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?

Valone123 said:

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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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.

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.

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.

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.

is a Nordhavn a FD?

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.

"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”.

+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.

Judy

And...

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

-Chris

Nomad Willy said:

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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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.

phillippeterson said:

is a nordhavn a fd?

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yes

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”?

Nomad Willy said:

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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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.

Valone123 said:

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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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/fleming-owners-travel-from-florida-to-europe-on-a-f55

I forget which one but either the Nordhavn 43 or 41 is equipped to cross oceans.

Most USCG patrol boats are semi displacement, yet they're on the ocean every day.

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.

Lepke said:

Most USCG patrol boats are semi displacement, yet they're on the ocean every day.

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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.

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?

skyhawk said:

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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No. SD hulls are very different than D hulls.

Sticking with SD hulls there are a lot of differences between boats. Some lean close to planning hulls, and some lean closer to D hulls.

In a planning boat the run of the bottom from amidships to the transom will be flat. In a D hull it will rise as water moves aft toward the transom for smooth water flow. The chines on a planning boat are generally sharp-edged, and those hard chines very often run up into the bow creating visible sharp-edged chines above the waterline. The chines on a D boat are very soft, to the point where often from the keel to the waterline the shape is round, or nearly so. So then in SD boats the chines can hard, or softened a bit at the edges. With that carried into the bow or not. Often there is some rise of the bottom toward the transom. Planning boats have no keel, or nearly none. D boats have a serious keel.

How much of all of that is present in a SD boat has a lot of impact on how the boat acts.

And comfort when / where? At speed a planning hull can feel quite stable. At rest not so much. Deep V planning hulls are very stable at speed, but roll an awful lot at rest. D boats very often have stabilizers, because those very round chines allow a lot of roll unless dampened somehow.

I would not generalize around comfort, but about ease to drive the boat with great fuel efficiency. Moving it with little power. But that too can be a gross generalization, and I'm sure someone will quickly point out a D hull that is quite comfortable without stabilizers.

Look at it this way. A normal sailboat is a D hull. It is designed to move with the best speed with only a puff of wind to push it. Substitute a puff of wind with a small HP diesel and you have a D hull trawler. Now if ever there was a gross simplification, that's it.

FWT wrote;
“In a planning boat the run of the bottom from amidships to the transom will be flat.”

Not flat. Just straight. Generally speaking zero degree quarter beam buttock line to be technical.
Lots (most) planing hulls have dead-rise and variable dead rise on the same hull. Take the older Mainship 34. Steep dead-rise fwd and eventually flat at the transom. This is generally called a “warped bottom” .. as though it’s been twisted. Each side as a mirror image.

Nomad Willy said:

FWT wrote;
“In a planning boat the run of the bottom from amidships to the transom will be flat.”

Not flat. Just straight. Generally speaking zero degree quarter beam buttock line to be technical.
Lots (most) planing hulls have dead-rise and variable on the same hull. Take the older Mainship 34. Steep dead-rise fwd and eventually flat at the transom. This is generally called a “warped bottom” .. as though it’s been twisted.

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Trying to say the same thing. You said it better.

Oh sorry,
SD is my favorite type of hull. But not the SD hulls you find on most trawlers. Very hard to design and are pretty much a one speed hull like FD. At the lower end of SD. At the upper end of SD there will be much more flexibility on power and speed. But as you add power the shape of the hull will cause control stability problems very much like slower SD boats.

But with trawlers some have a planing hull shape w a big heavy keel. The boat will only be able to run at SD speeds because of frictional drag from the big keel. And like any real trawler .. weight. So much drag and weight planing becomes more or less impossible.

A Banks Dory is another interesting hull form re classification. It has a totally flat bottom w 0 degree QBBL. The bottom is straighter than most planing hulls. And if you can control one it will plane. But it’s FD. And gets it’s FD function from the immersed sides. They are one of the most seaworthy of FD hulls ... w weight down low.

But IMO the best SD hulls have a degree of rocker. The transom is less immersed. The bottom is slightly higher at both ends and lower amidships. A typical rowing dory will have rocker such that the flat bottom will be at the WL (normally loaded) and the bottom will be deepest amidships.

It’s widely misunderstood that a Lobster boat w soft rounded chines is a SD boat. Not so if there’ rocker or lots of taper aft. Most Lobster boats are planing hulls. They have Lobster boat races in the NE US. Many have more than 500hp and do about 40 to 45 knots. Planing hulls.

yes, of course
but my point is...well question really.... isn't the primary or fundamental reason one design any of the SD variations into a boat would be to get more speed?
and in gaining that speed you trade some elements of sea kindliness....
so a trade off between speed and 'comfort at sea'

skyhawk said:

yes, of course
but my point is...well question really.... isn't the primary or fundamental reason one design any of the SD variations into a boat would be to get more speed?
and in gaining that speed you trade some elements of sea kindliness....
so a trade off between speed and 'comfort at sea'

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Personally, when the weather picks up I don't enjoy being on a planning boat. If you are trying to keep the speed up (where they do best, and are quite stable and comfortable in flat seas), bigger seas can beat the snot out of you. If you slow down, its not where they are at their best and you can pitch and roll more than in a D or SD boat.

Is that what you mean?

exactly!

although I should add.....it's more of an ask...I have no experience to speak of on full displacement hulls.

Pre planing hull-design development [invention of, if you will]... there was always and only displacement-design as the hull offered. Why?? Well... because of old school building methods, NAs' limited hull design learnings/capabilities, materials available, tools for construction and no high HP power sources to move a hull quickly. I'm sure there are additional reasons that other TF members can bring up.

Point I'm heading toward: Although displacement hull design will always be a fall back leader - IMO it's not necessarily the best, under many circumstances, for pleasure boats [e.g. trawlers and the like]. In that... once planing hull design became invented, only then did the wide range of semi displacement hull design emerge. And; that is my point. Thanks to the invention of planing hull design there became available a myriad of hull design capabilities wherein both D and P hull capabilities could be combined.

Having spent years in the 1960's on the best hull I've ever experienced, a 38' 180 hp diesel powered semi displacement hull, I choose a well designed semi displacement hull as the best universally-useful hull design available... for pleasure boats.

FWT said:

Personally, when the weather picks up I don't enjoy being on a planning boat. If you are trying to keep the speed up (where they do best, and are quite stable and comfortable in flat seas), bigger seas can beat the snot out of you. If you slow down, its not where they are at their best and you can pitch and roll more than in a D or SD boat.

Is that what you mean?

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It depends on the planing hull. Personally, I split them into 2 types. Fast and slow planing, as in design cruise over/under 20 kts.

Fast planing hulls tend to be pretty miserable at low speed, and unless a deep V with lots of power are often a nasty pounding affair in a head sea. If you can run over the tops a following sea isn't bad, just go fast.

Slower planing hulls may have a bit of keel, bigger rudders, etc. so running slower is much more manageable. Especially if you slow down to the low end of planing speed, the ride in a head sea is often decent. Below hull speed pitching may be excessive due to the more buoyant bow. Following seas will vary, but as long as you can at least keep up with the waves they can be a very comfy ride.