I don't know and I've never been on the Nam Tha, so take this for what it's worth.
The road is real good now and pretty good for a long time, most through transport went by road since quite a while ago.
A lot of the stories, reports, etc coming across the net are of people who private hire a whole boat (pricey) from Huay Xai. I'd think it could be done in the other direction. Often there are miscomunications with the boat owner and foreign passengers as to exactly what and who are along for the ride.
Huay Xai is close to where the river empties into the Mekong right? Fast boats run upriver to Xiengkok.
The Boat Landing website has great info on the Nam Tha boats, sorry I don't have the exact website address handy but Google should find it for you. You basically have to hire the whole boat or get lucky to join a group and there's an overnight stay in the boatman's village on the 2 day trip. I saw these boats while cycling on the dirt road that runs alongside the river downstream from Luang Namtha. They look uncomfortable and noisy and the scenery I saw for the 15 km I went on bicycle wasn't that great. Lots of new rubber plantations where there may have been opium production in times past. BAP Guesthouse in Huay Xai quoted me a price of 6,000 baht for the boat charter one way from Huay Xai to L. Namtha but I'd guess you could bargain it down a bit.
#3 LaoNow has been a member since 14/5/2010. Posts: 38
It used to be about US$ 100 per boat about 6 years ago, but fuel price increases have given the boatmen an excuse to increase the price. Yes, it isn't the most comfortable way of travelling, but it is a great two day trip, and an experience that may not be possible anymore soon. there are plans afoot to dam the river, whcih will severely affect this trip. Do it while you can!