From: Doug King

To: montgomery_boats@mailman.xmission.com

Sent: Wednesday, April 24, 2002 9:41 PM

Subject: Re: M_Boats: transducer position

Howard,

I've addressed each question below it. Let me preface by saying as a first-time real-boat servant (I would debate what owns who) and a never-time-working knotmeter owner, I've decided to start this boaty-sitting odyssey by erring on the side of caution. Or to put it succinctly, "written directions are the first refuge of the ignorant".
Then at some belated point I reach analysis paralysis, throw up my hands, and like-y Nike, "just do it".

At 06:11 PM 4/24/02 -0500, you wrote:

Doug:

Which part goes bad on the road trips. The transducer or the knotmeter?

As I recall from August 2000 when the unit went in for service, the meter went bad. I don't have the receipt, but just tonight I looked at the parts and the meter guts housing looks new and the impeller (paddlewheel transducer) looks used. I recall the technician saying that a competent person can stick multi-meter probes into the correct connector leads from the transducer, spin it and determine which unit is bad based on amount of electrical output. So I just sent it in. You may be able to call the manufacturer's service department and get the particulars, it's not my area. Might be a first step, though.

Are we talking removal of the transducer for road trips or disconnecting?

I'm just going by what the manual says from here on out (my model is Standard Horizon SL45):
"To protect the paddlewheel insert, use the long blanking plug when the boat will be moored in salt water for more than a week, the boat will be removed from the water, or aquatic growth build-up on the paddlewheel is suspected due to inaccurate readings from the instrument."

Under "Servicing the Paddlewheel Insert":
The water lubricated paddlewheel bearings have a life of 5 years on low speed boats (less than 10 kn) and 1 year on high speed vessels".

I take that to mean that more spinning, especially out of water, means more wear. The big question I have is, does the paddlewheel spin while running down the road? As easily as it spins when I just blow on it, I think it must (I know I'm a blowhard, but not THAT hard).

When I asked the boatyard guy, he said to take it out when trailering.
The model I have is simple to remove and replace the paddlewheel plug with the blanking plug, which will protect both the paddlewheel and the meter unit. Takes about 5 seconds, since I replaced one of the retaining rings to the locking pin with a hitch pin. Pull the hitch pin, pull the retaining pin, slide the impeller unit out of the thru-hull sleeve, and reverse the process with the blanking plug. Considering how easy it is to do, why take the chance? 

My knotmeter went dead and the new one I installed to replace it was just as dead, but I didn t replace the old transducer....yet.  That doesn t look like something you would want to do everyday.

I don't think it's that difficult on my unit. I can order just a paddlewheel kit. On mine, the thru-hull is a separate sleeve that is permanently installed. The transducer unit slides in and out of it, sealed with a few lubricated O-rings and locked into place with the retaining pin. The thru-hull sleeve doesn't have to be removed.

I use a GPS to determine speed, but that gets corrupted by current. It would be nice to know both the actual speed through the water and speed over ground.

My feelings exactly, with the exception of knowing Velocity Made Good, because that requires the additional wind speed instrumentation (WS45), which I'm not planning on springing for anytime soon, since WM sells it for $650. In the Series 45 system, it integrates through the NMEA-0183 interface. I'm pretty sure all of the systems must require the same instrumentation for VMG.

More importantly for me, is my need to learn how to trim sails, point optimally, dead-reckon, etc., and I want instant feedback on my adjustments. Plus I like multi-day cruises, sometimes out of sight of land, and want two different methods to estimate and double-check position.

I installed my depth transducer right behind the knotmeter transducer, and maybe one strake down. I used silicone caulking...just make a little puddle and press the transducer into it. It reads fine shooting through the hull, and it s easy to move if you want to. Just scrape it off with a putty knife.

Don't know about that. but since it works, it sounds like a great solution.

That would not work if the transducer involved has the knotmeter wheel on it. Those are almost universally mounted off the transom...even on those 70mph bass boats.

For some reason, that wasn't the impression I ended up with (transom-mounted on sailboats), but that is just my impression. I think exactly because they are fast powerboats, usually planing powerboats, they must mount well aft. Quoting the installation book again, under "Mounting Location":

"Turbulence-free water must flow over the paddlewheel at all speeds. Choose an accessible spot with a minimum deadrise angle."

Then, for different types of boats:
"Displacement hull powerboats--Locate the sensor amidships near the centerline.

Planing hull powerboats--Mount the sensor well aft to insure that it is in contact with the
water at high speeds.

Fin keel sailboats--Mount the sensor on or as close as possible to the centerline and forward of the fin keel 30-60cm (1-2').

Full keel sailboats--Locate the sensor amidships and away from the keel but still in the when the boat is heeled."

This little blurb is from the meter book, not the sensor book, unlike the rest of this stuff. Under "Impeller Installation":

"Sailboats
On sailboats, the impeller should be mounted close to the centerline of the hull and ahead of the keel by 12-24 inches (300-600 mm) so that the flow disturbance caused by the keel does not affect the flow of water past the paddle wheel".

Finally, this:
"Caution: Do not mount the sensor in a turbulent area; near water intake or discharge openings; behind strakes, fittings or hull irregularities; near the keel or behind eroding paint (an indication of turbulence)."

I think it was the "behind strakes" part that made up my mind and may make the M-boats an exception to the general rule.  Whether I would have put it in its current position had there not already been a thru-hull there, I don't know. But my transom  has the outboard on the port side, the rudder amidships, and a waterline step on the starboard side, where a rope swim ladder goes when anchored. I couldn't really find a place that I thought would be out of harm's way, and out of turbulence.

Granted, all of this may be for the benefit of racers, and not that important for the way I sail, but once again I adhere to my little motto at the top.

If you like, I can copy the sensor installation book and send it to you. It's four pages.

I'd like to know how the transom mount works, if you go that way. It's never too late for me to  buy MORE boat stuff and REDO something else AGAIN. ;)

Hope this helps Howard,

Doug



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Doug King
M-17 #404 "Vixen"

Montgomery Sailboats Owners Group Web site: http://msog.org
Email: mailto:msog@msog.org