I'm my experience you gain just a little top speed but lose some bottom end and a lot less grip for any sort of turn.
I don't prefer single unless I upsize and the plan is going fast.

It would have to depend on the amount of toe in in production boards

Some boards come over finned then it's still easy to sail. Others not so much and you won't get much low end or grip safety.

What's your center fin and sail size?

Think freestyle boards they run below 20. 19-21 is ok for waves. It depends on your style too (and weight).

A small fin is more reactive, faster top end and turn tighter. It'll spin out more abruptly and control could be an issue over irregular terrain such as a choppy wave face.

I have played with this for fun as well as with a lot of side-by-side testing here on Maui. I have found that taking out the thrusters and leaving in a smaller center fin like you find on most thruster setups really kills the low end for early planing and creating drive through turns. It can also lead to more spinouts. This can be easily fixed by going to a center fin that is 2 to 4 cm's longer depending on what size center fin you are starting with and what you want to achieve. There is no question that a correctly sized single fin will be noticeably faster in a straight line than a thruster setup. As far as turning on a wave, I find advantages to both single fins and thruster setups. I currently run my 75 and 82 liter waveboards as thrusters for more control in choppy conditions, and my 92, and 99 liter waveboards as single fins which makes these bigger boards feel looser to me. I am running my 77, 85, and 102 Freestyle Waves as singles for a more exciting ride due to the extra lift of a single fin. Adding thrusters to those boards slows them down and also makes them feel more planted on the water which can be an advantage in really rough conditions.
Just my experience after a lot of trial and error and side-side comparisons. Your mileage may vary.

gandalf99 said..
All else being equal, if you took the side fins off a thruster setup, put blanks in, and left the same center fin (NOT a bigger single fin)... would you expect to increase early planing ability due to less drag, or lose it due to less total fin surface area?

I have tested a bit but it's sometimes hard to tell since I obviously can't test side by side at the same time. It's also possible the difference is negligible and that's why it's hard to tell.

I tested a bit with a RRD FSW 90 in the last couple of months. With a 5.4 sail. Single 25 cm Maui Ultrafin X-wave is fastest with just above 25 knots 5x10". Single 25 with 6cm mini K4 has some more grip in turns and feels more stable in a straight, 5x10" is not significantly slower. Single 25 with 11 K4 sides (which is a lot of fin) has a much better low end and goes upwind like a rabbit, 2x10 speed drops only about 1 knot, ultra stable, but the set up feels draggy in overpowered conditions. I have yet to try a 21 X-wave with 11 Ultrafin sides, still a bit overfinned but I am curious.

Playing around with side fins is what made me think about small foiling side fins in a different thread ... can somebody please build some?

Grantmac said..

Manuel7 said..
[...]

Ever tried to carve a modern freestyle board with a freestyle fin?
Also freestyle fins are very thick for their size.

Yes an old 2012 I think. It's in my tests: windsurfing.lepicture.com/board-tests/

Fun times!