Get-Home Backup For Offshore Motorboats—Part 2, The Options

Artnautica LRC 58 "Broadsword" fitted with a get-home sail that proved unsuccessful. © Dennis Harjamaa

In Part 1 I revisited the efficient offshore motorboat that I have been writing about for a couple of years and discussed two alternatives to a motorboat with get-home backup before concluding that neither was a solution.

So now let's look at get-home options:

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Drew Frye

As a multihull sailor, I’ve done with both single and twin engine boats. As a sailor, if the engine dies you should still be able to get nearly home, if not right into the slip. But if I discard that possibly for the purposes of this discussion:

Twin engines. You are far more likely to head out with one engine misbehaving, so I had many more failures with twin engines. I never fouled both engines on fishing gear, always just one. Maintenance is double, though somewhat easier because the engines are smaller. Engine cost is more, though not double. Noise is increased, though it is different. Twin engines can steer with the rudders out–I experienced that a few years ago, after striking a submerged log. Docking is easier… unless one engine is out, in which case it is much, much harder (the boat wants to go in circles).

I have an article coming out in Practical Sailor in a month or two on steering with drogues. I originally thought of it in terms of sail, but it is absolutely applicable to a power boat with the rudder jammed or an engine out. Given the lack of sails to provide balance, I wonder if a small steering drogue isn’t even more sensible for a power yacht going far.

Running just one engine. With outboards or feathering props, it is common for catamaran sailors to run on one engine. It’s quieter and fewer parts are turning. The drag of second engine is slight, since it is either raised or feathered. I’ve seen a few studies that suggest that with fixed props the mileage drops, and I believe this. I do wonder, if I had a twin-engine boat intended for long distances, if I would investigate some manner of feathering prop and running on one engine on long passages. Probably not.

Bill Attwood

Hi John
This topic isn’t really one that sparks my interest ? but I thought I might throw a couple of ideas into the ring (or put my dog into the fight?).
1 Any solution requiring an engine, including any or all of the ancilliary items, compounds the problem, a bit like the philosophy of having a spare for everything,
ergo
2 The better solution is sail power. The possibilities for sail power considered above could be expanded to include less common sorts of rig – gaff, gunter, crab-claw, even junk. Requirements, short mast, easily stowed spars, minimal low-tech rig. Windward ability will never be good, but becoming embayed one could deploy the drogue and pray for a windshift.
Yours aye
Bill

Bill Attwood

Hi John
Definitely not a motor sailer! I am a fan of some motor sailers such as the Fisher range, a pity that the 46 is no longer produced, although they are probably more sailer than motor boat. In view of the oft expressed view here, that all boats are compromises, I don’t think this should rule out the concept of sails as a “get you home”. The more real problem may be that a sail solution that works cannot be found. I look forward to your revelation in the next episode. As a by the by, I am 72 and my chum Noel is 82, both of us own Rustlers, and neither have yet reached the stage of looking at motor boats.
?
Yours aye
Bill
ps I hope I haven’t just challenged fate.

Nikolas

Hi John,
On the drawbacks for the twin you state that ‘Exacts at least a 10% fuel-burn penalty when measured against a single’. I think this is an oversimplification and it depends on the actual comparison of a single prop vs twin prop installation on the same boat eg: 150 hp engine turning a 30 in screw coupled to a 3.96 : 1 reduction gearbox or twin 75 hp engine turning twin 30 in screws coupled to same reduction gearboxes. Although my example is still an oversimplification if one uses the empirical Slip method (Crouch) or the Bp-delta method it can be proved that twin installations can serve as more efficient solutions. Twins tend to be less efficient when used to reduce the drought and hence the prop diameter (increasing prop revolutions) of the same boat. Moreover twin props utilize a better water flow as there is no deadwood to disturb flow in front of them.

Nikolas

John,
If both kind of installations are behind protective skegs than the arguement is nullified. Even so two props outward the hull’s centerline are working within ‘cleaner’ less turbulent water. But this was not where I wanted to put emphasis on, but on what kind of results the actual methods of estimating prop efficiency can yield. If you would like I can provide a mathematical proof whose results are used in actual installations.

Nikolas

John,
This is a part of a study I am reproducing from the work of Watson naval architects of New Zealand for a particular vessel showing the mathematical solution. Other examples can be found in yacht design literature and I can provide references if you like for both the empirical and mathematical method.

Twin Screw
The following is a comparison of a single screw propeller and twin screw proposed for our Watson 54 vessel.
We have calculated the power coefficient in each case. The LOWER the coefficient the HIGHER the propulsion efficiency.

Single screw
Power 224 SHP @ 2100 rpm
Propeller RPM (N) = 453 rpm
Speed of advance(Va) = 8.85 knots
Power Coefficient (Bp) =√ power x N
Va 2.5

Bp = √ 224 x 453
8.85 2.5
Bp = 29.096

This power coefficient (Bp) yields an efficiency of 56.5% on a propeller diameter of 1250mm (49”)

Twin screw
Power 112 SHP @ 2500 rpm
Propeller RPM (N) = 505 rpm
Speed of advance (Va) = 9.00 knots
Power Coefficient(Bp) =√ power x N
Va 2.5
Bp =√ 112 x 505
9 2.5
Bp = 21.993

This power coefficient yields an efficiency of 60.5% on a propeller diameter of 977mm (38”).

