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

Continuing on with this brief series, France Helices asks for information related to the hulls. Additionally, before any orders are processed, we review a diagram or drawing of the hull, looking for things that might influence the hydrodynamics of the vessel, and hence resistance, performance, and speed. Hull characteristics, such as steps, can impact vessel speed by several knots, either up or down. Something that might seem minor, such as the size and location of spray rails, also influences speed and performance.

Surface drives operate solely within the realm of the laws of physics: We cannot violate them, regardless of whether we wish to or not. That is why this information is so critical.

calcs hull

1. Hull Type: Monohull, Catamaran, Trimaran: We need to know the hull type, since the vessel displacement, stability, and hydrodynamics are all influenced. An additional consideration is drive and engine placement, along with drive controls.

2.  Light and Full Displacement: This is often the most difficult figure for shipyards and naval architects to determine accurately. In fact, in 75% of cases where top speeds are not met, it is due to one factor: The vessel was built too heavy. Sometimes (quite often, in fact) this is the result of owners adding to, or changing the design after approval. Sometimes it is an honest mistake, such as neglecting to include the weight of wiring and cables (This is often forgotten… Cable runs on some vessels are quite large and add a lot of weight). Sometimes it is due to loose construction standards. There are hundreds of reasons, but the bottom line is simply: Higher Displacement = Higher Resistance = Lower Speed. Period. If the boat is built heavy, sometimes we can compensate with propeller or trim adjustments. Sometimes, the boat must be lightened.

3. L.O.A. and L.W.L.: We calculate hull resistance, and need to know the dimensions of the boat. Since draft and resistance will vary based on light or full load, this is why we ask for both lengths and both displacement limits.

4. Transom Beam and Vessel Beam: Wider hull = Higher resistance. With multihull vessels, the beam of each hull is required.

5. Draft: This is critical on all vessels, and greatly impacts resistance. On vessels that will be used in rivers, or landing craft, this figure is especially critical since we need to know if a surface drive is a viable option. Additionally, there must be enough water flow to allow the propellers to produce enough thrust.

6. Deadrise Angle: This is the angle between an axis running from the keel up the side of the hull and an axis running horizontally from the keel. The greater the deadrise (as in a V hull), the higher the resistance (and difficulty planning). Likewise, flatter bottomed boats experience reduced resistance.


7. LCG: This item is as critical as displacement, and is related to the ability of the vessel to plane. An LCG too far back can lead to the vessel “kiting” at speed, or trying to become airborne. Likewise, an LCG too far forward gives the vessel a tendency to “porpoise”, or dive. Surface drives are installed to provide level operation, and the LCG must be known. The point of planning is called the “hump”, and is the point of least efficiency of the vessel. A balance between LCG and power allows the vessel to plane easier and use less power to do so. LCG problems can be corrected by trim adjustments, or occasionally design changes, but in all cases the LCG must be known.

8. Transom Angle: We ask this to make certain that the engine and gearbox chosen can be mounted at the proper angle for surface drive use.

9. Spray Rails: Spray rails direct water flow along the hull, reducing resistance in most cases. We ask for the size and number of the spray rails.

10. Freeboard: Mostly related to design, but can be impacted by tight steering. Also related to spray pattern.

11. Hull Thickness at Transom: If standard, GRP is 65 mm, Aluminium is 11 mm. Thicker Hull = Higher Displacement. Cores of foam, Kevlar, or other materials also impact thickness. Finally, mounting of the drives is impacted. Minimum thickness for stability and even lengths of bolts.

12. Superstructure or Open structure: At full speed, wind resistance impacts speed. Superstructures increase this resistance. Some impact on LCG.

13. Stepped Hull?: Stepped hulls usually reduce resistance by aerating the hull, allowing quicker planning and reducing resistance. Improperly designed steps can also have a negative impact on speed. Steeped hulls can add 3 or more knots to speeed, so this is a critical item to know.

14. Construction material: GRP, Aluminium, Carbon Fiber?


This is most of what we require in order to make a reasonable resistance calculation. The next article will discuss engines and gearboxes.


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