stadiametric rangefinding

ác ống nhòm quân sự hay hàng hải chuyên dụng thường có 1 thang đo hiển thị trên khung nhìn, dùng để ước lượng khoảng cách. Mỗi nấc thang đo có đơn vị là mil (mili-radian – 1/1000 radian). Giả sử như biết trước chiều cao của đối tượng đang quan sát (e.g: 1 ngọn núi) là x, thì khoảng cách tới đó: y = x / k * 1000, với k là giá trị đọc được trên thang đo.

Đây chỉ là công thức tính nhẩm gần đúng, vì công thức tính toán chính xác có liên quan một chút đến cosine, nhưng cos của góc rất nhỏ xấp xỉ bằng 1. Ví dụ như, ta biết trước chiều cao của đỉnh núi Thánh Giá – Côn Đảo là 570 m, nếu như trong ống nhòm quan sát thấy núi có độ cao là 20 nấc, thì khoảng cách đến đảo xấp xỉ là: 570 / 20 * 1000 = 28.5 km.

somewhere out there

ồi trẻ, có 1 trò hay chơi là trồng chuối hít đất (chống đẩy). Trò này tôi rất là “tự hào” vì trồng chuối nhiều người làm được, nhưng trồng chuối hít đất: hạ xuống từ từ cho đỉnh đầu chạm đất, rồi lại nâng thẳng tay lên, cứ như thế 15, 20 lần, rất ít người làm được vì siêu khó, cần phải có cơ tay và vai rất khoẻ, không tin cứ thử làm xem, ngã gãy cổ đừng trách!

Bây giờ, sau 20 năm, không tài nào làm được như thế nữa (các bạn hẳn đã rất chán tôi luôn kể chuyện… ngày xưa, bao giờ cho đến ngày xưa!?) Chỉ một bức hình thoáng qua là bao nhiêu kỷ niệm, bao nhiêu trò đùa vui, xông xênh của tuổi trẻ hiện về! Ở một nơi nào đấy ngoài kia, “somewhere out there”, vẫn có một chiếc thuyền neo đợi ta bên bờ biển!

chiều về trên sông – 1

Chiều về trên sông - Thái Thanh 
Chiều về trên sông - Quỳnh Giao 

Chiều buông trên dòng sông cuốn mau, thương đời thương lẫn nhau trong chiều. Về đây bọt bèo muôn khắp nơi, vui buồn cho có đôi không nhiều…

ách đã vài tháng, có dịp thử cái “máng heo” – thuyền buồm nhỏ Optimist này trên một đoạn sông Sài Gòn gần An Phú Đông, quận 9. Một chiều gió rất nhẹ, vừa đủ để chiếc xuồng nhẹ nhàng chậm rãi lướt đi…

river sailboats

ail boats in the 60s, cut from a Vietnamese documentary film. Bamboo mast, sail of a very simplistic design, diagonally mounted rectangular – shape canvas, possibly river – only, transportation, sail and oar boats.

serene – 3, part 28

took the kayak out for several more paddling around my area, 12 ~ 24 km, and also took a few video shots, mostly from second – person positions. I want to observe the whole boat under action, its motions and its wakes, etc… I also want to see how the rudder works, to check if the hull need some trimming, as well as to observe my own paddling movements. Actually, I’m very lazy for video capturing (and editing), they’re time consuming jobs, I’d better concentrate just on paddling.

I made a new seat from plywood, of a very simple design, composed of 3 pieces of ply bended into curved forms around a frame made from MDF. This batch of plywood is really good, no cracks, no breakages even though tortured to extreme shapes. The seat is bolted onto two “hip braces”, connecting the cockpit coaming and the floor, providing a comfortable seat, and together with the rudder pedals, offer a very tight fit for my lower body, required to control the boat in turbulences.

The overall evaluation for Serene – 3 is very satisfying, the boat behaves very well, and very predictable under different conditions. Its stabilities enable me to lean the boat to extreme angles without falling, and I really enjoy its agility, which reacts instantly to my different paddle strokes. The hatches are completely waterproof, the rudder has (more than) enough steering power, and the electric – bilge pump system works just fine. More trialling ahead on this Lunar new year holiday.

serene – 3, part 27

etween the trials, I managed to complete a couple of things. First is to mount the compass, the tried – and – true type of compass used in all boats of the Serene series, this without an internal lighting. Then, I built another paddle, a simple one with rattan shaft, and plywood blades, a little bit bigger, weights at 1.1 kg, slightly heavier than the Greenland companion – my “storm paddle”, 0.9 kg. Still considered an Euro – type paddle, but with a smaller blade to resemble Greenland style.

