Why Aren't Semidisplacement Hulls Considered True Ocean Going Boats?

bayview said:

haven't seen much comment about hull shape.


Round chines combined with deep keel and ballast down low increase the tendency of a hull to remain vertical to the earth and not perpendicular to the water surface. Flatter hulls tend to remain perpendicular to the water surface even when the water surface tilts with waves.




Consider how a flat board will behave in waves vs a partially filled beer bottle. The beer bottle will tend more toward remaining vertical while the flat board will always follow the water surface.


Stability or roll resistances a big factor in being comfortable offshore. sailboats have a tall mast and wind loaded sails further reducing roll.

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Interesting, the bottle and board analogy really helped. Thanks!

BanB wrote;
"The beer bottle will tend more toward remaining vertical while the flat board will always follow the water surface."

The flat board is like the SD hull and the righting moment will overpower the paravanes resulting in only slightly reduced roll unless the paravanes are extremely over sized.

Yes I like the bottle and board analogy too. It's easy to keep the bottle oriented whereas it takes lots of force to counteract the righting moment (force) of the flat board.

Nomad Willy said:

So is it apparent to all why a wide flattish bottomed boat are not the best choice for paravanes?
You're right BandB I know little about active fin stabilizers and have been referring mostly to paravanes.

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I don't know much about paravanes other than I don't use them. Paravanes are really used by a very small segment of the boating population. There are many reasons not to use them on SD or Planing boats. One is speed. You're not going to want them hanging out if you're going 20 knots. There's also, at least in our area of the country, a strong trending away from them even for FD boats. They were most popular before stabilizer installations became more dependable. In 2004, for the Atlantic Rally, many of the Nordhavn's had fin stabilizers but had trouble with them as Nordhavn hadn't really perfected their systems. Now, fin stabilizers are pretty much trouble free on Nordhavn's, probably less trouble that with Paravanes on a long ocean crossing.

The stabilizer market is still developing too. It was fairly straight forward with Naiad, Quantum, and ABT-Trac having most of the market and very similar approaches, but now Seakeeper is gaining a lot of market with Gyro Stabilizers. Then you also have Sleipner/Side Power with Vector Stabilizers designed for boats between 50 and 70' or so with what they call "modern hull forms" so planing boats. By the design of the curved fin they are more effective. I thought it was probably hype at least to some degree, but after using them a while, I'm very impressed and they do everything they claim.

I don't even attempt to understand the science of Gyro Stabilizers but having tested a Sea Ray with Seakeeper and also been on one Sunkeeper with Seakeeper, I'm impressed with the job they do, just not impressed with the spool time and other factors.

A question about Paravanes. Are the majority of them either DIY or done custom by a yard? Are there no brands that are significant in that market? Or is there a "go to" product out there?

Nomad Willy said:

BanB wrote;
"The beer bottle will tend more toward remaining vertical while the flat board will always follow the water surface."

.

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No, I wrote nothing about beer bottles. I don't even drink beer.

Stabiliser paravanes come in different sizes, shapes and hanging styles
to accommodate different hull types and sizes.
They are also hung at various distances out the poles to vary the effectivness
of the fish.

Ted

Ted said:

Stabiliser paravanes come in different sizes, shapes and hanging styles
to accommodate different hull types and sizes.
They are also hung at various distances out the poles to vary the effectivness
of the fish.

Ted

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But no major manufacturer of them? I'm just curious.

There is a manufacturer of the fish in Seattle. They also make .. Oh I just remembered the name ... Kholstrand (SP?) they also make hydraulic anchor winches and other related fishing gear.

Nomad Willy said:

There is a manufacturer of the fish in Seattle. They also make .. Oh I just remembered the name ... Kholstrand (SP?) they also make hydraulic anchor winches and other related fishing gear.

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Kolstrand

The most high performance blue water ship I know of.

SD, FD or something else?

Most of the paravanes I am familiar with are one offs, or custom.

Pros: no holes in boat, no seals to replace, no hydraulics, no electrics, no computers, can be used at anchor, tens of thousands of dollars less than other options.

Cons: lots of manual labor, can't run in shallow water, ugly(opinions vary)

I can't imagine running fish at double digit speeds after watching them work at 6-7 kts. They are extemely effective on our hull.

Having had a SD boat before this one, I don't think they would work as well. That boat would fight the roll until it didnt, aka snap roll (lots of force needed in a short period of time). This boat is very linear in its resistance to roll and recover.

Hurrying Nowhere said:

In ports around coastal BC, big ships come in very very slowly.
It doesn't require a fast boat to catch them.
I see them coming in so slowly that over shooting them would be more likely than not catching them.
I was padding padding my kayak in a relaxed manner and passed 2 ships coming into the harbor last summer.
They tend to move just fast enough to maintain steerage....maybe a knot or so. I'm 45 and not an Olympic athlete.....if i pass you in my kayak you're barely moving.

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Here in Tampa bay the ships look to be going slowly, from a distance. Get up close and they are doing 20 Kts. They weren't gond slowly in SF when I was there either, IIRC.

