Campbell Boat Owners Website


Updating Cavitation Plates to Stainless Steel

Article by Campbell Boat Owner Ron Fasola, owner of "Strange Brew"




My 1977 24 ft. Campbell originally had a Gale Banks 800 hp turbo motor in it, and therefore it came equipped with a larger 1 1/8" diameter prop shaft, a stainless steel prop, a prop strut that is moved slightly forward, and full aluminum cavitation plates across the transom, (which are controlled by an electric-over-hydraulic pump).  Smaller flat bottom boats that use cav plates usually recess the plates into the bottom of the hull and use a foot pedal to mechanically bend the plates for the desired adjustment of the plates.  The Campbell's plates are attached to the transom with a huge stainless piano hinge that has a 3/8" rod connecting the two hinge halves.  No plate bending is involved. 

Soon after I bought the boat in 1981, the ¼" thick aluminum plates were replaced to improve their dull looks.  The new plates were made out of the same ¼" aluminum, then polished, then hard anodized in "clear" color.


These new aluminum plates looked great for a while, but eventually, the hard water of Havasu got to them and they too, began to look dull.  Polishing them with Mother's was a chore, and while it made them look a little better on the trailer, they never looked like they did when they were new, and a few days on the water brought them back to square one in terms of dullness.


So in 2013, it was decided to make yet a third set of plates, but this time the plan was to use polished stainless steel in the hopes that never again would hours have to be spent hand polishing, and shiny plates would always be smiling.


It was possible to remove the old plates from the boat with the boat on the trailer by loosening the load on the turnbuckles and then removing the piano hinge rod.   








Researching stainless steel grades over the internet, it looked like 316 stainless steel was "best" for this application.  316 stainless steel is apparently used in the medical and food industries, and has the highest resistance to corrosion.  However, when trying to source 316, it became obvious that it's not that readily available, especially in the small pieces needed for this project.  Apparently, most of the stainless used in marine applications is type 304, (bow rails, chocks, most deck hardware), and 304 is much more readily available.


A decision also had to be made about what thickness of stainless to use.


·       Obviously, stainless steel is much heavier than aluminum, and Campbell's don't really need more weight.  The plates measure 13" x 41" each, and each of these plates in ¼" aluminum weighed 13 lbs. So regarding weight, the thinner the stainless plates, the better. 


·       In addition to the piano hinge at the transom, each plate is supported at five evenly spaced places by turnbuckles.  Looking at the specs of the much smaller Bennett trim tabs, the plates really don't have to be very thick.  Bennett uses 14 ga. stainless on some of their tabs, which is about 3/32".  This meant the thickness of the new plates was not going to be determined by needed strength. 


·       Since this whole project was being done basically for "looks", esthetics were important.  A thin "razor blade" of a plate, even if it performed fine, just seemed like it would not look "right".  Also, the plates end up being used as a first step to get on board via the swim step when the boat is either beached or in deep water, and thin plates would be hard on feet.


·       The third consideration was that the plates are bolted to the turnbuckle bosses with special 5/16" counter sunk screws, so that the plate bottoms are flush for water flow consideration.  Consequently, there needed to be enough plate thickness to allow for these counter sunk screws.  This proved to be the major determining factor.


·       Considering all the above, 3/16" thick type 304 stainless seemed to fit the bill.  Most metal yards wanted to sell a full sheet of this material, (about 4 ft. x 8 ft.), or maybe a half sheet, at crazy high prices.  A piece of this material was finally located that was deemed "scrap" because it was less than half a sheet.  "Scrap" is priced by the pound, and Schorr Metals in Placentia, CA, sheared two pieces to rough size for $180.  When finished, these 3/16" plates weighed about 29 lbs. each; which is 2 ¼ times heavier than the same sized ¼" aluminum plates!  (Perhaps that's why there are not too many stainless airplanes!)


The plates needed to be trimmed to the exact shape, which included rounded or radiused outer corners, a 92 degree cut on each plate, (caused by the V in the hull, where they fit together in the middle of the transom), and rounding or "easing" all edges.  Using an angle grinder, and fine files, the plates were finished by hand to almost exact size.


