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Fixing a Loose Block

Fixing a loose block is a pain in the butt. For a start there is no way of knowing how long the foam will stay in place once you are done, many of these repairs hold up for years, but... The other thing is you will be working blind. Even with multiple inspection ports, lights, mirrors etc to see inside you will be working mostly by feel and checking after words. So get the heating pad for your neck ready before you start. Last, get some help. A second pair of hands and eyes will save you a lot of time and aggravation.

Before we start with the procedure, there are some things you need to know about your hull that will help you understand what we are trying to do.

Understanding Your Hull

The Sunfish hull has 4 major fiberglass parts: The hull, deck, tub and dagger board trunk. When the hull and deck are assembled they insert large foam blocks which are glued in place with expanding foam. This assembly becomes a sort of foam sandwich, with the blocks acting as stiffeners for the hull and deck. If they break loose the hull and deck will flex causing the boat to slow down in waves. In extreme cases it can weaken the whole structure.

FIG 1: Block Placement (dark rectangles are foam blocks)

Figure 2: Interior Perspective and End View


The most important thing is to make sure your hull is dry. And I don't mean sponged out. The hull should be as moisture free as possible. If you know your boat has a loose block in the fall, let it dry over the winter. If you haven't done this, then dry it out by sponge, and then use my Solar Dryer, Heat Lamps, Hair dryer etc. over the course of several days. In the following steps, we are going to glue the block back in place. The dryer and cleaner it is inside your boat, the better your adhesion will be. (This is a good time to cut in inspection ports if you don't already have some. See below)

Step 1

First you will need to cut access ports into your boat. The dark circles in Fig. 1 show possible locations for those ports. If you are cutting a port in the aft deck, make sure you are behind the storage compartment. You will need at least 4" dia. ports, and in the aft deck I recommend a 6" even though you will have to cut away part of one of the blocks. If you are doing the front I would cut (2) 4" ports in, one behind the splash rail and 1 in front of the mast step. Do not mount the ports until you are completely done so you have as big a hole as possible to reach through.

Step 2

We need to get the old expanding foam out. (fig.3)You won't get all of it or even most of it, but cut and scrape out what you can. I typically start with a utility knife and cut all the squeezed out blobs away from the block and hull. I then take a piece of old saw blade with a handle taped on and saw down the groove where the block and fiberglass meet.(fig. 4) Try not to cut into the block. The goal here is to have a nice clean groove for the new foam to flow into.

Figure 3: Foam Block Sitting in bed of Expandable Foam (dark is the expanding foam we are trying to remove)

Fig. 4: Use a Saw to Clean Out Between the Block and The Fiberglass

Step 3

Clean out the hull. Use a vacuum, paint brush, sponge whatever and get as much of the debris out as you can. When you finish you should have the block with a 1/4" space (more or less) top and bottom in which you can flow the foam in the next step. Take your time here it is really important to have the area where you are going to glue the block back in as dirt free and dry as possible.

Step 4

Gluing the Block In: There are 2 ways of doing this I will cover both below. The second is harder, but seems to work better if you are planning on keeping the boat for a long time. Get a second pair of hands either method, you will need them.

1) By 5 or 6 spray cans of "Minimally Expanding Foam" at your local home center. By plenty, you can return unused cans later, you don't want to run out midway through. Use the minimum expanding stuff because it has a higher finished density. Also get plenty of rubber gloves. The foam is a skin irritant and very sticky. Also where a long sleeve shirt to keep it off your arms.

Lay the boat on it's side with the block you are reattaching down. (gravity is your friend.) And push the block back into place.(fig 5)Insert the plastic tube nozzle of the foam can into the gap between the block and fiberglass and inject the foam. Start away from you and on the far side of the block and work your way closer. The foam will expand so you don't need to fill every square inch, the foam will help fill it as it expands. Work both sides of the block. (There is no substitute for practice here. If you are not sure how much the foam will expand, do a test shot on some scrap cardboard and watch it set before doing it inside your hull.) Let the block set and drink a beer, you will need one.

Fig 5: Boat on Edge w/Foam being poured into gap

2) Same as above but you will want to use a 2 part liquid foam and pour it into the gap. (You can order 2 part foam through West Marine) If you have a nice clean groove to pour the foam into you can get a better fill with a denser foam than the can. The trick is that you will be pouring it blind and you may need to construct a crucible(fig.6) to reach some of the farther spots. If you are not sure of your manual skills don't try this way.

Fig 6: Pouring Handle for Foam

Step 5

Let the foam set over night and then install the ports. You should be ready. The foam should stay attached, but there are no guarantees. This is a tricky job and the blocks seldom re-adhere quite as well as the original foam.

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