Boat Purchasing : Steel Thickness

[Bristolfashion]

We are embarking upon our first narrow boat purchase (the yacht is sold, so we are in a watery worry free zone for the first time in years!).

 

We'll be looking for a boat in decent condition, not a project (the project is to go boating!)

 

Now, broker's details often quote the steel thickness as 10/6/4 or 10/6/5/4, but this is presumably just the original manufacturer's specification. I appreciate that one should have a good old poke about to spot obvious problems, but there's no way of being sure about thicknesses in water and without a survey.

 

A survey will probably come up with some other, lower numbers. My question is what are your ranges for,

 

1. That's fine,

2. That's very minor, but is worth a few quid off

3. That's minor, but some work needs to be done by the vendor (or an equivalent cash reduction) before purchase.

4. Run away - and give me my deposit back.

 

I'm aware of the 4mm for insurance issue. I'd have thought that a surveyors recommendation to overplate (on a decent boat) would be option 4.

 

As ever, thank you for reading and for any advice given. I hope not to disprove the adage, "there's no such thing as a stupid question" during this process.

[Mike Tee]

One thing to look for in an advert is a previous survey 'which is available in the brokers office'. Whilst it is not to be relied on as a decision making point, it is a guide, particularly on steel thicknesses, as to whether or not to take the next step.

[matty40s]

5 minutes ago, Mike Tee said:

One thing to look for in an advert is a previous survey 'which is available in the brokers office'. Whilst it is not to be relied on as a decision making point, it is a guide, particularly on steel thicknesses, as to whether or not to take the next step.

That would depend on whether the previous survey was commissioned by someone wanting to buy it previously, or by the seller/ broker.

[Russ T]

I think a lot will depend on the age of the boat in question,and its past maintenance regime.

 

1. That's fine, - 10/6/4 with minimal pitting on a 20 year old boat. 

 

2. That's very minor, but is worth a few quid off - 10/6/4 with minor pitting on a 1-5 year old boat.

 

3. That's minor, but some work needs to be done by the vendor (or an equivalent cash reduction) before purchase.10/6/4 with remedial pitting work detailed by the surveyor.

 

4. Run away - and give me my deposit back. - Anything that mentions overplating.

 

FWIW. I sold a 25 year old boat last year that had virtually no pitting. A sellers survey was made available to potential buyers (Though some on here will tell you that it was a waste of money)

[David Mack]

4 hours ago, Bristolfashion said:

4. Run away - and give me my deposit back.

Be aware that if you have paid a deposit to a broker or boatyard the contract will state specific conditions when you can back out of the purchase and get your deposit back. Just changing your mind because of a somewhat negative survey is not always sufficient grounds.

[LadyG]

3 hours ago, matty40s said:

That would depend on whether the previous survey was commissioned by someone wanting to buy it previously, or by the seller/ broker.

Nobody is going to pay for a survey and walk away leaving the survey with the vendor.

[Peanut]

I walked away, and had my deposit back. The vendor wouldn't be keen to show the survey to another buyer, as it might put them ff.

[Bristolfashion]

12 hours ago, David Mack said:

Be aware that if you have paid a deposit to a broker or boatyard the contract will state specific conditions when you can back out of the purchase and get your deposit back. Just changing your mind because of a somewhat negative survey is not always sufficient grounds.

Yes, a good point, thank you.

 

I would, have, thought, however, that a plate that was advertised at 10mm, but is approaching 4mm would be adequate to withdraw (yes, I know all the broker caveats about their "details"), but I'll make sure I'm fully across the contractual details.

[Alan de Enfield]

43 minutes ago, Bristolfashion said:

Yes, a good point, thank you.

 

I would, have, thought, however, that a plate that was advertised at 10mm, but is approaching 4mm would be adequate to withdraw (yes, I know all the broker caveats about their "details"), but I'll make sure I'm fully across the contractual details.

 

Typically the statement to allow withdrawing due to repair costs is something like .............

