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Teakman
Deckhand

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9 Posts

Initially Posted - 06/05/2004 : 09:02:57

We just purchased a '77 Catalina 25. We have it on the hard, bottom painted and having the outboard serviced. We also intend to install a Raymarine ST40 knotmeter and depthmeter combined. My question is about the thru-hull fitting.

It appears you can use an actual through hull or one which mounts on the transom? It also appears the through hull version is correct for my boat. Now I have a knot meter through hull already but the instrument doesn't work. So, can I use the old hole? If yes, how exactly do you install it? Obviously I take out the old, re-cut the hole and install the new with 5200? If the old hole is too big, should I just leave it and start new?

I also have a coastal navigator depthmeter on the boat. It appears to have a transducer mounted internally on the bottom of the hull just under the starboard quarter berth.

Any help and advice will be appreciated.

Mike Masten

Edited by - on

dlucier
Master Marine Consultant

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Virgin Islands (United Kingdom)
7583 Posts

Response Posted - 06/05/2004 : 09:58:44

First, welcome to the forum, Mike!

This past spring I installed a new knotmeter on my boat and I drilled a new hole instead of using the old existing thru hull holes. I went this way because the old holes were, for one, in the wrong location and two, the old holes were bigger than what was needed. After removing the old fittings I patched the holes.

After drilling the new hole, the installation of the thru hull was pretty straightforward. The only thing that required a little extra work was the shim I had to fashion so the thru hull flange would sit flush with the outer hull. The shim was needed because of the curved inner hull and different hull thicknesses across the hole in the new location.

I highly recommend performing a dry test fit of your new thru hull to ensure that everything will fit properly.

Good luck.

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 06/05/2004 : 10:00:18

Transom mounted transducers don't work very well on sailboats. Too much turbulence back there, too many problems with heeling.

You should be able to use your old knotmeter thru-hull hole for the new unit if it's appropriately sized (2"). If the old hole is wrong (too big)... it can be permanently repaired using marine epoxy and glass cloth. There's another thread on this site that has details on how to do it. If the hole is too small, it is easily enlarged with a hole saw, roto-zip, round rasp, etc.

It's probably OK to leave the old transducer in there for awhile if you're anxious to splash the boat... but sooner or later, I'd fix the hole permanently if you can't re-use it for your new transducer unit. If you're already in the middle of a bottom job, now might be a good time.

If your ST-40 "Bi-Data" is like mine, you will have two seperate transducers to install.

One transducer is for the knotmeter function, and one for the depth. I installed the knotmeter transducer under the V-berth, at the aft end of the available space and just slightly off center to get a 'flat spot' for a good transducer seal. (pull the drawers out and have a look at that space).

It has been working well well in that location. The depth transducer got it's own thru-hull install in the flat hull area under plywood access panel tha lies below the aft quarterberth. It's almost a mirror image of the thru-hull for the sink drain, only on the starboard side of the hull.

When you install the new thru-hulls, I recommend using 3M 101 polysulfide sealant rather than 5200. Be sure NOT to paint the transducers with conventional bottom paint, the solvents in the paint can damage them. Use only water-based transducer paint. (available in small cans)

Note that 'paddlewheel' style transducers are very sensitive to fouling... if you're in salt water you will be cleaning it frequently (they can be pulled out from the inside of the hull while in the water). You unscrew a treaded locking ring, remove the paddlewheel and insert a 'blanking plug' to fill the hole while you're cleaning the paddlewheel.

It's pretty spooky at first to be 'pulling a plug' in the bottom of your boat, but with quick hands you only get a quart or two of water in the boat while switching the units. Bottom line is that you need to make sure the speed transducer is installed in an area where you can get at it easily. (hence under the V-berth). Your Raytheon manual also has some installation guidelines.

Edited by - on

Frank Hopper
Past Commodore

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Pitcairn Island
6776 Posts

Response Posted - 06/05/2004 : 10:06:19

If the hole is too big I think I would be inclined to patch the old hole and make a new one, I am no whiz on that topic but others are. I am curious why you want to install it at all. If you have a depth sounder already then wouldn't a GPS give you what you are looking for with a lot more versatility to boot? I have a cheesy Hummingbird depth sounder epoxied to my bilge and am using a $200 Garmin 76 for the rest. So far I think I am pretty happy. Can you take the RayMarine back? They appear to be about $250, that is a great start on a nicer GPS than mine.

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ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 06/05/2004 : 13:51:09

"wouldn't a GPS give you what you are looking for with a lot more versatility to boot?"

IMHO follows...

If you're on a lake... sure. Out on the Ocean/Bays/Rivers with significant current, 'speed through the water' helps you more with sail trim and tuning than speed over ground... which can be 'all over the map' depending on which way you're headed relative to the set of the current.

Of course, if you're really serious about your sailing, you'll be wanting SOG, water speed, and integrated wind data displayed too. That's a difference between the Raytheon (or similar) equipment and an inexpensive fishfinders/depthsounders. The pricier stuff is an 'instrument' that can be integrated into more sophisticated data systems. (Note: If you go that far, you start to talk fairly serious $$$)

Here's my personal breakdown on priorities/features.

Small Lake - Depth finder (fishfinder or inexpensive 'dash' display)
Large Lake - Depth finder + GPS
Anchor-out cruising - Anchor Watch features needed, either depth based, GPS based, or both.
Sailing in waters with current - Knotmeter very useful for developing sail trim, tuning etc.
Serious Sailing - Integrated system, true/apparent wind, drift, SOG, VMG, slip, etc. ($$$)

I doubt that many of us C25 folks are serious enough about racing to have the 'bucks up' systems. One of those is half the cost of my boat.

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Notice: The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.