Santo Antonio de Tanna (1697)
Country: Kenya
Place: Mombasa, off Fort Jesus
Type: Frigate
Identified: Yes
Dated: 1697 (historical accounts)
Depth: 13 to 16 m
Excavated: 1976-80 campaigns, directed by Robin Piercy
Uncovered hull: About 30 m long and 10 m wide
Collection: Fort Jesus Museum, Mombasa
Introduction
Santo Antonio de Tanna (spelled Santo António de Tanná by Green et al. 2024) is one of the few Portuguese wrecks of the India trade to have been excavated by archaeologists. It lies in relatively shallow water under the walls of Fort Jesus, and this location led to its early identification.
The excavation was recorded with tapes and film in poor visibility. Almost 50 years later, those records were reprocessed at Curtin University into a digital 3D model of the interior of the hull. This fiche summarizes that work, as published by Jeremy Green, Daniel Adams and Andrew Woods (2024).
History of the shipwreck
In 1697 the Portuguese Viceroy in Goa sent the frigate to relieve Fort Jesus, which Omani forces had besieged for two years. When it anchored alongside the fort in September, the captain learned that all the Portuguese inside were dead. The defense had been left to about eight Swahili men and 50 women.
The fort was relieved and supplies were unloaded under fire from the Omani batteries. In November the ship stranded on the shoreline reef next to the fort, capsized as the tide fell, and sank.
The site
James Kirkman reported the wreck in 1972 and Robin Piercy surveyed it in 1976. Excavation followed over the next four years under the National Museums of Kenya and the Institute of Nautical Archaeology, with the Department of Maritime Archaeology of the Western Australian Museum assisting the survey in the first three seasons.
The ship lies on the slope of the channel that passes Fort Jesus, with the bow pointing seaward. The bow is inclined about 20° down the slope and the hull leans about 54° to port. A keelson runs the length of the site. About 7 m of the port side and about 2 m of the starboard side survive along the length of the ship. Ceiling planking did not survive on the starboard side, so only the frames could be recorded there.
Mombasa Harbour has a tidal range of up to 4 m, and the moving sediments keep visibility poor on most days. Clear oceanic water reached the site only around high-water spring tides, when visibility could reach 10 m or more for a few hours.
Recording the hull, 1977-79
The stern was excavated in 1977 and 1978 and the bow was recorded in 1979. The site was backfilled after each season, so the whole hull was never exposed at one time. Five techniques were used:
- Widths of the internal planking and timbers, measured across the hull at each control point.
- Cross-sections of the hull at the same intervals, first with a large circular protractor clamped to the keelson and then with a T-shaped frame that gave better angular accuracy.
- 3D trilateration from the keelson, and from the top of a 2 m pole clamped to it, to control points at the port and starboard extremities of the site.
- A 3D measurement of the curvature of the keelson, using a wire survey line and a plumb bob.
- Systematic stereo photography from a neutrally buoyant steel tower with a 2 m square base, carrying two 35 mm Nikonos cameras.
Control points were 10 cm squares of white plastic fixed along the keelson, every 2 m from the mast step to the stern and every 1 m from the mast step to the bow. The photographic record comprises 832 images: 274 from 1977, 278 from 1978 and 280 from 1979. These were later scanned at 4000 dpi.
Reprocessing the legacy data
Early results included a photomosaic of the bow glued up by hand, in which the keelson shows a considerable false curvature, a later Photoshop mosaic, and 3D coordinates for 87 control points calculated in Site Recorder. PhotoModeler proved unsuitable because the photographs were largely vertical.
Digital 3D models were developed over about seven years:
| Model | Year | Result |
|---|---|---|
| 1 | 2014 | Trial model of part of the bow; dark and with little detail. |
| 2 | 2018 | Better model of the bow by Lachlan Shaw, with a digital elevation model. The lower-quality stern photographs would not align. |
| 3 | 2019 | Bow model from 31 high-level stereo photographs taken in 1979 without the tower; easier to process but less detailed than Model 2. |
| 4 | 2021 | First complete model of the site, by Daniel Adams, using contrast-enhanced images. Its scale proved wrong: distances differed from the 1970s tapes by -21% to 10% on the port side and -24% to 1% on the starboard side. |
| 5 | 2021-22 | Added photographs and original field measurements, but sections of the model remained hard to align where overlap was poor. |
| 6 | 2022 | Model built on the fixed coordinates of the corrected keelson control points. |
Model 6 rests on a reassessment of the original notes, which revealed transcription errors. Once corrected, the keelson showed a vertical curvature of about 500 mm and a horizontal curvature of about 200 mm, some of it probably caused by the collapse of the hull. Its control points span about 26 m. A new Site Recorder run with these points gave coordinates for 42 port and starboard control points, rejecting 28 of 180 tape measurements, with an overall RMS residual of 0.35 m.
