John Moore the owner of a 4100 SCB with Cummins 370B's had a "riser failure" in his boat. The failure caused the weld to fail bringing the engines HOT exhaust gasses as well as some of the engines cooling water into the engine compartment. moore4255@verizon.net
Additional text follows after the photos:

John's new exhaust riser, was purchased from his original exhaust riser manufacturer,
National Marine Exhaust, but has twice the number of cooling holes as the original. (no photo shown of new exhaust)

This information applies to all of us.

There are many possible causes of the failure of the elbow, one is that the elbow was improperly made. Possible, but not very likely. Another is that the exhaust riser and exhaust configuration may be the culprit, this is more likely to hold the answer. An explanation here is required to get you up to speed. Engines are sold to Maxum without exhaust risers. The boat builder can choose to make them dry exhausts, like a car. Or wet exhausts by adding water to the exhaust gas. Maxum makes wet exhausts, and therefore must add an exhaust riser to the engine, in this case on the Cummins Diesels just after the turbo. The Cummins Diesel produces test cell exhaust gasses temperatures of 898 degrees f for the 370B and 830 degrees f for the 450C, (source Cummins Marine SC.) The purpose of the riser is to spray engine cooling water through a "spray head" pattern of holes inside of the riser to cool the engines exhaust gasses. This spray pattern "ideally" blasts a perfect spray pattern of water hitting all of the exhaust gas, thus lowering the temperature of the exhaust gas under the rating of the exhaust elbows and exhaust tube, which is typically 250 to 350 degrees f. Exhaust risers are a maintenance item, and they only last so long because of their extreme exposure to corrosive raw water and extreme temperatures. This riser is just five years old and has 390 saltwater hours of engine time. Therefore wear and deterioration should not be the cause. Cummins points out that in salt water, if your engines coolant flow is low and you raw water temperature exceeds 130 degrees you will get the salt in the water transfer to solids and build up on the riser spray head clogging it. Cummins points out that this is a maintenance item. An exhaust tube failure is most likely caused buy the exhaust gas did not get completely cooled and there were voids in the spray pattern of the exhaust riser. The cause could be inadequate water flow through the engine to the exhaust riser due to anything from clogged engine water scoops to clogged sea strainers, blocked heat exchanger, to poor impellers, a broken piece of the impeller blocking flow inside of the engine, to kinked tubes and piping. BUT, It is suspected that the problem here is exhaust risers spray pattern in combination with the swirl of the exhaust gas, in combination with the geometry of the inside of the tube and its elbows produces could have caused a void in the spray pattern. The cause of the failure could be a combination of many of these factors leading to an inadequate spray pattern. Changing the components after the riser to 350 degree components will only mask the problem, it should be cured. This is not to say the upgrading to 350 degree exhaust tubes and 350 degree silicon elbows is a bad thing, it just would not have shown the base problem to the owners as quickly. 350 degree components will not harm the installation. The changing the exhaust tube from the original exhaust tubes to metals is NOT an option. A metal exhaust pipe transfers heat too well and can exasperate an acceptable design, into a dangerous design with the use of metal. This installation is being addressed by the original manufacturer, National Marine Exhaust has chosen to add additional holes in the elbow in the new riser in an attempt to more completely surround the exhaust gasses.

It should be noted that running the engines with the engines raw water seacock's closed will send 800+ degree f exhaust gas into the 250 degree f rated exhaust system. The results, well you can imagine that as well as I. If you're a second owner, this is an area surveyors ALLWAYS inspect thoroughly, just because it is a common point of failure. 1/2005

FYI, common exhaust materials are high quality 316L Stainless Steel., SMO, Inconel, Monel, Aluminum, Fiberglass / Composite Tubes. High Temperature Rubber, Silicone Hose is used as couplers, elbows and at times as exhaust tubes depending upon the application, 2/2001

Sources:

DeAngelo Marine Exhaust, Ft. Lauderdale, FL 800-952-8555 http://www.deangelomarine.com

National Marine Exhaust, Marysville, WA 360-659-2983 http://www.nationalmarineexhaust.com/

Marine Exhaust Elbows: I would have suspected that these were stock items. I just finished speaking to Delangelo Marine exhaust here in Florida, they do not stock Marine Exhaust elbows, they are all custom made to order. They do a "build to match" from your old unit and it takes about 2 weeks to get your new marine exhaust back. James