The Offshore Voyaging Reference Site

Artnautica 58—Design Analysis

LRC58 007 perspective views

In the last chapter, John wrote about a new motor yacht design that’s quite different from just about anything you’ll find coming off a production line. Now it’s time to put our preconceptions aside for a moment and consider if, from a technical perspective, the boat is likely to work as expected.


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Bob N

I’m aware that the helm station is well positioned in relation to the pitching centre but its low height means that vision directly over the bow will be poor. I like it being low because the roll arc at head height will be minimised but I’d opt for giving it a little more height and slightly more distance from the pitch centre to get better forward vision.

John Harries

Hi Bob,

I’m not sure that the “vision directly over the bow will be poor”. Dennis gave Phyllis and I a Skype video tour of the boat and the visibility from the inside helm station looked pretty good to us. These things are deceptive. I was on a motorboat the other day where I could have sworn looking at the drawings of said boat that the visibility from the helm would be poor, and it was in real life very good.

Also there is a raised outside helm station just aft of the wheelhouse on the Artnautica. I’m going to guess that the visibility from both her stations would be significantly better than that from the average aft cockpit sailboat helm.

Dennis Harjamaa

Hello Bob,
The boat has a fairly high bow and looking at the profile is would appear to impede vision a fair bit. I just posted a picture taken from eye height at the helm station on the Facebook page. You will notice that since the bow is fine the visibility is actually better than one would think. I also posted a picture from the exterior steering station.

Dougl
james

Here in Europe the cost of berthing this very fine vessel would be prohibitive that is if one could even find a berth.

Roger

Great looking design. Put a short, freestanding carbon spar on her and you’d cut the fuel burn in half when motorsailing off the wind.

Marc Dacey

I think that has merit, the short mast idea, as would a free-flying Parasailor chute off the bow for downwind “sail assist”. A short mast would also dampen roll in a quartering sea, something to consider in a narrow boat like this.

This comment, however, gave me pause: “In other words, fully 25% of the boat is fuel – an unusually large figure that implies a serious long-range cruiser.” It also implies (to me) a serious issue with stability numbers as said fuel, which by definition comprises part of the ballast, is used up. If you are down to your last 100 gallons, isn’t the actual waterline going to change along with handling characteristics? Or am I missing something?

She’s a beauty, regardless. I get the sense I’m looking at the future of distance powerboats instead of the prevalent “trawler/Nordhavn” model (not that I don’t like them as well).

Marc Dacey

I suppose the alternative might been ballast tanks that could be selectively filled and later blown with seawater to maintain trim…but that affects power requirements (for pumps and automatic valves), weight and fuel burn, and of course, interior volume.

I just recall one of the few times I’ve been in a “light load” powerboat in any sort (3-4 feet) of a running sea…it got pretty brutal and not just on the flybridge.

Dennis Harjamaa

Gidday Matt,
First, good job on writing this post.
I made a bit of a design change a while ago. My original plan was to have two 500 litre water tanks in the engine room but when the quote came in for fabricating said tanks I decided to purchase a simple water maker in stead. The extra cost was not much and the space saving benefits even greater.
The unit is an Open Ocean made right here in NZ.
So now I have the option to keep the built-in 1300 litre tank topped up at all times while on passage. This tank is as low in the boat as possible under the lower accommodation sole.

John Harries

Hi Marc,

Actually, I don’t think that using up the fuel would have much, if any effect on stability. The reason is that the fuel is in vertical saddle tanks and so when full I’m going to guess that much of the fuel is above the center of gravity. In fact the boat might even get more stable as fuel is used.

As to the waterline, with the plum bow and stern immersion is going to make very little difference to length, I would think.

Matt, Dennis?

Anyway, I gather from Dennis that the boat has great stability and recovery from capsize numbers—way better than most any trawler.

Marc Dacey

OK, John…I am familiar with saddle tanks. My buddy in a steel Goderich 40 ketch has them. He uses a cross-transfer pump to improve trim when he’s around half-tank level. I plan to do the same with my water tankage (my fuel tanks are all keel situated).

Thanks for the explanation.

Dennis Harjamaa

The self righting capacity of the boat is unaffected by the fuel load. If anything, she will right herself more readily at lighter loads.
No doubt she will feel more lively when the fuel load gets lighter. How much that will affect levels of comfort onboard is one of the more interesting questions I’m looking to answer when I get the boat in the water. I have not planned seawater ballast tanks but these could be incorporated in the design if deemed desirable – at a cost to interior volume of course.

Marc Dacey

Thanks, Dennis, and may I say it’s a privilege to have the fellow who drew and is building Hull No. 1 to go deeper into the design elements.

