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Slipway Update

1389151 orig
Published 21:58 on 25 Mar 2024

10 years after its commissioning in 2014, most of the Club's staging is holding up well - in line with the anticipated lifespan of 50+ years we were promised, subject to the need for ongoing replacement and maintenance. A formal inspection report commissioned last year spoke to a secure and enduring structure that is in good repair throughout, except for the intertidal portion (the sea end) of the slipway, which is now subject to remedial works.

Greenheart (Ocotea Rodiaci), the structural timber of both the staging and slipway, has been subject to biological attack from a marine organism known as Gribble Worm (Limnoria Terebrans), a yellowy pink crustacean about the size of a grain of rice. The intensity of the attack has increased exponentially over the last 12 months leading to a faster degradation of the boardwalk planks, and the main structural components of the slipway; the transom beams and to varying degrees, the vertical pilings.

The Gribble are not uncommon blights on marine structures. Yarmouth Pier, for example, has suffered a similar fate. Thankfully as Gribble do not survive in areas that dry out sufficiently, the majority of the staging is not at risk. It is just the end section that is submerged most of the time where we have a problem. 

The face of the enemy...

Gribble attack on decking boards.

A Gribble tea party!

An observed effect named socketing, where the fixing holes between main components grow as the wear and Gribble attack continues, led to significant movement under wave pressure, heightened stress on the structure, and more degradation. This continually worsening cycle led to localised critical failure this winter as a portion of the slipway collapsed, and we needed to repair it before a further failure caused harm to users, or disruption to the continuity of usage.

In more detail, the assessment of the slipway gave cause for concern in the following areas:

  • Five main pairs of pilings at the seaward end of the slipway show signs of Gribble worm degradation, in particular at the tops, where they connect with other structural components.
  • All transom beams associated with the above sets of pilings are heavily degraded, with evidence of the major socketing effect on all primary fixing points to pilings and evidence of gribble infestation leading to undermining of structural stability.
  • Boardwalk planks on the tidal area of the slipway have been partially eaten - in some instances >75% of the depth of planks deteriorated.

Slipway collapse!

Transoms out!

Socketing effects - note the bolt on the right!

Our first design challenge was how to protect the main vertical pilings. Whilst the pilings themselves continue to provide adequate support, the longevity of the wood cannot be guaranteed given the Gribble attack.

Replacement of these pilings is practically difficult, and materially very expensive. Replacement is also the most drastic option, and with years of life remaining in the pilings and little deterioration observed below the beach level, this option was discounted. Reinforcing these pilings, however, is readily achievable, and certainly necessary. Thus our approach of encasing the pilings with galvanised steel sections has been adopted. This steel, fixed to the pilings, running to their top, and embedded at the bottom of the pilings, provides a secondary, strong, secure, superstructure to build from.

The main transom beams on the slipway on the bottom five pairs of pilings will also be replaced. These were particularly subject to damage. Initially we looked at replacing these with lighter, more durable, and gribble-resistant composite material beams, to complement the GRP grating we have used at the bottom of the slipway for a few years. However the longevity of composite materials in marine environments is still relatively unknown, and this material choice was eventually dropped because lifespan could not be guaranteed. So, we have opted to use galvanised steel, replacing the transoms connecting the 5 pairs of pilings. These mate perfectly with the steel used to encase the pilings, with holes drilled for connections before the galvanising process, so the protective coating on all steel elements is preserved. Not only have they been engineered to withstand the elemental forces but will not be food for the Gribble.

All wooden deck boards in the tidal zone will be replaced with composite grating, including infilling the wooden middle section. The result will be a huge reduction on uplift stresses from wave action, and a further removal of the source of food for the gribble.

In its new form we believe, with annual maintenance, this is a long-term solution.

The project costs are broadly split into 2 equal parts; the materials (steel and composite decking) and labour/tools and plant costs.

The engineering and manufacture of the steel beams, precut and drilled for fixing pre galvanising accounts for half the project cost. The project management, installation labour, plant hire, other materials, and consumables account for the other half of the project. The total capex cost of the repair work was budgeted at around £100,000, with a contingency amount of £20,000, which appears to be largely unutilised as of writing. The financial underwriting of this project is paid from the Club's annual capex budget (we will be making savings elsewhere) and overall profitability. But this project also shows the need to build up our reserves by delivering a higher level of profit that we have achieved in the last decade. 

The Club project team consists of: Richard Dobbs (Commodore), Club Secretary (James Crofts), Nick Hewitt (Head Boatman), and William Robins (Finance Committee), along with, several relevant members and former Commodores with expertise and experience,  Nick Wakefield, John Edwards, Mike Smith, Richard Prest, and with special thanks to Hugh Eddowes.

External Contractors

Opex Construction Ltd Led by Aaron Hayward. Arron has historically delivered the following projects on behalf of the Club: Development of Boat Park and Training Centre, New entrance to the awning, Completion of new steps to staging, Enclosure of the awning by reconstruction of the wall adjacent to the Esplanade, ongoing maintenance work to staging and slipway. Opex is the main contractor and project management entity.

Aaron was a foreman for the original staging installation project some 10 years ago, and has integral knowledge of working in marine and tidal areas and on our infrastructure, which allowed us to derisk the execution of the project

Filers Engineering Ltd - Operated and led by Graham Filer. Filers operate locally and nationally in the fabrication and engineering of steel and iron.

Filers have led the installation of the main steel elements of the project and have consulted on the project as primary engineers.

Thus we were able to achieve an entirely Isle of Wight sourced solution.

The Project has taken or will take place during the following tidal windows, in 3 main stages:

Phase 1: 11th-14th February Demolish and remove existing Gribble-damaged elements. 

Phase 2: 10th-13th March

Installation of steel cladding for piles, new transoms, joists, and GRP decking. 

Phase 3: 8th-10th April

Completion of Slipway works to include rubbing strips and finish.

We are confident that the slipway will be operational as intended for the beginning of the season at the end of April. Members should note that, in line with our normal practice, we will not operate the slipway at Easter, but the Egg Throwing will go ahead on the staging. Our launch service begins on Saturday 27th April.

Best wishes for Easter,

James Crofts

Club Secretary

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