25 Bertram Prop Pocket Installation

Once the pocket is built and free of the mold it's time to move on to the installation. The goal of the installation is to "trap" the pocket in the original transom in such a manner that the pocket cannot come loose under any circumstances. The design I use is based on maximum strength and resistance to a high speed prop/shaft/strut impact. This is the worst case scenario for an inboard boat. The installation will focus on creating a vertical trust/impact resistance. In the unfortunate event that the boat strikes an object at high speed the energy of the impact is transferred into the strut and could "push" the strut up and into the hull, in some cases penetrating the hull completely...leaving a BIG hole.

This has been proven time and again on the 31 Bertram, as well as many other inboard designs. The hull design on the 31 is considered weak by some in the area that the strut base is attached. This has become known as "the classic 31 strut problem". Basically the original designers made little, or at least insufficient efforts to strengthen the hull above the strut. As defined above, when you hit something hard the strut tries to penetrate the bottom and enter the hull.

So logic would dictate that the pocket installation resist this force. The pocket insert itself is considerably stronger than the original hull. Why, because it is constructed from epoxy resins and multiple layers (10-15) of laminating cloth that has a smaller, tighter weave. More layers + tighter weave = stronger. Bi-axle or Tri-axle cloth would be the best. The original hull lay-up consist of a few layers (3-4) of woven-roven and the resin was polyester. Let's take advantage of the latest stuff and get the improvements in strength via technology that have developed over the past 40 years. I suppose Tri-axle Kevlar cloth and epoxy would be the ultimate.

Below is a diagram that represents the basic transom installation. Notice that the pocket insert is "under" the original transom. In the event of an impact as described the strut will not be capable of lifting the pocket insert up and out of position. The pocket insert is now in a shear position and it must be literally torn apart before the strut could enter the hull. The plywood coring distributes this support over several inches. The hull came with a design element in the transom that is referred to by many as "the bubble". Take advantage of the bubble and fill it with coring.

Considering that the pocket insert is considerably stronger than the original hull and that it is also built considerably thicker, I think this is all that can be done to avoid a hull breach from an impact. The goal will be an over-all pocket thickness of somewhere between 1^1/4" to 1^3/4". The original hull is between 3/4' to 1" thick and that holds true for the 31 Bertram as well. Basically this will create a stronger strut support for the 25 than ever existed in the 31....Ok by me.

This is the basic configuration that I used on the SeaRay and the boat has, on a few occasions impacted objects at speed. The boat has also been grounded and the stern literally picked up out of the water on the strut/prop. No damage to the pocket what so ever has occurred over the 4 years of running the boat. I can't say as much for the prop. The fabricated stainless strut has also proven itself worthy. The SeaRay actually did not have the exterior tie-in and STILL has survived without damage. The SeaRay did however, incorporate this transom installation schedule. This installation design is stronger. I believe that positioning the pocket insert "under" the transom is one of the keys to a strong installation.

When designing anything that you, your family or your buddies may be 75 miles offshore in....over kill is ALWAYS a good policy.

Getting Started...

The first thing that is required before you can install the pocket insert is tearing out the existing transom system. The 69' Bertram that I am reconfiguring originally came from the factory with twin IO's. I selected this version for the large fiberglass stringer system that came in the boats. The Bertram is exceptionally well built and getting this stuff out is no simple task. I have torn out several transoms on various boats and none are better built than the older Bertram's. I wish there was and easy way to do this but I haven't found it as yet. Cut, chisel, cut some more, grind, pry the wood out, grind and grind some more. Be patent and try to avoid damaging the transom itself...you'll get there.

With the top layer of glass removed take a look at the photo on the right. This is an excellent example of just one of the little things that separated Bertram from other builders. Little or no voids. Between the hull laminate and the plywood transom coring Bertram used a glass putty compound to fill the voids and create a monolithic boat. This cannot be understated as relates to the strength of any boat. It should be the goal of any builder to create a "void-less" boat. This really increases a boats capacity to stay together. The entire boats reacts as a single block of fiberglass. Voids trap water, create rot and allow the hull to flex excessively.

The wood in this transom was in amazingly good condition. The 35 year old Bertram could have been put back into service with no transom work needed. This is actually one of the worst jobs that you will run across when rebuilding older boats. Once you get the transom core removal taken care of the rest of the transom work will be reconstruction, which I always prefer to destruction.

By the time you find the aqua blue/green glass of the outter hull...your one tired camper and your hands and fingers are shot. I hope there's not an easier way to do this because I would hate to find out about it now. The transom core removal and clean up takes every bit of 8-10 hours. Well it took me that long anyway.

The good part about this portion of the project is that it's over. There are many long days and evenings ahead but one of the worst parts is behind you once the transom core is out.

