- An unusually low displacement-to-length ratio-48:4-reminiscent of pre-World War II, very high-speed destroyers
- An exceptionally slack, nearly Vee-shaped midship section
- Disproportionately large bilge keels
- Relatively limited main-deck freeboard
- An exceedingly low angle-of-entry forward waterline
- An unimmersed, well shaped, narrow cruiser stern
- A typical commercial ship-type bow bulb
- A conventional vertical profile bow with minimum flare A dramatically faired and shaped long and unusually deep forecastle
Except for its unique forecastle and bow bulb, the HSM hull form and hull proportions are similar to those of World War I four-pipe destroyers. These design features combined to:
- Minimize total hull resistance at maximum speed
- Generate adequate beam to arrange container cells nine-across within the hull Provide protection against deck wetness for containers stowed above the main deck
- Provide excellent freeboard forward
- Minimize its metacentric height and therefore maximize its roll period
- Damp roll motions at high speed
- Delay the onset of hull slamming
The overall result of these design features was relatively good seakeeping performance, particularly at relatively high speed, although very little of this may be attributable to the HSM's unique bow design. The high-speed seakeeping performance of the HSM hull form has limited importance for a warship, which generally operates at slow to moderate speeds.
The HSM hull form's resistance was relatively unimpressive, about 33% greater resistance per ton than present frigate hull forms at comparable speed-length ratios. The displacement-length ratio of the most recent U.S. Navy frigates and destroyers has varied from about 52 to 80, with the trend favoring short, fat ships because of their superior low speed powering characteristics. Generally, the lower the displacement-length ratio the lower the waterline angle of entry. For warships, the HSM's displacement-to-length ratio and angle-of-waterline entry are technically unattainable without accepting a substantial penalty in low-speed fuel consumption and acquisition cost.
The HSM derivative offered for the fire-support demonstrator incorporates significant above-waterline tumblehome and a more extreme wave-piercer bow, both designed to reduce radar signature. Unfortunately, these features combine to compromise severely the ability of the ship to withstand underwater damage. Severe tumblehome significantly reduces the reserve buoyancy of the above-waterline hull, and incorporating a wave-piercer bow will result in relatively high trim after experiencing underwater damage forward. It would be very difficult for a future surface combatant using a wave-piercer hull form such as the one proposed for the demonstrator to meet the U.S. Navy's present damaged-stability criteria for combat ships. Fortunately, the proposed demonstrator design has relatively high freeboard, probably caused by its underwater shape and the depth of its vertical-launch cells. Nevertheless, its high freeboard tends to offset its above-waterline signature-reduction features.
The speed of all ships in head seas is generally limited by ship motions. When their ships slam, most captains will reduce speed, change heading-or both. Seakeeping performance also affects the ability of ships to perform other evolutions at various speeds and headings.
Length and displacement have been the primary features that affect seakeeping performance. Modern research indicates that monohulls with flared full-waterplane hull forms, well designed moderately flared bows, adequate freeboard, Vee-shaped forward sections, and roll-stabilization systems will provide relatively superior seakeeping performance. All available evidence suggests that using a wave piercer hull form probably will reduce seakeeping performance, not enhance it.
Speed was important tactically when ships were armed with short-range guns and torpedoes. Today, speed provides strategic mobility, but has less tactical value. Increasing speed from about 22 knots up to 32 knots nearly quadruples the required propulsion power and has both significant life-cycle and acquisition cost impacts. Continuous high-speed operations also generate detectable wakes. Reducing fuel costs will be paramount in the design of new ships, which means that low-speed resistance will be relatively more important than providing one or two knots additional speed. The HSM hull form, therefore, which is biased toward high-speed operations, will be a less-than-optimum solution for future combatants.
The future Surface Combatant-21 will be a relatively large combatant whose length and displacement will provide significantly improved seakeeping performance. Its primary armament will be very long-range standoff weapons. The HSM hull form will have relatively limited applicability to such future combatants either from a speed, resistance, or seakeeping standpoint.
The use of severe above-water tumblehome significantly reduces internal useable deck area, which compromises the development of acceptable general arrangements. The standoff distance between the outer shell and inner magazines also is reduced, which compromises survivability. In high-freeboard ships, such as the proposed demonstrator, the upper strength of the hull girder will be concentrated in relatively narrow and vulnerable strakes of deck plating outboard of the vertical launch system and inboard of the deck edge.
Militarily useful stealth at sea is difficult to achieve; wake detectability compounds the problem. An exaggerated wave-piercer solution will provide marginal operational value, particularly when operating in littoral waters, or when operating at higher than loitering speeds. Stealth is tactically important in terms of improving the effectiveness of countermeasures and decoys-but tactically significant levels of signature reduction against surface or airborne search radars do not justify severe tumblehome or the use of a wave-piercer bow.
Seakeeping, speed, slow-speed powering, resistance to underwater flooding, general arrangements, survivability and realistic signature management all combine to suggest that the wavepiercer/HSM hull form should not be a candidate for use in future surface combatants. As reflected in the fire-support demonstrator, it is an unnecessary design feature.
The "new" wave-piercer hull form is not new at all. Unless carefully-even meticulously-assessed in an objective fashion, its premature selection could doom tomorrow's Navy to 40 years of frustration.