Wednesday, December 12, 2012


First, a little aside - we signed the contract today to build N6062.  Decoding that number, it means we are building a Nordhavn N60, hull number 62.  And now, back to hydraulics....

After having active fin stabilizers in our Grand Banks, I'll never consider a boat without them.  In fact, I don't know anyone who has had them who doesn't feel the same way.  N6062 will be no exception, and once again we are going with ABT/Trac.  Fortunately, ABT is the standard option for Nordhavns, so it was an easy choice.  I've had nothing but excellent experience with ABT and wouldn't consider anything else.

But this time, we are going one step further and including Stabilization at Rest, or STAR.  Historically, fin stabilizers only work when the boat is moving.  The fins work against the flowing water to counteract roll and hold the boat steady.  The slower the boat is moving, the less effective the fins are.  Once the boat comes to a stop, the stabilizers lose their effectiveness.  At anchor, they do nothing.  If you are in a rolling bay, you are in a rolling boat.  This situation is where gyro stabilizer manufacturers tout the advantage of a gyro - they help stabilize when the boat isn't moving.

However, a few years ago ABT got clever and figured out how to make the fins doggie-paddle while the boat is at rest in such a way to counteract roll.  Pretty cool, eh?  From owners I've talked to, it works pretty well, and we've decided to include it as an add-on to our stabilizers. Most of the equipment is already there with the basic stabilizers.  To add STAR, you need some extra brains in the control unit and some extra hydraulic controls.  And perhaps the biggest - you need hydraulic power while at anchor - which brings us back around to the hydraulic system as a whole...

Looking back at the basic stabilizers, they need to be powered, and the sensible way to power them is via a pump connected to the main engine.  Anytime you are underway the engine is running, so it's  a sensible place to get the hydraulic power.  But what if the engine fails and you have to run off the wing engine?  It sure would be nice to still have stabilizers, right?  From what I've seen, the most common solution is to put a second pump on the wing engine.  For the most part, it's a sensible solution, but there are a  few complications.  First, running the hydraulics takes power, and that's power that's not available to propel the boat.  Wing engines can only move the boat 5 kts or so, and robbing power for hydraulics will only slow things down more.  Often the solution is to up-size the wing engine, which is certainly an approach.  Remembering back to the previous post, the other gotcha with a wing engine is that it only has a puny alternator on it - about 40 amps - which isn't enough to run the electric power on the boat.  Remember, the main engine has a 175 amp alternator for house power.  If you are going to be on the wing for more than a short time, you need to run the generator for electric power.  With the generator running, it becomes another candidate power source for backup hydraulic power should the main engine fail.  With that, let's go back to hydraulic power for the STAR system.

Sitting at anchor, you will have occasion to run the generator, but no reason to run the wing engine.  And the generator is quieter with it's sound shield.  Putting all this together, we've decided to put the second hydraulic pump on the generator, not the wing.  This will give us both electric power and STAR when anchored.  Now keep in mind that we don't expect to run the STAR all the time we are anchored, but rather only when conditions demand.  But there is a price to running the hydraulics off the generator.  Just as it draws power from the wing engine, it draws power from the generator engine resulting in a de-rating of the AC power available.  Instead of 20KW we will only have about 12KW available to us.  For the most part that won't be a problem, but under some circumstances it will require management of the larger electric loads like AC, the oven, and dryer.

That covers stabilization, both underway and at rest, and power to keep it all going in a sensible way.  But on bigger boats, there are several other candidate devices for hydraulic power.  These are the bow thruster, stern thruster, and windlass.  The big advantages to hydraulic power for these devices is two fold;

  1. More power.  Hydraulic thrusters are 20-25hp where electrics are 15hp.
  2. 100% duty cycle.  Electric thrusters work well, but if you run them for more than a few minutes, they overheat and shut down.
This last point is the big concern.  If you are using your electric thrusters so much that they overheat, it's because you need them.  So by definition, when you need them most, they are most likely to thermal-out on you.

Given this, you would think hydraulic thrusters would be a no-brainer, but guess what?  They cost a small fortune - probably 2-3x the cost of electric thrusters.  After some initial back and forth, and to contain costs, I decided on electric thrusters and windlass.  My current and previous boats both have electric thrusters, and I've never timed them out.  In fact, I use then so little that they accumulate growth and by the end of the season up here in New England, they are nearly inoperative because of the crud on them.  This thinking lasted a few weeks, until I talked to a couple of owners who convinced me otherwise. One described a couple of situations where they really needed the thrusters, and sure enough, they thermaled-out.  The other consideration is that some things on a boat can be easily changed, added, or upgraded in the future, and some things can only be decided at build time.  Hydraulic thrusters fall more into the later category than the former, and is what finally tipped the scales in favor of a full hydraulic package.  In addition to the thrusters and windlass, the system also includes an anchor washdown and a high-capacity crash pump.  It's a painful hit financially, but I don't think I'll regret it.

