On the Mark: Improvements - A Swim Platform


Designing the swim platform

The requirements for the platform design were:

  • Fit with the curves of the Riviera. This meant it would have to be curved accordingly.
  • Fit with adding spray rails / bumper guards along the rear cockpit. The amount of tumble home in the hull makes it hard to protect when rafting together with other boats. The spray rails would protect the hull from dings and dents.
  • Be easily removable. The boat can't be moved in and out of the garage with it on. The platform will come off for the winter as well as any refinishing work.
  • Low maintenance. This led to using Ipe for the wood and stainless mounting hardware.
  • Strong. It needs to support at least 500 lbs. That is 2 large skiers / boarders and equipment with minimal deflection.
  • Hand grips. Hand slots are very nice for getting on the platform and eliminating the need for another ladder underneath.
  • Roomy. Having 1 or 2 people on it and putting on a board or skis requires some room. The overall size of the platform is approximately 69" x 20".
  • Near water line height We wanted it low to the water so it would not interfere with the lines of the boat while underway and make entry easier.

The search for hardware

The next step was coming up with hardware to mount the swim platform. It had to look good, be quite stiff and removable. After scouring the web, posting queries on rec.sport.waterski, etc., I was a bit disappointed. Basically, either I could:

  • Order replacement OEM hardware. (e.g. from Mastercraft or Correct Craft) and make blocking to allow for the transom angle differences. This looked like about a $260+ option.
  • Go with permanent triangular brackets . These were fairly ugly and the non-removability was not good.
  • Make full custom brackets. I did not want to have to weld aluminum or stainless, so this was not a very attractive option.
  • Adapt / modify something else. After digging around at Kendor Marine (local marine parts and salvage parts dealer), I found some solid stainless heavy duty bimini top hardware pieces that looked like they would work. I do not have a manufacturer name or part numbers. Like many items at Kendor, they were unlabeled. If you need some, ask for the "heavy duty bimini top hardware".

The bimini top brackets are 2 piece assemblies that are made to mount to a flat surface and then hold onto a 1" diameter tube. The tubular end swivels in the rougly U-shaped bracket. A screw with a shoulder is the pivot pin. The shoulder prevents the threads from damaging the mating swivelling tube piece. These are fairly heavy and solid stainless. Some other units were merely plated zinc or Zamac which I thought would surely break under the loads that I would apply.

The forward edge of the swim platform is attached to the boat in 3 places. There are also 2 diagonal supports that hold up the outside edge. To attach the forward brackets to the platform, I had to make three metal brackets from 1/2" x 1" stainless. These pieces were notched to mate with the brackets at a 5 degree angle. This straightens the alignment to compensate for the curvature of the transom. Note however, that the castings are not terribly precise and each had to be machined slightly differently for a snug fit. Once they fit, the pivot hole was drilled. Each bracket is held onto the swim platform by several 2" #12 wood screws. The brackets are drilled and countersunk to provide a nice smooth look. The brackets were finally belt sanded to remove the mill scale and give them a nice brushed look. The only stainless bar stock locally available was #304; this stuff is terrible to work. It work hardens easily and is very tough on milling cutters and drill bits. The side brackets are 6" long and the center one is 12" long. They really could all have been the same length.

The diagonal braces are 1" stainless tubing and are approximately 14 1/2" long. It is easily cut with a standard plumbing tubing cutter. After final assembly and fitting, there are small set screws that lock everything together.

Total cost for the hardware including replacement bits was about $160.

Constructing the platform

The platform is made up of parallel 1&q uot; x 4" (3/4" x 3 1/2" net) slats for the top and then 1" x 4" support pieces arranged roughly perpendicular underneath. The construction is very simple lap joints, which raises concerns about rigidity and structural integrity. At each support point, there is a slat that runs the width of the platform. I did not want a support simply fastened to the top layer and all of the fasteners then held in tension.

There are seven front to back support slats on the bottom. They are arranged with two at each outside edge, one in the center and then one on each side, approximately 25% of the way in from each side to mount the diagonal braces. The slats are spaced 1/4" apart to allow for expansion and for water to drain off. The finger slot is created by spacer strips that are 3/4" wide and have the same 1/4" spacing between the planks. This leaves a new finger slot size of approximately 1 1/4" x 12". This is easy to grab and wide enough to prevent fingers getting trapped, which may occur if the slot was too narrow. I am not sure if this is a real consideration, but the newest ski boat swim platforms no longer have finger slots.

The two layers are held together with 3M 5200 and #12 1 1/4" flat head stainless wood screws. Screwing it together was probably the hardest part. The pieces need to be clamped together as the 5200 is very viscous and does not easily compress evenly for full contact. The Ipe is VERY hard and requires good pilot holes and countersinks. It can split near the ends if the pilot is not right. At each board junction I used 2 screws, placed diagonally across the joint to minimize the chances of splitting.error-file:tidyout.log

The area between the lower layer support slats is filled at the rear edge with blocks. This gives the appearance from the edge that the swim platform is a solid 1 1/2" thick piece. The lighter appearing pieces on the lower layer are the infill. The darker pieces are the ends of the cross support slats.

