[Packinair]

When looking at a boat in the 70' range with the big diesels at what point do you have to start thinking about a rebuild? I know a lot depends on how it is maintained etc, but at what point if the motors have not been redone would you shy away?

[nilo]

I reckon by big diesel you are referring to small but high output diesels, E rated engines. I am not a technical person, just an owner with some experience. So, I believe other members with better technical knowledge will come forward for more precise information. Anyway, as to my knowledge one of the main factors for a need of overhauling any engine under normal conditions is very much related with how much fuel the engine has burned. So, hours is not the only factor, but the revs the engine has been used is just as important.

Anyway, if we consider that the engines are used in a planing boat, say about 80% of maximum rev most of the time you should be able to use them up to 2500 hours without major overhauling, if they have been maintained properly.

[TK-F430]

Diesel engines are built to fine tolerances. They will accept a lot of abuse and often deliver, if well maintained, 8,000 hours of hard work before requiring a major overhaul. Theoretically, a well-maintained diesel may last the life of your boat. Since the average recreational boater will only log about 200 hours per year, the 8,000 hour diesel can last for 40 years.
The average marine diesel engine will run for an average of 5,000 hours. The number of hours that a marine engine runs is very dependent on the amount and quality of maintenance over the years.

[K1W1]

Hi,

Nilo hit the nail on the head with his statement that TBO is based on F.O. Consumption. I take it from his mentioning of an E Rating that he has some experience with the Caterpillar range of engines.

The actual TBO depends a lot on how the engines are used anything in the CAT other than A Rated needs to be run at less than 100% output all the time. running a B,C,D,E Rated engine at full welly for long periods will shorten it's life considerably as will things like overload- running on one engine whilst trying to get up to full speed on the one left, excessive back pressure, exhaust arrangement with regard to sea water vapors getting back into the engine when it's stopped and the list goes on...

If you are buying a boat ask for historical Lube Sample reports and Maintenance records and have them and the engines checked over by a reputable professional, this will not stop the occasional genuine failure but it will stop you getting lumbered with a pair of old patched up dungas in your new toy.

If you can please stay away from ultra high output engines in a particular range - you will have a lot less heartache long term.

[Dhowdodger]

Would a cat 3412e giving 1400 hp be considered ultra high output and what kind of heart ache would you forsee after 1200hrs of running?
Ran not above 80% except for very short bursts, 4 years old, fully maintained since new.
K1W1 Your opinion would be most appreciated.

BTW I have a 10hp Bukh, 28 years old raw water cooled, if only all engines could last like they do.. Wouldnt push a 70 footer too well thou.

[goplay]

If you like the boat otherwise, just factor the cost of an engine rebuild into the cost of the boat then determine whether it is still a good buy.

[yachtbrokerguy]

Which engines are you considering and how old are they? I have seen many engines that were over 20 years old and running fine, and I have seen rebuilds on engines only a few years old. The type of yacht is also a factor, is it planning or displacement?