1975 13-footer Transom Construction Details

Repair or modification of Boston Whaler boats, their engines, trailers, and gear
KeepOnTruckin
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Joined: Sun May 12, 2019 8:49 am

1975 13-footer Transom Construction Details

Postby KeepOnTruckin » Sun May 12, 2019 9:07 am

At age 61 and recently retired, I have purchased my first boatL a 1975 Whaler Sport 13 with a 2000 Mercury 25 FOURSTROKE engine I have also purchased a Garmin Striker4. The boat will be docked from May through September in a saltwater tidal river in Maine.

I expected to mount the SONAR transducer using either a Stern-Mate or Stern-Pad type of thing to avoid drilling holes in the transom. But having figured out that the curvature of the transom will not allow this sort of installation, I will need to drill holes after all, and I have no idea what to expect. I'm a retired machinist and reasonably handy, but know little of boat construction.

Q1: how thick and strong is the skin of the transom?

Q2: Is there anything solid to screw into?

Q3: Is there a danger of easily over-tightening the hardware, or under-tightening and losing the transducer?

From the hardware that comes with the Garmin, I assume that I don't drill holes all the way through, with nuts on the other end.

Any assistance is most appreciated.

KoT

jimh
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Location: Michigan, Lower Peninsula
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Re: 1975 13-footer Transom Construction Details

Postby jimh » Sun May 12, 2019 9:17 am

A1: the gel coat skin of a Boston Whaler boat is about 20-mils thick. Below the gel coat there is a layer of fiberglass laminates. The thickness of the fiberglass laminate layer varies depending on the location in the hull.

A2: in the transom of a Boston Whaler boat there will be a large embedded plywood reinforcement. To learn the location of the reinforced areas, view the WOOD LOCATING DIAGRAM. You will find the WOOD LOCATING DIAGRAMS in the REFERENCE article on the 13-foot hull. See

http://continuouswave.com/whaler/reference/13/

A3: assuming the plywood reinforcement wood is in good condition, a #8 self-tapping pan head screw should obtain good purchase in the wood.

You assumption that a SONAR transducer is not through-bolted to the transom of a 13-foot Boston Whaler boat is correct.

To learn more about installation of SONAR transducers on Boston Whaler boats, read my article in the REFERENCE section this topic:

SONAR Transducers
http://continuouswave.com/whaler/refere ... ducer.html

Jefecinco
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Location: Gulf Shores, AL

Re: 1975 13-footer Transom Construction Details

Postby Jefecinco » Sun May 12, 2019 10:46 am

I solved the curved transom problem on our 1981 Sport 13 by cutting a 3/4 inch leftover Starboard Lumber scrap to size to act as a mounting pad. I used a larger than necessary scrap in order to provide space for a different transducer or relocating the current transducer should that become necessary. I cut several kerfs to one side to a about 80% of the thickness of the mounting pad. This allowed the pad to be bent to a curve with minimal effort and without breaking the pad. I cheated and used a mounting screw in each corner of the mounting pad to prevent movement while the adhesive cured. I used 3M 5200 adhesive.

Some experimentation was required to find the kerf depth and spacing necessary to get the flexibility needed. If you use 3M 5200 use some caution as clean up is difficult and the pad will require a lot of work if you ever need to remove it. I used relatively short mounting screws of about 1 inch length. The screws were stainless steel.
Butch

KeepOnTruckin
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Joined: Sun May 12, 2019 8:49 am

Re: 1975 13-footer Transom Construction Details

Postby KeepOnTruckin » Sun May 12, 2019 10:58 am

Thank you for the helpful links.

Now that I know where the plywood is in the transom, life will be easier, assuming that the construction remained similar over the varying years. I'm still concerned with what kind of torque to put on the screws, but I guess the base of the mounting bracket will spread the force out enough that I hopefully won't do any damage to the fiberglass.

jimh
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Re: 1975 13-footer Transom Construction Details

Postby jimh » Sun May 12, 2019 1:33 pm

Regarding the effort to drive a self-tapping screw into the transom of a Boston Whaler boat:

First, be certain to relieve the hole diameter in the gel coat layer to be large enough so that the screw fastener is not trying to force itself through the gel coat layer. If you do not do this, the screw fastener will be extremely likely--almost certain--to create a radial crack in the gel coat.

Second, drill the pilot hole through the laminate layer so the threads of the screw fastener will be cutting into the laminate layer but not so small that the screw ends up crushing the laminate.

Finally, drill the pilot hole into the wood reinforcement so that most of the screw fastener will be cutting threads into the wood--in other words, a small pilot hole.

My usual procedure is to use one particular self-tapping screw to start the threads in all the holes. I usually use a Phillips head screw. Driving this initial screw into the hull usually produces some deformation to the head of the screw. Once this screw has tapped all the holes, then I use new screws to actually fasten the work to the hull. This allows the screw heads to be undamaged by the process of initially cutting the threads. If you can find Pozi-Drive screws, they won't have a problem with the tool slipping off the screw head. If in Canada, try a Robertson Drive. Of course, use stainless steel fasteners.

For screws that will be below the water line, be sure to use some sealant on the screw threads for the final screw installation. The sealant will usually make driving the screw easier.

If you use an intermediate block of plastic for the mount, I would drill and tap the mounting holes for the transducer in the plastic and make sure the holes are not through holes. In some plastics, particularly the ultra-high molecular weight plastic material (such as KING StarBoard and generic kitchen cutting board material), you might need some thread-locker adhesive to keep the machine screws in the plastic. I haven't actually tried that myself, but that advice is from my impression of how the plastic seems to be perpetually slick.

When you choose a location and drill holes, be sure you allow some room for the transducer to immersed more deeply into the water if needed to fix poor performance. The usual problem with a SONAR transducer losing its bottom echo is from insufficient immersion.