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Old 04-19-2021, 06:43 PM   #1
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Confused by semi-displacement

My understanding has been that a semi displacement vessel is not suitable for ocean crossing, however, I’ve read and seen people using Flemings to cross. I’m just curious about this discrepancy. Does Fleming do something different with their semi hulls that make them able to cross oceans?
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Old 04-19-2021, 06:53 PM   #2
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My understanding has been that a semi displacement vessel is not suitable for ocean crossing, however, I’ve read and seen people using Flemings to cross. I’m just curious about this discrepancy. Does Fleming do something different with their semi hulls that make them able to cross oceans?
Welcome aboard. Some semi displacement boats can go very far by slowing down. Maybe not as far as a full displacement hull but maybe far enough. I know I would love to have a Fleming.
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Old 04-19-2021, 08:43 PM   #3
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Actually the term should be LESS SUITABLE.
Also a better term for the hulls often called Semi displacement would be SEMI PLANING because with enough power it can plane.
Even with a huge amount more power a F.D. hull will NOT get on plane.

More attention must need to be paid to fuel useage and range, the handling of rough conditions and so on with a Semi Planing hull.

However that does not mean that a F.D hull gets off scot free. The owners must still plan, watch the weather, take range into account and plan properly.

THe owner of a S.D. type hull must do more so but many S.D. can go if the owners work is done properly.

JMO.
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Old 04-19-2021, 08:53 PM   #4
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It's not that an SD hull is inherently unsuitable for crossing oceans, it's just that many SD boats aren't meant for that purpose and lack the range or other capabilities you'd want.
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Old 04-19-2021, 09:31 PM   #5
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Ocean crossing is best done w a heavy FD designed hull. And the biggest reason may not be it’s seaworthyness but it’s range. If you only make it 3/4 of the way across you’ve failed.

But FD gives you much more than fuel efficiency.
Room to live and a nice (relatively so) motion is golden. Also keeping the helmsman alone and in a dark wheelhouse has a lot of merit.

We’ve only recently began to cross oceans w relatively small powerboats. Only sailboats could go the distance.

Nuff said as this is Trawler Form, not Passage Maker Forum.

Get the book “Voyaging Under Power” by Bede for more information.
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Old 04-19-2021, 09:40 PM   #6
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is a Nordhavn a FD?
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Old 04-19-2021, 09:56 PM   #7
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Ocean crossing is best done w a heavy FD designed hull. And the biggest reason may not be it’s seaworthyness but it’s range. If you only make it 3/4 of the way across you’ve failed.

You guys aren't keeping up with trends in sailboating, for example the Volvo race or the expanded use of catamarans for trans Atlantic crossings. All kinds of crazy and small vessels have crossed the Atlantic, even row boats. Given the right weather window just about anything can cross as long as the fuel holds out. And obviously most didn't see my thread about the guy(s)? who paddle boarded from Seattle to Alaska, hats off to him/them.

Looking at the Aspen power cat boats, their specs are impressive as to speed/gph. I'm beginning to think they are the perfect PNW, coastal BC, SE Alaska boat. What holds me back with my enthusiasm is the lack of space at most marinas for boats that wide, most marinas here are old. What I like is speed plus low gph performance. And more and more trawlers are becoming fast trawlers, there is a new preference for speed versus slow and slower. You can just see this trend at boat shows, particularly Seattle and Vancouver with a significant drop of sailboats and an increased number of power boats.
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Old 04-20-2021, 04:47 AM   #8
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"We’ve only recently began to cross oceans w relatively small powerboats."


Captain Marin Marie – his actual name was Paul Emmanuel Durand Couppel de Saint-Front (1901-1987) – was a famous French painter, writer, naval architect, and sailor. In 1936, he single-handedly brought the Arielle, his wooden power yacht of 42’7” with a 75hp diesel engine from New York to Le Havre, bringing his boat under power into port without a mishap. Marin Marie has the indisputable record of being the first captain crossing the Atlantic Ocean under power on a leisure yacht! In 1936 he received the Blue Water Medal Award from the Cruising Club of America. And in 1945, he wrote his best seller, “Wind Aloft, Wind Alow”.
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Old 04-20-2021, 06:02 AM   #9
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+1 to the many of the posts above, good points well stated.

Keep in mind that a hull is designed for a purpose and the engines are matched for that purpose. A SD hull is not going to handle large seas in the same way that a FD hull will. Additionally, cruising at a speed not optimal for which she was designed is a detriment. Yes, I know about running up to engine load recommendations every hour or so, but the bottom line is the range and handling on a SD yacht is designed for a different purpose, different usage than a FD hull.

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Old 04-20-2021, 06:40 AM   #10
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And...

