Another busy and productive week here at Naval Yachts with continued great progress on building the hull of our eXpedition Passage Maker XPM70-1 also known as mv Möbius. Plus Christine and I went on two eXpeditions of our own this week and I’ll put that up in a separate post. Work this week concentrated on putting the bottom row of hull plates, which in its upside down state right now means the plates at the very top, These are all 12mm / 1/2” and 15mm / 5/8” thick plates so they take a bit more effort to wrestle into position and tack in place as you’ll see. With this final set of plates now in position the beautiful slender and slippery shape of Miss Möbius is now clearly visible and Christine and I spent some time standing up on the scaffolding running down both sides to take in her sexy curves and imagine even more vividly how fabulous it is going to be to have this hull under us and slicing so efficiently through the seas of the world. Oh the places we will go!!
You can also now see the aft prop tunnel and skeg keel taking shape as their framing is now mostly complete. The tunnel and skeg surfaces will be plated in next week most likely but lets stop with the typing and get on with the pictures so Möbius can show off her curves to you.
The bow is now fully plated from top to bottom on both sides.
All the slots have been fill welded to the underlying stringers, the nose cone is fully welded in and all that remains is for the curved transition plate to come back from the plate benders and be welded into the open triangle you see here.
Here is a quick screen grab of the model to show you what this curved transition bow plate will look like when we’re done.
Working our way aft on the Starboard (right) side we pick up where we left off last week as the bottom/top row of plates are pulled in tight against the underlying framework
You can see how the 10mm x 100mm flat bars are tacked along the entire edge of the plate just below where the new plate butts up against it to ensure that the plate stays perfectly flat as the seam is welded up.
Here is a a nice angle looking towards the bow with the Starboard side bottom plating now all tacked in place.
Standing in the same spot on the scaffolding and turning 180 to look aft, this shot will give you a better idea of the finished hull shape and you can see the framework for the fin like skeg up on top.
This screen shot of the model shows how the green skeg holds and protects the blue prop shaft tube and how the hull makes the transition to the scooped out tunnel for the propeller and rudder.
Here you can see how the 25mm keel bar which runs the entire length of the hull, extends in this area to form the backbone of the skeg and the vertical 25mm frames show the carefully engineered foil shape of the skeg is formed. This will soon be plated over with 15mm thick plates that have been wheeled to these complex curved shapes and will be slot welded to the vertical frames you see here.
Looking from the aft Port corner and with Uğur and Mehmet providing some scale you can see where the prop tube will welded into the middle of the skeg framework. To keep everything perfectly aligned during the build you will notice how there are several vertical fingers spanning the space where the blue prop tube in the rendering sits. These were purposely not cut by the CNC machine and will be cut out when the hull is finished and the prop tube is slid in place and welded.
Looking forward and straight down the center keel bar shows the curve of the prop tunnel
Looking forward along the Starboard side of the skeg framework you can see the remaining bottom plates stretching out in front patiently awaiting their turn to be fitted and tacked in place.
Zooming out a bit, this shot shows how the tunnel exits right at the very rear edge of the hull at the end of the swim platform.
One extremely important aspect of an eXtremely efficient through the water hull design is the shape of the hull on the waterline. This is difficult to gauge when you are looking at the whole hull in these pictures so I made this quick section drawing to contrast the shape of the hull at the outer edge of the deck rub rails in blue and the profile in yellow on the WL.
Makes it very clear just how long and skinny, high Length to Beam ratio (78.2/13.7 = 5.7) Möbius will be and why I use words like slender, skinny and slippery to describe this kind of hull shape. You can also see why we refer to this as “a sailor’s motorboat”. eXtreme to be sure, all by design and all in be best of ways to match our equally eXtreme use case and personal preferences and provide us with the just right for us Goldilocks boat.
Your fast framed video summary of the week is below and thanks again for taking the time to join us on this adventure. Please be sure to add your comments, questions and suggestions in the “Join the Discussion” section at the bottom here. See you next week.
She is starting to look like a boat and not “just” a hull, great progress! Can’t wait to see her turned right side up and of course in water eventually.
ps. L / B -ratio 18.8? Must be a typo, that would be skinny for a catamaran ama…
Mea culpa! Tried mixing metric with imperial. Now corrected Length/Beam ratio to 78.2/13.7 = 5.7 Good catch Andy, thanks.
