The big visible change this week was putting the plating or “skin” on the skeg that protrudes down at the aft end of the hull to house and protect the propeller shaft and also provide protection for the CPP Controllable Pitch Propeller itself and the rudder. However there was also much work elsewhere that just isn’t as obvious as the finished welding continues unabated on the inside finishing up all the baffles in all the tanks that make up all the area below the waterline. This is also in prep for welding the top plates on the tanks and getting them all in place so that all the 14 individual tanks, water and diesel, can be pressure tested for any leaks before all the access ports are added and then retested.
With Christine away doing more research for her new book series in Beirut and Doha all this week I was able to spend the time to create a guided walkthrough tour of Möbius such as she is right now as many of you have been asking for this. So there are links to two YouTube videos at the end of this post, one the usual time lapse overview of the progress this week and the other a guided tour narrated by yours truly. I am also working at learning a new video editing software that should enable me to do a better job of annotating and creating better videos so let me know what you like and what you do NOT like about the changes you’ll see.
One lovely change was having my cousin Lynda, staying shaded under that fashion setting blue hat, come in to visit us for a week from her home in Biel/Bienne Switzerland. Christine was only here for the weekend before she flew out to Beirut but it was a wonderful visit.
This beautiful waterfall is rather unique coming over the cliffs right into the ocean and doing so right in the center of Antalya right near the old original town center and port. We spent the day driving and walking throughout Antalya and getting to know this wonderful city better and better and liking it more and more.
Meanwhile back at Naval Yachts on Monday morning Team Möbius was hard at it and here is the overview of all their progress this week of October 15 to 19 ……………….
First a quick look at rough rendering to help you visualise this aft portion of the hull to the Skeg and Prop Tunnel in green 12mm / 1/2” plate and the prop and rudder aft of that.
This is where what the framework of the Skeg looked like at the beginning of this week, all ready to be skinned with 12mm / 1/2” plate such as the one you see leaning against the aft end of the skeg here.
First though, the rest of the prop tunnel plates need to be tacked in place as the Skeg plates butt up against these.
Uğur and Nihat use a crank jack with different lengths of angle iron attached to press these carefully curved plates into just right alignment
and then tack them in place.
The other side gets the same treatment
Then the Skeg is plated over and the Skeg is now complete and ready for Sezgin to come in and do all the final welding.
It is difficult to capture in a single photo so be sure to watch the videos at the end to see how all these different surfaces of the hull, the prop tunnel and the foil shaped Skeg all make the transition from one to the next to create these beautiful complex curves. A think of beauty and a testament to the craftsmanship of the plate benders and all of Team Möbius who transformed all of Dennis’s design work in producing the 3D model into this reality.
I climbed up into the Engine Room area to get this shot of what it all looks like from this top down perspective. You can also appreciate how complex the eXtremely robust framing needs to be in this area in particular. This very aft end of the boat will have aluminium floor planks set in place to create a stand-up Workshop area that I can’t wait to start using and enjoying.
Moving forward from the Workshop and Engine room area, past the Guest Cabin and Christine’s office allows us to see the work going on in the Basement area under the SuperSalon floor. As with all the area below the WL it is all tanks and Umit is busy cleaning up all the top surfaces prepping them for the tank top plates which are waiting to be welded in to completely close in all the tanks.
Moving up into the SuperSalon area above and looking down through this large hatch into the Basement you can see some of the tank top plates in position and being tacked in place. The slots all line up with the centerline of the flat bars underneath on top of each baffle so they can be welded together and then the whole perimeter of each plate is welded to close and seal the tanks completely.
Enver on the right and Uğur are carefully positioning each tank top plate and tacking them in place. The three large bays you see running down the centerline of the Basement will become the battery boxes for the 24 individual 2V volt battery cells which make up our 54kWh battery bank. The battery boxes will have the same 6mm aluminium plate bolted on top to keep them sealed up in case any water should get into the Basement and then each box is also vented to keep air flowing through to help keep the batteries cool. This massive amount of lead in these batteries, all 1640kg / 3615 lbs of it, is actually a “feature not a bug” as they go all the way down alongside the keel bar and act as part of our ballast. These are all Gel type batteries so fully sealed and require no maintenance so I don’t imagine I will need to remove their top plates more than once a year or so for annual inspection.
