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Catalina/Capri 25/250 Sailor's Forums

Catalina 25 Specific Forum

Thru hulls and backing plates

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Bladeswell
Captain

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USA
490 Posts

Initially Posted - 08/11/2015 : 08:44:59

Hello Again,

At the risk of sounding stupid here goes. The first post at the top of tech tips is from the good boat Dora. And it is a detailed report of how to go about the proper replacement of the To-hulls and gate valves. And he did a great job with pictures and and explanations.
However, I have a problem with the grinding away of the hull in order to use a "much weaker" 1/4" backing plate because the thru hull is too short to use with the backing plate. Now I do fully understand the concept of "spreading the load". But, I can't get behind the idea that 1/4" of plywood is ever going to be equal to 1/4" of solid fiberglass.
So, your thoughts and opinions will be appreciated. By the way, WM agrees with me. Thanks again.

Bladeswell

C25 TR FK Hull #973 1979 L-Dinette. So.Cal.

blanik
Navigator

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Canada
210 Posts

Response Posted - 08/11/2015 : 17:38:57

the C25 hulls are not solid fiberglass, they're sandwiched plywood

1984 C25 FK/SR #4593
Lake Champlain

Davy J
Master Marine Consultant

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USA
1511 Posts

Response Posted - 08/11/2015 : 18:13:18

quote:
the C25 hulls are not solid fiberglass, they're sandwiched plywood

Can't help the OP, but as far as I know, only the topsides have a core.

Davy J

2005 Gemini 105Mc
PO 1987 C25 #5509 SR/SK
Tampa Bay

wanderer13
1st Mate

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USA
76 Posts

Response Posted - 08/11/2015 : 19:21:31

I am pretty certain the Catalina 25 hulls are solid fiberglass. My 1981 hull does not have a wood core laminate. The deck, however does have a plywood core.

Regarding the through-hull replacement - there are several schools of thought on the backing plate. The backing plates should be formed to fit the hull contour - IMO there is no reason to grind any hull fiberglass material out at all as doing so only weakens the hull around the fitting. The primary reason for using a backing plate is not to disburse any loading on the hull, but to give the flange a flat surface to mate. I recommend using brass through hulls and a flanged valve because that's what Don Casey said to use (http://www.boatus.com/boattech/casey/installing-seacock.asp). There are some who counter-sink the flange mounting screws in the hull and some who recommend setting them in the backing plate (like this guy: https://www.youtube.com/watch?v=a9ZBrBWnNAs).

IMO, the through hulls are one of the most important upgrades on older model Catalina 25s. Good luck with your project!

1981 Catalina 25 FK/TR

blanik
Navigator

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Canada
210 Posts

Response Posted - 08/11/2015 : 22:43:45

nah, on my 84 when i changed my knotmeter on the hard, i removed the whole plastic fairing that holds the screw-in paddle wheel to recaulk it and the plywood was well visible, it was also confirmed by the pro tech who changed my tru-hulls (which are below the waterline)

1984 C25 FK/SR #4593
Lake Champlain

hewebb
Admiral

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USA
761 Posts

Response Posted - 08/12/2015 : 03:45:52

In my 1988 boat I drilled a hole for an instrument thru hull just off center from the keel and it was solid fiberglass and thicker than I expected. Perhaps the type of keel may change the lay-up of the hull, just a guess.

1988 WK/SR w/inboard diesel Joe Pool Lake
Hobie 18 Lake Worth

Life is not a dress rehearsal. You will not get another chance.

blanik
Navigator

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Canada
210 Posts

Response Posted - 08/12/2015 : 04:31:39

if there is a plywood sandwich it most certainly won't be the whole hull, my guess is that the center line, the joints between the keel area and the rest of the hull or between the stern and the hull would be solid fibreglass

1984 C25 FK/SR #4593
Lake Champlain

dasreboot
Admiral

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803 Posts

Response Posted - 08/12/2015 : 05:32:48

wish i had kept the plug from my knotmeter install to take a pic. no core in my 86 hull.