Bob Cofer said:

View attachment 53282

The most high performance blue water ship I know of.

SD, FD or something else?

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That would be FD.
Why do you mention "high performance"?

Hurrying Nowhere said:

In ports around coastal BC, big ships come in very very slowly.
It doesn't require a fast boat to catch them.
I see them coming in so slowly that over shooting them would be more likely than not catching them.
I was padding padding my kayak in a relaxed manner and passed 2 ships coming into the harbor last summer.
They tend to move just fast enough to maintain steerage....maybe a knot or so. I'm 45 and not an Olympic athlete.....if i pass you in my kayak you're barely moving.

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They take on the pilots long before they enter ports and therefore probably long before you ever see them. And, at much faster speeds than you think (or could possibly paddle).

Nsail said:

They take on the pilots long before they enter ports and therefore probably long before you ever see them. And, at much faster speeds than you think (or could possibly paddle).

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Here's a pretty good example of a routine boarding. From a distance, it might not appear that the ship is moving very fast, but up close is a different story.

If a pilot boat was limited to hull speed, it would be useless. It has to be able to run at least 10-15kts.

Interesting thread. It's given that most of us here have had enough conversation about FD and SD hulls that while individual characteristics exist, performance assumptions don't. There seems to be a gray area of either hull that closely approaches the performance of the other. I've had the opportunity to observe a Krogen 39 with 6 sq. ft. fins operating in almost identical ICW water traffic as the only successfully fin-stabilized Krogen Manatee I've ever known of, also with 6 sq. ft. of fin surface. These are both Krogen FD hulls but are so vastly different in design and behavior as to make them nearly incomparable. The 39 is a true "tub" with a easy, predictable roll. The Manatee is hard-chined and has a better "initial" roll resistance, but being inside each hull and watching the fin angle gauges working is all you'll ever need to get a perspective on how much more work it is to stabilize the "initially stable" Manatee design over the 39 tub. With the passing wakes of the typical center-console boats, you'll see the Manatees fins pegging to keep a lesser roll from happening while the 39's fins remain in a relaxed, but perhaps slightly longer swim. The only other fin-stabilized Manatee I've read about ended up being an abandoned effort for that reason. Once it got on a beam wave for a long enough duration, it's tendency was to "fall off" the wave with a sudden, increased roll, noting that the fin on the rising side occasionally came out of the water. This left the remaining fin on the downside of the wave still resisting and was likely responsible for the sudden increased angle of roll. The text went on to indicate that it didn't feel any more dangerous than having no fins, but why even have them in the first place. After the forth set of seals in as many months, they removed the system. I don't know how the fins were mounted or even what brand the system was, but the report gave credence to the idea that flatter-bottomed hulls, FD or not, will eventually take the shape of the wave.

For my own Manatee, I asked the best known NA on this site if he could design a paravane rig for me and he noted, affirmatively, that the vessel's design was almost "too stable" and would require a heck of a rig to do it. When I look at other FD hulls like the Great Harbour, for example, it seems logical that when attempting to stabilize any hull designed to have more initial (I said initial) roll resistance would need a rig able to transfer much greater leverages, as pointed out by Eric, perhaps leverages that would make the hull design itself the limiting factor.

I have to concur that the effectiveness of an anti-roll device would probably work equally on FD or SD hulls, provided that both the device and the hull were designed to take the stresses. Also, that the average FD mono-hull is likely a better choice for open water than the average SD mono-hull at hull speeds, but at greater than hull speeds, I'm not aware of any true FD mono-hull candidates to speak about.

I have a DeFever with hard chines and Naiad stabilizers. I installed them after I got the boat and the difference in the ride is almost unbelievable. Before, the slightest beam sea would cause it to roll and in bigger seas we quickly reached our comfort limits. After installation, it rides on rails, barely rolling. We have the latest electronic controller that has an adaptive setting that adjusts to the optimal level based on the sea state. No manual adjustments needed, but are an option. So at least for DeFevers, active stabilizers work on FD and hulls with hard chines.

Eric,

I think 40+ knots qualifies as high performance. I've seen them take off like ski boats during emergency break away drills.

Larry,
Re post #46 very good points are made. Your comparison of the KK39 and the Manatee is excellent. But imagine how much more so it would be with two boats further appart in natural stability. Like an IG or other wide and flattish bottom boat compared to a typical PNW Salmon troller .. deep and narrow. The effectiveness of stabilizers will be even more pronounced.
However the thing that mostly helps the FD boat offshore is the shape of the stern. The bottom of a FD boat dos'nt stick down in the water giving something for the waves to get ahold of and take over control .. of the stern and hence directional stability. Some FD boats have flat bottoms and aren't shaped like the bow but have most of the seaworthyness of boats w pointy sterns. Because the stern rises up so much that on most FD boats the transom is out of the water at rest. So waves can pass under the stern w/o having a great effect on directional stability.
Fairly narrow boats w round bilges, curved sides (fore and aft), long keels, a deep forefoot and big powerful rudders do best in big seas. Many SD boats are very seaworthy but the most seaworthy boats are FD. Larry is right though .. it's not black and white. It's not unusual to find boats that don't fit neatly into either catergory. Most boats are fairly easy to classify .. FD or SD. but probably 10 or 15% could be called either.