And that’s the week that was October 15 to 19 here at Naval Yachts in Antalya Turkey. As I noted above there are two videos for you this week, the first is the usual weekly update time lapse video, though I have included last week’s videos to make up for not getting them into the last post.
Then there is a longer 15 minute guided tour of both the completed Skeg and full walking tour of the deck, Pilot House and SkyBridge.
Hope you enjoy and thanks again for spending your valuable time joining us.
As I mentioned at the beginning I’ve been learning and using some new software, Cyberlink Power Director 16, to help me improve my videos so please do let me know what you like and don’t like about these latest videos and the changes from the videos in previous posts. You can add these along with any questions or suggestions in the “Join the Discussion” box below.
Wayne, I like the flowing words, a nice touch however I wish you would leave them on the screen a little longer. I can’t read them as fast as they disappear and have to stop, go back and then stop the video to see what you are saying. Having said that, I really enjoyed the videos, keep up the good work and the updates.
Thanks Orville, just the kind of feedback I’m looking for. I may have gone a bit overboard with the flying in/out text so will keep an eye on that and I debate with myself just how long to leave all the text parts up in a video from the title to the credits and then these intermittent text inserts. Don’t want to bore people with too much time with “just text” but as you point out too little time is disruptive if you need to pause the video, rewind, and go back so I’ll add more time for these in the future. Keep an eye on these as they come up in future videos and let me know if the timing is just right or too short/long.
As with myself, these videos are a work in progress and I’m enjoying the learning and finding ways to capture the build better so stay tuned for more and please keep your comments and suggestions coming.
Lets not get into Lithium vs. lead acid discussion again (or yet…), but lets take as an example a known made in USA brand straight lead acid replacement battery with integrated everything that you can just swap in place and that many sailors in youtube feeds (*) seem to use to replace old lead acids: a Battle Born 100Ah 12V, 1.2 kWh each. It is 13 kg each, so 91.2 Wh per kg. Your 1640 kg would make it 150 kWh battery, plus around double the usable capacity due to DOD allowed, so roughly 5-6 times the capacity.
Just stating for comparison, you have explained your choice well before…
* e.g. Sailing Uma, they are running 100% of their propulsion needs with 12 pcs of these
BTW can you comment a ballpark figure for price of 54 kWh gel battery bank?
My guess is roughly 10 large for such a bank.
These 2volt OPzV Gel batteries are made by a lot of different manufacturers so pricing varies similar as with any other battery size and type. But I’d say your $10k estimate is about right as I have suppliers who would range between a low of about $7k and a high of about $13k and so $10k would be right in the middle.
As with most of our equipment we will wait till close to launch date, or as late as possible in order to benefit from the typical price/performance curves which usually advantage waiting, plus we’d like to get everything to be as “fresh” and new as possible and be able to get the full warrantee out of them, though we are rarely in a position to be able to take advantage of warrantees with our preference for very remote locations. But in the first year or two we will probably be closer to ports where we could get things shipped in and be able to take advantage of warrantees. I suspect we will also see the typical “infant mortality” with new equipment failing or having issues so being able to get shore side shipping and assistance will be more necessary.
Andy, I hate to disagree with you but your example of Sailing Uma was a bad choice, I have seen them use their dingy as a tow boat on nunumerous occasions when their batteries ran out of juice. I haven’t seen any examples yet, where boats (at least long range sail boats) have successfully replaced their engines with an electric motor.
Agreed, it was a bad call to bring electric motors to discussion, didn’t mean to. Different place and time for that discussion.
