Transducer separation from prop wash

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Ray Clark

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Hi all.

I've just done a search of the forum with no result for this specific question, so thought I'd post it here this morning.

I just bought a new H'bird Helix 7 Mega SI. It's going to replace an old Lowrance Elite 4 on my 14' Lund. The old transducer is mounted on a plastic mounting plate and has worked well in thee couple of years I've owned the boat.

I was reading the transducer instructions yesterday, paying particular attention to transducer placement with regard to strakes, rivet lines, and the prop circle. The H'bird instructions state that "it is best to have a minimum of 15" between the edge of the prop and the transducer." I added the emphasis to the term "best"; note that the H'bird tech writers did not say "must be" or "required". The farthest I can mount the transducer on the existing mounting plate is about 11" from that prop circle. I have limited space to move the plate farther away, as the draft on this small boat is fairly shallow and moving the plate to the right very much will begin to bring the level of the transducer up and potentially above the water line. I also have potential placement limited by the cautions about strakes, chines, and rivet lines.

My question: How much can I squeeze that 15" recommendation when installing the transducer? I really don't see using the SI at speeds over no-wake speed, and the motor is only a 20HP Merc. For my boat, "no wake" equates to less than 5 mph. Obviously, when I'm on the trolling motor, this is not an issue at all.

I will probably call the H'bird customer support line tomorrow and get their take on the issue. But I'd also like to hear "real-world" experience if possible.

I'm looking forward to getting out on the water with this new FF. I'm moving the old Lowrance up to the bow and hooking it to the US2 on the troller while I have the Helix mounted midships. Can't wait to chase the crappie and walleye after ice-out.
 
After reading your post I measured my Helix 7 transducer (doesn't have SI). I was actually surprised it was just about 15" form the outer edge of the prop to the transducer. But it is only a few inches outside the prop circle, if that makes any sense.

Let us know what Humminbird has to say.
 
It sounds like you don’t really have an option to go further, correct? I think you hit the nail on the head in that Hbird recommends certain installation parameters, but those aren’t hard-set to ensure your electronics work.

I did A BUNCH of research before rigging my new Garmin (both on this forum and others), and I couldn’t find anyone that actually said their sonar was noticeably influenced by fudging transducer locations or wiring placement (for electrical interference). Can it happen? Sure, but when it’s hard to find people talking online about an actual issue they experienced, my guess is that it’s quite rare.
 
After reading your post I measured my Helix 7 transducer (doesn't have SI). I was actually surprised it was just about 15" form the outer edge of the prop to the transducer. But it is only a few inches outside the prop circle, if that makes any sense.

Let us know what Humminbird has to say.

Yes, that makes sense. The instructions are written from the perspective of the boat looking straight on at the transom. However, our boats are three dimensional objects. I was never very good in Mr. Braz's 10th grade geometry class, and that was 50 years ago.

Perhaps a picture would help. All measurements are taken with the transom saver removed and the motor in the down position. The old Lowrance transducer is still in place.Transducer Mount annotated.jpg



A is the distance between the keel (centerline) and the outer edge of the mounting plate. That distance is about 19 inches.

B is the distance from the outer edge of the prop circle to the outer edge of the mounting plate. That distance is 14 inches.

C is the straight line distance between the outer edge of the prop circle to the approximate position of the new transducer. That distance is about 17 inches with the motor in the down position.

D is the prop depth below the keel; that depth is about 10 inches.

In the down position the leading edge of the lower unit is 5 inches behind the transom.

So I have to be concerned with the prop wash and potentially the lower unit depending on how wide the Side image signal fan is when it passes the outboard.
 
My transducer is about the same relative place as your current transducer. I wouldn't hesitate to mount the new one in the same place. Again, my Helix 7 has DI but not SI. I think what Makaw says about reality dictating the location is very valid.
 
