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Old 06-22-2013, 06:38 AM
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Default Cavitation Plate on Tunnel Hull?

Guys, I have a pathfinder 15t with 25 merc. four stroke I am thinking about putting a jackplate on it. Would a after market cav. plate such as permatrim or shaw wing help with keeping my water pressure up when running with the jackplate elevated?

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Old 06-22-2013, 06:56 AM
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Shawwing or a manta ray cav plate are great! And it will help down here in the LLM we use them all the time. I run one on a shallow sport and a scooter. Had one on every boat I have owned.
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Old 06-22-2013, 07:04 AM
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Great. So they do help with keeping your water pressure up? That is my main concern. Also which do you suggest I read on old thread that the shaw wing was better on 90hp + and the permatrim was better for the smaller motors such as mine. Oh and I've seen on here and 2cool you got a new merc. 150 how are you liking it so far?
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Old 06-22-2013, 07:23 AM
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It will do nothing for your water pressure. It may help with holesshot and that is all.
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Old 06-22-2013, 08:46 AM
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They are designed to hold water around the prop when it's elevated, thus it will support your effort to maintain water pressure, while letting you run more shallow. If there is no water, then there cannot be water pressure!
I use a ShallowBlaster on a 200 HPDI mounted on a tunnel hull and it preforms, as intended.
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Old 06-22-2013, 12:33 PM
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Thanks bay fly that's what I was hoping for.
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Old 06-22-2013, 01:04 PM
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Quote:
Originally Posted by Bayfly View Post
They are designed to hold water around the prop when it's elevated, thus it will support your effort to maintain water pressure, while letting you run more shallow. If there is no water, then there cannot be water pressure!
I use a ShallowBlaster on a 200 HPDI mounted on a tunnel hull and it preforms, as intended.

Your water pressure does not come from the prop. Water comes from the intake grates on the sides of the cases or in some instances, holes in the nose of the case. These devices WILL NOT help you keep water pressure. Once you are on plane, they do nothing.

I also disagree that they are designed to "hold water around the prop" when its elevated. Once you are up on plane and the foil/plate/etc. is out of the water, its just along for the ride. They do "hold" water around the prop on initial holeshot and, act as a large trim tab to keep the bow down for better holeshot.

I run a full tunnel boat and have my engine mounted higher than anyone here. I have tried it all!
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Old 06-22-2013, 05:14 PM
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The roof of the pathfinder tunnels are not parallel to the running surface of the water and the water comes out of the tunnel at an upward angle spraying water around the anti cavitation plate of the motor. The larger fiberglass plates (shallowblaster etc) extend forward of the anti cavitation plate bridging the gap between the end of the tunnel and the leading edge of the plate keeping that spray down around the lower unit. With a 4 inch set back and a jackplate the motor is elevated 4-5 inches higher than possible without the plate allowing the lower unit to ride in the bubble. When you look aft with the jackplate the upper mounting holes for the motor are well above the transom. The downside is the need for a cupped or 4 blade to control the slippage on your hole shot.This describes the running attitude of the 17t with a 60hp 2 stroke. I don't know if you will benefit as much with the 15t and a 25 hp motor given the weight of the jackplate unless you buy a manual micro Bobs
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Old 06-22-2013, 06:37 PM
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Thanks for all the input guys. I'm gonna stay away from the cav plates for now install the jackplate and try it I do have a water pressure gauge so I'll just see what happens and go from there as I mainly use this little rig in the colder months so I have time to dial it in. I am currently running the powertech SRA 3 blade it has worked well without JP. Not sure how it will bite elevated though we'll see.
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Old 06-25-2013, 07:21 PM
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I havnt beat on the new mercury yet but this weekend it will be getting a full work out. Have you thought about making one out of 3/8'' aluminum ? I made one for a small motor one time it really helped the water pressure when the motor was mounted high. I also drilled out the lower water intakes and plugged the top water intakes. I'll get you a photo of that motor.
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Old 06-25-2013, 09:44 PM
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Or....
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Old 06-26-2013, 05:47 AM
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I put a Boatright compression plate on my 115 Merc 4s and it did nothing at all for water pressure or holeshot. The only thing it did was improve handling (tight turns) with the motor jacked all the way up. This is with a Baumann four-blade Blaster prop.

For maintaining/increasing water pressure at full height, I installed the Merc Racing water intake shrouds. They give me 30psi at full jackplate height, where before I had just under 20psi. The downside is that the water pump is harder to prime at idle or in reverse. Next time I may just try laying a bead of silicone right behind the intakes - just enough to coax the water into the intakes instead of flying by...

I also did the tape trick (prior to installing the Merc shrouds). Run a strip of electrical tape around your lower unit - start at the top two intake holes on one side, wrap the tape around the nose cone and finish it at the top holes on the other side. Two strips of tape should be enough to cover the top top 2-3 holes. Make sure the lower unit is clean and the tape should stick well if it is quality tape (mine lasted several months). Just be sure to watch your water psi gauge.
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