SailNet Community - View Single Post - Interesting Sailboats
View Single Post
  #4229  
Old 04-20-2013
PCP's Avatar
PCP PCP is offline
Senior Member
 
Join Date: Dec 2004
Location: Portugal, West Coast
Posts: 16,160
Thanks: 21
Thanked 95 Times in 79 Posts
Rep Power: 10
PCP will become famous soon enough
Chalmers study on bulbed keels.

Quote:
Originally Posted by Anders B View Post
My conclusions from the three Chalmers investigations are:
The first conclusion is that maximum thickness of the bulb must be placed well back from maximum thickness of the fin. Otherwise an extended lowpressure area is generated on the suction side (with contributions from both the bulb and the fin) that causes flow separation. Loss of lift is obtained and a really bad keel from the hydrodynamic point of view is obtained. Two T-keel designs were investigated, one with coinciding maximum thicknesses of the bulb and fin and one with maximum bulb thickness behind the rear end of the fin. The results clearly showed that the second one was best.

The second conclusion is the a flat bottom with sharp edges (chines) of the bulb is not a competitive design. A optimization was peformed where the shape of the bulb was allowed to vary from flat bottom (with sharp edges) to rounded. Rounded showed higher performance.

The Chalmers investigations were performed about 2 years ago, the results are public and should be known by serious designers (especially as Lars Larsson was the supervisor). Fixed keel design for recent custom yachts do not show an impact of this and my personal conclusion is that keel design is an ad hoc thing.
After looking better at it I found some more interesting stuff.

As you know they have made the fin keel in lead while all others are made of cast Iron (for the same RM and draft). This of course creates a problem because being of lead allows it to have a much smaller volume and less drag and they make clear that this one is only used for comparison purposes but they use it anyway comparatively with the others and that allows us some surprising results:




The Cast iron bulbed keel nº4 was the best performer, surprisingly even better that the lead fin keel that I thing everybody would thought would gave better results (because it was made of lead). Probably it has to do with the superior lift of the bulbed keel.

This is amazing and shows that an old designed lead fin keel, the type that was used on performance boats a decade ago has about the same hydrodynamic performance (or even worse) than a modern cast iron torpedo keel, the type that is used in some modern mass produced cruisers like the Hanse 415.

Of course if they have used a modern performance keel with the foil in steel and the bulb in lead, the performance would be much better in what regards drag.

But we are only talking about hydrodynamic performance since all keels had the same RM. In reality lead allows with the same drag of an Iron keel a much lower CG and that in fact counts a lot more.

The other interesting point related with the last one is that all keels had a very similar performance and the differences in time on the simulated race were really very small. Those differences are important in what regards racing but I don't think that they are meaningful in what regards cruising, even performance cruising.

They don't say that but it seems to me clear that the one that was clearly outperformed was the Fin keel (non bulbed). Regarding this one they did not even try to make an Iron fin because it would have to be so thick that it would have a huge drag.

But I really don't agree with that point in the methodology: If all keels were made of lead the proportion in what regards drag and lift would be the same as if all were made of Iron. Why use a lead fin when all others are iron? This introduces distortions in all study when comparing the bulbed Iron keels with a lead fin.

The more important conclusion we can take is that a fin non bulbed keel is clearly outperformed in all situations.

Fin non bulbed keels were and still are used in top boats on handicap races at high level not because they are efficient but because they permit to obtain better results in compensated time...and that is sad

Regards

Paulo
__________________

To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.

Last edited by PCP; 04-20-2013 at 05:32 PM.
Reply With Quote Share with Facebook