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Author Topic:   Mounting a Garmin Transducer on a Montauk
blackdog54 posted 02-04-2007 10:58 AM ET (US)   Profile for blackdog54   Send Email to blackdog54  
I am installing a Garmin 498 GPS with optional transducer for FF and depth capabilities. [Complains that a prior article was removed, then later recognizes it was not.--jimh]

The instructions for both the 498 and the transducer are very vague, in terms of mouting of the transducer on the transom. It does state that air bubbles from the prop affect readings and that an improperly placed transducer can cause prop cavitation.

Does anyone have a 498 with transom mounted transducer on an 80's Montauk? If so, where did you mount the transducer?

I have a 1985 Montauk, same year Johnson V4 90 HP, 2-stroke, normal (vs counter) rotation. Any input would be appreciated. I am trying to complete the install today.



jimh posted 02-05-2007 09:25 AM ET (US)     Profile for jimh  Send Email to jimh     
Are you under the assumption that the SONAR transducer from Garmin is somehow different than all other transducers in some unexplained way so that all of the advice already provided regarding mounting a SONAR transducer does not apply to this transducer? If so, you will need to explain how the Garmin transducer is different.
Tom W Clark posted 02-05-2007 09:39 AM ET (US)     Profile for Tom W Clark  Send Email to Tom W Clark     

The practical reality is that you can mount that transducer on either side of your Montauk and it will work fine. Generally, the port side if preferred but if it is easer to route the cable by mounting it to starboard, it is not going to make any difference. I say this as the owner of three Montauks over the years with numerous transducer installations mounted to either side of the transom.

There is no way a transducer can cause cavitation. I think what the manufacturer meant to say is that it may cause ventilation, a completely different phenomena.

Regardless of what side you mount the transducer on, do not put it at the apex between the sponson and the central hull as air will tend to be gathered there as the hull runs. Locate it some ways down the slope of the main hull at least 12" away from the hull's centerline.

Also see the information in the Boston Whaler Owner's Manual. Though it is rather dated, it does offer some relevant advice: html#fathometer

ratherwhalering posted 02-05-2007 12:44 PM ET (US)     Profile for ratherwhalering  Send Email to ratherwhalering     

You may want to mount a small piece (4-inch x 4-inch?) of 1/2-inch thick King Starboard to the hull, using 3M 4200 and 1.50-inch stainless steel wood screws, then mount the transducer to the Starboard, using machine screws. If you ever need to change transducers, you simply re-drill the starboard, not the hull.

If there are already incompatible holes in the hull due to a prior transducer mount, take a piece of paper, outline your Starboard block on the paper, then position the paper on the hull. Highlight the existing holes on the paper, and then use it as a template to drill holes in the Starboard. Mount the starboard using the old transducer holes, and then mount the new transducer to the Starboard.

bsmotril posted 02-05-2007 07:36 PM ET (US)     Profile for bsmotril  Send Email to bsmotril     
I've got the same boat and motor. This is what I did with a Lowrance transducer, and it works fine. 1984%20Montauk%20Updates/?action=view¤t=IMG_6309.jpg
jimh posted 02-05-2007 08:54 PM ET (US)     Profile for jimh  Send Email to jimh     
Three of the best minds in all of Whalering! Thanks fellows.

Let's hear how the installation went. Linda?

blackdog54 posted 02-05-2007 10:22 PM ET (US)     Profile for blackdog54  Send Email to blackdog54     
Many thanks to the folks who have responded. The navigation and electronics manager @ West Marine said that as transducers are becoming more sensitive, the sonar features more integrated with GPS, their displays more refined, that transom mounting is becoming increasingly more specific. Hence my questions.

In fact, they have a display that shows artifact from a variety of bad transom mount choices. It was significant.

I had already looked at the references Jim is referring to. They were in conflict with the manual supplied by Garmin with the GPS unit. They were further contradicted by Garmin's transducer mounting instructions. 2 different sets of instructions, same vendor. Since it involves drilling into my hull, I want to do it right, once.

Tom, the Garmin instructions specifically refer to cavitation on the prop, not ventilation. Can you explain the difference? Robert, I like the starboard idea. Finally, BSMotril-my transducer is about 5 x the size of the one you have. They have an even larger one that measures water speed, which I felt to be irrelevant to small power boating, but it was huge.

Unfortunately, I will not have an opportunity to work on the install until 2/17 now. However, given Jim's fascination with this request, I am happy to provide a detailed account, for the reference section.

Royboy posted 02-05-2007 11:03 PM ET (US)     Profile for Royboy  Send Email to Royboy     
If I understand it correctly, ventilation occurs when a stream of bubbles gets sucked into the prop thus wrecking its bite on the water. Cavitation is when air comes out of solution from the water by the action of a prop that has exceeded its design limitations, i.e spinning too fast or at the wrong angle to the direction of the boat. I can see perhaps an extremely bad transducer mounting causing ventilation, but certainly not cavitation. Chalk it up to electronics geeks writing the manual and not knowing the difference.

Regardless, your transducer is a sound sensing device and thus needs a relatively quiet place to listen for the returning echos. It doesn't have anything to do with your GPS. That part of the electronics has its own antenna (either eternal or internal). The only place the two signals (GPS and SONAR) come together will be on the device's monitor.

Back to your transducer. Mount it in a location where it is not directly in line with any part of your hull that will generate bubbles (hence noise), such as a strake or any other sharp members such as the point of a sponson. Also, as Tom pointed out, stay away from features that will collect and channel bubbles. Mount it somewhere as far from the engine as pratical along the big smooth round part of the hull, and you'll be fine.

The advice about auxilliary mounting plates is excellent, and you'd do well to heed it.


Royboy posted 02-05-2007 11:06 PM ET (US)     Profile for Royboy  Send Email to Royboy     
Oops, as far as we know, GPS antennae are not eternal; they do eventually return from whence they came; dust to dust and all of that. What I meant to say was: external.


jimh posted 02-06-2007 09:05 AM ET (US)     Profile for jimh  Send Email to jimh     
Garmin is the new kid on the block when it comes to SONAR, so I am not surprised they have confusing instructions and very large transducers. Lowrance has a great transducer, their "Skimmer" design, and it is probably protected by patent. The Skimmer works well even at high speeds. I hope your Garmin will perform as well.

You ought to get that West Marine store fellow to author a treatise on how the installation of a transducer on the hull transom varies according to the brand of transducer. He sounds like an expert. Does he have a website?

bluewaterpirate posted 02-06-2007 09:28 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
garmins transducers are made by Airmar. Here are their install instructions.


bluewaterpirate posted 02-06-2007 09:30 AM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
Some more info ....... asp?ProdID=48&Man=Garmin&PageNo=91


phatwhaler posted 02-06-2007 10:10 AM ET (US)     Profile for phatwhaler  Send Email to phatwhaler     
BWP beat me to it with the Airmar instructions. I just mounted a P66 Airmar transducer for my Furuno according to the above instructions and it works great. I mounted it on the starboard side of the boat.

PW out.

jimh posted 02-06-2007 01:24 PM ET (US)     Profile for jimh  Send Email to jimh     
Now we're cooking. AIRMAR is the Deluxe line in transducers.

For everyone keeping track, my current transducer is mounted on the starboard side of the transom, but not because I think that a right hand propeller is a consideration.

jimh posted 02-06-2007 02:09 PM ET (US)     Profile for jimh  Send Email to jimh     
I reviewed the AIRMAR instructions, and I found they were in the whole congruent with the advice from Boston Whaler, to which reference was made above, viz.:

BOSTON WHALER: "Be sure the transducer is not in line with any spray strakes. A smooth flow of water must pass over the face of the transducer....Some transducers are designed to hang just below the boat's bottom and pierce the flow of water."

AIRMAR: "Caution:: Do not mount in an area of turbulence or bubbles...Behind strakes, struts, fittings, or hull irregularities...."

It seems odd that Garmin did not include explicit instructions for their device. Does Garmin acknowledge that their transducer is actually an AIRMAR product?

bluewaterpirate posted 02-06-2007 02:31 PM ET (US)     Profile for bluewaterpirate  Send Email to bluewaterpirate     
No, Garmin does not explicitly say who makes their transducers. They like all of the other name brand marine electronics manufacturer's have turned to Airmar for transducers. Here is a list from the Airmar website who they provide transducers to.

The Airmar P58, P59 & P66 models are the transmon mounted transducers choosen by these companies: Stardard Horizon, Raymarine, Garmin, Furuno, Northstar, Lowrance, Simrad, and JRC. asp?Type=FF

The only way you can tell who manufactured your tranducer is to look at it.

It can be confusing.


blackdog54 posted 02-07-2007 09:05 AM ET (US)     Profile for blackdog54  Send Email to blackdog54     
I did not see Airmar marked on the transducer anywhere. Will take a look this weekend.

The link that BlueWater attached was for the original transducer that I bought, with water speed capabilities. It was HUGE. I took it back and got a smaller one, as I did not feel water speed was relevant for my use. The replacement was still a Garmin, smaller than the original shown by BWP but still MUCH larger than BS Motril's.

Garmin's install instructions from the GPS unit contradicted those from the transducer unit. However, both had them close to center, behind the prop. One set suggested slightly starboard side.

I thought at some point that Jim suggested port side but now I can't find that correspondence.

I have already patched holes in the transom from the prior FF transducer. I think that auxillary mounting plate is a great idea. Again, I do not have another opportunity to work on it until 2/17. Will send an update and photos.

*wishing the bottom paint was re-done as I know my boat is not detailed like the rest on here...*

jimh posted 02-07-2007 09:30 AM ET (US)     Profile for jimh  Send Email to jimh     
The recommendation to mount on the port side of the transom comes from Boston Whaler, and they explain their reasoning:

"Mount the transducer on the PORT SIDE of the transom. The transducer in most cases will act as a mini-trim tab and can affect hull trim. When mounted on the port side, particularly on 17-foot models, the transducer will counteract port listing tendencies from propeller torque providing better hull trim." --source was cited above in the article hyperlinked.

If you have a large transducer with a large flat surface, the trim tab effect is greater than what one obtains with a small, hydrodynamic transducer like a Lowrance Skimmer. On my boat, the Whaler Drive acts as a 6-foot by 2-foot trim tab itself, so there is not much concern about the influence of a transducer, and especially a Lowrance Skimmer, which is what I am using.

The transducer was initially installed on the port side by the prior owner or a marine installer (who was likely following Whaler's instructions), but I moved it to starboard in order to improve its mounting, as it was too far from centerline. When the boat was on hydroplane the transducer was running in aerated water and did not work well. I did not want to make more holes in the transom on the port side--there were already five or six--so I moved to starboard. This also cleaned up the cable path because the transducer cable was running across the splash well to reach the port side. My transducer is mounted about 13-inches from keel centerline. This is also congruent with the AIRMAR recommendation that the transducer be located 3-inches beyond the radius of the propeller tip. I have a 15-1/2-inch propeller. The minimum distance by the AIRMAR formula would be 10.25, so I am beyond that by a couple of inches. The location has proven to work very well. The echo returns are strong enough to maintain a bottom echo contour even at the boat's maximum speed of 42-MPH when in water of moderate depths, say to 50-feet. The depth sounder reading works to greater depths, by the raster display of the echoes becomes a bit ragged as the water gets deeper and you are at top speed.

The topic of transducer mounting has been thoroughly discussed, and other than the discovery of the AIRMAR heritage of the Garmin transducer and their literature, I do not think we are breaking new ground. However, it has been an interesting discussion, and I am anxiously awaiting the results and pictures.

blackdog54 posted 02-17-2007 10:47 PM ET (US)     Profile for blackdog54  Send Email to blackdog54     
So, I patched holes on the transom from the old Hummingbird FF and used Marine Tex. This was about 10 days ago. One batch for all holes. The patch on 2 of the old screw holes it is not quite tacky but it is not rock hard. Next steps?? Do I try to remove it and how?

Also, where can you pick up starboard? Is it strictly a Marine product?

The 498 is a great unit for a boat with a smaller console like the Montauk. Very happy with it, now if I can just finish off the *@#(*%^ transducer.

jimh posted 02-17-2007 11:02 PM ET (US)     Profile for jimh  Send Email to jimh     
MarineTex is an epoxy. If it hasn't cured in ten days, it probably is not going to cure.
jimh posted 03-13-2007 02:30 PM ET (US)     Profile for jimh  Send Email to jimh     
May we please have an update on the installation?
blackdog54 posted 03-14-2007 09:23 AM ET (US)     Profile for blackdog54  Send Email to blackdog54     
Well, in my usual "analysis paralysis" style, I spent too much time trying to select the mounting location. I did not use starboard to back it as the ONLY product that will bond to starboard was almost $50/tube and I know I am not moving this transducer again. That's right, 4200 and 5200 do NOT bond to starboard.

So I mounted it directly to the transom, port side.

When I prepped the area, I sanded some of the bottom paint. I then drilled my pre-marked holes from the Garmin template. Voila- a small drip of H2O. Sanded some more and found about 7-10 rubberized plugs in the transom (probably a 3m 4200 fill). I removed all of these plugs. They were threaded as a result of having filled screw holes. They looked as if they had filled their respective holes. Got 2 more holes with drips of H2O. The odd thing was lower holes did not necessarily drip, where you would expect water to settle, it was at random heights.

I tapped all over the stern of the boat, NOTHING SOFT.

Then I took some swabs used to perform PAP smears. (I am sure I will lose most of you guys here), pushed them into these holes, very little moisture. I hit hard plywood, nothing soggy.

I have let them dry for 2 wks. in warm San Diego weather. Will be sealing this weekend.

That's the update.

Royboy posted 03-14-2007 07:34 PM ET (US)     Profile for Royboy  Send Email to Royboy     
You really don't need anything to bond to the starboard to use it as a backing for your transducer. All that needs to be sealed are the screws that hold the starboard onto the hull. In other words, the sealant only needs to stick to the screws and the hull to form a seal.


jimh posted 03-14-2007 10:10 PM ET (US)     Profile for jimh  Send Email to jimh     
Fill those old screw holes with wooden plugs and use epoxy as an adhesive and sealant.
blackdog54 posted 03-15-2007 09:20 AM ET (US)     Profile for blackdog54  Send Email to blackdog54     

The West Marine folks said to fill these holes with fast set 5200. Why wood and epoxy?

jimh posted 03-15-2007 02:56 PM ET (US)     Profile for jimh  Send Email to jimh     
It's the Whaler way.
Royboy posted 03-16-2007 11:54 AM ET (US)     Profile for Royboy  Send Email to Royboy     
The real answer for the wood and epoxy method is that someday either you or someone else will get around to needing a hole drilled nearly where the old hole was. When this happens, they (or you) will have to fill the hole with something solid, like wood and expoxy, before you'll be able to drill there. It is the correct way to fill a hole in the sense that it is then filled with material very much like the material it's made out of, namely, wood and epoxy.


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