Why would you use side imaging directly behind the boat where the motor is running ?? My Lawrance does not have SI, only going by what I have watched on fishing shows they show left, right and front, never saw anyone use rear. Have saw them turn the boat sideways to see what was behind but dont remember seeing anyone talk about looking back toward the engine. On the tv shows they normally only use the si when going slow, I would think the transducer would be fine where it was as it worked before!
 
I'd mount the transducer to the side of the plastic as far as I could away from the propeller keeping it away from the strakes as much as I could. You want to keep the one you have now for the FF up front, correct?
 
Why would you use side imaging directly behind the boat where the motor is running ?? My Lawrance does not have SI, only going by what I have watched on fishing shows they show left, right and front, never saw anyone use rear. Have saw them turn the boat sideways to see what was behind but dont remember seeing anyone talk about looking back toward the engine. On the tv shows they normally only use the si when going slow, I would think the transducer would be fine where it was as it worked before!
This is the first time I've ever had side imaging, and I'm no expert, so this is all based on what I've read and understand right now. If I misunderstand what you're saying, please jump in.

There's only one transducer for both DI and SI, at least on the Helix 7, and transom mount is what is recommended by the manufacturer. They do offer other transducer configurations if the transom mount doesn't work for some reason. SI only looks out to either side of transducer while DI looks straight down from the transducer. The only systems I've seen look forward and all around while operating are the livescope systems. I don't have livescope capability.

I agree that SI use will be in low-speed situations, though with only a 20HP motor, how high speed could I get, right?

I'd mount the transducer to the side of the plastic as far as I could away from the propeller keeping it away from the strakes as much as I could. You want to keep the one you have now for the FF up front, correct?

I won't reuse the old transducer for the bow-mounted Lowrance Elite 4. I'm going to us the Universal Sonar 2 that's built into my Minn Kota Powerdrive trolling motor. That will require an adapter cable, but it can be done. I've not seen how that performs, but I'll give it a shot.

I guess I loves me a good science project....
 
I’m in a similar boat (pun intended) with mounting a Helix side view on the transom, that’s where I had it in the previous boat and it was fine at low speed but I lost signal around 15mph. I have to assume placement from the prop circle just helps with getting a clean signal but I’m definitely interested to know more because I’d like to be able to read the side view at higher than 15mph if possible. I’m also interested in how that universal sonar works, I just bought the adapter cable to do the same thing but more as a back up plan since I have another Helix with side view on the front of the boat that id like to take advantage of I can.
 
I’m in a similar boat (pun intended) with mounting a Helix side view on the transom, that’s where I had it in the previous boat and it was fine at low speed but I lost signal around 15mph. I have to assume placement from the prop circle just helps with getting a clean signal but I’m definitely interested to know more because I’d like to be able to read the side view at higher than 15mph if possible. I’m also interested in how that universal sonar works, I just bought the adapter cable to do the same thing but more as a back up plan since I have another Helix with side view on the front of the boat that id like to take advantage of I can.

I have never used SI, yet, so my perspective is only based on what I read.

The Helix 7 manual says that "Best side imaging performance" is at speeds 2-6 mph, running in a straight line, with minimum turning time and wave turbulence. There's that indefinite wording in H'bird's documentation again; that's what kicked off this thread.

Most of the lakes I fish are smaller and many are no wake lakes, so slower speeds are fine by me. The two lakes I fish most have 5 mph speed limits.

The finder I'm hooking up to the US2 is just a downscan model. I don't think US2 will support side imaging. I may not understand what you plan to do with the second Helix head. Are you going to put the side image transducer on the motor with one of those mounting setups?
 
I called H'bird and spoke to customer support. They acknowledged the situation that Makaw mentioned above; reality forces some tradeoffs and they don't have a hard/fast rule about minimum lateral distance from the prop circle. Their advice was "do the best you can while also avoiding chines, strakes, and rivet lines."

So, that's the vector for now.
 
I called H'bird and spoke to customer support. They acknowledged the situation that Makaw mentioned above; reality forces some tradeoffs and they don't have a hard/fast rule about minimum lateral distance from the prop circle. Their advice was "do the best you can while also avoiding chines, strakes, and rivet lines."

So, that's the vector for now.
I ran across this thread while checking on one of mine. I just installed a Garmin on my 16/52 jon boat and sweated every move. I removed my old Lowrance transducer from the stern saver pad I have on the transom and the new one wouldn’t fit on it without being to low which I fixed with some aluminum. But going back and forth and posting here I’ve learned a lot. I bet you’re fine where you’re at. I found out the more important thing is putting it on the correct side of the motor according to which way the prop rotates. From behind the boat most props spin to the right and I was told that’s the side to mount it where the prop was in a downward motion. According to what I was told if it’s mounted on the side the prop is coming out of the water is when it really pushes the wash toward your transducer.
Just thought I would share. I hope you got it working good.
 
I ran across this thread while checking on one of mine. I just installed a Garmin on my 16/52 jon boat and sweated every move. I removed my old Lowrance transducer from the stern saver pad I have on the transom and the new one wouldn’t fit on it without being to low which I fixed with some aluminum. But going back and forth and posting here I’ve learned a lot. I bet you’re fine where you’re at. I found out the more important thing is putting it on the correct side of the motor according to which way the prop rotates. From behind the boat most props spin to the right and I was told that’s the side to mount it where the prop was in a downward motion. According to what I was told if it’s mounted on the side the prop is coming out of the water is when it really pushes the wash toward your transducer.
Just thought I would share. I hope you got it working good.
Thanks. The new unit is not working, yet. I've had to put the installation aside as I finish up some woodworking commissions; those commissions pay for some of this foolishness, so....

We're still about 3 weeks from ice out, so I've got time to "git 'r done" before it's possible to test anything. I have a lake close that will be a good test place; I don't fish that lake because they seem to have periodic episodes of e coli contamination. It has a well-mapped bottom, so it will give me chance to learn and understand what I'm looking at against a "known quantity."
 
I understand. I haven’t been able to get mine wet yet either. But mines a rebuild. I’m finishing up this phase by completely rewiring everything and adding new toys here and there while I’m at it. Really close to being finished but life keeps happening and I have to stop and put out fires for the wife. This time it’s her car.
 
Your transducer needs to be in solid water, no air bubbles mixed in. Thus the reason of keeping it out of rivet lines and strakes which can mix air bubbles into the water. Wash from the prop stays pretty close to the prop, just look at underwater videos of boat props in the water in action. You will see a tunnel of turbulence almost like a tube behind the prop. As long as the transducer stays away from the prop wash "tube" by a foot or so you will be fine. Unless you have an inboard, your prop is well behind the transom anyway.
 
I think you should take H’Bird recommendations with a grain of salt. After all, the same company mounts transducers directly on bow mount trolling motors. Granted the water is less disturbed when mounted on the trolling motor but, a few inches in front of the prop is a few inches front of the prop. When you consider the beam angles coming off the SI transducer mounting farther from the prop may actually cause more problems. Maybe you can try a temporary installation with some high quality double stick mounting tape before drilling any holes.
 
Your transducer needs to be in solid water, no air bubbles mixed in. Thus the reason of keeping it out of rivet lines and strakes which can mix air bubbles into the water. Wash from the prop stays pretty close to the prop, just look at underwater videos of boat props in the water in action. You will see a tunnel of turbulence almost like a tube behind the prop. As long as the transducer stays away from the prop wash "tube" by a foot or so you will be fine. Unless you have an inboard, your prop is well behind the transom anyway.
Also found the angle of the transducer is important to get a good reading
 