Though as a get home engine backed by large batteries, solar panels AND a generator an electric engine might have its place on a long range motorboat as well. Either an outboard or one fixed to main shaft. Surprisingly low power would do, 10 kW would move a large boat quite nicely – in good weather of course. Silent cruising mode would be another benefit, 5 mile hop between anchorages would be easy with reasonable battery size.
Lets double that. 20 kW would move this boat at few knots. 10 kW would need quite a lot lower displacement, it just isn’t enough.
BTW just now having looked at their most recent videos, are you 100% sure Orville you are talking about their latest setup, not the one they had before the (battery) upgrade?
I am asking because the new setup look pretty good and seems to do what they want from the system pretty well – though with some improvements like better cooling still to be implemented:
this is also informative, if a little long:
ps. my all time favourite of their videos, not on batteries but on anchoring, I would say quite informative for everyone:
Thanks Andy and Orville. I’ve seen some of Uma’s blogs and YouTube videos and great to see them and there many young sailing friends be out there sailing and sharing their experiences. I think you worded it just right when you said “… the new setup look pretty good and seems to do what they want from the system pretty well”. However there isn’t too much comparison or similarity to be of too much use or comparison with what we are doing as their use cases, boats and sailing are so very different.
As for electric propulsion I’m very happy to see more boats pursuing this and seeing the various manufacturers progress and make great advances over just the past 10 years or so. GreeNaval GN series of boats which are the flip side of the Naval XPM type boats which Möbius represents are all electric hybrids of different types so I am gaining more and more first hand knowledge and experience with this side. Naval has added the XPM type of boats to compliment the GN series and provide great solutions based on the same set of fundamental principles of maximum efficiency, safety and comfort and minimum maintenance for both ends of the boating spectrum. So the GN series is aimed at those who are mostly doing day and coastal trips and the XPM series is aimed at worldwide open ocean passage makers. The capabilities and characteristics of electric propulsion suit the GN boats and owners extremely well whereas high efficiency diesel engines is what fits the XPM series best.
There are of course a lot of factors involved and it is a MUCH more complex discussion, but in my mind at least, the primary factor that determines this fit of propulsion to different use cases is that of range. With the current capabilities of electric power, batteries and solar panels electric propulsion is not an option for long range passages and crossing oceans without having large generators onboard to supplement the solar output.
I won’t go off on the tangent right now but I also think that we (the boating world in total) will be able to best compare different solutions and make the best choices of the best fit for a given use case and boat by taking the total life cycle of boats into account. This would include everything from energy, materials, labour, recycling, etc. that goes into the initial building of all the equipment and materials that go into building the boats themselves and then continues on through their use, storage and maintenance, and then through to their end of life cycle with recycling and otherwise dealing with the boats and all their equipment once they are no longer able to be used. Not doing so seem to most often lead to poor choices, poor matches and waste, but I’ll leave that discussion for another time.
All three of these technologies have certainly improved dramatically and I’d say that at the high end, motors and batteries are up to the challenge of long range passages but solar is not there yet for all but a very few boats. Specifically the Watts per meter ratio is still too low for most boats. Or said another way, there is simply not enough physical area on most boats to fit enough solar panels to have the output needed to power a boat 24/7 for days let alone weeks at a time.
There have recently been some very interesting exceptions to this such as the big catamarans, 55-80 feet, which Silent Yachts has recently been promoting. Once get up to this size, with say 17m/55′ LOA and 8.4m/28′ Beam, if you cover every bit of that surface area with solar panels, the math starts to work and electric only ocean crossing becomes possible. I have no doubt that solar panels and all the other components will continue their upward evolution curves and it will become more and more possible but at this point in time, ultra reliable and efficient diesel engines are the best and Greenest solutions for passage makers.
It is very exciting times to be a part of this rapidly evolving situation and I’m delighted to be a part of it.
Well said Wayne and I agree. What the future holds in store for electric motors, solar and battery advancement is unknown but I am sure technology will advance at a much faster rate than it has already and we probably won’t even recognize what will be coming in the next few years. Just look at the advancement we have had in the last 20 years in these fields to get a glimpse into the future.