With this last addition, it's worth looking back one more time at how the whole package is powered.  Thrusters use a LOT of hydraulic power.  As a first step, the pump on the main engine needs to be increased from 28cc to 75cc.  That's almost 3x the fluid flow.  But when you are operating thrusters, the engine will typically be at idle, yielding minimal flow from the pump.  That's one of the reasons the displacement needs to be increased so much, but it's still not enough.  Just the main engine can deliver enough power to run either thruster at full power, but not both at the same time.  To get the extra power, we will once again be turning to our generator, which will also get a more modest pump upgrade from 28cc to 45cc.  As a side note, this is where many boats choose to draw the extra hydraulic power from the wing engine.  When entering or leaving port, you fire up the wing and rev it up to 2000 RPM or so, and it provides the extra hydraulic power.  It's also a great way to ensure the wing gets regular exercise.  But given the need to power the STAR system, coupled with the need to run the generator anyway to supplement electric power when using the wing, we think the generator is the more sensible source for supplemental hydraulic power.

Another consideration in selecting electric or hydraulic thrusters and windlass, is fault resilience.  There is a good argument in favor or electric devices.  Each thruster and the windlass are separate devices, and a failure in one won't impact the other.  With hydraulics, if you lose hydraulic power, it takes out several important devices.  I spent a fair amount of time looking over the hydraulic schematics and discussing them with ABT, and ultimately concluded that it's a manageable issue.  In a later post I'll go through the whole failure analysis that I did on the boat, but focusing on the hydraulics, there are a few key points.  First, there are valves that segment off parts of the system.  This allows for containment of failures in a variety of ways.  Second, there are redundant sources of hydraulic power, so a pump failure is not the end of the world.  Third, the main hydraulic loop where a failure would take out everything, is entirely within the engine room and laz.  This means it's accessible for repairs.  My plan right now is to identify the hoses that would kill the whole system if they were to rupture, and carry spares.

So that's one more piece of the puzzle sorted out.


  1. Hi Peter,

    Thank you for a very informative and well-written blog! As an aspiring boat geek, your Nordhavn design and commissioning process is fascinating.

    With regard to at-anchor stabilisation and given your preference to be generator-free at anchor, I'm surprised you don't seem to have considered flopper-stoppers?

  2. All of us have various fears - sometimes rational and sometimes irrational. When you talk to someone about which failures they have protected themselves against, and which they have hand-waved away as unlikely, you usually catch a glimpse into their fears.

    I've got a thing about large unsecured objects on a moving boat. Now take a large object and hang it from a cable, and I really start to freak out.

    Yes, that scene from The Perfect Storm is Hollywood drama, but I have also heard first hand accounts of birds coming out of the water in heavy seas and hitting the boat.

    Birds strike me as a accident waiting to happen. And not even as dramatic an accident at George Cluney dodged. All it takes is dropping one of those things on a toe while hauling it around the boat, and you have a potential problem. Now place yourself in a remote area with a crew of two (that's us). If someone gets hurt it can be a real problem.

    Now lots of people use paravanes (stabilization while underway) and flopper-stoppers (stabilization at anchor) with great success, so I'm happy to classify my feelings as irrational fear, but I'm still clinging to it. The "price" of running a generator is worth it to me.

  3. I feel slightly guilty about asking this question :). I can quite see your fear of paravanes while under way and presume that, in case of a stabiliser malfunction, a Nordhavn wouldn't need stabilisation for anything other than comfort reasons.

    I understand the advantages of flopper-stoppers are twofold: passive when deployed, as well as more effective than fin stabilisers courtesy of the extra distance between the flopper-stoppers, the booms add when swung out. Apparently only one will already be enough in many circumstances.

    Perhaps of academic interest: this vessel seems to have the most bullet-proof flopper-stopper arrangement:

    FPB 78

    Still, some steel-toe boots for those owners... :)

  4. No reason to feel guilty. It's good question.

    You are correct that stabilization is all about comfort, not about keeping the boat afloat. I'm not aware of any boat that needs stabilization to make the boat seaworthy - just to make it more comfortable.

    These tradeoffs are rarely clear, hence so many different approaches, and so many opinions and debates.

    When anchoring out, I'd say around 1 out of 20 nights are uncomfortably rolly and would justify some form of stabilization. So to me, running a generator 1 out of 20 nights is just fine. I already run it more frequently than that for air conditioning in hot weather.

    Flopper stopper advocates would use a similar argument. If it takes some work (15-20 minutes seems to be the magic number) to deploy flopper stoppers 1 out of 20 nights, it's no big deal. I can't argue against that other than to say I'm lazy and pushing a few buttons is easier than deploying fins.

    Where I really think the difference comes into play is when it's rough and rolly, i.e. exactly when you would want flopper stoppers. So by definition, you will be deploying under adverse conditions. And worse yet, what happens if deteriorate such that you decide you need to move? Now, as you bring in the fins, conditions on board go from bad to worse - all while you are trying to handle heavy,lose equipment. I just want nothing to do with it. With stabilization at rest, you just raise your anchor and go, which by the way can be a real challenge itself in rough conditions for all the same reasons as retrieving fins. Then you just motor away and the system transitions from at rest to under way stabilization.


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