chrischch,
yeah doesn't matter what repellent you have, it's feeding time at the days end and nothing stops the buggers. the bushmans will, like and repellent and sunscreen, wear off through chaffing etc while you are mining. nothing beats covering up as much skin as possible.
no matter what your sluicebox set up is you should be able to observe the black sand "dancing" in behind the riffles. if the black sand is not jumping up and down chances are it's not set up properly. it is in this fashion the debris (ie. all the crap you want to get rid of as tailings - stones, small rock, excess black sand) will start to accumulate in your box thus stuffing up its effective operation.
i usually try and bed in to the stream bed some flat rocks for starters to support the box. then i chuck the box down and and weigh it in place temporarily with a good sized rock to stop it floating away. then i work out the height for the bottom end of the box. this needs to be clear of the water height in the stream so tailings flow out. the more drop here the better as tailings will build up in the stream as you work. then i work out the height for the top of the box. you usually only need a small amount of water flowing over the top end into the box but this will depend on the size of the gravels you put through as you want most of it to free tumble away thus avoiding having to hand pick the bigger stuff out. this is where you may need a little (or big) wing dam of rocks across the stream so you can adjust the water height into your box as you need to get the angle right so that the material dances around behind the riffles, thus letting it discharge away. try it out as you go. everything should wash away except a little blacksand and fines which will remain behind the riffles. by no means should the gravels simply sit over the box as it will block up and by useless in trapping gold. on the other hand the largest rocks you put through (presuming you aren't preclassifying all the material) probably shouldn't go flying out the end. if they do you probably have to much flow.
so its a bit of trial and error. a wing dam is great to adjust the flow though especially if the river flow goes up and down over the day. you could also get a sluice box with a flare which will gather more water flow and eliminate too much wing dam building.
lastly. once i have the flow right i place stones and rocks each side of the box up to the boxes top. i then place some rocks over these that overlap the edge of the box thus holding the box in place as there will be a bit of water pressure trying to pop it up and you don't want it to float away downstream. don't have too much rock overhanging the box though as you won't be able to see what's in the box or easily be able to pour in your gold bearing gravels.
the weight of the gravels you place in the box may eventually make the box settle a little on the rock base under it. to counteract this simply find some thin pieces of rock and slide them under between box and base rock and its fine again. this also allows for fine tuning as the water flow goes up and down over the day. without a rock base you may see your box quickly settle into the sand which will waster half your propspecting time.
hope this helps.
