My extra long single-handed orginal one is Oak and has lasted since 1948. The shorter family friendly new one is Ash (Ash has the highest shock loading of any of our native species).
PS.Answer to rot is a tiller cover and loadsavarnish
I laid an ash deck on seaspray , after about six months the water was getting in everywhere and after a year the rot started . Once it got hold you could pull the ash apart . Once it started going i knew i was going to have to replace it so didn't bother to do anything about it , thats probably why it went so far .
I wouldn't worry , most seem to think it is good on boats . I was just saying i won't use it again .
Ash is often specified due to its resistance to shock loads, but this would only really be relevant in something like a racing dinghy where the tiller was made as slender as possible to reduce weight, and the applied forces were large and sudden.
In a cruising / work boat the dimensions of the tiller will probably be such that any species would be capable of handling the normal working loads. In other words, ultimate strength is not the main design criteria.
Looking at the (fairly long and slender) tiller on my boat I think that if things get so hairy that the tiller is in danger of breaking then I'll have lots of other more important things to worry about. Like swimming.
Durability and aesthetics are likely to be more important, depending on the finesse of the boat and the prevailing maintenance regime.
I, too, have had ash on a boat. It is not a suitable marine timber, and (apart from situations like that described by Bodfish), I would use oak. If you go to a real timber yard you may well be able to choose a piece with suitably curved grain.
Incidentally, am I the only person who really hates those laminated tillers which use light and dark timber?
What ever oak you chose someone will say it's the wrong type so you may aswell get the available American white oak . The wooden boat forum seem to think its ok .
Is teak not suitable. I made my tiller from teak to replace a laminated tiller and it does the job well. It is a thick piece of timber so certainly strong enough although there is very little strain on the tiller as the boat is very well balanced.
Ok I wasn't boasting (particularly). Made of oak, it has also been designed (not by me) to swing upward and to have other self steering attachments (Tiller pilot stub, etc.) Oak varnishes to a light golden colour, and is perfectly good for a heavy old cruiser.