One thing I *think* I see from the 3D drawing are cambered decks…is this the case?

Dennis Harjamaa

Hello Marc,
Yes, the fore deck is cambered apart from an 800mm flat strip along the centreline. I did this to make hatch and ventilator installations easier.
The pilothouse roof is in three flat sections and the cockpit sole is flat but with one degree fall aft.

John Harries

Hi Roger,

Actually, in my experiences, and I have motor sailed a lot offshore, motor sailing does not work off the wind because the apparent wind is so low, particularly in the lulls, that the sail starts crashing about. It also has very little fuel burn benefit when off the wind for the same reason. A sailboat may wish to tack down wind in this case, but that would not make sense for this boat because the increased distance would outweigh any fuel burn savings from filling the sail.

Motor sailing does work well reaching but of course not dead up wind. So we have to ask ourselves whether or not the additional cost and complications of a rig and the design changes required—you can’t just stick a rig in the boat as is, it will have effects on the whole design—would be worth it. I don’t know, but I’m going to guess not. Matt, Dennis?

Dennis Harjamaa

Hello Roger,
I’m planning an emergency backup sail rig to help me drift controllable towards land in case the engine gives up.
The plan is to use one of the two booms as a mast, hoisting it up and securing it up with a forestry and shrouds to the aft corners of the cockpit.
This is not fully detailed up yet, it’s a bit of a tricky one since I want the whole shebang to be able to fold down for getting access into the European canals.
An alternative could well be a kite by Omega Sails in France. If anyone has first hand experience with one of these I would be very keen to hear. The French Ecotroll uses one.

pat synge

A minor point but why the forward raked front to the deckhouse? Surely the wind resistance is significant. A sleeker profile would provide more space below while reducing wind resistance. Perhaps not significant in calm conditions but when pushing into 40 knots it would be noticeable.

John Harries

Hi Pat,

As part of my project of learning a bit more about power boats I have spoken to several experienced owners, including one circumnavigator. They have told me that reverse raked windows are highly desirable and much better than the those where the bottom of the window is forward of the top for three reasons:

  • Improves visibility by reducing reflections from gear in the wheelhouse.
  • Sheds water better.
  • Is much cooler in hot climates.

I guess those reasons outweigh any slight drag advantage.

This is probably why most commercial vessels that I’m seeing today have reverse slanting windows like this boat.

Dennis Harjamaa

All the necessary comments have already been made in favour of the forward slanting windows except my favourite one: they look the business!

John Harries

Hi Dennis,

I secretly really wanted to say that!

Eric Klem

Hi Matt,

Thank you for the analysis. I was wondering whether you could comment on how you think that this boat will do going right into bad weather as you have mentioned that this will necessarily be an important storm tactic. I guess that my question is really about how much energy is dissipated by the motion of the hull and the wave action against it at different speeds as opposed to how much power is available. In my own boat which pitches a lot more than I would like in these conditions, we need a lot of power to maintain a very slow speed (and sometimes we don’t have enough and can’t keep the bow into it using the engine). A few years ago, I was sailing up the New England coast and passed Wind Horse as it was headed south into quite choppy ~8′ waves. While the conditions were certainly not very bad, I was impressed by how level the boat stayed and how little horsepower it appeared to take to move along at a nice pace.

I have no experience on this type of hull form but I have been on a number of boats with much higher power to displacement ratios and reasonably quick lines that are unable to stay head to wind in conditions that I would not consider very bad. What I am really getting at is how do you think this boat would do in really rough conditions? If the main storm tactic is to slowly jog into it, is 75 hp enough for the hull design to keep the bow into 30’+ breaking waves and 50 knots of breeze should you get trapped and be unable to outrun the storm?

I am not particularly familiar with this specific design but a vessel that relies on a single form of propulsion does scare me a bit. Maybe I missed a get-home option but it seems like you would need this and a backup option for heavy weather as well. In general, I think that it is a really appealing design and maybe I am just being too conservative.

Eric

John Harries

Hi Eric,

On the issue of backup power. I discussed that with Dennis. For his own boat he is planning to experiment with the new French kite sails that are designed for that purpose.

He also stated that there is plenty of room on the port side to fit a small get-home engine. Having said that, and talked at length with several motorboat owners, I think that get-home engines are often an illusionary security, particularly in the situation you postulate, since they don’t have enough power to be useful.

I think that if I were to buy an Artnautica I would first need to satisfy myself that she could safely lie to a Jordan Series Drogue off the stern in storm force conditions without power. My guess, and it is just that, is that she would do fine in that configuration since the JSD would prevent her from accelerating down a wave face to a speed where any tendency to bow stuff or steer would become a problem.

