One thing you can always count on as a boat owner is that you'll have nuts, bolts, and screws that freeze solidly, even when you've just used them. Getting them loose is a whole art, and here are some tricks to simplify your problems, reduce your costs, and soothe your temper.
Let's assume that you've found a frozen bolt. The first, and by far the most important, maxim to remember is that brute force almost always results in a great deal of damage.
If you encounter resistance when attempting to remove a fastener, stop! Don't keep forcing, don't go for a bigger wrench, don't hit it with a hammer! Instead, thoroughly analyze the problem. After all, you can out-think a piece of metal, can't you?
There are three key questions you need to answer before you proceed further. First, what condition is locking the fastener in place? Second, what materials are the fastener and the fitting? Last, what is the best no-slip way of applying force to the fastener?
Let's look at the first question: conditions. What can you see that might make the fastener stick? For example, a bolt that passes entirely through a threaded fitting and which has protruding threads on the far side may have corrosion, rust and gunk in the threads. Clean the exposed threads with a wire brush and use a thread chaser if the threads appear damaged. A final spray with lubricant and the bolt will probably come right out.
You may also find that there are several different conditions affecting a single fastener. For example, a bolt that has been overly tightened may also have rusted in place. You have to deal with the problems separately. You might find that you can pound out a split or lock washer under the bolt head, which releases the internal loads. The remaining slim gap between bolt head and fitting will enable a rust penetrant to attack the next problem, and you'll soon have the bolt out.
The types of metals involved will also determine the methods you can use (and can't use) to free a frozen fastener. Engine parts are usually made of iron or steel, with occasional brass or white metal castings. You may also encounter bronze, stainless steel, aluminum of different grades and, heaven forbid, common pot metal.
A brass bolt threaded into a steel part will respond differently than a steel screw in a brass fitting. While you're considering the problem, make a "doomsday" estimate and decide which is the most expendable of the frozen parts. Check to see that it actually can be replaced before you take the fateful step and sacrifice it. Some bolts may be specially threaded, but generally the fitting is the most expensive part and one that you'll want to save.
The last part of the problem revolves around having (and knowing how to use) the right tools. More damage is caused by adjustable wrenches, locking pliers, and worn screwdrivers than by all the rust and corrosion combined.
As a matter of course, you should have good wrenches aboard your boat, and these are invaluable for loosening bolts and nuts. The 6-point wrench or socket is far superior to the 12-point, since it gives you more surface contact and is less likely to round off the bolt or nut. The 12-point is fine for removing bolts that are already loose, or for tightening new fasteners, but nothing else. You should also have box-end wrenches rather than open end, since the open ends can sometimes spread slightly under load and round off the head. Don't use cheap tool sets...they are usually poorly machined.
You should have more than one size and type of screwdriver, in length as well as head size. The screwdriver should just fill the slot in both directions, or you'll tear out the slot under load. Last, a good set of taps and dies is invaluable for cleaning the threads.
Let's assume you've encountered a frozen bolt. The best start is to soak it daily in Liquid Wrench or a similar penetrating oil. Do it for at least a week, if you can, before you take further action. If the bolt is in a level area, build a small dam around the fitting with silicone sealant to keep it in constant solution. Check it regularly, and wire brush away any loose rust or corrosion to further aid the oil. Brass screws or bolts may respond better to one of the liquid copper cleaners used to shine pot bottoms, but penetrating oil can't hurt. If a paint build-up seems to be the problem, apply paint remover rather than oil.
After the first soaking, tap the fastener gently with a small hammer to set up vibrations to aid the penetrant. Pick the right tool and try a "gentle test". If you still encounter total resistance, go back to oiling and tapping the fastener. If it turns slightly, add more penetrating oil and work the fastener alternately tighter and looser to spread the oil inwards. Don't try to force it past the point where it re-freezes. You might try tapping it while you apply pressure. One word of caution: wrenches are designed at specific lengths to allow normal forces to be applied. Slipping a pipe over a wrench for more leverage will probably break the fastener or deform the wrench.
If the above treatments are unsatisfactory, heat may be a better solution. Use a small propane torch to alternately heat and cool the fastener. The different expansion rates, even among similar metals, may break up some of the corrosion and free the bolt. Obviously, you can't use this method for a fitting on your teak deck, nor around your bottled gas or fuel lines.
Now let's look at that common problem: you applied some pressure, there was a sickening lurch, and the bolt head sheared off.
The best retrieval method now is the screw extractor, a reverse threaded and tapered rod somewhat like a tap, that bites into a hole you drill inside the broken fastener. As you twist the extractor into place, the reverse threading also unscrews the bolt (you hope!). Every screw extractor is made for a specific size hole. Be sure you use only that size drill bit, and pick the largest size that will still fit inside the broken bolt. Start the hole by marking the center with a punch, and use a very sharp drill bit. A dull bit may wander before biting, and you'll have an off-center hole.
If possible, drill completely through the bolt so that you can pour oil into the chamber below it. To insert the extractor, you can either tap it gently into place, or you can thread it in by using a tee-grip from a tap set. Soft metals are usually not feasible for screw extractors because they will expand and tighten the threads, but there aren't many soft metal bolts these days.
Turn the extractor gently and, with a little luck, you'll retrieve the broken bolt. Treat the extractor with care, and be sure that you don't bend it sideways, since it is very brittle metal.
The court of last result for a broken bolt is to drill out the fastener and then re-thread the opening to accept a larger bolt. You might want to take this problem to a machine shop which can drill the hole precisely.
Dealing with a frozen nut is another ball game. The easiest method is to use a nut-splitting tool, which will neatly cut the nut away from the bolt without damaging the threads. In the absence of a nut splitter, carefully use a good cold chisel. The best method seems to be starting at the top of the nut and drive downwards as close to the threads as you can get. If the nut still doesn't split, you can then chisel in from the side to break it away.
If both bolt and nut are disposable, you can drill holes through the flats of the nut into the bolt threads to allow penetrating oil to enter the center threads.
By the way, if you have to cut off a bolt, the easiest way to restore the threads is simply to put a stainless-steel nut on above the cut. After cutting, back the nut off and it will rethread the damaged areas.
Assuming that someone (not you, of course) has already damaged the head of the fastener before you got to it, you'll have to find a way to keep your tools from slipping under pressure. If the bolt head is only slightly rounded, you can use a center punch in each angle of the hex to re-define the point enough for a 6-point wrench to grip. If the head is badly deformed, the most successful solution is to drill a hole horizontally through the bolt head, insert a steel pin, and use a wrench to remove the bolt. You can also file the sides of the head square rather than hexagonal, but you're removing strength-giving "meat" from the bolt.
If a screw slot is deformed, you can cut a new one by making a slotting tool from an old fine-toothed hacksaw blade. Tape the handle for comfort and grind out a notch so you don't damaged the surrounding areas. Be sure you don't cut the new slot too deep and weaken the screw head. This will also work if the screw head has been sheared off. Simply file a slot in the shank of the remaining screw and remove it.
One tool not already mentioned is the impact driver. Resembling a thick screwdriver, this tool turns a vertical impact from a hammer blow into several hundred pounds of turning force on a screw or bolt. I'm wary of these tools, since they can shear the fastener because of the tremendous power available but, used carefully, they can save the day.
Perhaps the best advice to remember when faced with a frozen fastener is to proceed slowly and think out the problem completely before taking any action. And don't let the rust and corrosion build up in the first place!