Nordic tug vs Helmsman

[rslifkin]

Have been told and come to believe there’s no free lunch when discussing efficiency. If you want efficiencies get a FD hull or a very long thin boat. If long/thin you give up space and pay for LOA every where you turn. If FD you give up speed. You will then need longer weather windows to avoid unpleasantness and potentially have troubles going the wrong way against currents.
So the NTs are said to have industry leading efficiency for SD boats but the difference is minor c/w a FD boat.

FD will definitely have better low speed efficiency. But outside of the really efficiency optimized FD designs, the lighter weight of an SD or planing hull typically offsets a good part of the difference (provided the hull is decently efficient at low speeds). Particularly if you're comparing to FD stuff like a Nordhavn where it's not skinny and has as much volume as the SD stuff.



Now if someone built an efficiently shaped, somewhat narrow, light weight FD hull, that would be a totally different story. But most FDs that are out there are pretty heavy.

 

[mvweebles]

A few displacement data points from long range deliveries/passages:

• Nordhavn 57 (Lugger/Komatsu 6-cyl) - approx 6 gph @ 8-3/4 kts (200 nm/day)
• Nordhavn 47 (I think it too had a Lugger/Komatsu 6-cyl) - approx 5 gph @ 7-1/4 kts (175 nm/day)
• Willard 40 (JD4045TA) - approx 1-1/2 gph @ 7 kts (170 nm/day)
• Willard 36 (Perkins 4.236) - approx 1.0 gph @ 6 kts (150 nm/day)

The N57 was definitely the cheetah of the fleet; the N47 a bit of a pig. The W40 with it's JD and double-ended hull was the fuel miser - it carried 700g of diesel and could make SoCal to Marquesas (~3000 nms) if desired using similar amount of fuel as Hippocampus' NT42 did from RI to Cheseapeake (roughly 400 nms burning 625g), though he likely took a very gentlemanly trip with decent stops along the way: being able to run at +10kts opens-up lots of options, especially since the time-penalty in/out of ports are minimized.

Peter

 

[Retriever]

We were traveling mostly in the high-7-knot range, which may have been a bit faster than optimal for the Helmsman. I do think the NT37/39 is super efficient, perhaps because it has a lot of waterline for its length with the plumb bow and molded swim step and it is fairly narrow. Given that the H38 is a foot wider and 7500lbs heavier, higher fuel consumption isn't that surprising.

 

[Hippocampus]

Good comparisons. We did a straight shot with one brief fuel stop. Was time limited as crew had a sick wife to get home to.
With 600g (540g plus 10% reserve) at 6.8k get ~1600nm range. Would do long coastal hops but not passage like Weebles. Expect to go across NJ and GA as not a big fan of going in/out in either state. (Except cape may).
Of interest Weebles hull is efficient because it’s a great design doublended isn’t the reason from what I’ve read. There’s transom hulls equally efficient. C/w a sd hull it’s has much rocker and very clean lines. As a simplification sd hulls try to achieve a planing surface to the detriment of efficiency at displacement speeds. What you see on the surface and above doesn’t matter much. It’s what’s in the water that counts. Consider decades ago racing sailboats gave up on double Enders and canoe sterns. Per available hp many transom stern designs were faster and that’s important for sail. True even in open classes with no design restrictions.

 

[mvweebles]

Well, I have no data beyond observation, but my strong hunch is a double-ender is indeed more efficient. Attached is a great picture of a sister-ship to Weebles on SF Bay doing a little over 5-kts, about 1.0 S/L ratio. Look how clean her stern wake is. Contrast that to transom boats at 1.0 S/L and you will see a ton of eddies behind it. That is all wasted energy.

Where I absolutely agree is what is above the waterline makes no difference. The KK42 is famous for a wine-glass transom that is effectively a double-ender at the waterline.

I snapped a few pics of boats in the boatyard today to show how boats carry beam at the stern/transom. I believe the sailboat with tarp over the transom is a Catalina 38, unique in that it was not an in-house/Frank Butler design but rather a S&S design from the 1970s. A nice reverse transom with very mannerly double-ender at the waterline.

