Powerbelt fitting & demo


A short video showing the mounting and operation of the M-Pro Designs Powerbelt

Tips and Tricks for Fin Fitting


Some insight and tips on how the screw/plate system works to prevent the fins in a "long box" from moving once the screw is tightened up .


If you do an "online search for fin boxes there are at least a dozen different variations of what are described as "long boxes" . Apart from the Aluminium fin box i designed for our "Mega" kayaks around 8 years ago, they are all produced in injection moulded plastic, in 2 parts, and then glued together to produce the fin box. Herein lies the first problem. Plastic is unstable, has varying degrees of shrinkage and that combined with then needing to glue the 2 parts togeter by hand (human error) and then a dozen or more different manufacturers trying to make the same (similar) product, there is bound to be some dimensional variarions between products. To then compound the problem, there are then dozens of different manufacturers of fins, again some hand finished , some CNC machined, some plastic moulded , probably all working to different dimensions and tolereances of fit . Little suprise then that it is actually quite difficult to ensure the fins you buy are going to fit and "lock up" perfectly for any given fin and fin box combination. Hopefully the following pictures and explanations will enable you to "fine tune" your fins to suit your particular fin boxes.Firstly lets look at the typical internal shape of the fin box.  




The lower horizontal slot ( mot plastic fin boxes just have one horizontal slot) is where the square nut runs . As per the next picture. 

When the screw is tightened it is vital that the square nut can locate hard against the upper surface of the square nut track.

One of the most common reasons for the fins not tightening in the box is that the "square nut" contacts the lower face of the fin "lug" before it contacts the upper face of the square nut track. Yjis may only be a matter of 0.05 mm ( less than the width of a human hair) but it can be enough to stop the fin from locking up "tight" in the fin box. Considering all the variables in the fin and box manufacture its not surprising that this can happen. Rectifying this can be as simple as lightly sanding the lower face of the fin lug (shown below) 

Its important to understand that the fin is not physically "clamped" in position, but it is the load on the upper face of the square nut track that is pulling the base of the fin hard down onto the bottom of the finbox and that it is purely "friction" that keeps the fin in position. It is therefore important that the more of the base of the fin that you have in contact with the bottom of the fin box the better. The main influence on the amount of fin in contact with the bottom of the fin box is both the size and position of the fin "cross pin" in relation to the width of the square nut track.If the pin Diameter is considerably smaller than the width of the square nut track, then the fin can ride up and down in the box.

The ideal situation is that ,with the base of the fin on the bottom of the fin box , the cross pin will be in the centre of the "square nut track"and with only minimal clearance between the width of the track and the diameter of the pin. 


From experience the square nut track can vary anything betwen around 4.5-5.5mm , and the cross pin diameter on fins between 4.5 and 5.3mm, Giving anything from zero clearance and up to 1mm clearance, or worse still up to 0.8mm oversize pins.

As can be seen this can cause all sorts of "fit" problems

To demonstrate this we have sanded the base of this fin so that the pin is "lower" than ideal.

As can be seen in the next picture when the screw is tightened the effect is for the rear end of the fin to lift until the top of the cross pin contacts the top face of the square nut track, whilst the fin will not easily move, it will not take much effort to get the fin to slide due to minimum contact with the base of the fin box.

To overcome this situation you need to either put a shim into the bottom of the fin box or bond a shim to the bottom of the fin. This will raise the "fin lug" bottom face ensuring a good loadingonto the fin once tightened.

There are a number of reasons why fins will not fit into the fin box, aside from the width of the fins being to big, (his is also open to debate as the width of fin boxes can vary considerably.

Lets assume that the width is OK, If the pin is in the wrong position (to high from the base of the fin) but the pin diameter is ok, then sanding the bottom of the fin should cure the problem, this will also lower the fin lug face, so you may need to sand this also.

To check if it is the pin position preventing the fin from entering the box , carefully tap out the pin and with a pair of tweezers try to slide the pin into the track. If it fits Ok, then the pin is to high from the base of the fin , so sand the base as above.

If the pin wont fit into the track and it is a roll (hollow) type pin then you should be able to squeeze the size down a little with pliers or a vice. If it is a solid pin, then get out the file , and file the top face first.

Having established the correct pin position, the other feature that can prevent the fin from fitting in the fin box is the end radius of the fin, relative to the pin. If the radius is to small it will stop the fin from being able to rotate around the pin preventing fitting the fin.  

The fin should be able to rotate around the pin like below, if it is fouling on the bottom of the fin box then sand the radius a bit bigger

Hopefully the above tips will help you fine tune your fin fitting if you are having issues with them moving or not fitting in the fin box.

Finally, dont over tighten them as you will simply put extra strain on the fin lug, If they fit properly you wont need to.

© 2015 : M-Pro Designs/G. Fawcett