Bulb design/positioning

Discussion in 'Sailboats' started by Julio767, May 3, 2024.

  1. Julio767
    Joined: May 2022
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    Location: Argentina

    Julio767 Junior Member

    Good morning,Julio from Argentina.

    I have a cruiser/racer sailboat that I’ve been optimizing.

    I replaced the original rudder, which was 9cm wide, with one made of divinycell that’s 5cm wide, with a NACA 12/13 profile (attached photos).

    The boat sails well compared to others similar in downwind conditions, but it lacks speed on close-hauled courses. After adjusting the rigging and various other factors, I believe the problem lies with the position of the bulb. It’s 1.5 degrees downward, so on downwind courses, water enters with that slight deviation. However, when sailing close-hauled with a leeway angle of 4 degrees, the water enters even more deviated, causing drag and reducing speed.

    From what I’ve gathered, the bulb should be positioned upwards so that it generates a slight lift on downwind courses and allows water to enter more aligned on close-hauled courses. However, I can’t find information on the recommended angle. Unfortunately, I can’t consult with the designer.

    Any help welcome.

    Julio
     

    Attached Files:

  2. wet feet
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    wet feet Senior Member

    I find most performance issues on close hauled courses have their roots in the sails rather than the foils.Have you been able to try an alternative headsail from a similar boat that has known good performance?
     
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  3. Julio767
    Joined: May 2022
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    Location: Argentina

    Julio767 Junior Member

    Thank you for the prompt response. Yes, I have tried other sails, and even the ones I have are from the same manufacturer and designer as the rest (North Sails). The sail designer, who also trims for internationally racing boats, has even come to sail with us and made some adjustments, but still, we sail about half a knot slower than we should be.
     
  4. wet feet
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    Location: East Anglia,England

    wet feet Senior Member

    That's interesting.I notice that the keel has a foil section with a very severe reflex in the aft portion,in fact the most severe i think I have ever seen.Do the boats you compete against have the same keel?Another point to consider is that the joint of the keel to the hull isn't the tidiest ever seen and may be a contributing factor.
     
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  5. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    Is that a lifting keel? If that is fixed, i say you are losing a lot of lift due to its short depth. It looks as though there is about as much wetted surface drag on that bulb as on the keel, only a foil really helps lift to windward. If that bulb was cast into an extension of the foil you would more likely get better lift=speed. I sail luggers, so im just throwing my non-existing race theory into the mix.
     
  6. CarlosK2
    Joined: Jun 2023
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    Location: Vigo, Spain

    CarlosK2 Senior Member

    Hola Julio

    Bonito Timón nuevo

    Keel bulb should be Horizontal or slightly inclined Upward = Leeway x sin Heel

    4 Leeway x sin 20 (?) Heel = 1.36 ... Up

    1.5 Up (!)

    Screenshot_2024-05-03-19-36-18-38.jpg

    1.5 Down (?!)
     
  7. Julio767
    Joined: May 2022
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    Location: Argentina

    Julio767 Junior Member

    Thank you all for the responses!!

    No, we only have this boat and another one with this keel, but the other one is a similar design but not identical. The boats here are usually designed for ORC and have shorter bulbs although with similar weight. The connection with the hull is not the best, but being narrow, the fiberglass couldn't withstand compression, so this solution was sought (assuming that the flow near the hull is slower and turbulent, so it shouldn't be too serious).

    It's not a lift keel, and it has a critical wetted surface. It was even smaller before and was modified because it would stall very easily. Previously, with less than 1/1.5 knots, the boat would stall and had a lot of leeway.

    Thanks!

    I'm not sure if I understand correctly. If when I sail close-hauled my heel is 20°, according to the formula, the bulb should be inclined 1.36° upwards. So, if I sail at 10° heel, the bulb angle should be 0.7 degrees? Should I measure my heel and leeway then to adjust the optimal angle? Today, I have it inclined 1.5° upwards, which means it's off by approximately 3°, and in 2 meters, that would equate to moving the tip of the bulb upwards by 10 cm?
    Attached is a diagram.
     

    Attached Files:

  8. CarlosK2
    Joined: Jun 2023
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    Location: Vigo, Spain

    CarlosK2 Senior Member

    Yes, the Upward inclination of the Bulb may be equal to =

    Pitch Angle + (Leeway x sin Heel)

    Buuuut, of course, with little wind and little Heel ... the Bulb is very much tilted upwards, that is why i have not added the Pitch Angle.
     