Stabilizers of various persuasions can -- to a certain extent -- improve the way a semi-displacement hull handles seas...

-Chris
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Old 04-20-2021, 07:50 AM   #11
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Ocean crossing is best done w a heavy FD designed hull. And the biggest reason may not be it’s seaworthyness but it’s range. If you only make it 3/4 of the way across you’ve failed.

But FD gives you much more than fuel efficiency.
Room to live and a nice (relatively so) motion is golden. Also keeping the helmsman alone and in a dark wheelhouse has a lot of merit.

We’ve only recently began to cross oceans w relatively small powerboats. Only sailboats could go the distance.

Nuff said as this is Trawler Form, not Passage Maker Forum.

Get the book “Voyaging Under Power” by Bede for more information.

Range is definitely a big one. Until you get into a bigger boat (probably somewhere in the 55 - 60 foot range), you'll run into a few problems with SD, range vs speed being the big one.

By the time you equip it to cross an ocean, add enough fuel, etc. it's going to be so heavy that you'd need a ton of power to actually exceed hull speed by much (and you wouldn't be able to use that power on a crossing due to range concerns). So you might as well just go FD and not try for speed. In a bigger boat, that equation starts to change, however.

In smaller boats, there's also the challenge of making an SD hull ride well below hull speed. As you get bigger, this becomes a bit easier.
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Old 04-20-2021, 09:21 AM   #12
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is a nordhavn a fd?

yes
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Old 04-20-2021, 09:59 AM   #13
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rslifkin wrote;
“By the time you equip it to cross an ocean, add enough fuel, etc. it's going to be so heavy that you'd need a ton of power to actually exceed hull speed by much (and you wouldn't be able to use that power on a crossing due to range concerns). So you might as well just go FD and not try for speed. In a bigger boat, that equation starts to change, however.”

I agree w most and well put.
Hull speed? Crossing oceans will be done at least a knot below hull speed. Who is going to be “trying for speed”?
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Old 04-20-2021, 10:08 AM   #14
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I agree w most and well put.
Hull speed? Crossing oceans will be done at least a knot below hull speed. Who is going to be “trying for speed”?

On a smaller boat, that's absolutely the case. But for a larger boat where carrying more fuel is easier, maybe not. You could easily have an 80 footer that's meant to cross the Pacific at 10 kts (~2 kts below hull speed). But with the right SD hull form and power, you could probably cross the Atlantic comfortably at 12 - 13 kts and make shorter crossings at 15 kts. Think something like the Dashew FPBs. Not fast in an absolute sense, but fast compared to a Nordhavn or other typical slow, FD ocean crosser.
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Old 04-20-2021, 12:52 PM   #15
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My understanding has been that a semi displacement vessel is not suitable for ocean crossing, however, I’ve read and seen people using Flemings to cross. I’m just curious about this discrepancy. Does Fleming do something different with their semi hulls that make them able to cross oceans?
Any "semi-displacement" boat could cross the Atlantic given enough fuel, cojones and luck.

The Flemings are not particularly suited to this but it has been done. For example Beluga did and made it all the way to my home port in Menorca where it was was based for several years.

https://www.flemingyachts.com/news/f...urope-on-a-f55
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Old 04-20-2021, 10:24 PM   #16
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I forget which one but either the Nordhavn 43 or 41 is equipped to cross oceans.
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Old 04-20-2021, 11:57 PM   #17
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Most USCG patrol boats are semi displacement, yet they're on the ocean every day.
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Old 04-21-2021, 01:36 AM   #18
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For pleasure boats of most lengths:

I love a really well designed SD hull. I thoroughly enjoy well designed P hulls. I am not particularly a fan of D hulls [for general use purposes]; unless fuel reduction/economy is required to accomplish long distances.

The way a properly weighted/trimmed boat rides and handles is pretty much up to the expertise of the captain [whatever its hull and superstructure design as well as its power availability may be]. Speeds traveled at, techniques for dealing with seas at hand and capability to efficiently "tack" are boat handling attributes that can and will improve a particular boat's seafaring capabilities/ailities.
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Old 04-21-2021, 01:22 PM   #19
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Most USCG patrol boats are semi displacement, yet they're on the ocean every day.
USCG Patrol boats need speed....and they also need seakeeping ability. The best solution to the problem is a semi-planing hull. But they do not need ocean crossing range.
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Old 04-21-2021, 01:48 PM   #20
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isn't it a fundamentally a trade-off between speed and comfort?

Oversimplification perhaps, but I thought a semi displacement hull generally speaking can acheive greater speeds but at the cost of sea comfort in some conditions.

tankage/range, layout design, storage space, etc...are all kind of secondary or indirect to the primary difference in the design aren't they?
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