In both these cases the propulsive efficiency is very high. If the efficiency was less than 50% we would need to adjust the inputs to correct the deficiency.

These calculations show that a twin screw vessel has (or can have) the same or higher propulsive efficiency as a single screw vessel.

The above calculations reveal “true economy”. Forget miles to the gallon or gallons to the mile. These only serve to tell the cost in dollars and disguise the real performance of a vessel. When making comparisons of different models the propulsive efficiency is a key number.

So what other benefits are there to a twin screw vessel?

1. A symmetrical engine room layout.

2. Commonality of parts.

3. Allows for a centerline engine room access.

What are the disadvantages?

1.Slightly more expensive than single screw with wing.

Based on the above the Watson 54 is offered with a twin screw engine installation.

Matt

Whether twins really are less efficient than an equivalent single engine depends entirely on how you define “equivalent”.

Let’s say all the engines do 3600 rpm at cruise speed, and they’re all going through 2:1 reduction gears. One boat has one 100 hp engine with a single large propeller, and the other boat has two 50 hp engines with smaller propellers, all propellers being sized to match their engines. In this case, you would expect the single to use about 10% less fuel per mile than the twins at the same speed.

Now let’s use a different meaning of “equivalent”. The single installation has the largest propeller that will keep the draft under 3 feet. Let’s say it’s a 24″ diameter prop producing 10 kN of thrust. For the twin-engine version, we keep the same draft limit. We now have two 24″ props, each producing 5 kN of thrust. Those lower-load, lower-RPM propellers will be nearly 15% more efficient than the equivalent single. Add a bit to the overall drag of the hull to allow for the 2nd skeg, and it’s still a 10% improvement. (However, you’ve had to go to much steeper – thus heavier and more expensive – reduction gears than in the single-engine case.)

Taking this line of thought to its mathematical extreme, you end up with the AeroVelo Atlas – a human-powered aircraft with four propellers, each of which is 20.2 metres in diameter and produces only 0.3 kN of thrust. The thing can hover using just 700 watts of power (approx. 1 hp) from its furiously pedalling pilot, making it arguably the most efficient propeller system ever made.

If you want efficiency, you need large, lightly loaded, slow moving propeller blades.

RDE

I’m still waiting for someone to step forward and demonstrate why the combination of a generator (that you already have) directly powering an inexpensive variable speed AC motor coupled to a straight shaft folding propeller isn’t the cheapest and most elegant solution to this problem. It could be done at a fraction of the cost of a diesel back up system, and would be more reliable than a seldom used diesel. For the thrust requirements of the Artnautica you could get by with as little as 6kw, although 9kw would come closer to normal cruising speed.

Doug

The market wants a get home option, in the end, that all that matters if you want to sell your product.
One way to win back some efficiency is to add a Nozzle.
“A single 240hp Lugger diesel drives a 40-inch Kaplan 4-blade propeller that turns within the circumference of a Kort-type nozzle by NautiCan. Rare on pleasure cruisers but a common fixture on tugs and other workboat types, the ducted propeller typically is specified for the increased pulling power it generates compared to an open propeller. Variations in duct geometry, however, also offer potential gains in range between 10 and 30 percent, at speeds below 15 knots. Olds Engineering of Australia recently tested identical 72-foot trawlers, one fitted with a Rice Speed Nozzle—similar to the NautiCan unit—another with a conventional open propeller, and reports that the nozzle achieved a 23-percent reduction in fuel consumption and a gain in free running speed from 9.3 to 10 knots. While Evans was able to acquire his at a favorable price, the typical cost of a speed nozzle—estimated at upwards of $12,000 to $15,000—may well cause most owners to think twice about committing to a similar installation”.

Matt

Kort-style (accelerating) nozzles can be quite beneficial if the boat is relatively slow (max. 8 to 10 knots) and the propeller diameter is constrained. If both of these conditions are true, than a propeller in a Kort nozzle can be considerably more efficient and produce more thrust than an unshrouded one.
Well-built nozzles are quite expensive, though, and poorly-built ones are much worse than having no nozzle at all. And smaller ones, where the drivetrain isn’t powerful enough to just slice through rope and logs and other fouling materials, are a royal pain to clean if they get jammed up.
I’m not sure they’re all that relevant to the get-home backup question, but maybe this is fodder for a future article….