Has happened before… 🙂
Thanks, that makes me feel better I guess. Certainly the consequences of my mix up was just in text and pales to this story. Of course if the only country on the planet still not using metric would see the light and finally convert then life would be much easier for everyone but I’m not holding my breath on that one! 🙂
I’ve long been a “metric guy” by choice as well as being Canadian and living mostly internationally and it is just so much easier units to use and calculate when building things. Project Goldilocks is done in metric entirely and about the only place I add in the imperial numbers are here in the blog as I know many readers are not yet metricated or bilingual. And to be clear, we all do still have some units which we tend to revert back to what we grew up with. So things like your height and weight, fuel “mileage”, and things like that I too still often think of those in non metric units. Though even there it gets messier because I’m Canadian and so we use “proper” British Imperial units rather than “US Customary System of Units”. However in total there are not too many units which are different between these two, mostly weights and volumes but still makes for a lot of extra work and conversion and introduces, as I just proved, yet more ways for things to go wrong.
So I’ll stick to metric and try to just not mix them up when doing ratios here in the blog! Thanks again for catching this so I could change it quickly and not confuse others reading this.
The tunnel for the propeller and rudder reduces the draft of MOBIUS to facilitate “exploring the shallows”.
How much more draft would a conventional design have?
Besides draft reduction, were there other considerations to go for a tunnel design?
Thank you and best regards.
Quite right Markus, reducing draft was a high priority for us as it opens up so many more options and possibilities for us. Adds a nice safety factor as well in terms of how we would be able to go over an unexpected shallow spot or hard bits floating below us. Being able to get through shallow passes into beautiful atols that other boats just can’t and being able to anchor in idyllic or well protected little spots that have shallows to get to really makes a huge difference for many of the places we’ve been and intend to go. Right now our estimated draft ranges, depending on how much fuel/water onboard from 1.23m/4.04ft to 1.34m/4.4ft so we are VERY pleased with that and can’t wait to get out there enjoying it.
As with most parts of the design everything is interconnected so our path to this draft and the use of the prop tunnel largely originated with the high torque/low RPM Gardner engine which has max torque starting at just under 1000 RPM. This allowed/required us to go for a large 1000mm diameter CPP prop spinning at a leisurely 335 RPM (2.95:1 reduction gearing) all of which gives us tremendous fuel efficiency and lots of “grunt” power to get us off of an strong lee shore or through high speed current passes and the like. So to reduce the draft with such a large prop the tunnel was a natural way to go. We researched tunnels and efficiency extensively and Dennis did a LOT of work on the tunnel/skeg/prop/rudder design, running models through rigorous testing and bringing in experts to add their input and this is what we have ended up with.
A tunnel does intrude into the floor space in the workshop but this seemed like a reasonable trade off for increased propulsion efficiency, added safety and reduced draft and we continue to be very happy with this set of decisions. Of course it remains to be seen how this will all play out once we splash and start collecting real data in a variety of conditions over our extended sea trials and we join you in anxiously awaiting that data.
Couple of things
If you look at the stern on the wall paper picture you’re using on your home page, there is a falre and flat surface. While it is above the water line, water doesn’t always respect water lines.
Second thing. Water is a three dimensional medium. One way to picture what is happening is to calculate the volume of the water being displaced by the hull not to only look at a two dimensional cross section of the hull as it appears viewed from above. That volume, IMO, should vary as smoothly as possible over the length of the hull. Starting with fin, and including the tunnel in the hull, it appears to my eye that some interesting variations are taking place.
Considering those two things together, it’s going to be interesting to watch not only how the hull floats but how the drag is created by the flow on the water. Water doesn’t like to change directions abruptly.
I can’t quite make out if the tunnel transitions smoothly both Laterally and Longitudinally. If there are sharp enges, there are going to be some interesting vortexes generated at the sharp edges.
Gona be an interesting time.
Hi John, thanks for your continued comments and concerns. Way above my pay grade I’m afraid to be able to talk much more about specifics of hull design and hydrodynamics so I’ll have to leave it to Dennis to comment if he is able. Right now he is up to his eyeballs in Sikaflex and plywood as he works full time on his LRC58 “Koti” to finally finish what is usually his and Raquel’s full time home so he is hands off the computer and hands on the tools and materials for the next few weeks. And of course at this point all that any of us can do is speculate and theorize until we put the real boat in real water and see how she goes.
