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» Forum Name: PT Boats - General
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» Topic: For the experts:a question about hull grounding strips.
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Hallo guys another question that surfaced while researching lower hull details: according to Elco drawing 3178(roll1 scan 360) early and middle production 80' boats had grounding strips installed. It appears that there were two 11/2" strips running parallel to centerline with a 9" offset each side. What isn't clear on the drawing is the lenght of those strips;till now I couldn't find a specific drawing and research in the spare parts list didn't give any clue.According to people who know grounding cables for lightning rod should run vertical to the grounding srtip/plate/whatever is used to ground your craft.
So assuming the lightning rod was on the radar mast, at least one of the strips should extend to bulkhead 39. was it the port strip,the starboard strip,or both[:-confused2-:]
It seems-always according to people who know- that a 1sqft plate or a 3/4" x 6' strip is sufficent to gound a wooden wessel. Apparently the Elco guys wanted to play it safe.Anyone out there in posses of more information?Any help gratly appreciated as always.
P.S on later boats the keel shoe doubled as grounding strip(Elco drawing 2350-roll 4 scan 552)
Regards

Daniele Kläy

Posted By: Daniele Klay | Posted on: Nov 18, 2015 - 3:57am
Total Posts: 126 | Joined: Jun 23, 2015 - 12:43pm

The ground strip are not just for lighting .They ground through hull fittings shaft brushes for electrolysis an cathodic protection.

Posted By: RANDY SMITH | Posted on: Nov 18, 2015 - 6:50am
Total Posts: 182 | Joined: Jul 16, 2012 - 7:16pm

Thanks Randy,I thought that corrosion protection was task of the zinc plates.
The so called sacifical anodes but electrolisis has always been an obscure and abstract subject to me....

Daniele Kläy

Posted By: Daniele Klay | Posted on: Nov 18, 2015 - 7:09am
Total Posts: 126 | Joined: Jun 23, 2015 - 12:43pm

You have to connect all the fitting to the zincs or you have to put zinc on all fittings.

Posted By: RANDY SMITH | Posted on: Nov 18, 2015 - 7:43am
Total Posts: 182 | Joined: Jul 16, 2012 - 7:16pm

From what I could tell from ELCO_Roll_5535-4_72SCAN_00542.pdf, PT 139-196, 314-367, 372-383, and 548-563 used the keel shoe.

According to a detail view, the (14?) keel shoe sections were joined by a soldered on 1 1/2" x 2" x 0.064" tapered edge rectangular keel shoe jumpers on 1 side of the c/l, and by round 1 1/2" diameter x 5/32" keel shoe connector heads on the other side of the c/l. The drawing quantities list shows 17 heads and 11 jumpers, I'm not sure of the exact arrangement but there doesn't seem to be enough jumpers for every keel shoe section although 17 + 11 = 2 per shoe section. But there seems to be a lot more heads towards the aft end of the boat. The heads had a connection to the internal grounding.

ELCO_Roll_5535-4_72SCAN_00536.pdf for PT 103-138 shows the additional grounding strips, 65' 11 1/8" long. It looks like they run from the transom fwd, stopping shy of the bow. I couldn't find the 3178 drawing you mentioned, are you sure it is roll 1 scan 360?

According to ELCO_Roll_5535-1_72SCAN_00330.pdf, it looks like the keel shoe gradually widened from 6" to 9" from frame 15 fwd? It also shows the joints along the keel shoe.

Man, this is one of the most confusing construction detail I've seen! Good luck my friend.

Posted By: Jeff D | Posted on: Nov 18, 2015 - 9:32am
Total Posts: 2200 | Joined: Dec 21, 2006 - 1:30am

Daniele,
Also the strip serves as the electrical bonding path. Anything metal can build up electric charges that can cause sparks if given the right condition. Lets say a vent duct attached to the engine room vent fan builds up a high static charge. The bonding system connects this vent ducting to the main ground strap which allows the charge to safely dissipate, without causing sparks. One of our PT Boat crewmen, Jim Brunette Motoromac of PT131 and PT557 (both Elcos) went through and made sure we recreated a functional bonding path on the PT658 per the drawings. He said they were trained at the Melville MTBSTC to maintain this bonding system religiously due to the extreme danger of 100 octane gasoline fume explosions on the PT Boats. So I guess it was important. I hope this helps.

Also for what it is worth, we recreated our bonding system per the blueprints on PT658 using galvanized wire "bonding strap" as the main bonding path, and there are 2 of them, one on either side of the keel. Every bonding wire is connected to one of these two main bonding straps. There are also aluminum plates that are on either side of the main stringers on either side of the keel that are incorporated into the system. All of these are connected to the propeller shafts and the engine blocks to allow any built up stray charge to bleed safely off into the water.

By the way I have never seen any PT boat with an installed "lightning rod" either in historic references or photos. I don't think they ever had a purpose installed "lightning rod" or at least as far as I know.

Jerry Gilmartin
PT658 Crewman
Portland OR

Posted By: Jerry Gilmartin | Posted on: Nov 18, 2015 - 10:20am
Total Posts: 1473 | Joined: Oct 8, 2006 - 11:16pm

The more I learn, the more I appreciate the tremendous effort that goes into restoration. How are the rotating shafts grounded Jerry? Something like motor brushes?

Posted By: Jeff D | Posted on: Nov 18, 2015 - 10:39am
Total Posts: 2200 | Joined: Dec 21, 2006 - 1:30am

Thanks everyone especially Jeff for the fast and precise advice and drawing reference! Till now I focused on roll 1,2and3.
BTW drawing 3178 is roll1 scan 360.
Back to work!

Daniele Kläy

Posted By: Daniele Klay | Posted on: Nov 18, 2015 - 11:00am
Total Posts: 126 | Joined: Jun 23, 2015 - 12:43pm

I'm glad you found it helpful Daniele, those details will be a cool addition to your model. I'm enjoying learning these hull details too, I haven't looked into much in the way of hull fittings and had no idea these were there. Thank you!

Posted By: Jeff D | Posted on: Nov 18, 2015 - 12:26pm
Total Posts: 2200 | Joined: Dec 21, 2006 - 1:30am

Hi Jeff,
Yes there is a braided wire strap brush that rubs on the shaft. I don't know if we copied that part. I will try to photograph some of the bonding system tomorrow when I go to the boat.

Jerry Gilmartin
PT658 Crewman
Portland OR

Posted By: Jerry Gilmartin | Posted on: Nov 18, 2015 - 6:49pm
Total Posts: 1473 | Joined: Oct 8, 2006 - 11:16pm

Thanks Jerry, it reminds me of the old slot cars with braided wire pickups I enjoyed when younger.

This is interesting to me finding out how extensive the grounding system in PTs were inside and out. I couldn't find anything in the drawings or build manual but a guess would be that those (tinned brass) keel shoe heads on the Elco boats were left unpainted.

Posted By: Jeff D | Posted on: Nov 19, 2015 - 4:45am
Total Posts: 2200 | Joined: Dec 21, 2006 - 1:30am

Jeff, actually-always according to those who know- the whole grounding apparatus suold be left unpainted.
BTW the question abuot the keel shoe jumpers:11 jumpers connect 12 sections section 3 through 14 and those are the submerged ones;in my opinion there would be little sope in connecting sections that are out of the water.
Greetings.

Daniele Kläy

Posted By: Daniele Klay | Posted on: Nov 19, 2015 - 10:51pm
Total Posts: 126 | Joined: Jun 23, 2015 - 12:43pm