Fairing, Port hull

Let The Sanding Begin

Posted by Paul

Well today I had no choice. I started sanding the hull. Well started isn’t exactly accurate. I have done some sanding. But today I started the last stage of the port hull. Once it is sanded, faired and sealed, it is time to turn it.

After chatting with another builder (Alan building an Oram 44) and because he has used and recommended a Bohler sander, I have decided to give this new electric sander a try. I noticed it a few months back and wondered if it would be an effective sander for the boat. Using an orbital sander to remove the bulk of the bog is ok but is fraught with danger because an ordinary orbital has such a small footprint that it is very difficult to sand a large flat section without undulations. So the larger the footprint the safer or more likely you are to maintain a consistently flat surface. Furthermore, any sander that rotates has a sharp cutting edge that contributes to the risk of undulating rather than flat sanding.

The difference with the Bohler sander is that it is made up of 3 rotating discs the size of 1 normal orbital disc which in turn also rotate as 1 making the sand area around 300mm instead of the usual 120mm. Because all 3 rotating discs rotate the cutting edges are constantly in motion and this minimizes the chance of the cutting edge damaging the smooth flat surface you are trying to achieve. Finally, (and I didn’t know this until today) the Bohler operates at very low speeds, with each disc as well as all 3 rotating very slowly (only 100rpm). The Bohler sander is only available from Super Cheap Autos, which is not renowned for quality brand name products and as I had already killed a Ryobi sander I had my doubts as to whether the Bohler could stand up to the work rate needed to finish the hull. But to my surprise the sander works very well and seems to be able to stand up to the workload. The disc pads are a bit fragile (they sell spares) but other than that it works so far. The down side is it is slow and heavy.

bohler basebohler sander

I used the Bohler to flatten out the highs and lows of the bog and on the particularly high (thick) sections I reverted to the orbital, because it has a faster rotation speed it has a faster cutting action with the same grit paper. (I am using the coarsest paper I can get to start with W40). I managed to sand about half the outboard side of the hull today. So at this rate it will take me around 3 more days to finish the hull to the first stage of fairing.

The next stage is to fill the low areas (where the sander barely touches) with another thin layer of bog. Then I have to use the torture board to get a totally flat surface by sanding down the hull again but hopefully the bulk of the work will have been done by the electric sanders.

As you may know, I am making my own torture boards because the 3M ones are so expensive. I bought 2 cheap trowels because this was the easiest way I could think of to get the type of handles I wanted. In fact I searched Bunning’s and couldn’t find handles. I glassed them onto a 1.2 meter piece of Duflex the width of the sandpaper. I also got some dense foam from Clark Rubber to go between the sandpaper and the Duflex. I was planning to apply Hookit to the foam in order to attach the sandpaper. So far the trowels and the foam have only cost me $20 and 4 meters of Hookit (it doesn’t come in shorter lengths) is $30. I may skip the hookit and attach the sandpaper with blocks screwed to each end. Either way it is still way cheaper than a 3M board.

glassing handles onto torture boardglassing handleslines marked for sanding

I have marked the waterline and the line at the chine above the waterline on the outboard side of the hull. This chine is the only really important one. The rest are below the waterline and whilst I want to do a fairly good job, they are not visible (and painted over with antifoul paint) so I can afford to be less fussy. The plan is to sand to the line (on the above waterline chine) from above and below but leaving the line in place. This way I will have the line as a guide as it is very important for this line to be straight and level with the waterline. This line will be visible at around 500mm above the waterline when afloat so if it is not parallel to and straight in relation to the waterline it will show. In fact the antifoul line will be painted about 200mm above the waterline making the chine around 300mm above that.

Tomorrow I will finish attaching the foam (using resin) to the base of my board. I have done a small section to see if resin is effective with the foam. If not I will switch to contact. After that I will continue with the sanding using the electric sanders.

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