Fitting the Pocket...

Well it's been a couple of months but I finally got around to building the pocket. First round of glass that went onto the new mold was (8) layers of 20 oz cloth saturated with 1:1 laminating epoxy from Fiberglass Coatings out of Florida. This provided about 3/8" worth of glass. The final thickness will be around 1 1/4" so you can see there's plenty of glass work ahead.

As with most of my endeavors, some of this process was good and some was not. The good part is the pocket is a near perfect fit. I could not have wanted a better outcome with regards to how the insert follows the contours of the 25 Bertram's hull. The bad part was getting the insert off the mold! I used a paste wax for a release agent, 12-15 coats. Most of the paint came off the mold! Luckily the mold was undamaged...just needs a good sanding and repainting. Someone else can do that because I'm not going into the prop pocket insert business. The mold is going into storage until someone comes along and needs it.

More demo to the original internal supports was required to gain access to the area of the keel effected by the pocket. It all had to go anyway, so I might as well break out the grinder and get on with it. Here's a few shots of the trial fit

During the demo process I'm sorry to report that 100% of the plywood used for supporting the front of the stern drive engines was completely rotten. The only bright spot was the keel board insert. Although the 1" thick plywood used in the keel was completely rotten as well, the glass that wraps the keep plank is very heavy. More than enough support to keep the old girl's keel from flexing.

In most cases when I hear someone say "the wood just provides a shape and the glass provides the strength"...the first thing that comes to my mind is "WHAT". But the keel insert on this boat was exactly that, simply shape for the glass.

This particular hull would have been a tremendous amount of work for any engine configuration because of the excessive rotten wood.

Another disappointment was the glass box stringers are separated from the hull in a couple of spots. I have been on the lookout for this one because I hear it's pretty common on Mark II 25 Berts. I think that the boat could have continued to be used with the stringer tie-in de-lamination because the original glass is overlapping the into the keel section of the hull, so the flexing is somewhat contained and kept to a minimum. ...But I won't leave it that way!

Fire evidence. This boat had a fire at some point in it's 40 history. The discolored and slightly bubbled fiberglass is the tell tale sign. The fire apparently was contained, or went out on it's own.

One interesting find is "where" the fire was located. A bit of burn marks near the starboard stringer at the transom and more in the bilge. The fire was under the existing false bottom of the bilge. More than likely gasoline in the bilge ignited and then burned out, followed by the bilge water sloshing around and keeping the glass from burning.

Based on the location of the fire there would have been no way that an extinguisher could reach under the false panels to put it out. The previous owner's may have never known just how close they came to burning up, or exploding at sea. Maybe there is something to be said for a boat that always retains a couple of gallons of water in the bilge, sorta creates a built in fire extinguisher! Once the fuel is burned off the water puts it out. 

The lesson here is NOT to design a boat in such a manner that fuel can accumulate in areas you can not reach.

Because of the short wave period that we have in the Gulf of Mexico boats take a tremendous beating, and this one came from South Louisiana. The Gulf takes it's toll on boats.

Once I determined that the pocket was a great fit it was time to move on to the actual installation. Prior to glassing in the pocket the entire surface area of the inner bottom need to be cleaned and prepared to accept the pocket. Before you tackle a "grind" of this magnitude get everything you will close at hand and try to make yourself comfortable.

 I started by aligning the pocket into it's correct location. This was accomplished with a lot of measuring and by pulling strings from the stern to the forward end of the cockpit to determine the true center line. Note the photo on the left, the center line keel support installed by Bertram was way off! When boat builder are mass producing hulls they rarely take the time to measure and center components that basically just don't matter.

After the exact insert location is determined....make the cut. Are you sure you want to do this? Once this cut is made a change of mind would be tough, although I'll save the drop...just in case! I believe the drop could be reinstalled if you wanted to with little or no effect on strength, assuming it's reinstalled properly.

This is NO time to change your mind or second guess all your research and difficult decisions, but if your like me you'll still hesitate, sit back and take a deep breath....yeah I'm sure!

I traced 2 lines. One line around the perimeter of the insert and then removed the insert. Then I traced another line approximately 1/2" INSIDE the original pocket insert trace line. The cut will be made on the INSIDE line, this way the insert sits in the hull for the final fabrication.

There is a bonus to making the cut-out PRIOR to preparing the surrounding surface. The bonus is that once the cut is made and the drop is out you can put a chair in the hole and basically do all the prep work sitting down. In fact the area that you will need to grind and prep is at work bench level! The older I get the less I want to spend 2 or 3 hours bent over my work in a bad position, makes it hard to get out of bed the next day. So make the cut and then go back and prep the area.

