T&T: roll reducing fins

Richard P rpackard43 at gmail.com
Tue May 26 23:46:19 EDT 2020


We have recently completed the installation and initial testing of roll
reducing fins (aka roll chocks or bilge keels) on Lilliana our Willard 40.
Since this topic has been of interest I will give a report from our initial
test drive and overnight anchoring experience. If anyone wants to see
photos of the fins contact me off line since I don’t know how to post
photos to this site..

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.

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.

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 for me  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 a similar 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 90 degree turn to port in a fairway
less than two boat lengths. 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 Salter ( on this list) 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  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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