written by Paul Pachmayer of www.FastElectricRc.com
Installing a stuffing tube in any boat is very important part of your building process. Done correctly your rewarded with a free running drive line with the least amount of friction thus maximizing your motors potential. Also included in this "how too" is my modification of a Warehouse Hobbies strut (now available from Fine Design Marine). These struts were designed to use a 1/8" stub shaft and .098 flex cable which really isn't suited to higher powered motors we have today. What we want is to run a .130 flex cable and 3/16" shaft so a new bearing cartridge is in order, this is an easy modification and I'll show you how I do it.
One thing to keep in mind is I don't use Teflon tubing in my drive lines. My opinion is that a straight brass stuffing tube has less friction than Teflon tube. Try it yourself, put a piece of flex cable in a Teflon tube and spin it. Notice the rotations and try it again in a piece of 3/16"OD brass tube. Which one seemed to have less resistance? Another bonus to straight brass tubing is that it's much easier to keep water from entering the hull, later in this "how too" you'll see exactly why.
First thing to do is locate where the stuffing tube hole in the transom should be. For monos my general rule is 1/8" up from the chine. Some people like offsetting this location to the right on monos, the theory is that the offset helps counteract prop walk. I like running mine centered, not that it's better, it's just my personal choice. Make your first mark 1/8" up from the chine, from that mark measure up another 3/16" and mark that location. Between these two marks is where you will drill your stuffing tube hole. After you mark these locations it's time to install the strut mount to the transom.
Take the strut bracket and place it on the transom so the two marks you made are centered in the lowest hole on the bracket. Now you want to mark the location of the lower bolt hole, take your time and make sure your stuffing tube marks are centered and that the bracket sides are straight up and down. Once your satisfied with the bracket placement make your mark in the center of the LOWER bolt hole. Now drill a 3/32" hole. Mount the bracket with one of the countersunk 4/40 bolts in the hole you just drilled. Now check that the bracket is straight up and down on the transom and drill the upper bolt hole and install the other bolt.
Now that the bracket is installed drill the 3/16" hole for the stuffing tube in the center of the marks you made earlier. When your done it should look like this.
Now it's time to prepare a piece of 3/16"OD K&S brass tubing for the stuffing tube. First thing to do is anneal the tubing so it's easier to bend. Annealing is simply heating the brass tube until its glowing red. I use a propane torch to do this which works very well. The stuffing tube is about 3 5/8" long for this boat and I anneal all but the last 1/2" at each end.
Now you want to make the bend in the stuffing tube to match the angle of the motor. Install a motor with coupler to the motor mount and place it in the boat. If you use an Octura Flex Hex like I am be sure to remove the nut that tightens the collet. I use a short piece of left over flex shaft in the coupler to get a rough idea of what the down angle of the motor is.
Gently bend the stuffing tube so that the flex shaft fits right down the center of it. Ideally you want as little bend in the tube as possible, in other words as close to a straight shot out the back the better. Notice in the picture that the flex shaft runs straight down the center of the stuffing tube and that there is about 3/8" of flex showing between the stuffing tube and coupler.
Now it's time to build the new bearing cartridge for the strut. There's 2 ways to do this and both work equally as well. You'll need 1/4"OD and 7/32"OD K&S brass tubing, a piece of Dave Brown FG push rod tube and a Octura 3/16"ID lead/Teflon bearing.
For this install I made the bearing cartridge the same length as the original that came with the strut, you can make them longer or shorter to personal preference. Cut a piece of the FG pushrod tube 3/4" long. Take the lead/Teflon bearing and slide it into the end of FG tube.
Now slide the 1/4" brass tube into the FG tube until it touches the bearing. Cut the 1/4" brass tube 3/8" away from the FG tube. After you cut the brass tube apply a drop of thin CA at the edge of the FG tube to secure them together. Now take the 7/32" brass tube and slide it inside the 1/4" one until it touches the bearing and cut it flush with the end of the 1/4" tube. Again, apply a drop of CA to the seam of the two brass tubes to secure them together. It's helpful to lightly sand the outside of both brass tubes before assembly so that the CA gets more "bite".
A shot of the stuffing tube, strut, original bearing cartridge and home made bearing cartridge.
The other way to build the bearing cartridge is to use the brass tubing as your bearing surface rather than the Octura lead/Teflon bearing. In this instance you would just use all brass tubes nested inside one another all the way to the end of the FG tube. You could also use brass tube instead of the FG tube, just be sure you end up with an OD the same as the drive dog.
OK, now we will assemble the whole thing and fix it in the hull. With the stuffing tube in the hull install the strut on the bracket.
You'll notice that the stuffing tube is hanging out of the back of the strut. You want at least a 1/2" of the stuffing tube showing, this will be positioned correctly in the next step. Take the new bearing cartridge and slide it into the strut housing, don't be concerned if the stuffing tube is resting inside the lead/Teflon bearing. Now take your stub shaft and slide it into the bearing cartridge.
The step on the Hughey stub shaft is resting against the stuffing tube. These couple of pictures show what's going on inside the bearing cartridge. The first picture shows the turned down area of the stub going into the stuffing tube, the second picture shows the stub bottomed out against the stuffing tube.
Now you just push the stub into the bearing cartridge until it can't go any further. You just set the depth of the stuffing tube and are finished with this part of the install. The nice thing about this setup is the stuffing tube is aligned by nature because it nests inside the brass tubing in the bearing cartridge, it also acts as an additional bearing support for the front of the stub shaft.
One other thing of mention, because the front of your stub shaft rides in the stuffing tube there's no gap for water entry!
Now we move to the inside of the boat to fix the stuffing tube in place permanently. Check to make sure the flex shaft is still running down the center of the stuffing tube and make any necessary adjustment. It should look like this.
When your satisfied everything is aligned put a few drops of thin CA around the stuffing tube at the inside of the transom. This will hold everything in place and seal the hole you drilled for the stuffing tube. Now we epoxy the stuffing tube in the hull. I personally use an epoxy called Plastic Bonder from Power Poxy. This stuff gets nice and hard and sets in 3-4 minutes, it also doesn't get brittle like normal epoxy. I find myself using the Plastic Bonder on just about everything because it doesn't self level, in other words it stays where you put it. Mix some up and apply it to the area where the stuffing tube enters the inside of the hull and at the same time fill the area under the stuffing tube to support it.
That's it! Now you have a nice free running drive line. The only other thing I do is put a small piece of silicon tubing on the inside end of the stuffing tube to help keep out any water that may make it up the tube. You just need a small bit of the silicon tubing touching the flex shaft to do the job, too much touching just creates drag on the flex. Here's a shot to give you an idea of how much should be touching.
This procedure is the same one I use on all the monos I build, the stuffing tubes last the life of the model and require little maintenance. I think once you try it you'll be quite pleased with the results. I hope this has helped you some in understanding how to install a stuffing tube in your own boats.
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