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Acceleration: a new problem for high-speed boats
Written by Administrator    Thursday, 09 August 2012 13:06    PDF Print E-mail

Acceleration acting on a high-speed boat can be subdivided into three directions in relation to the boat:
- Longitudinal accelerations – horizontal increases or reductions in speed, extreme cases being crash stops and collisions
- Transverse accelerations – mainly during high-speed turns due to cen trifugal force
- Vertical accelerations – due to craft motion on waves Below, we will look at some important issues in more detail.
In the last few years, the availability of more powerful engines and lighter composite structures has led to an increase in speed for pleasure, commercial and special craft. On the seaway this increase in horizontal speed and associated considerations for emergency stops, sharp turns or collisions, often coupled with vertical acceleration (i.e. unsteady vertical motions) and comfort of ride considerations, are defining modern safety requirements. These acceleration factors are already used in car design, public transport and other fields, but are still fairly novel for boats. For the pleasure boater, speed is an important image factor and sales point, but unfortunately the average pleasure boater does not consider acceleration levels on his craft at that desired speed. Some commercial small boat operators in Asia are negligent about the impact of acceleration on human comfort and performance as well as on safety, though for fully class approved and certified high-speed commercial vessels, these factors are of primary concern.
Longitudinal acceleration and cases of collision
Statistics of boating accidents indicate that collisions between one boat and another boat or object are the most widespread accident type with small craft, comprising up to 60% of total accidents [9]. During collisions, two stages are considered:
1. Boats come into contact and keep moving, penetrating and damaging the structure of each boat's hull. Injuries at this stage are prevented by 'stronger structures' and by not locating passengers in dangerous zones of the boat.
2. The boats have almost stopped, but people and equipment on board continue moving, causing damage and injuries. Injuries at this stage can be minimized by proper accommodation design, use of seats and handholds in their correct locations, avoiding shifting of luggage and heavy items, etc.
Research carried out on pleasure boats with safety test manikins [5] indicated that acceleration levels during collisions can comprise 5-8g and even higher. Most injuries, including major damage to the neck and spine, occurred during the second stage of collision by contact between bodies and boat structures. Presently there are no regulations for this type of craft, and such collision events would not be possible to predict. However, good design should always take worst scenarios and risks into consideration.
For commercial passenger craft designed to IMO High Speed Craft Code [2] maximum levels of acceleration during collision are classified in two design levels:
  • Design Level 1 is characterized by longitudinal 'crash accelerations' below 3g. For this type of craft, sofa seating and bar counters are allowed in any direction and there are few other restrictions. This level of acceleration refers mainly to slower boats or to relatively large craft.
  • Design Level 2 refers to 'crash acceleration' levels between 3-12g, and restrictions are imposed on fore and aft seating, use of seat belts or other protective measures, other limitations on accommodations and limitations on luggage storage.
    Most of the speedboats used in Thailand are of Design Level 2. Though formal compliance to HSC Code is not required, it would be good practice to follow common sense and good design practices where feasible with regards to the size of the boat in question.
    Some of the most important (and evidently ignored on local speedboats!) safety issues on commercial high speed craft are:
  • Passenger seats should be located facing forward, or facing backward if seat belts with shoulder straps are used. For forward facing seats, belts are unnecessary if other seats or structures are protecting people from shifting forward.
  • The bow area of the deck, comprising a minimum of 4% of its total area (or more, depending on speed and hull material – there is a formula in HSC Code), should not be used to accommodate passengers or crew. This means that the front area of the hull/deck is sacrificial; damage to this area will absorb energy from a collision, leaving the accommodation area untouched. This limit also matches with require ments to have collision bulkhead placed in this section.
  • Luggage, bar counters, cargo and bulky items should be situated in such a way that they cannot slide during an emergency stop.
    The quintessence of safety features related to longitudinal accelerations is presented using one of our designs as a sample.
Transverse accelerations
Transverse accelerations are often checked for high speed commercial craft which are required to install rate of turn meters and are not to exceed a maximum of 0.2g of side acceleration during a high-speed turn. On smaller boats these meters are not used/checked, but experienced operators can avoid unfavorable effects by reducing turning speed or by using a larger turning radius. This helps to prevent passengers from falling out of seats, an event often coupled with increased heel during turns.
Vertical accelerations
Vertical accelerations are the most important of the three in the daily operation of boats and have a direct effect on passenger comfort, crew/ personnel performance and structural loads on the hull and equipment. Thus, vertical accelerations are one of the most important measures of seakeeping and structural loads for high-speed craft.
In engineering practice, vertical accelerations on high-speed craft are estimated by calculations using Savitsky- Brown or Hoggard-Jones formulas [11] or more sophisticated methods. Detailed information can also be obtained during tank tests and self-running tests of radio controlled models; we use both methods depending on the project's budget and level of requirements. After launching and during sea trials, vertical accelerations are also measured using three-component (or sixcomponent if angular accelerations are also of interest) acceleration gauges. Some sample results of measurements are presented for an 8m power catamaran. For a monohull of similar size and conditions, measurements indicate 30-50% higher acceleration values.
We used to perform seakeping analysis for all high speed craft at very early design stages, thus ensuring that the craft would be capable of performing the mission in designated conditions and at desired speeds [6-8]. Often customers' expectations and budgetary considerations for passenger craft or dive charter boat are to deliver higher speeds on smaller boats. We would certainly recommend an appropriate size of boat or catamaran as the way to increase ride comfort.
