Is a hull anode needed?

Tammany

Tammany

Well-Known Member
Joined
24 Oct 2015
Messages
694
Visit site
Getting to that point when going to need to sort out anodes. Boat is GRP sailboat.

Yanmar 1gm10 with small engine anode. Has hull anode at the moment but not connected to anything.

My intention is to fit one of those prop shaft anodes with rope cutter attached. Does it need a hull anode with this and engine anode??

No external metal apart from propshaft & prop.
 
You don't need an anode for the engine - it already has one cunningly hidden above the starter motor. May not be any need for an anode for the propeller if it does not currently have one and you are not experiencing any problems. If you want to be sure, bond the hull anode to the gearbox casing which will give it a path to the shaft, bridging the coupling if it is the flexible type. This is the normal way of protecting the prop.

Be wary of fitting one of those anode/cutters. Apart from not being very effective as cutters they need space in front of the prop and fitting it there may block water flow in and out of the stern tube and cutless bearing and prevent the shaft from moving forward when you go in gear. If you want a rope cutter fit one that works.
 
Tammany said:
Getting to that point when going to need to sort out anodes. Boat is GRP sailboat.

Yanmar 1gm10 with small engine anode. Has hull anode at the moment but not connected to anything.

My intention is to fit one of those prop shaft anodes with rope cutter attached. Does it need a hull anode with this and engine anode??

No external metal apart from propshaft & prop.
Click to expand...


If there is no corrosion of the stern gear and the existing hull anode is not connected to anything then there is no evidence that you need an anode, apart from the engine anode, at all.

You may chose to fit a shaft anode , or connect the existing hull anode, as a precautionary measure. Your choice as to which to do. The only downside is that you will have to inspect and replace it regularly in the future.

Connecting the hull anode as well as fitting the shaft anode may extend the life of the shaft anode very slightly.
 
Only one skin fitting and thats engine inlet and a plastic trudesign one. I'll remove the hull anode, the bolts are a mess anyways. My only concern is the engine one is so small so that's why I was considering the propshaft anode. I think it will fit as the prop is moving away from the cutless as fitting a r&d flexible coupling too which will give extra space. Yes their is plenty of space behind the prop too. I don't mind replacing a propshaft anode on a yearly basis.

I winder why the hull anode was fitted if they aren't really neccessary?
 
I was quite happy with the bottom of the boat. this pic was when she was first taken out. Nice clean prop & shaft. The rudder bearings were a bit worn but since replaced. The only downside was those poorly done keel shoes. Removed these though as they were leaking water behind them due to virtually no adhesion to the gelcoat of the keels. Re-doing next week in kevlar & epoxy.

HN7gDpwt.jpg
 
Tammany said:
Only one skin fitting and thats engine inlet and a plastic trudesign one. I'll remove the hull anode, the bolts are a mess anyways. My only concern is the engine one is so small so that's why I was considering the propshaft anode. I think it will fit as the prop is moving away from the cutless as fitting a r&d flexible coupling too which will give extra space. Yes their is plenty of space behind the prop too. I don't mind replacing a propshaft anode on a yearly basis.

I winder why the hull anode was fitted if they aren't really neccessary?
Click to expand...

A shaft anode and an internal engine anode are two completely different issues. One is to protect the prop, the other is to protect the engine cooling water circuit

I expect the hull anode was fitted by someone believing that it was necessary to protect the prop , but failing to realize that to do so the two must be electrically connected
 
Last edited:
It looks like it might have been originally fitted as it looks very old. The bolts even on the inside are very badly corroded. If I can fit a shaft anode I will, will measure up when re-fitting engine and flexi coupling. What is the recommended distance between the rear of a prop/anode & cutless bearing?
 
Tammany said:
It looks like it might have been originally fitted as it looks very old. The bolts even on the inside are very badly corroded. If I can fit a shaft anode I will, will measure up when re-fitting engine and flexi coupling. What is the recommended distance between the rear of a prop/anode & cutless bearing?
Click to expand...

Minimum 10mm. However if the boat has never had an anode connected then probably does not need one. As VicS says the engine is not protected by any external anode as it is not connected to anything in seawater. Many engines do not have, or need an anode, but the 1GM does as it is salt water cooled and the anode is to reduce internal corrosion.

If you are inserting an R&D, looking at the existing gap you will have plenty of room for a proper rope cutter. if you do want to fit an anode to the prop then suggest you use the type that attaches to the prop nut. you need a special nut and the anode screws on the end. Shaft anodes are for use on a P bracket arrangement where they are fitted ahead of the bracket, not between the bracket and the prop.
 
