Ok, it's been awhile since I wrote a really contraversial topic... so here it is.
There is a serious issue with the Airmar through hull fittings which are used on all the Raymarine ST60 series knotmeters and depth sounders.
We are talking about fizzure cracks that can lead to catastrophic failure of the fitting, and a 2.5 inch hole in the bottom of your boat.
And yes.... it happened to me, BUT, I caught it while the leak was still managable.
History of through hull fittings:
It used to be that the transducers and dummy plugs had about 2 or three O rings on them. The OD (outer diameter), of the O rings was just slightly larger than the ID (inner diameter) of the through hull.
A little bit of grease on the O rings, and when you pushed the through hull into the fitting, it was a very close fit.
A few years ago, Airmar re-designed the through hull fittings to incorporate a flapper valve. Basically, this is a very thin curved piece of plastic, which has the same curvature as the ID of the through hull.
It has a berellyium copper leaf spring, and a pivot point.
It is actually very clever to be honest, and the whole purpose is to shut off the water flow when you pull either the transducer, or dummy plug while the boat is in the water.
It efffectively shuts off at least 80% of the water flow.
The way that it works is that the leaf spring puts a certain amount of pressure against the hinged flap, so that it will start to swing as you retract the transducer. Once the transducer is out, the upwards water flow pressure continues to swing the flap, much in the same way as a normal gate check valve.
So far so good you say.
Well here's the problem, and I cannot tell you how much you need to pay attention to this.
When we haul our boat during the winter, prior to being placed in the slings, I pull the transducer, and put in the dummy plug. Obvioulsy this is good practice in order to avoid damaging your impeller.
So, you pull out the transducer, bit of a water gush, (which tapers off), and then push in the dummy plug until spring time.
Of course, in the spring after launch, you pull the dummy plug once your're in the water, and put in the transducer.
So, why did my boat almost sink this year??
Here's why.
Remember the original old style through hulls? ID of the hull fitting is the same size, (just about) as the transducer.
But, the new Airmar units have shut off flaps. And if you look at the through hull fitting, there is a lot of air space inside the transducer housing, to allow for the mechanics of the flap.
Whereas the older through hulls sealed off on the ID of the housing, the newer units have an O ring at the bottom, which presses against a tapered ID at the bottom of the fitting. The other O ring is at the top of the transducer, (or dummy plug), which seals off at the front end.
So here is what happens in slow motion.
Pull transducer
Water flow up fitting.
Start to put in plug.
Water flow tapers off, but fills up cavity space inside through hull fitting.
As you tighten the transducer cap, the water inside the fitting starts to get compressed. There will be no air inside, only water.
Now you leave this sealed unit for the winter.
I could go over the math with you, but I can assure you that there is a substantial structural weakness on the exterior wall of the through hulll fitting where the nut threads are exposed. If you want to see how easilly threaded surfaces fracture, do the following:
take a stainless 3/8" bolt, and clamp it in a vise, leaving 2" of exposed bolt.
Clamp a pair a vise grips onto the end, and give it a sideways smack. Now wiggle it 3 times and watch it snap. By the way, this is the easiest way in the world to shorten bolts under your deck that you might otherwise spend 5 hours with a hack saw.
So you've got water trapped in this through hull, under pressure from the screw down cap.
Guess what happens when it freezes? The through hull starts to expand, approximatley halfway between the bottom of the screw down cap, and the nut holding the through hull into the hull.
And they crack. Just like that.
And here's the really scary thing. You might not even notice it at first. It just be a few drips, as mine indeed started.
But then you go out sailing, the hull flexes, etc. and the fitting splits.
At first I couldn't figure out why this happened, so I just took the needed measures and started calling around for a travel lift schedule.
What really was odd that a number of people I spoke to said, "oh yeah, raymarine fitting... had the same problems"
So knowing this wasn't isolated, I decided to figure out what was causing this.
I then contacted Airmar, who said they were unaware of this happening. Frankly, I'm not surprised. Most people just fix their problem, and can't be bothered to alert the general masses. Then spoke to Raymarine.
They were very supportive. by the way, of me posting this objectively on the Hunter site.
Ok, so what do we do about this????
Very simple.
After your boat is hauled for the winter, pull the plug, let the water drain, and then replace. 30 seconds.
I am encourging Airmar and/or Raymarine to attach a warning tag on the transducer handles.
