Twin Prop Shaft Rotation
10-26-2004, 06:48 AM
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Twin Prop Shaft Rotation
Same question has been asked before. Just forgot the answers.
Who has experience with a Baja 280 and prop rotation? If the left hand prop is on the right with a twin my understanding is you are pushing water out. Correct? For speed, the left hand should be on the left. Corect?
Reason I ask, is I have 454 twins with alphas. Last year I replaced one with a counter rotating drive. This spring I had the drives off when I had the boat painted. The boat has been slower than last year all year. The boat sits on a lift all summer. Just winterized the boat and noticed the left hand prop on the right side.
Is it slower because of this?
Thanks, JimmyBaja
Who has experience with a Baja 280 and prop rotation? If the left hand prop is on the right with a twin my understanding is you are pushing water out. Correct? For speed, the left hand should be on the left. Corect?
Reason I ask, is I have 454 twins with alphas. Last year I replaced one with a counter rotating drive. This spring I had the drives off when I had the boat painted. The boat has been slower than last year all year. The boat sits on a lift all summer. Just winterized the boat and noticed the left hand prop on the right side.
Is it slower because of this?
Thanks, JimmyBaja
10-26-2004, 07:15 AM
#2
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Re: Twin Prop Shaft Rotation
On a vee spinning the props inward creates bowlift. Spinning outward
creates sternlift.
creates sternlift.
10-27-2004, 06:42 AM
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Re: Twin Prop Shaft Rotation
Spinning in is when the top of the props go toward each other. Spinning out is when the top of the props go away from each other.
When the left prop is on the starboard side, the right is on the port side, this is referred to as spinning in.
Most boats have the props spinning out. As a general rule this results in better handling. We have found the when the props spin in on a v bottom boat this causes stern lift. Thats why the boat doesn't handle as well. The stern isn't as deep in the water and the boat has a greater tendency to spin out.
There are exceptions to this rule. The best way is to try spinning the props in and out. Note the attitude of the boat, speed and handling. Then make a choice which way works best for you.
With the Alpha drive to change the rotation of the props you will have to swap the lower units from side to side.
Dock handling is best when the props spin out.
When the left prop is on the starboard side, the right is on the port side, this is referred to as spinning in.
Most boats have the props spinning out. As a general rule this results in better handling. We have found the when the props spin in on a v bottom boat this causes stern lift. Thats why the boat doesn't handle as well. The stern isn't as deep in the water and the boat has a greater tendency to spin out.
There are exceptions to this rule. The best way is to try spinning the props in and out. Note the attitude of the boat, speed and handling. Then make a choice which way works best for you.
With the Alpha drive to change the rotation of the props you will have to swap the lower units from side to side.
Dock handling is best when the props spin out.
10-27-2004, 01:06 PM
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Re: Twin Prop Shaft Rotation
I agree with Throttle up in regards to trying it either way would be the best way to go.
But yet I disagree in spinning them outward creates bow lift.
My theory on this is when the wheels are spinning in.The outer blades are creating a upward force against the water pulling the stern down. When the wheels are spun outward the outer blades exert a downward force wanting to climb the stern out of the water.
Without getting into a book on propeller theory Maybe Matt can chime back in and explain what cup,rake,Diameter may contribute to explain why there are various opinions.
Again what works well on your hull may not work on others.
X dimension may play a significant role on why there are different opinions
on this topic check this out.
Perhaps others may shed some light on this topic.
Just my 02
Gerry
http://www.offshoreonly.com/forums/s...=prop+rotation
http://www.offshoreonly.com/forums/s...=prop+rotation
But yet I disagree in spinning them outward creates bow lift.
My theory on this is when the wheels are spinning in.The outer blades are creating a upward force against the water pulling the stern down. When the wheels are spun outward the outer blades exert a downward force wanting to climb the stern out of the water.
Without getting into a book on propeller theory Maybe Matt can chime back in and explain what cup,rake,Diameter may contribute to explain why there are various opinions.
Again what works well on your hull may not work on others.
X dimension may play a significant role on why there are different opinions
on this topic check this out.
Perhaps others may shed some light on this topic.
Just my 02
Gerry
http://www.offshoreonly.com/forums/s...=prop+rotation
http://www.offshoreonly.com/forums/s...=prop+rotation
10-27-2004, 02:07 PM
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Re: Twin Prop Shaft Rotation
Your turning them "in"
"In" ='s bow lift.
In my dealings "in" has been slightly faster ( less than 1 mph)
And will always give you poorer docking manueverability.
jt
"In" ='s bow lift.
In my dealings "in" has been slightly faster ( less than 1 mph)
And will always give you poorer docking manueverability.
jt
10-27-2004, 10:31 PM
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Re: Twin Prop Shaft Rotation
The hull was painted from the waterline up. No bottom paint. last year the boat was faster and I am assuming it was faster because the left hand prop was on the port side? Was I turning in or out?
Thanks, Jim
Thanks, Jim
10-28-2004, 09:24 AM
#10
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Re: Twin Prop Shaft Rotation
Gerry,
I believe it has more to do with hull and drive set up than props.
We have worked with teams who will give up the 1-2 mph speed gain by turning the props in, in return for better handling. Many of the boats we work with will tend to have a tendency to spin out when the props turn outward.
Cats are another story altogether. With cats the amount of tunnel water that the prop encounters will change the way turning in or out lifts the boat.
The only way to really be sure is to try it both ways and depending on one's boating style and skill will determine what works best.
At least one aspect of turning in or out is undisputed. That's the fact that turning out is better for docking. For some that's important, for others the only thing that matters is the top speed number. It's all about getting the boat set up the way you like it.
Matt
I believe it has more to do with hull and drive set up than props.
We have worked with teams who will give up the 1-2 mph speed gain by turning the props in, in return for better handling. Many of the boats we work with will tend to have a tendency to spin out when the props turn outward.
Cats are another story altogether. With cats the amount of tunnel water that the prop encounters will change the way turning in or out lifts the boat.
The only way to really be sure is to try it both ways and depending on one's boating style and skill will determine what works best.
At least one aspect of turning in or out is undisputed. That's the fact that turning out is better for docking. For some that's important, for others the only thing that matters is the top speed number. It's all about getting the boat set up the way you like it.
Matt