The next picture is a Formula 41 (sistership to Comodave??) - note the steep dead-rise at the transom. Guessing this would move pretty nicely in 3-4 foot chop, but at displacement speeds, would have a ton of eddies dragging the stern.

The last pic is of a classic double-ended trawler similar to the Willard 36. Easy to understand how it slips through water with minimal disruption - let's face it, there's a reason fish are shaped the way they are.

Finally, in my opinion, threads like this often state something to the effect of "at displacement speeds, SD is roughly the same efficiency/fuel-burn as FD." I'm not sure thats true - it ignores effect of Prismatic Coefficient, a design parameter whereby the naval architect matches the hull-form to the intended operating conditions. This is probably why the Willard 40 can run almost 7-1/4 kts (1.2 S/L) in open ocean conditions and burn just 1.5 gph whereas a similarly powered SD would burn over 2-gph at same S/L. Nothing wrong with making that choice to go with SD, but saying efficiency doesn't matter likely isn't entirely accurate.

There are a lot of reasons NOT to own a double-ender such as lost storage and no swim platform, but there are very likely efficiency benefits. The KK42 solution of double-ended at waterline but transom above strikes a good balance.

Peter

EDIT - the reason I own a double-ender is because I fell in love with the Willard. Nothing too rational or analytical. As my wife says "Real boats have round butts!"

 

[rslifkin]

Lots of sailboats strike a similar compromise to the KK. Very few really drag much (if any) transom through the water unless they're heavily loaded or pushing hard speed-wise (at which point you often have enough wind that a little extra drag isn't an issue). The modern down-wind pizza slices may be the one big exception to that concept.


Quantity of submerged transom definitely makes a big difference in low speed drag. At the same engine RPM, from light on fuel and water to full tanks I see almost a 1/2 kt difference in speed with my own boat, as the tanks are located aft and submerge the transom a few inches more when full. Interestingly, at planing speeds where weight usually matters, the extra weight seems to make no noticeable difference. I've found the boat runs faster stern heavy (and with correspondingly more trim tab applied), so I'm betting the change in weight distribution ends up offsetting the extra weight in that situation.

 

[KnotYet]

Assumptions based on aesthetics are not very solid arguments.
Visual eddies in a boat's wake aren't really meaningful drag measurements.

Rather than wonder or debate which hull design details matter for efficiency I'm
more curious to see the tank test data that must exist in naval design databases.

 

[mvweebles]

Assumptions based on aesthetics are not very solid arguments.
Visual eddies in a boat's wake aren't really meaningful drag measurements.

Rather than wonder or debate which hull design details matter for efficiency I'm
more curious to see the tank test data that must exist in naval design databases.

At a qualitative level, they absolutely are valid. By how much? Feel free to cough up the $100k for full blown tank testing. Empirical? The W40 long range running 1.2 S/L at 1.5 gph.

Drive any interstate and youll see all sorts of contraptions on trucks to reduce eddies. Wheels are covered. Baffles beneath trailers. Even angled extensions off the back. Wind tunnel tests of new cars. Winglets on airplanes. This is 40 years past theory - there are examples of efficiently allowing flow to resume position all over the vehicular world. It is a well worn path to improved efficiency.

Peter

 

[klee wyck]

Well, I have no data beyond observation, but my strong hunch is a double-ender is indeed more efficient. Attached is a great picture of a sister-ship to Weebles on SF Bay doing a little over 5-kts, about 1.0 S/L ratio. Look how clean her stern wake is. Contrast that to transom boats at 1.0 S/L and you will see a ton of eddies behind it. That is all wasted energy.

Where I absolutely agree is what is above the waterline makes no difference. The KK42 is famous for a wine-glass transom that is effectively a double-ender at the waterline.

I snapped a few pics of boats in the boatyard today to show how boats carry beam at the stern/transom. I believe the sailboat with tarp over the transom is a Catalina 38, unique in that it was not an in-house/Frank Butler design but rather a S&S design from the 1970s. A nice reverse transom with very mannerly double-ender at the waterline.

The next picture is a Formula 41 (sistership to Comodave??) - note the steep dead-rise at the transom. Guessing this would move pretty nicely in 3-4 foot chop, but at displacement speeds, would have a ton of eddies dragging the stern.

