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Old 05-08-2007, 11:27 PM   #1
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Default Shoot through hull vs transom mount

I'm sorry I know this has been discused before I just cant find it. What will I lose by using a Lowrance shoot through transducer vs. a transom mount?

I know it does not matter in shallow water because I have used them on past boats. What about 300 feet of water. Am I going to lose definition by shooting through fiberglass? What about the slight angle of the bottom of my bay hull? If the angle of the hull is 3 to 10 degrees (for example) how much is the error at 300'?

I want a shoot through ducer but not at the expense of accuracy.

Need I be worried?
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Old 05-09-2007, 10:24 AM   #2
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Default Re: Shoot through hull vs transom mount

If my trigonometry is correct.... if you are in 300' of water and your transducer is at a 10 degree angle, it should show 305'.
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Old 05-09-2007, 11:03 AM   #3
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Default Re: Shoot through hull vs transom mount

I don't know about the Lowrance units, but the Airmar P79 let you adjust the level of the transducer to compensate for the angle of deadrise. You turn the top of the transducer to match deadrise numbers on the mount. If your Lowance has that, you're set. Perhaps check to see if there is an installation manual available for the unit you're considering.
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Old 05-09-2007, 01:20 PM   #4
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Default RE: Shoot through hull vs transom mount

Will the fiberglass hull impede sonar clarity or definition?
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Old 05-09-2007, 02:40 PM   #5
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Default Re: Shoot through hull vs transom mount

So long as it's solid, not cored, it should be minimal. Fiberglass density is very close to that of water.

Transducers intended for in-hull use are generally designed to compensate for the hull thickness. Performance will certainly be better than a transom mount. All else equal, a transom mount usually is ranked last in terms of performance for clarity, etc. because of the turbulence of water at the transom. An in hull unit will give clear readings at a much faster boat speed.
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Old 05-09-2007, 02:41 PM   #6
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Default RE: Shoot through hull vs transom mount

Double post. Where's a delete button when you need it?
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Old 05-09-2007, 11:11 PM   #7
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Default RE: Shoot through hull vs transom mount

So I guess shoot through is the way to go.

Thank yall very much.
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Old 05-10-2007, 06:13 AM   #8
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Default Re: Shoot through hull vs transom mount

Quote:
CAVelocity - 5/9/2007 9:24 AM

If my trigonometry is correct.... if you are in 300' of water and your transducer is at a 10 degree angle, it should show 305'.
No, your trig is not correct.
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Old 05-10-2007, 09:16 AM   #9
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Default Re: Shoot through hull vs transom mount

Use thishand calculator on the Furuno website to calculate beam coverage: http://www.furuno.com/Furuno/Rooms/D....entity.Entity[OID[AF575FE21F71AE45B6DE71C013424D5E]] Also a very interesting read is to download the Furuno transducer guide (approx. 1MB)

Also there will be loss when shooting through fibreglass. The "loss" is not really calculated in regards to the afect on output power but rather the loss when receiving. Keeping a transducer face "wet in the water" is always the best option so a properly installed transom mount will give excellent results. However the physical size of the dual frequency 1kW ducers is not always easy to mount on the transom so this is why you will see most users make the effort to install through hull or shoot through.

In summary, a properly mounted 1kW transom mount will out perform a shoot through. Also there is no difference in the technical specs between transom mount, shoot through or in hull.
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Old 05-10-2007, 11:28 AM   #10
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Default Re: Shoot through hull vs transom mount

Quote:
rotordriver - 5/10/2007 6:13 AM

Quote:
CAVelocity - 5/9/2007 9:24 AM

If my trigonometry is correct.... if you are in 300' of water and your transducer is at a 10 degree angle, it should show 305'.
No, your trig is not correct.
I'd say 305 is accurate.
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Old 05-10-2007, 11:58 AM   #11
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Default Re: Shoot through hull vs transom mount

How about 52ft circle on the bottom.

http://www.furuno.com/Furuno/Rooms/D...13424D5E%5D%5D
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Old 05-10-2007, 12:30 PM   #12
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Default Re: Shoot through hull vs transom mount

You'll be reading a depth of 305, but it will be about 52 feet off the centerline of your boat.
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Old 05-10-2007, 12:53 PM   #13
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Default RE: Shoot through hull vs transom mount

Looks like most of this issue has been covered but I will stick in my two cents anyway. Definitley go with the P79 adjustable to take the deadrise out of the equation if you are defintiely intyent on an in-hull tyransducer. At 3 degree's the depth accuracy problem would be negligable but at 10 degree's it may start to become a problem depending on your accuracy needs. Following your example in 300ft of water, with the beam pointing out 3 degrees your depth would read 300-301ft. At 10 degrees 305. The shallower the water the less then inaccuracy the deeper the more.

While fiberglass can pass acoustic energy pretty well multiple factors come into play on how well.

1. How well you actually seal the transducer to the inner hull. Any air bubbles, even microscopic could drastically effect your performance in a negative way.

2. Transducer position on the hull. You still need very clean flow over the hull where the transducer is, air bubbles or turbulent flow (do to thru hulls, intakes, etc) could also significantly performance in a negative way.

3. Sensitivity. In my experience over 20 years there is about a 20% reduction in sensitivity performance for a transducer in the hull vs. thru-hull. This is a combination of factors 1and 2 with the fact that fiberglass is good but not a perfect density match to water.

What I really suggest is that you look into the Airmar tilted element flush bronze thru-hull transducer. No fairing block is needed. Only 1/8 on an inch sticks out of the hull, making it great for trailered or rack stored boats (won't get crushed or broken). You install the transducer into the hull at the deadrise angle (so it looks like it would be shooting to the side) but internally the element is tilted so that it is actually looking down. I have done over 100 of these and have had great success. It even works great (supposing that you have installed it in the optimum location for your hull) at speed! they cost about $255-249. Worth every penny.

Good Luck.
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Old 05-11-2007, 05:52 AM   #14
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Default Re: Shoot through hull vs transom mount

One other thing one looses with a in hull ducer is a true reading for water temp.Its one of those things I like to know when running offshore.
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