In Part 1 of this series I concluded that yes, we really do need a get-home option for an offshore motorboat, and in Part 2 I took a deep dive into the benefits and disadvantages of each option.
Now I’m going to stick my neck way, way out and pick a winner.
Not Confirmation Bias
But first, one interesting thing in all of this is that if you had asked me to pick my preferred get-home two years ago, before I put all this time and analysis into this decision, the solution that is my winner today would have been at the bottom of my list then—once again reminds me of the importance of making these decisions using research and thoughtful analysis, not gut feel or crowd “wisdom”.
Enough said. Can I have the envelope please…cue the thumping bass music…
John,
OK, two motors and two drive chains. Deals directly with the problem of one motor failing and or one drive chain failing.
What are the chances of one drive chain fouling and not the other? I think you are being overly optimistic to expect only one drive chain to be at risk.
This might be another solution.
One main engine and drive chain with a standby engine or generator which can be geared at will to a hydraulic system driving a hydraulically retractable sterndrive (which can be retracted into a stern compartment if desired).
It would be sized to permit a boat to beat off a lee shore under severe conditions.
I have no idea how expensive this would be relative to other solutions however it would constitute a near fail safe rarely used solution, which is what you are looking for.
If you want to get home, you have to pay for the ticket.
Hi Henry,
That’s not really what I wrote. I acknowledged the risk of both props fouling at the same time, but decided that the likelihood of both being catastrophically damaged was low as long as they were properly engineered. That’s not conjecture, but is based on my experience of having had props badly fouled on three occasions. In each case after cutting the foul clear we were able to use the engine for several hundred hours, although in one case the shaft was slightly bent, but in that case it was too light anyway.
As to a retractable stern drive, sure that might work and might solve the problem. But it would be very complicated, custom, and would need huge power to overcome hydraulic losses so as to be effective on a lee shore, so it would not be my choice.
Bottom line, like most things in life and boats, there are no perfect answers that will work in every conceivable circumstance.
One of the big risks with any kind of complex “only for emergencies” drive system is that it won’t work when you need it.
Such systems, almost invariably, end up seeing less thorough and less frequent maintenance than the main engine does. They tend not to be tested at full capacity very often.
They can, in principle, protect you against the situation where mechanical damage (from grounding or fouling) takes out both main engines of a twin-engine installation but not the retracted wing / backup engine of a single-screw vessel. That, however, seems to be a relatively rare scenario.
Matt,
Your comments are very apt and sensible. However, the point of a backup system is that it can pull you out of any situation that might come up, no matter how rare the possibility, particularly something like having to beat off a lee shore in periculous circumstances when, for some reason, all normal systems have failed. Having to deal with two different systems may just be one of the costs of such an insurance system. And of course, the insurance system has to be routinely tested.
In any event, is the system suggested by me that complex?
Hi John,
As a layman reader who like many others has the dream of one day cruising and living aboard, I’ve eagerly read your (and the AAC community’s) analysis and discussion on this topic.
For backup propulsion of an offshore cruiser, it’s clear that finding tradeoffs that result in “maintainable complexity” is the sweet spot, and your argument for a twin engine system is convincing.
Having two identical engines adds propulsion redundancy without adding new technology, and most importantly IMO, allows the boat owner to become more practiced in a single maintenance routine. Far better for an owner to focus scarce time on the art of diesel maintenance, and do so on a single model of engine, than to split that time between multiple types of prop propulsion, each with different parts, maintenance schedules, and modes of failure.
For a diligent owner, I believe such economies of time, parts, and expertise will contribute greatly to the reliability of this system. The discipline would seem to be in the importance of keeping both engines in fighting trim at all times, and to never leave the marina with one of the engines operating below spec. Perhaps to think of the vessel more like a twin engine airplane in this regard…
The twin configuration has the bonus of being applicable to both mono and multihull designs. In addition to any more news on Artnautica and Boréal developments, I look forward to any future articles discussing stability and hull shape in this AAC powerboat vein.
