Boat Weight, Hull Design and Performance
by David Pascoe
Light weight equals faster speeds plus better fuel economy. That's the advertising hawk we see in boat ads over and over these days.
Is that really true? Actually, it is, but unfortunately that is only part of the story. The other side of the story is the one that's rarely told.
The truth is that achieving light weight is (1) good for a few types of boats, but not most, and (2) there may be a terrible price to be paid for lightness in most others. It should go without saying that in discussing boat weight, there are certain relevancies we have to deal with. Boats can obviously be either too light, or too heavy. And for each type, in most cases there is a more or less ideal compromise. That statement is modified by "each type" because it is important to consider how a boat is intended to perform, as well as the water conditions in which it will perform.
The central advertising claims give the impression that the lighter the boat, the better it will be. Now, anyone with a modicum of experience knows that this isn't true. A boat that is too light is going to bob like a cork, making it an extremely uncomfortable vehicle for passengers. It may be fine for racing where the occupants wear helmets and other protective gear, but not exactly what you'd want for cruising or taking some kids or older folks out for a tour on anything but a calm day.
The central issue here is the effect of water on the motion of the hull. On perfectly flat water, the issue becomes nearly irrelevant, but since calm water is usually the exception rather than the rule, this is an issue most boat buyers should consider. As an example, let's use a 40 foot boat heading directly into 3 foot seas of more or less average frequency and steepness, at a speed of around 22 knots. You already know that, depending on bottom shape, a particular boat will have some tendency to pound or slam into the water as the hull comes off the top of the wave and heads into the next one. The flatter the bottom, the more it will pound; the deeper the vee, the less so. The deeper the vee, the more comfortable the ride is likely to be.
The extremely fine entry of this 38 sport fish is similar to an offshore racer and yields superlative rough water performance.
But bottom shape is not the only factor. Both the bottom and the bow have to contend with the oncoming waves. An extreme analogy would be to compare dragging a knife through the water either edge-wise or dragging the blade sideways. Dragging it sideways is going to create a lot more resistance and slow the motion down. The very same thing happens when the bow of boat is so shaped that it, too, presents a lot of surface area to the wave. The bow smacks into the wave, and in meeting all those tons of water, the water inevitably slows it down. Unfortunately, that slow-down is a nearly instantaneous impact, rather like driving your car into a concrete wall.
Thus, you have the difference illustrated between the performance of a boat with a very fine (narrow) entry versus one with a very full, wide bow. In fact, nearly all boats built prior to 1950 had knife-edged bows. The shape of hulls were gradually changed because pointy bows mean very cramped interior spaces, and what people want are spacious interiors.
There are two parts of the hull to consider. The shape of the bow both above and below the water line, because these can vary significantly. By means of comparison, we can look at the typical Hargrave designed Hatteras versus something like a Sea Ray. The shape of these hulls couldn't be more different, but so is the performance of the two types. The Hatteras has a full bow above the water line, yet below it has a rather deep, fine entry. When the bow faces a big wave while moving along at displacement speed, the upper part of the bow will meet the oncoming wave and there will be something of an impact, thus slowing boat speed. Conversely, at planing speed only the fine entry part of the bow is meeting the waves and thus slices through the wave with relative ease, making for a nice, fairly smooth ride.
Now we can contrast this with the Sea Ray style bow which is almost a spoon-bill shape that is quite broad both above and below the waterline. A fine entry it is not, and when it meets a wave, it may do so with a resounding thud.
Contrasts in hull shape. These two could hardly be more different. These two boats are the same overall length. Also notice the closeness of the propellers. This one doesn't turn very well either.
It doesn't take a genius to see why you're going to get less interior space with a shape like this. In return, you get vastly improved performance. Notice the huge difference in beam at the water line.
Next, we move directly to the matter of boat weight. Our subject wave is made up of many tons of moving water; our boat also weighs many tons. Ultimately, how pleasant of a ride our passengers are going to have is a direct function of the moving mass of the boat versus the mass of the wave, relative to the shape of the bow of the boat meeting that wave. Which is going to win the battle, the boat or the wave? Hopefully, it's evident to you that our very heavy Hatteras (Most weigh around 1,000 lbs per foot, or more. See table below for comparisons.) is going to perform a whole lot better than our comparably sized light weight, broad bowed Sea Ray. The Hatteras is going to slice through the wave and flatten it. For our poor, old Sea Ray, the wave is going to win the battle of weight, momentum versus hull shape. In other words, it slams, giving our passengers a more uncomfortable ride.
