Friend has two TRS drives on his older scarab. The problem is that one of them revs up but goes nowhere and just feels like the prop isn’t spinning but when the boat is out of the water prop spins fine, Im sure that has to do with the load in the water but what could cause this and how do I figure it out? My first guess is the coupler like on a Bravo or Alpha but I have no idea on a TRS setup what is in between the flywheel and prop.

Also to help with diagnosing it the props are solid hubs so no rubber or breakaway.

TRS has a transmission between the flywheel and drive, no coupler. Sounds like the transmission is slipping bad. About all you can do is check that the fluid level is correct and that the linkage is shifting it all the way into gear. Other than that the motor and trans will need to be pulled. It's possible but unlikely the drive plate on the flywheel is bad but most likely the trans needs a rebuild.

Thats what I was afraid of. Is there any test that I can do before pulling the trans to make sure that it is not fried gears in the drive or something like that?

First, as stated previously, check the fluid, especially if it is a Merctrans. If you have changed the controls to a modern system, make certain you are getting full engagement of the F N R lever on the actual transmission. Even if its an original control system check that throw distance is equal for both trans.

Seriously doubt it is gears in the drive. It would be making all kinds of noise when in the water with the prop not turning with it in gear. Drive has to be removed first before pulling the engine/trans anyway. You can turn the drive over by hand with it off the boat and see how it feels but my money would be on it being the trans. What year is it? Most likely it's a Borg-Warner and not a Merctrans.

I don’t know the year of the boat but I think it’s a late 70’s. Thank you guys for the info, im going to pass it on to my friend and im going to bet before the week is over we have it torn out and at a shop by Friday.

Next question is do I need a marine shop to do this or can a regular transmission shop handle it? Even if they can handle it should I let them or do I need to find someone with experience with these things? The reason I ask is because we are along way away from any real marine shops.

A local trans shop could do it easily as they are much less involved than say a car automatic trans. If it's a Borg-Warner Velvet Drive parts are readily availiable.
http://marineengineparts.com/shopsit...l/page198.html

If it's a Merc Trans ll parts will be harder to find and you will probably end up having to go to this guy.
http://www.kcmarinetr.com/Default.aspx

You need to identify which one it is. MercTrans will have an ID tag centered on top and a B-W will have a tag more on the port side and is usually red or green. If it's a B-W it will have the model number on the tag. If it's a Merctrans it will only have the serial number on the tag which is useless.