Moving speed sensor to centerline

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jkalucki
Posts: 13
Joined: Sun Feb 08, 2015 9:24 pm

Moving speed sensor to centerline

Post by jkalucki »

Hi,

I'm trying to race to polars, and I've reached the tack-to-tack and other limitations of my existing circa 2015 Triton B&G system. My electronics guy is recommending that I move the Airmar DST800 paddlewheel speed sensor to the centerline. (And also upgrade to some Zeus components, the paddlewheel to a DST810 or Signet, etc.) I see that a few boats here have already moved the sensor. Two questions:

1) Is it worth (for some value of "worth") the effort and expense for the accuracy gained? It seems difficult, if not impossible, to get things calibrated as it is in the default starboard position. I have both AWA and boat speed issues tack-to-tack, several things are wrong.

2) Any structural hints in moving the sensor? There's a plywood rib right down the centerline under the V-berth that seems to add some structural stiffness to the whole bow area. It seems that static and shock load would need to be accounted for and re-distributed around the sensor somehow.

Thanks,
John
Lindo #195
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Vento Solare
Posts: 352
Joined: Wed Dec 31, 1969 8:00 pm
Location: Newport, RI

Re: Moving speed sensor to centerline

Post by Vento Solare »

John,

I currently have an analog Airmar speed paddlewheel mounted on the centerline as input to a B&G H5000 Hercules processor and it provides very good speed information. It is definitely worth the effort if you are serious on getting good readings for speed and the resulting true wind calculations. When I purchased Vento Solare from the previous owner in 2013, the speed sensor was off center and there definitely was a small error introduced on measured speed between tacks. At the time, Nexus instruments were installed.

The second off season when the boat was hauled, I moved the speed sensor and while doing that installed flush housings for all sensors. The depth sensor is faired smooth so there is no projection externally. On solid glass boats, the xdcr can actually be placed inside against the glass and the sound is transmitted through the glass with minor loss.

The plywood rib on Vento Solare was reinforced with a plywood doubler that supports shelf that provides a platform for the air conditioner (removed for racing season) only extends about 6" further aft, so I didn't see a problem cutting a section out for the hole on the centerline. Be aware there is a thick ground wire glassed in along the plywood seam that connects the headstay tang to the keel grid. I ended up cutting and splicing it, but ultimately replaced it with another wire. After cutting a notch in the plywood rib, I built up the area and created a flat spot internally so the nut that holds the transducer housing makes contact all around. The old hole was tapered, wet out with epoxy and filled with chopped fiberglass. The inner surface was tapered and 17oz biaxial cloth layers glassed in. When cured, this was ground smooth to match the existing hull profile. Externally this was faired smooth, barrier coated and bottom painted.

Old Speed Xdcr Housing - drill guide for circle cutter using closed cell foam and a dowel - use circle cutter same diameter as external threads on housing
Image

Removed "mushroom" after partial drilling using putty knife
Image

Hole drilled on centerline from outside in - note exposed balsa core and you can see where the plywood rib is glassed to the hull
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Hole drilled - note old speed xdcr hole is epoxy isolated and new centerline hole exposes core that needs to be epoxy isolated. I used a Dremel tool to grind back the balsa about 1/8", wet out the exposed balsa and filled the area with thickened epoxy. The new flush mount housing for the depth transducer is shown slightly recessed.
Image

Plywood rib cut to provide clearance for housing and xdcr removal (note cut wire). Afterwards the area was sanded to expose fiberglass and a few layers of 17oz biaxial cloth used to reinforce. A clay dam built provided a section where the glass and epoxy would be constrained until it kicked. The area was covered with release cloth and a flat board placed on top with weights so when the epoxy kicked, there would be a flat surface.
Image

External area faired and barrier coated
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Not a real good internal view but looking through vent installed in v-berth aft section where Vento Solare has an area cut for air inlet to the air conditioner, which also provides easy access for the transducers. In this photo I was calibrating the temperature sensor using ice water.
Image
jkalucki
Posts: 13
Joined: Sun Feb 08, 2015 9:24 pm

Re: Moving speed sensor to centerline

Post by jkalucki »

Bill,

Thanks for all the detail and especially for digging up the photos!

Its great news that over all these years, you haven't seen stress or flex issues in just cutting the plywood rib.

FWIW: I think on my hull, the plywood rib is glassed into the under-berth bulkhead where you have the door, making the whole area under the berth appear as a solid rigid structure. I hope a few other owners can describe their configuration, as by inexperience and ignorance, I'm a little apprehensive about having the boat yard just going and cutting-into various bones.

Thanks,
John
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Vento Solare
Posts: 352
Joined: Wed Dec 31, 1969 8:00 pm
Location: Newport, RI

Re: Moving speed sensor to centerline

Post by Vento Solare »

John,

What could be done is to build up the inside flat surface once the plywood is cut and tab that to the plywood fore and aft, thus extending the newly formed flat surface as a structural member that reattaches the area cut. Just make sure there are enough threads exposed on the plastic transducer housing to screw in the nut on the housing.
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