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

[dirtdoc1]

It appears that most builders refer to the semi-displacement hull as coastal and the full-displacement hull as ocean going. I have read many articles about both types of hull configurations and so far have not gotten my question answered, which is: Why is the semi-displacement hull - used by such well known builders as Grand Banks, Flemming and Defever - not considered an ocean going or true blue water passage maker? And my other question is: do they ballast only full displacement hulls?

Here's what I know about the beneficial characteristics of the full displacement hull. The keel has ballast for added stability. It sits lower in the water which further aids stability. The hull configuration provides better fuel consumption.

Ok fine and dandy but a semi-displacement hull with a keel could be ballasted and they can add larger fuel tanks to compensate for fuel consumption. So that leaves only one thing that the full has over the semi and that is - it sits lower in the water. That alone can't be the only reason not to use a semi for long ocean passages. I must be missing something or got something wrong. Some people consider the semi to be the best of both worlds.

Your feedback would be greatly appreciated.

Cheers!

 

[THD]

Boy are you going to get some interesting answers! Be prepared.

I will give you my opinion, but first a bit of a disagreement with a few of your points. I looked at the Fleming 58 and compared it to our KK 58. They weigh about the same (less than 5% difference), have about the same draft, 5'1" for the Fleming, 5'8" or so for the KK. As you know the KK is a true full displacement an d the Fleming is a semi-planning. The big physical difference shows up in the power and the fuel capacity. The Fleming is standard with Cummins QSC 8.3, 500 hp x2. The KK is standard with JD 6068TFMs, 158 hp x2. The Fleming carries 1450 gallons of fuel, the KK 1760 GPH.

The first issue is simply the hull capacity. At 1800 RPM (around 10 knots as best I can determine) the Fleming burns about 9.4 GPH. At 1400 RPM, bout 7.5 knots, the Fleming burns just over 5 GPH. At 7.5 knots, twin engines, 1450 gallons of fuel keeping a 10% reserve, that translates to a range of about 1100-1200 miles. At 1800 RPM, the KK is at about 9 knots burning 3.0 GPH, at 1400 RPM, about 7 knots, it burns about 1.8 GPH. With twins, at 7 knots, 1760 gallons of fuel with a 10% reserve, that translates to a range of about 3500 miles. A very substantial difference.

The second issue is physical, the hull shape. A full displacement hull is a better seagoing hull. There is little argument about that, I hope. That is not to say you can't cross oceans in a semi-plenty of people have, including Tony Fleming. A full displacement is simply gong to be more comfortable and better able to handle rougher seas. But I think is either boat, you will want out of the bad seas before the boat does!

Hope this helps and now folks here who know a lot more than I can add to it!

 

[Ski in NC]

To take advantage of the semidisplacement hull form, you would be traveling faster than hull speed. The burn rate above hull speed, whatever the hull form, would generally mean that you could not carry enough fuel to do an ocean crossing.

Since you are going to go at hull speed or below for a crossing, you might as well optimize the hull form for that speed. And that optimal shape is some flavor of full displacement.

 

[CPseudonym]

The first two paragraphs at this link explains it in a nutshell but naval architects can spend days discussing the nuances.

http://www.nordhavn.com/35/overview_hull.php4

 

[Donsan]

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.

 

[O C Diver]

My guess is size will be part of the answer. Above 100' a semi-displacement hull can likely carry enough fuel to travel at a speed consistent with their hull design and have stabilizers work correctly. I'm sure BandB can add to this as I believe one of his boats is 120' and can cruise at 30 knots.

Ted

 

[FF]

Weight does not automatically translate into strength but a surface boat must be light to climb out of the water.

The ocean vessel must be able to survive rough weather.

Waves landing on deck, for a few days , and the ability to survive being tossed on her beam requires scantlings that are hard (and very expensive ) to build very light.

At speed many of the on top boats burn at least 1 gallon of fuel per mile.

A 3,000G full fuel tank would be at least 18,000 lbs requiring a large volume in the hull.

Lots more rational to purchase a great auto pilot and chug at the Sq Rt of the LWL , and simply enjoy the ride for an extra week or two.

Murphy switch gauges mean anyone with eyes can stand a watch , see a boat? call the captain.

Hear an alarm bell, call the captain.

Poof,,, you're a seagoing watch stander.

 

[caltexflanc]

Two excellent texts on the subject are "the Nature of Boats" by Gerr and "Voyaging Under Power" by Beebe later modified by Leishman of Nordhavn. Stabilized SD boats like my old Hatteras can be great sea boats but have capacity and efficiency drawbacks for optimized long distance passagemaking.

 

[Rustybarge]

On the sea worthiness issue every patrol and pilot boat is semi-d which would suggest they are the pro's choice for atrocious sea conditions.

