From winged keel to bulb keel

Mikko,
thanks for reworking the bulbkeel file.
We´ll rework the wingedkeel file and send asap.

 

thx Mikko but found tspeers answer on my halfbulbwing question and better expressions and designs for adjustable keel lift
like daggerboards, rudder on keel, foiling canters, special appendages and more

cant find much on such results tho, most seem to preform pretty well but at what cost, meaning fysics not money ;-)
i cant find much on that searching for: "stability increasing upside down submerged counteracting sails"

this "winged keel to bulb keel" thread is fascinating enough without my "oops"

 

just as additional detail I am attaching the file obtained during the keel scanning.
the winged keel dwg sent before is result of this model reworked by mirroring the fin through the vertical plan and wing at horizontal plan.
the first solid was used to calculate volume, area and CG and was the plataform to cut the fil, calculate the required bulb size and later join the fin and bulb pursuitiong the same keel properties.
If the new geometry is beeter or not I leave the answer to Mikko who is now assisting me with the CFD calculation ...

 

Some pics from a first run with the bulb keel... I first ran a VPP with the boat data I had, with a bulb keel, and obtained a boatspeed of 6,7 kn + leeway of 7,4 deg + heel 25 deg, to use for the CFD.

 

The VPP and the CFD-run do compare nicely, even if the VPP does underpredict the keel lift a bit, and the CFD again overpredicts (?) the viscous drag. The CFD run has no free surface effects, hence no wave drag, so the wetted area is different and the viscous drag is not right either. And of course the hull is just something with the correct principal dimensions and DSPL.

Note how big a contribution to the lift the hull is, and even the bulb has some positive lift. If you add the VPP wave drag to the CFD total drag, you end up with almost exactly the same total for the two.

I think all credit goes to the VPP I wrote almost 25 years ago... with the knowledge available at the time. It can do bulb keels but not wing keels, so I intend to run the wing keel as I get the model, and adjust the leeway angle until I reach the same side force as with the bulb keel. Then I should run both upright and slowly, to simulate light air. My gut feeling is the bulb keel without its wings is small in area (shallow), as Joakim predicted, 7,5 degrees of leeway is a lot.

The analysis is not dissimilar to the one I did on the Finn dinghy last year: http://www.finnclass.org/index.php?option=com_content&view=article&id=204&Itemid=220

 

Splendid! I´ll try to speed up the wing model to monday so that you can compare the results. In the meantime some additional questions:
- where does the bulb lifting comes from? The flatter it´s the better? Should I consider to add a beaver tail to improve the lift?
- how do you like the vortex at the end of the bulb? Is there a way to change it? If yes this would reduce drag without reducing lift?

 

A Thin bulb trailing end is a maintaence problem. The little nose bends and snapps off when aground and makes it near impossible to reverse out of a grounding

Winged keels in general are less than desirable. Far better to go with a convetional Scheel type keel. Put fore and aft rocker in the bottom of the bulb and terminate with a beaver tail. and be careful not to put so much lead down there that you overwhelm the rigging.

 

From the fin... It's called lift carry-over. The fin carries over some of it's lift to the bulb. Similarly, the keel fin transfers lift to the hull - most likely the hull would not produce as much lift without the keel ubder it.

 

Lot´s to learn yet ...
what about adding some few inches to the bulb ahead the fin, make it flatter and add the beaver tail shape? do you think it will help the penetration and reduce drag?

 

I'm not a yacht designer... perhaps you should consult one ;-). Basically, flattening the bulb will increase drag a bit (?), but lower the CG and improve stability. I believe a beaver tail would be good, too.

Maybe you should also consider what they call an L-keel (yours is a T-keel), where the bulb is all behind the fin, with the leading edge extending all the way to the bottom. That should improve performance for a given draft. On the other hand, if a little extra draft is no issue, the T-keel shoulb be more performing.

 

Yes, I have been thinking of it!
Instead of only a bulb I am considering to build a bulb with a bit of fin which is the additional draft.
Doing that I assume the bulb size will be sgnificantly smaller so then I can make it small diameter, flatter, longer and add a beaver tail.
I expect to have the wing keel model today so let´s check its performance first ...

 

With pressures shown, the fin-bulb and fin-hull lift carry-over is more obvious. Blue colours are lowered pressures (suction), while red is high pressure. From the dark blue colours on the windward side you can see how the keel fin extends itself onto the hull and the bulb.

 

I dropped the fin away to see what it does to the hull & bulb... You can see from the plots that the dark blue suction areas are gone. In the original run with the keel fin:

- Hull drag= 55,5 kgf lift= 108,0 kgf
- Bulb drag= 9,8 kgf lift= 24,9 kgf

Without the keel fin:

- Hull drag= 48,9 kgf lift= 59,2 kgf
- Bulb drag= 9,1 kgf lift= 0,8 kgf

Clearly there is some serious interaction there. The bulb does rob the fin some of its efficiency, so that for a similar draft the clean fin would be slightly more efficient. While my VPP allows for that, I had missed the fact that the bulb also has lift of its own. Hence the VPP predicts too little lift for the bulbed keel. Likewise, the drag of the bulb is underestimated since it is calculated for a non-lift (zero leeway) situation/bulb. Now I should be able to improve the VPP to take into account the bulb lift and the additional drag associated. An exercise not all in vain.

 

From a performance point of view deeper is definitely better. Do you have shallow waters or other limitations for draft?

T-keel and a torpedo like bulb are not the most effective choice with the small draft you have. A torpedo bulb is optimized for small drag vs. volume, but not for a good lift drag ratio on a beat.

The best option would be to replace the whole keel to the original deeper option or to a new design. If you are not willing to do that, the second best would be to make an extension to the keel and continue to taper to shorter chord and then add a L-shaped bulb that is shaped to function as a lifting surface (2D NACA profile with flat bottom). This would work OK for 20-40 cm deeper draft, but the keel would have too much area and not as good as a keel originally designed to this draft. With more draft you can make the keel lighter and later add weight to bilge, if necessary.

Here you can see some good pictures of the keel of the boat I used to have. It was very efficient for its draft and very similar is used in many other successful boats with 1.6-2.3 m draft. Look also the pdf files at the bottom in order to see the difference to shallow draft version. Note the much bigger chord and area (and drag!) of the shallow draft keel. http://www.bellamer.com/_su/tempo.shtml

 

I would suspect such a flat bottomed keel would not be ideal in a seaway?