Will look into the prop nut anode, never seen one of those. Replaced the engine one when I serviced it, easy enough to do. It's not the original engine so maybe the hull anode was for the old engine? Good advice people, plan is now to remove hull anode, fit prop nut anode & rope cutter at some point. Your right about that other anode with cutter, it get's some terrible reviews.
 
Tammany said:
Will look into the prop nut anode, never seen one of those. Replaced the engine one when I serviced it, easy enough to do. It's not the original engine so maybe the hull anode was for the old engine? Good advice people, plan is now to remove hull anode, fit prop nut anode & rope cutter at some point. Your right about that other anode with cutter, it get's some terrible reviews.
Click to expand...

If the original anode was fitted with the intention of protecting the original engine, it was a mistake. The rule is that the anode needs to be able to 'see' the item it's protecting (and be connected electrically to it.)
 
Once all boats had some kind of anode , so what have change ?
What are the rules regarding having one or not ?
 
Bow42 said:
Once all boats had some kind of anode , so what have change ?
What are the rules regarding having one or not ?
Click to expand...

I don't think all boats did. It can actually be damaging for some wooden boats to have anodes fitted.

Anodes are to protect mixtures of less noble and more noble metals underwater. My understanding is that as a rule, proper bronze doesn't need protecting if it's not electrically connected to anything else. Anodes are (sometimes) needed when you've got a mixture of metals near each other underwater. Steel rudders and stainless shafts with bronze propellers etc all connected electrically would be an example.

I've had surveyors suggest I remove anodes as they were either doing nothing or they were actually doing harm.
 
Bow42 said:
Once all boats had some kind of anode , so what have change ?
What are the rules regarding having one or not ?
Click to expand...

There was a view in the past that all underwater fittings should be bonded in a circuit with anodes. Particularly popular with large MOBOs that also have lots of electrical equipment.

However, this has now shown to be unnecessary in GRP boats (and for different reasons not so good in many wood boats). skin fittings and seacocks are (or should be) made of corrosion resistant material and are not connected to any other metal under water so do not need anodes. Engines are not connected to anything in seawater, but may need an anode if any parts of the cooling system are made of mixed metals. The manufacturer fits them if necessary.

This leaves three areas of potential problems. The most common is stern gear where there is usually a potential difference between the prop and the shaft (as in the OPs situation). Sometimes this is a problem, sometimes not and the usual preventative measure is either a hull anode connected to the shaft via the gearbox (or direct to the shaft with a special brush device) or with a shaft or prop nut anode. Folding and feathering props usually need their own anode as they use mixed metals in the mechanism.

The second area is around the rudder if mixed metals are used under water. The three common metals are bronze, stainless and aluminium. Some designs only use one of these, usually stainless for stock and non metallic bearings, others use bronze bearings and stainless stocks, others aluminium stocks and stainless fittings such as quadrants. Most work OK without anodes, but some do benefit from them to remove the slight risk of electrolysis.

Third area is saildrives (and outdrives) which are a mix of aluminium and stainless and definitely need anodes which are built into the housing casting and in some situations (particularly older types with small anode volume) need frequent replacement.

On my saildrive boat the anode lasts about 3 years, although I usually change it every two, and the small button anode for the steering gear seems to last 2 years.
 
Over here on the left side-you won't find hull anodes on GRP shaft driven boats.
You will find egg shaped anodes on strutted shafts, donut anodes between prop & cutless-(with 6-10mm water flow space) &/or many prop nut anodes.
You will find anodes bolted or welded to mild steel rudders.
Steel & Aluminum hulls are another matter,as are Al. outboards, outdrives & saildrives.
 
I’ve read through the previous correspondence but I’m still unsure as to whether I need a hull anode:

The shaft is isolated from the engine by a flexible coupling. There is no room on the shaft for an anode. The space that was has a rope cutter fitted.

Apart from the prop there are no other exposed metal fittings. Everything painted over with anti fouling.

Thoughts would be much appreciated.

Peter
 
You don't need a hull anode, especially not connected to anything. When I bought my boat 23 years ago I removed a hull anode that was doing nothing. I have a prop anode, a shaft anode and nothing else. No corrosion problems.

Painting is good, although antifouling does not give much corrosion protection. If your prop is in good condition, no pitting or serious pinkness, leave well alone.
 
You must log in or register to reply here.

Share this page

Top