Fiberglass stern tube in a catamaran?

[Bruce626]

Yet another issue has come up in repowering my catamaran - it is apparently common practice to use a fiberglass stern tube to reduce electrolytic issues. The current tube in my boat is stainless so I have no experience with the use of fiberglass here. My concern is that catamarans are often beached, especially in third world areas where there are few places to lift boats out of the water. The yard that built my catamaran originally pushed it into the water using a series of logs as rollers; later, when they redid the antifoul they backed it onto the beach using ropes, and although they used sandbags to support it, I noticed that the propeller did come in contact with the sand (not while running) and they twice dug the sand out from under it. My concern is that a fiberglass tube - even a thick one might not have the same strength as the stainless. Does anyone have experience with beaching catamarans that have fiberglass stern tubes? Is this an issue I should be concerned about, or am I worrying mostly based on ignorance and the unknown? Informed opinion will be welcome. Thanks.

[Pauls]

Hi Bruce,

I've never used fiberglass tube for a stern tube, but I've used it for other applications. If the tube is heavy wall and well made it will be plenty strong enough. You don't say what your hull material is. For a fiberglass hull a glass stern tube is sensible, easy to bond in. If it is wood you will need to look at how it will attach to the structure.

[Bruce626]

The hull is plywood covered in fiberglass. The existing tube is at a significant downward angle since the transmission is a V-drive. It runs through a glassed in block of some kind under the hull - this will be filled with epoxy and a new tube put in at a shallower angle with the new engine installation - it is possible that the block will be reused to help support the new tube; but the exact details are up to the installer.

[Pauls]

Hi Bruce,

A question for you - why do you have a concern about the strength of the stern tube? Typically in a wood boat the stern tube is just there to protect the wood from worms, etc. The stern bearing normally is bolted to the wood structure and the shaft seal is likewise bolted to the the shaft log in the interior. The tube is just protection. If this is how your boat is built, then the stern tube is just there to protect the wood and it does not need to be real strong. In this case almost any fiberglass tube will work, just epoxy it in well.

Regards, Paul

[Bruce626]

I am concerned because I am really new at owning boats and likely too stupid to know any better. My only, and very limited, experience is with what I have seen on my boat and a couple of others in the yard where it was built - they all have stainless steel tubes and I didn't know they could use anything else, My catamaran has been on the beach several times for various reasons and I just want to make sure that changing from steel to plastic is not going to be a future problem. Your explanation is really helpful - I just want to make sure that the dealer and installer are clear on all the details and that I provide acceptable input when they turn to me and ask 'What do you want to do?'. likely I will learn a lot from this process and yet will probably never have to do it again - still, understanding the systems I will have to maintain is important to me.

[Pauls]

Hi Bruce,

Fully understood. Looking at your boat and how it is built in the stern tube area should make it clear whether strength is important. And then you can make a decision about how strong your stern tube needs to be.

Whatever you end up using for the stern tube, it is really important that the wood be full protected. Protected means isolated from contact with sea water. If the sea can touch the wood then worms can get into it and they will eventually eat away that wood. So you want to seal the wood well.

The stern tube hole is bored thru your keel. The bore exposes a lot of cross grain wood. Cross grain is very absorbent, it will soak up a lot of liquid. This is imortant because if you brush on some epoxy and then slide your tube in the wood can absorb most of the epoxy in the joint and you end up with a lousy joint. It's important to use a very slow setting epoxy on this, apply a good thick layer and allow some time for the wood to aborb what it will. Then apply another thick layer to both the wood and the tube before you slide the tube in. The first application should be regular, not thickened epoxy. If the tube has any significant clearance between itself and the hole then the second application of epoxy should be thickened enough that it will not sag and run out of the hole. So, first layer unthickened, second layer thickened if there is any noticeable gap between the tube and the hole.

Regards, Paul