STOPPING THE LEAKS

An environmental alternative to the packed gland promises to stop leaks on marine craft with inboard diesel engines

Throughout history, boats have leaked where the rotating propeller shaft has passed from inside the hull to the propeller outside. Although packed glands have reduced such leaks to regular dripping, they need regular manual greasing and are difficult to set up and adjust.

Most solutions cannot run without a pressurised water feed from the engine, and many have found that silt in the water has caused rapid wear on the sealing faces. Now, Halyard has launched a solution to the problem. Its HMI Propeller Shaft seal has been designed to stop leaks on craft with inboard diesel engines of between 10hp and 500hp – covering almost every boat.

The HMI seal itself uses straightforward lip seals, modified with stainless steel components to run in salt water, in a novel arrangement which incorporates a floating seal assembly running on a special sleeve which slides over the existing propeller shaft. The whole assembly is connected to the boat’s stern tube via specially moulded neoprene bellows which allow the shaft to vibrate in time with the engine.

The company recognised that seals running onto the existing propeller shaft were vulnerable if the shaft surface was at all worn, or damaged. The problem was solved by incorporating a stainless sleeve which slides over the existing shaft, providing a smooth surface for the lip seals to run on.

In the past, almost every shaft seal on the market depended on the provision of a pressurised water feed from the engine. But this was costly to install, and had the disadvantage that a propeller shaft turning idly with a yacht under sail would receive no lubricating water as the engine would not be running. Halyard’s oil bath seal, with a tiny oil pressure reservoir, makes the unit completely independent of the engine and any external form of lubrication.

Traditional packed glands also required greasing, from a manual remote pump, every 30 minutes or so. This grease had only one place to go – into the sea or river. Today’s environmental concerns do not permit such behaviour, and Halyard’s lubrication arrangement avoids it.

Alternative designs required customers to set-up and adjust the existing glands and seals on the market – meaning that yacht owners affected the successful installation and longevity of the product. Halyard solved this by supplying a product which the fitter cannot open or adjust. The user simply slides it down the shaft, and clamps it to both sterntube and shaft.

In the event of the main seal failing, the HMI seal has three layers of safety: a spare seal right at the back, which must fail completely to allow water to reach the main seals; the principal pair of seals, which contain oil and would both need to fail to permit a leak; and the bronze bearings, which run on a tiny clearance on the shaft and would only permit a tiny rate of leak if all three seals failed completely.

Most important of all was the way in which the seal could absorb fore and aft movement of the propeller shaft, and the lateral and vertical vibrational movements of the shaft. Almost every marine diesel has soft rubber mounts, so the engine will move quite violently and carry the shaft with it. In addition, the engine, and therefore the shaft, jerks forwards slightly every time gear is engaged, as the propeller bites in the water.

Halyard solved this by creating an unitary assembly supported by, and running on the propeller shaft, and connected to the hull only by a flexible neoprene diaphragm which absorbs such vibrational movement in every plane. Not only does this solve the problem of shaft movement, but it insulates the shaft from the hull and reduces overall vibration levels.

Figure 1: The seal fits onto the engine shaft and is clamped to the sterntube. Once in place, it is lubricated from an oil reservoir, mounted above the waterline

{{HalyardTel: Whaddon (01722) 710922Enter 430}}