Andy I must admit that I stopped following Uma when they had to use their dingy as a tow and seemed to change the layout of their channel. I am more interested in the DYI part of sailing and boat restoration projects, probably because that is the stage I am in at the present time, but I also try to stay up with what is new technology at the same time. This is one of the reasons that the Mobius Build is so interesting to follow, that and Waynes charismatic style LOL.
Quite right about the rapid pace of change in electric related technologies Orville. The two big ones I’m watching in this area are solar panel Watts/Surface Area (m2/ft2) and energy storage density, be that battery or otherwise. And of course it won’t surprise me if in the slightly longer term something other than solar and other than electric turns out to be the big game changer as in fuel cells, nuclear, etc. And on a much larger scale, one of the meta themes featured in my keynote talks for the past several decades has been that the rate of change of most things in our world is exponential and that we are in fact now living in a world characterised much more by exponential change rather than linear, and that we have very little experience with or sense of what exponential changes are like so this is one of the key new skills I think we would all do well to gain greater proficiency with. But I won’t gallop off on my favorite horse Tangent here.
When it comes to making decisions for things like equipment and systems on Mobius, in fact with most decisions I make, I try to take the emotional aspects out or at least consider them separately, and focus on the factual aspects. In the context of electrical and propulsion systems this boils down to putting the decisions into the context of our use cases and design parameters for the boat which I outlined in my early post here “Project Goldilocks” https://mobius.world/project-goldilocks-mission-impossible-or-just-right/ and then doing the math on the various key metrics such as efficiency, weight, cost, maintenance, etc. I’ve outlined my basic calculations on this in previous responses here and the conclusion is clear that for our use cases and this boat, the best combination is a single low rev diesel engine with a large slow turning CPP prop, no generator, maximum # solar panels and a very large house bank capacity made from high Ah 2v Gel “traction batteries”. We have used the same kind of logic and calculations for pretty much all the other critical design decisions such as going with all aluminium over engineered hull, unpainted exterior, right up to the 24m limit for LOA, tankage for all below the WL hull areas for safety and range, highest LWL/BWL ratio, “empty ended” interior layout, weighting and design for self righting, single oversized bow anchor, passive stabilization paravanes, and so on.
While it may seem counter intuitive to some, when you “do the math” and do so within the context of a given use case, we conclude that for our kind of use case, an XPM style of boat such as Möbius is the safest and most efficient combination of features whereas for the use case that the GN family of boats that GreeNaval also builds are created for, a very different set of choices for systems and equipment that includes hybrid propulsion, is the best choice. And I would be remiss if I did not make the point right up front that almost all the decisions one makes in finding or designing the Goldilocks “just right, just for me” boat, are eXtremely personal ones and therefore there is the “emotional” and deeply personal aspect that encapsulates the entire process. So there you have my 2 Canadian cents worth!
Hi Wayne, I just wanted to drop you a quick line before heading out for a few appointments. I was surprised by your statement that you aren’t having a generator on board, even a small one for standby. I think I may just be overly cautious but having just gone through two weeks of overcast and rain may have something to do with my outlook. The other comment you made about only having one massive bow anchor also surprised me. Is that the only anchor you have onboard? I would have thought you would have had a smaller stern anchor as well for storm conditions but then I have a 40 foot sailboat and I am sure our anchoring requirements are quite different from what yours will be on Mobius.
I totally agree with the rest of your comments about change, having spent 40 years in commercial construction I may have a little better understanding on that. Each stage of a construction project is a growth on the stage before with more work being done while building to the next step…a good example of practical exponential change. We also saw changes in technology and equipment so in many ways it is a constantly changing profession with the outlook for the same results.
Have to run or I’ll be late, I really am enjoying your project and the discussions on the different systems, it is alway good to hear other thoughts and ideas if one is to keep the mind active as the body slows down. Hahaha
Hi Orville, thanks for the note amidst your busy schedule.