 

"If the cost of necessary repairs is more than 10% / 15% (insert any number the broker feels like) of the agreed purchase price then you can renegotiate the purchase price, or have your deposit returned"

 

Other contracts can be found that are more in favour of the buyer :

 

5.2 Survey
5.2.1 The Buyer may, at a venue to be agreed and at his own cost, be entitled to require the Seller to arrange to slip or crane the boat ashore and open up the boat and her machinery for the purposes of survey which shall be completed within twenty one days of the signing of this Agreement. If any survey requires more than superficial non-destructive dismantling and limited removal of anti-fouling the consent of the Seller must be obtained before such work commences, such consent may not be unreasonably withheld.
5.2.2 Within fourteen days of the completion of the survey the Buyer may, by written notice to the Seller:
EITHER
5.2.2.1 Reject the boat outright if the Buyer rejects the Boat, this Agreement shall be automatically terminated and the Seller shall refund the Deposit to the Buyer in accordance with Clause 8.
OR
5.2.2.2 Reject the Boat but offer to proceed with the purchase and accept the Boat on the condition that the Seller agrees either:
5.2.2.2.1 to rectify specific listed defects or deficiencies in the Boat and/or the Inventory before Completion. All required items of work to be completed without undue delay and carried out to meet the expressly specified requirements of the survey; or
5.2.2.2.2 to a reduction in the Purchase Price to enable the Buyer to rectify such defects or deficiencies after Completion.
5.2.3 If the Buyer serves notice requiring the Seller to rectify specific listed defects or deficiencies, or to make a reduction in the Purchase Price, and the Seller has not agreed to make good such defects, or the Parties have not agreed a reduction in the Purchase Price within twenty one days, then this Agreement shall be deemed to have been rescinded on the twenty second day after the service of notice, and the Seller shall refund the Deposit to the Buyer in accordance with Clause 8.

 

 

Return of Deposit

7.2 In the event of the rescission of this Agreement by the Buyer following survey, and/or sea-trial, he shall at his own expense reinstate the boat to the condition and position in which he found her, and shall pay all boatyard and surveyor’s charges for this work, and the Seller shall return the Deposit as specified under Clause 8.

8 Within seven days of the completion of any reinstatement work required under Clause 7.2, the Seller shall return the Deposit to the Buyer without deduction and without interest, save that he shall be entitled to retain such part of the Deposit necessary to defray any reasonable boatyard or surveyor’s charges not paid by the Buyer. Neither Party shall thereafter have any claim against the other under this Agreement.

 

 

[Slim]

3 years ago I sold my 26 year old (owned from new) NB. and I had a pre sale hull survey conducted for my own peace of mind. Originally the steel was 10/6/4 . One single spot on the baseplate was down to 8.5 This was just where the engine/main cabin bulkhead was, and the corrosion internal. The corresponding spot on the lower hull was 5.4  and also internal. The rest of the sides were 5.7+. The rest of the baseplate was 9.2+ and had never been blacked. The boat itself had been blacked at 2 year intervals with bitumen. Minimum of 6 anodes throughout it's life. Galvanic isolator from new and fitted with an isolation transformer for the last couple of years.

 

[MtB]

19 hours ago, Bristolfashion said:

Now, broker's details often quote the steel thickness as 10/6/4 or 10/6/5/4, but this is presumably just the original manufacturer's specification. I appreciate that one should have a good old poke about to spot obvious problems, but there's no way of being sure about thicknesses in water and without a survey.

 

One point no-one has made clear is steel doesn't corrode uniformly. Nor does it necessarily even start off as thick as the nominal value. 10mm rolled sheet can be as little as (something like) 9.6mm straight from the steel mill if you look up the specs for rolled sheet steel. Then when as corrodes with age the thicknesses will be all over the place depending on where on it one measures. You might get a result of 7mm then move 30mm sideways and find it is 9.4mm. Totally randomly. 

 

The older the boat, the more variation I think you might find although there are a lot of exceptions. I recently sold a boat built in 1991 with virtually no corrosion degradation. Not even in the baseplate. 

 

 

 

 

 

 

 

 

 

 

 

[LadyG]

29 minutes ago, Slim said:

3 years ago I sold my 26 year old (owned from new) NB. and I had a pre sale hull survey conducted for my own peace of mind. Originally the steel was 10/6/4 . One single spot on the baseplate was down to 8.5 This was just where the engine/main cabin bulkhead was, and the corrosion internal. The corresponding spot on the lower hull was 5.4  and also internal. The rest of the sides were 5.7+. The rest of the baseplate was 9.2+ and had never been blacked. The boat itself had been blacked at 2 year intervals with bitumen. Minimum of 6 anodes throughout it's life. Galvanic isolator from new and fitted with an isolation transformer for the last couple of years.

 

And your point is?   Presumably, that a well maintained boat is more likely to be in better condition than one that has not been maintained?

Edited November 16, 2025 by LadyG

[Slim]

2 hours ago, LadyG said:

And your point is?   Presumably, that a well maintained boat is more likely to be in better condition than one that has not been maintained?

