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lesson on twin stepp take off

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Old 03-28-2009 | 08:33 AM
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Originally Posted by Shanghied Again
This may sound like a stubd queston but its not. first what is the make of the boat? Does the boat have a pad as well as steps? If your boat does have a pad do you have blow out vents under the pad with shut off valves in the engine compartment? Most step bottom boats that don't get on plane quick need to get the air that is trapped under the pad out. Donzi 35ZRs have blow out vents and I believe Fountain does the same. When you take off try neutral positon on your tabs that would be around 3-4 depending on your boat, make sure your drives are tucked all the way in and ease your throttles foward, If you feel the slip back off very slight on your throttles until you feel the grab and then push foward. Bravo 1 props are your best choice with 496s, Going 5 or 6 blade will not help with the slip and will destroy your X drive, 496s don't run XR drives. I have 496s in my 33 ZX I added blow off vents in my boat and have no problem getting on plane. The previous owner put bigger K-Planes on the boat and that did not help it actually made the boat worst on take off! If you bury the tabs all the way down on take off, you will actually lift the stern of the boat adding more air to the underneath.
sorry for the stupid question, but what are blow off vents?
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Old 03-28-2009 | 10:48 AM
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Originally Posted by boat teck
this may be just another stubd queston and may not mean much hear with theas boats but has any dealer R&R the drives lately ? at the rear trim ram pin with the black caps covering the nut's there is a white plastic spacer in between the nut hard wear that could be installed wrong and it will change the drive trim angel a bit not leting the drive tuck in as far.they do this becouse there is a differents between bravo 1 & 3.and some guys will change that around by not knowing where to put it.on bravo 1 the spacers need to be installed with the grove part pointing a way from the bow of the boat or a way from the boat in general. well i'm shur it's fine but if not it a easy thing to look at any ways.it dose sound like the x is high but that is where the top speeds come in.you would think the manufacturer,s would by now say to you oweners to install lower unit drive spacers by now to help stop the blow out. merc dose sell them spacers also.just an idea is all this is.good luck and have fun this year boating.
This is a good point and may be worth looking into. I completely forgot I changed mine a couple years ago. The spacers were positioned from the factory to prevent the drives from tucking under as far as possible. The reason is if the drives are tucked 'too' far at speed in a hard turn, it is more likely to spin-out/roll a twin stepped hull due to the added stern lift. I never have my drives tucked all the way in when turning (rule #1 of driving a stepped hull). I changed the spacers to allow the drives to tuck in further and it helped with planing. Although, the props do still slip which is the nature of a stepped hull and a higher x-dimension.
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Old 03-28-2009 | 11:36 AM
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just stick 1000# of sand in the bow.. that should fix it
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Old 03-28-2009 | 11:43 AM
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Originally Posted by boatnt
sorry for the stupid question, but what are blow off vents?
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Old 03-28-2009 | 11:51 AM
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Originally Posted by boatnt
sorry for the stupid question, but what are blow off vents?
On a step bottom boat on the rear pad there would be two 1/2 or 3/4 brass thru hull openings that runs through your engine compartment out the transom or sides, They will have shut valves so if they leak you can shut them down. What happens on take off excess air gets trapped in the padded area and causes excessive prop slip and blow out. The Blow offs get the air thats trapped out from under the boat and reduce the slip from start.
you will see on the back of a lot of number 6 Drives these 2" black tubes coming off the back of the drive and they come up over the drive, this does the same it release the excess air from a step bottom boat and reduces prop slip. Helps the boat get on plane faster.
Thats why I asked what kind of boat and does she have a pad on the rear of the boat.
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Old 03-28-2009 | 12:48 PM
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Originally Posted by packinair
just stick 1000# of sand in the bow.. that should fix it
actually if you read the thread,I need to add the sand in the back so I could keep the stern in the water.

thanks for the reply
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Old 03-28-2009 | 12:50 PM
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Originally Posted by Shanghied Again
On a step bottom boat on the rear pad there would be two 1/2 or 3/4 brass thru hull openings that runs through your engine compartment out the transom or sides, They will have shut valves so if they leak you can shut them down. What happens on take off excess air gets trapped in the padded area and causes excessive prop slip and blow out. The Blow offs get the air thats trapped out from under the boat and reduce the slip from start.
you will see on the back of a lot of number 6 Drives these 2" black tubes coming off the back of the drive and they come up over the drive, this does the same it release the excess air from a step bottom boat and reduces prop slip. Helps the boat get on plane faster.
Thats why I asked what kind of boat and does she have a pad on the rear of the boat.
thanks for taking the time to explain how blow off valves work!

the boat is a 2003 powerquest revenge with a small pad
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Old 03-28-2009 | 01:03 PM
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Originally Posted by boat teck
this may be just another stubd queston and may not mean much hear with theas boats but has any dealer R&R the drives lately ? at the rear trim ram pin with the black caps covering the nut's there is a white plastic spacer in between the nut hard wear that could be installed wrong and it will change the drive trim angel a bit not leting the drive tuck in as far.they do this becouse there is a differents between bravo 1 & 3.and some guys will change that around by not knowing where to put it.on bravo 1 the spacers need to be installed with the grove part pointing a way from the bow of the boat or a way from the boat in general. well i'm shur it's fine but if not it a easy thing to look at any ways.it dose sound like the x is high but that is where the top speeds come in.you would think the manufacturer,s would by now say to you oweners to install lower unit drive spacers by now to help stop the blow out. merc dose sell them spacers also.just an idea is all this is.good luck and have fun this year boating.

I've had that exact thing happen after having a drive worked on with the Gun. It definitely can be overlooked, and made my boat cavitate like hell until it was swtched back.
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Old 03-28-2009 | 05:46 PM
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Originally Posted by boat teck
this may be just another stubd queston and may not mean much hear with theas boats but has any dealer R&R the drives lately ? at the rear trim ram pin with the black caps covering the nut's there is a white plastic spacer in between the nut hard wear that could be installed wrong and it will change the drive trim angel a bit not leting the drive tuck in as far.they do this becouse there is a differents between bravo 1 & 3.and some guys will change that around by not knowing where to put it.on bravo 1 the spacers need to be installed with the grove part pointing a way from the bow of the boat or a way from the boat in general. well i'm shur it's fine but if not it a easy thing to look at any ways.it dose sound like the x is high but that is where the top speeds come in.you would think the manufacturer,s would by now say to you oweners to install lower unit drive spacers by now to help stop the blow out. merc dose sell them spacers also.just an idea is all this is.good luck and have fun this year boating.
Well I just checked the trim limit blocks and they were on backwards,once I switched them the outdrives lowered about an inch,hopefully that will do it,I will let you guys know the results next time I go out.

Thanks for all the replys!
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Old 03-29-2009 | 03:12 PM
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A friend had a 34' PQ that also has the 2 large steps. His drives had spacers on it when he bought it. Possibly for this very reason?
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