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Compass Marine How To | all galleries >> Welcome To >> Replacing Thru-Hulls and Seacocks > Making Backing Plates
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Making Backing Plates

Unless your hull is close to an inch thick, of solid fiberglass, you're going to need backing plates for the seacocks. Backing plates are designed and used to distribute any stress loads over a larger footprint and to reduce and minimize any flexing of the hull at the seacock.

While most boat yards and DIY boaters use wood, because it's quick and easy, there really is a more permanent method and that is fiberglass the same material your hull is made of. Of course if you have a beautiful old wooden boat don't use fiberglass.. Many wood backing plates can still eventually absorb moisture and then swell and contract, over time, and possibly compromise the fit of the seacocks and the water tight nature. Most wood's are also far softer than solid fiberglass so it can flex some with the constant opening and closing of a seacock. Many folks swear by coating the wood with epoxy, which I've done over the years, but, in my experience, it really does not take that much more effort to make them out of fiberglass.

I now use fiberglass for my backing plates so these instructions show how to do that. If you want to use wood just replace the "epoxy" or "kitty hair" step with a product like Sikaflex 291 or 3M 4200 and secure the wood to the hull with that.

Fiberglass backing plates are easy to make but yes they are admittedly a little more time consuming. The benefit is that once you have glassed these into the boat they will last a lifetime and never need replacing even if you replace the seacocks ten+ years down the road. My motto is "any job worth doing is a job worth doing right" Morris Yachts for instance, one of the highest quality builders in the United States, currently uses solid fiberglass for their seacock installations on both their cored and solid fiberglass hulls. I personally feel wooden backing plates are a compromise but please do remember what you paid for this opinion when making your own decisions.

Fiberglass Backing Plates:

To make solid fiberglass backing plates you will need the following items.

1- Fiberglass laminating resin & hardener + some finishing or waxed resin.

2- Fiberglass in both 18oz "Woven Roving" and a standard 6oz "Fiberglass Cloth".

3- A 24 X 24 inch piece of relatively thick glass.

4- A Fiberglass roller.

5- Mold release wax for the glass.

To make the backing plate board wax the glass and begin laying up, wetting out, rolling & alternating the layers of roving and cloth until you have a fiberglass board at least 5/8 of an inch thick. The whole laminating process may take you 30 minutes. The finishing resin goes on as the last coat so it does not dry tacky.

You may also be able to find scrap pieces fiberglass around your boat yard. This would save some time instead of laminating the backing plates yourself. Even two thicknesses of thinner fiberglass epoxied together is less work than actually doing the laminating. So ask around at the yard you never know what you might find.


O.K. I've had numerous questions about the use of the plastic marine lumber Starboard and it's many "knock off" iterations. Starboard is NOT a suitable product for a backing plate.

#1 Starboard is far to flexible to offer any sort of rigid stiffening to the hull.

#2 The manufacturer states that it does NOT stick well to any adhesives which includes bedding compounds such as 5200, 4200 or Sikaflex.

#3 The concept of a backing plate is to become one with the hull, make it thicker and add stiffness so the seal between the hull and through hull is not compromised and does not flex or move. Starboard fails in all these areas..

Starboard is a great product for many uses just not one where bonding to another substrate is required or rigidity is required. Use Starboard or other similar products at your own risk.

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Guest 15-Jan-2017 22:22
McMaster-Carr has a large selection of frp sheet, tubing and angle. They will ship small quantities, much easier than making your own.
John 07-Jan-2017 14:13
What about pre-made backing plates from Groco?
Jose Eugenio 09-Dec-2012 16:37
Many thanks for the articles.
Is possible use a backing plates in polyethylene (the same material to kitchen cutting boards)?.
Best regards.
Perry 17-Oct-2012 01:22
I bought the 1/2" fiberglass sheet from McMaster-Carr. I then had a local granite countertop shop use their waterjet machine to cut an assortment of backing plates to match my new seacocks and struts, as well as an irregularly-shaped backing plate for my underwater exhaust. Hiring the cutting means no fiberglass dust, dull saw blades, or burned fiberglass.
Howard 15-Dec-2010 01:59
Is there any advantage in glassing in the bolts (maybe use Bronze carriage bolts with heads imbedded into the backing plate)if one did not want to use the thru-the-hull method?
Dan Baisley 16-Apr-2009 16:20
Great website, it's been very useful to me. I used your alternate / no-though-bolts method, as I have a cored hull and wanted to minimize potential water intrusion points. I'll add a helpful tip back. I bought a pre-made fiberglass/epoxy sheet from McMaster-Carr. If you go to their website and search for GPO3 (that's the letter O, not zero) you'll find in in various sheet thickness and sizes. A 12 X 12 X 3/4 sheet was around 30 bucks. Shipping was cheap, around $7. Don't bother with next-day shipping, they have warehouses around the country, so standard shipping only takes 2 days in most cases.
John Martin 22-Mar-2009 11:06
I had occasion to talk with some of the builders from Morris Yachts at the 2009 Maine Boatbuilders Show. They said that they specifically use G10 Fiberglass for their backing plates. They cautioned that G10 will completely wear out blades and drill bits, but they use it for all the good reasons described here.
Ron 27-Feb-2008 14:36
I have found frp board in various thickness available from Msc supply corp. handy for applications like this. Comes in 12 x 12 (or larger) and usually arrives next day.
Joe M. 06-Jan-2008 04:28
Thanks for this. My wood backing plates are all wet and rotted and now two of my thru hulls are leaking. I guess they were never installed correctly. I'll be using your method of fiberglass. The waxed glass trick is the secret part I was missing.