Humminbird 981C 80ft transducer extension

[ChrisPreece]

981 plus towfish!

If you look back at the eighth reply on this thread and click on towfish.jpg, you'll see what you can do with the transom mounted transducer. At the time the tow cable had only been extended by thirty feet and worked quite well on both frequencies. I've now added another thirty feet and it still works quite well on the lower frequency but not the upper one. With a 997 unit you may have a problem extending the tow cable past what the manufacturer supplies with the transducer.

[The Pirate]

997

gainted.

Can i ask you where you've managed to pick up the 997 from in the states as i'm looking to get one as well, i'm actually torn between the 997 and the 987 as the 997 sidesacn capabilites are shorter that the 987???

Now here's a silly question so i would like a serious answer, The transom mounted transducer, do you loose sidescan signal from engine being in the way when fiited to the transom???

All answers would be appreciated

[rgecy]

The 997 is not available yet. Some web sites have the info and price, but they are on back order.

...as the 997 sidesacn capabilites are shorter that the 987???

I think you have it backwards. The 997 has better resolution and depth capabilities than the 987. The 987 uses a 455khz transducer and is rated to 100' and the 997 uses a higher resolution 600 khz transducer and is rated to 150'. The 997 will definately be the better unit.

Robert in SC

[The Pirate]

Rgecy, I was only thinking that the 987 has a better range than the 997 on its sidescan due to what humminbird have put on the spec sheet, The 987 has 360ft sidescan and the 997 says its only 240ft but they both go to the same depth of 150ft. Am i right in thinking that the higher the frequency the better the definition and the lower the frequency the further the range??? i.e the 997/455khz & 600khz better definition and the 987 262khz & 455khz so better range with less definiton.

Thats how i see it on the humminbird data sheet's, Please correct me if i;m barking up the wrong tree.

Other than that all comments greatly appreciated.

The Pirate.

[rgecy]

No, according to the data sheets you are correct. What I do know is the 987 was rated only to 100' and performs poorly at 262khz at this depth and is not even readable at 455khz. The specs according to the new sheets list 150' at 262khz. Maybe there have been some changes to the new 987 units since I purchased mine last year.

As far as the 360' side image, the clarity at 262khz is very poor and I have never imaged an object past 240' with any success. In fact I do not even use the 262khz because it is so poor.

The 987 is still an excellent choice since they will incorporate the new recording features of the 997 into the 987 as well. I think Bass Pro had the 987 on sale for like $1499. Thats a bargain because I felt like I stole mine when I paid $1750 last year.

Robert in SC

[Bryan]

Guys:

Nothing in life is free. If you increase the frequency for better resolution you decrease the size of the window (at some distance, here 150 feet).

If you read the specs re 2 sided split image for both the 987c and 997c you will see that the 987c spec is 360' at 262 kHz. The 997c works at both 455 and 600 kHz for 2 sided imaging, but they quote for 455.

If you multiply the frequency by the width (using the 455 kHz for the 997c) you get about the same value (the 997c being slightly 'better')

All that is happening is that the size of the transducer may be slightly different but otherwise they are both performing as expected by physical acoustics.

What you expect to be gaining is increased resolution of the target. However I don't hear Bob saying that the behaviour is as good as advertized for even the 987c.

Of course how they measure and we use the equipment may differ.

Bryan

[The Pirate]

Ok Fella's, Here's the question, Would you buy one of these unit's to look for shipwrecks in the 150ft region??, If so which one would you buy the 987 or wait until the 997 to come out??? and would you mount the transducer on the boat transom or make up a towfish. Now here's a question, I have a aquascan mc5 proton magnetometer would i be able to fasten the transducer to the towfish for the mag and use it theat way?.

Obviously some of you guy's have got these humminbird units so all answers would be much welcomed.

If anybody in the UK reads this and has got a 987 please could you let me know the best place to get hold of one.

Regards to all.

[ChrisPreece]

The right tool for the right job............

A lot of the people using this site are using their own individually selected equipment (be it Humminbird or something else) under differing conditions and with differing requirements. This inevitably results in different experiences and different points of view.

