Outboard jet tunnel design questions

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gabeskillzz05
Posts: 42
Joined: 20 Aug 2014, 20:58

Outboard jet tunnel design questions

Post by gabeskillzz05 » 29 Sep 2016, 15:16

I have a 90's Lowe roughneck 1752D with a tunnel. I have had trouble getting the motor set to the correct height and also with planing. Motor is 98' Yamaha factory 50/35. Has a new foot/liner and impeller is setup correctly.

When I set the jet foot so that the leading edge is even or slightly above the tunnel extension I get horrible cavitation. The only way I could get it to run was to set it almost 1/2"-1" below and use an excessive trim angle. I accomplished this with a set of transom wedges. Also added a spray plate to keep the inside of the boat dry. I can get about 21-23 mph on the lake like this.

The strange part is that on the river it jumps right on plane with up to 3 people. But on the lake it doesn't quite plane out unless I go over a wake. It is hard to explain but if I do a large loop and run back over my wake the nose will sit down and I pickup about 3 mph. You can feel it happen.

I have been thinking about this for a long time and the only thing that I can figure out is that my tunnel design is not correct. I think the problem could be that the tunnel is at the same angle from the beginning all the way out through the extension. See picture below. I believe that this angle should change when it exits the transom and be parallel with the bottom of the boat. Any thoughts? The way it is I think is making my lead in edge sit much too high compared to if it came out parallel with the planing surface. I think it is also pulling water up higher and in return pulling the rear of the boat down, almost the opposite of a trim tab??

Image

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JL8Jeff
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Joined: 26 Oct 2013, 14:56
Location: Ewing, NJ

Outboard jet tunnel design questions

Post by JL8Jeff » 29 Sep 2016, 23:10

I have a Roughneck 1652 tunnel with a Merc 60/45 jet. I had to use the transom wedges and have the motor mounted with the front of the foot just below the tunnel extension. I moved my center console forward and put the battery up front as well and it planes off pretty quick. The only time I get cavitation is if there are a couple of people up front. My tunnel looks the same as yours so I don't think the tunnel is the issue. Where are you sitting when you get the cavitation? When my console and battery were farther back, it would take longer to plane off, but now it happens really quickly. I think mine is a shorter transom than yours, I have a short shaft jet but I don't know why that would matter. I just run on the river but it sounds odd you would have problems on the lake.

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Attachments
Lowe_1652_5.jpg
2001 Lowe Roughneck 1652 VT with Merc 60/45 jet 2 stroke
1993 Jetcraft 13' sprint boat with Chevy ZZ3 350 engine and Hamilton 772 jet drive

gabeskillzz05
Posts: 42
Joined: 20 Aug 2014, 20:58

Outboard jet tunnel design questions

Post by gabeskillzz05 » 30 Sep 2016, 23:22

Yeah I have a forward console sitting on the drop deck. I have tried moving weight all around doesn't seem to make a differnce. My motor is a short shaft as well.

What kind of speeds do you see with your setup?

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JL8Jeff
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Joined: 26 Oct 2013, 14:56
Location: Ewing, NJ

Outboard jet tunnel design questions

Post by JL8Jeff » 01 Oct 2016, 10:27

Maybe your weight is too far forward. I don't have any way of measuring the speed but it really moves at WOT. I'll have to see if I can borrow something to check the speed. The only time I've had some cavitation was with 2 or 3 people up front. I wonder if the 50/35 has the smaller pump vs the 60/45 and if that makes a big difference in planning and top speed.
2001 Lowe Roughneck 1652 VT with Merc 60/45 jet 2 stroke
1993 Jetcraft 13' sprint boat with Chevy ZZ3 350 engine and Hamilton 772 jet drive

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handyandy
Posts: 538
Joined: 02 Jun 2015, 09:56
Location: Bedford Indiana

Outboard jet tunnel design questions

Post by handyandy » 06 Oct 2016, 15:22

Your yamaha might not be quiet enough motor I think it uses a medium size pump vs the 60/40 merc uses a large series pump. Check what model outboard jet your yamaha uses. sounds like you should experiment more with your setup too JL8Jeff gave some good advice as to what worked for him and yours sounds similar.

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