SYAS Performance Pty Ltd · SYAS Performance Pty Ltd ... with the current cable steering system and...

Phone: 0449200139 | 185-187 Panguna St, Trinity Beach,Qld, 4879 | ww.syasperformance.com

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SYAS Performance Pty Ltd

14/07/2014 The following report is to formally present existing problems with the current cable steering system and suggest ways to improve the setup in order to minimize the risk of cable failure. My interest in this subject is not purely academic as I recently was involved in an accident due to a broken steering cable on the 12th of June 2014. While towing my skier at 70mph during a training run in Cairns, the steering cable snapped resulting in the boat barrel rolling through the air and tossing both myself and my observer from the vessel. My observer and I were extremely lucky to have only sustained bad bruising and scratches. We were also fortunate that the boat sustained only minor damage. In my role at SYAS Performance I have been asked to design steering systems for outboard engine ski boats. During this time, I have inspected many existing systems so as to know what to improve on. I have investigated and replaced many existing cable steering systems over the last 5 years and have found there to be one issue that is always present. This is the issue of cyclic fatigue at the hooded pulley. The research I have undertaken in order to prepare this report has led to modifications of our components and influenced the way I will design components in the future. As an experienced manufacturer of these steering systems I would like the information contained in this report to be used to improve the safety of power boat racing. The information is to help Boat owners and drivers become aware of the systems they are using, and the potential for mismatching components. This report highlights some very simple and in inexpensive solutions to the cable steering system. It also highlights how high the safety factors are if the correct components are used in the system.

Yours sincerely, Shayne Young Director SYAS Performance


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23 June 2014

ASSESSMENT OF CABLE STEERING SYSTEM ISSUES IN OUTBOARD ENGINE SKI BOATS

Prepared by Shayne Young (MRINA)

Abstract The objective of this report is to address the issue of premature cable failure in Outboard Engine Ski Boat Steering Systems. This report specifically addresses the use of wire rope to control the steering of the outboard engine. The aim of this report is to highlight the mode of failure and how to address the issue. The outcome should be improvements in cable steering system reliability and safety margins.

1.0. Steering System Issues Currently all Outboard Cable Steering Systems use either 4mm or 5mm 7x19 galvanised or stainless steel wire rope. The wire rope is arranged in a pull/pull configuration to control the outboard engine for steering. Multiple pulleys in various configurations are used between the helm and engine steering bars to successfully achieve this. The cable steering system, just like a hydraulic one, must be set up inside some basic parameters to make it effective and safe. Using components outside their limitations and parameters can lead to premature failure of the cable steering system.

1.1. Pulley Generated Failures There is a wide variety of pulleys available for the pull/pull steering configuration with one exception. This exception is the hooded sheave located where the wire rope exits from inside the boat to the outside engine well. This hooded pulley is a Ronstan RF2417 (see figure 1) and is where most failures occur. The Ronstan pulleys RF2422(Fig-2) and RF2417(Fig-1) were designed during the 1970s for boats with HP ratings that rarely exceed 100hp. The issue of wire fatigue was not well known during this period. The design of the pulleys gave no consideration to wire rope fatigue.

1.2. Large Span Generated Failures The other issue capable of creating fatigue breakages is large unsupported cable spans between the front turning block and the rear turning block. This is the area running down the side of the cockpit. In rough water this unsupported span bounces up and down flexing the wire rope in the middle (fig 3). Excessive flexure has caused fatigue and failure.


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2.0. Investigation into the fatigue issues at the hooded pulley

A good internet search has presented a wealth of information on the issue of using wire as a rope. The most informative industry is the rigging industry with cranes as the main driver for development. Some recommendations from aviation can be found, but is generally directed back to the rigging industry and wire manufacturers. Phone calls to an Australian wire manufacture also referred me to information on the internet regarding fleeting angles and drum to diameter ratios. (Just Google fleeting angle and drum diameter ratios) Below is information and findings from searches. I have also included some calculations based on new information and results. .