Having no generator was a design goal from the beginning. At first we thought we would just be able to get to having the generator be a backup to our solar setup but as we worked at our design criteria and priorities for eXtreme remote lifestyle, energy efficiency, reliability and comfort we were able to successfully eliminate the need for a generator all together. This was possible thanks to continuing to focus on these priorities and with no small amount of effort design wise and thanks to the continued evolution of solar panels and charging stystems. Short story, for a change from me I know, is that we were able to get our capacities for solar output, battery capacity and alternator charging up to levels which rendered the generator unnecessary.
We were able to work with Dennis to fit a total of 14 full size (96 cells) solar panels onboard and at today’s panel efficiencies these will be about 360-355 Watts each so that would give us 5-5.4kWp output. We did some work with the models and putting them in different locations in the world to run shading and sunlight calculations and used these to figure out conservative actual daily outputs we can expect from these panels taking into account losses from shading, voltage drops in wires, MPPT efficiencies, etc. and net out with expecting to see a low of 15.5 to 21.7 kWh per day. The lower figure is based on what is referred to as a 5 hour solar day and the higher is a 7 hour solar day. I’ve compared these estimates to the real world experiences by other boats with similar setups and these seem to be realistic and very conservative. For reliability the solar cells will be divided into multiple banks and MPPT controllers possibly all the way down to one MPPT per panel.
Our house battery bank will have a total capacity of 55kWh coming from 24 OPzV 2volt Gel cell industrial “traction” batteries which are commonly used throughout the world for things like fork lifts, building backups and more and more off grid installations. Based on our expected maximum daily consumption of 400-500Ah per 24 hour day we should be able to have very limited duty cycles, likely not going below 75-80% DoD most days and being able to fully charge these batteries every day so they will be very understressed, longer lasting and give us lots of “fudge factor” if needed.
And then finally in the overall scheme of electrical power onboard Möbius, we will have two massive 250A @ 24V 6kW industrial alternators driven by the Gardner. We will derate these down to about 200A each to increase their longevity and so they will provide up to 4.8kWP each so almost 10kWP in total whenever we are underway or in the rare instance that we don’t get enough solar though more likely when we have something go wrong with the solar system.
So that’s our rationale behind going with no generator which pays us huge dividends in reducing our overall equipment budget and fuel burn, increasing our comfort and that of others that may anchor near us, free up space in the Workshop and eliminate an entire engine and system to go wrong and for me to service and maintain.
For anchoring, over the years anchoring almost every day in a huge range of conditions and bottoms, I became a huge believer in going with a single “way too big” anchor on the bow with equally oversized chain, windlass and ground tackle. So Möbius will have a 125kg/275lb Rocna/Manson anchor with about 150m/500’ of 7/16 or ½’ chain on the bow. We will have at least one anchor for the few times we might want to have a stern anchor along with at least one reel of webbing or line for going to shore in fjords and the like. The big bow anchor and lots of chain enables us to anchor in much deeper water and with much less scope in pretty much any bottom conditions. Given our size we need to anchor further out and it is our preference in any case whenever we are in places with lots of other boats. So we won’t be in marinas or mooring fields and more likely out with the commercial tankers and such which suits us just fine but necessitates this kind of ground tackle.
Appreciate your insights and experiences with exponential change and glad I can be provoking some good mental gymnastics with you. Keeps us mentally fit from all the research I read so all a good thing to as you noted keep the mind active. Fortunately for Christine and I we are also still keeping our bodies as active as ever and we’ll keep exercising both mind and body for as long as we can.
Thanks Wayne for the complete and informative answers, I can see you have your bases well covered and you don’t need a generator. I am really enjoying the series and the comment section. I think in some ways I learn more from questions asked and your replies that just the video commentary although I do like that too. I wish we could get more questions asked because the dialogue is lively and it becomes a great source of knowledge and information. All the best….
Thanks for a long response, again. It is amazing you find the time for this in the middle of boatbuilding, big thanks!