Hi Scott,
I think that’s a good analysis of the issues. As you point out, when we boil all of this down, maintainability probably trumps all, as it often does around boats.
Hi John, very good article, great research. Just a quick thought/question: you will already be bringing a substantial outboard for your tender. Could that be given a transom mount on the mothership so it can bring you home, as a third option. You could even bring an ultra low-pitch prop for this purpose, easy to swap out. In fact, you could take it one step further: Two outboards! One in active duty for the tender, one in spare, and together they are a capable Get-Home team. Also circumvents the problems of twin-engine fouling, catastrophic diesel fuel contamination and even rudder failure. Think the new 2017 Yamaha F25, great for a substantial tender, 2 x 25 = 50HP, but you need a crane to lift it as it still weighs 60Kg.
Hi Gerben,
Yes, that might work. In fact Dennis and I have discussed just that. However, there are a couple of problems:
First off, it will only work in smooth water since in a swell there is going to be high risk of the either the power head getting dunked or the prop coming out of the water. And second, it means carrying huge quantities of petrol, unless we can find a viable diesel outboard.
For these reasons, particularly the first, I don’t think that putting an outboard on the vessel herself is any more functional than using the tender on the hip, or certainly not enough more to justify the complications.
That said, your though about carrying a low pitch prop for this use makes sense.
Well, I certainly agree with your point about the coupler; that was part of our reasoning for an AquaDrive installation. The twin Beta 50s are also reasonable, but allow an interesting calculation of what you don’t like: increased complexity. I’m thinking that two engines allow two PTOs, with the option of belting in either spare or additional alternators or to run compressors or watermakers as desired. The rationale being that this sort of load is “parasitic” over two drivetrains, not one.
On the other hand, I’ve done precisely what Gerben suggests: put a RIB Zodiac on the hip of an ailing Atomic 4-driven sailboat, fired up the Honda 100 and trucked an hour or so back to our dock on a dead-calm day. I think his suggestion is reasonable and in some ways more flexible, save for the need to carry sufficient gasoline, which is annoying and problematic offshore. Which leads to my last observation: is there a market for a diesel outboard and could it be made light enough to be used on a typical tender?
Yanmar is about to release diesel outboards again? http://www.boatingmag.com/yanmar-re-enters-diesel-outboard-motor-market
Hi Mark,
An AquaDrive, much as I like them (we own one), does not add any robustness to the drive train that I can see. You still need a Drivesaver, or the like, to have the desired “fuse” in the drive train.
Yes, I get that, and they can be used together. So far, I’m having good results with a hydraulic transmission and the AquaDrive, but it would not be difficult to throw a Drivesaver in that line.
Marc,
Complexity:
A sailboat engine room containing two Deutz V-8s, each with a hydraulic PTO of about 80hp, driving anchoring gear, forward and aft thrusters and retractable stabilizers and Hundestadt variable pitch props. In addition, two giant alternators per engine. Every motor in the vessel sized to accommodate 50 or 60 cycle. Three 220v generators with a total capacity of 190kw. Twenty communications devices and radars on board. A 24/7 satellite link continually monitoring and broadcasting 100 vessel functions to the main control office in New York.
The perfect expression of a sailboat that will never be used by it’s owner.
KeepItSimpleStupid!
Hi Richard,
Sounds like the start of a Stephen King novel to me.
As I’ve suggested, choices always start from intended use. For a ULDB powerboat planning to go to remote places the KISS principle still leads to twin engines being the best choice. And a pair of little BETA’s or Yanmars are a far cry from an engine room crammed with two V-12’s and two or three generators.
One thing I learned from trying to nurse one of those monsters 900 miles down the Baha where no fuel is available is that widely spaced big props don’t allow low speed operation on a single engine. At idle speed (8knots) you will simply drive in circles, and full rudder is barely sufficient to maintain course. With two small inline 4 cylinder engines one would have the option of keeping the props close together. I’d study very carefully the trade offs between maneuverability, interference flow, and single engine utility.
Hi Richard,
I agree, the key to a really good outcome is going to be doing some really good modelling and design work to get those variables right.