Have a look at these safehaven boats made in Ireland. There are some spectatular videos on the site.

SAFEHAVEN MARINE

So....

At displacement speeds a good semi- d boat could potentially carry enough fuel to make sea passages., it definitely has the sea keeping qualities.

 

[sunchaser]

Dirtdoc

The NA business moved well beyond simple labeling of SD vs FD years ago. It seems that the intended use of the vessel determines hull shape, build materials, propulsion systems, weight and tankage. And some builders go for creature comfort volume and height and add ballast to compensate. Adding oodles of ballast and tankage then allows stability to be regained and boat speed relegated to a crawl due to more and more dead weight.

A read of Dashew's website shows that speed, small engines and range are not mutually exclusive. He calls his designs FPBs, a reminder of the pilot boats mentioned earlier in this thread. Now comes the Nordhavn CP 59 a true SD design that has many FD purists clucking their tongues. But, N claims 5 new order commitments for this just introduced apparition!

Even in ski boats there are variants galore. Again, intended purpose.

On what basis do you call, say a newer DeFever 52 or 46 a SD? Or what would you call a Great Harbor? Point being, the designers would struggle with affixing labeling per us on TF's attempts at same. Maybe our resident expert Tad Roberts could once again weigh in on this oft discussed subject. If he does, we should listen carefully.

 

[Nomad Willy]

It has mostly to do w speed, weight, space and seaworthyness.

Very few yachts will be FD as one or more of these elements will be unacceptable to most pleasure boat buyers (PBB).

However very few PBB really care about seaworthyness and they aren't willing to pay for active stabilization or put up w paravanes. Serious seaworthyness requires a stern that looks much like the bow. And they don't like boats that roll much at all.

As for space most all PBB are very keen about space. Square sterns, full bows and wide beams are the formula for lots of space. The wide beam not only gives lots of space it provides the stability that is so high on the PBB minds .. especially the female half of the buyer.

One can't have a deep hull that is light and gives the speed and stability that PBB really want. And boats that are tops in seaworthyness will be deep hulled boats w long keels and big rudders. More anti-speed features. One can't have a heavy and fast boat. How many trawlers plane? Almost none. They are much heavier than a planing boat but light and shallow compared to a FD boat.

Nothing much about a FD boat caters to the demands or wishes of the PBB. That's why there are so few FD pleasure boats. Rolling around in a really slow and heavy boat is not a winning formula for most all pleasure boat buyers.

When most PBB contemplate seaworthyness they think mostly of a good boat .. a well designed boat and for those that troll TF that would be a DeFever or a close equivalent.

 

[sdowney717]

The first two paragraphs at this link explains it in a nutshell but naval architects can spend days discussing the nuances.

Nordhavn - Power That Is Oceans Apart

The hull is hand- laminated featuring solid fiberglass with vinylesther resin below the waterline to prevent blisters

You might think they would spell he word correctly for a high end boaat.

 

[FF]

"On the sea worthiness issue every patrol and pilot boat is semi-d which would suggest they are the pro's choice for atrocious sea conditions."

These are excellent boats , but the range is usually limited '

 

[bayview]

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.

 

[dhays]

On the sea worthiness issue every patrol and pilot boat is semi-d which would suggest they are the pro's choice for atrocious sea conditions.

I think there is a flaw in the logic there. You have hit on an association, not a causal relationship. Pilot and Patrol boats are semi displacement but one of their primary requirements is speed. That speed can't be achieved in a displacement hull so they can't use that hull shape.

 

[CPseudonym]

You might think they would spell he word correctly for a high end boaat.

Ironic that you pointed out a spelling error with two

 

[BandB]

There are plenty of ocean going and ocean capable semi-displacement boats with more than capable hulls. It's not capability, but rather choice of ocean goers. Range becomes the biggest issue and so a semi-displacement boat crossing an ocean is generally going to have to do so closer to displacement speeds. Also because more semi-displacement boats are used for other types cruising they often have very limited fuel supply. To get the range necessary it's going to have to be a larger boat to accommodate the fuel.

Let's keep in mind too that most displacement boats don't cross oceans but are used for coastal cruising.

I saw a comment about stabilization not working on semi-displacement and have no idea where that came from. It works quite well.

Fleming's cross oceans all the time.

