I mostly cruise between 8-9 kn. Can get to 10 easy enough but fuel use jumps. My recent 93 day cruise was 263 hours of engine time, average 1.33 nmpg and 7.7kn over th entire trip. The latter includes idling in and out of marinas etc. For the first half of the trip I pushed it a little, on the way back I eased off to see how much difference it would make. Attached is a graph I have posted before, which shows performance with fresh antifoul, clean props etc. This is with John Deere 6068, 201HP each. The original Cummins was less fuel efficient at the lower end.
Most boats were delivered with a pair of 120HP Lehmans or 270HP Cummins 555. Some had 210HP Cat 3208. With the Lehmans you could get 11kn, but they were probably working a bit harder than ideal in the 8-9kn range. If punching into waves with a headwind they can feel downright underpowered. For the Cummins, and some were 320HP rather than 270HP, you could get 16kn provided you had trim tabs and they were down. When mine was new it topped out at 15.5kn using 40GPH total (Cummins 555). To run in the high teens I think more like 800HP total should be installed, and it might not use that much more fuel. The 15-16kn mark seems to me that its not really over the planing hump as it were. Not that they will really plane - a lot of boat and a big wake at those high semi-displacement speeds.
With my 201HP John Deere's I still only get 11kn, like the Lehmans, but there is plenty of reserve power for the adverse wind and wave scenarios. The 3208's would have been the same. An issue with V8's, Cat or Cummins, is that there is not much space outboard of the engines. Sure you can get one arm in there, but.....
Hull is solid GRP. I have not heard of anyone who has had hull issues. They are well built.
Given that they were built from around 1978-1984 only, rather than focus on original build pros and cons it is more important to determine what the specific boat has done, and had done to it, during its 30+ year life to date. For example, if it has had some hard groundings then the hull might have weak spots.
There will have been various 'improvements' by DIY owners and perhaps even professionals (well meaning or otherwise) that will need critical appraisal. Electrical systems at the outset probably were never as good as Grand Banks. Now they might be a complete mess, like mine was.
The Cummins 555 are at times maligned. Parts are expensive (eg raw water pump ~$1200) but there is still a lot of them about in the PNW and Alaska and aftermarket stuff fairly readily available. They were never a great truck engine, perhaps the same as the Lehmans in that regard, and should not be worked hard all the time. For Cummins, although rated at 3000rpm you would not want to run above 2500rpm continuously. The Cummins marinisation was not all that good. Most installs replaced the exhaust crossovers with SS custom fabs. Standard price for this back in the day was $12k. The PO of my boat said he thought he got a good deal on a $6k job, only to have to rip them off and put the $12k ones on. He had a rueful look on his face when telling me this.... Then the exhaust mixer elbows were often an issue. The PO had one fail on mine, and that engine needed a turbo rebuild. The total job was $10k back in early 1990's. A good mixer elbow these days will have the water injection done in a downward sweep of the exhaust so that failure and water leak will not get back to the turbo.
As to other original build issues, nothing additional that bugs me, other than the internal door heights. They could have easily been a couple of inches higher which would have saved me a lot of knocks on the head!
