Different Hull Designs - A Basic Summary

Posted Date: September 04, 2013     By: Sanlorenzo Americas

Hull Design: a Basic Summary
In the true spirit of the Made-to-Measure philosophy of the yard, Sanlorenzo Yachts cater to all owners that seek the extraordinary, by offering three important hull options:

- Planing Hulls, Semi-displacement hulls, & Full displacement hulls.

Which are the most important attributes, that best suits your specific yachting needs and desires?   Speed? Stability? Fuel Economy? Cruising Range?

Brief Synopsis of Hull Design:

Displacement hulls:
• A displacement hull moves through the water by displacing the water, while a planing hull runs on top of the water.
• The overriding fact about displacement yachts is that their speed is limited by their length. Speed in knots = 1.34 x Square Root of Hull’s Waterline Length (in feet).
• These hulls are very fuel efficient; they require little power (relatively) to reach their maximum speed. Attempting to exceed hull speed will waste fuel and gain little if anything in speed.
• The Sanlorenzo 42EXP Expedition Superyacht, the 46STEEL and 52STEEL superyachts have displacement hulls.

Planing hulls:
• Designed to be fast, when they achieve planing speed, the yacht rides on top of the water.
• However, they are not designed to be as fuel efficient at slow speeds as other hull shapes.
• When a yacht is planing, the water flows off the bottom in a sheet and the wave breaks behind the yacht.
• To aid performance, trim tabs can be used on the hull at the transom / very stern.
• Sanlorenzo SL range from 78ft to 118ft, and the 40Alloy Superyacht have award-winning Planing hulls.

Semi-Displacement Hulls:
• There are yachts that fit between displacement and planing yachts; yachts that exceed the speed limitations of displacement hulls, but do have higher speed potential of full planing hulls.
• These yachts tend to be less efficient than displacement yachts at slow speeds, and less efficient than planing yachts at high speed, but the Sanlorenzo SD Range from 92ft to 126ft, offers fantastic advantages in:
- Unrivalled Fuel efficiency & Cruising range;
- Award-winning design;
- Lower carbon footprint attributes.

Further Comments & Advantages:

Planing Hulls
A planing hull runs the majority of the time on top of the water, achieving high speeds, getting from point A to point B very quickly. Rather than going “through the water” they go across the top of the water and are subject to the state of the surface of the water at any given time. Planing hulls travel at fast speeds, though are therefore less fuel efficient, so are not geared to longer range cruising.  
Example = Sanlorenzo SL106

Planing hull advantages:
- The more horsepower you add, the faster the yacht will go
- You can travel from A to B in the fastest possible way

An important question to ask yourself: Is cruising range and fuel efficiency of greater preference, than higher speeds?

Semi-Displacement Hulls
The Semi-displacement hull design is arguably, the best of both worlds. While typically most semi-displacement vessels do not cross oceans, many are capable and have the range and capability to do so.
A semi-displacement vessel can take advantage of increased power and can achieve greater speeds when needed. Applying additional horsepower consumes more fuel, but typically most semi-displacement vessels only take advantage of this available speed when needed. Most of the time, semi-displacement vessel operate in displacement mode and are very fuel efficient.
Due to the lesser draft as compared to full displacement vessels, semi-displacement vessels have less “wetted surface area”, requiring less horsepower to propel the hull through the water. A semi-displacement hull can achieve about 35% more speed with the same engine load requirement as compared to a full displacement hull form, or conversely at the same speed use significantly less fuel and energy.
Example = Sanlorenzo SD126

The semi-displacement hull advantages:
- Combines some attributes of displacement hulls with some features of planing hulls.
- The semi-displacement hull never completely planes, but relies on a flat forefoot and lots of horsepower to drive the yacht a little bit over its bow-wave, generating some lift.
- The yacht can be heavier than a full planing yacht, so it can be a little more stable in a sea or at anchor, but not as stable as a full displacement yacht.

Full Displacement Hulls
Full displacement yachts typically have deep drafts and rounded hull forms. These design characteristics are best suited for long offshore passages and low speeds. Displacement hulls are not able to plane, therefore are restricted to their “hull speed” which means that regardless of how much horsepower is applied to the drive train, the yacht can never go above the designed hull speed of that particular vessel. (Speed in knots = 1.34 x Square Root of Hull’s Waterline Length).
Obviously the deep draft of the full displacement hull form is not favorable to coastal cruising, especially on the Eastern Seaboard and the Bahamas, though the sheer cruising range possible opens up numerous options not possible in other yachts.
Example = Sanlorenzo 52Steel (171ft)

Displacement hulls have several advantages:
- They require relatively little horsepower to drive them through the water, so the fuel consumption of the yacht is much lower.
- The yacht pushes aside the waves rather than riding over the top, its ride is smooth and seaworthy.
- There is plenty of hull below the waterline, therefore the yacht tends to be stable and comfortable at anchor (where yachts spend most of their time).
- The lower speed of the yacht means that appendices such as zero-speed stabilizers are added to Sanlorenzo Yachts, increasing to the stability and seaworthiness of the yacht.
- The world is your oyster in larger displacement superyachts (52m Steel for example).
However, if a high top speed is important to your yachting lifestyle, then you need either a planing hull or a semi-displacement hull.

Concluding Remarks

When purchasing a yacht, choose a yacht to suit the cruising lifestyle that you plan to do for the majority of the time. Before buying your yacht, it is important to know something of the different types of hull design and bottom profiles, and to understand what set of compromises these will offer you. No single hull design will offer you all the advantages in one package.

That is the beauty of a Sanlorenzo Yacht – the design is a one-off, never two have been built that are the same, so each can be tailored to your specific needs and desires – much like a tailor-made suit.