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View Full Version : prop rotation for twins



mcollinstn
09-03-2001, 09:38 PM
okay, which is it?
on a relatively speedy deep vee (75 plus), which rotation gives best bow lift? PortCCW/StbdCW *or* PortCW/StbdCCW ? It *looks* like the "first" choice would create more stern lift at first glance, but would also keep the blades biting better at the bottom of their rotation arc (cause the blades would enter up high, pull air with them, be free of air by halfway, then cleanly pull thru the bottom of the arc). On a boat such as mine ('91 Formula 311SR1/T502Mag/Bravo1/1.36/25"Mirages) a high trim angle is required for best speed. Any way to increase bow lift at less extreme trim angles would have to increase speed. I have not tried swapping props, but wonder if anyone else has done a fairly well-documented attempt at doing so.. Also, who has tried L&S Stern Jacks? The concept looks sound, but I have replaced my gimbal pivots (at the top) on both sides (wave jumping, I guess) and am concerned about the added load with Stern Jacks. Also, would need to re-rig external steering rams if added the Stern Jacks. How about extension boxes on a long deepvee? I am open to ideas. I am fixing to add prochargers, but want the rigging worked out first.

ToffenG
01-30-2005, 06:38 AM
Hi!
Whether to turn the propellers inward or outward is a choice to be made by testing. Today, there is a trend with Hi Perf V bottom boats to turn the inward given the unusual somewhat elevated installation height. Turning inward proponents feel that a litte more top speed is achieved because there is a little more bowlift. The upward (outboard) swinging blade sees more water than the downward (inward) swinging blade because of the bottom dead rise masking off more water as you approach the keel. The greater push upward results in a down force at the prop which results in bowlift. Some also fee that a little better control comes with turning inward.
Cheeers, Toffen G - Norway :2purples:

LHC30Victory
01-30-2005, 09:10 AM
Toffen G is right - props in (viewed at the top) provide bow lift, props out = stern lift. the advantage with out is that low speed handling is greatly improved. You should read the threads on Offshoreonly.com on this topic, there is a ton of info there. also, look to talk to Julie at ThrottleUp - you can find their info on offshore.
I am looking to try the Hydromotive P5X props. they are the improved bow-lifting 5 blade prop.

ChumpChange
02-09-2005, 11:29 AM
Wow, this question took four years to get a response!

INSman
02-10-2005, 01:30 PM
Wow, this question took four years to get a response!
Too funny, but looks like you are right !!