I have a 1957 Chris Craft Runabout repowered with a 327 and at 5000RPM the nose dives and the boat will do a sever lerch right.
I know that Dr Millar has written about this and I would like to contact him for his suggestions including prop size etc
Thanks
Troy Marshall
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Troy,
Try [email protected] or (386) 788-0213
Try [email protected] or (386) 788-0213
Troy,
Before you reply with Dr. Millar's solution I would like to take a crack at my own theory of what is happening.
As your boat is accelerating and the engine is approaching 5KRPM it is hydroplaning on the last couple of feet of the hull. The keel is almost all out of the water and engine torque is twisting the boat (keel has little effect in mid-air) so the aft is pushed port-side and forward is twisted to the starboard side. Now as the boat reaches top speed with the prop continuing to push the remaining couple of feet of hull out of the water, gravity brings her back in suddenly with the keel out of alignment with forward motion, causing the jerk to the right as the keel finds water.
Solution? I haven't a clue. You probably have a very well matched prop now for high speed but the boat can't handle it. The engine would over rev with less pitch so maybe the fix is more pitch, thus, limiting the max RPM's and its top speed.
Al
Before you reply with Dr. Millar's solution I would like to take a crack at my own theory of what is happening.
As your boat is accelerating and the engine is approaching 5KRPM it is hydroplaning on the last couple of feet of the hull. The keel is almost all out of the water and engine torque is twisting the boat (keel has little effect in mid-air) so the aft is pushed port-side and forward is twisted to the starboard side. Now as the boat reaches top speed with the prop continuing to push the remaining couple of feet of hull out of the water, gravity brings her back in suddenly with the keel out of alignment with forward motion, causing the jerk to the right as the keel finds water.
Solution? I haven't a clue. You probably have a very well matched prop now for high speed but the boat can't handle it. The engine would over rev with less pitch so maybe the fix is more pitch, thus, limiting the max RPM's and its top speed.
Al
Boat twist
I had this same problem on my 1970 XK 19. I installed trim tabs and eliminated the problem as well as having a great deal more options as to the nose postition at different speeds.
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