7.5 lower unit question

[chris]

Iam trying to run a 7.5 foot on a htb 290 with a novi rossi and I cannot seem to get the right set up. I have tried every depth and angel on this lower unit and the boat seems to be dragging and it will not release and go. The motor turns up really well with a 1440 prop, but a friend of mind is using a lawless and with the same prop he is a good 8-10mph faster. I have seen this set up work really well, but does anybody have any suggestions?

Thanks,

Chris

 

[Mark Anderson]

I have never run that combination but a couple of things come to mind. The skeg on the 7.5 lower is very large, if you have not trimmed it you may want to cut it down to the same size as a 20 lower. Also the 7.5 lower has a very large bulge on the side of the fin to strengthen it. Remove the bulge then using the prop shaft as a visual guide sharpen the skeg front and rear.

Due to the size of the bearing housing if it is in the water you will have alot of drag you can trim it down a little and vee the front edge to stream line it.

Mark

 

[chris]

Mark,

I have tried all those suggestions. The boat corners great, but when it starts to reach top end the back of the boat comes out of the water and the prop cavitates. I have run tunnels for years and I cannot figure this one out. I have tried non lifting props and the boat acts the same no matter what I do.

Chris

 

[synthon]

Chris...this may be out in left field but...is the BOTTOM of the skeg sharpened? I had a problem with on outrigger doing something similar, and determined that the flat spot on the bottom of the rudder was serving as a planing surface. When sharpened to a point that behavior went away.

Just a thought.

Rick

 

[chris]

Rick,

Thanks for the reply. The lower unit is not completly sharp, but it will be the next time I try it out.

Chris