Wet Exhaust System Issues

By Cameron Sharpe — Seastar, 32-128, Port Ludlow, WA
and Jim West — Kotuku, 32-197, Blaine, WA

Summary:  This report covers the investigation and potential solutions for wet exhaust system leaks on Nordic Tugs manufactured in the 1998 to 2002 time period.  Due to continuous product improvement by Nordic Tugs, the following may apply only to the specific boats of the vintage covered by the report.

From time to time, we've read of the fiberglass reinforced pipe (FRP) in wet exhaust systems cracking or leaking.  When we purchased our 1999 model Nordic Tug, Seastar (NT32-128 with a 220 HP Cummins 6BT5.9M), in December 2005, the survey indicated seepage from the FRP section of the wet exhaust system under the settee, just aft of the 90-degree elbow exiting the engine room.

In January 06, the shipyard wrapped fiberglass mat around the leaking FRP section, halting the seepage.  Although cracked and leaking FRP is often blamed on over-tightening of the T-bolt connecting clamps, ours was chemically bonded with no clamps. We soon discovered another prominent failure cause.

At the '07 Nordic Tug Rendezvous at Roach Harbor, WA, Joe Franett, VP of Operations at Nordic Tug, came aboard the Seastar to look over the area that had been seeping.  As we peeled back the insulation, we discovered the fiberglass wrap was charred and the FRP was seeping again.  Joe suspected we were getting excessive heating from engine exhaust gas, and suggested we gather actual temperature data on the FRP.  Joe advised that 110° F was the target operating temperature for FRP.  He also advised us to review the exhaust inspection and maintenance steps suggested in the Tech Tips of Nordic Tug's newsletter "Waypoints #11."

NME wet exhaust water injection elbow showing additional 18" lift. (Click to Enlarge Photo)

Soon I was in contact with Jim West and several other tuggers who had the same or somewhat similar issues.  Jim and I (working on our respective boats) spent most of the summer of '07 consulting with Cummins, Nordic, National Marine Exhaust, Unicraft Marine Products (mufflers and FRP) and Sherwood Pumps to gather technical and engineering data.  We also conducted many flow tests, spray tests and sea trials to document what actually was happening.  A number of other tuggers contributed technical information, shared their experiences and helped us locate resources. 

First, a short description of the typical wet exhaust system. An engine-driven pump (1:1 ratio on the Cummins) passes seawater through the transmission oil cooler, then the engine heat exchanger and finally injects the water into the hot exhaust gas from the turbocharger.  The water cools the exhaust gas from the 350-800° F range to a much lower temperature that can be handled by the FRP used in the exhaust system downstream. In many cases, a water lift muffler is mounted adjacent to the engine, achieving the same objective, but taking up valuable engine room space.  Briefly, here's what we found. 

1. Water lift muffler typical of Nordic Tugs 42 (Click to Enlarge Photo)

   Cracking and leaking of the FRP wet exhaust pipe appears to be caused by hot exhaust gas overheating the FRP (we measured operating temperatures on the FRP from 245° F to 275° F).  The water injection elbow, with a proper hole-pattern and adequate water flow, should quickly cool the exhaust gas down to an operating range of around 110° F.  Depending on the manufacturer and specific materials used, we have been advised that the thin-wall FRP is generally rated for 125° F continuous and 150° F intermittent. Some newer types of FRP are rated as high as 200-350° F continuous.  Higher operating temperatures, like those we measured, appear to bake the resin out of the older type FRP, causing brittleness and pinhole leaks.

2. The M71 Sherwood pump (#10615 impeller) used on our 220 HP Cummins 6BT5.9M engines appears to be marginal for this application. It is rated at 22 GPM at 2600 rpm, but only about 10 GPM at 1200, and 11.8 GPM at 1400 RPM.  Nordic used a well-constructed National Marine Exhaust (NME) wet exhaust elbow in many NT 32s, which rises about 18" higher than the original Cummins setup.  This provides improved surge protection from seawater backing up into the turbocharger, as well as more room in the engine compartment.  But it also adds 3/4 lb. additional head to a marginal raw water supply.

