I am looking for my first boat, and I have run across a late 1960s Commander 27 that seems to be in good shape. It has what seems to be the original Crusader 350. If I were to buy it, I'd probably want to repower it down the line, as I would prefer to cruise in the mid 20s (lots of ground to cover!).
Can the forum help me understand my options? The only older thread I could find was discussing changes on a twin screw....and my sense is that any change to a larger HP motor could potentially be a tight fit.
If the boat has a 350 Crusader it's been repowered with a Crusader, which is a premium engine, as Chris-Craft didn't use that brand in their boats of that era.
I always compare power to weight by going back to my 1968 35-foot Sea Skiff with the twin 327F motors with 210-hp each.
That boat would get up and go, was the fastest cruiser at our yacht club and outran a twin 454-powered aluminum Marinette one day with witnesses aboard each boat. We were side by side running at "full speed" until I decided to push the throttles all the way forward and we just pulled away. Boy was there some talk around the club that evening !!!
That boat was probably in the 31 to 32 miles per hour range, and it weighed 12,500 pounds with a 30 pounds of weight to each horsepower.
By comparison, a 27 Commander at 6500 pounds with a single 327F has 30 pounds of weight to each horsepower, so it should perform quite nicely too, but the Skiff "footprint" was pretty widely spread out and it had a lapstrake bottom which could give it a statistical edge.
By comparison, a 23 Commander has 15 pounds of weight per horsepower, and is rated at 40-mph give or take a couple.
If you added 300 horsepower with torque to work in the marine power range (down low) you would be in the 21 pound range, and with 350 horsepower you would be in the 18 pound range. The twin engine 27 models are the ones packed with motor, they're tight, and it's a LOT of iron in a 27 foot hull. With a pair of 327F motors at 210 each, they would be in the 17 pound range due to the fact that there is a considerable amount of additional weight.
On my 427 Ford FE motor destined for a 23 Lancer, I discovered I could easily make that motor equal or LESS weight than the original small block if I did three things:
1. Replace the 80-pound iron Ford intake manifold with a 25-pound aluminum Edelbrock Performer RPM.
2. Replace the massive iron exhaust logs and risers with Glenwood aluminum logs and risers.
3. Eliminate the massive brass and iron circulation pump alltogether and let the sea water pump do all the work of moving water through the motor.
You could do item 1 and 2 easily, but item 3 is quite involved in order to make it work properly.
Therefore it's possible to install a GM 454, as an example, that would weigh in nearly the same as the original iron small block. That motor would not put any more stress on the prop shaft if it were run the same speed as the small block, but of course with the added power and torque you will want to put a larger pitch prop on it and perhaps larger diameter too, testing would tell what works best.
NOTHING like this is EVER as easy as you think it's going to be at first. If it were me and I was going to add a big block, I would go for a Borg Warner 72c transmission, they are available for less than a rebuild of a Paragon and they are rated for bigger power. I got one for my 427 project.
It may be possible to keep the original shaft size and go with a higher grade of stainless steel, but probably best for a boat that weight to move up in shaft size.
Now having gone through all of that, it's possible to pop in a motor of the exact same size as the original motor but with larger displacement, power, and torque, and I'm thinking of a 383 stroker. They're available with big power but you MUST not creep up to higher compression in a cruiser our you WILL burn up the motor (a cruiser is like running a fully loaded dump truck up hill all the time with your foot on the accelerator). A 383 stroker with compression in the 9.0:1 range would be a nice upgrade to the original small block, and you could use all of the original Chris-Craft pumps, exhaust, risers, etc., but some of the aftermarket cylinder heads out there are NOT drilled for all of the fittings Chris-Craft used so you'll need to be looking for what some people call "truck heads".
With boats we allways want to see more speed, and more power. It takes a LOT of additional power to make a boat run faster, and using the example of a 38 Commander at 20,000 pounds and 600-horsepower, adding 20-horsepower per motor by changing out intakes and ignition would be fun, but you would hardly know the difference at wide open throttle by looking at the speed rating. The main advantage I would see going to a big block on your project would be the fact that a RV (or marine) camshaft that delivered maximum torque down in the 2800-3000 rpm range would allow pretty effortless cruising, and you could prop the boat to take advantage of the peak torque range of the motor. The 383 stroker would be fun too but it's always going to be a smaller motor trying to be a bigger motor, and just looking at the torque curves between motors would tell the tale pretty well. Torque is a main issue. Horsepower that comes on at 5000 rpm is pretty useless in a cruiser.
Hope these observations give you some perspective.