The last pic is of a classic double-ended trawler similar to the Willard 36. Easy to understand how it slips through water with minimal disruption - let's face it, there's a reason fish are shaped the way they are.

Finally, in my opinion, threads like this often state something to the effect of "at displacement speeds, SD is roughly the same efficiency/fuel-burn as FD." I'm not sure thats true - it ignores effect of Prismatic Coefficient, a design parameter whereby the naval architect matches the hull-form to the intended operating conditions. This is probably why the Willard 40 can run almost 7-1/4 kts (1.2 S/L) in open ocean conditions and burn just 1.5 gph whereas a similarly powered SD would burn over 2-gph at same S/L. Nothing wrong with making that choice to go with SD, but saying efficiency doesn't matter likely isn't entirely accurate.

There are a lot of reasons NOT to own a double-ender such as lost storage and no swim platform, but there are very likely efficiency benefits. The KK42 solution of double-ended at waterline but transom above strikes a good balance.

Peter

EDIT - the reason I own a double-ender is because I fell in love with the Willard. Nothing too rational or analytical. As my wife says "Real boats have round butts!"

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Is Katherine Jane (Trawler Cap) in Ensenada now with a Garibaldi hailing port? A few miles under her keel since last year at this time.......

 

[mvweebles]

Is Katherine Jane (Trawler Cap) in Ensenada now with a Garibaldi hailing port? A few miles under her keel since last year at this time.......

No, I should have been clearer. I pulled that pic off TF to show a double ender.

Wish she were here. I'd enjoy being aboard. Fine example of a classic west coast trawler yacht.

Peter

 

[KnotYet]

At a qualitative level, they absolutely are valid. By how much? Feel free to cough up the $100k for full blown tank testing. Empirical? The W40 long range running 1.2 S/L at 1.5 gph.

Drive any interstate and youll see all sorts of contraptions on trucks to reduce eddies. Wheels are covered. Baffles beneath trailers. Even angled extensions off the back. Wind tunnel tests of new cars. Winglets on airplanes. This is 40 years past theory - there are examples of efficiently allowing flow to resume position all over the vehicular world. It is a well worn path to improved efficiency.

Peter

Your examples of aerodynamics are not hydrodynamic data.
I don't feel I am arguing with you, only cautioning that 'gut' feelings should
not be confused with real data. It is entirely possible that the drag on wetted
surfaces matters more than the shape of the transom. In that case a squared off
stern could drag less than a tapered one. There are any number of factors that
either cause or reduce drag that may matter more.

I don't think I need to pay for testing since this has likely been studied for a very long time.

 

[mvweebles]

Your examples of aerodynamics are not hydrodynamic data.
I don't feel I am arguing with you, only cautioning that 'gut' feelings should
not be confused with real data. It is entirely possible that the drag on wetted
surfaces matters more than the shape of the transom. In that case a squared off
stern could drag less than a tapered one. There are any number of factors that
either cause or reduce drag that may matter more.

I don't think I need to pay for testing since this has likely been studied for a very long time.

I'd be interested in actual data too. At a concept level, I understand that hydrodynamics differs from aerodynamics - an airplane prop bears little resemblance to a boat's prop.

That said, my premise is that naval architects design hulls for certain use-cases. Prismatic Coefficient is a variable based on S/L - Dave Kasten has a decent discussion HERE. I can tell you the only flat surface on my Willard 36 is the bottom of the keel and it would take a pair of Pratt & Whitneys to get her to plane. For high S/L multiples (planing), pretty inefficient. Why would the inverse not be true, that a hull designed for planing would be ineefficient at displacement speeds? May not be as bad as getting a W36 to plane, but still, there must be a difference.

Attached is a picture of a cargo ship - likely optimized to run at a certain speed around 1.3-1.5 S/L. Note the double-ended strern at waterline despite the fact it has to relinquish cargo space to achieve. Certainly not data, but if the difference were diminimus, would think the cargo ship would go for the extra cargo space.

Peter

PS - apologies that my response came off argumentative. Uncalled for.

 

[rslifkin]

Visibly turbulent water flow is an indication of drag. However, it's not a good indicator of total drag. It's just one thing you can see, but in an extreme comparison, there could be other sources of less visible drag that would be just as bad.

