Adding a bulb to keel

[Laser28]

Hi, I'm considering adding a bulb of around 200 lbs. to the keel of my Laser 28 racer/cruiser. The boat displaces about 4250 lbs of which 1500 is in the keel. Draft is 5 feet. Boat is high performance with SA/D ratio of around 26.5.
The boat was designed to race with a crew of 6 but I often sail single handed or with a small crew, therefore the boat is quite tender and I need to reef early.
Does anyone have experience of adding a bulb? Will a bulb of approx. 15% of the keel weight stiffen the boat up considerably?
Thanks

[Dutch Peter]

Can't you experiment?? Fit different weights on your keel and sail with it, till you find the one you feel comfortable with and then fix it permanent.

About the amount you propose: 15% of keel weight, almost 5% of total weight.
I would say, yes, you start changing stability considerable.

[sorenfdk]

Be sure to check if the keel attachment (keel bolts, floors etc.) can take the extra load.
And also remember that when you do sail with a full crew, the loads on the rigging will increase too.

[dionysis]

Why put a bulb on the keel? With such a small amout of lead, you would be better off lengthening the keel itself with this amount of lead. You get the same added righting moment, with a little less induced drag, and you do it without the the drag associated with a bulb. A bulb needs carefull design if you are to gain righting moment without losing speed.

[Dutch Peter]

Originally Posted by dionysis

Why put a bulb on the keel? With such a small amout of lead, you would be better off lengthening the keel itself with this amount of lead. You get the same added righting moment, with a little less induced drag, and you do it without the the drag associated with a bulb. A bulb needs carefull design if you are to gain righting moment without losing speed.

I do notfully agree with you, Dionysis, for the same amount of rightning moment you need to add more weight when lenghtning the keel as a bulb is fitted much lower!
For what the drag is concerned, I 'm not to experienced with that, but it seams to me that lenghtning the keel will create more area, i.e. more resistence, then when fitting a bulb.
When the bulb is resembling a drop of water or a torpedo (same??) it shouldn't be such a big problem, should it?
And fitting the bulb is easier!

[dionysis]

Dutch Peter. I don't know the dimensions of the keel, so I may be talking out of place.

I do not fully agree with you, Dionysis, for the same amount of rightning moment you need to add more weight when lenghtning the keel as a bulb is fitted much lower!

I suggested that Laser28 lengthen - make his keel deeper - that is, shape the lead the same as the airfoil section tip of the keel. Since the keel will become deeper doing this the righting moment will increase just as much - in fact more that a bulb. (it's center of gravity will be lower)

For what the drag is concerned, I 'm not to experienced with that, but it seams to me that lenghtning the keel will create more area, i.e. more resistence, then when fitting a bulb.

The teardrop shape of a bulb has less wetted surface than the keel extension true - but it would be marginal (only a little bit).

When the bulb is resembling a drop of water or a torpedo (same??) it shouldn't be such a big problem, should it?

What about the area where the bulb joins the keel? - unless filleted carefully, could lead to more resistance. Think of the bulb at 3-5 degrees leeway.

And fitting the bulb is easier!

Could be right there.

Don't get me wrong. I think bulbs are wonderfull. If you have a draft limitation they are the best way to go, but without a draft limitation, you get less resistance for the same righting moment, just by lengthening the keel.

200 lb = 0.28 ft^3 = very small volume.

I may be wrong.

[Dutch Peter]

Originally Posted by dionysis

I suggested that Laser28 lengthen - make his keel deeper - that is, shape the lead the same as the airfoil section tip of the keel. Since the keel will become deeper doing this the righting moment will increase just as much - in fact more that a bulb. (it's center of gravity will be lower)

Yeaah, that's lenghtning too!! Sorry, didn't think of that.

If the area is just marginal effecting the amount of drag, isn't the keel-bulb connection having the same marginal effect. Resulting in a + for area and a - on connection radius thus, resulting in 0 on extra drag compaired to lenghtning?
Told you, it's not my area and I'm sailing for the "piece and quiet" on the water, so, not interested in .5 knt extra.

[sorenfdk]

I wonder why I didn't think of lengthening the keel...

Making the keel deeper will add area and (let's not forget this) increase the aspect ratio, making the keel more efficient as the coefficient of lift is increased. In other words, you'll be able to point higher. Off the wind, however, you'll go a bit slower.

The bulb will also increase the coefficient of lift, but not as much.

[Dutch Peter]

Originally Posted by sorenfdk

I wonder why I didn't think of lengthening the keel...

Yes, why didn't you??? You're the designer!? Something else on your mind, I guess.
So what is the best option? Or is the easiest option the best option, as without modeling or testing the arrangements it can not be determined?!

[dionysis]

The best option, from a hydrodynamic point of view is a deeper keel. (If, a deeper keel is no problem.)

The "proportionately less induced drag of a deeper keel / greater effective span of the bulb end effect" tradeoff must go to the deeper keel. The bulb will not maintain streamlines unless designed properly. You will get a lot more drag. It is not worth the trouble.

Anyway, you will get a better righting moment in the deeper keel. The bulb distributes its weight sideways as well as down since it has a circular section. The deeper keel option, since it is a thin section distributes its weight deeper than the bulb, and hence it's center of gravity is lower.

[sorenfdk]

Originally Posted by Dutch Peter

Yes, why didn't you??? You're the designer!? Something else on your mind, I guess.
So what is the best option? Or is the easiest option the best option, as without modeling or testing the arrangements it can not be determined?!

Most likely because I'm busy right now designing a bulbed keel! Sometimes, it's not that easy...

Apart from that, I agree with Dionysis. From a hydrodynamical point of view, the extended keel is best, because it will have a better lift/drag ratio. So if there's no restrictions on draft, this is the best choice.

[SeaDrive]

Originally Posted by dionysis

With such a small amout of lead, you would be better off lengthening the keel itself with this amount of lead.

Can you lenthen a keel without re-casting the whole thing? Mars is/was marketing "half-bulbs" to be bolted to the bottom of a keel. The original idea was to replace ballast weight after a keel bob - i.e. a shortening in favor of shallow draft.

p.s. I read recently on another board that Mars was difficult to work with. Any comment?

[sorenfdk]

Originally Posted by SeaDrive

Can you lenthen a keel without re-casting the whole thing? Mars is/was marketing "half-bulbs" to be bolted to the bottom of a keel. The original idea was to replace ballast weight after a keel bob - i.e. a shortening in favor of shallow draft.

p.s. I read recently on another board that Mars was difficult to work with. Any comment?

It depends on the keel material. If it's lead, you'll have to re-cast, if it's cast iron, you probably don't.
You could remove the keel (and as you may have to replace the keel bolts with new, larger ones this has to be done anyway), drill holes in the bottom, cut threads and bolt on the extension from its bottom. In other words exactly as you would do it, if you were adding a bulb.

[dougfrolich]

Laser28, Adding a bulb is simple, and it will help with your stability. The added drag will be overcome by the ability to carry more power in the sails. The Laser 28 is a fine boat, and will benefit from the modification. I would attatch the bulb in two halves, split longitudonally. Make a template of the tip section were the bulb will go. Shape the inside face of the halves so they match the tip section, and thru bolt them together, then do any nesc. fairing with thickend epoxy. Happy sailing.

The added drag of the increased surface will be more than overcome by the increase in aspect ratio (sailplanes have long slender wings for good reason).

Just take the leading edge, the trailing edge, and a few lines along the span and extrapolate them, then fair the intersections of those lines with horizontal (chordwise) planes to get the same airfoil shape.