Because these new stainless plates were thinner that the aluminum ones, a 1/16" shim plate was needed under the piano hinges to ensure that the bottom of the new plates would be in the same place, (level with the hull bottom), as the aluminum plates were.  Two 2 ¼" x 41" strips of 1/16" thick 304 stainless were purchased and cut for this purpose.


Each plate is supported and controlled by five stainless turnbuckles that vary in length and are attached to the plates by bosses.  Glenwood Marine carries all the cavitation parts just about any boat will ever need, and they sell either chromed brass bosses or stainless bosses.  My original bosses were chromed brass, and only the bosses directly under the exhaust tips were dull.  The polished stainless bosses only cost $2 more than the chromed bosses, so ten new stainless bosses were purchased.  New special 5/16 stainless counter sunk screws that attach the bosses to the plates were also ordered.




To attach the plates to the piano hinges and to the turnbuckles, each plate needed nineteen ¼" counter sunk holes drilled in them.  Drilling stainless steel is difficult, and takes special tools.  Researching the internet confirmed that cobalt drills would be best to use, along with special cutting lube, a very slow drill speed, and a firm but not too firm, drilling pressure.  Basically, it's almost an art.  If the drill speed and/or pressure is not exactly right, stainless steel almost instantaneously "work hardens" and becomes almost impossible to drill and can easily ruin the drill bit.  Or, the drill bit can get very hot immediately with the wrong speed or pressure, which again can quickly ruin the drill.  All sources on the internet indicated that it was not good to see smoke during the drilling process, (which is the cutting fluid burning or getting too hot).  It was decided to first drill a 1/8" pilot hole, and then drill 17/64" holes to allow just a little clearance for the ¼" holes.    Cobalt drills were purchased from McFadden Dale hardware, ($2.70 for a 1/8", and $6 for the 17/64" drill).  The ¾", 6 flute, 82 degree counter sinking bit needed was only available in carbide, not cobalt, but supposedly it would work on stainless.  It was $65!


Experimenting on scrap stainless, it was found that watching the curl of the metal chip coming off the drill bit was the best indicator of the correct speed and pressure.  All the sources on the internet recommended a drill speed of about 775 rpm for drilling a ¼" hole in stainless.  The drill feed rate, (which translates to pressure, or how hard one bears down on the drill press handle), was given in terms of distance per time, which did not help in this situation because a manual drill press was being used.  Compared to drilling in mild steel, more pressure is needed for drilling stainless, and one can't let up or the metal will instantaneously work harden.  The cobalt drills worked fine, in conjunction with stainless cutting fluid, but drilling was stopped after each hole to both re-position the plate for the next hole, and to let the drill bit cool off.  A little smoke was unavoidable.   Once in a while, if the drill bit felt too warm, a wet rag was used to cool it off.  Counter sinking the holes was difficult; the bit cut fine, but it chattered on the work piece.  The slowest speed the drill press had, which was 300 rpm, was used with lots of cutting fluid, and the work piece seemed solidly held, but it still chattered.  Eventually, an old monstrous, HD ½" drill with a big D handle and cross bars from the 1950's that had an unknown but very slow speed was tried.  The plates were supported on 2x4's on the floor, and the huge hand held drill was leaned on with a best-guess pressure.  Cutting fluid was used, and the drill with the counter sinking bit was slowly "wobbled" as the counter sink cut was made.  That method worked just great.


The piano hinges were used as drilling templates, along with some clamps, to drill the needed holes in both the plates and the two shim pieces.  Naturally, the port and starboard plate hole locations were not exactly identical on the two plates.  Campbell's are indeed "custom", (although the piano hinges were bought pieces, and THEY determine the hole locations).



Although the stainless plates were covered on one side with a protective, plastic, adhesive film, and the protected side was fairly smooth, it was not polished.  Because of the size and weight of the plates, it was never even an option to attempt polishing them at home.  They were sent out to Sihilling Polishing in Santa Ana, and while they were in the shop, the other smaller parts, (both piano hinges, all nuts and bolts), were re-polished at home.