 

[Hippocampus]

Unfortunately $275 for the book but the abstract belies the benefits of double Enders for efficiency
https://nap.nationalacademies.org/read/1841/chapter/20. This is for ships. For boats read Kasten, Perry, Gerr or whoever is your favorite NA.

Current understanding from computer modeling and old school tank testing shows the current slice of pizza hull to have less parasitic drag then either prior traditional transom sterns or double enders. Even traditional transom sterns are more efficient. Point to single buoy or ocean race won by a double ender in the last 50 years if other forms of hulls were allowed in the race.

Similarly the risk of pooping is increased not decreased in true double enders. Even canoe sterns fair poorly when c/w modern or traditional transom sterns. There’s usually less reserved buoyancy in those designs. Dynamic wetted surface is decreased and laminar flow better preserved with non double enders correctly designed.

Finally boat don’t run on a level plane. Even with stabilization they heave, corkscrew, pitch, and roll. Given this movement the double enders increase their parasitic drag and disturb their boundary layers more than other designs.

Historically for plank on frame the double ender was a easy way to construct a very strong hull. We don’t commonly build in wood anymore. The paradigm has shifted. The dynamics of ships and boats is different in several regards. I’m not a NA and have limited schooling in this subject. But believe Bob Perry who drew more recreational canoe and double ended recreational boats then anyone to my knowledge. He said he wouldn’t own one for his personal use and given current available building materials and knowledge they didn’t make sense for recreational boats. He definitely did a good job as you continue to see Valiants in all corners of the world. I owned and cruised a Crealock designed Pacific Seacraft. A canoe stern fairly close to a double ender. We saw some weather in her and she did great but the Outbound far exceeded her performance. Schumacher applied the evolution of naval architecture eschewing the canoe/double form. For years now on Attainable Adventures there’s been a ongoing debate about “the benefits of a balanced ended transom hull v a slice of pizza. Double ended sterns aren’t considered any longer as design has moved on.

Yes double enders are drop dead gorgeous. Prettiest girl at the dance every time. There are better sea boats and more efficient ways to handle a stern. A little time with google would confirm that opinion.

Still think the Willard is the epitome of that hull form and a most remarkable boat. If they still made them especially if done in the 40-45’ range I’d buy it right off. A truly great boat and design.

BTW personally think Williards are canoe sterns not double ended.

 

[mvweebles]

Hippocampus - I don't argue with much of what you've pointed out. Double enders (or canoe sterns) have many shortcomings that make them an unattractive choice.

My point was limited to efficiency when moving a boat at displacement speeds in the 1.0-1.2 S/L range, especially one loaded for cruising, a use-case that is arguably at the heart of a forum named "TrawlerForum." A racing sailboat is designed to shed traditional bow/stern wave entrapments. In a way, it sort of supports my idea that hulls are optimized for certain S/L speeds. The pizza slice speedsters are designed for much higher speeds than 1.2 S/L.

The last couple months have had countless threads about people seeking to save on fuel costs and plan to slow down to something in the 1.0-1.2 S/L range. Some boats were designed to operate at that speed multiple. Many were not. I can give chapter and verse on the tradeoffs about no swim platform and limited laz storage. Rationally, I fully understand why double enders are no longer made. You won't hear me say they track better in a following sea (though full keels do). But to say they both have the same/similar efficiency at displacement speeds is doubtful.

My hunch is Bob Perry was stunned at the success of the Valiant 40. He recovered nicely and smiled all the way to the bank with subsequent designs.

Peter

 

[Hippocampus]

If the concern is fuel burn we’d all be on long light thin boats. By far and away he most efficient power craft at displacement speeds.
Issue is berthing expense and our desire for displacement and useable space. As proven they can be self righting, efficient even above square root of lwl x 1,32, and with available stabilization technologies quite comfy in a seaway. Issue isn’t the physics. There’s a reason LM is yet another to join that movement.
Wouldn’t be surprised to see 50’ers at a reasonable cost for a retired mom and pop with limited air draft be in the works. Aimed not at voyaging but rather long coastal, European canals, Loop and Ditch. The smaller Blue Waters are pretty close to that and even cost competitive to. N41. North Americans tend to be very traditional in thinking about boats. Fe never took on where as the Dutch Fe boats in power ace a big following. Kanter and others have tried but aren’t big players. If electricity costs come down so AL is less dear perhaps Americans will become more accepting of that paradigm.
See some yards billing for layup by the square foot not just loa. Probably a response to multihulls. Still that could benefit efficient monohulls .

