Author: mtashby

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Sizing the Bow Thruster

In preparation for installing a bow thruster during the next haul-out, I need to estimate the needed capacity. The box thruster must be capable of countering wind. The force applied by the wind onto the boat is determined by the factors including the wind speed, angle of wind attack, and lateral wind draft area of the boat. The wind pressure is given by the formula:

P = 1/2 × ρ × V² (lbf/sq.ft), where:

ρ (rho) represents the specific mass of air = 0.0752 lb/ft²

V is the velocity of the air in ft/s, where 1 knot = 1.688 ft/s

The wind-draft of the boat can be determined by multiplying the wind pressure by the wind draft area. The wind draft area is determined by the shape and the dimensions of the of the boat (its windage) and the wind angle, and the greatest wind-draft is created if the wind is at 90 degrees to the boat. An efficiency reduction factor is generally applied to account for windage, and a value of 0.75 is frequently used. To calculate the required torque to rotate a boat, we need to know the wind pressure and the length of the boat.

The Aegir-Ran has a LOD = 36 ft and the freeboard is about 3 ft, giving wind draft of 36 x 3 = 108 sq ft. Adding the cabin, hard dodger, and the sail pack adds perhaps 50 sq ft, which is why the efficiency reduction factor is often used. I will assume the lateral draft is 200 sq ft and I will not use an efficiency factor. Assuming we want to counteract a wind force of 25 knots, the wind pressure is:

P = 1/2 × (0.0752 lb/ft²) × [25 knots x 1.688 ft/(s x knot)] = 1.59 lbf/sq.ft

To counteract this wind pressure, we sill need the following amount of torque:

T = 1.59 lbf/sq.ft x 200 sq/ft x 36 ft/2 = 5,724 ft.lbs.

The thrust-force required is the torque divided divided by the distance from the bow thruster to the pivot point. In our case, we will assume we want to pivot about the stern, do the distance is essentially the LWL = 33 ft:

F = 5,724 ft.lbs. / 33 ft. = 173 lbf.

I have decided to install a 12 V Ventus bow thruster. Examining the specifications of the models, it seems the BOW7512D is the best model with a maximum thrust of 180 lbf.

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Hike to Elefante de Trompa Grande

While anchored in Bahia San Padro and waiting for a weather window to head south, I noticed a rock arch in the hills above the bay (circled in red).

Asking the locals, I am told it is called “Big Trunk Elephant” (“Elefante de Trompa Grande”). It took about two hours to hike to the arch, first through washes to the base and then bushwhacking to the arch itself. Once there, I discovered the rock to be loose and dangerous to climb. Also, it appeared that rock climbing would be needed to actually get to the hole. Perhaps the backside would be more accessible.

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Stress Fracturing of Block Nut

Sailing from Bahia Algodones to Bahia San Padro, the starboard jib block failed. It’s nut suffered unseen crevice corrosion, causing the pin to pop out and the sheave to be slingshotted across the cockpit. The nut that failed was split in four places! Chloride ions can cause localized corrosive attack (pitting and crevice corrosion) of stainless steels.

Fortunately, the winds were light at the time , all of the parts were recovered, and a replacement nut was on-board.

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inReach Activated and MapShare Page Created

A Garmin inReach GPSMAP 86i has been activated. The “safety” plan was selected, which is the least expensive subscription. The plan allows unlimited SOS messages, ten text messages/month, and unlimited check-in messages. The check-in messages are sent to selected e-mail addresses, the Facebook page, and the Instagram page.

The check-in also updates the MapShare page.

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Topping Lift Upgraded

Since the solar panels were installed in the hard dodger, I have been concerned that the topping lift was not beefy enough. Quarter inch double braid seemed to be insufficient and it was not possible to increase the diameter without significant modification of the boom. Also, the single block did not deliver sufficient leverage to lift the boom with the extra weight of the sail pack when the mainsail is lowered. The boom vang struggles with the extra weight as well. To deal with these issues, I replaced the double double braid with block with Dyneema and the single with two blocks, a Schaefer 3 Series Single Block Stainless Steel Cheeks with Becket and Swivel Shackle and a Schaefer 3 Series Single Stainless Block with Front-Side Shackle. A Brummel Lock Eye-Splice was used to attach the Dyneema to the becket.

A Spinlock XAS single clutch was mounted on the boom the finish the upgrade:

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EPIRB Registered and Mounted

I have been focusing safety equipment this week (ditch bag, InReach, AIS, etc.). Today I registered an Emergency Position-Indicating Radio Beacon. The beacon was registered with US National Oceanic and Atmospheric Administration (NOAA). Since a Category 1 EPIRB is being used (which means it is capable of automatically deploying, it was mounted out-of-the-way on the deck.

Category I EPIRBs are housed in a special bracket equipped with a hydrostatic release. This mechanism releases the EPIRB at a water depth of 3-10 feet. The buoyant EPIRB then floats to the surface and begins transmitting.

Upon deployment, the EPIRB’s built-in GPS will fix your position to within a few meters and then utilizes a powerful 406 MHz signal to relay a distress call to orbiting satellites. When triggered, the EPIRB broadcasts a unique registered distress signal (that includes the vessel’s MMSI) that not only tells rescuers where you are, but who you are.

An additional feature of the EPIRB is a 121.5 MHz secondary homing transmitter which means that once Search and Rescue teams have been deployed, they are able to home in on your exact location. The rescue is also assisted by a a LED strobe built into the EPIRB.

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Walker Bay 8 Dingy Replaced with AB 8UL RIB

When I purchased the boat, it can with a hard-chined rowing boat for a tender, a Walker Bay 8, which was poured by a Yamaha 2.5 hp engine. While the Walker Bay was perfect for the Sea of Cortez, it was inappropriate for where I plan to go. I needed a dingy that could carry provisions, that I could beach in a surf, and that I could dive off of. The new dingy had to fix on the foredeck (9′ or less) and had to be light enough to launch single-handed. I also wanted a RIB (that could plane, and thus be relatively fast. I settled on an AB 8UL, which is constructed of Hypalon and has an unfinished aluminum hull. I saved $800 in taxes and $800 in shipping (and perhaps a month) by driving to Oxnard California to pick it up. I sold the Walker Bay to a local for $150 today and the RIB is now in its place. I need to sew chaps for the new dingy and a cover to store it on the foredeck.

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Sewed Case For Folding Bicycle

I have been struggling with ideas for storing my folding Dahon Mariner D8 bicycle. Eventually, I decided it is best to store it below deck (to avoid corrosion and thievery), but I wanted something to protect it and its surroundings. Most generic folding bicycle cases as simply too big and bulky, so I sewed my own. The case consists of two layers of Sunbrella material in-between which is quilted 1/4″ closed-cell foam. Most of the case was sewn with a Sailrite LSZ-1, but one of the sides and the zippers has to be stitched on by hand using a Speedy Stitcher.

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Cover For Tiller Pilot

I am preparing to install a Simrad TP32 Tiller Pilot. I have read that the lifetime of the autopilot can be expanded markedly by keeping it covered, so I sewed a cover. It has a piece of vinyl that allows access to the control buttons and it is held on with a piece of velcro.