Polishing stainless is just like polishing aluminum, only about five times more difficult.  The procedure is the same; wet sanding with progressively finer paper, and then using a wheel with the appropriate compound.  Stainless is not really that hard and it scratches fairly easily.  Even Mother's metal polish, if used aggressively, was seen to leave fine visible marks on 304 stainless once a fine polish was established.  Progress was not being made with a 1/3 HP 1750 rpm electric motor equipped with a polishing wheel and stainless compound, so a switch was made to a 1 HP 3450 rpm motor.  That did the trick.  The bigger motor was hard on the polishing wheel, (lots of wheel material got thrown off), but at the higher speed, it could take greater pressure from the work piece.  The fact that almost immediately the parts got too hot to hold without gloves proved that work was getting done and polishing was taking place.  So it appears that high speed and high pressure, along with the correct compound, are the keys to polishing stainless.


The plates came back from Sihilling Polishing looking fabulous; they looked just like big mirrors.  It apparently took two polishers working together to hold each plate against their big wheels – they do it the time-honored, old fashioned way.  More modern shops would probably have used a moving polishing mechanism over the plates held stationary.  The cost was $175 for both plates, which seemed like a great deal.



Installing and adjusting the new plates meant that the plates needed to be clear of the trailer bunks so the up and down travel could be observed.  The boat normally rests on the trailer bunks with about four inches of trailer bunk extending past the cav plates.  So the boat needed to be reset on the trailer about 16 inches back from its normal position so that the transom would barely overhang the end of the trailer bunks.  The boat was taken to the launch ramp, and floated off the trailer the needed 16 inches, and the bow was strapped down tight to the trailer using the bow eye before being hauled out as a precaution against anything weird happening with this altered trailering method.  Then the rig was very slowly towed back the short distance to the work place.  Since the boat was now not supported by the front "V" in the trailer, one wouldn't want to tow very far or very fast like this.  When disconnecting the boat from the truck, a check was made to ensure that there was some tongue weight on the hitch so that the tongue would not fly upwards when disconnected.  Freddie Kuerner had warned that when Campbell "retrailered" boats to work on them like in this case, depending on the particular boat/trailer, the tongue could fly upwards when disconnected.  Although there was positive tongue weight with the boat moved back 16", the tongue could be lifted with two fingers.  Therefore the rear of the trailer was blocked so that it could not tilt backwards when one climbed into the boat and moved behind the axles.  The trailer usually carries a tongue weight heavy enough so that one person cannot lift the tongue, and moving the boat back "just" 16 inches had a very dramatic effect on the tongue weight.


The electric/hydraulic control rod coming out of the transom that turns the main cav plate shaft was seen to have a 3" stroke, which translated to plate movement of about 2 inches.  Using a straight edge along the underside of the hull, the turnbuckles were adjusted so that the plates were about level with the hull at half the control rod's stroke.  This means that the plates could move about an inch up and an inch down from level, therefore inducing a one inch hook or rocker in the back of the hull.  This set up provided enough control of hull attitude for this boat.  There are other Campbell's with Bennett trim tabs that can bury the nose deeper than this set up, but there doesn't seem to ever need that drastic a setting to me.  Because of the "V" angle of the hull at the transom, the plates come together closer when they are in the full "up" position, and the plates were ground and filed in place to fit as close as possible, without actually touching each other, when in the full "up" position.  It was recommended that the outer port and starboard edges of the plates, (when viewed from the rear of the boat looking towards the transom), should be adjusted by the turnbuckles so that they are slightly lower than the inboard edges.  This, in theory, should add stability to the boat while on plane.  However, at this time, it was decided to set the plates up straight, (from the center of the boat outward), using a straightedge.  A final check was made to ensure that the two plates were set up identically by measuring at various points from the main turning shaft to the back edge of each plate.


The final test run confirmed that the boat ran just as well as it did $800 before.







                        Personal labor & time                                 Priceless.00

                        Type 304 Stainless steel plates                            214.00

                        Final shearing & shim plates                                 75.00

                        Cobalt drills, cutting lube                                      90.00

                        Polishing                                                               175.00

                        Stainless bosses, spare E-clips, screws                238.00

                                                                        Total                   $792.00