We got an excellent demonstration of this recently with the opportunity to sea trial a 50' Sea Ray express and a 50' Magnum a few weeks apart. The Sea Ray weighs 35,000 lbs and was ocean tested in 3 foot seas. The Magnum, a similar style boat, weighs in at 50,000 lbs, has a much deeper vee (25 versus 18 degrees) and a finer bow entry. It was tested during the approach of a hurricane in 5-6 foot seas, nearly twice the size of the Sea Ray conditions. I'm not going to tell you that the Magnum at 24 knots heading into these seas that we got a very smooth ride. That's not the case. Yet the motion of the Magnum was tolerable, and one did not have to hang on for dear life.
In the Sea Ray, the motion of that boat was positively violent in every direction. The reason was a combination of boat weight and hull shape, the later being designed to provide for maximum interior space. The 18 degree deadrise carries forward almost all the way to the forefoot, providing a huge amount of buoyancy to the bow. When it meets a wave, the bow just goes flying upward with great velocity, whereas the Magnum would punch through it. Hence the Magnum gave a better ride in conditions that were twice as bad at the same speed.
So it is that the mass of the boat (weight relative to volume) has a major effect on overall performance. Heavier boats simply resist the force of waves better. Speed and efficiency are indeed important factors to consider in any boat purchase, but not to the exclusion of performance and sea keeping ability on rougher waters unless one is boating on a small lake or river. To make a general rule of this, overall a heavier boat with a reasonably good hull design will offer a substantially better ride.
As a general rule, deep vee hulls are usually heavier because the builder intends to offer a superior handling vessel.
Deadrise is the term used to describe the angle of the hull bottom, normally measured at the stern. Of course the angle of the hull extends along the entire length and can be highly variable. In the example shown above, the 18 degree deadrise is nearly the same at all points. But for most boats, the deadrise angle will gradually increase toward the bow. Typically, just forward of the midsection, the deadrise will be close to double, and even more at the forefoot, up to around 70 degrees. A boat with this configuration is said to have a "fine entry" and will be far less prone to pounding.
You may notice that in many of our reviews we point out the superior performance (meaning speed relative to motion) of many of the bigger, heavier sport fishermen and cruisers such as Hatteras and Bertram. Conversely, we've said relatively little about flatter, lighter boats like Viking which are beginning to show the effects of trading off speed for sea keeping. Recently we've got three known instances of Vikings getting their towers thrown off in rough water. These towers were torn off as a result of being thrown FORWARD, not side to side. And we have no doubt that the culprit is anything but a function of lightness and hull shape, resulting in an extremely fast moment of inertia. We don't find Bertrams or Hatterases throwing their towers off in anything but severe storm conditions.
Frankly, we don't like to be so blunt as to make statements like, "The ride and sea keeping of this or that boat is positively atrocious." Unfortunately, many of them truly are.
Bottom line: When shelling out big bucks for a boat, it pays to pay attention to the performance of the boat under less than ideal conditions. The lighter weights and flatter bottoms that yield higher speeds may at first be very appealing. And so are those cavernous interiors. In the long run, these features won't mean much if the boat is going to beat you black and blue, or force you to stay tied to the dock. It all comes down to a battle between boat and waves, and which one has the right stuff.
Some weights for comparison*
|Hat 46 Conv||52,000|
|Bert 46 Conv||46,000|
|Fairline Squadron 50||38,000|
|Luhrs 40 Tournament||30,000|
|Tiara 43 Conv||31,000|
|Silverton 46 Conv||25,000|
*Boats selected for comparison are approximately the same vintage.
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Comparing the performance qualities of different boats is not easy because most people rarely have the opportunity to do so. This is one reason builders get away with putting out so many poorly performing boats; the average boater doesn't know any better, and doesn't get much opportunity to compare.
Give careful consideration to ideas such as "I'm never going boating in rough water." Those are good intentions that are subject to change as time passes. Compare what the weather is usually like in your boating area against your intentions. Do they match up?
Whether you're buying new or used, try to get a demonstration ride in something other than calm water. If at all possible, attempt to get a ride on a boat that has an established reputation for being a "sea boat."
For newbies, making such decisions can be decidedly difficult, particularly when issues of interior volume and spacious accommodations are concerned. When taking demo rides, pay less attention to the gizmos and and goodies, and more attention to how it feels to be on that boat. Is it relatively comfortable, or are you holding on for dear life all the time?
For the budget minded, fuel economy is always a major consideration. The question to be answered is whether the need for extra speed and/or economy is worth the compromise in sea keeping ability. Bear in mind that making a mistake gets expensive to correct.
For the serious cruising yachtsman, this should be the quintessential no brainer. You do not trade off sea keeping ability for accommodations, speed or economy lest too many well-planned cruises be aborted or screwed up because of weather.
If you're really serious about all-weather performance, opt for the heavier, more powerful boats, even if that means cutting back on your appetite for size. Heavier always costs more in every respect. But when all-weather performance is the issue, compromises have to be made.Originally posted April 19, 2000.