Sonic 27 ft chine walking

[Preformer 40]

My buddies 27 sonic was chine walking and getting dangerious at 65 mph and above,he has 2 250 efi mercs,did all the different things to try and help it and none helped,we flipped the lower units and the problem is completly gone,it will go 74 mph and stay as steady as a boat going 25.Hopes this helps some one with twin setups.By throwing the thrust to the outside has made it stable,can someone tell me why? in stupid terms.

[BMSYachtDesign]

Dear Preformer40,

With counter rotating props the boat is basically in balance, so the dynamic unbalance causing the chine walking had no resistance (except damping).

When one lower drive was switched a thrust unbalance was created with enough force all to one side to overcome the force that created the chine walking.

That would be my guess. Did the boat list slightly when the boat was underway after lower unit switched.

Jerry

[Frosty]

BMS Hy, welcome to the forum. Actually I dont think he said that , he said he flipped to two gearboxes changing the rotation of both drives. I assume that what your suggesting is that by having the rotaion of both drives the same way put torque into the boat and held it in one chine.
I would love to know the answer to this as I have a catamaran ( not 70 mph) but a mere 24 kts. I have had the surface props running both ways and made not the slightest bit of difference but to put a different load on the rudder hydraulics. Left is left and right is right

[Frosty]

Preformer 40,-- Would you please tell us how high do you have the motors.? What gap is there between the motors?Are they bang centre? and what props you are using? What steering do you have?

[im412]

just a guess
could be that before counter rotation was lifting the transom ..now counter rotation is changed over its making it sit with down force of prop torque or water flow?
only thing i can think of

[Preformer 40]

The motors are on a twin stainless marine bracket,Standard spacing 25" ?
23 pitch rapture ss,The motors are mounted up as high as possible,it has low water pickup(standard on 250s)stock case.Steering is seastar, twin center rams and the largest helm 2.4 ? before switching L/u cases,large quanities of water would be pushed from the props at the transom in the center at speed.the water was pushed as high as the swim platform and would dump out between the motors like a river,maybe 30 gallons a second.Now the motors are clean and no spray.