# From winged keel to bulb keel

Discussion in 'Sailboats' started by Mad for sail, Nov 15, 2011.

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### Mikko BrummerSenior Member

Actually, my VPP allows for "heel dependent keel drag", even if I maybe saw it more as induced drag or hull-keel interference drag than wave drag. From the VPP manual:

"Accurate calculation of the keel drag and sideforce is very difficult, but crucial to the performance prediction, since it involves also leeway. Professional VPP breaks keel drag calculation into two components: The drag due to side force (vortex induced drag, drag due to lift), and the profile drag. These two are interrelated, since profile drag also depends on keel lift.

The induced drag of the keel is summarized into determining a factor introduced by the author, called span efficiency (see also Keel-derived parameters). This is a complex factor which depends not only on geometric properties of the keel, ie. aspect ratio, taper-ratio and sweep angle, but also to a great extent on the attitude of the boat, ie heel angle and the boatspeed (Froude nr). The reason for this is that as the boat speed increases, the hull wave system creates a through midships. This brings the keel root nearer to the surface of the sea, and results in a loss of efficiency. The heeling over of the yacht increases this effect so that in strong winds the keel can lose almost a half of its efficiency compared to light wind conditions.

The calculation method used by the VPP is based on systematic series of tests in a towing tank (see ref. 1, bibliography), as well as some theoretical extrapolation. It should able to deal correctly with very high sweep angles and low aspect ratio (often found on oldfashioned, long type of keels), as well as inverse taper (an "upside down keel” in style of Australia II), but no provision is made to deal with wing keels. The drag increase of the eventual bulb is added in a simple manner based on frontal area. The relatively important hull-keel interference drag is included into the span efficiency, since it was not discerned in the formulation of the equations based on the systematic series. The profile drag coefficient depends on leeway angle and profile thickness%. The Reynolds nr (Rn) of the keel effects both the keel lift and drag properties. For lift, the influence comes through the profile lift curve slope, and for drag Rn has a direct effect on keel profile drag coefficient.

VPP deals with the side force balance between the hull and the sails as follows: First, the rudder generated sideforce is subtracted from the sail induced sideforce. The hull-keel combination (without the rudder) is balanced against the remainder of the aerodynamic sideforce by adjusting the leeway angle. Note that leeway angle also influences sailforces, since the apparent wind angle decreases as leeway increases."

The VPP manual was written in a page layout program not running anymore on existing computers... at some point, I've extracted the text of the manual, and include it here if someone would be interested. It may shed some light on the physics of the sailboat. The illustrations are missing, so there's a lot of blanks in the text.

Yes, the simulations shown here were run with the same code as that australian service appears to be offering. A RANS-solver very easy to use and fairly accurate it seems for all but high lift cases with adverse pressure gradients, where it fails to predict separation satisfactorily.

Open Foam is great, since it's free, but it comes at a cost: I've understood that setting it up for your computer system easily takes a month for the unitiated. And I wish I had those 1-2 days to spend sometimes, on boat design forum project ;-)

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2. Joined: Nov 2011
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### Mad for sailJunior Member

Mikko and Joakim, I am really impressed tge degree of technical discussion my keel proposal has triggered. This is the first time I use a forum to discuss my ideas and if I knew I would receive so much information I would have entered yars ago.
From the results so far I conluded that my keel is not that bad for a shallow one. Last week I had one of those perfect "brazilian summer's day" with 15 knots wind with zero waves when you can really trim the sails to point high. The hull is clean and we were three in the boat achieving 5,5 to 6 knots very easily. My speedo is not working so I cannot say about true wind but from the telltales I can tell you the boat has done well.
I have no right to ask Mikko to perform a third simulation on the proposed L Keel but would be interesting to see the results a it's about 400 mm longer.
For the last my engineer soul is a bit disappointed because there are several terms used here which I cannot follow. Would be nice to have a link to a kind of VPP and CFD glossary.

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### Mikko BrummerSenior Member

Don't worry, I'm just as interested in knowing how the L-keel performs as you are. Here's a link to the ORCi-VPP http://www.orc.org/rules/ORC VPP Documentation 2011.pdf - rather a technical paper, but it describes pretty well how a VPP works.

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### Mad for sailJunior Member

Thanks! I'll read carefully. Look forward to seeing the L Keel results.

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### yipsterdesigner

sure does and thanks for the reads
trial'd some cfd programs a year back and just when i got some handles on http://www.tecplot.com/ it expired
forgot why/what was hard with open foam but feel i want to check again

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### Mad for sailJunior Member

After reading the ORC document some concepts became clear to me as how to define the correct keel. I have made the following execise:
- used a Vacanti Demo SW to create an all new keel similar to J111
- applied the 223Kg Lft force to define the lee angle
- based on the new lee angle extract the new drag force fro the SW chart
The result is that the new keel is able to generate the 223 kg at 4,5º instead of 5,3º from current wing keel. At this angle the drag force would be reduced from 43Kg to less than 6 Kg.
If the numbers are correct I should them go for a new loop where the new drag will allow a higher speed and so on? It means that the boat would point higher and faster with a new keel and I am wasting your time asking for so many simulations? ;-(

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### JoakimSenior Member

Yes a deep keel is much more efficient. That's why racing boats have deep keels. But now your keel has already a 1.94 m span (~2.4 m draft) and it doesn't use the current keel fin. That's a totally different concept and you probably need to make some modifications to the hull as well, since that short and narrow keel fin will load the hull much more.