2.1. Investigations show that there is a ratio between the wire diameter and sheave/drum diameter that should be maintained. This is commonly expressed as the d/D ratio. This will apply to the pulleys and steering drums used in our steering systems.

2.2. The internet search also revealed an article from New Zealand maritime about steering sheaves being too small in boats causing failures. (appendix A)

2.3. Loose and Co wire manufactures show a simple and realistic chart as a guide

for using wire as a rope and the recommendations of d/D ratios. (Fig-3)

2.4. Investigations show that sheave groove design is another area that needs special requirements. Fig-4. (It should be noted that all readily available pulley blocks from Ronstan or Harken in the larger diameters, have sheave groove profiles suitable for high performance rope not wire).

2.5. Investigations show that a minimum critical d/D for the wire that is typical used

in our steering systems is 18:1. This critical 18:1 is considered not to substantially fatigue the cable. Testing would need to be done to determine fatigue life.

2.6. Further investigations of existing steering cables do not show any sign of

fatigue failing at the larger sheaves in the steering system. Only at the hooded pulley. The hooded pulley has the smallest turning surface at 45mm. The lack of fatigue failure on other components are most likely due to SRA’s 12month renewal of cable, now reduced to 6 month renewal of cable.

2.7. d/D ratios currently in use in our steering systems works out as follows:

2.7.1. Using 5mm wire around a 45mm sheave gives a d/D ratio of 9:1 this is below the recommended critical ratio.


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2.7.2. Using a 4mm wire around a 45mm sheave gives a d/D ratio 11.25 again

way below the minimum critical ratio. It does however highlight that the smaller diameter wire is better.

2.7.3. To achieve an 18:1 ratio with 5mm wire we would need a sheave

turning surface of 90mm

2.7.4. To achieve an 18:1 ratio with 4mm wire we would need a sheave turning surface of 72mm

2.8. A cable that is used around a sheave with a d/D ratio less than 10:1

permanently bends the wire. In the case of steering systems this means the wire is bent (not flexed) and then straightened again, as the boat is turned. This happens repeatedly in the same area along the steering cable causing it to fail prematurely.

2.9. Typical working load of a steering system has many different influencing

factors. The maximum load that could be put through the steering system was dependent on what the driver could hold on to. A super fit driver is realistically only able to apply 25-30kg of torque to the system for any length of time. This input can then be simply used to look at system loads.

2.10. Typical system load calc. Steering wheel diameter = 300mm, Steering drum

diameter = 100mm. This is a mechanical advantage of 3:1, so the steering cable working load is 30kg x 3 = 90kg.

2.11. Using a 4mm 7x19 304 wire that has a breaking load of 950kg (fig 5) gives us

a safety factor of 10.5:1. Well within acceptable limits.

2.12. There has been no alternative to using the RF2417 to date. SYAS Performance is now designing a replacement for the RF2417 that has a 75mm sheave and will be able to be retro fitted in to the RF2417 slot. The sheave has also been designed to accommodate 4mm wire with all the information above.

2.13. Cyclic load testing needs to be done on 4mm and 5mm, gal and stainless

wires to find out a service life for these wires. This way a more accurate steering cable swap out program can be established.

2.14. Determine from testing if a sheave smaller than 75mm can be used with a 12

or 6 month change over program. 2.15. Moving to a system that uses SK75 Dyneema gets rid of fatigue around any

sheave size and increases the breaking load size for size with wire. Larger sizes of SK75 Dyneema can be easily used as there is no issue with pulleys or drums. The problems with fitting SK75 Dyneema instead of wire rope which include abrasion, heat/fire, termination, would need to be investigated further.


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3.0. Investigation into the fatigue issues at large spans

3.1. There is much less information available in this area. While it is acknowledge

as a contributor to fatigue failure there are no simple hard and fast solutions

3.2. Any span that is large enough to have the wire self-weight contribute to bounce needs to be reduced.

4.0. Recommendations for updating your steering system

4.1. A strong recommendation is made use 75mm sheaves where possible. Look

for sheaves with a wire groove profile instead of rope where possible.

