Transom mounted Transducer

[Frank C]

I was at the marina today, so curiosity bit. The only holes thru my hull below the waterline are those of the dealer-installed transducers, one each side of the outboard, and the ballast gate valve. Since a leak of the gate valve screw holes would admit water into the ballast tank, it's a non-event. I too was amazed ... the two lower motor-mounting bolts are ABOVE the waterline.

Frank, the bracket that the triducer is mounted with is already hinged and designed to kick up if it hits an obstruction. Unless yours is a different design, couldn't you just manually kick it up when you need to beach?

Well, mine was a (hinged) bi-ducer for depth/temp, plus a uni-ducer for the paddle wheel, one mounted to each side of the outboard. In answer, Yes for the skimmer, but NO for the spinner, which has no hinges! Further, the kick-up skimmer would remain in the water, so both would still grow hair.

I guess I was thinking they might both mount on a single board, which could be attached to hinges above the waterline. Prolly more effort than reward.

[ronacarme]

At first screwed it onto apiece of 1/4" piece of ply clamped on the transom with a widemouth screwclamp, but over time weed started covering it and it looked pretty scuzzy.
So later mounted it on the leading edge of my piggyback shoal rudder, which is normally up, unless I want to monitor water depth or /and low water depth may put standard rudders at risk. Shoal rudder piggybacks on same pivot axis (extended pivot bolt)as the stbd standard rudder and one or both can be up or down as desired.

[Dimitri-2000X-Tampa]

Well (going against the well-intentioned heartfelt advice of several folks), I went ahead and drilled it last night and sure enough, the 3 holes go right into the ballast tank as predicted. The only hole you can see from inside the boat is for the cable clamp which is almost at the top of the transom. I just couldn't see gluing a piece of wood there, figured even if it didn't bust off on the first wavy day, it would look rather unprofessional...

It was a bit of a nuisance to fish the wire through the conduit tube without breaking away too much of the silicone that seals the wire bundle but I finally did it with a cut off piece of fish tape and then used up some more of my red RTV silicone that I got for my engine job to reseal it. I'll finish mounting the control unit on the pedestal this morning but I wanted to get the transducer on last night so the sealer would be dry enough today to take it for a test drive!

With no extra shims, I have the front of the ducer pretty much flush with the bottom of the hull and then the rear drops down a couple millimeters. Should work well according to the instruction manual and I guess that implies a Mac26X transom is pretty close to the standard 13 degree tilt. I'll test out the newly fixed motor and the new transducer/fishfinder later today...maybe I'll even trailor out to Ft Desoto for Mother's day tomorrow but I figure I better do a test drive closer to home in more protected waters first with all the big works lately. Maybe soon I can get back to installing the stereo system and second electrical panel i finally bought 3 months ago....been too busy fixing broken stuff lately to do new stuff yet.

One more thing on the fishfinder, it has NMEA support but I don't see much of a reason to connect it to my GPS-Autopilot network. Maybe it would send the water temp to the GPS but otherwise, I think it is mostly just to receive lat/long from the GPS and display it on the screen. Don't see much use in that since it is already on the GPS. Figure it might get me in trouble by sending conflicting information to the A/P. Anyone think its useful hooking up the fishfinder's NMEA to any other gadget?

[nemo]

I also have a Raymarine ds400x. You're right, the screw holes go right into the ballast tank so no big deal. Here's some pictures of the mounting. Yes, it's a pain to fish the wire up through the steering pedestal, but possible with perseverance.

http://www.brownz.com/transducer_2.jpg
http://www.brownz.com/instruments-2.jpg
http://www.brownz.com/finder_wires_2.jpg

[Jeff Stagg]

I mounted my Apelco (Raymarine) transducer under the waterline at the stern on the starboard side with stainless screws, short ones, read, short ones, eight years ago (works great).

The screws don't go through the glass, meaning, you can't see the screws from inside way back there, just into it enough to hold the plastic piece on there.

This allows the depth to be found without ballast, a common scenario for me, especially if under power to an anchorage at approaching darkness.

While I have had problems with 3M 5200 sealant on the sunny parts like the chainplates, I haven't had problems below the water line on tiny screws that are sealed into the hull or rudder bolts that I had to re-install after driving up the ramp with the rudders tied down one time a long time ago.

BTW, removing the brackets and being a blacksmith with a sledgehammer worked great on the stainless steel.

The only blister on my hull came with it new. on the port side in the mid section, below the cockpit, still the same as then.

[Dimitri-2000X-Tampa]

Took the new fishfinder for a spin yesterday, not real satisfied with the triducer install. The speedometer is not accurate as it shows about 3-5 mph slower than what the boat is really doing. The temperature seems to show a few degrees warmer than it really is. And the depth reading drops out at higher speeds. It's a bit inconsistent too but for the most part, does not read well when the boat is planing but does ok at hull speed and below.

I don't know if I can do anything about the temperature...unless the unit has a way to calibrate inaccuracies. As for the speedometer/depth, I was trying to install the unit as flush with the bottom as possible to minimize extra drag. That must have been a mistake. The speedo actually gets more accurate when I pull the stern down right before planing. This kind of implies that the stern of the triducer (where the paddle wheel is) should be lower than what it is now (a couple millimeters below the front). The install manual had said that you may get better high speed performance by using a shim to tilt the triducer back more (4-9 degrees) however, my low speed performance is not good either. I had figured when they talked about high speed, they really weren't talking about Mac speeds.

Then there is the issue of the depth cutting out at higher speeds. Seems like to fix that, I will need to drop the whole transducer a few more millimeters so that it is well below the hull. What a nuisance, but I do want it to work right, and others in this thread said theirs worked well at all speeds so I must need to tweak mine. Hopefully, it isn't an issue with turbulence from the motor or from the ballast valve otherwise I have it in the wrong place. Oh well, its low priority so I'll wait until the next time I have a good reason to put it on the trailer again. At least my motor isn't overheating any longer...and the gadget does seem to work well at finding fish when the depth is reading.

