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IOM Alignment and Measuring Jig (UPDATED)

I need to start to consider how I'll mount the fin and bulb, and get it all straight, so I made a 3D modeled alignment jig:

Ian Dundas in Scottland sent me pictures of his setup. I just bought a laser level too! He does amazing work and has been a great help in this project so far.

There is also another jig featured on page 14 of the winter 2008 Canadian Radio Yacht Association (CRYA) newsletter that features one. It's by Lawire Neish. Read it here.

I took both those designs and made my own version that features sliding cradles for the hull, a fixed waterline sight, proper depth to cradles for the bulb, etc... Most of the parts can be laser cut, and the two end pieces and the base can easily be cut on a tablesaw.

It has fixed waterline sights that are 420mm above the top edges of the bulb sliders in the base. The fore and aft cradles can be slid up and down to adjust the waterline of the hull. You'll see that the overall interior length is 1003mm to accommodate the aft slider thickness. I had to come up with a way to account for rudders that might be flush with the transom. But the total distance between the inside of the aft slider and the inside of the forward board is 1000mm.

The bulb sliders can be slid fore and aft to account for varying bulb diameters and bulb cant. The TOP edge of the bulb sliders are longer than needed, and are 420mm below the waterline, so you have a visual reference for the bottom of the bulb and can adjust as needed. 

Here you can see the bulb sliders. They are identical, and the top edges of them are exactly 420 mm below the waterline sights. You slide them forward or aft to adjust the bulb, and bulb cant, as needed. The top edge of the bulb sliders are extended so that you can look at the bulb from the side and see how close the bottom is getting to that 420mm limit.

Here is the forward cradle slider that the bow fits into.
Here is the aft cradle slider.

Here you can see how the transom is against the aft slider and not against the end board. This is to accommodate rudders that are flush to the end of the transom.

I haven't actually made it yet. Hoping for some initial feedback from people more experienced than I in IOM stuff. But might be able to laser cut it by next week and see if I can get it to work.


UPDATE: I built it and it worked very well! When used in combination with a cheap laser level, you can get millimeter accuracy. You can also much more easily set up your fin and bulb for mounting.

After the next picture was taken, I received the adapter bolt that fit my tripod and the laser level, and I can now raise the laser to the proper waterline level too. It all works really well. 

Here is a link to my DWG file if you wish to laser cut your own version from 3mm (1/8") Lite Ply. This file only includes the parts that need to be laser cut! You will still need to make, cut, assemble the MDF parts yourself. The parts that are colored as plywood in the pictures above are included, the rest aren't. Good luck! 



  1. Might I suggest making accommodations in the forward slider for a 10mm (or larger) spacer for boats that have not yet attached the bow bumper.

  2. Not a bad idea, but probably just as easy to use a 10mm spacer that you cut yourself. If you're worried that the boat won't fit, the two sliders each extend 20mm into the middle. The 3D boat that I'm using in this example doesn't have a bow bumper, and it sits easily on the slider. Let me consider this though! Thanks!


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