This is a follow-up to the post Sizing an anchor windlass where we calculated the mass that our anchor windlass must lift given the scope, chain and anchor sizes we are working with. The end result was that we needed to pull up 10,140 lbs.
The following calculations were worked out and sent to me by my Dad. You can catch a picture of him in another earlier post with the smudge on his nose, helping me install hull plates!
Required Power Output
Must lift (and lower) 10,000 pounds at 1 foot per second.
Thus required power output to chain = 10,000 lb-ft/sec.
Convert lb-ft/sec to hp:
From http://www.engineeringtoolbox.com/unit-converter-d_185.html#Power
1 hp (English horse power) = 745.7 W = 0.746 kW = 550 ft lb/s = 2,545 Btu/h = 33.000 ft lb/m = 1.0139 metric horse power ~= 1.0 KVA
Power output = 10140 lb-ft/sec * ( 1 hp/550 ft-lb/sec ) = 18.43 hp
Required Power Input
Assume hydraulic motor is 80% (mechanical and volumetric) efficient. Then the hydraulic motor must be receiving more input power from the hydraulic fluid, some of which is lost as heat in the motor:
Power input = 18.18 hp / .8 = 23.04 hp
Required Pump Power Output
Our system is comprised of a hydraulic pump that generates the pressure in the fluid and the hydraulic motor that generates the work. The hydraulic motor connected to the windlass is in the bow. The hydraulic pump is located at the other end of the ship in the engine room. The hydraulic pump producing the flow also must be delivering at least 23.04 hp. As some energy is lost to heat in the fluid line due to the drop in line pressure between the pump and motor, the pump must produce more than 23.04 hp to make up for line drop.
Required pressure and flow
Required input energy received from hydraulic fluid can be expressed in terms of Q*P where Q = flow (gallons/min), and P = pressure drop (psi) across the motor. Q*P is measured in units of energy.
Convert gal-psi/min to lb-ft/sec:
Hydraulic power delivered from 1 gallon/min at 1 psi
= Q*P = (1 gal/min * 1 psi) * ( 0.1337 Cu.Ft./gal ) * ( 1 min/60 sec ) * (1 lb/Sq.In / psi) * ( 144 Sq.In / Sq.Ft ) = 0.3209 lb-ft/sec
Convert gal-psi/min to hp:
Input Power (hp) = 0.3209 lb-ft/sec * ( 1 hp / 550 ft-lb/sec) = 5.834e-4 hp = (1 / 1714) hp
Thus power (hp) = Q (gal/min) * P (psi) / 1714
For example, to deliver 1 hp to the motor, Q*P = 1714
As calculated above, motor’s input power required is 23.04 hp. Either P or Q can be calculated based on the other. Lets assume P = 2000 psi. Then Q must be:
Q = power (hp) * 1714 / P (psi) = 23.04 * 1714 / 2000 = 19.75 gal/min

So now to find a pump and motor combination that match this flow and horsepower requirement….
Update on Sept 27, 2009:
I found a nice website – Ideal Windlass that has a nice listing of windlasses that might work for the tugboat.





























I am still working my way around the starboard side stern quarter. It is slower going as it gets to be more overhead welding and there are more TC bolts per square foot. I found a TC bolt that had no weld on it at all! Most of these TC bolts have an existing single pass of weld on them already. I was such an amateur welder when I first welded these that I would grade my work a D+ or C-. Going back over the welds, I definitely have got the technique down and they are now A+.
Hi – Today it was raining pretty good all day so I stayed home to work on my upcoming tour with the band 

Sundays are the quietest days at the shipyard as all the regular workers are off. So no sandblasting or forklifts running around. I am getting to the stern on the starboard side of the hull now. And the hull is going to start to slope back so I will start to have to do more overhead welding which is the most strenuous. Its like trying to do needlepoint overhead under a shower of hot sparks. At least this side of the boat is on the east, so its shady most of the day.
I used TC (Tension Control) bolts in lieu of rivets where I have replaced the wrought iron plate with steel plate. Then have a nice rounded head that looks like a button head rivet and they are threaded so I could use a large pneumatic impact hammer to tighten them down. I forgot how many kegs of these 3/4″ TC bolts I bought, but it was quite a few. There are 300 bolts with nuts per keg and I maybe used 5 or 6 kegs… So now I am going over the entire hull welding the bolt heads to the plate so they will never leak. Here is a close up of one of them welded.
I have started this photoblog in the middle of the actual restoration which leaves me in a quandary as how to start and what past pictures to post here. But I really like this one from October 2005 of her back end. All the stern plating has been finished as you can see by all the TC bolts. And the rudder has been redone as well. But the port gunwhales are still getting repaired.









Ring around the rosey
So I have been working my way around the stern – rewelding all the TC bolts. These are the hardest ones as they are virtually overhead. This is where the rudder post goes up through the hull. Only a few left to go in this area and then I can move back forward to the starboard side bow. I am looking forward to those easy ones as my arms, neck and back are very sore from holding the welding rod overhead all day.
Imagine standing on top of a 9 foot high scaffold, blindfolded (you cant see with the welding helmet glass down). Now take both hands and grasp them together so that they are at eye level. Now simulate the slow burning of the welding rod by slowly and exactly moving your hands directly overhead at a slow but constant rate. Move them exactly one foot from your starting position over the next minute. Go too fast and you will drive the tip of the welding rod into the hull and it will stick. Go too slow and you will lose the arc and have to start again. Don’t forget to breathe and don’t fall off the scaffolding! Also don’t forget to visualized the sparks of molten steel at 1370 deg C (or 2500 deg F) raining down on you! And don’t flinch if you get a spark through your glove, shoe or even down your pants! I wont show you those burn marks. You have now just completed welding one half of a TC bolt. Grind off the slag scab over the weld and repeat for the other side of the TC bolt. Then repeat and repeat and repeat, etc….