely compromising its cruising ability. There's virtually no bilge and very little space underneath the teak and holly floor - not enough space to run a big hose. The tanks would be well above the waterline so would only be helpful at modest angles of heel (perhaps counterproductive at 30 degrees or more)
Also, I am a CYA Instructor and I teach the CYA courses on this boat so another complication might confuse learners - plus there's the occasional gybe all standing when beginners have the helm! I think the heeling which would occur with a lot of weight on the "wrong" side of the boat would freak them out.

 

There a solent based Laser 28 that had a new keel it was done by Mark Mills (http://www.mills-design.com/other.htm) he spent a fortune on that and rig alterations will find out which one and let you no as I have forgotten its name.

What no-one seems to have mentioned is that if you make the boat more stable then you have to redesign all the way up the rig the chainplates standing rigging etc is all worked out on the righting moment. You'll be eating into your factor of safety.

Good luck and good racing

 

Thanks for the recent thoughts. The winglets idea is a good one. I think I would need some decent design work done for this solution and that would push the cost up substantially. Also, fixing a winglet in place would be a bit trickier than a bulb, wouldn't it? That's why I was thinking of a simple bulb - Mars Metal has already made one for a Laser 28 some years ago so the design work is already done.
With respect to the additional load that a bulb will put on the rigging, I realise that is a consequence. Having said that, Laser 28's are raced in OD with a 1002 lbs crew weight limit and are regularly raced in phrf with 1200 lbs or more of crew on the rail. They need lots of rail meat to be competitive in more than 10 knots of wind. Adding a 200lb bulb to the keel would be equivalent to one or two guys on the rail (depending on the angle of heel) wouldn't it? So as long as I sail with no more than 5 or 6 bodies on board I won't be pushing the boat too hard.
One more comment - I recently received an e mail from a Laer 28 owner who fitted a 200lb bulb to his keel 12 years ago, usually sails short handed as I do, and says he's has had no problems. His boat still regularly surfs/planes to over 10 knots with spinnaker on a broad reach.
To give you an idea of Laser 28 performance, I was out with four other guys in 20 knot winds a few days ago and we consistently hit speeds of 8 - 8.6 knots when reaching with main and genoa - old cruising sails at that. Getting beyond the theoretical hull speed of 6.5 knots is no problem.

 

Tend to question the statement you just made. 200lb on the rail is substainly more efficent on the keel than on the rail and it will substainly improve performance. Its a much larger lever than the half beam or less as on a laser 28 even in normal sailing angles.

Remember the rigs only going to go under shock loading like a broach or waves. It probably worth the risk as its likey to be within the factors of safety as most things in this industry are over engineered and it will improve the competive life.

I'd e-mail Mark he's a friendly chap and will help you out. Winglets and stuff like that are just thing to keep Naval Architects in a job and I feel make very little difference maybe half a tack and are very vunerable. What Plain Sailing said is correct you will have more drag with a bulb but you can get away with a thinner keel section that wil help compensate.

Hope this helps

 

Just be careful when adding righting moment that you don't add too much. If you do, you can exceed the rigs capacity and lose your mast. You could do a rough had calc and compute added righting moment from six 200lbers on the rail and try not to exceed that - at least for a 1st cut guess.