Between 1609 and 1718 there was a wooden ramp to the west of the Hondsbossche Sluis to drag ships over the dam from the Binnenzaan to the Voorzaan: the Zaandam 'overtoom'. Overtooms already existed in the early Middle Ages, but were generally intended to help smaller ships get over a water barrier. The overtoom in Zaandam was the only one in the world that was specifically intended for large seagoing vessels.
The demand for large ships grew rapidly at the beginning of the 17e century through the development of Amsterdam as a trading center. Almost all Zaanse yards that could build such ships were located on the Binnenzaan. In that period, what we now know as Voorzaan mainly consisted of swampy areas that were only made suitable for shipbuilding and other industrial activities in 1636. In 1647, the area now called the 'island' was created. The Zaan meandered around it on the west side, along the Hogedijk. The Voorzaan, as we know it today, was not dug until 1883.
In 1608, thirteen shipyards formed a shipping company to build an overtoom specifically for unfinished sea-going vessels that were too large to sail through the then covered Hondsbossche Sluis. It went into business in 1609.
Ship hulls larger than 82 feet and/or 16 feet and five inches wide (the dimensions of the lock chamber) were towed to an underwater ramp of transverse beams, where the bend in the Damstraat in front of the entertainment center of Zaandam is now located. From there it went slowly up about 23 meters to the top of the dike. After a flat stretch of about 5 meters, the ramp made a small angle to the east and then descended quite steeply for 14 meters to the water of the Voorzaan. Just behind where the Zaans Theater now stands and the Hogedijk bends to the south.
The space between the houses on the dike was just over seven meters. This tightness and the fact that the ship had to make a small angle on top of the dike limited the maximum length of the ships on the overtoom to approximately 124 feet. To allow the ships to make that small angle, a longitudinal beam was fitted to the transverse slope beams to force the keel around the corner on the greased flat piece. Like a train being pushed aside over a switch.
There were 'pivots' on either side of the ramp. They had wind poles (spokes) of five meters in length and were turned by 24 to 30 men per spindle, depending on the weight of the ship. The pulling power of the tow ropes was increased by using two pulleys with three rollers per rope. With this combination of muscle power and modern mechanics, ships of up to 300 tons were pulled up about 2.5 meters to the highest point of the overfly. Downhill, after the bend, it was of course a little easier, but it was not without risks. A long cable was attached to the bow to pull the ship to portside as soon as it was in the water. Straight ahead was a shipyard on the Hogedijk.
If the rise took too much time, the necessary high tide had sometimes already fallen and the ship became stuck in the mud of the very shallow Voorzaan. That was also the fate of the ship that was pulled over when Czar Peter and his entourage came to watch in 1717. They had waited so long for the Czar to finish his extensive lunch that the tide had gone out.
De overtoom was a great success in the first decades after its opening and attracted a lot of attention. Sometimes three ships crossed the dike a day. Apparently the extra costs were not a commercial disadvantage for the position of shipbuilding on the Binnenzaan. But the ships became increasingly larger and the number of yards on the Binnenzaan gradually decreased. At the end of the century the frequency dropped to three per month and from 1700 until March 19, 1718, when the last ship passed the overtoom, the number had dropped to five per year. In 1719 the overtoom was demolished and the dike was repaired and paved. Now there is nothing left in Zaandam that reminds us of what was once the greatest overtoom in the world, not even a street name.
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