Have you gotten this set yet? The SI beam is not wide enough that an inch or three in placement will likely make a significant difference, but depending on the angle of the v-hull and distance between the lower-edge of the stern, and the front of the lower-unit, its possible to block the beam on the left side with the motor lower-unit--that'll likely be the only real showstopper problem youd run into. If the engine in your photo is down it looks like you will have clearance. Anything beyond this may affect your image quality, but it wont be a total no-go.
Side imaging really only works at low speed, and when going in a relatively straight line. it doesnt work at all when not moving, and wide swings in direction will distort the image. At higher speeds (15+?) you lose the resolution very quickly and 2d (traditional sonar) becomes more useful and less prone to problems with turbulence. You will probably want to switch to 2d sonar when running anywhere with the big motor, anyway. On bigger boats many (most?) people use a splitter cable in order to utilize separate transducers for side-imaging and 2d sonar, with the si ducer mounted directly to the jack plate--completely out of the water when on plane, but totally protected and in clean water--and a shoot-thru 2d transducer glued into the bilge above the pad for 2d. That's the typical setup for a boat with a pad hull. If you have a 14' boat it's probably not generating so much turbulence the way a 250hp or something would, so a bit closer to the prop if needed wont kill you. Also, unless your boat is planing only on a pad (yours is not) you wont run into issues with the transducer coming out of the water if you mount it a bit toward the chine. As long as you can strike a balance between keeping the transducer in the cleanest water possible, while keeping its field of view to the sides clear, and protecting it from impact by keeping it only as low as needed to maintain a 2d view at higher speed, you should be fine--try the far outside of your mounting plate and I bet you are OK. Worst case, its sensitive enough that you can see turbulence in the water as a haze on your sonar screen, so it will simply decrease your resolution as if there were static in the water column--this will interfere with your ability to see stuff like bait balls and fish, but at low speeds I doubt it is all that significant and you will be able to adjust your speed to account for it to a large degree. My lund was a lot bigger than yours, but I had the SI transducer mounted more or less where yours is, with the inside side of the transducer almost even with the bottom of the hull, and the outside side with the seam on the transducer about even with the hull--this got me ok 2d sonar on plane up to 45mph or so, and the SI was fine.
Also for what its worth, assuming a good install side imaging works fine on a trolling motor on the bow from an interference/clean water perspective, the issue you will see is that the image will be distorted and +/- unusable if you are steering much--the image relies on a constant speed/constant movement, so every time you steer or start/stop the image is significantly distorted. the 180-degree livescope stuff is newer and will look forward and alleviates some of these issues. I havent mortgaged my house yet for the livescope stuff so I generally keep my bow unit on 2d for this reason, while the stern unit is used on SI at low speeds and switch to a memorized screen with gps and 2d for higher speeds or specific uses. You will almost certainly find yourself switching between various screens to better see certain stuff, especially on a small screen like that, especially as the water column gets deeper. If this is your first time using SI check out the humminbird image interpretation forum and install forum at bass boat central, there are some great resources for rigging, setup and figuring out what the heck you are looking at.
 
Yes, that makes sense. The instructions are written from the perspective of the boat looking straight on at the transom. However, our boats are three dimensional objects. I was never very good in Mr. Braz's 10th grade geometry class, and that was 50 years ago.

Perhaps a picture would help. All measurements are taken with the transom saver removed and the motor in the down position. The old Lowrance transducer is still in place.View attachment 113598



A is the distance between the keel (centerline) and the outer edge of the mounting plate. That distance is about 19 inches.

B is the distance from the outer edge of the prop circle to the outer edge of the mounting plate. That distance is 14 inches.

C is the straight line distance between the outer edge of the prop circle to the approximate position of the new transducer. That distance is about 17 inches with the motor in the down position.

D is the prop depth below the keel; that depth is about 10 inches.

In the down position the leading edge of the lower unit is 5 inches behind the transom.

So I have to be concerned with the prop wash and potentially the lower unit depending on how wide the Side image signal fan is when it passes the outboard.
Pretty much every jon boat looks like yours. It won't be an issue.

