Trimming jet motor up

[optaylor823]

I bought a used 1666 Snyder with a 175 Evinrude converted to a jet 3 years ago and when I first got it anytime you trimmed the motor up it would lose a lot of power, so I never trimmed it. Well we are in a major dry spell here and the Duck River is probably as low as I have ever seen it, so today I decided to try to trim the motor up a little going over shoals. WOW what a difference it seem to make. The motor would trim up some and it did not seem to lose power and the boat would run what seemed to be shallower then it has ever run. I was able to run over shoals and leave mud trails behind and not suck up the rocks like I normally do. The only thing I can thank of is that I have bent one of the fins on the foot this year and have not straightened it and also I took the UHMW extension on the tunnel and turned it over since it seemed to have a pretty good bow in it from having rocks stuck in it. I thought the fins where to keep the motor from cavitateng, so with the fin bent up I would think it would cavitate more or I miss understanding the purpose of the fins. Also do you think turning the UHMW extension over helped on letting me trim the motor up. Here is a picture of the bent fin. The picture is also of the motor trimmed all the way down.

 

[Ranchero50]

Yep, the cover over the front of the foot keeps it from drawing air instead of water. Similiar to a tunnel that force feeds the intake water through hydrodynamics.

Jamie

 

[optaylor823]

So should I worry about the fin that is bent. If so should I get it bent down and rewelded or just reweld it. Also your saying the fins and the cover have the same purpose?

 

[Ranchero50]

My thoughts are that the outboard jets are a compromise at best. From my conversations with Kevin Turner and the way my build has went that ideally you want smooth undisturbed flow into the jet inlet. On an outboard jet you have the break of the hull at the transom disturbing the water flow and the action of the water flowing around the foot. Plus since the foot is depending on the water getting forced into it there's a good chance of sucking air as well, especially if it's trimmed out a bit. Add that the hull can capture air into the top 1" of water and the outboard can suck that as well.

What I'd love to see is a collar that goes around the foot that mates with the UHMW on the transom to keep any air away from the foot. Ideally an upsweeping tunnel would act like a spoon to force feed the intake. That's how my inboard runs best. The spoon causes the undisturbed water 2" under the hull to get sucked up into the jet inlet. Keeps it from cavitating and runs fastest.

Jamie

 

[optaylor823]

Thanks for the reply. So everything does act as I thought and I see where a collar around the jet would be nice. Also just read some where that when starting out with a jet that is not always best to have the motor trimmed down. I was wondering if this is so, because I did notice a couple of times taking off that my boat did seem to stay more level and get in plane quicker, maybe because I still had the motor trimmed up a little.

 

[fender66]

optaylor,

I'd love to see another picture of your set up....straight from the side (looking along the back of the boat) when you have time. In the photo you posted, the foot looks VERY close to the back of the boat. Curious minds want more info, and are, of course, curious. :?

 

[optaylor823]

Hope these pictures are what you are looking for. I tried this weekend to get the pics but wife had the camera hide. The motor is close to the tunnel, but is sitting about 6 inches from the boat since it is on a jack plate.