Even planing hulls can cross oceans. Want a very European semi-planing hull that is capable, go to a 155' Sunseeker. At 22 knots it could never make it even with it's 15,000 gallons of fuel, but at 10 knots it has a range of 4000 nm. A 164' Westport, top speed 24 knots, cruise of 20 knots, 20000 gallons of fuel. Range is 5000 nm at 10 knots. Even a 130' Westport, cruise of 20 knots, top speed 25 knots, 9500 gallons of fuel. 2700 nm at 13 knots, 3600 at 11 knots, 5000 at 9 knots. So why not a smaller boat. Well, look a moment at a 60' Hatteras MY. Even at idle of 7 knots it only has 1000 nm range. A 100' Hatteras MY only has range of 1400 nm at 13 knots. To reach 2000 nm it has to slow to 1000 RPM and 10 knots. To reach anything greater it has to slow to idle. Take something like an 85' Westport/Pacific Mariner, and at 12 knots your range is only 1280 nm. The boat cruises at over 20 knots with a range of just over 600 nm. To get 2000 nm range you'd have to slow to idle and 8 knots.

Until you get to large boats, the purpose of design of a semi-displacement or planing hull is often contradictory to long range cruising. The boat is designed for greater speed which uses more fuel which can cover less range. If one was designing to go slow and have range then why make it semi-displacement, why have the larger engines.

This really isn't an issue of hull shape as much as one of design intent. I'm not going to design a planing hull or a semi displacement then I'm not going to start adding tons of ballast.

I'm amazed at the number of 100' plus European boats with very limited range. A 122' Riva has a range at cruise of 440 nm, but cruises at 26 knots. It only has 4800 gallons of fuel. It's designed to get you around the Mediterranean quickly and in style, not to cruise long distances.

Back to Fleming a moment though as the best example of a semi-displacement that is most definitely an ocean going boat.

 

[dhays]

Not sure why, but I saw a bunch of Flemings out a couple weeks ago. For a while it seemed as if there was a Fleming in sight at all times. Regardless of their other other qualities, I think they are hard to beat as far as simple beauty.

 

[Nomad Willy]

BandB wrote;
"I saw a comment about stabilization not working on semi-displacement and have no idea where that came from. It works quite well."

SD hulls are usually too stiff to be a good platform for stabs. Will they work on SD? .. Of course .. but much more effective on a heavy, deep, narrow and round bilge boat.

What is it that the Fleming boat has that makes it so seaworthy? They are wide and big but beyond that ....?
I think you just like a big beautiful boat. We have a member here that crosses big chuncks of ocean with a relatively flat bottomed Bayliner. Dos'nt make it seaworthy.

 

[BandB]

BandB wrote;
"I saw a comment about stabilization not working on semi-displacement and have no idea where that came from. It works quite well."

SD hulls are usually too stiff to be a good platform for stabs. Will they work on SD? .. Of course .. but much more effective on a heavy, deep, narrow and round bilge boat.

What is it that the Fleming boat has that makes it so seaworthy? They are wide and big but beyond that ....?
I think you just like a big beautiful boat. We have a member here that crosses big chuncks of ocean with a relatively flat bottomed Bayliner. Dos'nt make them seaworthy.

SD Hulls are too stiff for stabilizers? Wow. Then I guess planing hulls definitely are? Or aren't? I don't know. Guess we should remove all our stabilizers that we thought were working fine since you've declared they obviously aren't.

Why Fleming? Perhaps their proven history of ocean crossing? Perhaps their incredibly smooth ride in rough seas?

Once again you're criticizing boats different from yours, boats you don't personally like, but with no actual experience on or with them. I've got actual experience with SD and planing hulls and stabilizers. I've not crossed the Atlantic yet, but I'm also not limited my boating to the protected waters of Washington. I've covered over 60,000 nm in the last four years including many ocean miles and all on boats you state are too stiff for stabilizers and are unfit for the conditions I've used them in.

You don't see me jumping on here and saying Nordhavn's and KK's are unfit for this or that, simply because they're not my choice to own. You don't see me criticizing your boat even though it's as opposite mine as could be. But again and again you make statements critical of every type boat not your own and you do it based on your theories, not on actual experience.

 

[GFC]

You guys are simply amazing. It's threads like this where you all share your knowledge with each other that make TF a great place to hang out.


Thanks to each of you.

 

[Nomad Willy]

Here's a boat that works very well w stabilizers (paravanes).
All the same boat except the last pic .. a Fisher.

Wide flat boats can benefit from stabs also but to a much lesser degree. And the loads on the stabilizers goes way up. Can you imagine a boat in very rough seas w broken stabilizers? Soon the other side would probably break and cables would find the prop.

Thanks Mike,
I got a thanks for this post for post #11 too.

 

[bayview]

Pilot boats need to be able to catch ships. a displacement hull wont do that. they also don't care about range. At speed they do achieve dynamic stability.

 

[dhays]

SD hulls are usually too stiff to be a good platform for stabs. Will they work on SD? .. Of course .. but much more effective on a heavy, deep, narrow and round bilge boat.