The sides are curved to extend the contour of the side of the boat. The piece removed from the front cockpit deck frame was used to set the curve. It was handy and looked about right. The sides step in about 1 1/2" in 18".

The back is curved slightly and the curve starts about 1/3 of the way across and cuts off about 1 1/4" at the outside edges. There are really no critical dimensions. Play with the layout while it is sitting on the bench or floor until you have something pleasing. The curves were cut on the band saw and with a sabre saw. The edges were planed and scraped smooth.

Scribing and mounting


Once the platform was assembled and brought up to the boat, temporary 3/8" spacer blocks were laid on the exhaust ports to support it above the exhaust. I did not want it permanently resting on the exhaust ports or rubbing against them. The outer edge was held up by a piece of plywood clamped vertically in my Work Mate. This setup was quite stable and then allowed me to scribe a line along the forward edge to match the boat transom. A spacer was used with a pencil to mark the scribe line on the platform. The scribe marking spacer was cut to width so that the line ended about 3" in from the ends. This means that the very ends of the platform are flat (not perfectly conforming to the transom), but has the advantage of avoiding small points that would break off over time. The curve was cut on the band saw and then scraped.

The forward edges were routed with a 1/8" round-over bit. This approximately matched the edge round-over of the planks. The rear edge was routed with 1/4" round over bit. The sides were left square for the moment to make alignment with the spray rails easier.

The assembly was again brought back up and braced in position. The outer brackets were temporarily clamped on and the holes were marked on the transom. The outer bracket holes were then drilled and sealed with epoxy. The brackets were bedded in 5200 and through bolted. The alignment is such that the bolts passed though the transom plywood and the curved interior frame pieces. This meant that interior blocking was not required. The center bracket position was simply drilled and screwed in place. It was not through bolted as it was not accessible on the inside without removing the gas tank (which was still full for the winter).

I like the idea of through bolting as it is easy to swing the swim platform into boats, docks, etc while turning and did not want it to come off. error-file:tidyout.logThe brackets were screwed to the underside of the platform with #12 2" wood screws and bedded in more 5200. Did you know that 5200 also makes a nice screw lubricant to assist in driving screws?

The angle brackets were screwed in place both on the platform and on the transom. The brackets on the transom are screwed through the transom into some very substantial interior framing and are simply held with #12 3" screws and bedded in 5200. error-file:tidyout.log

The above photo also shows the angle bracket and the top center mounting bracket as well.

The right photo is another view showing the cross piece that the diagonal bracket is mounted to as well as more clearly showing the infill piece at the rear edge.

Spray rails

The spray rails are cut from a 2" x 4" piece of Ipe (1 1/2" thick net). They are approximately 1 1/4" wide at the front and 1 3/4" wide at the back. They are scribed to fit the hull. This fitting took about an hour each as the sides of the boat curve in both the horizontal and vertical directions. Hand plans and scrapers were used. A curved scraper was very handy in error-file:tidyout.logdishing along the length of the piece to match the vertical curvature (tumblehome) of the sides. The Ipe is extremely stiff and there was really no pulling the gap shut with screws.

The spray rails were through bolted and fender washers were used on the inside to avoid crushing the okoume plywood. The spray rails were bedded in Sikaflex (they may have to be removed some day). The bolt holes were plugged. With the mahogany colored Sikaflex, the joint line is nearly invisible.

At this point, the final fitting of the outer contour was done by planing the spray rails to match the swim platform. The edges of the spray rails and swim platform were then routed with a 1/4" round-over bit. The above photo shows the joint between the swim platform and the spray rail.

Stainless half oval trim was applied for wear resistance. The cut ends were beaten flat (big hammer and steel plate). Next, they were filed to shape, sanded with 600 and 1000 grit emery paper and then polished. The half oval pieces were screwed onto the spray rails. I chose not to put any on the swim platform.


The entire surface is scraped smooth. All the corners are rounded over as well. The wood was coated with a couple of coats of Penofin. It is recommended by a friend of mine who works for the importer (http://www.ironwoods.com/). I have used this on outdoor Ipe furniture in the past. It makes for an amazingly good color match to the mahogany.

In Use


My wife says that the swim platform is the best addition I have made to the boat. It is so easy to get people in and out of the boat with out having to fool around with the ladder. We gained storage space, reduced hassle when changing skiers / boarders and reduced damage to the boat. Additionally. when we anchor out in the lake, the ladder would often come loose due to wave motion. This required that it be tied on or a ski handle draped over the tow eye. No more problems with that.

There aren't photos of the actual construction process. I was in a bit of a rush to get the boat in the water. With everything that needed to be done this spring, platform, pylon and refinish, I completely forgot to shoot anything until we had the boat in the water.

The photos on the boat were taken with a Sony digital still camera and underwater housing. We use this camera extensively while on the water. The movie clips are great for troubleshooting new tricks and teaching beginners what is going on with their deep water starts.

Next month

I will cover the wakeboard pylon installation and use. After that, the learning experience of stripping the varnish and refinishing with Imron.

We are very pleased to have Mark sharing his expertise with his fellow boat builders. If you would like to comment on this article to Mark, he can be reached at "builders at bronkalla.com". Replace the at with @. (Just trying to dodge a few spammers). See Mark's great web site for information about his Riviera project.