Fastening Mats for On-deck Fuel Tanks to the Deck

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Author Topic:   Fastening Mats for On-deck Fuel Tanks to the Deck

rivarat posted 11-02-2012 12:30 PM ET (US)
I am planning to replace two rubber fuel tank mats under the bench on my MONTUAK 17. Why were they riveted on?
I can drill the heads off the rivets to remove the mats. Do I just push the base of the rivet down into the hull? It is hard to extract it if it expanded under the deck. Should I use rivets again? Can I use stainless steel screws?--glenn
L H G posted 11-02-2012 02:09 PM ET (US)
I did the same thing you are doing on my 1979. I bought new mats from Twin Cities.
As to why Whaler riveted them on, I don't know, but my guess it was a cost saving technique. I have noticed they used a lot of riveting in the earlier models.
I completely drilled out the rivets to get them out of the wood underlayment in the boat floor. That leaves a larger hole than needed for #6 or #8 screws to install the new ones. You have two choices. You can carefully tape off the hole and fill it with Evercoat Formula 27 polyester resin putty, and then re-drill for the screws. Or you can do what I did which was to counter sink the hole and fully seat flush a #10 flat head screw into the old hole, set in clear Life Seal. Then relocate the new mats by offsetting them about 1/2" and install the screws in completely new holes. The old screw holes will still not show. I used #6 truss head (low profile) screws from West Marine and the same fender washers.
jimh posted 11-02-2012 02:13 PM ET (US)
When a rivet fastener is drilled out, the portion inside the Unibond hull cannot be extracted through the original hole, so the only reasonable option is to just push the rivet more deeply into the foam. I suppose you could enlarge the original hole to greater diameter and fish out the rivet. Then you will have to repair the hole. This may be in the works, anyway; see below
A screw fastener is generally best used when there is a reinforcement embedded under the deck to receive the threaded fastener. If you want to install a fastener in an area of the deck where there is no reinforcement, the thickness of the laminate is typically only going to provide limited holding. You can get a screw fastener to have some purchase into the laminate, but you have to be very careful about the pilot hole size. You want the pilot hole to allow the threads to cut into the laminate, but not so small that you force a crack. To hold a mat in place, the force is mainly a shear force, so you may be OK with a screw just threaded into the thin laminate.
Sometimes you can make your own reinforced area by digging out the foam around the hole. You can usually do this with an a small Allen wrench or L-shaped bit of steel. Insert the short arm into the hole and dig out the foam in a small radius around the hole. Then instill some thickened epoxy resin into the pocket you have created. Don't use pure resin. Thicken the resin to a mayonnaise consistency, then inject it into the pocket through the hole until the void is filled. When the resin cures, you can drill a pilot hole into the resin, and you'll have more material to create holding for the screw.
I don't know if there was a practice from the factory for riveting the fuel tank mats in place. Clearly someone at some point decided they needed to be retained and added the rivets. If could have been done at any point in the boat's history.
rivarat posted 11-02-2012 02:15 PM ET (US)
Thanks for the information. Is there anything special about the original mats that would make me need to get exact replacements? [I] was thinking of foam anti-fatigue mats. Being located in [C]anada comes with the curse of not having ready or economical access to OEM type parts.
jimh posted 11-02-2012 02:16 PM ET (US)
By the way, there should be wood reinforcement under the deck area in the cockpit of a MONTAUK. See the wood locating diagrams for various models and years in the REFERENCE section at
http://continuouswave.com/whaler/reference/16-17/
rivarat posted 11-02-2012 02:21 PM ET (US)
What diameter and length rivets should be used? It sounds like it may not be required to secure the mats at all; is this what [jimh] is saying?--glenn
andygere posted 11-02-2012 02:30 PM ET (US)
The mats are not under much lateral load, and the rivets or screws are there to simply keep them in place. There is plywood embedded in the liner structure under the RPS, so there is sufficient material for a self-tapping screw to fasten into. The original mats were sized to keep the steel 12 gallon fuel tanks from marring the deck and from sliding around. The friction from the rubber and the small lip on the mats are quite effective for that purpose. I'm sure some other type of mat could be substituted. The OEM mats had a tidy look and did a good job so they are frequently retained or replaced on older Montauks.