Todd Lewis
Eowyn 87 TR/WK C25 #5656
ARWEN 84 TR/SK C25 #4031
www.mainsailsailingschool.com

Steve Milby
Past Commodore

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USA
5851 Posts

Response Posted - 08/12/2015 : 06:42:42

quote:
Originally posted by blanik

if there is a plywood sandwich it most certainly won't be the whole hull, my guess is that the center line, the joints between the keel area and the rest of the hull or between the stern and the hull would be solid fibreglass

The joint between the keel and the hull (called the keel stub) is plywood, sandwiched between layers of fiberglass. The keel bolts protrude through that plywood core, and, occasionally moisture from the bilge would get around the bolts and into the core, causing the plywood core to delaminate and rot. It could be replaced, but it was a fairly big job. If I remember correctly, in the very last years of production, I think the keel stub was changed to solid fiberglass, because the plywood in the keel stubs of a number of C25s and C27s (which also had plywood in the keel stub) became soggy and had to be replaced. Catalina provided a line drawing that showed owners how to replace the plywood keel core.

My recollection is that the transom was also reinforced by plywood, to support the motor mount and prevent the transom from oilcanning when you pressed on it, which was characteristic of some other smaller boats.

The plywood that you saw when you re-caulked the knotmeter thru hull was probably a piece of plywood that the thru hull installer put there to back up and support the hole cut for the thru hull.

I never saw or heard of any plywood coring in the hull, other than in the keel stub. Plywood was used to core the decks and seats, to support the weight of people walking on them, and to support the cleats, winches and other hardware.

Steve Milby J/24 "Captiva Wind"
previously C&C 35, Cal 25, C25 TR/FK, C22
Past Commodore

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/12/2015 : 07:31:37

I was writing almost word-for-word what Steve wrote, when my computer shut down to update Windows.

The only difference on my '85 was that the gap between the transom and the inner liner was filled with foam, not plywood--probably a variation on the "mid-year" boats. Anyway, the hulls are solid fiberglass with (up to the later years) a fiberglass-encapsulated plywood backer for the fin keel bolts.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 08/12/2015 10:23:58

OLarryR
Master Marine Consultant

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USA
3367 Posts

Response Posted - 08/12/2015 : 09:14:33

I also believe the hull is solid fiberglass (at least in the later years) since my inside hull transducer to my fish finder has a strong signal thru the toilet bowl wax ring media slapped onto the inside of the hull. It would not have a strong signal if there were any air gaps which would be the case if the hull had an inner plywood laminate or was of core construction. Then again, I have an '89 and Steve's comments addressed the changes they made in the later hulls.

Larry
'89 Robin's Nest#5820, Potomac River/Quantico, Va
http://catalina25.homestead.com/olarryr.html

dmpilc
Master Marine Consultant

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USA
4593 Posts

Response Posted - 08/12/2015 : 15:05:48

Ditto my 84 and inside-the-hull-mounted depth finder aka fishfinder with the transducer smushed into a toilet wax ring seat against the hull. No plywood core or it would not have worked at all due to air gaps., so solid fiberglass hull. also, my transducer was mounted almost directly under the companionway beside the swing keel trunk and it worked very well there.

DavidP
1975 C-22 SK #5459 "Shadowfax" Fleet 52
PO of 1984 C-25 SK/TR #4142 "Recess"
Percy Priest Yacht Club, Hamilton Creek Marina, Nashville, TN

Dave5041
Former Mainsheet Editor

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USA
3754 Posts

Response Posted - 08/12/2015 : 18:48:32

Pearl's hull is solid, the coachroof is cored, the cabin sides have pieces of plywood here and there as spacers. I have not verified other areas. Bronze, not brass, is the most desirable metal.

Dave B. aboard Pearl
1982 TR/SK/Trad. #3399
Lake Erie/Florida Panhandle

jduck00
Captain

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USA
313 Posts

Response Posted - 08/13/2015 : 13:11:34

Can't comment on a fixed keel. I have a swinger and they are solid glass. I've drilled several holes in mine.

When I put new thru hulls in mine, I layed up the area with glass using several layers of cloth, then mat, then cloth. Made a nice formed backing plate.

Jeremy Duck
The Lucky Duck
1980 SKSR Hull # 1850

blanik
Navigator

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Canada
210 Posts

Response Posted - 08/13/2015 : 14:09:18

i must have a unique C25 then because i clearly remember seeing a layer of wood sandwiched between two layers of fiberglass where my knotmeter passes through :-)

1984 C25 FK/SR #4593
Lake Champlain

Edited by - blanik on 08/13/2015 14:09:38

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/13/2015 : 17:43:13

quote:
Originally posted by blanik

i must have a unique C25 then because i clearly remember seeing a layer of wood sandwiched between two layers of fiberglass where my knotmeter passes through :-)

In most solid fiberglass boats, the "layup schedule" includes a combination of materials, such as woven roving for strength and thickness, biaxial mat for smooth outer surfaces, and sometimes a composite material like Coremat (with microspheres) for additional thickness and weight reduction. What you may have seen is several (like 8-10) layers of woven roving sandwiched between something like biaxial mat--maybe two layers on each side. Impregnated with polyester and/or vinylester resins, these different materials have different textures within the layup. I seriously doubt you were seeing wood, except in the bottom of your keel sump where a piece of plywood is encapsulated as a backer for the bolts (which I believe is the primary contributor to the "Catalina smile").

