Fibreglass Tubes

Fibreglass Tubes

Advantages and Disadvantages

Fibreglass tubes bonded in place with epoxy resin are a modern alternative to the traditional Bronze tube. They have the advantage of being completely inert and not subject to the electrolytic corrosion that is sometimes seen with bronze. They are also cheaper - much cheaper if the builder makes them up himself.

The disadvantages of glass tubes are that they are not suitable in all applications. If you have a traditionally build boat whose sternpost and deadwoods are large chunks of semi-seasoned oak, it obviously wouldn t be sensible to rely on a tube bonded in with epoxy. In an all-glued boat with laminated backbone, on the other hand, it would be fine.

A glass tube will also have a larger outside diameter than a bronze one. In some instances that can be a serious drawback. If the designer hasn t made allowance for the extra diameter in sizing the sternpost and deadwoods, it is easy to find there is very little wood left either side of the hole. It is a good idea to check the tube O.D. with your supplier before ordering. On the positive side, a fully bonded tube will add strength to the structure in a way the traditional Bronze tube doesn t.

Most stern gear manufacturers will supply fibreglass sterntubes with the cutless bearing ready installed outboard and with the inboard end machined to receive either a rubberneck type stuffing box, or a modern dripless gland.

When ordering fibreglass sterntubes bear in mind that the tube must project outboard of the sternpost about one third the bearing length in order to provide access to the locking screws that hold the bearing in place, and also to get the propeller far enough back for good water flow.

This last is very important if the propeller is to operate efficiently. Ideally the back of the propeller hub should be not much more than one shaft diameter aft of the bearing in order to avoid excessive side load, which means that with a glass tube the propeller gets pretty close to the sternpost. The cast bronze bearing carriers used with a bronze tube get the bearing much farther aft. When using glass tubes I prefer to cut back and fair the deadwood, rather than have an excessive length of exposed tube, which seems to me a weakness. See Figure 2 for an example of stern deadwood faired for a glass tube. http://www.tycopper.com/