What I can add to some of your points is that renderings like the one you referenced in the slider bar on the blog can be very deceiving with different angles, shadows and lighting. So that “…. flair and flat surface” you noticed is a visual anomaly and you can see what the hull is like at the stern much better in the coloured renderings in the blog posts and of course best of all in the photographs of the actual hull plating now. And rest assured that pretty much all the work on designing and testing the hull has been very 3Dimensional and with the hull at every one of the 360 degrees of roll, pitch and yaw. I put in the 2D profile view of the hull on the WL to contrast the difference in shape and size of the hull on the WL vs up on deck and to show how slender this hull is. As she rolls and moves that profile changes significantly to provide some natural resistance to movement by the surrounding water and conditions and provide us with the best compromise for our use case between stability and comfort. Still no guarantees of course and always some distance between theory, computer testing and reality, but this is the best we can do and all so much more than with past methods and tools.
The shape and transitions of the aft of the hull around the skeg, tunnel, prop and rudder will come into view for all of us over the next few weeks at these parts receive their plating and things like prop and rudder are set in place in the coming months. And I will try to include more renderings from different angles to show all this as clearly as possible.
Interesting times indeed and part of living life fully for me!
Interesting post at
Doing the best compare and contrast that I can helps me to see that which I didn’t notice before.
I strongly endorse the comment in the article ” We want to emphasize that we do not recommend taking your FPB or any other yacht purposely into what could be harm’s way.”
Interesting comparison is the location of props and rudders:
On Cochise they seem to be hidden quite noticeably far from the stern. This is a tradeoff, but makes it pretty sure they never “breathe” even when green water is present.
Thanks for these revealing shots Andy. As the comparison between these shots of the FPB78 and the photos and models of Möbius here on the blog show very clearly, these two boats take a very different approach to hull design and resulted in hulls with very little in common other than sharing similar sets of First Principles of design and a lot of similar use case scenarios. As you’d now from following the FPB’s for so long they have all used quite long “swim extensions” since the first bolt on ones were added as a refit after the first few FPB64’s were launched. These apparently worked very well as the subsequent 95, 78 and 70 models extended these much further and they became welded parts of the overall hull. So I think the rudder/prop location was perhaps set relative to the original 64’s length prior to the extensions being added on and this was continued with the design of all the FPB models that followed.
Let me be clear that I have NO expertise in hull design, just going by a bit of engineering and lots of sea miles on different boats, but I think both designs can and will work very well, they will just do so very differently. With all but the 64’s being twin screw hulls this would have been a significant factor in the aft end hull design and with the props being fully exposed, no prop tunnels, I suspect that is why the later FPB’s have that very big arc of the hull bottom upwards to clear the tops of the props and then that large flat section aft. Our case was completely different with just a single screw and the use of a prop tunnel and hence our aft hull area is completely different. As you noted, hull design and most things about boat design is a constant seeking of balance between the various trade offs and compromises which is why it seems so important to me to start by determining your design criteria and use cases first and then using these to help guide you through the plethora of decisions big and small throughout the design and even in the build process.
Dennis did extensive testing and modeling as we were determining the location of the prop and rudder and we think the resultant design will work very well. Amongst the many factors we looked at very closely is the one you noted about the possibility of the prop breaking the surface of the water, or “breathing” as you noted, and we did indeed move the prop and rudder a bit forward to achieve what we think is the just right position. With the combination of the prop tunnel and the long smooth curved hull surfaces along the entire length and on either side of the prop tunnel, we should have significantly less lift and forward pitch than the FPB’s would and we think the prop will stay fully immersed in all but the largest waves we might be slicing through.
Time and in the water testing will tell and we very much look forward to those days when they get here.
That post from Steve was interesting to me for his use of the new cameras he has mounted outside now. Very helpful to be able to have multiple perspectives at the exact same time with the real time stamps on each frame. As for the rest of the article I’ll choose not to comment for now as I try to understand the intent and purpose of this one.
But I must say that I have long been puzzled by comments such as the one you endorsed so strongly John because I remain unable to understand who such comments are aimed at or why they are being made? I know of NO sea going Captain who would
and I hope never to meet such a person.
Seriously, do you know a Captain who would chose to put his crew and ship purposely in harms way? Each of us will analyse risk differently and we will make different decisions, and all of us who have been at sea for any length of time have found ourselves unexpectedly “in harms way” and we become better Captains with the learning such experiences provide. But to PURPOSELY put your boat and crew in harms way will remain unimaginable to me.
Well they have done it before: