If you own your boat long enough, you will have to replace and renew various components to keep her at her best. I try to maintain mine such that if I decide to undertake a major cruise it does not require an undue amount of preparation or additional expense to do so. Having a boat that is ready to meet your cruising or racing aspirations is the whole point of owning it. It is sad when a boat is allowed to decline and the owner's boating plans are forced down with it. I have found that if I undertake some upgrading project and expense every year, beyond the usual maintenance, then my boat is pretty much always ready to go. Last year it was a new sail, this is 'the year of the engine'.
If your boat has a good solid structure and you maintain it well, the depreciable parts are engines, electronics, safety gear, interior appointments (ie. fabrics & upholstery) and on sailboats - sails. Keeping each of these updated, depending on your usage and inclination is the key.
This has come to mind as I have just had a major job done on what I still think of as my 'new' engine. I have owned my boat for 32 years and in 1991 the original engine packed-up. In the spring '92 a shiny new Westerbeke 63 (hp) was installed. It does not seem possible that it was 19 years ago.
I use my boat a lot and almost two decades and 4,040 engine hours later oil leaks prodded me into having the engine taken out of the boat for an over-haul. When I shared my intention to do this, I received a lot of advice. Some recommended that I discard the engine for a new one; but it works perfectly, notwithstanding the oil leaks.
Discussions with the engine manufacturer confirmed that the engine has much more life left in it. Annual lubricating oil analysis showed nothing abnormal. I therefore decided to keep the engine and undertake the work needed to get more life out of it. It has proven to be a bigger project than I had intended, but aren't they always.
Regular maintenance had resulted in replacement in the past of the exhaust-manifold, heat-exchanger, external water-pump; and I have the injectors cleaned every other year. This year my mechanic has replaced the engine-mounts, the damper-plate and the internal water circulating pump along with the leaking oil seals that started all of this. The engine's distributor recommended the replacement of the lower push-rod bearings while we were at it; so we did that too. The 'may-as-wells' expanded the scope of work considerably such that the total cost will be about $3,000. Certainly this is a significant expense but I rationalize it as a good investment. What I have spent is the fraction of the price of a new engine and the cost of its installation. Keep in mind that the installation cost for a new engine often approaches the price of the engine itself. If I can get another ten years out of my engine because of this effort, it will be well worth it.
Note that this was not a 'rebuild' of my engine. As my mechanic explained, a rebuild would have involved returning it to original factory specifications. As we did not touch the valves, pistons or rings the work that was done falls well short of a rebuild. There was no apparent need to do this work, so I did not have it done.
If you are considering the purchase of a brokerage boat with a 'rebuilt' engine, it is important to know what that is, or what it should be. Ask for the invoices for the work done on the engine and seek out a mechanic to help you determine how much was done and whether it lives up to the contentions of the seller. Of course knowing why the work was needed in the first place is important too. It's all about attaining a 'comfort level' with the boat and engine that you are buying. As a seller, keeping detailed records and the supporting- documentation of any work done will be invaluable in reassuring a buyer.
My observation is that most diesel engines on recreational boats do not wear out. They suffer from a lack of use not excessive wear. I would rather buy a boat with an engine that has been well used and well maintained than one with few operating hours that has been ignored. In the end, the deciding factor in changing engines is likely to be the availability of parts. Once an engine is of 'a certain age' and parts become scarce and expensive, re-powering with a current model becomes necessary. The popularity of the engine model will affect parts availability and therefore longevity. This should be kept in mind if one is either buying a brokerage boat with a used engine or purchasing a new engine for an existing boat.
Another thing to consider, besides the reliability of the product, is the service reputation of the engine manufacturer and the distributor for your area. I chose my engine for a variety of reasons but topping the list was the stellar reputation of the distributor for eastern Canada, Hansen Marine of Marblehead, Massachusetts. This included their parts department. Seeking out the opinions and experiences of owners of engines similar to the one that you are considering is well worth the effort.
What I am talking about here is about protecting your investment and assuring trouble-free time on the water. Keeping your boat upgraded and as ready to go as you are is the goal. Qualified yacht brokers can assist you in accessing expert advice and prioritizing projects. It is just one of the many services that they offer.