It is generally accepted that average vertical accelerations of 0.2g are acceptable for passengers. In this case, the MSI (motion sickness incidence) parameter will not exceed 10% of people in two hours having signs of seasickness, such as vomiting and task performance degradation [10]. The effect of acceleration is also evaluated using MII (motion induced interruptions) criteria, which shows how many times per minute a person must stop task activity to maintain balance.
Unfortunately the MSI criteria used in ISO [4] uses the root-mean square procedure, which 'smoothens' the peaks of accelerations, so it does not say anything about the possible injury that is likely to happen precisely due to those peaks. Thus, for special service craft, other criteria such as ICI (impact count index) [1] is used, based on the full spectrum of acceleration values and their possible effects on personnel.
So what are the features of crafts that result in lower or higher vertical acceleration values? Of course in design practice all of these are evaluated numerically, but there is some information that is useful for boaters. First of all, a reduction of speed in accordance with encountered wave heights will reduce acceleration levels; choose comfortable speeds for certain seastates and wave headings. Reduced beam of bottom (i.e. beam of chine) and increase of deadrise (especially at the front 40% of hull length) will definitely reduce wave impacts and accelerations. Catamarans generally possess narrower demihulls and higher deadrise angles, thus their acceleration levels are lower. In general, larger and heavier craft are more comfortable following vertical acceleration criteria. The most uncomfortable area on high speed boats is the front section, where accelerations due to pitching motions can be 20-100% higher than those at the center of gravity (CG). The most comfortable area of planning craft is from CG to stern, where accelerations are the lowest and are best suited for passengers.
Effect on structural design
In Albatross Marine Design we use different rules and standards defining acceleration levels and loads on structures. The two most important are vertical accelerations defining loads mainly on the bottom (peak loads of 1/100th highest are used), and the impact of acceleration at bow affecting loads at the bow area during collision.
Pleasure planing craft below 24m covered by international standard ISO12215-5 [3] will generally be in range a1/100 <3.0g of 1/ 100 of vertical acceleration values. This assumes that the operator is reducing speed values according to sea state conditions to avoid 'overloading' the boat. Another common approach is to limit loads on structure based on crew performance limitations - say, in Bureau Veritas Rules for yachts level of a1/100=2.5g is used as the upper limit of structural loads, that might be good reference for relatively large motoryachts. Similar limits are always used for commercial high-speed craft, where a1/100=2.0- 2.5g are common values derived from passenger comfort considerations. Here again, common practice is to assume that these craft are not operated far beyond the comfort limits of passengers.
Smaller craft, RIBs and some sportboats are operated at higher relative speeds, and thus levels up to 4-5g can serve as good reference points. These craft can be out of water for short times (become airborne) during operation.
For special and patrol craft, equipped with shock mitigation seats or designed for standing operation, those 'comfortable' levels of acceleration might be exceeded, as the craft are expected to perform rescue/patrol missions in robust environments. This will result design acceleration levels up to 6-8g.
What is the difference? For a 15m powerboat, an increase in design speed from 25 to 55kts will cause an increase in design acceleration level from 1.9g to 5.4g, thus causing an increase in weight of structure by 35% (if the same materials are used).
Conclusions
Acceleration levels are to be considered at all stages of boat design – from defining the general dimensions to the design of general arrangements and structures. Negligence of these issues can result in craft that is not functional and unsafe.
Understanding of issues related to acceleration is important for operators of both pleasure boats and small commercial craft. For passengers, our general advice is always to choose larger/heavier craft or catamarans for ride comfort, avoid sitting at the bow and sideways, and watch for some of the security features discussed above.
References
  • 1. Dobbins T. et.al. High Speed Craft Motion Analysis – Impact Count Index (ICI)// 43rd United Kingdom Conference on Human Responses to Vibration - Leicester, September 2008.
  • 2. International Code of Safety for High Speed Craft (2000 HSC Code) – IMO, 2008 Edition
  • 3. ISO 12215 - 5 Small craft - Hull construction and scantlings - Part 5 Design pressures, design stresses, scantling determination.
  • 4. ISO 2631: Mechanical vibration and shock - Evaluation of human exposure to whole-body vibration - Parts 1-5
  • 5. Lucas S.R., McGowan J.C., Salzar R.S., Planchak C., Getz G.E. Biomechanical Assessment of Small Craft Collisions// 2nd Chesapeake Power Boat Symposium - Annapolis, USA, 2010.
  • 6. Nazarov A. Application of catamaran concept for small commercial, special and pleasure craft// 16th High Speed Marine Vessels Conference (HPMV-2011) - Shanghai, China, 2011. – E18.
  • 7. Nazarov A. Power catamarans: design for performance// 2nd Chesapeake Power Boat Symposium - Annapolis, USA, 2010.
  • 8. Nazarov A. Practical Small Craft Design: Combining Art with Science// International Conference on Marine Design, Coventry, UK - September 2011
  • 9. Recreational Boating Accident Statistics – USCG, 2010.
  • 10. Ross J.M. Human Factors for Naval Marine Vehicle Design and Operation. Ashgate Publishing, 2009.
  • 11. Savitsky D., Brown P.W. Procedures for Hydrodynamic Evaluation of Planing Hulls in Smooth and Rough Water// Marine Technology, Vol.13, 1976.

About the author:
Albert Nazarov is a naval architect (1996) and managing director of Thailand-based design office Albatross Marine Design. He has a Ph.D. (2004) in the field of sailing craft hydroaerodynamics, and also possesses a Yacht Captain License. Together with his team, Albert is developing pleasure, passenger and special high-speed craft for builders in more than 10 countries.