Question: I have just bought a used trimaran that uses waterstays. Is there an easy way to calculate the required strength for such an item?
The maximum load on a waterstay will depend on a number of factors, many of which will vary with the physical layout of each design. However, a generally conservative approximation can be obtained as follows:
Take the full buoyancy of the ama (float) at total immersion and deduct the ama dryweight. Call this UT for upthrust. Use this figure in lbs allowing 64 lbs/ft³ of volume for saltwater application.
Take the ½-beam of the total boat, measured from the centerline of the centerhull, out to the centerline of the ama. Call this ML for max.lever (in feet).
Measure the vertical distance of the forward waterstay attachment at the main hull, down from the center of the aka (crossbeam) in feet. Call this WSL for waterstay lever. (If the aft waterstay distance is VERY much less than the forward one, then you may need to calculate them separately, as less distance, will increase the waterstay load.)
The estimated maximum load (EML) in the most critical forward waterstay would then be:
EML = 0.67 × UT × ML / WSL (in lbs)
Note that I suggest using 2⁄3 (0.67) of the total aka volume on the forward waterstay as a design figure, as the forward one typically takes a higher portion of the load. But depending on the layout and also location of the aka, this 2⁄3 value could vary upwards, and on a few designs might even justify using up to 1.0 in the given equation.
To this EML you need to add a significant safety factor. Dropping into waves can more than double the static load and degradation with time is another factor. Multiply by 3 at the VERY least and preferably by 5, to find the required breaking strength of the waterstay cable and its fittings. It's often the forged end or the attachment that fails, not the wire itself. Depending on service, replacing such waterstays every 10‑15 years (shorter period for synthetic fibres) is also smart, as their failure can lead to catastrophic failure of the whole boat.
The exception to attachment failure, is when the waterstay is poorly handled ashore and picked up by an end fitting only. This cable of often heavy enough that this so loads the cable entering it, that wire strands can fracture as they exit the fitting. So a waterstay with heavy end fittings, MUST be handled with both arms outstretched or with two persons to avoid stress at the fitting exit.
This is the last place I personally would use any sort of synthetic fibre. This is mainly due to the risk of being impacted and abraided by floating debris. A steel wire rope is more durable for this and as the weight is also low down, there seems far less reason to replace this SWR with a synthetic one, compared to rigging up a mast.
If replacing an existing waterstay, I would either use a size derived from the above calculation with safety factor, or use the same size as the existing cable—whichever gives the larger size. To ever justify a replacement with a smaller size cable, (eg for weight or cost saving), this would require a more sophisticated calculation requiring more specific detail of the specific boat design, geometry and rig.
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