One interesting coming development that might shift the equation in near future is if/when direct diesel fuel cells come to market. There are plenty of working prototypes already and its not rocket science, main challenge remaining is the fuel purity or lack of it and not getting your membrane blocked by this. After solving this, we will see 50%+ efficiency in converting fuel to power, all in complete silence and roughly doubling the motoring range for each litre of diesel spent. These would also fit perfectly together with solar panels, both driving the same electric engine and supplying all the hotel loads of the boat.
And one additional comment Wayne, I would not hold my breath too much for solar panel efficiencies to jump up that much in near future. They may go up few percent from current 20%+ , but that won’t make dramatic difference in boat applications.
The thing is, you can already buy very much higher efficiency panels for very very much higher price, they are literally of the shelf for projects like cubesats (*) but cost around the same as the famous space shuttle toilet.
The driving trend in solar panel development is right now residential and utility solar, and there the efficiency per area is much less important than efficiency per dollar plus reliability/life time issues etc, unfortunately for us. Higher efficiencies require all sorts of exotic techniques that drive the costs up, sadly.
Hi Andy, thanks as always for all your comments. I live life with what I refer to as “no expectations but unbounded hopes” but I won’t go off on that tangent right now. Lucky you!! So I’m not expecting any huge jump in current solar panel efficiency but with 14 full size panels we’ll take very percentage point they do go up as they have been for the past 5 years or more. Output goes up and prices go down so what’s not to like right? So we will take the benefits we can from the continued upscaling of solar installations around the world which create great economies of scale and reduced pricing for us. And given the changes in just the past 5-10 years, who knows where we will be at when it comes time to replace our solar panels?
Hi Möbius team,
Such an awesome project you have!
I was wondering would you like to share some estimated stats of range and speeds what you expect?
CPP is very interesting solution..
Whats displacement and would be interesting how much is the weight of the hull?
All the best!
Hi Jo, thanks for joining in with your kind comments and questions.
If you go to one of my early posts here on the blog called “Project Goldilocks: Mission Impossible or Just Right?” you can read over our distillation of the “mission statement” for this boat and the XPM line of boats Naval will be building and then all the layers of the design pyramid below this that goes through our priorities, use case and design criteria which amongst other things will give you the stats and performance figures.
Short list of stats is as follows:
•LOA: 23.8m / 78.2’
•LWL 23.8 / 78.2’
•Max BEAM 5.13m / 16.8’
•DRAFT 1.3m / 4.2’
•DISPLACEMENT w/ half tanks 41,000 kg / 90,000 lbs
•CRUISING SPEED 10.5 kts
•MAX. SPEED 12 kts
•RANGE @10kts 7000+ nm
•FUEL CAPACITY 14,400 L / 3800 USG
•WATER CAPACITY 5300 L / 1400 USG
Be glad to answer any additional questions you might have once you’ve looked these over.
Thanks for info,
I have missedbthat post!
Still some q on my mind:
How mutch weight was the aluminium parts? The bare hull n structure?
Did you had to made any decisions regarding weight on the structure?
Im intersted coz i have my project on progress with some similarities with your
(Aluminium slender LDL voyager)
I try to keep Displacement quite low 20-30k
When you are waiting your project to be “finished?
Sorry for this delayed response Jo. I would estimate that the XPM78 has about 16 tonnes of AL plate and probably a total of 18 tonne for the entire AL hull with all the superstructure, bimini, arch and interior aluminium. When planning the design we think of weight like having a budget; we do our best to save weight everywhere possible so that we can “spend” it in all the right places. So we try to keep the weight that is higher up as light as possible and put weight down low wherever we can. Examples would include our massive keel bar and plating below the waterline, going with big heavy Gel batteries that are encased in sealed/vented boxes down on the very bottom of the hull on either side of the big keel bar so they act as part of our ballast and having all the hull below the waterline be integral tanks so that the very significant weight of all the eXtreme volumes of fuel and water we cary are also down as low as they can get.
