Fender whips are the ropes that hold a fender in position. They attach to the ends of fenders and allow them to lay against the hull at the appropriate position.
The questions that circulate around fender whips tend to involve length and material.
Length is a rather simple you need enough line to lower the fender into position and tie it to the boat. This may sound like an over simplification, but that is the goal of the length. Each boat is different and therefore needs a different length. Keeping this simple target in mind will take all the guesswork out of selecting your fender whips.
The lowest position you will tie a fender is sea level, so the whip needs to be long enough to lower the fender into the water. The other end of the whip needs to tie to the boat. If you have a toe rail that can serve as an attachment point, this will be your upper mark. If you have a smooth rail or teak rail, you will probably need to attach to the railing or lifelines. Now that we know how far the whip must travel to connect the rail to the waterline, we need to add in enough line for the knot. I personally prefer to have 2 feet for the knot, allowing me flexibility and ease of tying. If the line is too short, the knot can be hard to tighten.
In conclusion, the length of the fender whip should be the distance from the rail to the waterline plus two feet.
If you have a low freeboard sailboat, this distance might only be 1 or 2 feet; making the whip length 3 to 4 feet respectively. If you have a high freeboard boat, such as a Kadey Krogen 58, you will need a much longer whip for the midship fenders.
Sadly, there is no hard and fast rule for fender whip lengths. They need to be made in accordance to what is needed. A whip that is too short is worthless, and a whip that is too long is cumbersome to deal with. The ideal would be the distance from the attachment point to the waterline plus 2 feet.
The next decision to make involves fender whip material. Once again, the choices are plentiful and ambiguous. Any rope can serve as a fender whip, even spare line you have lying around.
The common choices are Three Strand Nylon and Double Braid Polyester. Both of these choices are easy to purchase and widely available in any length. Some "dedicated fender whips" come with eyes spliced into one end, allowing you to easily connect it to the fender. These are nice, but a bowline works just as well if you don't have a spliced line and don't feel confident performing the splice yourself.
Both of these choices are easy to come by, but can be rather bulky when you tie them. This is where another alternative material comes into play. You can remove the core from double braid polyester lines and make your fender whip out of the cover. These whips will be plenty strong to hold a fender in place and will not form bulky knots, allowing you to tie them up through small holes in the toe rail with ease. These whips are easy to splice if you wish to make eyes for the fender end.
Now you know how to select the proper length and material for your fender whips.