 

[mvweebles]

If the desire is how to efficiently go significantly faster than 1.2 S/L, then long skinny without excess weight is an answer. Different conversation.

There is a common thought that by slowing a SD trailer down to displacement speeds, you will more or less achieve the efficiency of a FD hull. "Best of both worlds" is a common refrain. Slowing down will certainly increase MPG, but that does not get you to the efficiency of a boat optimized to run at displacement speeds. It's why prismatic coefficient exists in the naval architecture world.

Peter

 

[Hippocampus]

It’s not only prismatic coefficient. No single number defines a boat. It’s the totality of all its characteristics. There’s a common misperception that full keel boats tract better than other designs. Actually testing, modeling and use has shown that to be untrue. In both camps some do some don’t. Put a skeg at the back of a pencil and it will tract well. Similarly I’ve heard diatribes about hard chine v radius chine v soft chine that aren’t justified by the results of data.
But you’re correct . A boat optimized to function at a variety of speeds will not be a master of any. A boat optimized to function at a certain displacement, operated at a certain speed and certain loa that more perfectly preserves laminar flow throughout its entire boundary layer at all expected angles of heel will have less parasitic drag and be more efficient requiring less hp and hence fuel burn. True for sail, power FD, SD or planing.
Little appreciated is the impact of appendages in these forum discussions. We all recognize keels, fins fish and rudders add drag but so do exposed shafts, rest of supports for running gear. Similarly any piercings of the canoe body(hull body not stern) induces drag. Not only the holes for bow and stern thrusters but even each and every through hull. There’s a reason for that indentation aft of bow thrusters commonly seen on sail. Nordhavns pay a penalty to expand head room in engine rooms. Anything disturbing smooth flow costs fuel burn.
Your Williard is smooth. It’s a “balanced” hull. So are the DDs with their hard chines and transom sterns. It’s efficient but still handles well. NA is well above my pay grade but the concepts are simple physics. It’s the combination of a multiplicity of factors, not just one, that produces a optimized hull for efficiency.

 

[rslifkin]

If the desire is how to efficiently go significantly faster than 1.2 S/L, then long skinny without excess weight is an answer. Different conversation.

There is a common thought that by slowing a SD trailer down to displacement speeds, you will more or less achieve the efficiency of a FD hull. "Best of both worlds" is a common refrain. Slowing down will certainly increase MPG, but that does not get you to the efficiency of a boat optimized to run at displacement speeds. It's why prismatic coefficient exists in the naval architecture world.

Peter

The SD hull is still less efficient at the lower speed than the FD hull. However, weight factors into efficiency. If the SD hull is less efficient, but only moving 40k lbs of boat instead of 60k lbs, that will make the end result difference in fuel burn smaller than the hull shape alone would lead us to expect.

Because weight becomes more of a performance impediment at higher speeds, faster boats (SD and planing) tend to be built lighter. FD is often built on the idea of "build strong and add weight".

Of course, if you lightened up the FD hull to match the weight of the SD, you'll widen the efficiency gap. There just aren't many light weight FD hulls actually available out there.

 

[mvweebles]

For the standard 42-foot trawler cruised by a couple, I wonder what the amount of weight is added in gear and stores. In the sailboat community, dirty secret is that car owners lose much of the speed benefit due to adding too much weight.

To be clear, I'm not advocating double-enders/canoe-stern designs. Willard, with their canoe stern, was a bit of a unicorn in the market and exited the recreational trawler business over 20 years ago as a result of their design decisions.

There are many, many benefits to a pure slab-transom boat. Even loving my canoe stern, I believe the efficiency penalty is more than offset by those benefits. The Defever 44, a displacement boat with a pronounced transom, is a great example and a boat I've admired for years. With RT Firefly listing his Cheoy Lee 46, I'm aware of another displacement/transom (BTW - at $130k for a stabilized boat from a respected builder, a helluva deal in my opinion).