SD Hulls are too stiff for stabilizers? Wow. Then I guess planing hulls definitely are? Or aren't? I don't know. Guess we should remove all our stabilizers that we thought were working fine since you've declared they obviously aren't.

Sarcasm aside, I think you misread what Eric said. He certainly did NOT say that stabilizers don't work on SD hulls.

I would think that stabilizers would have a greater effect on a FD hull compared to a SD hull, but then a FD hull will have more roll to begin with. That is not the same as saying that they are ineffective on a SD hull.

 

[caltexflanc]

Having had stabilizers on what some would call an "SD" planing boat (very similar hull form to a Fleming, having seen both on the hard a few yards from each other) I would contend that they actually lend themselves to, and in many ways benefit more from active stabilizers than an FD hull, by the very nature of their "stiff" or "snappy" harder chine hulls. I can say this also having been on stabilized and unstabilized FD hulls. Seaways vary greatly in their composition and severity, so of course one has to generalize some, so no I can't say I have been on all the flavors in all different seaways, but certainly a few where the roll characteristics of the boat were a definite factor.

 

[BandB]

Sarcasm aside, I think you misread what Eric said. He certainly did NOT say that stabilizers don't work on SD hulls.

I would think that stabilizers would have a greater effect on a FD hull compared to a SD hull, but then a FD hull will have more roll to begin with. That is not the same as saying that they are ineffective on a SD hull.

But stabilizers don't lend themselves better to FD. Perhaps if he limits his definition of stabilizers to Paravanes that might be true, I don't know, as I've never seen Paravanes used on other hull forms. But there are many forms of stabilizers for every type boat out there. We have different types on different boats. However, all are very effective.

 

[Hurrying Nowhere]

So here comes the total lack of experience guy who has never owned a boat with stabilizers.
But how is a hull design that is stiff going to make stabilizers not effective?

And even dumber question is this.
Unless its really rough would you not pull them up for most cruising to reduce drag, thus increasing fuel range?

I have read a little on them but would think if anything a stiffer hull would be a benefit to the effects of stabilization.

 

[Hurrying Nowhere]

Pilot boats need to be able to catch ships. a displacement hull wont do that. they also don't care about range. At speed they do achieve dynamic stability.

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.

 

[psneeld]

Gross generalizations tend to be full of gross errors.


Some dwell on what they know and have never expanded much beyond that from what I can see.


Roll reduction is just that...does the stabilizer brain know what type of hull you have? Maybe you program that in much like an autopilot....and it reacts according to what it is programmed for. As to paravanes...most commercial fishing boatd around here whether full or semi displacement are all hard chined.... the results of roll reduction seem to be about the same but hard to measure as hardly one setup is similar enough to another or get 2 out under similar conditions.


NEVER in all my years of pro and amateur boating have I read or heard that stabilizers are more effective on one type of hull or another...unless you of course assume because full displacement hulls will roll farther but slower and more comfy that they are more effective.


But reduction is reduction and if they reduce both hull types to a tolerable level...then what the heck is the discussion over? Semantics?


I agree with Bands post where some view this forum through blinders and spend a lot of time passing info while true, is so limited in scope it is hard not to respond.

 

[Nomad Willy]

Most all the time fishermen and native cruisers put the fish over when it gets rough enough so that if it got any worse deploying them could be dangerous. Usually they are not used to cruise with unless they are about to cross bad waters w a sour weather forecast. They usually cost about one knot or a little less .. and run about 15' deep.

The reason paravanes are far less effective on SD boats is that when more water is under one side of the hull it causes lift and the boat rolls. Two forces (basically) the lifting force or the righting force created by the boats volume X the distance from CL. With the FD fish boat I showed pics of the lifting force is small but the force necessary to maintain a level boat is easily achieved w paravanes. Lighter weight, smaller poles and rigging can be used. With a wide SD boat like a GB36/42 the righting moment or lifting force w more water under one side is huge. That's why when you step onto a cap rail the GB responds so little you may not even notice it moved. Won't right away on the Dixie (FB) either because of inertia but if you stand on the cap rail in 20 to 40 seconds the boat will roll or list far more than the GB. Look at my post #11 and see Dixie w a pole extended. You can clearly see the list.
To put it another way it's easy for the paravanes to keep Dixie from rolling. But on the GB w the same fish and the same water under one side the extreme flotation on one side of a wide hard chine hull (GB) will force the water to have it's way w the boat. She'll rise to the occasion .. smartly. Paravanes or no. The GB will roll much like it would w/o the paravanes but a somewhat better (less).
Dixie can probably go through big steep waves almost like they wer'nt there w her paravanes deployed.

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.