Keel design advice for a 33’ performance cruiser: bulb shape

[Julio767]

Hi all, I’m designing a keel for my 33’ performance cruiser. I’m currently in a very similar boat using a 1.6 m draft keel and an 850 kg bulb (I’ve attached the ORC polars for reference). The new keel I’m considering has a 1.8 m draft and a 950 kg bulb, since the previous boat needs stability.

The keel planform I’m looking at is similar to the one in the attached image, with about 0.88 m² of lateral area.

My question is specifically about the bulb. I’m hesitating between:

• A traditional symmetric style bulb, or
• A less symmetric bulb I’ve seen on some designs.

I like the second option because it lowers the CG and seems like it could generate some lift at low heel angles. Do you have experience with these asymmetric bulbs? Pros/cons? What would you recommend for this type of boat and usage?

Thanks!

 

[gonzo]

The first thing to consider is whether the keel to hull attachment is strong enough for the increased stresses.

 

[Julio767]

The first thing to consider is whether the keel to hull attachment is strong enough for the increased stresses.

Yes. Since it has a larger keel (Kr in the graph) base and it will have an extra pair of keel bolts it won´t be a problem.

 

[fcfc]

Just some thoughts :

Beware of rating rules. If your boat is 5 sec/mile faster with yours mods, but get a 10 sec/mile penalty by rating rules, you will do a losing modification. Current fashion / trend either IRC or ORC is to have a bulb weight no more 50% of the ballast. See latest JPK / Pogo /Italia Yacht racing sailboats. Rating rules seems to give penalty to heavy bulbs because heavy bulbs means more expensive sailboats. (Or keel issues).

Increasing righting moment will lead to rig modification, since rig is classically sized from RM30.

 

[Julio767]

Just some thoughts :

Beware of rating rules. If your boat is 5 sec/mile faster with yours mods, but get a 10 sec/mile penalty by rating rules, you will do a losing modification. Current fashion / trend either IRC or ORC is to have a bulb weight no more 50% of the ballast. See latest JPK / Pogo /Italia Yacht racing sailboats. Rating rules seems to give penalty to heavy bulbs because heavy bulbs means more expensive sailboats. (Or keel issues).

I'm aligned with what you're saying, but I tend to think we have less righting moment than what might be optimal under ORC. The Italia 9.98 has a righting moment of 105 kg·m, while mine is 89. With this keel increase, I shouldn’t quite reach the righting moment of the Italia 9.98.

Additionally, there was a boat here with a 10% increase in keel (keel righting moment), and it had very good results over the two years it raced; afterward it was used for cruising and didn’t race anymore.

That said, taking your point on board, I think I’ll lower the bulb to 900 kg. I might not reach the Italia’s righting moment, but within my fleet it should be fine without being too different.

Thanks for your thoughts!

 

[philSweet]

That's a pretty big increase in lateral area. Do you also want to improve light air handling? Are you fiddling with sail area as well? I'm a fan of light air performance, but that cuts against the grain these days. I suspect the drag increase will completely offset the power carrying gains in moderate breezes. You generally try to use less lateral area if you increase power carrying ability. As you sail faster, you need less lateral area to produce the same amount of side force.

 

[fcfc]

There was an article , perhaps in seahorse magazine (I do not remember) , by Matteo Polli saying that increasing keel area lowers ORC rated performance (computed in steady conditions) , but in real life, in dynamic conditions (cited was tack maneuvering), it increases performance a little.

See there : OCEC 2023
Sensitivity to small keel area change


About stability, ORC onces rated stability as 1/2 measured RM + 1/2 rated RM. To give advantage to sailboats with better stability than rated RM. Then it 2/3 measured RM + 1/3 rated RM. reducing the effect of rated RM. And now it is only measured RM, since most sailboats also have to comply to ISO STIX regulations, mandated in ISAF OSR requirements.

Bulb weight should be written on IRC (not ORC) certificates.

 

[fcfc]

Back to keel bulb : Beyond this Chalmers University content and the document titled "COMPARISON BETWEEN EXPERIMENTAL AND CFD DATA APPLIED TO A BULB KEEL" by Jose L. Ortin Montesinos who deals the the very same keel bulb, with some further informations, there is not that much public information available about keel bulbs.

 

[Julio767]

That's a pretty big increase in lateral area. Do you also want to improve light air handling? Are you fiddling with sail area as well? I'm a fan of light air performance, but that cuts against the grain these days. I suspect the drag increase will completely offset the power carrying gains in moderate breezes. You generally try to use less lateral area if you increase power carrying ability. As you sail faster, you need less lateral area to produce the same amount of side force.

Actually, it seems there’s a big increase in lateral area because I’m starting from a keel I used with 0.62 m² of lateral area, but it was so thin that it had to be shimmed up to 0.72 m². Before adding that shim, the boat would round up below 1.5 knots when trying to maneuver in the harbor, and it had a lot of leeway compared to other similar boats. The boats of this model that sail best have 1.0 m² of area, although that’s too much for me, which is why I’m reducing it to 0.88 m².

One thing I didn’t mention is that I have a spinnaker and large asymmetrics, so my performance on a reach is good, but not so much upwind; that’s why I’m trying to balance the boat with better righting moment.

There was an article , perhaps in seahorse magazine (I do not remember) , by Matteo Polli saying that increasing keel area lowers ORC rated performance (computed in steady conditions) , but in real life, in dynamic conditions (cited was tack maneuvering), it increases performance a little.

See there : OCEC 2023
Sensitivity to small keel area change


About stability, ORC onces rated stability as 1/2 measured RM + 1/2 rated RM. To give advantage to sailboats with better stability than rated RM. Then it 2/3 measured RM + 1/3 rated RM. reducing the effect of rated RM. And now it is only measured RM, since most sailboats also have to comply to ISO STIX regulations, mandated in ISAF OSR requirements.

Bulb weight should be written on IRC (not ORC) certificates.

Thanks, fcfc.

What I’ve heard—but haven’t found anything to support—is that the keel type I’m going to use, with a large surface area near the hull that then tapers, is favored in ORC and, in practice, doesn’t penalize as much because the larger area is right next to the hull where there’s more turbulence and slower water flow. I think that’s why you see it quite a bit. The boat Katara, which had good ORC results last year, also installed this type of keel.

Back to keel bulb : Beyond this Chalmers University content and the document titled "COMPARISON BETWEEN EXPERIMENTAL AND CFD DATA APPLIED TO A BULB KEEL" by Jose L. Ortin Montesinos who deals the the very same keel bulb, with some further informations, there is not that much public information available about keel bulbs.

Thank you very much. Both articles are very interesting. In the Chalmers paper, they mention that upwind at 6.8 knots, 35% of the losses are due to waves. I’m trying to design a bulb that helps lift the boat as it rises and isn’t completely flat on the way down. I’m thinking of something similar to what they tested, but with a bit more curvature on the bottom, which is why I have so many questions.