Penn Yan Tunnel Drive Exhaust Problem

[joremi]

"I've installed new cowlin

"I've installed new cowlings on my twin tunnels of my 26' Vindicator. I decided to replace the cowlings because much of the chaulk was worn away and the cowlings were begining to break apart from the tranasom. I have chaulked the new cowlings with 5200 and have had a significant drop in high end power. Both engines were rebuilt this summer and operated at 35-3700 rpm @ 25 knts before the change. Now both engines struggle to get to 1500 rpm @ 14 knts. The only change I've made is the cowlings.


Upon inspection, the exhaust port alone seems too small. I also notice that the transom plates are tappered toward transom. Should I NOT chaulk the cowelings and allow exhaust to exit all around the bottom of the coweling? Has anybody experienced anything like this this before?"

 

[bill999]

"Joe,
In your first posting,

"Joe,
In your first posting, It was explained that what you have is a " tunnel drive" and this extremly important! What you are talking about is replacing the metal exhaust covers that are bolted around the exhaust port and are curved down?
If this is correct, is the new curved nozzel the SAME length as the old nozzel? The new nozzel may be a universal to be cut to length depending on its postion in relation to the bottom of your particular application. It must not extend below the bottom of the boat or you will not only get restriction but pump water back up the exhaust system. The nozzel opening is usually just under the water line at rest, this keeps the exhaust noise down when not at speed.
Since you say you are loosing rpm and speed at wot, I am guessing your NEW nozzel opening is beyond the bottom of your boat causing the engine to use much of its power just to pump exhaust.
If this "exhaust nozzel" is your project, please let us know. And what is the difference its length compared to the old one?
Thanks Bill"