20-30kg would be extremely light displacement for a 22m boat but depends on if this is displacement when fully loaded or the empty hull? We hope to launch XPM78-01 into the water before the end of next year. (2019)
Best wishes with your new boat and do let us know if you have a blog or other ways we can follow along with your build.
Thanks for an estimate on aluminium content. That is a lot of cans to drink…!
I think 25k kg is entirely realistic figure for 22M LOA/LWL boat, not even light to the extreme. If beam is kept moderate, say 4M or 4.5M max and freeboard low(ish), and interior length quite small, say max 10M long with long empty ends, even 20k kg displacement should be doable pretty easily without any exotic materials etc. I think 10-15k kg range is what could be called extreme for 20-24 LOA, that would certainly require carbon fiber hull and look like a racing sailboat. Pogo Loxo 32 is 1650 kg to give perspective: http://www.pogostructures.com/en/fiche-bateau/loxo-32/
Also lighter boat moves with smaller engine burning less fuel needing less tankage weighting less… And less weight means lighter equipment required and lower construction bill, to a point. So there are definitely advantages at aiming as light as possible.
And don’t get me wrong, Mobius displacement is still very very light in context of boats in that size range, just commenting on Jo’s set goal of sub 30 tons which I think is not unrealistic at all for a 22Mish LOA aluminium long range motorboat.
Some extreme examples on lower bound for displacement:
– 100ft maxi sail trimarans with two hulls two many are slightly over 10 tons. I would bet sub 10 with amas and rigging removed. Converted to power would make a very interesting long distance boat.
– Nigel Irens design 21m LOA trimaran was 5 tons
– 24m LOA Earthrace was 13 tons and had a range of 12k nm
– Could not find a figure for new 60M LOA Earthrace II, but that is gonna be one good looking machine:
But of course those are all carbon fiber and very spartan camping rather than liveaboard type of boats.
Appreciate your enthusiasm for going as light as possible with boats Andy and all the links to some good examples of doing so.
Weight, as with pretty much every aspect of a boat has to be put into the larger holistic context of the total boat and the total experience and use case any given boat is aimed at. In the case of Mobius and the XPM line of boats weight is one of our topmost priorities and considerations as we designed and now build the boat. I’ve also come to understand that not all kilos, or pounds or tons are the same as there is “good weight” and “bad weight” so this needs to be factored into your calculations and design for overall displacement. As I’ve noted in previous posts we work with a “weight budget” and we practice a kind of “save to spend” approach where we reduce weight as much as possible in some areas and use those gains so we can “spend” the weight in others. In general and as you have summarized very well, light or low displacement is good but I’m wanting to add that for our type of boats lightest is not best.
Our previous boat, an all steel 52′ Bruce Roberts designed sailboat taught me many lessons and lived up to my naming her “Learnativity” VERY well. One of those lessons was that there is a relationship between weight and what I think is best called “sea kindliness”, how a boat rides when at sea and at anchor to some extent as well. I don’t think there is a formula or number for measuring this but my point here is that for live aboard boat in particular and an ocean going passage maker as well, the Goldilocks displacement is not the lightest possible and is somewhat more “medium”. Hull shape certainly influences how any given boat rides and how sea kindly it is to live aboard for years and years through all parts of the world. As you noted at the end of your comment on some of the extreme examples of low displacement boats “But of course those are all carbon fiber and very spartan camping rather than liveaboard type of boats.” And I often remind people that we are not camping, this is our full time home.
The XPM boats and others like them are very much in the LDL camp where we strive for very low Displacement to Length ratios but it is equally important to understand that this does not mean lightest or that the lower the number the better and that as always a great boat design and build is a matter of finding that just right balance in the sea of compromises and balancing act of weighing (sorry) all the various pros and cons and choices against each other. My best guess is that when we launch Möbius and have her at start of passage trim with full fuel and ¼ water and all our gear and provisions, she will weigh in at about 45 tons. Certainly not the lightest but with a D/L of about 94, she is still very much a LDL boat and one that we think will be very “kind” to us under all sea and anchor conditions.