Garmin GT51M & deadrise single vs pair.

adhdan · 5 Apr 2024

Ive bought a garmin GT51M TH transducer & on reading the instructions after it arrived it turns out i should of bought a pair of them as my deadrise is 15degrees.

Anyone fitted a single gt51 on over 5degree hull?

Will running a pair give a better image or does it just fill in the blank on the side image?

Is the single unit the same transducer that come in the double pack or are they handed left & right?... If i bought another single unit would it work as a pair.

The fairing block stands the transducer off the hull so im guessing that will 'in effect' reduce my deadrise by a little bit, is it 5degrees measured from the side of the transducer & any hull in the way of that line will create a dead spot in the sidevu image?

thanks
Dan

jfm · 5 Apr 2024

It won't work well if you mount it well away from the keel on 15 degree deadrise. It will simply not see one side, regardless of whether you use the fairing block.

Depending on your boat, you could mount it on a fairing block really close to the keel so it can see both sides. Depends on your hull shape and whether the transducer will hit things, etc. The fairing block stands it off the hull as you say, and can change the 15 degrees to 5, but you still get a shadow/dead spot caused by the keel if you mount it too far away from the keel. The only way to fix that when using one transducer is to fit it really close to the keel so it can see "past" the keel. You could even fit it on the keel, but I appreciate that might cause other problems with lift slings, trailer, beaching, etc

Fitting a pair will let you see both sides, not improve the image.

The transducers are not handed left/right. So if you buy another it will be identical to the one you've already got. The left/right handing is done in the Y cable that you have to buy as well. The Y cable lets you plug in two transducers to the one transducer cable socket on your black box, and the two "tails" of the Y cable are handed (labelled) port/starboard.

"Should have", not "should of" - can we stamp that one out? (ffs!)

Best of luck.

Portofino · 5 Apr 2024

They are all on the keel midships on my 23 degree deadrise boat .

adhdan · 5 Apr 2024

jfm said:
It won't work well if you mount it well away from the keel on 15 degree deadrise. It will simply not see one side, regardless of whether you use the fairing block.

Depending on your boat, you could mount it on a fairing block really close to the keel so it can see both sides. Depends on your hull shape and whether the transducer will hit things, etc. The fairing block stands it off the hull as you say, and can change the 15 degrees to 5, but you still get a shadow/dead spot caused by the keel if you mount it too far away from the keel. The only way to fix that when using one transducer is to fit it really close to the keel so it can see "past" the keel. You could even fit it on the keel, but I appreciate that might cause other problems with lift slings, trailer, beaching, etc

Fitting a pair will let you see both sides, not improve the image.

The transducers are not handed left/right. So if you buy another it will be identical to the one you've already got. The left/right handing is done in the Y cable that you have to buy as well. The Y cable lets you plug in two transducers to the one transducer cable socket on your black box, and the two "tails" of the Y cable are handed (labelled) port/starboard.

"Should have", not "should of" - can we stamp that one out? (ffs!)

Best of luck.

The only place i might get away with a single transducer then is about 2' behind the bow thruster where the keel runs in.

Sorry i should of not said that

jfm · 6 Apr 2024

OK but normally that would be considered too far forward. It is so close to the bow that it would get a lot of air bubbles/turbulence as you speed up. You normally want the transducer aft of amidships sort of area.

adhdan · 6 Apr 2024

jfm said:
OK but normally that would be considered too far forward. It is so close to the bow that it would get a lot of air bubbles/turbulence as you speed up. You normally want the transducer aft of amidships sort of area.

I’m back at the boat & wondering about this spot, it’s between the keel & the skeg, it’s at the side of the engine & 5’ infront of the propeller .
**Edit: scrap that idea..I thought it was a temperature sensor but it’s the water intake

jfm · 6 Apr 2024

Ok having now seen the hull I think you should get the second transducer and the Y adaptor, if you want it all to work well.

adhdan · 6 Apr 2024

jfm said:
Ok having now seen the hull I think you should get the second transducer and the Y adaptor, if you want it all to work well.

Yes sorry I should have mentioned at the beginning it’s semi-displacement.

colhel · 6 Apr 2024

That looks like a high spec transducer, do you need all those functions?
Below is a picture a picture of my setup on the same boat as yours. But, I'm only set up to show depth. The inside (nearest keel) is the depth transducer and the outside is the paddle wheel. From the inside of the boat, they're just forward of the battery compartment.
I'm wondering if you moved yours further away from the keel it would have more vision

jfm · 6 Apr 2024

Col, he already has the hardware. No point debating whether he needs it! (though it is very good kit imho)

Moving it away from keel doesn't solve anything with these sidescan transducers. The pictures on this Garmin page show the problem - keels and high deadrises create blind spots for these sorts of transducers.

adhdan · 11 Apr 2024

colhel said:
That looks like a high spec transducer, do you need all those functions?
Below is a picture a picture of my setup on the same boat as yours. But, I'm only set up to show depth. The inside (nearest keel) is the depth transducer and the outside is the paddle wheel. From the inside of the boat, they're just forward of the battery compartment.
I'm wondering if you moved yours further away from the keel it would have more vision View attachment 175115

I thought about a flush through hull but I like gadgets & want the sidevu. I reckon I’ll get a pair of Gt56uhd transducers as they are slightly cheaper then a pair of 51’s.

