Raw water intake valve for Starboard 300 hp, 427 cubic inch

After performing some minor mechanical work (plugs,filters and oil change) I decided to run engines for a once over look prior to putting back in water.  I noticed on the intake valve for the starboard engine there was a slight leak coming from under the wood block that stabilizes the valve. Leak was coming from between the block of wood and the hull. Wood seems to be fiberglassed in place. In order to replace I have to remove old piece of wood (size is about 4" x 6" x 3/4") removeand refiberglass. My Dremel should be useful.

Has anybody experienced this problem and solved it.

All help is appreciated. Thank you.


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Sorry...Boat is a 1972 CC Commander with 427 engines. Thanks


Without a pic, James, it’s hard to imagine, but it sure sounds like you’re talking a backing plate. What’s throwing me off is they’re not usually glassed in place. Unfortunately, the right way to fix a leaking backing plate is to remove everything and rebed. But if you’re out of the water, that greatly simplifies. I have one myself that will get redone this offseason. Good luck!


Thank you for your advice. I plan to repair tomorrow and will take some pictures as they may be helpful to other Commander owners. Hopefully this small repair will not turn into a  more then I expect,...............................Jim


Something you may consider doing if your replacing the backing block is to improve on a realitively common but dangerous plumbing issue.

Many boats both old and new alike often have a mismatch of threads between the threaded portion of the thru hull water intake scoop and subsequent plumbing fittings and valves.

The thru hull scoop will have NPS (straight) threads but most standard plumbing parts have NPT (tapered) threads. 

While the taperd parts will seem to thread onto straight  threads, it will only tighten down a few threads at best. 

The solution is to either screw a legitimate sea cock (which will always have female NPS threads) directly onto the NPS Threaded thru hull or use an adapter which would screw onto the thru hull but have male NPT on top which allows the safe usage of all NPT fittigs and valves.

In the picture attached is the original plumbing I removed from my 35's engine intakes.

You can see that there were only 3 full threads of mismatched threads keeping my boat floating for all these years. 


The best solution is to replace the whole thing with the boat out of the water.  I was faced with a similar situation shortly after acquiring our 38' Sedan.  It did not have seacocks on the raw water intakes for the engines, and the raw water intakes for the A/C units were fitted with valves attached to the thruhulls.  

Thruhulls are made with NPS threads to allow for a nut to be tightened against the backing plate and hull.  Valves are NPT, and the taper will not allow them to mate properly to the NPS threads on thruhull fittings.  Groco does make an adapter, and I considered going that route, but decided to use proper seacocks, since that would increase the lever arm above the fitting, and I was replacing the backing plates anyway and I felt the features of the safety seacock were worth the added expense.  I did a fair amount of research on backing plates, and decided to use 3/4" thick G10 epoxy board.  I considered Starboard, but it didn't have the strength nor the adhesion characteristics, and experts advised against its use.  After struggling with an impact wrench and deep socket without success, I finally ended up using a grinder and cutoff wheel to remove the inside nuts from the thruhulls and used a 2X4 and a hammer to remove the old thruhulls. I installed Groco SBV-1500 Safety Seacocks for the engine intakes ( I went with 1-1/2" and used a reducer bushing in case I decided to repower later and needed the larger diameter; the 427 uses a 1-1/4" intake).  I used BV-750 seacocks on the A/C intakes.  I was able to remove, refurbish, and reuse the Groco SVS-1000 Sea Strainer that was connected to the Genset, and I reused its backing plate (which was still in good shape).  I installed the new thruhulls using a Groco THT-530 Installation Tool.  I bedded them with 3M 4200, since it's less permanent than 5200 but still has excllent adhesion properties.

If you are desparate and in the water, you might try a penetrating epoxy on the wood backing plate or a try to wedge some quick setting epoxy in there if you've removed too much with your Dremel tool, but that should be viewed as a temporary expediency, as there really is no substitute for doing the job properly.


Thank you all for your advice which I tried to follow. I removed everything and then installed a new backerplate which I had fiber-glassed in place. So far so good. Would post pictures if I new how to post them. . Thanks again for all your help.


Hi James,

Here is a link on how to post photos.


Use the second icon on the left or use the upload files

James E Romer said:

Thank you all for your advice which I tried to follow. I removed everything and then installed a new backerplate which I had fiber-glassed in place. So far so good. Would post pictures if I new how to post them. . Thanks again for all your help.


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