Reply
 
Thread Tools Display Modes
 
Old 06-18-2016, 12:28 AM   #1
Veteran Member
 
dirtdoc1's Avatar
 
City: Palo Alto
Country: United States
Join Date: Apr 2016
Posts: 62
Why Aren't Semidisplacement Hulls Considered True Ocean Going Boats?

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!
__________________
Advertisement

dirtdoc1 is offline   Reply With Quote
Old 06-18-2016, 01:35 AM   #2
THD
Guru
 
City: Seattle
Country: US
Join Date: Apr 2012
Posts: 1,142
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!
__________________

THD is offline   Reply With Quote
Old 06-18-2016, 01:38 AM   #3
Guru
 
Ski in NC's Avatar
 
City: Wilmington, NC
Country: USA
Vessel Name: Louisa
Vessel Model: Custom Built 38
Join Date: Jan 2014
Posts: 3,888
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.
Ski in NC is offline   Reply With Quote
Old 06-18-2016, 02:00 AM   #4
Guru
 
City: Hotel, CA
Country: Fried
Join Date: Sep 2011
Posts: 8,328
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
__________________
Craig

It's easier to fool people than to convince them that they've been fooled - Mark Twain
CPseudonym is offline   Reply With Quote
Old 06-18-2016, 05:08 AM   #5
Guru
 
City: Satsuma FL
Country: United States
Vessel Name: No Mo Trawla
Vessel Model: Hurricane SS188
Join Date: Jul 2014
Posts: 1,681
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.
Donsan is offline   Reply With Quote
Old 06-18-2016, 05:59 AM   #6
Guru
 
O C Diver's Avatar
 
City: Fort Myers, FL... Summers in Crisfield, MD
Country: USA
Vessel Name: Slow Hand
Vessel Model: Cherubini Independence 45
Join Date: Dec 2010
Posts: 4,817
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
__________________
Blog: mvslowhand.com
I'm tired of fast moves, I've got a slow groove, on my mind.....
I want to spend some time, Not come and go in a heated rush.....
"Slow Hand" by The Pointer Sisters
O C Diver is offline   Reply With Quote
Old 06-18-2016, 06:24 AM   #7
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,530
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.
FF is offline   Reply With Quote
Old 06-18-2016, 06:36 AM   #8
Guru
 
caltexflanc's Avatar
 
City: North Carolina for now
Country: USA
Vessel Name: Small Incentive
Vessel Model: Boston Whaler 130 Sport
Join Date: Aug 2011
Posts: 3,801
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.
__________________
George

"There's the Right Way, the Wrong Way, and what some guy says he's gotten away with"
caltexflanc is offline   Reply With Quote
Old 06-18-2016, 07:44 AM   #9
Guru
 
Rustybarge's Avatar
 
City: I need a bigger boat!
Country: Ireland
Vessel Model: Cheetah 25' Powercat.
Join Date: Aug 2014
Posts: 925
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.
__________________
Peter.
Rustybarge is offline   Reply With Quote
Old 06-18-2016, 08:42 AM   #10
Guru
 
City: Carefree, Arizona
Country: usa
Vessel Name: sunchaser V
Vessel Model: DeFever 48
Join Date: Apr 2008
Posts: 6,371
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.
sunchaser is offline   Reply With Quote
Old 06-18-2016, 09:30 AM   #11
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,714
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.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 06-18-2016, 11:20 AM   #12
Guru
 
City: Seaford Va on Poquoson River, VA
Country: United States
Vessel Name: Old Glory
Vessel Model: 1970 Egg Harbor 37 extended salon model
Join Date: Jan 2016
Posts: 1,304
Quote:
Originally Posted by CPseudonym View Post
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
Quote:
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.
sdowney717 is offline   Reply With Quote
Old 06-18-2016, 11:23 AM   #13
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 16,530
"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 '
FF is offline   Reply With Quote
Old 06-18-2016, 11:31 AM   #14
Guru
 
City: gulf coast
Country: pinellas
Join Date: Jun 2014
Posts: 2,199
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.
bayview is offline   Reply With Quote
Old 06-18-2016, 11:55 AM   #15
Guru
 
dhays's Avatar
 
City: Gig Harbor
Country: United States
Vessel Name: Kinship
Vessel Model: North Pacific 43
Join Date: May 2015
Posts: 5,093
Quote:
Originally Posted by Rustybarge View Post
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.
__________________
Regards,

Dave
SPOT page
dhays is offline   Reply With Quote
Old 06-18-2016, 12:28 PM   #16
Guru
 
City: Hotel, CA
Country: Fried
Join Date: Sep 2011
Posts: 8,328
Why Aren't Semidisplacement Hulls Considered True Ocean Going Boats?

Quote:
Originally Posted by sdowney717 View Post
You might think they would spell he word correctly for a high end boaat.

Ironic that you pointed out a spelling error with two
__________________
Craig

It's easier to fool people than to convince them that they've been fooled - Mark Twain
CPseudonym is offline   Reply With Quote
Old 06-18-2016, 12:33 PM   #17
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 13,171
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.
BandB is offline   Reply With Quote
Old 06-18-2016, 12:57 PM   #18
Guru
 
dhays's Avatar
 
City: Gig Harbor
Country: United States
Vessel Name: Kinship
Vessel Model: North Pacific 43
Join Date: May 2015
Posts: 5,093
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.
__________________
Regards,

Dave
SPOT page
dhays is offline   Reply With Quote
Old 06-18-2016, 01:12 PM   #19
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 13,714
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.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 06-18-2016, 01:35 PM   #20
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 13,171
Quote:
Originally Posted by Nomad Willy View Post
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.
__________________

BandB is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





All times are GMT -5. The time now is 04:28 AM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012