Bob Cofer said:

Eric,

I think 40+ knots qualifies as high performance. I've seen them take off like ski boats during emergency break away drills.

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Bob,
OK but the image you show looks FD to me. Does the bottom rise up to the WL at rest? Looks like it does in the image and that's a dealbreaker in this question.

Randy w the GB32 on our float in LaConner painted his hull sides what appears to be black but he said it's blue I think. Looks great.

Nomad Willy said:

Bob,
OK but the image you show looks FD to me. Does the bottom rise up to the WL at rest? Looks like it does in the image and that's a dealbreaker in this question.

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Beware - thread creep ahead . . .

Eric,

OK, here is a modern 389' naval vessel with a published speed of 45 knots, a speed/length ratio of 2.3. It does not plane, nor does the stern squat above 1.34. The link below shows that it pretty much remains on its resting lines at high speed. Do you suppose it is FD or SD? Or do those terms only apply to smaller vessels?

https://i.kinja-img.com/gawker-medi...rogressive,q_80,w_800/1503305973800324899.gif

Hi Larry,
By all the standards I've ever applied to the question this big boy looks to be about 95% planing hull. Remember re what Larry (healhustler) said that there was no black and white but perhaps 30% of boats we may ask this question of are in a grey zone. Here the ship has such an astronomical amount of power it can attain high speeds that we generally consider to be planing but remain mostly in the water and amazingly level running. The "reynolds" number probably has a fairly large effect on this vessel compared to boats our size. Plus any boat w an high aspect ratio (long and narrow) like this will be quite to very level running.
So I'll say planing.
That's funny,
The water spouting out the anchor hawse hole. Makes her look a bit like a whale.
In the linked vid she looks to be running bow down some and I suspect there is an adjustment to the angle of the jet discharge stream up or down to have some control over pitch or running angle .. like trim tabs. On calm water they may direct the jet stream up to squat the stern a bit giving a higher angle of attack to the hull bottom producing lift or more lift so reduced wetted surface would reduce drag and increase speed. Just think'in ......

Semi displacement stabilizers great

Donsan said:

Don't forget stabilization. Most true blue water cruisers have some form of stabilization that don't work well or are inefficient on a semi displacement boat.

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I have an Ocean Alexander semi displacement hull with stabilizers. I can tell you that the stabilizers work great and the ride is as comparable or better than that of a full displacement hull with stabilizers.

I have just completed a trip both down the ICW with a full displacement Katie krogan and return with my ocean Alexander, and the ride on my ocean Alexander is every bit as good.

Gordon

Bob Cofer said:

View attachment 53282

The most high performance blue water ship I know of.

SD, FD or something else?

Click to expand...

Bob,
How long is that thing? Must be about 600'. 40 knots is'nt very fast from a speed/length ratio standpoint. Hull speed for an aircraft carrier is only a little faster. I don't know of that ship ... what kind of vessel is it.

I believe the original poster was asking about ocean or blue water experience.....ICW not so much. BTW its Kadey Krogen. Until youve been hundreds of miles offshore, its hard to describe it to others. The sheer size and weight of the water affxts both hulls very differently. We are not talking about a steep chop or boat wake on the ICW

Looks like an updated version of a Spruance class destroyer Eric

https://en.m.wikipedia.org/wiki/Spruance-class_destroyer

Gordon J said:

I have just completed a trip both down the ICW with a full displacement Katie krogan and return with my ocean Alexander, and the ride on my ocean Alexander is every bit as good. Gordon

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Atta boy, Gordon!

Ok then. Now I pose this question. A planing hull is more efficient than a semi planing hull at planing speeds(more lifting area). Why would you choose to design a semi planing hull that you intend to do planing speeds and take the efficiency hit???...instead of designing a planing hull???

I have my thoughts but just curious what yours are?

PS.... Most pilot boardings are done with the ship anchored or at least hove to.

During my maiden voyage home, from Miami to Norfolk, Va, our new to us, 2003 OA 456 averaged 4 gal/hour going between 8 and 9 knots.

The biggest pita for our boat is that a sd hull produces significant waves at 7 knots. On the ICW with the many no wake zones, we were reduced to idle speeds. On our 43-foot sailboat, I never worried about wake....there was none.

Gordon

"Why would you choose to design a semi planing hull that you intend to do planing speeds and take the efficiency hit???...instead of designing a planing hull???"

The true plaining hull would be even less efficient at displacement speeds and would probably require 2x the engine size of the SD boat , and get 1/2 the MPG.

The main users of full plaining hulls of a good size seem to be the Sport Fish folks , who frequently have OPM paying the fuel bill, and for the shorter engine life.

A boat that burns 60GPH makes no sense for a cruiser that will attempt the 7K cruiser crawl and pray for a -4GPH fuel burn.