Jeff, I put tape on my drill bit so that I wouldn't drill more than 7/8 inches deep but the drill still went all the way through into the ballast tank. The screws that came with it look to be about 1.25 inch. I guess the extra length is needed if you use the shims....which it looks like I may need to use.

[Dimitri-2000X-Tampa]

Nemo, thanks for posting the pics. Looks like you have that 26M inside a garage...that must be one big garage (tall as well as deep).

Its hard to tell from the pics how low you have it mounted. Do you have any of the problems I just noted and do you recall how low you mounted it and whether you used any shims? You can see the increased dead rise with the 26M versus the 26X but I wonder if the transom angle is the same. I would guess that it is similar.

[Catigale]

Frank - my Standard Horizon Vertex speedo/depth gauge/timer has a calibration function where you can set it to match a GPS speed - I would think most every paddle wheel speedo would have this function since the relationship of flow at the wheel to boat speed is a function of boat hull shape of course.

[Frank C]

Right, mine too Steve. But I just never bothered to try calibrating it ... and now Lorrie (my Lowrance GPS) has given up her ghost - thinks we're near Block Is - always. I keep telling her it's Angel Is, but she refuses to believe me!

Dimitri, I should have left the bi-ducer right where the dealer placed it ... where readings were always strong. If you think about it, these gizmos are used on just as many powerboats as blowboats, so it WILL work reliably. My bracket was flush at hull bottom ... meaning the skimmer wasn't skimming at all. It ran in a clean stream below the hull, plus the 2 more inches of the skimmer's body depth - and prolly 6 to 8 inches deep on port tack - moved it so long ago, simply cannot remember if starb'd tack caused any issues??? - YMMV - (see next).

Cavitation is a major obstacle to achieving high speed operation. If the flow of water around the transducer is smooth, then the transducer sends and receives signals normally. However, if the flow of water is interrupted by a rough surface or sharp edges, then the water flow becomes turbulent. So much so that air becomes separated from the water in the form of bubbles. This is called "cavitation." If these air bubbles pass over the face of the transducer (the part of the housing that holds the crystal), then "noise" is shown on the sonar unit's display. You see, a transducer is meant to work in water - not air. If air bubbles pass over the transducer's face, then the signal from the transducer is reflected by the air bubbles right back into it.

The cavitation problem is not limited to the shape of the transducer housing. Many boat hulls create air bubbles that pass over the face of a transom mounted transducer. Many aluminum boats have this problem due to the hundreds of rivet heads that protrude into the water. Each rivet streams a river of air bubbles behind it when the boat is moving, especially at high speed. To fix this problem, mount the face of the transducer below the air bubbles streaming from the hull. This typically means you have to mount the transducer's bracket as far down as possible on the transom.

[nemo]

I mounted the transducer just as specified in the directions. The left bottom edge is flush with the bottom of the boat, and the right side is exposed more (beneath bottom) due to the curvature of the hull. I did use one shim during mounting to get the angle as specified in the directions.

My water speedo also indicates a "slow" speed similar to yours. There is a calibration function on on the DS400x to fix this. I haven't got around to it yet, but will on my next trip out. Just read the directions and it uses a scaling function by a percentage to correct. Makes me wonder how the accuracy will scale across a variety of speeds..

I've never noticed the depth finder to flake out at higher speeds - always has worked steady.

Yes we're fortunate to have a big garage to hold the boat and other stuff. Was built for an RV originally.

[Dimitri-2000X-Tampa]

Thanks for the tips guys. I'll hunt for that calibration feature for the speedo and see if that makes it good for now. I'm not gonna put it back on the trailer for while but next time I do, I'll adjust it some. Depending on how it looks, I'll have to decide whether to just try to drop it a bit, or shim it and drop it. Thinking back, I think I probably did it in too much of a hurry trying to get the boat back in the water as quickly as possible. Its possible that the front left edge is not below the hull enough.

Regarding the temperature, that seems to read a bit high all the time too (in air or water) and shouldn't care how it is mounted. I would have at least expected that to work right

[nemo]

You can also calibrate temperature. See page 62 in the owners manual. Here's a link to the online version:

https://www.raymarine.com/Raymarine/Sub ... 4_2www.pdf

Link to Transducer manual:

https://www.raymarine.com/raymarine/Sub ... 6_2www.pdf

I mispoke a bit in my previous post where I said the left bottom edge is flush with bottom of boat. I double checked and it *is* hanging below by an 1/8" or so - this is consistent with instructions in the manual.

[nemo]

My fears about non-linearity of the speed calibration were confirmed yesterday when out on a lake I calibrated my Raymarine DS400x to my GPS at 10 kts. This required a 125% calibration. When I slowed the boat to 5 kts according to GPS, the Raymarine says I was going 2 kts.

I don't thing currents were a factor, as I saw this independent of what direction I was heading.

I didn't have much of chance to play with it at higher speeds as I only started dinking with it late in the day and ran out of time, but my current hypothesis is that these water-wheel transducer speed indicators just don't work at low speeds.

Or is mine just broken?

[Scott]

Sidebar question for this thread,


"How to keep my Transom mount Transducer from growing a Beard??"

The first year it worked fine all year, cleaned it when I pulled the boat. Second year worked fine for around 3 months then clogged with green hair algae. Third year, boats been in the water around 3-4 weeks and went sailing the weekend. So much green hair that the paddle wheel wont rotate.

Any suggestions. Mine looks and is mounted like the one in the earlier link??