If you were to employ the Humminbird 900 series in lakes, where the possibilities of calmer conditions are at least somewhat greater than at sea (I have the Western Approaches/Channel to cope with) then, given the dictates of sidescan use, if the water wasn't that deep, you could get away with a transom mount and still get very good results. One thing you might like to be careful about in this scenario would be propwash and boat turbulence (the installed position is very important).......as I understand it some people have found that 'aperture blocking' occurs because the system can not as effectively 'see' through the boats wake, as it can through undisturbed water.....the result being image distortion on one side.

You aren't going to use it under those conditions though.....looks like you'll be operating off the North Welsh coastline and are interested in having something that will work to 50m/150ft. Transom mount? Don't think so!!

The Irish Sea isn't a lake and you will have to cope with similar conditions to myself. This effectively means that you have to find some manner of means by which boat motion is removed......because it is this, that over 50m or so, is going to make the image look like something a five-year old drew at playschool.

Although Bryan has an great idea for a gadget that eliminates boat motion (and this is the basis for some commercial/military systems) it gets away from the fundamentals of sidescan use......those being that; more information is derived about the scanned targets when the 'angle of sight' is shallow, thereby throwing sonar shadow. This means (for us at least) the use of a towfish.

CP

[Bryan]

Very helpful comments, Chris.

Since I posted my intention to build a motion-compensated platform, I was asked by a friend (from whom I know a little about side-scan sonars) if the idea could be expanded to 'deep' tows. At first I didn't see how but with some quiet time I realize that it is relatively straight forward.

Consider a platform in the middle of some gimbals which are themselves supported on an x-y slider arrangement (so far 4 DOF). My first idea was a vertical tube, fixed at the centre of this platform. Through that ran another post which went down to the transducer on the bottom of an 'up-side down' hydrofoil.

The inner tube would be free to rotate with boat yaw and move up and down (say on something like linear bearings or a nylon lined bushing) (now 6 DOF).

To stop the hydrofoil from pulling the entire inner assembly down, one attaches a bungee cord off some structure emanating from the top of inner tube (or gimballed platform) (now 6- DOF as have introduced some lag). Also the drag on the assembly underwater will pull the lateral (y) arm back in the fore-aft (x) direction and so it needs another restraining elastic link (say a spring or bungee cord). (Now 6-- DOF)

That system is good if you just want to get the sensor a few feet into the water. However there are many cases where, as Chris points out, you want to get the fish down so you can shadow the object. OK, what to do?

First let's replace the inner tube with a wire (which ultimately will be connected to a winch). That wire is held with a (say chinese finger web) grasp, and supported from our friend the bungee cable. If we want to set or change the depth, the weight is taken on another arm (say fixed arm with an openable eye) and pulled in or out by a winch on the deck nearby.

As before, I suggest something like a pontoon boat with the apparatus forward, the x axis mounted first on vibration pads, the y next up (ie riding on the x arms), (dual tracks in x and y by the way), then roll gimbal (on y tracks) outside, pitch gimbal inside it, (leaving room for rod or wire to move within gimbal. Might as well have inner gimbal connected directly to the inner tube/platform.

The strength you have to allow for is determined (for me) mostly by the lift/drag ratio of the inverted hydrofoil. The hydrofoil will pull down and forward, very similarly to a water-skier pushing out and developing a force forward which is greater than his drag back until he/she gets almost abeam the towing boat.

The beauty of this arrangement is that you don't need as much cable out (power (signal?) loss) and you know exactly where it is. If the weight of the hydrofoil (and line) is significantly less than the lift of the hydrofoil, the arrangement is 'independent' of boat speed (so you don't have to 'fly the fish'.

So far I have spent about $125 US on ebay - mostly for nylon lined sliders and a few bearings/bushings. I plan to use aircraft grade aluminum tubing at 3/4" OD for my strength requirements. Of yes, I also discovered smallish, light, hydrofoils used to liftl rear outboard engines up to get a boat to plane. It cost me a princely $25. I'm planing on using my aircraft floats for a test pontoon boat. A few more fittings, possibly some welding/machining to get to make some structure to place the gimbal bushing in, and we are ready to give it a bath.

However that isn't possible until April around here.

Bryan

[Unregistered]

Sorry Folks:

The idea of an upside down hydrofoil - and one borrowed from the outboard motor group - is old hat. See Sportscan and the 'depressor fin'.

Good idea - just years late.

But for $10 on ebay I'm glad to know I've reached the level of modern technology.

Bryan