How accurate is the model?
At short range the model performs well. Nine distances measured on a rectified photograph of the mast step, which includes a 1 m grid frame, were compared with the model. Most agreed within about 10 mm, and the largest difference was 30 mm (15%).
Longer distances were less reliable. Eleven keelson-to-side distances in the bow section differed from the 1970s tape measurements by up to 34 cm, probably because of gaps in the photographic coverage. Neither record can be treated as the true value: tapes bow and snag, divers misread and mistranscribe, and some control points moved between seasons. The software is also a closed system and can give slightly different results from the same photographs.
Conclusion
The authors expect this work to be the starting point for a re-examination of the hull. The model should allow new cross-sections to be compared with the original hull profiles, and further searches of the archive may fill the gaps in coverage. The artifact collection is in the Fort Jesus Museum, with the photographic data at the Institute of Nautical Archaeology and the Western Australian Museum, and copies in Mombasa.
3D models
Santo Antonio de Tanna 3D models, Curtin University HIVE
Sources
Green, J. N., 1978. "Hull recording methods used at Mombasa," International Journal of Nautical Archaeology 7.4: 311-314.
Green, J. N., 2019. "Legacy data in 3D: The Cape Andreas survey (1969-1970) and Santo António de Tanná expeditions (1978-1979)," in J. McCarthy, J. Benjamin, T. Winton and W. van Duivenvoorde (eds.), 3D recording and interpretation for maritime archaeology. Coastal Research Library, Vol. 31. Springer.
Green, J. N., Adams, D., and Woods, A., 2024. "The Use of Modern Technology to Obtain New Information from the 1970s' Excavation of the Portuguese Frigate Santo António de Tanná, Wrecked in Mombasa in 1697," International Journal of Nautical Archaeology 53.1: 53-73. https://doi.org/10.1080/10572414.2023.2245913
Green, J. N., Shaw, L., Hollick, J., Woods, A., Helmholtz, P., and Belton, D., 2020. "Working with legacy data: Santo António de Tanná," in J. N. Green and A. Paterson (eds.), Shipwrecks of the Roaring Forties: Researching some of Australia's earliest shipwrecks, pp. 239-245. University of Western Australia Press.
Kirkman, J. S., 1972. "A Portuguese wreck off Mombasa, Kenya," International Journal of Nautical Archaeology 1.1: 153-157.
Mousa, Y. A.-k., 2020. Underwater measurements and adjustment of the Santo Antonio de Tanna. Report, Department of Maritime Archaeology, Western Australian Maritime Museum, No. 337.
Piercy, R. C. M., 1976. "The Mombasa shipwreck," Institute for Nautical Archaeology Newsletter 3.3: 1-5.
Piercy, R. C. M., 1977. "Mombasa wreck excavation: Preliminary report, 1977," International Journal of Nautical Archaeology 6.4: 331-347.
Piercy, R. C. M., 1978. "Mombasa wreck excavation. Second preliminary report, 1978," International Journal of Nautical Archaeology 7.4: 301-319.
Piercy, R. C. M., 1979. "Mombasa wreck excavation. Third preliminary report, 1979," International Journal of Nautical Archaeology 8.4: 303-309.
Piercy, R. C. M., 1981. "Mombasa wreck excavation. Fourth preliminary report, 1980," International Journal of Nautical Archaeology 10.2: 109-118.
Shaw, L., 2018. The Photogrammetric Analysis of the Santo Antonio de Tanna: A research project supported by the Curtin HIVE Summer Internship Program (2017-18). Report, Department of Maritime Archaeology, Western Australian Maritime Museum, No. 326.