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Calculation information… and why we need it (Part I)

When a shipyard or naval architect contacts France Helices for the purpose of equipping a vessel with SDS, we ask for a fairly substantial amount of information, some may say excessive, about the vessel and its’ mission.


Well, because in order to obtain and information that has any real meaning, we simply MUST know this detail. Without it, we are merely making guesses. In the case of high speed and high performance vessels, small differences in information that may seem insignificant or irrelevant can yield big differences in the ultimate speed and applicapility to mission of the vessel.

Therefore, we always strive for complete accuracy in our calculations, and re-review vessel specifications before a purchase order is issued.

What do we ask, and why does it matter?


The first group of questions we ask is related to the climate where the vessel will operate the majority of the time. In most countries, the operating environment is usually fairly consistent, but not always.

1. Water Density (Fresh water or salt water): Salt water is denser than fresh water, and therefore produces greater hull resistance than fresh water. A vessel that will operate on a lake will experience a bit more speed than in the open ocean. Likewise, rivers with heavy sediment suspension have greater density than seawater, and hence greater resistance and slightly lower speed is the result.

2. Air Density: A boat operating on Lake Titicaca at 4,000 meters elevation experiences a lower air density, and lower resistance (Along with more difficult combustion, a different problem entirely) than a vessel at sea level.

3. Sea State: This varies on all bodies of water, but, obviously, rougher water means slower speed… Sometimes significantly. Why do we ask? Virtually all military or police agencies require measuring performance in sea state 2 or 3 (moderate chop and swell), as opposed to sea state 0 (like glass). Additionally, in certain regions, the seas are rarely in a calm state and a true performance calculations must take this into account in order to provide rasonable predictions of performance. Finally, as described in a recent article on this site, vertical acceleration, the g-force experienced by passengers and crew from slamming, becomes a critical factor for consideration in vessel design and performance.


4. Air Temperature: This is related to engine power. Engines cool less efficiently in hotter climates. However, since most engines (not all) are manufactured and rated in cooler climates, there is often a loss in usable horsepower. A vessel operating in the UK, with average air temperatures below 25 deg C will simply perform better than when it is operating in Dubai, where air temperatures often average above 40 deg C. This power loss can be significant, as much as 15%, depending on the engine design and rating, perhaps reducing speed by several knots.

5. Water Temperature: Just like air temperature, water temperature means less efficient cooling and reduced power output. This power loss is more relevant and acute on high speed vessels with high RPM propulsion than container ships or other displacement hull, low RPM vessels.

The climate information above is just the start. The next article will discuss the hull data we request, and why.

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