Diver with a sluice full of gold
Ahh, the age old question - what size dredge should I buy, and what brand is best?
A question with as many answers as there are dredgers. Each dredger will tell you their particular size is best, and their brand is best. Chevy versus Ford. The real question is what size is best for you?
First let's tackle what brand. Every now and then a dredge manufacturer will come up with an improvement, which is quickly copied. The only dredge which really looks different is the Dahlke. It's pretty much designed from the ground up. However, all the dredges do a pretty good job at capturing gold, but you have to know what you're trying to capture. If you're in a river which just runs fine gold, then you need a dredge designed for fine gold, if you're stuff is pretty chunky then you need something different. Some rivers have a lot of gold on quartz, primarily the old hydraulic tailings rivers, and gold on quartz tends to blow right through the sluice.
No one dredge or sluice box is going to be designed for all situations. Some people want a utility dredge, one that will work wherever they drop it figuring a dredge is a dredge, but look a little closer and you'll see there are differences. Like the old El Camino which wanted to be both a pickup truck and a car it did neither very well and was ugly to boot. Same for dredges.
Where do you plan on dredging is your first question. Deep water means bigger dredge, but you should also be familiar with the type of gold being recovered. Is it coarse; fine or very fine? The hydraulic operations lost up to 25% of the gold from their sluices, even using mercury. If you've ever recovered some of the very fine flour gold you know it is the consistency literally of flour. I've dredge creeks before where the majority of the gold was flour gold, but there was a lot of it. The old triple box would blow this gold right through it and I was probably returning to the creek about 1/4 ounce a day. The riffles weren't set up to catch the flour gold and when you started the engine you got a rush of water right through the sluice which I was always convinced would blow gold out the back.
It may sound like a great idea to run out and get the biggest dredge you can afford, you may find yourself regretting that. The largest dredge allowed in California is the 8". There's a significant number of people, about 20%, who run eights. You're gong to find them on the big rivers and for the most part they do pretty well, but it's not likely they started with an 8". No one brings in an 8" unless they know there is gold there to make it worth their while.
Bigger dredges allow you to hit the deep holes full of gold
A significant number of dredgers work alone on their own mining claims. If you're working alone, and packing your equipment in, then you need to have a dredge you can pack by yourself, and yes weight does matter. 100 pounds of lightweight gear is a 100 pounds. The reality of dredging is gold isn't uniformally distributed, especially in the smaller creeks. You're going to find yourself moving - a lot. A big dredge sounds great until you have to move it.
I've found the 5" dredge is the biggest I can work alone. It breaks down into small enough parts you can carry by yourself, although the engine is a real slog, especially going downhill, it's much harder than packing it up hill. The 5" dredge is a great dredge for working by yourself and most of the 5" dredges today come with two engines for better portability. It's still a lot of weight.
From a prospecting perspective, not recovery, the 2.5" dredge is ideal. It's light enough you can pack one in about two trips, maybe one trip but you'd have to be pretty tough to do it, and real efficient. It's not just the dredge you're packing, it's all the other equipment such as wetsuits, tools, food, water and gas. The 2.5" has enough power to get you about 3' under water and some of the 2.5" have air which can extend your reach, but of course that adds a bigger engine and more weight.
Use the 2.5" to determine where the gold is running, then bring in the bigger dredge. Use prospecting dredges for what they were meant for and use recovery dredges for what they were meant for. The 2.5" is highly portable and you can cover a lot of territory with it. In my opinion, having run dredges for some number of years, the 3" dredge is an oddball. It's too close is size and weight to the 4" to really make a difference and it takes as much work as a 4" so why bother with a 3" dredge. Either get a 2.5" or a 4", but skip the 3".
The innovative design of the Dahlke 2.5" makes it easy to pack and set up.
People like to poke fun at the 2" backpack dredge, but I've got one. Over the years you find yourself picking up different dredges for different reasons. The 2" backpack dredge is a one pack dredge, but why bother with something this small? The 2.5" I've found takes me two trips to pack one in, the 4" dredge takes me about 4 to 5 trips but the 2" dredge I can put on my back and pack in one trip. It's easy to move and it gives you something you really need - the ability to clean off the bottom and work cracks.
The old reliable 2" dredge prospecting a creek
If you're prospecting the backpack dredge is the ideal tool. If I had to trade the 2" off for only a 4" I wouldn't do it. Think of every ravine, gulch, gully and puddle of water you can run a 2" in, but then think about trying to get a bigger dredge into the same spots. For a day trip the 2" dredge is my tool of choice. If I find decent gold then I'll probably return with the 2.5" but there's going to have to be enough gold there to make it worth my while to bring in the 4".
Now there's some who believe size matters in dredging. It doesn't. What matters is gold recovery. If you can do a half ounce a day with a 2" why would you run a bigger dredge and get the same? A half ounce a day with a 2" is completely realistic and I've had more half ounce days with my 2" than I've had with my 5". Some believe you have to be in big, deep water to get good gold. Sometimes that's true. Sometimes really good gold is found in the deep holes on the big river and let's face it a 2" dredge ain't going to cut it. You need the power and ability of a big dredge to get big gold, but there's a lot of nice gold to be had in the smaller creeks with the smaller dredges.
Prospecting is the art of finding gold, mining is the act of recovering gold. You don't use little bitty dredges when you're on the gold. You use big dredges which can move the most material, recover the most gold and spend the least amount of money doing it. Once you're on the gold it's time to bring in a real dredge. Some consider this a 5", some consider it a 6" but most professional dredgers use the largest dredge size which is legal, right now that's an 8" dredge. Most 8" dredges are custom built to the operator and for a specific purpose. Most folks who run the 8" dredge I've talked to don't really move much. They have found the hole that has the gold and they may spend a summer working down to the bedrock, they're after power and reach.
It's likely if you're reading this you're considering an 8" dredge, but a lot of people end up with a 6" or 8" dredge once they find the spot they really want to work. Until that time you're in the 2" to 5" range. Some think the 4" dredge is the best all-around utility dredge, but I'm not one of them. The 4" has limitations as well. It's pretty heavy and bulky, about the same as a 5" and for whatever reason that extra inch of nozzle size makes a big difference in moving gravel. I really like the 5", have accepted the 4", love the 2.5" and the 2" is my favorite tool.
There is no answer to the question what size is best. You need to evaluate your projected prospecting, and if you think you're going to be actually mining a placer deposit. If you know where the deposit is then you're going to lean to a bigger dredge. If unknown then lean towards the smaller dredge. Don't shy away from having a couple of dredges. The truth is you need a couple of dredges, the brand doesn't matter so much as the size and your ability to get the slope of the sluice box right. Once you tune your dredge to your particular conditions then you can forget about brands, focus on your task, not someone else's task. Good luck out there.