  9. Julio767
    Joined: May 2022
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    Location: Argentina

    Julio767 Junior Member

    Perfect, thank you very much. I should also consider, besides the heel and leeway, how the boat is inclined (pitch angle I understand). Generally, when there's more wind, people tend to move towards the bow to prevent it from lifting with each wave. However, I understand it's better to disregard that effect because otherwise, we would need to raise the bulb more, and then when sailing downwind, it would be very misaligned.
     
  10. skaraborgcraft
    Joined: Dec 2020
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    Location: sweden

    skaraborgcraft Senior Member

    I can imagine. There is good reason some boats also have "canards" to aid efficiency when the ballast bulb is on the end of a long yet narrow "keel" (more of a strut).

    Carlos baffles me with math, but his sums add up.
     
  11. Julio767
    Joined: May 2022
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    Location: Argentina

    Julio767 Junior Member

    Good morning everyone. I'm going to undertake the modification of the keel so that the bulb ends up with a 1-degree upward angle.

    Is there a way to estimate the potential impact on speed? I have all the ORC polar curves, and the VPP doesn't take this into account.

    If in the ORC VPP the bulb is replaced with a larger one to account for the increased area 'pushing' against the water, would that serve as an approximation? Today assumes it's perfectly aligned.

    thanks
     
  12. Howlandwoodworks
    Joined: Sep 2018
    Posts: 234
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    Location: USA MO

    Howlandwoodworks Member

    [/QUOTE]I find most performance issues on close hauled courses have their roots in the sails rather than the foils.Have you been able to try an alternative headsail from a similar boat that has known good performance?[/QUOTE]


    wet feet,
    I think you are right about the severe reflex and the joint of the keel to the hull isn't the tidiest.
    It's is hardly every just one or two things that are the problem.

    Julio76,
    Look at the photo from my easy chair it's looks like a down wind bulbed keel to me.
    Star Class | History https://starclass.org/history/the-development-of-star-boat-keel
    upload_2024-5-17_16-48-1.png
    Keel Bulb Design Software https://www.boatdesign.net/threads/keel-bulb-design-software.64702/

    https://vacantisw.com/2020/09/23/keel-and-rudder-design-methodology/?v=7516fd43adaa
     

    Attached Files:

    Last edited: May 17, 2024
  13. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    Without wanting to upset our OP too much,I have a feeling that the main reason for the bulb's presence is that the keel is quite short and a long,slim bulb is about the only way to get sufficient mass down there.That the boat seems to sail well compared to others is perhaps evidence that there isn't too much wrong with the bulb's shape and relationship with the flow of water.The sails are reported to be quite good and so the point I made earlier about a lack of sufficient area on the keel to generate lift may be worth investigating.It certainly reduces the result of the lift equation and I wonder if the keel cross section has a small operating window.Some guidance to what the section actually is would permit looking at the lift curves to see if there are particular characteristics that give some clues.One matter that does make me twitch a bit is that the new and rather impressive rudder appears to be a bit deeper than the keel and could be prone to scraping the bottom in really shallow water.
     
  14. Julio767
    Joined: May 2022
    Posts: 14
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    Location: Argentina

    Julio767 Junior Member

    Thank you all for your responses, I believe each one adds valuable insights.

    The keel was designed short because there isn’t much depth where I sail. Without such restrictions, it would have a draft of 2.2 meters instead of 1.6.
    It’s also true that it seems more suited for downwind sailing and lacks sufficient lateral surface. Originally, it had even less lateral surface but was modified. Previously, the boat sailed at the same angle as others but experienced significant leeway, which improved after the modification. Attached is a photo where you can see that the daggerboard was lengthened, increasing the surface area by 16%.

    It’s also correct that the widening at the hull junction is inefficient, but without it, the fiberglass wouldn’t withstand the compression. Is there any way to reduce turbulence there without narrowing it?

    Lastly, regarding the bulb, would you suggest adjusting it from 1.5 degrees downward to 2 degrees upward or at least level?

    Here's an article that, although it's about one-meter boats, talks a lot about this topic. It discusses how the boat's stern sinks the bow and changes the angle compared to when it's still, and the same happens when sailing close-hauled.

    Bulb cant https://www.onemetre.net/Design/Bulbcant/BulbCant.htm

    Thanks.
     

    Attached Files:


  15. Howlandwoodworks
    Joined: Sep 2018
    Posts: 234
    Likes: 88, Points: 38
    Location: USA MO

    Howlandwoodworks Member

    My understand is that the sailboat keel to hull and bulb root fillet shape is very important for reducing drag.
    What is a good sailboat keel to hull and bulb root fillet shape shape?
     
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