RDE

Hi John,
Looks like the use profile you have in mind is more demanding than the one I was envisioning. If your get home system needs to substantially replace the primary propulsion system in expedition conditions or during similar usage to what you’ve done with Morgan’s Cloud over the years that is completely different than something designed to bring you home with a little more reliability than a sail hoisted on a stabilizer pole.
And yes, if you want to design a green boat around solar and wind DC systems, that precludes using a generator as the back-up drive system. Personally I’ve never met a motoryacht owner who could live on board for 24 hours without a generator running, but I assume that like black swans, they must exist. LOL

RDE

Well, maybe—
Thing is, once you start building a DC system that is solar intensive you have to store the energy in batteries. Then you have to buy them, maintain and replace them, and carry their weight. Add a 30hp diesel engine powering a DC generator to the system, and aren’t you back in the efficiency/complexity trap that was deemed unacceptable in the many permutations of the earlier discussions? And you have no ability to regen while underway—harvesting wind energy which is one of the benefits of electric sail.

Love it or hate it, nothing begins to approach the energy density/cost of diesel.

On your other point, a hydraulic motor should be more reliable than an AC motor and not too much more expensive, but judging by the power losses I’ve been quoted for the hydraulic drive systems designers used to put on IOR race boats I doubt if the power losses are any less than electric conversion losses. Funny how I never seem to find a free lunch or a free boat. Anyway, it does come back to the required use profile first, and then considerations of simplicity, budget, and how much one is willing to pay to feel green.

Eric Klem

Hi John and Richard,

Like Richard, I tend to largely ignore the propulsive efficiency in the get home system as provided that you don’t run into range issues (probably only an issue for trying to go uphill with one of these systems), your only concern is getting to the next port safely. Like you guys have mentioned, the efficiency numbers I have seen for hydraulic drives are not very good.

If someone has a large enough generator, I see no reason why you couldn’t run a serial hybrid system with it in an emergency. For my own tastes, the generator would be enormously oversized for its normal use to make this viable. Maybe on a boat that can utilize more power at once such as with lithium batteries and AC? There have been several ~30′ sailboats that have reported making their electric boat a hybrid by hooking up a 2kw generator and the ~3 knot speeds they are claiming just are not high enough for me for even a get home solution. I would think that the generator would need to be in the range of 1/3-1/2 of the main engine output to make it viable which gets big fast.

I have seen a few boats (Roseway pre-rebuild being one example) with an interesting setup that may be applicable. They had 2 main engines coupled to a single shaft through a gearbox that allowed either engine to be clutched in or out. During normal operation, a single engine was run relatively hard. In heavy going, the prop load would be too high so you would start the second engine. This was claimed to provide both good propulsive efficiency, ability to punch into heavy weather as well as backup. I am not so sure on the backup part as there is a non-standard gearbox and still a single shaft but certainly interesting to think about. With a good set of spurs and a drivesaver, it might be pretty reliable.

Everyone has their own comfort level with get home options. For me, I have no problem taking a chain wrench down and locking a shaft (speaking from experience) whereas other people would want to simply hit something on an engine control panel. Obviously, I am willing to run some level of risk of a failure where I need the backup running right away and I don’t have time to enact it.

Part of me feels like we should simply have a setup to run a longtail like you see on Thai boats. No drag during normal operation and you could have a small diesel mounted in a sealed box in the stern that you hook the longtail up to when it is needed. It would be hard to put together when you needed it and it would be a challenge to make the shaft stiff enough and light enough (composite?).

Eric

RDE

Hi John,
Just noticed your request that the idea of a hybrid system not be brought up, so I’m guilty of dragging up an issue that you feel has been beaten to death! But this is an entirely different application than a primary system based upon diesel-electric drive. The purpose of a get-home system is to get home when the single engine drive system fails, not to replace it. It might be used once in five years, so concerns about efficiency loss and fuel economy are irrelevant. What does matter it that it be economical to install, provide enough thrust to get you home, and be reliable when called upon. Being instantly available when the primary drive engine sucks air in the midst maneuvering through a reef isn’t a bad feature either—-.

Nikolas

John,
I am not affiliated with Watson naval architects in any way but like you I admire their work as I believe they are very close to commercial standards and practices and I perceive that their designs are by far the most seaworthy of the ‘trawler yachts breeds’. I would like to see an ‘attainable’ version of their work as well, but our specifications in regards of that are about opposite. For me the design of a long range displacement ocean crosser begins from the prop and that is where I would compromise the least and that means large diameter slow revolving prop, which means relatively deep draught (min 15 % of WL) and so on. Through extensive research in nautical literature and some hard earned sea-miles (some deliveries of production frp sailboats mostly and some production frp trawler yachts) I know what will serve me best, why is that and what I can afford as it is within my restricted budget. At some point I would like to send to your personal mail my design parameters and specifications along with my reasoning for my ‘attainable’ project in anticipation for your comments.
Best
Nikolas

RDE

Hi Eric,
Once we get beyond hypotheticals into actual desired usage patterns I suspect you are entirely correct– the compromises in needing far to large a generator (unless the boat is large enough to have two generators) would prove the Achilles heel of the simple get home system I postulated.

If I might tread softly around the edges of the whole electric drive issue, I suspect that the one case where it might prove viable or superior is for a catamaran used for typical Caribbean cruising. Lots of solar panel surface area and good breeze for windmills at night, most of the time spent at anchor partying, and plenty of wind for direct regeneration when it is under sail.