This is Captain Paul Foer of Annapolis, MD On The Foerfront with another blog post about helping you realize your boating dreams. How old is too old for a diesel? This question or similar such ones are often posed on online boating forums by prospective boat buyers. The latest is from a person buying a sailboat for $150,000. I see such questions often, but may I take a different approach not only with the usual answers but with the usual question?

This is a sailboat, with an auxiliary diesel engine. The real engine of the boat are the sails, spars, rigging, winches, sheaves, etc. etc. The responses to this type of question from other boaters are remarkably uniform--which is amazing for a boating forum (just try it on anchors or any other equipment), but it is telling. The question is almost always posed in the same manner as engine, engine, engine and answered with something such as "engine hours are not as telling as maintenance and a diesel can last many thousands of hours etc. etc. " 
I suggest you take a step back first and ask lots of questions about and to determine the condition of the rig and sails. How come nobody else recommends that? Are you getting a rigging survey? When was the boat re-rigged? Have you inspected the mast step? Gone up to the masthead? 
You can spend more to re-rig a boat than to rebuild the engine. You can spend more to replace a mainsail than to do an injector and valve job. A new winch can cost more than a new heat exchanger and alternator. 
I believe that many sailors place sails and rigging in secondary or tertiary order with engine in first place. That is likely wrong IMHO. So yes--get a diesel survey but don't forget a full rigging and sail inspection as well. The key word is "auxiliary" engine. 
An engine is not exposed to the constant atmospheric elements as are sails and rigging. Diesels are made to function with lots of heat and pressures and are constantly lubricated because diesel is an oil but the auxiliary engine is not exposed in the same way to UV, salt, wind etc. as is the rest of the boat. An engine problem or minor inconvenience or basic maintenance issue is more likely than is an engine catastrophic failure. When a diesel fails, it will generally show obvious signs of wear and distress over time. This is not necessarily so with a sail or rigging. An engine catastrophe is rare but could be severe in some cases. Most diesel problems are unlikely to ruin your day whereas a rig failure or catastrophe is probably more likely to occur---and can do a lot more than ruin your day.
In conclusion, one should definitely get an engine survey to some extent but tell-tale signs are generally just that--tell tale signs and more easily discovered than are rigging accidents waiting to happen. Don't focus on your engine and drive train and the risk of disregarding rigging inspections when considering a boat or when owning and operating a boat. Visit www.foerfront.com for help in realizing your boating dreams.