Replacing through hull transducer - deadrise issue

[Ulysses II]

I am replacing my old instruments and find that the new DST 800 through hull transducer won't cope with the 30 degree deadrise of the old speed sensor site (my mistake in not checking the angle pre-purchase). My plan is to build up the hull with fibreglass so that the fitting can be angled at about 15 degrees to the hull surface and get to a deadrise angle that the tranducer can cope with. This will avoid the need for a bulky fairing block and protect the transducer fitting. Has anyone done this, and has any tips?

 

[salar]

Why not mount it in-hull, much easier. You can mount it in a plastic pipe that is cut at a 30degree angle and glued to the hull, then filled with a liquid of your choice. Otherwise you will have more work to build up the thickness in that area, and it would have to cover a larger area for strength purposes and avoid risk of damage/delamination. If the transducer allows an adjustment of 15 degrees then you could in theory do what you are planning to do. The face of the transducer has to be in clear water flow so must not be below the hull face.

 

[pvb]

Why not mount it in-hull, much easier. You can mount it in a plastic pipe that is cut at a 30degree angle and glued to the hull, then filled with a liquid of your choice.

I doubt that the paddle-wheel would work properly mounted in-hull.

 

[pvb]

I am replacing my old instruments and find that the new DST 800 through hull transducer won't cope with the 30 degree deadrise of the old speed sensor site (my mistake in not checking the angle pre-purchase). My plan is to build up the hull with fibreglass so that the fitting can be angled at about 15 degrees to the hull surface and get to a deadrise angle that the tranducer can cope with. This will avoid the need for a bulky fairing block and protect the transducer fitting. Has anyone done this, and has any tips?

Would it be possible to find an alternative location with less deadrise? The DST800 will cope with up to 22 degrees of deadrise, although for best results you should aim for 7 degrees or less.

 

[salar]

I doubt that the paddle-wheel would work properly mounted in-hull.

Definitely not! I wasn't aware the transducer also has a paddle wheel. Looks like the hull will have to be "modified". On reflection I think it will need a fairing on the outside whatever you do on the inside.

 

[Ulysses II]

Thanks for the comments. On balance, I've decided to use the DST800 for speed (and temperature) using the existing hole in the hull and add an NMEA2000 in hull transducer for depth. The system allows me to choose which depth source to use and display, so the DST800 depth can be ignored. This avoids the need for a fairing on the hull and limits additional expense! I'll have to check that there won't be a problem with interference between the two depth sensors.

 

[ibbo]

The depth will probably work for shallow depths, or on whichever tack puts it most upright. I know ours only worked up to 30m on the wrong tack when we mistakenly did the same thing.

 

[Ulysses II]

I suppose that the beam will find the bottom when the boat is heeled over at 8 degrees or more - but not ideal! Hopefully, the in-hull will be more helpful.

 

[Buck Turgidson]

Good idea.
This cost an arm and a leg.

 

[Ulysses II]

Really interesting to see a real life example of what I had thought of doing. The idea of a triducer packaging speed, depth and temperature into one is very appealing - until a few extra degrees of deadrise throw a spanner in the works!

 

[bedouin]

Given you are going that way it is probably worth checking whether the depth transducer works at that angle in spite of what the specs say

 

[salar]

Given you are going that way it is probably worth checking whether the depth transducer works at that angle in spite of what the specs say

It won't. A signal leaving the boat at an angle won't bounce back, it will bounce away from the boat and readings will be unreliable at best. There is a risk of interference if two transducers are on the same frequency, but you may get away with it depending how the signals are transmitted. I have three on my setup (I'm a fisherman) and have to be careful what frequencies I am using.

 

[pvb]

It won't. A signal leaving the boat at an angle won't bounce back, it will bounce away from the boat and readings will be unreliable at best.

The OP's DST800 transducer has a wide fan-shaped beam which covers an angle of 44 degrees, which is why Airmar say it will work up to 22 degrees of deadrise. Obviously, performance at this limit will be less good, but at smaller angles of deadrise it will work fine.

 

[bedouin]

It won't. A signal leaving the boat at an angle won't bounce back, it will bounce away from the boat and readings will be unreliable at best. There is a risk of interference if two transducers are on the same frequency, but you may get away with it depending how the signals are transmitted. I have three on my setup (I'm a fisherman) and have to be careful what frequencies I am using.

Don't be so sure - they have a pretty wide cone of transmission. My transducer works quite happily when healed 30 degrees on either tack.

You are likely to get a reduced depth range but I would expect it to work just fine

 

[Tradewinds]

I had the same conundrum. I've made a nacelle that brings it to about 5 degrees and this will be epoxied in place.

However, if you read the small print in the fitting instructions, Airmar recommend a stronger thru-hull bronze housing instead of the more usual black plastic if you are going to use a fairing.

Now, obviously you could stick with the black plastic and take your chances . . .

 

[pvb]

I had the same conundrum. I've made a nacelle that brings it to about 5 degrees and this will be epoxied in place.

However, if you read the small print in the fitting instructions, Airmar recommend a stronger thru-hull bronze housing instead of the more usual black plastic if you are going to use a fairing.

Now, obviously you could stick with the black plastic and take your chances . . .

I can't see that a DST800 would work very effectively with a nacelle, as the water flow will be greatly affected.

 

[Tradewinds]

I'll let you know. Unfortunately, personal family circumstances means that it's highly unlikely I will bother launching this year. BTW nacelle is probably a bad description on my part.

 

[salar]

The OP's DST800 transducer has a wide fan-shaped beam which covers an angle of 44 degrees, which is why Airmar say it will work up to 22 degrees of deadrise. Obviously, performance at this limit will be less good, but at smaller angles of deadrise it will work fine.

OK fair enough, I'm more used to high resolution, narrow cone transducers on fishfinders.

 

[Ulysses II]

By way of an update. Both the DST800 (mounted at about 30 degrees to horizontal) and the in-hull (directed vertically down) are fitted. In the marina, the in hull recorded about 2.5 metres to DST 800 3.5 metres. In calm, open water conditions, they read almost exactly the same. My conclusion is that the DST 800 will measure depth at higher deadrise angles than advertised but I wouldn't be confident about relying on it. I'm not regretting fitting, and using, the in-hull.

 

[pvb]

By way of an update. Both the DST800 (mounted at about 30 degrees to horizontal) and the in-hull (directed vertically down) are fitted. In the marina, the in hull recorded about 2.5 metres to DST 800 3.5 metres. In calm, open water conditions, they read almost exactly the same. My conclusion is that the DST 800 will measure depth at higher deadrise angles than advertised but I wouldn't be confident about relying on it. I'm not regretting fitting, and using, the in-hull.

Thanks for the update.