Blue Bill

BOAT REPAIRS & MODIFICATIONS

New Lifting Strakes for BLUE BILL

Note: The following was published in a slightly different form in as the first part of an article by Eric Sponberg called “Case Studies in Redesign”, PROFESSIONAL BOATBUILDER magazine, issue #45, Feb/March 1997.

Mr. Richard Oster of Providence, RI, owned a Wilbur 34 called Blue Bill which he bought new in 1986. A few years later, he upgraded the 3208 Caterpillar diesel engine to the turbocharged version, increasing power output by 80 HP. Blue Bill's speed increased from 15 knots to 21 knots, which was Mr. Oster's objective, but he also unwittingly gained a very bad spray problem. So much spray and fog engulfed Blue Bill's stern at planing speeds that Mr. Oster and his guests got soaked every time they went out. The boat was unusable.

Photo 2. Blue Bill before modification, full speed, 2800 RPM. The spray from the hull extends halfway up the superstructure.

Coming to me, Mr. Oster asked if spray rails or lifting strakes could be built onto the hull to reduce or eliminate the spray problem. He had seen other boats where this had been done—some nicely, others looked rather agricultural—and wondered if they really were effective. The quick answers were: yes, they could be built; and yes, done properly, they would be effective. Their shape, position on the hull, and structural detail was what I had to design and engineer.

Lifting strakes are different from spray rails in that, besides deflecting water and spray from the hull, they also provide dynamic lift--they physically help lift the hull higher out of the water. Spray rails only deflect spray; they do not provide any dynamic lift. Blue Bill already had spray rails at the bow and stern that, in fact, appeared to be causing at least part of the problem. I felt that lifting strakes would provide the proper cure.

There is very little guidance in technical literature regarding the design and configuration of lifting strakes. Even the U.S. Navy, which probably has compiled the most data from planing powerboat model testing and full-scale testing than anyone else in the world, has virtually nothing regarding lifting strake design. A few years ago, I asked one researcher from the Carderock Division of the Naval Surface Warfare Center, David Taylor Model Basin, in Bethesda, MD, how to design lifting strakes, and he eyed his outstretched thumb at me and said, "Put them wherever they look good. No one really knows, it's never been systematically tested."

I began by photographing Blue Bill at various speeds to determine the changing wave pattern along the hull. We ran Blue Bill back and forth in front of a photo boat at 200 RPM increments from 1200 RPM to full speed at 2800 RPM. While I took pictures from the photo boat with views from ahead, profile, and astern, the crew on Blue Bill radioed back the corresponding RPMs and boat speed. Back at the dock, I also measured the freeboards to determine the position of the flotation waterline. I mounted the photos on an artist's foam board and plotted the speed and power results on graph paper. From this complete picture of the wave profile along the hull and the speed/power performance curve, I could design the lifting strakes and place them in the proper position on the hull, based, of course on my previous knowledge of lifting strakes and using proper thumb technique.

We moved Blue Bill to Tom Denick's Island Marine Service shop in Wickford, RI, where, during the winter of 1995-96, he built the lifting strakes onto the hull. I provided Tom with a complete design package and materials specification. We leveled Blue Bill and marked the actual flotation waterline. I had made full-size cardboard templates of the lifting strakes at two-foot intervals from the stem to the transom. We cut these to their final shape where they would fit on the hull and glued them into place with a hot glue gun.

Photo 3. The lifting strake templates glued to the hull. We did this on one side only following the masking tape guideline running at the heels of the templates. Note the original spray rails above the templates.

Beginning at the stem, the section shapes of the lifting strakes had about a 3-1/2" wide bottom surface with a 15-degree down-angle. As they swept down the hull, the lifting strakes grew wider and the down-angle became less such that by amidships they were 7" wide with a dead flat horizontal lifting surface that continued all the way to the transom. We positioned them lower down than the original bow and stern spray rails. The bow spray rail was made of wood and was simply screwed into the hull. This we removed and discarded. The stern spray rail was molded into the solid fiberglass hull, and we left it where it was.

Construction required a fair bit of artistry by Tom Denick to carve foam core into the appropriate shapes in 4'-long sections, and then glue them onto the hull using epoxy high-density adhesive.

Photo 4. The first built-up foam section is ready for shaping. By reversing the one set of templates, both port and starboard lifting strakes could be cut and shaped. Female templates, shown taped to the hull, were also made to inspect the final section shape.

As directed by the plans, Tom filled and faired all the corners of the foam to the hull with epoxy resin and microballoons.

Photo 5. The foam sections are completed on the port side, and a strip of G-10 fiberglass is glued to the outermost edge for superior impact protection.

Then he overlaid them with heavy triaxial fiberglass fabric and epoxy resin, and vacuum-bagged the laminate onto the hull.

Photo 6. The fiberglass lay-up shown before filling and fairing.

After sanding, filling and fairing, Tom painted the entire hull from gunwale to keel and installed 1" wide stainless steel rubbing strips on the outside edges of the lifting strakes.

Photo 7. This bow shot shows the finished faired shape with a bit of paint on the port side to show Mr. Oster how it is going to look.

Photo 8. The job finished with paint on and hardware installed.

The result was so successful that I had difficulty getting the "after" photographs--Mr. Oster had Blue Bill in constant use all summer. Finally, though, I scheduled another photo session to see the differences in performance. The spray problem was completely eliminated, and at some RPMs the boat speed increased: 0.7 extra knots at pre-planing speed, and 2.2 extra knots at cruise planing speed.

Photo 9. Blue Bill after modification, at full speed, 2800 RPM. She rides higher out of the water, and all the spay is below the gunwale.

The hump speed (where the boat climbs onto plane) and the top end speed remained about the same. At slow speeds the lifting strakes are totally submerged by the waves formed by the hull, but they were having some effect because we noticed slight improvement in boat speed. But beginning at hump speed and continuing through to full speed, Blue Bill rides higher out of the water and all the spray remains below the level of the gunwale. We also noticed that Blue Bill is much more stable now both sitting at the dock and when running. She does not heel over nearly so far. The project was a complete success.

Photo 10. Blue Bill’s speed-power curves for before and after modification.

Blue Bill was originally designed as a heavy, slow-speed (less than 15-knot) trawler. Our natural quest for performance, however, sometimes takes us into new regions where we try to make our boats do things differently. Sometimes, it is simply easier to buy another boat. In this case, Mr. Oster enjoyed Blue Bill immensely and was loathe to replace it. We made a change to the hull that is not all that uncommon; it required only a bit of common sense engineering and good-quality boat building skills. I am sure many boats on the water would benefit from the addition of lifting strakes. The one consideration, of course, is cost. Blue Bill's modification for design and construction was something over $20,000. You probably would not recover that cost on selling the boat, but like Mr. Oster, you would get additional enjoyment out of it.