Reply
 
Thread Tools Search this Thread Display Modes
 
Old 06-23-2018, 07:59 PM   #1
Senior Member
 
toocoys's Avatar
 
City: Seabrook
Country: United States
Vessel Name: Final Choice
Vessel Model: 1996 Mainship 37 M/Y
Join Date: Oct 2016
Posts: 422
Question Semi-Displacement Hulls?

What were some of the better built semi-displacement hulls, say... pre-1990's?
__________________
Advertisement

toocoys is offline   Reply With Quote
Old 06-23-2018, 11:20 PM   #2
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 14,715
Most of the SD hulls common to rec trawlers have a relatively straight run aft. And a straight run aft is a promenent feature of planing hulls. But they're considered SD because of their big keel and relatively heavy weight.

One that is much closer to a FD hull is the CHB.

I took this pic a few days ago in the yard while this CHB was being hauled for a potential purchase.
Notice the bottom curve's up almost to the WL at the transom. This is a relatively unusual feature of a SD trawler. There is very little submerged transom in this design. She's not noticeably slimed down beam wise aft so she's as roomy as most others w the exception of her shallow Lazerette she's as big as her middle of the road SD trawler cousins.

The advantage of this design variant is considerably less drag at hull speed and more importantly at a knot below. So for the same displacement, WLL, and beam She's more efficient. More range and less speed. Another advantage is better directional stability especially in following seas. My opinions all.

This is a design feature though .. not a build quality assessment.
Attached Thumbnails
Click image for larger version

Name:	DSCF2613 copy.jpg
Views:	198
Size:	122.8 KB
ID:	77810  
__________________

__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 06-24-2018, 05:33 AM   #3
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: 1,731
Semi displacement is not very good hull shape for fuel efficiency.
That rocker in the hull where it it rises still leaves an immersed transom, so it sucks the water (creates resistance to forward motion ) as it moves through the water since it is still FLAT across the back. Then when you try to go fast it sucks it down much more and they can not get over the hump, so you end up just plowing and throwing up lots of water and uses up huge amounts of fuel like a FD hull does trying to go faster.

You can not go fast as in a planing hull either. So you sort of get the worst of both.
My boat is a semi displacement, I find I end up going at displacement speeds as the fuel burn is ridiculously higher than a planing hull would be at higher speeds and the speed is no where close to what a planing hull can do, so just negatives all around.
If your willing to go very slow, then it is still not as efficient as FD hull, but most likely people don't know the difference. Boat designers sure churn out lots of them, maybe they are cheaper to make than FD boats. If you like the FD hull shape, and can not afford a trawler, just get a used sailboat with fixed keel, remove the rigging and use it as a power boat.
sdowney717 is offline   Reply With Quote
Old 06-24-2018, 05:38 AM   #4
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 17,748
Semi displacement boats are frequently a compromise that is a disapointment.

If you want to go fast there are many sport fish with OK accomidations that aren't too squirely at displacement speeds.

Within the capacity of an AP to steer.

The concept sounds great ," I can put put at cheap fuel burn and then "outrun any storm", a huge wake at WOT at 12-14K seldom can.
FF is offline   Reply With Quote
Old 06-24-2018, 05:39 AM   #5
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 17,735
I would lean towards a SD hull is not as efficient fuel wise, but there are small plusses for it over either planing or displacement hulls.

Like the glass half full or half empty discussion.....

Every boat is a compromise.....its all about which way is better for you.
psneeld is offline   Reply With Quote
Old 06-24-2018, 05:45 AM   #6
Senior Member
 
toocoys's Avatar
 
City: Seabrook
Country: United States
Vessel Name: Final Choice
Vessel Model: 1996 Mainship 37 M/Y
Join Date: Oct 2016
Posts: 422
Quote:
Originally Posted by sdowney717 View Post
So you sort of get the worst of both.


It's funny, or strange rather, that you mention that. I've heard them described as the best of both worlds. Fuel efficient at cruise speeds but fast enough to outrun weather if needed.
toocoys is offline   Reply With Quote
Old 06-24-2018, 05:47 AM   #7
Senior Member
 
toocoys's Avatar
 
City: Seabrook
Country: United States
Vessel Name: Final Choice
Vessel Model: 1996 Mainship 37 M/Y
Join Date: Oct 2016
Posts: 422
Quote:
Originally Posted by sdowney717 View Post
If you like the FD hull shape, and can not afford a trawler, just get a used sailboat with fixed keel, remove the rigging and use it as a power boat.
Ohhh if it were only that easy! LOL
toocoys is offline   Reply With Quote
Old 06-24-2018, 06:09 AM   #8
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 17,735
Quote:
Originally Posted by toocoys View Post
Ohhh if it were only that easy! LOL
It is...more than a few do it.....some take it to extremes and remodel quite a bit.

