Under contract - 2003 Neptunus 56 w/ cat 3406e - top speed/rpm issue

I have C18 engines on 62 Neptunus 2008
Port rebuilt under warranty at 800 hours
Both engine have new aftercoolers in 2016
I had both aftercoolers cleaned by Cat 2 years and 50 hours ago
Obviously don’t put on a lot of hours
I was planning on new ones next year
Is that unreasonable?
During survey to sell boat buyer wanted $26,000 credit to do 1000 he service and replace aftercoolers. Is that customary?
Shouldn’t it be split? Suppose that was done 3 years ago would they expect $13,000 credit because it would need to be done in the future.
The boat was very light but got up to 2390 RPM and 30 knots
Thanks

BTW
Burns 50 gal/hr each at WOT

He just wants to pay $26k less. The justification is presented but doesn’t really matter. You can take it or counter. If you got a good offer then I’d say take it as the market is volatile. Or you could tell the brokers you will cover half and they fill in a portion?

its reasonable because the cat recommendation is every 6 years and they’re due now.

If they are selling, then they are reasonably priced. If not selling, then overpriced. But reasonably priced doesn't mean it's what you're willing to pay.

This. Cheap for you to close the deal with a negotiation rather than gamble on another buyer materializing quickly....unless you know of one.

Sort of hijacking the thread, but short answer is "what does shaft/prop in bad shape actually mean?" If pitting and electrolysis, you need a quality electrical survey performed.

The Neptunus 56 with 800 CAT's should top out at 29 knots and cruise at 24 kts. That is with clean bottom 50% fuel, 50% water. I work for Neptunus and we build probably 35 of them.