Full Displacement or Semi Displacement

[Art]

DAMN! Am I ever convinced after reading all the posts in this thread!

D, FD, SD, SP FP, P: All acronyms that either carry a D or a P as their base... which means to me... the hull is either D or P. It either is or it ain't one or the other! If it can't plane ever it is a D. If it can plane ever it is a P.

I/we simply LOVE our very stable and pretty quick (whenever desired) Tollycraft P boat!

Happy Boating Daze! - Art

 

[Underway]

If it can't plane ever it is a D. If it can plane ever it is a P. /QUOTE]

I'd revise that to say if it can't plane ever it's a D. If with a large application of power it can skim along the surface with the majority of the hull exposed, then it is a P. If the hull can clear the bow wave, but cannot skim along the surface, even with application of an inordinate amount of power, then it is probably an SP. I have doubts as to whether many of these very deep forefoot (non-displacement) hulls could be lifted free and clear of the surface even with a pair of high powered, light weight turbine engines. But it does depend on how you wish to define "plane".

 

[Nomad Willy]

Great stuff here and I was a bit afraid the thread wouldn't get a single post as so many don't like beating the dead horse. But I always say as long as posts keep coming a thread isn't dead.

Spy I agree. Semiplaning. Semi means "partly" to me and a SP/SD hull that we are most familiar w is "partly" planing (to a small degree). Tried to float that about 4 or 5 years ago. SD for hulls closer to planing and SP for boats closer to FD. Made sense to me but I got negative feedback so let it slide.

Skidgear re your post .. sure you can build a canoe w a perfectly flat bottom but the hull is 100% FD. And the QBBL shows a Full blown planing hull. Like psneeld says definitions and even formulas won't hold water at times. But if we had the QBBL angle and a good cross sectional section area ratio (transom/midsection) we could nail it down for typical trawlers such as most here have.

Tom hard Chines have nothing to do w FD/SD. And basically pointy stern or perfectly right angled stern don't either. But the transom being in or out of the water is very definitive. If the whole transom is out of the water it's FD. Look at the older sailboats w their transforms fully out of the water. FD.
And yes even most trawler people don't like tippy so the typical trawler has a wide fairly flat bottom aft for a good purchase on the water for good inital stability. Almost never do we need secondary stability. But the notion of a big wide flat stern being more seaworthy is rubbish to these ears. May make a better yacht though.

My computer's in the shop and I'm on my i-pad (no pics and awkward). But I'll be reading these posts for several days as there is so much to absorb.

 

[Tad Roberts]

Some of DeFever's earlier FD designs were vessels with canoe style sterns much like Eric's. He was not pleased with the canoe stern vessels' hobby horsing, stability at anchor and lessened carrying capacity aft. So in the late 60s and into the 70s he started designing very large FD yachts with no canoe stern and gasp, active stabilizers! I was fortunate this past summer to be on one of his earlier canoe stern designs, his first yacht type vessel. A DeFever FD design all the way

I'll be a bit nit-picky...... It's (another) fine point but I wouldn't say DeFever ever designed a canoe stern. If you google "canoe stern" you'll get a lot of pictures of double-enders and boats (as the Willard) with narrow, tight-radius round sterns. They used to be called "Cruiser" sterns in the UK. These could be called canoe sterns. The early DeFever stern was that of Marda, pictured below. This is a wide, rounded shape, with large transverse flat, usually with a knuckle well above waterline where the hull bottom ends. Here in BC we call this a Horseshoe stern, it's common on trollers and other fishboats.

The Horseshoe is very definitely a displacement hull feature, designed to ease the return of water from under the hull to the "at rest" level somewhere astern. At the correct speed this stern leaves no wake at all. The deeply immersed transom of DeFever's later designs is far more "draggy", but does allow greater speed without squatting. It costs money to drag that around though......of course I know you toy boat owners don't care about such things

 

[sunchaser]

I'll be a bit nit-picky...... It's (another) fine point but I wouldn't say DeFever ever designed a canoe stern. If you google "canoe stern" you'll get a lot of pictures of double-enders and boats (as the Willard) with narrow, tight-radius round sterns. They used to be called "Cruiser" sterns in the UK. These could be called canoe sterns. The early DeFever stern was that of Marda, pictured below. This is a wide, rounded shape, with large transverse flat, usually with a knuckle well above waterline where the hull bottom ends. Here in BC we call this a Horseshoe stern, it's common on trollers and other fishboats.

The Horseshoe is very definitely a displacement hull feature, designed to ease the return of water from under the hull to the "at rest" level somewhere astern. At the correct speed this stern leaves no wake at all. The deeply immersed transom of DeFever's later designs is far more "draggy", but does allow greater speed without squatting. It costs money to drag that around though......of course I know you toy boat owners don't care about such things

View attachment 27193

Right you are Tad, it is a horseshoe in the early DFs. But using 4.5 gph at 7.7 knots and 1/2 that at 6 knots in our DF 48, even with our draggy buttocks, is not too shabby.

