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Author: Jason
Subject: Knot Meter Thru Hull Location
Info: (2375 views) Posted: Friday 8-11-17 08:15:04 PM
Hi Brad,

So I just went through this myself last year. I had the old knot meter housing from way back in the day, as well as the previous owner didn't buy the triducer and ended up blasting 2 more holes through the bottom of the boat. 3 holes is 2 too many, so I paid to have 2 of them filled.

These holes were all in the solid fiberglass area under the aft most middle hatch of the V berth. I left it in there so it would be protected as JB mentioned. It also meant I could re-use 1 of the 3 holes that were already in the boat.

Unlike JB, I went ahead with the DST800 and I feel I had had pretty good success using the tapered housing (link below) for a DST800 triducer. It definitely took a bit of time and patience to get it done right but in the long run it came out really nice. As JB mentioned there are definitely some compound curves in the hull that need to be dealt with. It is also, in my opinion, mandatory to build up the fiberglass in the area where the thru hull goes through to help with the compound curves of the hull and such.

The other thing I had to do to get the housing in the way I wanted it was to build a 3 inch diameter G-10 washer. I used 3/16 because that is what I had, but if I had to do it again, I would use 1/4 G-10. Once the washer was cut out, I then had to taper the washer so that on one side of it was nearly the full 3/16 think and at the other side of the washer it was almost like a knife (maybe 1/32 or so). Keep in mind that the washer remained circular. The tapering was done on the thickness axis of the washer.

This washer allowed for me to be able to rotate the housing in such a way to accommodate the compound curves of the hull while also having a flat surface for the housing nut to sit on. It also helped to reinforce the area just a bit as well give more surface area for the sealant to adhere to.

One of the reason's I went down this path is that the DST800 seems to be a fairly common triducer and is used by multiple manufacturers. I have RayMarine ST60s that came with the boat but I have been consider either the Garmin or B & G instruments at some point and they both use the DST800 as well.

Link to the P314 housing - The housing does fit the DST800 even though it is not listed.

I don't have any photos of the underbody right now but I do have a couple of photos from inside the boat. I will see if I can post them up in the photos area.

Jason Hull 59

:: Hi Brad, there's some good information on this message
:: board using the "search" feature.
:: From the handful of Expresses I have inspected, there is
:: a dedicated location for thru hulls. It is built up
:: solid fiberglass and raised a bit (about 5" by 5"). Mine
:: (#100) is located beneath the most forward part of the
:: floor board which is OK since my Signet speedo unit is
:: shallow and does not protrude (though it was necessary
:: to cut a hole in the floorboard).
:: An earlier express hull number had their dedicated
:: location (solid core, no foam) just forward of the floor
:: board underneath the first V-berth hole. Mine has a
:: battery tray platform in that place.
:: Another Express has theirs mounted well forward of the
:: of that V-berth hole and keel, and it appeared that it
:: was a dedicated location since it was built up solid
:: core.
:: My considerations for mounting a thru hull were:

:: 1. Could I accidentally damage it by stepping on it or
:: heavy gear slamming into it.

:: 2. How flush was the unit on the waterside of the hull.

:: 3. Self-installation.

:: 4. Easy inspection for any damage that could lead to
:: water intrusion.

:: 5. One hole only in the entire hull.
:: When I wanted to replace my signet knotmeter with a
:: triducer, I considered the Raymarine DST800, but I did
:: not want to increase the diameter of my existing hole or
:: have it protruding out of floor board or hull where I
:: normally step. Plus, the DST800 did not have a flush
:: mount option and because of the radius of the hull, the
:: mushroom would not be flush against the hull. If
:: installed professionally, they can recess and flush
:: mount them well.
:: Eventually, I kept the Signet unit since they still make
:: them and just added a inexpensive Norcross, inhull
:: epoxied depthsounder. I mounted it in the dedicated
:: location right next to the thru hull signet since the
:: Norcross unit is fairly small. I was unable to find
:: another location on my boat that the Norcross would
:: shoot through.
:: I will try to add a picture to the files.
:: JB
:: :: Looking for advice on the best location to place the
:: :: thru hull fitting for a knot meter. Thanks

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