The values 6 kg and 43 kg must be just for the induced drag (note how the drag approaches zero in the graph). You must add friction (or foil drag) drag to these.

How did you calculate the 43 kg value? Or is it from the CFD data by Mikko? That included friction, which is probably around 12 kg for the wing keel and 7 kg for your newest design.

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### Mad for sailJunior Member

Yes, I have used the Mikko´s data to make a rough calculation.
I just wanted to know how better the new keel would be.
I am very impressed ...

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9. Joined: May 2006
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### Mikko BrummerSenior Member

Results for the L-keel. With 40 cm more draft, the L-keel appears to perform really well: it needs a leeway of 4,8 degrees only to produce about the same side force as the wing keel, and at considerably less drag. Draft is indeed an important performance factor. In the last attachment a summary of the forces on all keels.

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10. Joined: May 2006
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### Mikko BrummerSenior Member

Adding a rounded bottom to the L-keel improves performance very little compared to the flat bottom L-keel, even if draft is increased by 100 mm. The flat bottom L-keel seems to be best if draft is limited, or penalised by the rule, although my guess would be the rounded bottom keel will perform better in a seaway, when the keel is moving around a lot.

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11. Joined: Nov 2011
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### Mad for sailJunior Member

Mikko,

nice work. on my previous post I was wondering I should quit this discussion and go to an all new keel as the preformance from a keel 1,9 long was significantly better than the bulb or wing. The new results have given me hope to continue with my quest ...
Some questions:
- did you reduce the span as discussed?
- we have mostly discussed the induced drag what about the friction drag? what should be those keel performances downling? should I assume that the different projected areas from the 3 keels will define the friction drag?
It´s summer here and I still have about 5 to 6 months to winther when I plan to change the keel. Besides your simulations I am considering to make some "home made tank tests".
Therefore I am building a 20:1 hull and will cast the wing and L keel models from SLA samples ( perhaps dóesn´t work but will be a nice toy for my office .
Would you consider to simulate those keels downwind?

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12. Joined: Nov 2011
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### Mad for sailJunior Member

my little boat is evolving fast. I wish Santa Klaus could bring me a tank!

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13. Joined: Nov 2011
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### Mad for sailJunior Member

New year ... new challenges! The hull model is ready and I have just received the keel. Keel was made in a 3D Printer usind the IGS file used by Mikko at the CFD calculation. Next step is to melt the model using a Sn_Pb alloy and assemble it on the hull model for some measurements.
The simulations made by Mikko made it clear that the wing keel was not that bad so my conclusion is that instead of cutting the current keel I must consider an all new L keel as discussed before. CFD analysis was very important to me to understand the flow beahavior of those keels and give me confidence on going ahead with a new keel.
I am working on some keel designs and intend to generate 3D models and print some prototypes. My target during the coming weeks is to be able to test at least one new keel shape in the same model hull and compare the results.

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### CRMBoat Builder

Mad for Sail, Brazil.

Jumping in. ---->I've found a Masters thesis of a study on bulb keels "Investigation of Keel Bulbs of Sailing Yachts" Goteborg, Sweden. http,Publication Bjorn Axford.pdf. report no.x-11/263.

Involved here is work with Larsson & Eliasson, authors of the fine book "Principles of Yacht Design. In English, it 'Masters thesis' reads well, and as a 'technical work' all the supporting math is found enclosed buttressing conclusions, naturally a part of it's 68 pages.

Recently I've recaptured a table of offset off a Laurie Davidson keel I developed back in1990, for a IMS,MORC type 29 footer. Owning the production tooling of the boat a renewed interest in this fine little cruiser/race had me searching for the old cement keel mold. No luck running down the long lost mold. I luckily took off the keel foil and profile shapes during a recent haul out. Bang, Bang the foil is within 1/32nd of 64-series NACA foil after these past 20+years. The original working drawings I used didn't state the NACA foil shape, as is the usual shop practice.

What is relevant here, was that I found genius behind the design of this bulb keel. The utter perception of Davidson working back in 1989, without CDF, wind tunnel tests, etc to verify, and predict his design is the courage of a master. The paper I referred to above clearly matches the subtle preferred features I found in LD's keel. 1. A bulb type keel not a 'T' type. 2. Yet is a fin that incorporates circular bulb of 1000Lbs. of lead displacement. 3. Positioning it's 'bulb's' max dia. aft of the blades 64-12% foil. The bulb foil also a 64- but of 14%max thickness. 4. the root foil of a 0009/ 9% design I made of bronze 'T-plate' type for nest up into the hull.

I also see in Mikko Brummer's fine CDF flow line snaps support the performance prediction in Axfords' paper. However the subtle elements of these keels 'T' ; Wing ; Bulbs ; and Trap planner fin keels can be understood Thur the good works of all the above peoples efforts. One last, listen to Paul B input, that fellow worked close to Doug Peterson and the bulb shape he commented on as a little off is valid.

Craig

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15. Joined: Aug 2009
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### CRMBoat Builder

Bulb keel.

Bulb keel.

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