Get a sailboat with extensive rig damage from a storm for pretty cheap and if it doesnt have water dsmage it could be turn key.

Some sailboats can enclose or alter their cockpits (center or aft or even pilothouse) to be comfortable.
psneeld is offline   Reply With Quote
Old 06-24-2018, 06:14 AM   #9
Senior Member
 
toocoys's Avatar
 
City: Seabrook
Country: United States
Vessel Name: Final Choice
Vessel Model: 1996 Mainship 37 M/Y
Join Date: Oct 2016
Posts: 422
Quote:
Originally Posted by psneeld View Post
It is...more than a few do it.....some take it to extremes and remodel quite a bit.

Get a sailboat with extensive rig damage from a storm for pretty cheap and if it doesnt have water dsmage it could be turn key.

Some sailboats can enclose or alter their cockpits (center or aft or even pilothouse) to be comfortable.

I know itís physically easy, but the reason we done have a sail boat in the first place is because my partner doesnít like the cave feeling of the interior, or the lack of deck space and confined cockpit areas, center or otherwise.

I would rather a center cockpit sailboat than anything else on the water, but because of compromises Iíll never be able to sail unless I do it alone.
toocoys is offline   Reply With Quote
Old 06-24-2018, 06:19 AM   #10
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: 5,864
Fuel efficiency for semi-displacement hulls at displacement speed, has a great deal to do with the engines. In a 45' boat, if you want to plane (sort of), you'll probably need 500 to 700 horse power. In displacement (7 knots), you'll probably burn 3 to 4 GPH. My 45' semi-displacement boat with 135 HP will cruise 7 knots at 2 GPH. A full displacement 45' boat will probably cruise 7 knots at 1.5 to 1.75 GPH.

With the right engine, semi-displacement can be reasonably efficient, but then you loose your 12 to 14 knot top end. You have to decide what your priorities are.

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 online now   Reply With Quote
Old 06-24-2018, 06:32 AM   #11
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: 1,731
Quote:
Originally Posted by toocoys View Post
It's funny, or strange rather, that you mention that. I've heard them described as the best of both worlds. Fuel efficient at cruise speeds but fast enough to outrun weather if needed.
Marketing hype from people who make semi displacement hulls to their gullible customers.
sdowney717 is offline   Reply With Quote
Old 06-24-2018, 06:34 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: 1,731
Quote:
Originally Posted by toocoys View Post
I know it’s physically easy, but the reason we done have a sail boat in the first place is because my partner doesn’t like the cave feeling of the interior, or the lack of deck space and confined cockpit areas, center or otherwise.

I would rather a center cockpit sailboat than anything else on the water, but because of compromises I’ll never be able to sail unless I do it alone.
I have been on an Island Packet 43, and boy was that a nice layout, did not feel claustrophobic or tight. Sailing and power cats are nice, but hard to find a slip to fit.
sdowney717 is offline   Reply With Quote
Old 06-24-2018, 07:30 AM   #13
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: 4,537
Anybody here have a naval architect's technical description of "semi-displacement" ? I have never seen one. Some function of Speed/Waterline (S/L) or Displacement/Waterline (D/ L)?
__________________
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 06-24-2018, 09:11 AM   #14
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 14,715
Quote:
Originally Posted by caltexflanc View Post
Anybody here have a naval architect's technical description of "semi-displacement" ? I have never seen one. Some function of Speed/Waterline (S/L) or Displacement/Waterline (D/ L)?
Cal,
It’s expressed in numerous ways.
The way I prefer for a long time in the past is a very simple but not very accurate method. Observe the amount of flat transom that is underwater at rest. If more than an inch or two (depending on boat size) one would be look’in at a SD.

A much better way that I hear on Boat Design .net is to observe the angle of the bottom aft of a bit behind midships. As in X number of degrees. Many here dislike the expression “Quarter Beam Buttock Line” but that’s what it is. It’s a line at the “quarter beam” like half way between the keel and chine running fore and aft. A qbbl of 0 degrees would indicate a planing hull. A line of 2,3 or maybe 4 degrees is SD and 5+ degrees Displacement (depending on draft) and with the transom out of the water or in some cases (mostly heavy boats) with a small amount of thranom submerged .. like an inch or two on a 35’ boat would be FD.