How would describe your PL designs in light of this discussion?

 

[Tad Roberts]

How would describe your PL designs in light of this discussion?

Oh boy...are you sure you want to incite this? I'm not at all interested in making any sales pitch but I'm happy to try and shed some light on questions of naval architecture.

The PL (Passagemaker Lite) design, like Dashew's FPB, the Vripack Ned, and Nigel Irens LDL designs, is a "High-speed" displacement hull. It's ratios, shape, and performance are similar to a naval Frigate. Somewhere between a Frigate and a high-speed displacement catamaran. A lot faster than typical heavy "Full" displacement hulls, these hulls achieve speed by being small (in cross section), relatively high-powered, and light weight for their length. Unlike a semi-displacement hull, they are not intended to lift with speed at all. Rather the idea is to reduce wetted surface and wave-making to allow the hull to slip along at higher speed. They lack the bottom area to get any useful dynamic lift as big wide (and usually heavy) semi-displacement boats do.

You might find some reference to this in the Dave Gerr link above. He couples it to displacement/length ratio. The lighter (displacement) a hull is for a given length, the higher it's "Hull Speed" will be.

 

[markpierce]

I prefer the motion of going through waves rather than bouncing over.

 

[psneeld]

Oh boy...are you sure you want to incite this? I'm not at all interested in making any sales pitch but I'm happy to try and shed some light on questions of naval architecture.

The PL (Passagemaker Lite) design, like Dashew's FPB, the Vripack Ned, and Nigel Irens LDL designs, is a "High-speed" displacement hull. It's ratios, shape, and performance are similar to a naval Frigate. Somewhere between a Frigate and a high-speed displacement catamaran. A lot faster than typical heavy "Full" displacement hulls, these hulls achieve speed by being small (in cross section), relatively high-powered, and light weight for their length. Unlike a semi-displacement hull, they are not intended to lift with speed at all. Rather the idea is to reduce wetted surface and wave-making to allow the hull to slip along at higher speed. They lack the bottom area to get any useful dynamic lift as big wide (and usually heavy) semi-displacement boats do.

You might find some reference to this in the Dave Gerr link above. He couples it to displacement/length ratio. The lighter (displacement) a hull is for a given length, the higher it's "Hull Speed" will be.

Thank you...one more intelligent explanation that the world does not revolve around 3.14 and QBBL.

 

[psneeld]

I prefer the motion of going through waves rather than bouncing over.

ever ride a honking Hobie Cat or a 40 something, trip engine go fast from wave top to wave top?

you may not change your opinion but you'll see another way to travel comfortably though surprisingly large chop.

 

[sunchaser]

The PL (Passagemaker Lite) design, like Dashew's FPB, the Vripack Ned, and Nigel Irens LDL designs, is a "High-speed" displacement hull. semi-displacement boats do. .

So given this frigate design, how does the stern look and behave in the design?

 

[Underway]

Oh boy...are you sure you want to incite this? I'm not at all interested in making any sales pitch but I'm happy to try and shed some light on questions of naval architecture.

The PL (Passagemaker Lite) design, like Dashew's FPB, the Vripack Ned, and Nigel Irens LDL designs, is a "High-speed" displacement hull. It's ratios, shape, and performance are similar to a naval Frigate. Somewhere between a Frigate and a high-speed displacement catamaran. A lot faster than typical heavy "Full" displacement hulls, these hulls achieve speed by being small (in cross section), relatively high-powered, and light weight for their length. Unlike a semi-displacement hull, they are not intended to lift with speed at all. Rather the idea is to reduce wetted surface and wave-making to allow the hull to slip along at higher speed. They lack the bottom area to get any useful dynamic lift as big wide (and usually heavy) semi-displacement boats do.

You might find some reference to this in the Dave Gerr link above. He couples it to displacement/length ratio. The lighter (displacement) a hull is for a given length, the higher it's "Hull Speed" will be.

Yes, the linked Gerr paper above spends a fair amount of time on this. Another related Gerr paper that compares various hull types is here:

More efficient power voyaging? - Ocean Navigator - May/June 2012

 

[healhustler]

Fascinating conversation. Among the marketing Catch Terms of Super Displacement, Twin Displacement, Ultra-Displacement, Mega Displacement and this and that and the other displacement, I wondered if anyone cared to comment about the Schulz Reverse Deadrise design (SRD). It looks familiar. Here's the link:

SRD is a better hull shape, fast, fuel efficient, stable

 

[Nomad Willy]

Larry wev'ed talked about the SRD before .. On of my favorites.

Mark you'd have liked the "Tin Can Navy".

My favorite TAD R design is the 28hp 38' Yellow Cedar. Really like YC.