4.2. Minimum Steering drum of 75mm

4.3. A 4mm diameter wire only to be used. Unless 100mm sheaves and drums are used to allow for larger 5mm wire rope.

4.4. That 304 grade stainless and galvanized wire be used and 316 grade wire not be used if possible.

4.5. Wire construction must be of flexible 7x19 or better. Wire that is designed to be

flexed. 4.6. Service life of cable is a maximum of 1 season. Or x number of hours

(depending on results from testing).

4.7. No span of wire be larger than 1.5m 4.8. Look at replacing the wire rope with UHMWPE rope in the future.

5.0. Conclusions Cable steering systems for outboard ski racing boats is precise and direct. In some cases it is the only safe solution to use. Understanding the requirements of the components in the system is essential, so as not to create premature failures. The findings in this report would also apply to inboard boats using wire rope in the steering system. While making a d/D ratio of 18 or more into the rule would be ideal, it will take a little time to redesign system components to suit. Eliminating 2 of the major issues immediately is possible with low cost and ease of implementation.


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FIG 1 – RF2417

FIG 2 – RF2422

FIG 3 – Cable Span


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Fig –4 Fig – 5


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Fig- 6


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Fig.-7


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Fig. – 8


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APPENDIX –A

Maritime New Zealand Guidelines

SAFETY BULLETIN ISSUE 17 – January 2011 (updating September 2008)

Small craft wire cable steering controls

This safety bulletin is for:

owners, operators and crew of commercial vessels and jet boats ship

surveyors

safety auditors authorised persons

safe ship management companies maritime

safety inspectors

manufacturers of vessels and jet boats with wire cable steering controls.

Purpose The purpose of this bulletin is to recommend that all operators of boats fitted with a cable and pulley type

steering system consult with the wire cable manufacturer, or supplier of the system, to ensure that the

pulley’s diameter (it’s sheave diameter) is large enough for the wire cable being used.

Safety concern: the pulley-to-cable diameter ratio Following a steering cable failure on a commercial jet boat in 2008, Maritime New Zealand carried out extensive testing and analysis into the cause of that failure. The results of these tests, and their analysis, has brought attention to the critical importance of the correct

pulley-to-cable diameter ratio to be applied when employing such systems. If the pulley diameter is too

small for the cable then the risk of fatigue failure, due to higher internal stresses, is extremely high and the

service life of the cable is dramatically reduced.

Wire cable materials, dimensions and pulley diameters The length of time a metal wire cable can be safely used as part of a boat’s steering gear arrangement,

before being renewed, is determined by a number of factors set by both the design and the owner’s

maintenance of the steering equipment. The main factors are:

what the wire cable is made of (the type of metal and what that metal’s properties are) how it is

constructed (how many strands it has and in what type of geometry) the dimension of the wire cable (its overall diameter ‘d’)


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the size and type of loads applied to it (the repeated fatigue loading of the wire cable)

the pulleys the wire cable passes through and how tightly the wire cable is bent by those pulleys the

diameter of the sheave groove where the wire cable rests in the pulley the abrasion, damage and corrosion the wire cable is exposed to. The relationship between the dimension of the pulley (D) and the dimension of the wire (d) is critical for each

type and material of wire used. Examples of ratios between these dimensions would be “30:1” or “25:1”. If the ratio of pulley diameter to wire diameter is too low it bends the wire too tightly and the repeated stresses

cause the wire to break. The diameter “D” is measured at the innermost profile or root of the pulley’s sheave

and the measurement “d” is the maximum external diameter of the cable. The type of metal that the wire is made from also makes a difference to the fatigue life of the wire. Stainless

steel wires generally need a larger pulley-to-wire diameter ratio than galvanised steel to avoid fatigue

failure. This is because the material properties of the metal, when the wire is bent too tightly, cause it to

break sooner. When assessing the suitability of a wire for installation, and when sizing the pulleys to be used with this wire,

the wire manufacturer’s recommended pulley-to-cable ratio for the wire involved should be used. The

manufacturers often also state a minimum ratio that is acceptable, however, the minimum ratio should only

be applied if the design of the steering gear prevents the recommended ratio being used.

