The Mushy Vermiculite Problem
John T. Custer, Customer Service / J Boats, Tillotson-Pearson, Inc.
the weight requirements for the J/24 Class, there has been interest In
the process of replacing the vermiculite material in the sumps of
early boats. The focus of this attention has been not on the
possibility of strengthening the keel joint of the boat or on
improving the hull's stiffness and longevity but rather upon the
effect this operation can have in relation to the boat's total weight.
purpose of the vermiculite mixture found In the keel sump was to
support the keel bolts both vertically and laterally In some
boats, (hull numbers undocumented) this same material was spread
throughout the area under the main cabin sole and aft. As this was
done in a relatively small number of boats, I think we can assume that
the hull stiffening properties of a resin/vermiculite goo were found
to be less than Ideal. Given the level of technology available at the
time (1978-81) and the still experimental nature of fiberglass boat
building, the use of this material was seen as a lightweight, simple
way of providing a platform for the bedding of five keel nuts. Some,
not all, of these early hulls have, over the course of time (let us
not discuss the effects of hard sailing, groundings, poor maintenance,
etc. etc.) developed the annoying tendency to retain water within the
vermiculite mixture, and to ultimately become "mushy". The
keel flex, this condition can allow is needless to say, slow and hard
on both the boat and the owner's nerves.
are many steps one can take to assess the existence or extent of a
vermiculite problem. All will provide good clues, but short of
removing the original material there is little that can be done to
prove the lack of a problem or to locate the beginnings of such a
problem. With the boat in the water it is possible to check the
Integrity of the sump material by trying to drive screwdriver or
similar object into the surface of the vermiculite. If it is
easily possible to penetrate the outer layer, obviously there is a
problem in the upper parts of the sump it may extend throughout the
sump or be just a problem at the surface. Further Investigation can be
completed by drilling small (1/8" - 3/16") pilot holes Into
the vermiculite. If wet, mushy material is removed, it is probably
past time to consider a complete removal and replacement of the
existing vermiculite mixture ‑otherwise known as doing a 'verm
job'. Again, remember that by drilling into the sump, one only
determines the condition of the vermiculite in that particular area.
Regardless of the result of the above tests, any opening through the
gelcoat coating of the vermiculite should be resealed with gelcoat or
an epoxy based resin or paint. Cracks In the existing gelcoat should
always be treated in the same manner, with the boat out of the water,
check the lead/keel sump joint (about 9" below the hull itself)
for signs of hairline cracking. This may be an indication that the
keel is not adequately supported. It may also be simple stress
fractures, or a result of past impacts. If the boat is suspended with
the base of the keel free, assess the integrity or the keel joint by
"shaking" the keel. Is it firmly attached to the boat, or is
there some play? Mushy vermiculite is only one possible explanation
for such play. A loose keel may be just that a result of loose keel
nuts, a broken seal between lead and fiberglass, or a break in the
band of glass around the keel joint.
not too technically demanding, a verm job is rather labor intensive.
When done by Tillotson-Pearson, it takes 17-20 hours. Someone without
the experience or facilities found at the factory can expect to spend
much longer on the effort. Keep In mind that experience in working
with fiberglass and resin is critical to doing a proper job.
effect a verm job may have upon boat weight is difficult to predict
except on a case-by-case basis. By doing a verm job as suggested, one
will add some weight as compared to the original weight of the
vermiculite installation. If a boat is carrying considerable water
weight within its sump material, one can then reasonably expect the
job to reduce total weight. Also, removal of the vermiculite spread
throughout surrounding areas on some boats will reduce weight
Stage boat securely on boat stands, trailer or cradle making sure that
the keel is supported.
Mask off interior of boat (this is going to eliminate quite a bit of
clean up time later).
Cut out cabin sole along nonskid back to the cooler location leaving 1
1/2" lip all around the edge.
4. Remove the nuts from the keel bolts and protect the threads with two or three layers of duct tape. Remove vermiculite filler from sump and bilge area as far as possible up under remaining sole. Use of an air chisel or rotary hammer (available at most tool rental shops) will save a lot of time and effort. Great care should be used when nearing the bottom and sides of the sump so as not to damage the hull laminate.
Sand away any remaining filler left in the sump and bilge area to
expose fresh, clean laminate. Be careful not to remove any good
laminate. It is critical to the rest of the process that the area be
clean and free from any residual filler, water, oil or other
6. Install the floor supports (8) at the following locations measuring back from the main bulkhead: 12 1/2", 26", 40". Tack them in place with polyester filler so that the tops are flush with the top of the lip left from the original sole. This ensures that the replacement sole will be supported evenly with minimal shimming. Using the polyester filler, form a small radius where the floor supports meet the hull (the supports will not contact the hull directly), glass these in place with two layers of 1708 biaxial cloth or three layers of 1.5 oz. mat alternating with two layers of 10 oz woven roving.
The sump must now be filled to the threads on the keel bolts using a
mixture of resin (70-80%), chopped or milled fibers (10-15%), and
cabosil or aerosil (10-15%). The mixture should be just pourable but not
so stiff that it will not fill into the corners. The curing process of
this mixture will produce a great deal of heat making it necessary to
fill the individual areas in several stages to prevent excess heat
buildup and shrinkage that will cause crystallization and cracking in
the resin. A catalyst ratio of 1 to 1.5% by volume should be used
(10cc/qt) of resin=1%) depending on the ambient temperature. The warmer
it is, the less catalyst you will need. Do not use less than 1%
catalyst. Catalize no more than two quarts of filler mixture at one
time. Allow the previous batch to cool before proceeding with the next
layer. Repeat the process until the sump is filled to the original
level. Remember to leave an indent between the aft two keel bolts to
allow installation of the lifting shackle or hook.
Coat all exposed bilge areas with air dry gelcoat or epoxy barrier coat
after lightly sanding all areas to be coated.
Install keel nuts and tighten to 100-120 ft/lbs.
Install cabin sole and shim as needed. Screw sole to floor supports and
seal around the edge with RTV silicone sealer.