Repowering a 27' Chris Craft

How much does the 4.3L weigh in comparison to the 5.4L?
What's the difference in BHP between the two?

I'd be more concerned with the additional weight of the larger motor and it effecting the attitude of the boat in the water.

Reading your post quickly...
Retro-fitting twin 4.3L's will double your original installed power ?
You are asking if twin 5.3L's will give you a significant increase in speed over the 4.3L's ?
Have you looked into prop sizes / shaft issues yet ? Doubling the original power alone will raise some eyebrows...let alone going bigger still.
And yes, the weight distribution will be 'interesting'.
Intrigued.

All of these engines are basically small block Chevrolet engines, and consequently the basic engines are all pretty close in weight. There are a lot of differences between a modern engine and your old 283's so you could see a weight increase due to the cooling system differences and accessory differences, but it won't be a ton. According to one source on the web, the 283 weighs 585 pounds and a 327 weighs 615. The 350 is going to be close to the 327, so weight isn't really a big issue, it's not like you are stuffing in a couple of big blocks that weigh a couple of hundred pounds more each. Most new marine engines have a heat exchanger and I have no idea if your boat had a freshwater system in it or if it just used lake water for cooling. If you upgrade to engines with a closed cooling system you are better off anyway but it could cost a few pounds.

Torque will be higher (you are going from 283's to 350's)and the gearbox, shafts and props will all need to be looked at. If the 327's that were an option used the same gearbox and shafts as your boat, then the change isn't huge, but there is still a bit more torque that is coming into the shafts and gearboxes. Props are going to have to be different since you have a higher top speed potential, you will need more pitch for sure.

If you look at Crouchs formula, the potential speed increase from the power boost is a square root function, so the potential speed increase is just less than the square root of two, or about 1.325 x the original top speed. I have no idea how fast the boat went before, but if you did 24 knots before, you would do about 32 with the 325 hp engines. That makes a lot of assumptions about the hull, trim angles and things like that, so in reality it isn't going to be that great, but you would likely expect a top speed of around 30 with the new engines and a properly tuned setup. Consequenty you will burn a good bit more fuel, to go faster, but if you slow back down to your original speed you will burn close to what you did before, a bit more, (like 10% or so) but not a lot.

I would see if I could find a similar boat with the optional 327 engines and find out what props he is using, if the gearboxes are the same and then go from there. Finding a good shop that will do the swap and address all of the myrid of small issues that upgrading to a newer engine will entail is the key to successfully upgrading your boat, but it does, at least at a first glance, appear to be feasible.

One last point, if you upgrade to the bigger engines and don't bend the throttles, your engines will, to a larger extent, be loafing and that can contribute to a significant improvement in engine life. If you are running at a high power setting most of the time, then a bigger engine with some power and speed potential isn't a bad thing. You don't have to use all the power all the time.

Shaft, transmissions, prop size, harmonics, ... the list could go on.
You're at least looking at it in the right way.... the speculation phase, rather than having two crate motors sitting there waiting for you on the shop floor while you *then* go to the drawing boards to figure it out.

What speed does the boat do at cruise and WOT with the 283's and the 327's (there must be factory specs out there). I would guess that the 4.3's would cruise the boat at what it did WOT with the 327's. Keep in mind the 4.3's probably don't have the torque the 327's did, but should still have a lot more power then 283's and top end then the 327's. You shouldn't have a shaft or transmission issue with the 4.3's

27 roamer

steenburgr,
I have the same boat and have done some investigation on repower also. One engine guy said on a boat there is no substitute for cubic inches and torque of a V-8. Also there is a guy over here on the west side that will take your existing engines and trans and mate it up to a 350 block and if he can get a aluminum intake under the deck can get a lot of extra horespower. Call Macatawa bay boat work and they can put you in contact with him or they can set you up with new Crusaders. I also checked with Kalamazoo marine gear and the Paragon Trans. will handle the power with no problem.

Gordon

The original 283 and the 5.7 (350) are both small block Chevy's and will have very similar weights. The 4.3 is a V-6 Buick which is basically a 350 Buick with 2 cylinders lopped off and there is not that great of a weight savings there, so weight will not be a considerable issue in this repower. IIRC you have Velvet Drives, and when you repower, you may as well trade them in for a new set, no sense putting in new engines in front of old tranny's. The shafts should be ok to use, just check the cutless bearings. You will want to reprop. The fuel used will not change for the same speed, actually, you may get slightly better fuel economy from the 5.7 as it's torque band is lower and props are more efficient at lower speeds. This is of course given the same speed. If you take advantage of the extra power for extra speed, it will use more fuel.

Roamer

If your engines are in a 1962 roamer 27', I would think they are identical 283 C.C., to mine. The problem is they are sitting in backwards, flywheel forward and the transmission is driven off of the front of the engine where the harmonic dampner should be, also the lube for the trans is fed through the crank shaft and is engine oil. Usually what takes the motor out is something wrong with the trans. The trans are a C.C. converted Paragon. Also if you post this on the engine section of the "Roamer Forum" farther down the page, you might find others that have already done all this and are more familiar with your boat design.

Good luck