If you choose not to build any rear sponsons for your Kep's 21 Freebie rigger I recommend you to build this type of prop-support center "shoe" instead... This rigger needs to be light in the rear and it also needs some lift (in the rear). On a nitro rigger the basic lift (in the rear) is done with the propeller, that means you need to run a lifting prop like a ABC H4, H5, H6 or Octura 1445 on it. If you run a non-lifting prop, like a Octura X-series, it will most likely run too wet (in the rear).
The rear sponsons (or a right designed center "ski/shoe") also adds stability and makes the ride angle of the complete boat to be controlled. It accelerates also better out of the corners (with rear sponsons or any other type of prop-support).
This prop-support "ski" will also cover up the flex shaft brass tube and reduce drag. Do note we have never raced our Kep's riggers without any rear sponsons ourselves... Yet. ;)
This illustration shows where the prop-support "ski/shoe" is ment to be attached to the Freebie.
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