Here we explain a few technical aspects of design, and bust some urban myths along the way. We’ve tried to explain it correctly but using terms a non-technical person can understand.
A more meaningful definition is the effective deadrise angle, which is the angle from keel to the outer corner of the chine. Naval architects and various authorities who approve designs use the effective deadrise angle to calculate the design sea pressure loading on the hull bottom. The effective deadrise angle can be measured through any transverse section along the length of the boat.
The best measure of how a vessel will ride is the effective deadrise angle at a section through the centre of gravity of the boat. For a small pleasure boat the centre of gravity is generally around 40% of waterline length forward from transom. Again, this is the measurement used by classification societies like Lloyd’s Register and others to calculate design vertical accelerations (or ride softness) for the vessel.
A reverse chine is a chine or spray rail set at a downward angle to deflect spray down and away from the boat. It can be any width or down angle, but as the angle and size increases, the effective deadrise angle is reduced, which makes the ride harsher.
Many boats with large reverse chines will be quoted with around 18 degree bottom deadrise, but the effective deadrise angle is only 12-14 degrees.
In layman’s terms you could think of a conventional deep vee hull with moderate chines as like a 4WD with nice long travel, progressive rate springs. A hull with large reverse chines would have shorter travel springs with a hard bump stop at the end. The reverse chines effectively ‘trap’ water rather than allowing it to be pushed out the side, which causes a spike in the wave impact pressure at the chine.
We use moderately sized chines with enough reverse angle to keep spray down, and spray strakes on the hull bottom forward. Our effective deadrise angle on monohulls is typically in the range of 20-22 degrees, which gives a markedly softer ride than a hull with only 13 degrees.
Many production and kit aluminium boats don’t have them fitted, and a variety of reasons are given as to why. We do fit them because they improve fuel economy, and help keep wind-blown spray down. The main chines on the boat are the primary spray deflector, but the closer to waterline the spray can be peeled off and turned downwards, the less chance it will be picked up by the wind and end up in your lap.
Contrary to what you might see on various internet forums, if they are correctly designed and importantly, not too big, they won’t:
- make the engine ventilate
- cause constant problems with your sounder
- make the ride harsher
- make sharks angry and cause them to bite your boat
We also incorporate a specially designed flap into the transom which can be closed to keep the ballast in, if you simply want to enjoy a smoother ride when conditions are really rough.
Note that stern drive versions aren’t ballasted because the engine weight in the hull bottom naturally lowers the boat’s centre of gravity.
Much of our discussion on this topic is about giving lift forces at the bow in following seas and preventing broaching. When the bow of the boat runs into the back of a wave, both static and dynamic forces are created. The static forces are due to the buoyancy of the immersed part of the bow. If you imagine suspending the boat from a crane pointing a bit nose down, and dipping the bow in the water – this is the static or buoyancy force.
Dynamic forces are created by moving an angled or shaped surface through the water at speed, and tend to be much larger than static ones. When you’re cruising at say 25 knots and run into the back of a steep wave which is doing 10knots, it isn’t hard to see that dynamic forces are going to dominate what happens next. The point is that you need the correct bow shape with angled bottom surfaces to provide dynamic lift. It isn’t good design to provide a high bow with lots of buoyancy up top to compensate for what’s wrong with the bottom shape below – by then it’s too late (see broaching!).
If a boat is directionally stable it will tend to continue in the same direction even if there is some force like wind or waves trying to turn it one way or the other. If a boat is directionally unstable it will naturally wander from one side to the other, and require constant attention to keep on course. If it is very unstable it will dive-off to one side rapidly without warning – ie. broach.
An example of a boat set-up which is directionally unstable would be a 3.7m car topper with two people sitting on the foredeck, and another person on the seat trying to row. Loaded this way the boat would skate all over the place and be almost impossible to control. If you moved the two crew onto the back seat, you wouldn’t get anywhere fast but the boat will hold course quite nicely. The reason is that with the boat nose down and transom almost out of the water, the centre of lateral resistance (CLR) shifts way forward (think of pushing the boat sideways through the water with one hand to find the balance point – this is the CLR). Having the CLR forward creates directional stability problems – its like firing an arrow backwards.
So, when you bury the nose into a wave, the CLR shifts forward, and the boat will tend to broach. So why do some boats show this behaviour and others don’t?
Firstly, any boat can be made to broach if conditions are bad enough or if poorly handled, so be extremely wary of claims that a boat cannot or won’t broach. Its more that some boats have certain characteristics that they tend not to.
In order to prevent a broach you need to keep the CLR from shifting too far forward. To do this you need first to stop the bow from digging in too far, which means the bow needs the right shape to create lots of dynamic lift when it runs into the back of a wave. The other main factor is the bow shape shouldn’t be too sharp. Fine shapes tend to have a stronger effect on the CLR, so a sharp ‘v’ has much more effect than a flatter ‘v’ or ‘u’ shape section (think of pushing it sideways through the water).
An extreme example of an ‘anti-broaching’ bow shape would be some of the European coastal rescue boats, which operate on coastal bars and surf-like conditions. The stem is curved in profile with no real straight upper part, and the overall bow shape is almost spoon like. Offshore racing boats are similar but are more like a ‘v’ shaped spoon, where the ‘v’ is not too sharp and will pop the bow up if it ducks into a wave.
Of course our bow shapes are more moderate than this, but the principles are the same.
OK – end of essay. We hope all of this makes some sense!