See full version: Crunching Numbers: Defining Displacement
Once you have a reasonably reliable displacement figure to work with, you can use it to run various formulae that give you a much clearer idea of how a boat will perform. We'll discuss these in future Crunching Numbers posts. more
But a boat’s displacement tells us more about the boat than just its underwater hull volume. It also influences a boat’s speed, in that lighter boats are generally faster than heavier ones, its load-carrying ability (heavier boats can more easily carry big loads than lighter ones), and how comfortable it is (heavier boats normally have an easier motion). To get a realistic sense of these characteristics, however, you first need to get a realistic displacement number to work with--i.e., what the boat will actually weigh when you are using it. In most cases, you can be certain that the displacement number published by the boat’s builder will be a low-ball light-ship figure of some sort. They do this to make their boats look faster compared to other boats, and because they know other builders will be doing the same thing--a classic case of the power of the lowest denominator.
The bottom line, in any event, is that, unless you have specific information to the contrary, any published displacement number you see will need to be adjusted upward, usually by a significant amount. The best way to get an accurate number is to load the boat with what you want to have aboard and then weigh it yourself. In most cases, however, particularly if you are trying to evaluate a boat you don’t already own, this is not feasible. But whenever you do happen to have your boat hauled by a Travelift with a load cell (or, even better, a boat you are having surveyed), you should be sure to make a note of what its actual weight is.
A 12,000-pound boat, for example, has an underwater hull volume of 187.5 cubic feet when floating in salt water (12,000 ÷ 64 = 187.5) and a volume of 192.3 cubic feet when floating in fresh water (12,000 ÷ 62.4 = 192.3). In both cases the boat weighs the same, as does the water it displaces, but the amount of water displaced is different. The end result is that the boat will float slightly higher on its lines in salt water, as slightly less water is actually being displaced by the boat. In fresh water it will float slightly lower on its lines, as slightly more water is being displaced, as per the drawing up top.
Absent such a glorious opportunity, you can only estimate how much extra weight you will put aboard and add it to the published figure. Nigel Calder, in his very useful Cruising Handbook (International Marine/McGraw-Hill, 2001), suggests that the best average adjustments to make to arrive at a reasonable half-load number (i.e., the boat is loaded to cruise with tanks half full) are plus 2,500 pounds for a lightly used coastal cruiser and plus 3,750 pounds for a bluewater cruiser. For a more heavily equipped coastal boat, or a long-term liveaboard bluewater boat, he further suggests that these corrections be increased to 3,750 and 5,000 pounds respectively. These estimated average corrections are as good as any I have seen. Alternatively, if you want to be really anal and try to make a much more detailed itemized estimate of the weight you will put aboard a boat, you can use the handy worksheet that appears on page 13 of Nigel’s book.
The volume of water displaced by an object is also equal to its immersed volume, which, in the case of a floating boat will be its underwater hull volume. If we know a boat’s displacement, we can therefore also calculate its underwater hull volume. This volume, however, will vary depending on whether the boat is floating in salt or fresh water, because salt water is a tad heavier than fresh water. (It weighs 64 pounds per cubic foot, compared to just 62.4 pounds per cubic foot for fresh water.) here
How much should I insure my boat for? here
Start with a little fact-finding. Ask your boating friends which company they use and how their claims have been handled. The way an insurer has handled claims in the past is a good indicator of the quality of service you can expect in the future.
Ask your insurance agent if they offer what is commonly referred to as a “lay-up” discount. You have year-round coverage with a discount for the months your boat is in dry storage. [links]
You should insure your boat for the amount it would cost you to replace it with like kind and quality. This is called "Agreed Value" or "Stated Value" coverage, and in the event of a total loss, will pay the full insured amount. Beware of policies providing "Actual Cash Value" (ACV) coverage, which means the value of your boat will be replacement cost less depreciation. here
Many homeowners policies include perfectly good coverage for smaller boats and motors, usually with a horsepower limit of from 25- to 100-horsepower. While homeowners riders are normally adequate for these boats, be careful to ask the same questions you’d ask any other insurer about damages to your vessel and how they will be paid. Also, most homeowners insurance riders apply only to use in inland waterways, lakes and rivers. Coverage seldom extends outside a coastal inlet or along the beach. If you plan to boat “outside the inlet” you definitely need an experienced marine insurer. Many other factors will lead you to a qualified marine insurer, too, and here are the things to consider.
Beyond that, the basic configurations of the different watermakers currently on the market are all quite similar. As a first step, after entering by way of a through-hull, the feed water travels to a strainer, which like the raw-water filter for an engine serves to remove grit, grass or anything else that would harm either the membrane or any other part of the system. After that the feed water continues on to a booster pump, which sends it through another filter, or filters, to the high-pressure pump, whose job it is to force the water through the semi-permeable membrane.
Beyond that, another major difference you’ll find is in the area of watermaker controls, with some systems offering a fully automated, backlit digital interface while others are more basic and require you to manually turn and adjust various buttons and valves to run the system correctly. Obviously, for those interested in ease of use, an automated approach is the way to go—although it’s going to cost you more. Some units can even be controlled remotely, via a smart phone or tablet, which in turn makes it possible for a manufacturer to troubleshoot a system from its home office in the event you run into problems. [links]
Fortunately, in recent years sailors have been able to enjoy the benefits of modern watermakers that in many ways do for water stowage what GPS did to navigation: make a whole world of problems go away with the push of a button.
Freshwater fishing boats come in all shapes and sizes, from tiny one-man self-propelled craft to snazzy bass boats that are juiced up with hundreds of horsepower. If you’re interested in buying a boat for fishing in sweetwater, you’ll have countless to choose from and many decisions to make. How do you get started sorting through the options? Never fear, dear boater—we’re here to help.
If you want a new freshwater fishing boat, the choices can be overwhelming.
So, which came out on top, aluminum or fiberglass? Please don’t say it was a tie—in that case, you’re in for some serious soul-searching before you can make a decision. But there’s one more thing that might help: take a look at Aluminum Fishing Boats: Light, Economical, and Seaworthy. This article goes into some in-depth detail about aluminum fishing boats, and may push you one way or the other.
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