External pick up vs. bravo pick up
02-02-2010, 02:30 PM
#11
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Eddie
02-02-2010, 02:45 PM
#12
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Platinum Member
The water could potentially come back out of the drive, or the pickup for that matter. I can see where one could overpower the other. I guess it will all depend on what the strainer pressure is. What I do when I use a 2 stage pump is to feed it with a 1 1/2" hose, instead of 1 1/4" hose. The 1 1/2" can keep up with the 2 stage pump. If you are using both stages for the engine, the exhaust usually restricts the flow enough so that the pump will never empty the strainer. It has always worked well for me. I always at least check strainer pressure to see what it is. This gives me an idea of what's going on. Is the pump taking all of the water?(low pressure in strainer). Is the pickup to deep causing high pressure in the strainer?(high strainer pressure). This will tell me what to do. I like to see around 10 psi in the strainer. This ensures the pump will always stay full of water. In your case, I would just use the transom mount pickup for the engine and use the outdrive pickup as a drive shower. You have to do something with the water from the drive. You don't want to just plug it up because it will want to blow water back out of it and could cause the prop to cavitate. Best case it will increase the slip by not giving the prop clean water. Hope this helps.
Eddie
Eddie
How do you run the water back to the drive shower? Is there a kt or a diagram or a photo of this somewhere?
Thanks for the help,
Jeff
02-02-2010, 05:09 PM
#13
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So, the farther back you go with the drive, the higher up you can go. General rule of thumb is for every 12" back, you can go up 2-3" and still have the drive in the same relationship to the water. In effect, you are not really changing the X in relation to the water.
I also wondered about the difference between the "technical" X dimension and the "practical" X in relation to the water flow, because I was curious to check what the X (19") of my boat meant in terms of position of cavitation plate and prop shaft.
So, I made a calculation based on Mercury original drawings, with the result attached below: the brown line shows the alignment with the hull bottom (just 2" 13/16 above the prop shaft).
And since I also have the DWP, it's pretty obvious that if the water wouldn't bend upward after leaving the bottom of the hull, the side holes would suck air. Surely the notched transom also helps in this respect.
Anyway, the water pressure is steadily high, even at top speed, so what I can confirm is that at least up to this X level the DWP works just fine (at least of a Fever 27 hull).