As I sometimes need larger power faces to propel the boat under unfavorable conditions. The rudder control lines are modified to be more easily adjustable, see the second image below. The pedals are attached to small segment of chains, to adjust the tension, you simple move the shackle to another chains’ eye, this could be quickly done while you’re in the field. I also replaced the bungee cords holding the pedals back by 2 large steel springs, providing better “suspension”.

Just release one pedal and the springs would pull the rudder blade back to its neutral position. I added 4 tiny 3.7V LED bulbs (series wiring) to the electric box, in an attempt to indicate the batteries’ remaining capacity (as there’s no easy way to do so). This is a trick I learnt from experiences, when 1 or 2 of the LED bulbs won’t turn on, or their brightness is drastically reduced, you’d need to plug in the solar panel then! 😀 Not all works has been completed, but it’s time to clean up the workshop a bit!

serene – 3, part 26

aunch the boat today, first day of the year 2018 😀! Took the kayak out to water on a sunny, breezy afternoon for a 12 km paddling, the temperature was around 25 ~ 27 Celsius, considered “cool” with this hot – all – around – the – year tropical climate. The short paddling is just to get some first impressions, to check if everything works, and how it feels, the new hull shape, something radically different from my previous boats. Yet all confirms things I’d known from the design phase.

The kayak is nimble and very predictable, probably not too agile compared to Serene – 1 and Serene – 2, it’s also a bit more stable across all its axis. It’s easier to get in and out of the cockpit with these stabilities. The rudder works nicely, changing course instantly with just some slight pedal kicks, though I’ve come up with some new ideas to improve the “fine – tuning mechanism” for the rudder control lines’ tension (the cleats used are fine, but not too convenient for adjusting the tension).

The rudder pedals offer very good seating position, together with the plastic seat (used for testing), fixing my lower body well to the hull. I would decide later whether to just use this plastic seat, or build another lower one to facilitate reentry actions. The hatches are absolutely water tight, an important criterion! The overall feeling is very satisfactory, yet more trials are needed, in different conditions, especially on rougher water, to really understand the capabilities of this new boat.

serene – 3, part 25

lmost ready for the water, but the tide is unfavorable for the weekend (its heights are at night), and the Tembin cyclone is crossing the East sea, threatening the area. So I take the time to complete a couple more things. Attaching the electric box through cable gland is quite straight forward, everything works out right, in essence the bilge pump and reed switch. I added one master switch to the electric circuit, in order to turn everything off and not to drain the 18650 batteries to exhaustion.

One critically important issue with the 18650 batteries is that, never used them to exhaustion, when the voltage drops below 1 ~ 1.5 V or so, the battery just becomes dead, you simple could just throw them away, a costly lesson the last time I hadn’t paddled for some weeks, and let the batteries run until dead 😢. Next, I “refurbish” one of my old paddles to match the new boat style, turning it to black, and make it a bit stronger (and slightly heavier) with an extra layer of fiberglass.

The paddle was specially built from rattan and balsa wood, and is light enough already at 0.8 kg. I would try to use this paddle first, before deciding if it’s necessary to make an extra new one, weight and durability are two major concerns. Third image: a “smoke test” to verify if the bilge pump and the solar panel all properly works. Now wait out for a few more days before going to water, just some 10 ~ 12 km of paddling, to see how the kayak behaves, how everything would feel out there.

serene – 3, part 24

askets fabrication: the critical thing in a hatch’s waterproofness is its gasket, and I realized that the best material for gasket is silicone: flexible, durable and offer a very good fit. And for a custom size and shape hatch, somehow I’ll need to make the gaskets too. It turned out to be pretty much easy, using the 2 – parts silicone available here on local market. You mix up component A to component B (catalyst, around 1 ~ 4 % by weight) and pour it into a mold, wait for about 2 hours, and it’s done.

1st image: the mold (cut from plywood) is filled with silicone, I use the “Silicone” (red bottle) spray as a mold release, it’s “strange” that silicone is used as a release agent also for silicone. 2nd image: the product, the O – ring released from the mold. 3rd image: the gaskets inside the hatch lids. The gaskets are 15 mm wide, 5 mm thick, and are pressed down to the hatch rings with those belt locks. I hope I would end up with excellent waterproofness, but let wait until “sea trial” to know better.

I reuse the bilge pump from my previous boat, it still sees very little use, the installation is quite straight forward using some cable ties fastening the bilge pump into its mount. The flexible water pipe leads to a hose right on the right side behind the cockpit, the hose is protected by a screw – in cap, to prevent water from leaking back into the boat when waves wash over the low aft deck. The bilge pump really offers lots of convenience and confidence when you are out there, in the roughness.