It was given to aid the OP as an example of what might be out there. Sorry if you weren't able to understand that!!! 

[David Mack]

25 minutes ago, Slim said:

It was given to aid the OP as an example of what might be out there.

But it's also an example of what's available at the better end of the market. Other boats of similar age may have had multiple owners and completely unknown maintenance regimes.

[mrsmelly]

43 minutes ago, David Mack said:

But it's also an example of what's available at the better end of the market. Other boats of similar age may have had multiple owners and completely unknown maintenance regimes.

No two boats are alike. I have said before, I bought a twenty year old colecraft, built in 1981 from 6/6/3 steel from an honest chap who had owned it 18 years and never even docked it so never been blacked. Lots of stuff needed doing on it so i paid peanuts for it and took it to Johny Pinders to do hull survey and do all the necessary plating work. He surveyed the Hull and worse bit was 5.7 mm, no work realy needed but I put a bit of rubbing strake on the chines arse end for good measure, hull was near mint in reality. You simply never know, there seems to be no rhyme nor reason why some boats rot away and others do not. 

[WulfNut]

On 15/11/2025 at 20:23, David Mack said:

Be aware that if you have paid a deposit to a broker or boatyard the contract will state specific conditions when you can back out of the purchase and get your deposit back. Just changing your mind because of a somewhat negative survey is not always sufficient grounds.

Depends on the broker.  ABNB for instance certainly used to accept any reason.  Rugby accept virtually none. 

[matty40s]

1 hour ago, WulfNut said:

Depends on the broker.  ABNB for instance certainly used to accept any reason.  Rugby accept virtually none. 

Rugby as far as I know accept any repairs identified on survey needing over 10% value of the boat and you can walk away.

[Peanut]

The RYA contract states the same, if the survey identifies repairs which would cost over 10% of the purchase price, then the buyer can walk away and have their deposit back. You might need to get a quote for the cost of repairing the faults indicated in the survey, to a satisfactory standard. In case of a dispute, the surveyors might be able to assist.

Other contracts may differ, and you should check the terms before signing.

[agg221]

Quality of steel and maintenance really do have a major role in condition - far more so than age alone. There are quite a few boats about from the 1970s/80s which are in good overall condition with minimal loss of thickness. The counter conversion was done on ours in 1974 (John Pinder) in nominal 1/4" and the thinnest areas are 5.7mm, majority is over 6mm, with no overplating or even pit welding.

 

Two things I would consider when looking at older boats. The first is that most insurance companies require a hull survey for fully comprehensive insurance when boats pass a certain age (depends on the insurer but typically seems to be 40-50yrs). The owner may well have had a survey done for this reason. I wouldn't rely on it as a buyer, but it would certainly be enough for me to strongly consider paying for a survey on a boat I liked. Secondly, when people value their boat, they tend to enter a continuous maintenance regime of some sort. This will be a series of regular surveys (you end up with a relationship with your surveyor which isn't just a tick box exercise - it's what needs doing now, what can wait until next time); there will be evidence that the remedial work was done, although that can be a case of physically looking as not all boatyards itemise the bill, which doesn't mean they are not good, it just means they don't like paperwork!; it is also increasingly likely that the boat has been grit blasted and epoxied - again a very good sign as it dramatically reduces the likelihood of ongoing corrosion, although if when surveyed the epoxy all falls off in sheets it indicates poor preparation.

 

The presence/absence of the above would, for me, be strong indicators of whether it was worth spending the money on the survey, because they will probably be good indicators as to whether it will be 1,2 or 3 (in which case you are going to buy the boat) or 4 (in which case you are not).

 

Alec

 

Edited to add: another thing to be aware of is that on many older boats the guard irons are continuously welded at the top but only tacked at the bottom. The thinking at the time was that water would drain out that way, but this turned out to be an error as what happens is the corrosion between the guard iron and the hull gradually causes a wedging action and either the hull distorts or the welds crack. This isn't something you will get money off for, but it is something to look out for and consider the cost of having addressed by having the bottom edge welded continuously too. It is pertinent to the survey because it is an area which cannot be inspected from the outside, so if it is thin behind the guard irons you won't pick that up in the figures.

Edited November 22, 2025 by agg221

[Alan de Enfield]

1 hour ago, agg221 said:

depends on the insurer but typically seems to be 40-50yrs)

 

I have found it to typically be 25 / 30 years.