Mahogany boat in fresh water

Hi I was wondering if any one can help, I’m looking into buying a 52ft ex-navy Pinnace which I have been told (by the current owner) is made of double diagonal teak on oak frames and is copper clad below the water line.
It is currently moored in sea water and I want to move it and live aboard it on a river, I have been informed by a local dock yard expert(?) that if it is copper clad it definitely wouldn’t be teak but probably mahogany, he also told me that if I moored a mahogany boat on the river it would have rotted away within 5 years.
My questions are- would the navy clad a teak boat in copper?
And why would a mahogany boat survive 47 years on the sea, but not last more than 5 on a river?
I look forward to any information that can be provided.
Thanks Ed

 

Mahogany is class II wood and one of the most common used and versatile hardwoods to be found.
Before the FRP era, most boats were built from mahogany (or teak) over ash or
oak frames.
It is definately nonsense that a mahogany boat would rot away in a time lapse of 5 years.
It has not the technical properties of teak or iroko, but nontheless the mahogany constructions last very long, if maintained well. If the bottom is copperclad, it is against the teredo worm that goes with waters in the tropical hemispherfe and might spoil a wooden hull in a few years time when not properly protected.
With the modern below-waterline compounds one might easily build up a protective barrier; if the bottom is copperclad, even a problem less!

 

Very big topic wrapped in tons of mythology and superstition. Some basic points:
1 Wood does not "rot" in sea or fresh water. The fungi that we call rot requires specific moisture contents (among other requirements) in order to live..underwater it is too high. (Be aware that you can have rot occur from the inside outward due to a supply of fresh water. A good deck and proper ventilation will forstall this.)
2 Copper sheathing is/was put on to protect a hull from marine borers and damage from barnacles etc. where they attach themselves to the wood. Be aware that copper sheathing is not very effective at sluffing off those creatures that attach themselves to hull and would still require some scrubbing and scraping.
3 Actually damage to the wood is much more likely from reaction to the metal fastenings in the copper if they were improperly installed.
4 A good surveyer is in order here...no matter how cheap the boat, he could still save you from being saddled with an environmental liability.

 

I have some experience with HMSs, double diagonal mahogany planked pinnances and Harbor Service Launches. Usually copper sheathing was only applied to those vessels being shipped to tropical or semitropical waters - like India. When was she built? What was she used for?

 

My reaction was the same as D'Artois and Dgreenwood. Some one is feeding you nonsense. Wood boats are built out of mahogany every day, and used on both fresh and salt water. I would also be more concerned about the fasteners. On any wood boat that has been around that long the fasteners need to be checked and maybe replaced. Unfortunately this means hauling the boat and pulling fasteners. But it will be worth it.

 

Thanks for your replies
Dgreenwood, I'm having it surveyed on Tue so that should enlighten me to any fastener problems.
artemis, she was built between 1962 & 66 I’m not sure of much of her history except that she was used to ship sailors between 2 naval bases on the south coast (of the uk), then later on (not sure when) she was sold to a dock yard and was used as a tug for barges, then about 10 years ago was converted to a live aboard by the current owner who lived on her for 7 years so for the last 3years she has been used as a weekend break boat. I don’t remember the owner saying she had ever been aboard though he did mention some of them being used in the Falcons and I have seen one in France.
The owner did say it was double diagonal teak which as I gather from your replies is the better timber and it was only what I was told by a local marina owner that made me wonder if it was mahogany but just in case I have got my brother (an expert and highly skilled in all things wooden) to go and look, on Tue as well, at the general condition of the timber and to identify if its teak or mahogany.
Many thanks Ed

 

Hi
I recently was talking to another local marina owner and he said the problem with timber boats that are in salt water when they are moved to fresh water is that the salt would have got into the timber over the years and as it is hydroscopic it attracts water into the timber which would help to rot it.
I’m not sure how that fits in with the information you posted Dgreenwood but I would appreciate any more response from any one to this suggestion.
Many thanks
Ed

 

First of all these discussions must adhere to some standards of terminology. Rot occurs due to the consuming affects that an organism, (or rather a family of fungi) have on wood. As I said before, despite what some would have you believe, these organism can only propagate within certain environmental limitations. Eliminate one requirement and the fungi cannot live. Those requirements are the food source (the wood), a specific moisture content range (fresh water). a specific temperature range, and, it seems, still air.
Specific wood types and access to endgrain also make life easier for rot fungi.
Rot doesn't happen underwater. However, there are other things that happen to wood underwater that can affect its stability and strength. Contact with metals can cause a breakdown of the cell structure that can look a lot like rot.
And yes long periods of submersion seems to take away some of the structure of the wood. I saw this when I did some restoration work at the museum is San Fran (way back when) and since then on some other yachts.
I don't know for sure that what you friend says is not true, but I am skeptical. First of all, the cell walls are a semi-permeable membrane. I don't know if the salt coud even get into the cells. If it is not inside the cells then why can't it just dissolve and dissipate in the same way it arrived?
I am not really very well versed in the topic of electrochemical deterioration of wood fiber, and am just going on my own observations. I do know this problem is well researched by the people that are attempting to preseve the Vasa and other historical ship presevationists.
It all boils down to---if the boat is in decent condition, I don't think it will melt when you bring into fresh water.

 

Salt is also used as a preservative and many folks actually pour salt along the interior joints to prohibit the growth of rot. Any fresh water that works its way in gets turned into salt water and therefore unable to support rot, even in stagnant air areas.

Steve

 

This was used extensively in the 19th century in wooden vessels. The space between the outer planking and the inner "ceiling" would be filled (and kept filled) with rock salt. Kept the fungi inactive. Many small (under 50') commercial fishing vessels that used both ice and salt to keep the catch fresh had very few problems with rot.