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

blanik
Navigator

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Canada
210 Posts

Response Posted - 08/14/2015 : 06:53:26

maybe but it sure looked and felt like wood, about 1/4" thick, sandwiched between two slightly less than 1/4" fibreglass layers and i also remember checking out a old crappy C25 for sale on the hard (many years ago, way before i knew anything about boats) and i remember that the freeboard was soft near the bow, i could push it with my finger and it would bulge in, back then i thought boats where made like that, with hindsight i'm quite positive that this C25 had a fully rotten wood core...

wood or not i don't really care, when i changed the knotmeter socket what looked like wood was very dry, my bilge is very dry, the insurance inspector check the humidity levels and all was good, my trough-hulls where redone professionally 3 yrs ago, it's still an awesome boat ;-)

1984 C25 FK/SR #4593
Lake Champlain

Edited by - blanik on 08/14/2015 06:57:38

Stinkpotter
Master Marine Consultant

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Djibouti
9013 Posts

Response Posted - 08/14/2015 : 07:52:14

quote:
Originally posted by blanik

...the freeboard was soft near the bow, i could push it with my finger and it would bulge in, back then i thought boats where made like that, with hindsight i'm quite positive that this C25 had a fully rotten wood core...

I suspect it had bow damage that was covered up, or serious delamination due to something like leaks at the deck and freezing. "Oil canning" is virtually unheard of with a Catalina hull.

Dave Bristle
Association "Port Captain" for Mystic/Stonington CT
PO of 1985 C-25 SR/FK #5032 Passage, USCG "sixpack" (expired),
Now on Eastern 27 $+!nkp*+ Sarge

Edited by - Stinkpotter on 08/14/2015 07:53:48

wanderer13
1st Mate

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USA
76 Posts

Response Posted - 08/15/2015 : 17:43:59

quote:
Originally posted by Dave5041

... Bronze, not brass, is the most desirable metal.

Yes, sir. You are correct. Bronze is the proper material for through hulls - my error.

1981 Catalina 25 FK/TR

Bladeswell
Captain

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USA
490 Posts

Response Posted - 08/16/2015 : 06:41:53

Thanks again guys,

Your insights and opinions are always helpful and educational. I am glad to have them. When I get to that portion of my restoration/upgrade, would you like me to take a picture of the drilled core ? I will need to seal up the original holes in the hull as Catalina liked to install their "too-hulls" vertically rather than at 90 degrees to the hull wall. Thanks again.

Bladeswell

C25 TR FK Hull #973 1979 L-Dinette. So.Cal.

OJ
Master Marine Consultant

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USA
4382 Posts

Response Posted - 08/28/2015 : 19:15:39

http://www.pbase.com/mainecruising/replacing_thruhulls&page=2

http://www.pbase.com/mainecruising/backing_blocks

1989 C25 TR/WK, #5822
1973 McVay Minuet 19
1975 Jester 12
1981 C25 SR/SK, #2428
1981 C22 SR/SK,
Tanzer 16
Sunfish

"There is nothing, absolutely nothing half so much worth doing as simply messing about in boats." Kenneth Grahame

ct95949
Captain

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Aruba
300 Posts

Response Posted - 08/29/2015 : 09:30:20

I probably didn't read carefully enough but it doesn't seem like the original question was answered. The only thing you should grind away is the 'volcano' that's around the old pipe and a little to rough up the surface of the hull for the new epoxy. When you add the plywood backing plate with it's bed of epoxy the hull in that area will be almost twice as thick and super strong. I have never done any hole in the hull or transom without using a backing plate and a nice bed of thickened epoxy as well as over drilling and filling any old holes with epoxy.
Looking at the Forespar spec sheet, 3/4" Marelon thru hulls are 3" long, more than enough to go through the hull and backing plate and room left for the nut and valve. The gap between the straight plywood and the curved hull is where the epoxy bed sits.

'90 SR/WK #6017. PO of '79 SR/SK.

Edited by - ct95949 on 08/29/2015 09:45:18

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