But there really isn’t a black and white .. this boat is FD .. this boat is SD. The above is sort of a rule of thumb with a grey area between. The whole boat, disp, shape, power, the size or design of the keel all enter into it. There are SD trawlers that have a planing hull shape but are diesel powered w a fairly large rudder and a big keel that are clearly SD but would be a planing hull w the features mentioned above quite different. I would be fairly or somewhat comfortable calling the old 34’ CHB a FD boat (see my original post) but an IG would not. They are much closser to a 0 degree qbbl. The shape of a planing hull. Dress it up the IG hull as a planing boat and it is. Twin gas engines, almost no keel, much lighter weight w small tanks, smaller rudders for less drag at speed ect ect but w the same hull form/shape the IG could be a planing boat. So a hull w/o the big keel, heavy diesel engine/s, big fuel and water tanks you’d have a planing boat. But the same hull form w/o the heavy trawler features is known in dock talk and on brochures is known as a SD boat.

But for those with minds that require a black and white definition you will be forever frustrated. You’ll adopt a theroy and the one day (maybe in a few days) you’ll see a boat that clearly dosn’t fit your theory. So some would call the CHB 34 a SD and some a FD. Both would be right in the grey scope of boat type/hull type talk.
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 06-24-2018, 09:45 AM   #15
FF
Guru
 
FF's Avatar
 
Join Date: Oct 2007
Posts: 17,748
A nice big old dismasted sail boat could easily be converted to a power cruiser , but the keel will probably have to be lightened a good deal.

The mast and rigging removal could make the boat too stiff for comfort.

A snap roll can create a vomitorium, weather its too stiff from form , a flat bottom or hard immersed chines or just too much weight in the keel.

An old center boarder might be best as the weight is shallow.

The big hassle will be resale, remember Winny the Poo!
FF is offline   Reply With Quote
Old 06-24-2018, 11:16 AM   #16
Senior Member
 
Woodland Hills's Avatar
 
City: Jacksonville
Country: USA
Vessel Name: Alzero
Vessel Model: Hatteras 63' CPMY
Join Date: Apr 2017
Posts: 381
Quote:
Originally Posted by FF View Post
A nice big old dismasted sail boat could easily be converted to a power cruiser , but the keel will probably have to be lightened a good deal.

The mast and rigging removal could make the boat too stiff for comfort.

A snap roll can create a vomitorium, weather its too stiff from form , a flat bottom or hard immersed chines or just too much weight in the keel.

An old center boarder might be best as the weight is shallow.

The big hassle will be resale, remember Winny the Poo!

What he said X2.......
Woodland Hills is offline   Reply With Quote
Old 06-24-2018, 02:00 PM   #17
Guru
 
Nomad Willy's Avatar
 
City: Concrete Washington State
Country: USA
Vessel Name: Willy
Vessel Model: Willard Nomad 30'
Join Date: Oct 2007
Posts: 14,715
IMO itís hard to do but if one starts w a good (appropriate) sailboat I think it can be done. A non-typical sailboat hull is a good start. But it would take time to find such a boat. I bought Willy (my trawler) before I found a really good sailboat to dismember.

Not being a NA I donít know most of the features needed. One would be full ends. The 27í Albin is such a boat but they have the prop aft of the rudder. And so it goes.

Iíll try and post something I picked up on facebook about an interesting conversion.

https://www.facebook.com/photo.php?f...9041918&type=3
__________________
Eric

North Western Washington State USA
Nomad Willy is offline   Reply With Quote
Old 06-24-2018, 04:28 PM   #18
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: 4,537
Quote:
Originally Posted by toocoys View Post
What were some of the better built semi-displacement hulls, say... pre-1990's?
Getting back to this innocent (?) question, if it means heavy boats with the ability to get on plane to, say around 150% of theoretical hull speed, or attain an S/L of over 200 (which pretty much defines a planing boat) , but maybe not as much as 300, then on my list #1 (surprise!) would be Hatteras and #2 Grand Banks.
__________________
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 06-24-2018, 04:38 PM   #19
Guru
 
BandB's Avatar
 
City: Fort Lauderdale
Country: USA
Join Date: Jan 2014
Posts: 15,048
Quote:
Originally Posted by caltexflanc View Post
Getting back to this innocent (?) question, if it means heavy boats with the ability to get on plane to, say around 150% of theoretical hull speed, or attain an S/L of over 200 (which pretty much defines a planing boat) , but maybe not as much as 300, then on my list #1 (surprise!) would be Hatteras and #2 Grand Banks.
Glad to see one person actually answer the question. I'd second those two, toss in perhaps some of the tug models and a lot of Sea Rays out there.
BandB is offline   Reply With Quote
Old 06-24-2018, 04:44 PM   #20
Guru
 
psneeld's Avatar
 
City: Avalon, NJ
Country: USA
Vessel Name: Freedom
Vessel Model: Albin 40
Join Date: Oct 2011
Posts: 17,735
And boatloads of lobster hull designs.....
__________________

psneeld is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
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 09:21 PM.


Powered by vBulletin® Version 3.8.8 Beta 4
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Search Engine Optimization by vBSEO 3.6.0
Copyright 2006 - 2012