Inspection of cable and pulley steering arrangements Wire cables are consumable items of equipment. They have a limited lifespan and owners are responsible for

undertaking regular inspection of the wires to check for any wear, kinks, compressions, abrasion or corrosion.

Pulleys also have a limited lifespan and their alignment, sheave wear, balanced rotation and secure

foundations are very important. Checks must be completed on the boat’s full steering system prior to

operations, especially behind any pulleys. If any broken strands of wire are discovered, the cable should be

replaced immediately. Where a pulley is hinged, the tension of the wire is important to ensure the wire is aligned properly with the

groove of the pulley; this should be checked during the inspection of the cable and steering arrangement. It is

recommended that the alignment of the pulleys is within 2 degrees of alignment of the cable. Cable and pulley steering arrangements should be closely inspected at regular intervals paying particular

attention to the length of wire that works with the pulley, the tension in the wire and how securely the wires

are terminated. Any cable showing signs of significant wear, damage or corrosion should be replaced at the

earliest opportunity, regardless of the next planned renewal.


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Safety Bulletin issue 17 – January 2011 (updating September 2008)

Importance of lubrication Corrosion from water, and abrasion from dirt or dust contamination, can greatly reduce the lifespan of both

the wire and the pulley. A suitable lubricant will help in protecting the wire cable from this type of damage.

Lubricants can also significantly prolong the fatigue life of a cable that is subjected to cyclical stress reversal

(as is experienced in a cable and pulley application).

What should be checked and considered

If you have an existing boat you should check to see what diameter of pulley and what diameter of wire you have in your steering system. You should find out what type of wire you have installed and take note of the dimension, type, grade,

material and manufacturer. This information is important for when obtaining advice from the

manufacturer on what size of pulley is appropriate for the wire being used. Some boats may have pulleys with a diameter of 75mm. Where this is the case the material and

dimension of the wire should be selected appropriately to the wire manufacturer’s recommendations or the

dimension of the pulley increased to suit. In all cases where it is possible to increase the diameter of the

pulleys, this should be fully considered as it increases the life expectancy of the wire. It should be kept in

mind that for a given diameter of pulley, a thinner dimension of wire may sometimes promote a longer

fatigue life. Where stainless steel wire is being used the diameter of the pulley is considered critical. Where the

dimensions of the steering equipment do not permit the use of an appropriate diameter pulley for stainless

steel wire, it is important that an alternative material of wire is installed. Galvanised steel wire is generally

less prone to fatigue failure than stainless steel and can therefore normally be used with slightly smaller

pulleys, but the manufacturer’s recommendations on pulley diameter should always be considered.

Recommendations

All operators of boats fitted with a cable and pulley type steering system should consult with the wire

manufacturer, or supplier of the system, to ensure that the pulley diameter is within acceptable limits for

the wire being used. Wherever possible, the wire manufacturer’s recommended pulley-to-cable ratio should be applied, in preference to the minimum ratio, when sizing the pulley for a given wire.

Wherever practicable, the largest possible diameter of pulley for that wire should be considered. The condition of the pulley’s sheave, its bearings, and the foundations for the pulley are all important to check at regular intervals. Operators of boats with a steering gear that use a 5mm diameter stainless steel wire running though a 75mm diameter pulley should urgently seek the advice of the manufacturer of that wire. Operators should carefully consider, based on advice from the wire manufacturer, the running hours at which the steering gear wire is renewed.

Cables should be lubricated in accordance with manufacturers’ recommendations. Any wire showing signs of significant wear, damage or corrosion should be renewed at the earliest opportunity.

Regular inspection of steering gear arrangements is critical to boat safety. Further Information For further information please contact our Wellington office: Phone: 0508 22 55 22 or (04) 473 0111. Fax: (04) 494 8901 Email: [email protected] Safety Bulletin issue 17 – January 2011 (updating September 2008)

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