Boat Transom Problems – Everything You Need To Know!

What happens to a fibreglass boat that has a wet or rotten transom, and why is it important to fix it? We run through how a transom becomes wet, pictures of major telltale signs and cost to repair the transom correctly.

What is the transom?

A transom is the structural section of the back of the boat that the engines mount to. With an outboard, they mount to a bracket on the back. With a sterndrive, they mount via a hole through the middle and a transom assembly.

How does my transom become wet?

Water ingress to a transom can be caused by numerous reasons. All of these take a period of time to develop, and getting water in your transom once will not necessarily rot it away overnight.

The most common failure points we’ve seen at BoatBuy are:

  • Damaged/cracked drain fittings to splash wells (outboards)
  • Perished sealant (silicone) to mounting bolts (outboards)
  • Transducer and trim tab mounting screws allowing water past
  • Leaking steering pin seals with unsealed transom shield mounting holes (sterndrive)
  • Through transom anode bolts (as these anodes are changed annually, without the utmost of care it’s quite likely to spin the bolts on removal and fitment of the anode – the spinning of the bolt breaks the sealant between the bolt’s washer and the transom)

It’s not just the outer transom that needs to be sealed with any fixtures going into the transom, the inner transom and stringers should also be sealed as build up of bilge water can seep into any non-sealed fixtures.

How to determine if my transom is wet?

Some telltale signs your transom has been affected by water ingress and needs attention could include:

  • Rust/brown stains below screws or bolts
  • Visible compression of the transom where mounting bolts secure an outboard
  • Visible compression or cracks noted to an inner transom around the sterndrive transom assembly
  • Gelcoat cracks around upper transom and splash well corners (suggests compromised integrity and flexing to the transom)
  • An audibly dull note and feeling a lack of rebound when performing a percussion test of a transom
  • An audibly tinny (delamination) note when performing a percussion test of a transom
  • Elevated moisture readings in a particular area (water, salt, metals and ablative coatings can interfere with the accuracy of moisture meter testing, results should be achieved on a dry transom and from the inside wherever possible)
  • Remove a screw an observe for corrosion and water entry to the transom

These are a series of indicators we look out for at BoatBuy when completing a Pre-Purchase Inspection.

Does it matter that there is water in the transom?

Yes, once water has entered a timber-cored transom it travels between these layers that create the integrity of your transom. If you can imagine, with every gear change and with the boat thrusting forward and aft every time you accelerate and decelerate, the water between these layers has a hydraulic effect, pulsing and pushing water with extreme pressures further throughout the transom. This is known as water migration and hydraulic erosion. The migration of this water causes timber core shredding, further areas of delamination and eventually an irreversible complete core failure and the entire transom must be replaced.

How to repair a wet transom?

The decision on whether a transom can be repaired should be decided by a shipwright or boat builder willing to complete and warrant the work. This could include core samples to be extracted and examined, to various areas of the transom to determine the extent of moisture spread. Sterndrive or anode mounting holes could be over bored to eliminate areas of wet, corefilled and re-glassed, these spot repairs should be noted to be a repair of temporary nature, as the integrity of the remainder of the transom is unknown. Ultimately, the most complete and thorough repair to a transom is a transom replacement. Chris Austin from FibreFinish has a good article about it here.

How much does it cost to replace a transom?

On a small outboard driven boat with easy access, you may be looking around $4000-$6000. It’s important to remember that any major transom work is going to require the motor removed, and the hull taken back to bare to allow access to cutting the entire timber core section out (without damaging the outer skin).

On a larger boat, you could be looking anywhere from $5000-$15,000. It’s not uncommon to find the water has tracked further into the stringer system (if timber cored too), compromising the hull further and adding additional costs.

What percentage of moisture is too high for a wet timber cored transom?

Provided there are no other factors affecting the readings for example anti-foul coatings, metal, residue salt:

5-20% – Considered normal

20-30% – Transom has deteriorated and is letting water in

30% + – Basically underwater!

Percussion testing should be used in conjunction with a moisture meter to get the most accurate results. Timber has a natural moisture content of between 5 and 15% so these levels are considered normal. When an elevated reading is returned the next step is invasive testing such as core sampling, which involves drilling a number of holes into the transom and assessing the shavings produced from the drill bit to see how far the rot has travelled.

Professional inspections

The older the boat, the more likely a wet transom will be uncovered. It takes a mix of a trained eye, special tools and experience to uncover a transom issue, and our entire team are trained to detect these issues.

Having one of the BoatBuy team assess your boat not only covers the hull, but a number of other important areas including a sea trial to ensure you’re making an informed purchase. We provide our comprehensive services in both Queensland and New South Wales. Feel free to get in contact or book in a Pre-Purchase Boat Inspection today.

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