In terms of the repair, LHG provides 2 viable alternatives. I like using polyester or epoxy resin to fill the holes of the drilled-out rivets because I think it does a better job of keeping water from wicking into the under deck plywood. Use new stainless steel self-tapping (sheetmetal) screws offset from the original holes.
jimh posted 11-02-2012 02:51 PM ET (US)
I think the original fuel tanks were probably metal tanks, and the mats were used to keep the deck finish from being marred, as Andy suggests. In you are using plastic fuel tanks you can forego the mats, but I would use some sort of tie-down strap to hold the tanks in place.
Yes, don't leave any holes in the deck unsealed. You want to keep the water that inevitably accumulates on the deck away from the wood lying under it.
Get a West System fiberglass repair kit. It will have six packets of epoxy. You will only need one packet to fill about a dozen holes. The advantage of the epoxy is manifold:
--the shelf life of the unopened sealed foil packets is practically unlimited
--the kit has all the stuff you need: fillers, mixers, gloves, everything in one nice kit;
--the epoxy is a much better adhesive and will make much stronger bonds to the other materials
--epoxy is completely water resistant and needs no protection against water.
These are all advantages of epoxy over polyester resin for repairs, which is why it is so widely used. Once you open a small can of polyester resin it has a very limited shelf life, you will have to get additional thickeners, and it is not as strong an adhesive when making secondary bonds to already fully cured resins and laminates. It isn't even water resistant, which is why it is always recommended to apply epoxy topcoats to hull bottoms if you are going to leave the boat in the water all the time. Let me quote from the owner's manual published by Boston Whaler about this:

quote:
Any boat left in salt or fresh water for more than a few days should have the gelcoat surface protected with a two-part epoxy paint...
http://continuouswave.com/whaler/reference/manual18-25/maintenance. html#bottom
rivarat posted 11-02-2012 03:40 PM ET (US)
[T]his is all good info for sure. [R]egarding using epoxy, [I] have scads of experience with it and have a cabinet full of resins and fillers, so [I] should be able to make the holes go away if needed. [I] think [I] will [make] new holes as it appears if there is ply[wood] under the floor simply pushing the rivets down out of the way would not be an option.
Chuck Tribolet posted 11-06-2012 09:54 AM ET (US)
The mats on my Montauk have a lip around the edge that would
keep the tank from sliding around.
Chuck

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Author	Topic: Fastening Mats for On-deck Fuel Tanks to the Deck
rivarat	posted 11-02-2012 12:30 PM ET (US) I am planning to replace two rubber fuel tank mats under the bench on my MONTUAK 17. Why were they riveted on? I can drill the heads off the rivets to remove the mats. Do I just push the base of the rivet down into the hull? It is hard to extract it if it expanded under the deck. Should I use rivets again? Can I use stainless steel screws?--glenn
L H G	posted 11-02-2012 02:09 PM ET (US) I did the same thing you are doing on my 1979. I bought new mats from Twin Cities. As to why Whaler riveted them on, I don't know, but my guess it was a cost saving technique. I have noticed they used a lot of riveting in the earlier models. I completely drilled out the rivets to get them out of the wood underlayment in the boat floor. That leaves a larger hole than needed for #6 or #8 screws to install the new ones. You have two choices. You can carefully tape off the hole and fill it with Evercoat Formula 27 polyester resin putty, and then re-drill for the screws. Or you can do what I did which was to counter sink the hole and fully seat flush a #10 flat head screw into the old hole, set in clear Life Seal. Then relocate the new mats by offsetting them about 1/2" and install the screws in completely new holes. The old screw holes will still not show. I used #6 truss head (low profile) screws from West Marine and the same fender washers.
jimh	posted 11-02-2012 02:13 PM ET (US) When a rivet fastener is drilled out, the portion inside the Unibond hull cannot be extracted through the original hole, so the only reasonable option is to just push the rivet more deeply into the foam. I suppose you could enlarge the original hole to greater diameter and fish out the rivet. Then you will have to repair the hole. This may be in the works, anyway; see below A screw fastener is generally best used when there is a reinforcement embedded under the deck to receive the threaded fastener. If you want to install a fastener in an area of the deck where there is no reinforcement, the thickness of the laminate is typically only going to provide limited holding. You can get a screw fastener to have some purchase into the laminate, but you have to be very careful about the pilot hole size. You want the pilot hole to allow the threads to cut into the laminate, but not so small that you force a crack. To hold a mat in place, the force is mainly a shear force, so you may be OK with a screw just threaded into the thin laminate. Sometimes you can make your own reinforced area by digging out the foam around the hole. You can usually do this with an a small Allen wrench or L-shaped bit of steel. Insert the short arm into the hole and dig out the foam in a small radius around the hole. Then instill some thickened epoxy resin into the pocket you have created. Don't use pure resin. Thicken the resin to a mayonnaise consistency, then inject it into the pocket through the hole until the void is filled. When the resin cures, you can drill a pilot hole into the resin, and you'll have more material to create holding for the screw. I don't know if there was a practice from the factory for riveting the fuel tank mats in place. Clearly someone at some point decided they needed to be retained and added the rivets. If could have been done at any point in the boat's history.
