roll reducing fins-a long post


richarddalaska
 

We have completed the installation and initial testing of roll reducing fins (aka roll chocks or bilge keels) on Lilliana. Since this topic has been of interest I will give a report from our initial test drive and overnight anchoring experience. I have uploaded several photos of the fins. They are i an album labeled “Liliana" in the photos section of this site.

cost: the materials cost was $1010 . this was dominated by 9 gallons of epoxy ($812). I could have used polyester for half that cost.
Labor was $1700 which covered one master fiberglass person and his assistant working for almost 5 weeks. They typically worked about four hours each day because the epoxy required curing time after application. 

Construction: Each fin is made from two shaped pieces of 3/4” plywood epoxied together. They are covered with three layers of fiberglass and attached to the hull with thickened epoxy and 3 or 4 layers of fiberglass tabbing. They feel very sturdy. Total thickness is slightly less than 2”. The outboard edges of the plywood were beveled to a 1/2” radius. The bevel extends about 2.5”. With hind site I wish I had made them knife edged because after covering with fiberglass the edge is probably about 7/8”. Sharper is better to encourage turbulence upon rolling.The fins are about 11” wide and 12ft long.

The most difficult part of construction is matching the curve of the hull such that fore and aft lines on the fins are parallel to the water and the outboard edge needs to be parallel to the center line of the keel. This is done to minimize drag as the boat  moves though the water. A more sophisticated shape would have the fins curved and following a streamline. However knowing the streamlines requires a sophisticated computer analysis. Luis Soltero (from the Trawlers and Trawlering site) had such an analysis performed for his non-willard hull. That analysis shows that toward the center of the hull the streamlines are not very curved. So I concluded that straight fins would be satisfactory.

Initial test: Since launching Lilliana, we went out for a test drive and overnight anchoring in a bay that would be open to moderate but short swell on the beam. I can make only qualitative comments on performance because there is no practical method to compare the motion with and without the fins.
1. The boat feels noticeably stiffer. When passing a side wake the roll is not very large and recovery is rapid. No  underdamped rolling. 
2. At anchor with beam-on waves of period near 3 or 4 seconds the rolling seems less than normal but the more rapid recovery makes for a larger deceleration. Some people might prefer more roll but slower recovery. Note that since the natural roll period of a W40 hull is about 3.5 secs. this is the worst possible type of beam sea at anchor.
4. Underway in moderate conditions, about 15 knots of wind and beam sea of about 2ft at 3-4secs, there is still considerable rolling, although it seems less than without the fins. The  only real measure of the fins effectiveness is to observe after  considerable cruising if we use the paravanes less than without the fins.
5. While rolling at anchor I could observe turbulent upwelling at the water's surface. This indicates that the flow perpendicular to the fin edge is turbulent, which is exactly what we want. The turbulence transfers the  energy in the rolling hull into eddies in the water.
6. Underway in beam seas there is an occasional thump that is felt under the hull. A friend with fins on a similar hull noticed the same effect. This may be caused by trapped air under the fins collapsing, or possibly trapped water making a sort of water hammer.
7. I anchored again in a spot with a longer period swell. Here the rolling was clearly less than normal. The sea of Cortez is a particularly nasty place regarding rolling and pitching because the typical waves come at 3-4 secs intervals On the ocean the wave period is at least three time longer.
8. I could detect no difference in the boat's steering characteristic. Backing out of our slip requires a a 90 degree turn to port in a fairway about 80ft wide. I could make that turn using the usual “bump and stop” technique. The boat's speed at 1650 rpm seems unchanged. So I am guessing that fuel consumption will not be noticeable changed. Note that the forward cross section of the fin is only 1/12 of a sq. ft and is tapered. That is much smaller than the area of a paravane. The surface area of the fin is a small fraction of the surface of the hull. The main drag would come from any change in turbulence in the stern wave. 

I want to mention that Luis Soltero gave me invaluable advice and encouragement on this project. I hope he will write up his experience with roll reducing fins for some boating publication.

These are just my first impressions of the roll reducing fins. My opinions may change when we use the boat more. At present due to the extreme (for me) heat we won’t be doing much boating until next winter. I can’t imagine how you boaters on the East Coast, especially south of the Chesapeake, can survive the summer.. I understand why so many people have air-conditioning on their boats.  

Richard



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Richard P
Willard 40 -Lilliana-Sea of Cortez, Mexico
Willard 30- Puffin- SE Alaska
Tiffany Jayne 34-sailboat- Dancer- SF Bay

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