Three questions:
Why do skinsuits make you faster than just swimming in tri shorts?
Is there a conventional wisdom about how much time the save over an ironman swim?
Are they like wetsuits in that they help poor swimmers more than good swimmers or is it the same advantage for everyone because there is no added bouyancy?
They make you faster by giving you a lower drag coefficient. The textiles used are coated with a coating that doesn’t allow water to penetrate the material very easily. Thus a smoother skin for the water to run over. Somewhat the same as an SCS coating on a wetsuit, except that it is required to be permeable by water in a very small percentage, small enough that you can put water on the suit, and it will just sit there for a while.
Swimming with just tri shorts is fine, but you will see a little bit of drag. A speedsuit is better. Plus, have you tried to put spandex on when you are wet? Very tough. (if so, I suggest stepping into your top and pulling it up, rather than over your head).
Results will vary, but I think it is safe to say that you will save somewhere between 2-5 seconds per 100m. Some will see more, some may see less.
Good swimmers will see a benefit from this too. It really depends on how you look at it. A good swimmer will see a great benefit from a Speed suit by having good mechanics and body position, giving the optimal positioning for water to have the least amount of body to drag across. A poorer swimmer will see a benefit by covering more body dragging through the water, causing less drag. If you could cover you body in this stuff legally, we would be making suits for you to wear.
Some people claim that these WTC approved suits are just marketing hype, but I can assure you that there is enough testing done to say otherwise. Including independent tests here on the board.
jake