The photo on the right shows you what the glass in the hull should look like before you are ready to install the pocket. Take a look at this glass and don't stop sanding, grinding etc. until the bottom is 100% clean and your down to virgin fiberglass. Say, that's a BIG hole in the boat!

Notice the delaminated glass that wraps the box stringers has also been removed. This is the perfect time to clean the entire area because your sitting on a chair! Nice and comfortable, productivity is excellent when your not in a nasty position.

I have saved the transom work for AFTER the pocket is installed. Once the pocket is in I will add the plywood coring into the "bubble"...trapping the pocket. Once the plywood is installed it will provide a backing plate for the glass work on the transom.

Photo below shows the box stringer AFTER the excess, separated fiberglass has been removed. Now the stringer repairs can be facilitated at the same time.

Sand, sand and sand some more. You'll only do this once and you don't want to second guess your work. Taking the time to properly prep the surface areas is the key to a forever bond.

After the surface is prepared I vacuum away ALL the powder, wash with water and let it dry thoroughly. Prior to applying any epoxy I wipe the entire area down with de-natured alcohol...and then let it dry thoroughly.

Now it's time to "stick" the pocket. Not completely glass it in, but rather putty it in. The insert needs to be securely in place before it's possible to apply the fiberglass. Once I have the pocket in position I used 2 cinder blocks to keep it from moving during the putty application.

Photo's below show the process. I used 1:1 laminating epoxy mixed with milled fiber and cabosil. First mix the resins until well blended. Then add the milled fibers...these are for strength. Now add the cabosil a little at a time until the mixture will no longer flow when you tilt the mixing pot. Now use a flexible putty wedge and apply about 3/8"-1/2" of epoxy putty around the perimeter of the insert, packing it into the gap.

Now GO AWAY and let this stuff set up. I know I'm always tempted to move ahead as quickly as possible, but this is not the time to be in a rush. Notice the pin sticking out of the front of the pocket. I drilled a 1/4" hole on dead center of the pocket and installed a bolt to use for alignment. I also installed one on the top of the pocket near the stern. This way I could pull a string line down the center of the boat and align the pocket insert correctly. Accurately positioning the insert will pay big dividends later. If the insert is off by as little as a 1/4" the engine will be WAY OFF, or the shaft will not be coming out of the pocket in the center of the stuffing box pad...ugly.

While your mixing & running epoxy go ahead and putty in the damaged stringer area. The repair schedule for the stringers will be the same as installing the pocket insert. Use the putty to hold everything in place, come back later and apply the fiberglass. This use of putty prior to applying the fiberglass has another benefit, it creates a "void-less" repair. Keeping everything you do void-less will make a for strong boat.

Once this stuff cures I thing it would probably hold the boat together forever....even without fiberglass top coats!

I have never seen this mixture pull loose, even under extreme stress, providing the surface prep was good. Of course I'll be using a bunch of fiberglass over all of this initial bonding.

One thing to mention is never leave epoxy exposed to sun light for days on end. Epoxy is fantastic stuff but it has very little resistance to UV's. Once painted it's there forever. UV's will quickly breakdown unprotected epoxy, so cover your work if this adventure is happening outdoors. 

The final stringer modifications to accommodate the inboard installation are not quite worked out as yet. So I may add 1 or 2 layers of cloth to this stringer joint for a little insurance, but I'm not going to do a major build up of this area any time soon.

Take a look at this photo taken from the stern of the boat during the pocket insert installation. Notice the gaps along the joint. This can't be helped and makes a good case for the epoxy putty, void filling. As mentioned early these are production boats. As good a Bertrams are they still have a very uneven thickness in the hull laminates. The thickness in my boat ranged from a 1/4" to over a 1/2". In this situation all you can do is set the pocket correctly and fill the voids. Notice the head of the bolt that was used for centering the insert is directly aligned with the 1/4" hole that I drilled in the transom for alignment. It's harder than you may think to find the center of a fiberglass boat!

The hull is starting to take shape with the pocket finally in. If your undertaking a project of this size you'll get excited once you can see what it's going to look like! I'm ready to see a shaft and prop hanging under the boat but I know that's many months away. At this point the exterior of the hull has not been shaped to accept the glass that will be applied outside. So far, so good.

As discussed on the Prop Pocket Design page, this IS NOT a huge pocket. Primary design goal for the pocket is to provide the correct engine location and alignment....not to run around in 1 ft of water.

Below is the first look at a 22" wheel placed in the pocket. That's a big prop. If you look closely you can see the tab I added to the top of the pattern. This tab is 2.75" and establishes the correct prop tip clearance. Estimated draft is 38"-40". This draft is absolutely the max I can live with in my area.

[Back to the Site Directory]