Understanding Your Boat

By Rob Naysmith Down South Marine

IN the last edition of SKI-BOAT we discussed hull types and the effectiveness of the chines on a planing hull. It’s all food for thought as we wait in anticipation of a time when we can all go fishing again. In this issue we will look at other important features so that the next time you are scolded for gazing lovingly at your boat, you’ll be able to say you’re studying the chines, strakes or deadrise. And believe me, just using those words will stop any further questioning.

We finished the last article looking at features that give a planing hull lift, and as lift is the fundamental feature of a planing hull, let’s discuss other contributing factors ...

Strakes — those ridges running parallel to the keel, lengthwise along the underside of a ski-boat hull — are designed to serve two purposes. Firstly, they aid lift as the water pressure on the flat surfaces forces them upwards, and secondly, they help deflect the spray when the boat is planing.

The problem with strakes is that they create drag due to their increased surface area, and if they’re not perfectly in line with the water flow, their drag effect increases still further.

With this in mind, many designers make use of the “stagnation line” — the theoretical waterline under the hull when the boat is planing — for determining the ideal number and lengths of the strakes, all in an attempt to minimise this drag. This is evident on many hulls where the strakes end a third of the way before the transom, usually in a V formation due to the hull shape and stagnation line.

In practice what is achieved is lift and spray deflection in the bow to midsection areas where it is needed, and a reduction in water-drag on the aft planing area.

One final aspect to check on the strakes is that they do not come to an abrupt, square type end before the transom, and if they do then the strake should not be in line with the engine.

Unless a strake has been tapered off in a neat transition to the hull, air bubbles will be generated. This stream of bubbles can play havoc with your engine by increasing prop slippage and, in severe cases, causing the engine to overheat.

Another important point is never to position echo-sounders or water speed transducers in line with a hull strake because the turbulence coming off them will adversely affect the readings.

Unless the boat has a completely flat hull from bow to stern, it has a deadrise. Deadrise is the angle of the V from the keel to the chine and is found on both mono- and catamaran hulls, usually quoted at the transom. The deadrise angle of a hull determines the way the boat rides. The sharper the deadrise, the easier it knifes through the water, also making the re-entry into the swells more comfortable. The shallower the deadrise angle, the more stability the boat has and the easier the hull will lift out of the water, but the re-entry into the swells will be harder.

This is where differences in boat design really begin to come to the fore.

Let’s start with the most evident use of deadrise, the deep ‘V’ hull. A deep V has a sharp deadrise running the entire length of the hull. As they run deeper in the water, using the sharp hull to slice their way, these boats are great for use where the sea conditions are often rough there are big swells to contend with. This hull type is well suited to travelling long distances as the ride is softer and more comfortable for the crew. A deep V will commonly have a transom deadrise of around 21 to 26 degrees.

A planing or semi ‘V’ hull is better suited to less rough conditions where it can use the lift of its relatively shallow deadrise to ride and stay up on the plane. These hulls are more manoeuvrable and can run in shallower water than a deep V, making them perfect for surf launching, estuaries, rivers and inland waters. These hulls have a much shallower deadrise from midships towards the transom, while still making use of a sharper deadrise in the bow section. Commonly the transom deadrise will be around 12 to 20 degrees.

Cats have a clever design — with the narrow hulls they can make use of the best elements of both designs. Here you can have a nice sharp deadrise in the bow for cutting into the water and either retain the sharp angle to the transom for use in rougher conditions or flatten it out to a shallow transom deadrise for manoeuvrability and shallow water work.

Cats mainly rely on tunnel lift to get the hull out of the water to allow the sponsons to do their work. This is the clever bit — cats still make use of the deadrise effect in their tunnels, although it’s not called that, nor would a scholar be associated with anyone proposing that notion. But yes they do, and that is determined by the height of the tunnel relative to the water line. Cats with a shallow, round tunnel have the semi V, low deadrise characteristics and make good use of water pressure for lift ,while those with big square tunnels, with less internal water resistance, rely more on air for lift, ride deeper in the water and have more of the deep V, sharp deadrise influence.