There is a serious issue with the Airmar through hull fittings which are used on all the Raymarine ST60 series knotmeters and depth sounders.
We are talking about fizzure cracks that can lead to catastrophic failure of the fitting, and a 2.5 inch hole in the bottom of your boat.
And yes.... it happened to me, BUT, I caught it while the leak was still managable.
History of through hull fittings:
It used to be that the transducers and dummy plugs had about 2 or three O rings on them. The OD (outer diameter), of the O rings was just slightly larger than the ID (inner diameter) of the through hull.
A little bit of grease on the O rings, and when you pushed the through hull into the fitting, it was a very close fit.
A few years ago, Airmar re-designed the through hull fittings to incorporate a flapper valve. Basically, this is a very thin curved piece of plastic, which has the same curvature as the ID of the through hull.
It has a berellyium copper leaf spring, and a pivot point.
It is actually very clever to be honest, and the whole purpose is to shut off the water flow when you pull either the transducer, or dummy plug while the boat is in the water.
It efffectively shuts off at least 80% of the water flow.
The way that it works is that the leaf spring puts a certain amount of pressure against the hinged flap, so that it will start to swing as you retract the transducer. Once the transducer is out, the upwards water flow pressure continues to swing the flap, much in the same way as a normal gate check valve.
So far so good you say.
Well here's the problem, and I cannot tell you how much you need to pay attention to this.
When we haul our boat during the winter, prior to being placed in the slings, I pull the transducer, and put in the dummy plug. Obvioulsy this is good practice in order to avoid damaging your impeller.
So, you pull out the transducer, bit of a water gush, (which tapers off), and then push in the dummy plug until spring time.
Of course, in the spring after launch, you pull the dummy plug once your're in the water, and put in the transducer.
So, why did my boat almost sink this year??
Here's why.
Remember the original old style through hulls? ID of the hull fitting is the same size, (just about) as the transducer.
But, the new Airmar units have shut off flaps. And if you look at the through hull fitting, there is a lot of air space inside the transducer housing, to allow for the mechanics of the flap.
Whereas the older through hulls sealed off on the ID of the housing, the newer units have an O ring at the bottom, which presses against a tapered ID at the bottom of the fitting. The other O ring is at the top of the transducer, (or dummy plug), which seals off at the front end.
So here is what happens in slow motion.
Pull transducer
Water flow up fitting.
Start to put in plug.
Water flow tapers off, but fills up cavity space inside through hull fitting.
As you tighten the transducer cap, the water inside the fitting starts to get compressed. There will be no air inside, only water.
Now you leave this sealed unit for the winter.
I could go over the math with you, but I can assure you that there is a substantial structural weakness on the exterior wall of the through hulll fitting where the nut threads are exposed. If you want to see how easilly threaded surfaces fracture, do the following:
take a stainless 3/8" bolt, and clamp it in a vise, leaving 2" of exposed bolt.
Clamp a pair a vise grips onto the end, and give it a sideways smack. Now wiggle it 3 times and watch it snap. By the way, this is the easiest way in the world to shorten bolts under your deck that you might otherwise spend 5 hours with a hack saw.
So you've got water trapped in this through hull, under pressure from the screw down cap.
Guess what happens when it freezes? The through hull starts to expand, approximatley halfway between the bottom of the screw down cap, and the nut holding the through hull into the hull.
And they crack. Just like that.
And here's the really scary thing. You might not even notice it at first. It just be a few drips, as mine indeed started.
But then you go out sailing, the hull flexes, etc. and the fitting splits.
At first I couldn't figure out why this happened, so I just took the needed measures and started calling around for a travel lift schedule.
What really was odd that a number of people I spoke to said, "oh yeah, raymarine fitting... had the same problems"
So knowing this wasn't isolated, I decided to figure out what was causing this.
I then contacted Airmar, who said they were unaware of this happening. Frankly, I'm not surprised. Most people just fix their problem, and can't be bothered to alert the general masses. Then spoke to Raymarine.
They were very supportive. by the way, of me posting this objectively on the Hunter site.
Ok, so what do we do about this????
Very simple.
After your boat is hauled for the winter, pull the plug, let the water drain, and then replace. 30 seconds.
I am encourging Airmar and/or Raymarine to attach a warning tag on the transducer handles.
