I just re-documented our WS42 and the NVDC came up with a different Net Tonnage than was originally posted on our hull when initially documented. Using what I believed was the correct model to calculate GRT and NRT produced the same NRT as posted on our hull: 23 Net Tons. However, the NVDC came up with 17 Net Tons. I have included a table that I hope is both readable and helpful.
Good news is that our boat is now documented, but our placque is not completely correct, as it has the wrong net tons. So, what to do? Well, most importantly, the documentation number is what is important. The USCG has told me that they will not fine, imprison, or keel-haul me for having an inconsistent net tonnage number. Based on the what the USCG told me, if I am worried about the placque, I should see a dentist!
However, the different results for NRT got me to wondering why.
Gross Register Tons (GRT) is a volume measurment that combines hull volume and deckhouse volume. For sailing vessels, deckhouse volume is almost universally excluded because deckhouse volume must be equal to or greater than hull volume to be included. Most sailing vessels don't have that substantial a superstructure. Net Register Tons (NRT) represents GRT adjusted for a machinery factor, which for a sailing auxillary is .90. The GRT and NRT figures are truncated for the documentation form
Simply, the GRT equation is (hull volume + deckhouse volume)/100 and the NRT equation is GRT * machinery factor. For our vessels the detailed GRT is (length * breath * depth to keel bottom * sailboat shape factor of 0.5 * sailboat keel factor of 0.75) divided by 100 and truncated. The detailed NRT equation is GRT * machinery factor of 0.9 and truncated. I added the table below:
| Gross Tonnage to Net Tonnage Calculation: |
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Westsail 42 Ketch - S/V Harmony |
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Overall Length |
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* |
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42.83 |
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42' 10" |
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Overall Beam |
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* |
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13.00 |
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13' 0" |
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Overall Depth |
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* |
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9.25 |
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9' 3" |
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Sub-total |
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= |
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5150.31 |
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(L*B*D) |
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Shape Factor |
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* |
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0.50 |
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Shape Factor for Sailing Vessel |
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Keel Factor |
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* |
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0.75 |
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Keel Factor for sailing vessel with integral keel |
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Divide by |
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/ |
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100.00 |
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| Gross Tonnage |
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= |
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19 |
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Machinery Factor |
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* |
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0.90 |
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Machinery Factor for auxillary Sailboat |
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| Net Tonnage |
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= |
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17 |
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| All values on Certificate of Documentation are truncated to the nearest integer |
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What my calculation method was missing was an adjustment for "keel factor" which is .75. However, I do believe that when our WS42 was originally documented the deckhouse volume may have been included. Possible because the NVDC forms were not as refined as they are today.
I hope this is helpful.
Doug