Its not hard to measure for the presence of moisture in your transom.
Take a decent quality volt/ohm meter and set it to the highest (usually 20 to 40 M ohms) resistance setting. Then probe any two metal penetrations that you know are in contact with the transom plywood. Start with penetrations that are close to each other. They can be on the interior or exterior of the transom. Note that they must be isolated from each other. For example, if you use two engine mounting bolts they are electrically connected with a low impedance path to each other through the engine so you will read a short circuit. Instead use one engine mounting bolt and, for example, a depth transducer mounting screw as under normal circumstance there should not be a electrically conductive path between those two points. Metal thru hulls (ie garboard drain) are good for this as is any screw that goes into the transom. If there is a continuous field of moisture between the two penetrations the meter will register something. If it’s dry wood the meter will measure the same as it would for open circuit. Dry wood has a resistance in the giga ohm range which is well beyond what most low end ohm meters will measure, hence you will read an open circuit reading. The table below translates resistance into % moisture content. Normal "dry" marine plywood should be around 12%-14%. Moisture content exceeding 26% is caused for concern as that is the generally accepted threshold for supporting fungus growth aka transom rot. Fully saturated wood is in the 35-40% moisture content range. If you do measure high moisture content you can “map” it across your transom to trace it back to its point of origin. I used this exact method to prove to Boston Whaler that my brand new 2006 Whaler as well as its 2008 replacement had saturated wood in their transoms. Story for another day.
Here is the conversion table
Resistance (Mohms), Moisture Content %
18.6, 15
11.2, 16
7.1, 17
4.6, 18
3.09, 19
2.14, 20
1.51, 21
1.1, 22
0.79, 23
0.6, 24
0.46, 25
If you have access to a pin type moisture meter you can do the same type of measurements. A pin type moisture meter is really just a high quality ohm meter that translates resistance into a rough measure of % moisture content. Another tool that is often used (or should I say abused) to make transom moisture measurements is a dielectric moisture meter. Unless you first calibrate it on a known dry sample of your transom its readings are next to meaningless. The dielectric type moisture meters measure the dielectric constant of the material it’s held up against. Wet wood has higher dielectric constant than dry wood. But since fiberglass is a very good insulator and has a low dielectric constant it will result in the meter reading too low. How low it reads is a function of how thick the fiberglass on your transom is, hence the need for taking baseline measurements of known dry wood.
Note that my business is designing and manufacturing wireless moisture monitoring equipment – see www.omnisense.com if you are curious.
Take a decent quality volt/ohm meter and set it to the highest (usually 20 to 40 M ohms) resistance setting. Then probe any two metal penetrations that you know are in contact with the transom plywood. Start with penetrations that are close to each other. They can be on the interior or exterior of the transom. Note that they must be isolated from each other. For example, if you use two engine mounting bolts they are electrically connected with a low impedance path to each other through the engine so you will read a short circuit. Instead use one engine mounting bolt and, for example, a depth transducer mounting screw as under normal circumstance there should not be a electrically conductive path between those two points. Metal thru hulls (ie garboard drain) are good for this as is any screw that goes into the transom. If there is a continuous field of moisture between the two penetrations the meter will register something. If it’s dry wood the meter will measure the same as it would for open circuit. Dry wood has a resistance in the giga ohm range which is well beyond what most low end ohm meters will measure, hence you will read an open circuit reading. The table below translates resistance into % moisture content. Normal "dry" marine plywood should be around 12%-14%. Moisture content exceeding 26% is caused for concern as that is the generally accepted threshold for supporting fungus growth aka transom rot. Fully saturated wood is in the 35-40% moisture content range. If you do measure high moisture content you can “map” it across your transom to trace it back to its point of origin. I used this exact method to prove to Boston Whaler that my brand new 2006 Whaler as well as its 2008 replacement had saturated wood in their transoms. Story for another day.
Here is the conversion table
Resistance (Mohms), Moisture Content %
18.6, 15
11.2, 16
7.1, 17
4.6, 18
3.09, 19
2.14, 20
1.51, 21
1.1, 22
0.79, 23
0.6, 24
0.46, 25
If you have access to a pin type moisture meter you can do the same type of measurements. A pin type moisture meter is really just a high quality ohm meter that translates resistance into a rough measure of % moisture content. Another tool that is often used (or should I say abused) to make transom moisture measurements is a dielectric moisture meter. Unless you first calibrate it on a known dry sample of your transom its readings are next to meaningless. The dielectric type moisture meters measure the dielectric constant of the material it’s held up against. Wet wood has higher dielectric constant than dry wood. But since fiberglass is a very good insulator and has a low dielectric constant it will result in the meter reading too low. How low it reads is a function of how thick the fiberglass on your transom is, hence the need for taking baseline measurements of known dry wood.
Note that my business is designing and manufacturing wireless moisture monitoring equipment – see www.omnisense.com if you are curious.