Having said all that. I’m just as interested as you in her ability to jog upwind into storm conditions. Here I think that she might surprise us to the positive side because the rig in sailboats, as you would know, has a huge effect on pitching moment—no rig, much less pitching.

This was graphically demonstrated to a friend of mine two falls ago when motoring into a nasty swell trying to get out of Buzzards Bay in a 60-foot ketch equipped with 150 hp engine and a big fixed prop. Another friend in a Nordhavn 46 came past them like they were standing still, despite being a much smaller boat with less horsepower. Of course there are a lot of variables, but as far as the two friends could see, the big difference was that the motor boat was just pitching less.

We have also noted the same thing in our own boat: since we replaced our old cracked aluminum mast with a carbon mast, our speed under motor upwind into waves is much improved even with no sail up and we just don’t get stopped by pitching, even though we only have 87 Hp in a boat that is much heavier than the Artnautica.

Erik de Jong

On the topic of “getting home”; the majority of the cargo ships that log around 1.5 to 2 million NM during their economical lives, do with just a single engine as well. They just have to fix it to such a level that it can bring them to port. In the old days, major main engine repairs had to be done on a monthly basis or even more frequent, in the modern days this barely ever happens.

I don’t think that there is a big probability that one will encounter an engine failure that can’t be (temporarily) repaired at sea if the proper tools and spare parts are carried. Assuming that the level of maintenance of the engine and attached systems is raised to an art.

John Harries

Hi Erik,

I would agree with that. In some 45 years of going offshore with diesel engines I have only had two failures and in each case we were able to fix them at sea.

The secret seems to be good maintenance, some parts and tools, and a bit of basic common sense.

My friend Bob T, a professional mariner, fleet fisherman, and sailboat owner tells me that in his some 50 years (he started young, fishing with his uncle) he has never had a diesel engine fail to bring him home. Bob has just bought a single engine powerboat and actively rejected twin engine versions of the same model.

Eric Klem

Erik and John,

While I agree with both of you that a single diesel engine is actually remarkably reliable, I fear that this would not work for a large portion of the cruising community due to a lack of comfort with making repairs at sea. Maybe the answer would be that the cruising community needs to become better educated about repairing these engines and their systems as well as caring appropriate tools and spares. Realistically, rebuilding an engine is really not any harder than setting up a jury rig if your mast just went over the side if you have ever done one before (I might be a bit biased here as I used to design industrial air compressors which are basically diesel engines minus the fuel system). On my own boat, the most difficult part is that I would have to take the engine out to pull any major components as I can’t get the head or pan off due to height constraints in the engine compartment.

Something else to think about is that many twin engine setups are not really independent and give a false sense of security.

Eric

Eric Klem

Hi John,

Thank you for the reply. I think that all of the points that you make are good.

You make a very good point about the relationship between the polar moment of inertia about the pitch axis and the ability to motor to weather. Yet another reason to keep the weight in the rig and the ends of the boat to a minimum. I couldn’t see in the pictures of the design whether the anchor chain is stored back from the bow but that would probably help a lot.

As you suggest, the combination of a JSD and some form of sail as the get home option seems like the best route to go to me. I wonder what Dennis’ actual plan for heavy weather is and whether it is the JSD?

Eric

Dennis Harjamaa

I have read a lot of good things about a Jordan Series Drogue and one of those will likely be part of the inventory.

John Harries

Hi Dennis,

You might want to consider incorporating the strong points required for the JSD now, while you are building. Also, I would think about adding built in storage so that it can remain ready to deploy at all times. We detail all of that, together with chapters on deployment and retrieval in our Heavy Weather Online Book.

Dennis Harjamaa

Hi John,
I’ve had in mind that the bollards in the cockpit would work well with a JSD. I’ll read the book to confirm what else I can incorporate into the boat to make handling the gear easier.

Eric Klem

Matt,

Thanks for ballpark guesses, they seem quite reasonable and confirm my suspicion that the ultimate heavy weather tactic cannot be reliant on the engine alone because it would require more power.

I have always thought that the design of boats is funny in that it often results in an unstable set of forces when pushed past normal limits. The fact that the thrust is back aft and the resistive forces are mostly up forward when pushing into weather mean that the boat wants to spin around. And then at anchor, a lot of boats have a lot of windage forward and a CLR quite far aft (not talking about the Artnautica here) so they are again unstable. I wonder whether someone will eventually be able to come up with a way to overcome these issues without introducing even worse ones or badly compromising everyday efficiency. Overcoming the instability of the location of the thrust could go a long way towards making a boat that can hold its own motoring into heavy weather without a lot of power, just look at how many boats with narrow sterns actually back to weather better than they go forwards because they are not putting a ton of energy into steering.

Eric