Both transducers losing bottom

[JMacLeod]

We ordered an in-hull transducer as backup to be able to read depth at higher speeds than our transom mount, but both units lose bottom around 12 kts/14 mph.
Both transducers, Garmin GT51M-TM and GT15M-IH, are mounted on/in the starboard sponson, and to a sonar neophyte like myself, seem to work properly below that speed.






Since the TDs seem to function at low speed, this leads me to think this may be just a matter of positioning?
Do either of these positions look like they could be improved on/in the Tomcat hull?

 

[ssobol]

My guess would be that there is ventilation/turbulence under the sponson that causes air bubbles which disrupts the sonar beam. It appears from your photos that at high speed you show zero depth which is the sonar reflection from the air in the water stream.

 

[JMacLeod]

Yes, that was my thoughts as well.
Both transducers are mounted on the flat of the first hull step up from the bottom.
I was considering moving the TM a little lower toward the middle/bottom of the V, and the IH directly to the bottom near the bilge pump.

Where have the rest of the Tomcat owners found to be the best placement with the least amount of turbulence?

 

[gulfcoast john]

No worries, you have a bit of positioning angle experimentation to play with.
My GT30-TM is mounted in the exact same position on that same chine and holds depth (but not CHIRP image) up to my usual cruising speed of 25MPH or so.
From about 8MPH to about 14MPH a TC255 is climbing out of displacement mode and onto plane. The angle of attack (if it were an aircraft) is changing from zero to about 5 degrees bow high, and then at max speeds back down towards 3 degrees depending on your engine trim. You don’t want to spend much time in this transition zone, as you’re throwing a massive wake and wasting gas pushing a wall of water, and many Florida boat drivers will throw their empty beer cans at you. (Some of them travel with a surprising number of those onboard, as you will soon discover).
Unlike sailors, I contend that most trailer boaters at cruising speeds don’t bother with watching the depth readout, since by the time you see an underwater obstruction on the screen at 25 MPH your prop has already passed over or hit it and you are already 10 boat lengths past it. Watch the depth and charted obstructions on your charts instead, they are on the low side (MLLW is the avg of the past 17 years lowest low water). If the height of your predicted low tide is a negative number on saltwatertides.com you’ve lost that MLLW cushion.
Smart boaters don’t go 25 MPH in 3 ft waters, and smart fisherpersons don’t fish at 25MPH regardless of depth, even with CHIRP. Use a long straightedge along the chine and beside your TM50 to level it for displacement speeds using the adjustment nuts. Enlarging your pic, looks to me like the aft portion of your ducer is too low, which will having it pinging way too forward of the ducer at planing speeds ie lose bottom. Try adjusting the aft portion upwards by 3-5 degrees using the angle adjustment nuts and I bet it will work better. These units will hold bottom in some simple boats at 70MPH (but not if the angle of attack changes to the point that the return ping is not bounced back to the unit). I believe our issues are multifactorial, and that if you must choose you’ll choose to have great performance at displacement speed (idle to 1500RPM or so, where most well designed powerboats start to plane), and some at 25MPH. At 45MPH just hope you don’t run over a small branch or the delicate piezo is toast.
Put the IH puck in a baggie of water and try it just aft of the starboard bilge pump. I have an old Garmin Intelliducer that works there at displacement speeds, not cored there per several old posts.
Have fun experimenting!
John

 

[thataway]

For the "transom" mounted transducer a good reason why I always recommend using a Starboard mounting block. When you have to move the transducer down there may be new holes to be drilled. Are the current holes epoxy plugs with core routed out? They may be the basis for the Starboard mounts.

My Tom Cat--and other boats held bottom up to max speeds. I believe that your transducer is a little high--just from the photos.

Agree, that turbulence can be an issue as well as mounting angle.

John, I believe that the one transducer is installed "thru the hull", not shooting thru the hull. The hulls are cored--and you have to be in an un-cored area for a thru the hull shooting transducer to work.

Many power boats are not planing well at 6 mph (I hope that the comment was a bit tongue in cheek). I have seen fish caught at 12 to 15 knots when trolling in the S. Pacific.

 

[drbridge]

While we have a 22 cruiser and not a Tomcat, We do have the same transducer. We had the same problem and the solution ended up being to position it about a 3/8" lower than the hull instead of even with it as the instructions state. It works perfect now.

 

[JMacLeod]

No worries, you have a bit of positioning angle experimentation to play with.
...
Have fun experimenting!
John

Thanks, John.
All good information, and I'll definitely be putting it to use.

For our purposes, the fancy TM one was meant for future fishing fun at idle speed (we're still a loooong way away from that goal at the moment), while the IH was meant just to give a current depth at speed to confirm what is listed on the charts.
I'm just starting to learn a little about the waterways around here, and I figure if I'm cruising along and start seeing depths that don't jibe with the charts, i.e. reading 5' where the chart says it should be 12' or 14', that's my cue to slow down and get my bearings/find the channel.
That sounded good in my head during the planning phase anyway.

For the "transom" mounted transducer a good reason why I always recommend using a Starboard mounting block.

Sorry, the detail on that photo wasn't enough.
We did take your advice and asked the dealer to mount the TM on a board, but there must have been some miscommunication in the process.
When I discussed it with the installer I thought we had agreed to not drilling any holes and epoxying on a small Transom-saver block to use, but when the job was done they had attached a large piece of Starboard with 6 big screws.
Not much to do about it now except hope they're epoxied and sealed well.

John, I believe that the one transducer is installed "thru the hull", not shooting thru the hull. The hulls are cored--and you have to be in an un-cored area for a thru the hull shooting transducer to work.

I've got a call in to the factory to ask about any places that are uncored on the 2018 hulls.
I will definitely update if I get an answer.

My Tom Cat--and other boats held bottom up to max speeds. I believe that your transducer is a little high--just from the photos.

While we have a 22 cruiser and not a Tomcat, We do have the same transducer. We had the same problem and the solution ended up being to position it about a 3/8" lower than the hull instead of even with it as the instructions state. It works perfect now.

Thanks for the consensus.
I'll give it a try next time we're able to get on the water and let you know the results.

 

[thataway]

Thanks for the clarification ref mount. I would also look at any turbulence from the Block of Starboard--I always make sure the bottom is cut flush with the bottom of the boat.

Second--what is the prop rotation--the transducer should be on the Least turbulent water from the prop--which would be on the down stroke of the blade.

Cat outboards are usually mounted opposite to standard inboards. Left hand rotation prop on the Stb side and visa versa.

 

[JMacLeod]

Thanks for the clarification ref mount. I would also look at any turbulence from the Block of Starboard--I always make sure the bottom is cut flush with the bottom of the boat.

Trimming, sanding, and rounding the board is a simple enough task.
I'll add that to my todo list.
Thanks, Bob.

Second--what is the prop rotation--the transducer should be on the Least turbulent water from the prop--which would be on the down stroke of the blade.

Cat outboards are usually mounted opposite to standard inboards. Left hand rotation prop on the Stb side and visa versa.

Looks like mine is opposite of what you say it should be.
The motors rotate outwards, meaning the TM transducer is on the inside/upward stroke side of the starboard engine.
Do you think that is as much of a factor with the added length of the Armstrong bracket separating the transducer and prop?

 

[thataway]

It would be the normal practice to mount the transducer outward of the starboard rotation prop. Hard to say what difference the set back of a bracket/hull extension makes. One would think with that much set back it should be OK--and changing the motors or mounts would be way down the list of what you would want to do,

First be sure you can lock on bottom to at least 300 feet depth (OK, you have not gone 100 miles offshore yet!
:

And be sure the bottom is held to at least your top cruising speed. The ICW and entrances change with storms and currents--so a working depth sounder is very important--only if digital display for depth.

 

[JMacLeod]

I've got a call in to the factory to ask about any places that are uncored on the 2018 hulls.
I will definitely update if I get an answer.

According to the callback I got today from the mfg, the 2018 TC 255 has NO area in the hull that is uncored.
I'm a little shocked the dealer/installer didn't mention this when I asked for an IH to be placed in the uncored depression of the sponson I read about on C-Brats.

Would the transducer work as it does now through a cored hull?
It doesn't look from the photo that they cut anything out to mount the transducer inside the hull.
I imagine I could cut out a section of the inner glass and core, and fill with epoxy to avoid installing another thru-hull?
If it comes to that I'd like to be darn sure that I'm doing it in the right area first.

Now I've got another call into the dealer.
Hopefully I'll be able to speak to the installer(s) that wired the motors and/or transducer to find out how these things were installed right from the horse's mouth.

 

[thataway]

first my bad--I did not realize that you had an inside the hull transducer. I thought it was a transducer which went thru the hull.

Second--if that area is cored, and you are getting readings Garmin has denied the laws of physics or there is a solid core. They will not shoot thru air--and the foam core is like air to the transducer.

It is possible that the installer used a hole saw and cut the inner fiberglass and core out--then installed the transducer into a puddle of epoxy resin. I think it is better to re-enforce conductive media (oil, water, epoxy, silicone etc--avoid any trapped air bubbles...)

On my older Tom Cat there was no area which did not have a core.

Looking at your photos of the sounder screens, It appears if they are all of the TM transducer. ????

 

[JMacLeod]

first my bad--
...
Looking at your photos of the sounder screens, It appears if they are all of the TM transducer. ????

:shock: :shock: :shock:
Your bad?
Ha!
I think you meant MY bad.
:roll:
I had been digging around the menus, flipping it back and forth from GT15 to GT51, and I must have experienced a dyslexic senior moment when I took those pics.
I could've sworn one of them was on 15...

...and This is why I hate getting stuck with the new guy on the job.


Thanks, and good eyes, Bob.
I'll revisit the IH after sorting out the TM suggestions above.

 

[JMacLeod]

I guess all's well that ends well, even if I'm still confused as hell.

According to the factory there's no uncored areas of the hull.
According to the installer, the IH transducer was just surface mounted as normal.
According to the chartplotter (after clearing the transducer errors, re-seating all the cables, and changing around the settings for the TDs) we are now violating the laws of physics!
Obviously someone is mistaken.

The IH is holding bottom all the way up to our WOT of 39mph.
The only time it lost bottom was when we were cruising in other boat's prop wash on the way back in from the ICW.
The transom mount (after adjusting) now holds the bottom to around 20-23mph before it starts sputtering.

So long as it keeps working, I have no desire to tear it apart.
It can keep it's mysteries for now.

This time I noticed that when trying to change the display to the GT15 from the GT51, it changed itself back and logged a disconnected error.
I must have failed to catch that the first time around.
Thanks again for pointing it out, Bob!

We'll have to wait til we learn to cross the bar before I can test it out on anything deeper than 15-20'.

 

[thataway]

My guess is that the area at he flat to reversed chine is either solid glass. or very resin rich..saturating what foam was there?? I have seen this in some of the boats where there is a thin band of glass/high resin concentration at the turn of the bilge or where the transom and bottom intersect.

Glad that things are working out--a few more adjustments may give you bottom up into the 30's.