Sometimes, when I am making QuickDraw countours, I'll trim my engine up a little, just to make sure I get perfect images to the left of the boat. Not that I have ever seen a difference with it down, just being extra-careful, hoping I don't miss a single bottom feature.

This year, I'm determined to hunt down and find more brush piles, stumps and other stuff that holds crappie on my home lake.
 
Have you gotten this set yet? The SI beam is not wide enough that an inch or three in placement will likely make a significant difference, but depending on the angle of the v-hull and distance between the lower-edge of the stern, and the front of the lower-unit, its possible to block the beam on the left side with the motor lower-unit--that'll likely be the only real showstopper problem youd run into. If the engine in your photo is down it looks like you will have clearance. Anything beyond this may affect your image quality, but it wont be a total no-go.
Side imaging really only works at low speed, and when going in a relatively straight line. it doesnt work at all when not moving, and wide swings in direction will distort the image. At higher speeds (15+?) you lose the resolution very quickly and 2d (traditional sonar) becomes more useful and less prone to problems with turbulence. You will probably want to switch to 2d sonar when running anywhere with the big motor, anyway. On bigger boats many (most?) people use a splitter cable in order to utilize separate transducers for side-imaging and 2d sonar, with the si ducer mounted directly to the jack plate--completely out of the water when on plane, but totally protected and in clean water--and a shoot-thru 2d transducer glued into the bilge above the pad for 2d. That's the typical setup for a boat with a pad hull. If you have a 14' boat it's probably not generating so much turbulence the way a 250hp or something would, so a bit closer to the prop if needed wont kill you. Also, unless your boat is planing only on a pad (yours is not) you wont run into issues with the transducer coming out of the water if you mount it a bit toward the chine. As long as you can strike a balance between keeping the transducer in the cleanest water possible, while keeping its field of view to the sides clear, and protecting it from impact by keeping it only as low as needed to maintain a 2d view at higher speed, you should be fine--try the far outside of your mounting plate and I bet you are OK. Worst case, its sensitive enough that you can see turbulence in the water as a haze on your sonar screen, so it will simply decrease your resolution as if there were static in the water column--this will interfere with your ability to see stuff like bait balls and fish, but at low speeds I doubt it is all that significant and you will be able to adjust your speed to account for it to a large degree. My lund was a lot bigger than yours, but I had the SI transducer mounted more or less where yours is, with the inside side of the transducer almost even with the bottom of the hull, and the outside side with the seam on the transducer about even with the hull--this got me ok 2d sonar on plane up to 45mph or so, and the SI was fine.
Also for what its worth, assuming a good install side imaging works fine on a trolling motor on the bow from an interference/clean water perspective, the issue you will see is that the image will be distorted and +/- unusable if you are steering much--the image relies on a constant speed/constant movement, so every time you steer or start/stop the image is significantly distorted. the 180-degree livescope stuff is newer and will look forward and alleviates some of these issues. I havent mortgaged my house yet for the livescope stuff so I generally keep my bow unit on 2d for this reason, while the stern unit is used on SI at low speeds and switch to a memorized screen with gps and 2d for higher speeds or specific uses. You will almost certainly find yourself switching between various screens to better see certain stuff, especially on a small screen like that, especially as the water column gets deeper. If this is your first time using SI check out the humminbird image interpretation forum and install forum at bass boat central, there are some great resources for rigging, setup and figuring out what the heck you are looking at.
Thanks for the advice.

My boat's top speed is only about 22 or 23 mph; and I won't be using the SI at that speed. More like no-wake speeds, so I'll be good on that.

I took the old Lowrance that was transom-mounted in the pic and moved it up to the trolling motor. I'm finishing up that install now. Need to figure out how I'm going to get the transducer cable to play acceptably with the spot lock and inevitable twisting around the TM shaft.

You all know how it is; life seems to intervene when we want to get the boat out. Maybe next week.
 

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