rivarat	posted 11-02-2012 02:15 PM ET (US) Thanks for the information. Is there anything special about the original mats that would make me need to get exact replacements? [I] was thinking of foam anti-fatigue mats. Being located in [C]anada comes with the curse of not having ready or economical access to OEM type parts.
jimh	posted 11-02-2012 02:16 PM ET (US) By the way, there should be wood reinforcement under the deck area in the cockpit of a MONTAUK. See the wood locating diagrams for various models and years in the REFERENCE section at http://continuouswave.com/whaler/reference/16-17/
rivarat	posted 11-02-2012 02:21 PM ET (US) What diameter and length rivets should be used? It sounds like it may not be required to secure the mats at all; is this what [jimh] is saying?--glenn
andygere	posted 11-02-2012 02:30 PM ET (US) The mats are not under much lateral load, and the rivets or screws are there to simply keep them in place. There is plywood embedded in the liner structure under the RPS, so there is sufficient material for a self-tapping screw to fasten into. The original mats were sized to keep the steel 12 gallon fuel tanks from marring the deck and from sliding around. The friction from the rubber and the small lip on the mats are quite effective for that purpose. I'm sure some other type of mat could be substituted. The OEM mats had a tidy look and did a good job so they are frequently retained or replaced on older Montauks. In terms of the repair, LHG provides 2 viable alternatives. I like using polyester or epoxy resin to fill the holes of the drilled-out rivets because I think it does a better job of keeping water from wicking into the under deck plywood. Use new stainless steel self-tapping (sheetmetal) screws offset from the original holes.
jimh	posted 11-02-2012 02:51 PM ET (US) I think the original fuel tanks were probably metal tanks, and the mats were used to keep the deck finish from being marred, as Andy suggests. In you are using plastic fuel tanks you can forego the mats, but I would use some sort of tie-down strap to hold the tanks in place. Yes, don't leave any holes in the deck unsealed. You want to keep the water that inevitably accumulates on the deck away from the wood lying under it. Get a West System fiberglass repair kit. It will have six packets of epoxy. You will only need one packet to fill about a dozen holes. The advantage of the epoxy is manifold: --the shelf life of the unopened sealed foil packets is practically unlimited --the kit has all the stuff you need: fillers, mixers, gloves, everything in one nice kit; --the epoxy is a much better adhesive and will make much stronger bonds to the other materials --epoxy is completely water resistant and needs no protection against water. These are all advantages of epoxy over polyester resin for repairs, which is why it is so widely used. Once you open a small can of polyester resin it has a very limited shelf life, you will have to get additional thickeners, and it is not as strong an adhesive when making secondary bonds to already fully cured resins and laminates. It isn't even water resistant, which is why it is always recommended to apply epoxy topcoats to hull bottoms if you are going to leave the boat in the water all the time. Let me quote from the owner's manual published by Boston Whaler about this: quote: Any boat left in salt or fresh water for more than a few days should have the gelcoat surface protected with a two-part epoxy paint... http://continuouswave.com/whaler/reference/manual18-25/maintenance. html#bottom
rivarat	posted 11-02-2012 03:40 PM ET (US) [T]his is all good info for sure. [R]egarding using epoxy, [I] have scads of experience with it and have a cabinet full of resins and fillers, so [I] should be able to make the holes go away if needed. [I] think [I] will [make] new holes as it appears if there is ply[wood] under the floor simply pushing the rivets down out of the way would not be an option.
Chuck Tribolet	posted 11-06-2012 09:54 AM ET (US) The mats on my Montauk have a lip around the edge that would keep the tank from sliding around. Chuck