Peter

 

[rslifkin]

For the standard 42-foot trawler cruised by a couple, I wonder what the amount of weight is added in gear and stores. In the sailboat community, dirty secret is that car owners lose much of the speed benefit due to adding too much weight.

We're not full time on the boat, but I've tried to ballpark the weight of "stuff" on the boat at some point. I never came up with a terribly close estimate, but as best I figured, it can't be over 1000 lbs for us (if it's even that much). That's all of the gear, spares, clothing, tools, food, and other stuff on the boat. Being full time would add a bit more weight, but realistically, not a lot. There's only so much more that we'd want with us and could actually bring before running out of space.


We've also added probably 1000 lbs worth of additional equipment weight to the boat (larger water tank, house batteries, solar, inverter, etc.).

 

[mvweebles]

We're not full time on the boat, but I've tried to ballpark the weight of "stuff" on the boat at some point. I never came up with a terribly close estimate, but as best I figured, it can't be over 1000 lbs for us (if it's even that much). That's all of the gear, spares, clothing, tools, food, and other stuff on the boat. Being full time would add a bit more weight, but realistically, not a lot. There's only so much more that we'd want with us and could actually bring before running out of space.


We've also added probably 1000 lbs worth of additional equipment weight to the boat (larger water tank, house batteries, solar, inverter, etc.).

Oh boy, but this is worth its own thread. When I unloaded Weebles for the refit, I was stunned at how much stuff I'd squirreled away. Part of it was being pretty involved in the trawler industry for years and collecting all sorts of spare parts. Spare pumps, helm rams and pumps, windlass, bronze valves and pipe fittings, prop, all sorts of tools, you name it. Dozens of spare jackets for guests, sewing machine, you get the idea. A good 4 inches of waterline appeared when I offloaded.

But I think it has less effect on a FD than a SD operated at FD speeds. No data, just a hunch.

Peter

 

[rslifkin]

Oh boy, but this is worth its own thread. When I unloaded Weebles for the refit, I was stunned at how much stuff I'd squirreled away. Part of it was being pretty involved in the trawler industry for years and collecting all sorts of spare parts. Spare pumps, helm rams and pumps, windlass, bronze valves and pipe fittings, prop, all sorts of tools, you name it. Dozens of spare jackets for guests, sewing machine, you get the idea. A good 4 inches of waterline appeared when I offloaded.

But I think it has less effect on a FD than a SD operated at FD speeds. No data, just a hunch.

Peter

With SD, I think the location of the weight will matter a lot. On plane, my boat is very sensitive to weight placed forward, but added weight aft makes very little difference (other than climbing onto plane a little slower). My theory on that one is that with our very large trim tabs, being bow heavy means less tab deployed and the boat runs deeper in the water due to less total lift. Shifting weight aft means more tab deployed and more total lift for a given amount of weight, getting the boat higher out of the water and reducing drag.



At displacement speeds (around 1.15 S/L for a typical slow cruise for us), it's very different. Total weight doesn't seem to make a big difference from what I can tell, but the more bow-heavy the boat is, the faster it runs at a given engine RPM (and with less wake). The difference from 3/8 fuel and water to full tanks is almost a 1/2 kt (those tanks are all aft). At full tanks, 1300 RPM gives us 6.4 - 6.6 kts in calm water. Once we're light on fuel and water, we're up to 6.8 - 7 kts at the same 1300 RPM.



For a flat transom SD or planing hull, loading it stern heavy leads to more submerged transom and more drag. Loading a bit bow heavy may hurt handling, but helps efficiency, as there's less transom in the water and also often an upward slope of the hull bottom to the transom in that condition, both of which reduce drag.

 

[White Gold]

I own a Helmsman 38. I had an American Tug which I’m led to believe would be Nordics closest competitor.
My honest opinion I’d that both the AT and Nordic are too tight. Both however are well built vessels. I’m thrilled with my Helmsman. Quality wise it’s as good, if not better than the Banks I once owned.
That’s just my opinion based on experience. Too many positives for the Helmsman to write herein. Hope this helps.