Hello All, I have read through this thread with great interest given that I just finished installing a brand new pair of rolling chocks on my benford 40 full displacement trawler (MV Bliss).    Before taking the plunge I spent two years researching the subject.  It still amazes me that given the age of this technology so little technical information is out there.  There are a few interesting technical papers such as https://www.sciencedirect.com/science/article/pii/S2092678218300426 which contain valuable information but for the most parts most of the posts out there are of the form..."I pasted these things on the bottom of my boat... and they seem to work..." So... because I was quite unsure about how well bilge keels work... and before spending good money to have them installed I thought it would be best to hire naval architects to perform fluid dynamic modeling on my hull with fins mounted at different locations at different speeds comparing results with the plain hull without fins under the same conditions. I must say the results are quite illuminating.  Bilge keels are big and proper installation is essential to guranty performance and to minimize detrimental effects. Let me thank the folks at Orca3D.com for doing a great job with the modeling and Jay Benford for providing digital copies of the hull to the modelers. Here are things that came out of the molding. 1. Placement on the Hull is critical.  If done correctly the amount of drag added is just the surface area of the fins.  For displacement speeds this results in very little fuel consumption and additional power.   For Bliss with a 35' waterline running at 7.2 knots the difference in HP 11.9 without fins and 12.5 with.  At 6 knots the difference is 5.0 vs 5.3 HP.    For me that is less than 1/10 of a GAL per hour. 2. the Fins should be placed on the Hull stream line and not all stream lines are equal.  So.. its important to model the fins on different stream lines to find the location of turbulence and least resistance.  For example using the example above the difference at 7 knots between two adjacent streamlines was found to be .2Hp.  Not much but best to find the stream line with smallest impact. 3. the edge is important.  What causes the later resistance to the roll is turbulance along the edge.  Therefore the sharper the find edge the better.  My fins have an edge which is 1/2".   Increasing the edge width drastically decreases the performance of the fin.  I have seen pix of bilge keeps made for 2x6" planks with edges that are almost 3" wider and quite square (as opposed to sharp)... these fins will do little to reduce roll.   My understanding is that the Navy specs 1/2" steel plate for the edge of the rolling chocks placed on their vessels. 4.  The fins should not be installed too close to the bow. This can negatively impact pitch performance of the vessel.  The should be at least 30% (if not further) aft from the bow.  Similarly you don't want them running too far aft although the impact is less further aft then it is forward.  Rule of thumb is for the fins to be be about 30% the LWL and centered round midship. 5. The fins do not need to be very wide.  In my case they are 12"... 10" would have worked as well.  Again... the model will show what is best.  Making them to wide creates drag with little performance improvement.  Mine are 13'x12"x 2->1/2". see attached. Stabilizing small boats is difficult... Bilge keels provide a simple (and fairly inexpensive) way to achieve this.    In addition to the keels we also have a SeaKeeper 5 which works remarkably well. Our hope (and expectation) is that the keels will work in conjunction with the SK to provide even a more stable ride when surfing down big waves and serve as a backup if the SK5 were to fail.  We will report back when we have more experience running this combo. For details on SK5 performance on our displacement vessel you might refer to https://www.panbo.com/bliss-100hp-diesel-powers-trawler-seakeeper-and-huge-battery-bank/ In the mean time I am happy to answer any questions. Take care and stable sailing. --luis