Took the boat out & im getting plenty of white smoke at higher rpm’s , max rpm’s 3400 @ 19knots. Should I start with the turbo or the injectors? Doesn’t smoke until about about 2000 so turbo?

Scubadoo · 11 Apr 2024

adhdan said:
I thought about a flush through hull but I like gadgets & want the sidevu. I reckon I’ll get a pair of Gt56uhd transducers as they are slightly cheaper then a pair of 51’s.

Took the boat out & im getting plenty of white smoke at higher rpm’s , max rpm’s 3400 @ 19knots. Should I start with the turbo or the injectors? Doesn’t smoke until about about 2000 so turbo?

Have you considered a transom mounted transducer?

adhdan · 11 Apr 2024

Scubadoo said:
Have you considered a transom mounted transducer?

From what I read they don’t work as well on the transom because of the turbulent water, seems a bit fragile as well on the bracket

colhel · 11 Apr 2024

adhdan said:
I thought about a flush through hull but I like gadgets & want the sidevu. I reckon I’ll get a pair of Gt56uhd transducers as they are slightly cheaper then a pair of 51’s.

Took the boat out & im getting plenty of white smoke at higher rpm’s , max rpm’s 3400 @ 19knots. Should I start with the turbo or the injectors? Doesn’t smoke until about about 2000 so turbo?

19 knots at 3400 rpm is pretty good. Could you get anymore rpm? Max rpm should be 3900. I'm making the assumption your boat and engine are the same as mine. Nimbus 3003 with a tamd41a or tamd41b?

Scubadoo · 11 Apr 2024

adhdan said:
From what I read they don’t work as well on the transom because of the turbulent water, seems a bit fragile as well on the bracket

I have a transom mounted old Garmin transducer fitted to my planning hull for my fishfinder which if I remember correctly works up to around 12knots and another thru hull transducer raytheon tridata for higher speeds. I am about to upgrade my Garmin Fishfinder and transducer with a GT52HW-TM as after 10+ years the old one has recently failed.

As you are a semi-displacement boat, I think it should work fine, if I was you I would experiment by temporarily fixing a piece of wood (e.g. using clamps) to the transom with the transducer mounted to it, and perform a few test runs until you find the best position. It will certainly be a lot cheaper than your current dual transducer idea.

As for being fragile, never had an issue in the 30+ years of boating with a transducer fitted to the transom. The transom bracket is designed to kick up if you strike an object underwater, but never had that issue.

adhdan · 11 Apr 2024

colhel said:
19 knots at 3400 rpm is pretty good. Could you get anymore rpm? Max rpm should be 3900. I'm making the assumption your boat and engine are the same as mine. Nimbus 3003 with a tamd41a or tamd41b?

I think I've sorted it , had a spare air filter & changed it over and smoking stopped. I had 19knots @ 3500rpms which was maxed out, if I held it for longer I think I'd creep up a little more. Ad41b.

& I'm being daft I was looking for max rpms running in gear ,

out of gear I should hit 3900rpms then but in gear I should never hit 3900

adhdan · 11 Apr 2024

Scubadoo said:
I have a transom mounted old Garmin transducer fitted to my planning hull for my fishfinder which if I remember correctly works up to around 12knots and a another thru hull transducer raytheon tridata for higher speeds. I am about to upgrade my Garmin Fishfinder and transducer with a GT52HW-TM as after 10+ years the old one has recently failed.

As you are a semi-displacement boat, I think it should work fine, if I was you I would experiment by temporarily fixing a piece of wood (e.g. using clamps) to the transom with the transducer mounted to it, and perform a few test runs until you find the best position. It will certainly be a lot cheaper than your current dual transducer idea.

As for being fragile, never had an issue in the 30+ years of boating with a transducer fitted to the transom. The transom bracket is designed to kick up if you strike an object underwater, but never had that issue.

I've sent the other back so I'll give it some thought before I buy another & lift the boat again

colhel · 12 Apr 2024

adhdan said:
I think I've sorted it , had a spare air filter & changed it over and smoking stopped. I had 19knots @ 3500rpms which was maxed out, if I held it for longer I think I'd creep up a little more. Ad41b.

& I'm being daft I was looking for max rpms running in gear ,

out of gear I should hit 3900rpms then but in gear I should never hit 3900

You should be able to get 3900 in gear, underway, but 19 knots at 3500 sounds right. At WOT (wide open throttle) you should get 3900rpm and about 22 knots. Is the hull and sterngear clean?
This is useful info for me too as it looks like I'm underpropped

adhdan · 12 Apr 2024

colhel said:
You should be able to get 3900 in gear, underway, but 19 knots at 3500 sounds right. At WOT (wide open throttle) you should get 3900rpm and about 22 knots. Is the hull and sterngear clean?
This is useful info for me too as it looks like I'm underpropped

19.5knots on gps @ wot 3700rpm seems about it for me. Old fashioned speedo was 20ish .
Out of gear it easily reaches 3900 pushing 4000.

Hull is clean but not smooth

colhel · 12 Apr 2024

I'd say, but I'm no expert, there's nothing to be too concerned about there. Enjoy the season

Garmin GT51M & deadrise single vs pair.

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