Sunday, August 7, 2011

Bilge Keels

In regard to a couple of articles I wrote recently, Mike sent in this comment. The comment was made in the context of some queries made about ballasting for my Whimbrel design (not yet available, but she promises to be an interesting boat).

An un-finished drawing of Whimbrel, showing her general proportions. As you can see, she carries leeboards, which have just enough ballast to sink the boards reliably, but are not intended to add significantly to the boat's stability.

Ross - I have often thought that stubby little bilge keels combined with leeboards might be just the ticket. The bilge keels could have through holes or threaded inserts in them. When ultimate stability in heavy weather was called for, a long heavy steel plate could be bolted to each keel. In light weather, etc the steel plates could be left off. And the little bilge keels would more or less allow her to sit level when the tide is out. If designed just right the boat could travel on a flat trailer or a traditional boat trailer. The steel plates could just stay right on the trailer in position to be bolted on or off as needed - no lifting or grunting required.
I am not any sort of designer or engineer but I have often thought that this sort of arrangement would offer lots of flexibility and simplicity and be budget friendly.
Feel free to shoot holes in my thoughts - I am just always dreaming of my ultimate boat on a budget.
Thanks Ross!

I read Mike's comment with interest, but my thoughts were limited by a prejudice I've long held against bilge keels. My understanding of the behaviour of bilge keels had been based purely on the comments of others, without any personal experience on my part. Not a good way to form opinions!

Picture of a typical bilge keeler, taken from a sales brochure on the web
My feeling had been that bilge-keelers (quite popular with the English) tended to be slugs - slow with poor handling, and only good for drying out level in areas of high tidal range. The theory was that the wetted surface area required for bilge keels would be substantially higer than for a well designed single keel. More importantly, I had been led to believe that the flow of water along a displacement hull swept in towards the centreline from the bow to midships, and then out towards the bilge or chine from midships to the stern. If you look at the hollow formed in the waterline of a displacement vessel travelling at speed you will get the idea.

A boat we built, running just over displacement speed - note the hollow in the waterline.
The problem shown up by this theory of flow around the hull is that while the bilge keel may be positioned parallel with the centreline of the boat, the water flow would not be parallel with the centreline. In effect, the bilge keel would be passing through the water at an angle to the flow lines i.e. being pulled through the water in a partially sideways fashion, causing great drag.

Now, I had accepted this theory without any significant thought, and I was going to write back to Mike with such an answer. However, to illustrate my thoughts, I decided to do a flow analysis on the computer using DELFTship Professional software with Whimbrel as the example. The flow prediction facility in DELFTship Professional is a much simplified system than that used in CFD (computational fluid dynamics), but the results are said to be remarkably similar to those gained from CFD, and in this case we only need to get an impression of flow.

A perspective view of Whimbrel showing the predicted flowlines
Well, to say that I was surprised is an understatment! The flow lines, at least on the undersuface of the hull, were almost parallel with the centreline. According to the computer, bilge keels would not necessarily be such a problem after all, as long as surface area was kept to the minimum for the required lateral resistance. There are clues to this when one looks at the performance of some of the current ocean racing designs which use bilge boards, which are in effect, retractable bilge keels.

Here is a link to an article from Bray Yacht Design and Research in Canada about the advantages of bilge keels

If anybody has more information on this subject, I am very keen to learn! In the meantime, I'll just wipe egg off my face....

1 comment:

  1. Ross - GREAT blog! Thank you! You could have chosen to just go with the established "group think", but instead you did some real homework/research that has actually validated my layman's ideas. Awesome! And thanks for the link to the bray yacht design article - very good reading and real food for thought.

    If you come across any research or have any insight into the relative efficiency of various bilge keel sizes and shapes, please share it. Most of the photos of bilge keels that I have seen show relatively short (as measured from fore-aft) and relatively deep (hull to bottom of keel) bilge keels. What is the effect of making the keels twice as long and half as deep? I am sure that some of the answer has to do with the shape of the hull below the waterline and the shape of the keels, but is there a way to actually measure the differences in the cost/benefit of various bk dimensions using computer modeling?

    And on a completely separate note, is Whimbrel desiged with a pram/scow bow? I think that is what I am seeing in your drawings, but I could be wrong?

    Thanks again Ross!