Hi John 123,
You could use a shorter vertical fin, but it would not have the area nor the depth to give your F-159 anything like the same feel that the stock 70 cm fin does.
The 70 cm fin is longer than 1/2 the width of your board and that extra 20 cm "balances" the fact that your footstraps are set so far off center on the formula board.
If you put a 40-50 cm vertical fin on your formula board and try to run a big formula sized sail (9-0 m2-
12.0 m2) the big sail will simply overpower the fin.
If you use a 40-52 cm weed fin, with much more area, you won't get the same feel as the 70 cm, but the big sails also will not overpower the larger area of the big weed fin.
Also, sailing in shallow water (sandbars and other obstructions) the leading edge rake angle on the weed fin really helps if you get into the shallows and run the fin aground.
On the vertical 70 cm, you will come to an abrupt stop when the tip of the fin hits the sand and sticks.
Could break the fin or damage your board/finbox, or hurt yourself in a very sudden catapault type stop.
With the leading edge rake angle on a weed fin (normally 45-50 degrees back from vertical) your fin will
slide into the sand and lift the rear of the board a bit as you slowly grind to a halt.
Much slower decelleration and something that gives you some time to sheet out, get the power off the sail, and step off the board.
Much safer in really shallow water with the weed fin.
Since you say you have weeds as well, then you won't collect huge clumps of weeds on the leading
edge of the fin and have to stop and back up to get the fn clean all the time.
Hope this helps,