Transom angles are measured from the bottom of the hull to determine how far off a 90 degree vertical the transom leans out. This angle will determine the attitude of the bow of the boat when powered along. Most outboard transoms are around 12 degrees off vertical; this allows the skipper to trim the engine in to bring the bow down while also using less outward trim to bring the bow up once planning. The greater the angle of the transom, the more the bow can be controlled.

To help compensate for a low transom angle where extreme weight is situated aft due to engines and fuel, boats often make use of trim planers to get the bow down and help level the ride. Trim planers, those plates that protrude off the back of the transom, work by raising the transom whilst on the plane, forcing the bow down, and helping the boat run with a more level attitude.

Bulkheads are those sturdy supports that run from side to side across the hull below the deck. Their function is to give lateral strength to the hull, create watertight compartments to contain flooding and give support to the deck.

Stringers are found attached to the top and bottom of the bulkheads and run from bow to stern, with their function being to give longitudinal support to the hull. Much lighter than bulkheads, the bottom stringers are fixed to the inside of the hull for strength while the top stringers are used for supporting the deck.

This combination of bulkheads and stringers gives the hull enormous strength and reduces flexing on the water. During the production of resin boats, the bulkhead and stringer combination is fixed to the inside of the hull while it is still in the mould; this stops any distortion when it is removed.

The waterline is the theoretical line at which the water will come up the sides of a boat while the craft is floating on the water. Often a boat builder will calculate the position of a waterline based on weight, design and size, and include this marking in the finish. This line is particularly important over time when you notice your boat “lying deeper” in the water which is usually a good indication of compromised flotation and water in your hull.

Freeboard is the measurement between the waterline and the gunwale. Skippers who frequently venture out in bad weather rely heavily on a decent freeboard height to stop water coming in over the side.

Ski-boats have what we term “wet” or “self-draining” decks — that’s where the deck is able to take on water and drain off without causing any serious issues. The height of a ski-boat deck, determined in relation to the waterline, can be perfect on an unloaded boat, however, this may not be the case once the boat is loaded. If the deck level is below the waterline, water will rush in and swamp it.

How often do you hear folk complaining about their scuppers not working? That’s because water is coming in and not draining out which indicates that the deck, or a portion of it, is below the waterline.

Deck heights are important as they form the boat’s centre of gravity, determining balance and stability. A low centre of gravity will give better stability, which is why heavy items such as fuel and fish (on some boats) are stored below deck level. Everything stored above water level will cause the boat to rock more and be less stable, and the higher it is above the waterline, the more unstable things become.

The nature of the wet deck is to be able to take on water without too much concern, but there is always an urgent need for it to drain off as fast as possible. This requires large drain holes, usually through the transom, to allow a quick outflow of water.

However, as these holes are subject to submersion and thus allowing water back into the boat, scuppers are fitted on the outside. They take many forms with the most common being a cage with a ball that floats on the water and rises to block the hole each time the water threatens.

A gunwale (aka gunnel) is the flat surface that runs round the boat, on top of the sides. These add shape, support and strength to the hull while providing a place to mount all kinds of necessities such as railings, rod holders, etc. Gunwales are so named from the old sailing ship days where the crew would rest their guns on the sides of the boat in the fury of battle. This battle fury at sea is still evident today, displayed by the skipper when the crew cut bait on the gunwales!

Running all the way along the outer edge of the boat, from bow to transom, are what we term “rub-rails”. Not only to they protect the boat when it’s alongside a jetty or another boat, but they also cleverly disguise the join between the sides and gunwale.

Then we have “bow rails”, those shiny stainless-steel rails on top of the gunwales in the front of the boat which, as aesthetically pleasing as they may look, serve a number of very useful purposes.

Primarily a safety feature designed to keep the anchor man in the boat, the rail is also used to hold on to the bow at the dock or in the surf and, as we all know, if you have control of the bow, you have control of the boat.

Although mono- and cat hulls look different in every way, they employ the same basic principles using the same basic features, all that changes is how. Next time you look at a boat you’ll have a clearer vision of how her features will affect her performance, making your buying decision that much more precise. And, as I said in the previous article, we all need two boats.

Should you have any questions or require assistance regarding any boating issues, please feel free to contact Rob Naysmith at Down South Marine on 083 235 9550.