3. The 90-degree elbow directing the combined gas and water exhaust aft out of the engine room creates spiral turbulence with hot spots at the top of the pipe about every 18".  The hot exhaust gasses cool about 10° F each time they pass the cooler raw water flow in the bottom of the pipe. 

4. Vetus exhaust gas mixer

   On Seastar, with a new impeller in a good pump, clean intake strainer, clean engine heat exchanger, but with about 10% obstruction in transmission cooler, we measured 245° F on the top of the FRP just aft of the engine room.  Later, with the entire system squeaky clean, temperatures dropped to 182° F which was still way over the desired temp of 110° F. 

5. Adding a Vetus Injection Water Mixer just aft of the 90 degree elbow leading from the engine room, and some improvements on the NME water spray nozzle design cooled NT32-128 Seastar's hotspot to 105° F at 1200 to 1800 RPM, and no more than 118° F during an extended run at 2200 RPM with 750° F Turbo outlet temp.

6. In Jim West's tug Kotuku, NT32-197, a spiral water discharge nozzle was added to the wet exhaust elbow itself with impressive results, but he still has some work left to achieve the desired temps in the FRP portion of the system.

Spiral water discharge nozzle on Kotuku (Click to Enlarge Photo)

Conclusions: It appears that the original design specific to our boats, with: a) the small Sherwood M71 raw water pump and; b) the increased height of NME Wet Exhaust elbow and; c) the water injection spray pattern used at that time led to inadequate cooling of the exhaust gas before it entered the FRP section of the wet exhaust system. The result was overheating and ultimately leaking of the FRP, often near the T-bolt clamps. The initial Cummins design appears to be at the upper limit of the M71 Sherwood pump's capacity when used with the stock Cummins water injection setup.  Adding the 18" of additional lift with the NME Water Injection Elbow, results in 3/4" of extra head to the raw water system.  The turbulence issue downstream of the 90 degree elbow going aft from the engine room exacerbated the problem.  (Current designs are very good, and are well documented by NT during sea trials.) 

Tirdent VHT high-temp silicone hose on Seastar (Click to Enlarge Photo)

Adding a Vetus Injection Water Mixer appears to be a simple and inexpensive solution for the 220HP Cummins application. (http://www.fisheriessupply.com/online/  Go to Products, search "Vetus" then to page 8.)  Also for the Seastar we substituted Trident VHT high-temp (350° continuous) silicone hose in place of the FRP in this area as insurance. (Some other brands of pipe also now meet this high temperature rating.)

It is also important to configure the water spray nozzles in the wet exhaust riser to provide a larger number of smaller streams, concentrating them at the top of the turbo outlet (hottest exhaust area) to improve heat transfer.

For Seastar, we are currently working on a complete water pump upgrade package to add the GC5 pump with #15000 impeller.  The larger pump adds 33% more capacity (and opens a host of additional engineering and cost issues).  If you already have the larger pump with the 15,000 impeller, you probably don't have the cooling problem.  We are also working on a more extensive report with upgrade part numbers, flow charts, test data, photos, and discussion of the additional engineering issues dealing with the pump upgrade.  We will post on this forum when the upgrade package is ready.

For questions for comments, the author may be contacted at: SharpeC@cox.net

Readers' Feedback:

Date: January 10, 2008
From: Bob Bevins — NT32-009, Anacortes, WA

After reading the report I have the following suggestion:

Remove any rise in the exhaust system and put a vertical loop in the exhaust just before it discharges overboard.  This keeps a small amount of water in the exhaust (FRP) section and aids in the cooling.  The loop also prevents water from going into the exhaust system in a following sea.
 
My tug has had the loop in the exhaust for the past twenty years and I have not had any issue with the FRP getting hot.