TAD I participated on a thread saying my 30 Willard was not a canoe stern. Most pushed me to accept it as such. I frequently refer to it as a " bathtub" stern.
Can you and would you comment on the Sumner Craft of Amitiville NY. The most interesting design I ever had experience with. Mine was 29' w 120 Sabre.

 

[Art]

Fascinating conversation. Among the marketing Catch Terms of Super Displacement, Twin Displacement, Ultra-Displacement, Mega Displacement and this and that and the other displacement, I wondered if anyone cared to comment about the Schulz Reverse Deadrise design (SRD). It looks familiar. Here's the link:

SRD is a better hull shape, fast, fuel efficient, stable

Hi Larry, thanks for posting this – SRD seems a great new hull design in many respects! I am impressed. Following is a quote from link you provide. Sections I highlighted interest me and via three (3) excerpts I explain how or why. - Art

1. “...the most fuel efficient powerboat on the market at speeds over 10 knots/12 mph.” From further on in copy that seems potentially correct (see #3); and, if so that is great!

2. “At 10 knots, the SRD obtains almost 2 miles per gallon...” I’m not sure the LWL of SRD tested for these results and wonder if 10 knots is its hull speed? That mpg is similar to what my 34’ Tolly with twin 1977 gassers gets at or just below her hull speed of 7.5 knots.

3. “Traveling at 15 knots, the SRD provides over 1.6 miles per gallon...” That IS impressive!!! Because – at 16 knots our Tolly gets 1 mile per gallon... but that’s pretty well loaded. If with near full load RFD still gets 1.6 mpg fuel use rate then it has surely very improved fuel economy at fairly quick cruise speed.

"SRD Fuel Efficiency

Based on independent testing, the SRD hull, with a single engine or twin engines, is the most fuel efficient powerboat on the market at speeds over 10 knots/12 mph. While the SRD hull can easily reach speeds of 25 knots/30 mph or greater depending on engine horsepower, the hull provides exceptional fuel economy in the 10 knot /12 mph to 18 knot /20 mph speed range. At 10 knots, the SRD obtains almost 2 miles per gallon, or the same fuel burn found on a full displacement trawler at top speed. Traveling at 15 knots, the SRD provides over 1.6 miles per gallon, whereas a modified vee planing hull struggles to provide 1 mile to the gallon at the same speed. The fuel efficiency of the SRD hull translates into 60% less fuel burned, increased range and a significant reduction in pollution."

 

[Art]

Larry wev'ed talked about the SRD before .. On of my favorites.

Mark you'd have liked the "Tin Can Navy".

My favorite TAD R design is the 28hp 38' Yellow Cedar. Really like YC.

TAD I participated on a thread saying my 30 Willard was not a canoe stern. Most pushed me to accept it as such. I frequently refer to it as a " bathtub" stern.
Can you and would you comment on the Sumner Craft of Amitiville NY. The most interesting design I ever had experience with. Mine was 29' w 120 Sabre.

Eric - List of Sumnercraft boats...

http://www.fiberglassics.com/wiki/images/b/b9/Sumnercraftbb001.jpg

 

[Daddyo]

How about we go with HB (high fuel burn hulls) or LB (low fuel burn hulls)? We burn 1.9GPH pushing our 60,000lbs through the water at 6.8 knots. Call it what ever you want, I call it a amazing! Thank you Art DeFever

 

[FF]

>ever ride a honking Hobie Cat or a 40 something, trip engine go fast from wave top to wave top?

you may not change your opinion but you'll see another way to travel comfortably though surprisingly large chop.<

The big buck , damn the cost , Navies and Coasties do this very thing , but even with $6,000 + seats , special bottom designs and using kids on the boats , the problem of PEOPLE breaking during the ride has not been solved.

6+G loading to and from the local picnic hole doesnt sound like there will be many boat sales.

 

[psneeld]

>ever ride a honking Hobie Cat or a 40 something, trip engine go fast from wave top to wave top?

you may not change your opinion but you'll see another way to travel comfortably though surprisingly large chop.<

The big buck , damn the cost , Navies and Coasties do this very thing , but even with $6,000 + seats , special bottom designs and using kids on the boats , the problem of PEOPLE breaking during the ride has not been solved.

6+G loading to and from the local picnic hole doesnt sound like there will be many boat sales.

It's driving the boat that's the issue much of the time. Sure not all conditions are suitable but many of the go fasts learned that going faster was the answer...don't let the boat develop the vertical motion to create the G-forces. You can do it in seas that are of similar size and reasonable spacing which isn't all the time but enough that the technique is worth mastering. The rest of the time you just have to slow down like the rest of us.

 

[Art]

Talking about taking a wave-pounding in go fast boats...

Don Aronow, my hero when I used to race around in some fasties (Donzi) during 60's in my teens, was world famous speed boat designer and a renowned off shore speed boat racer through the 60's, 70’s, 80’s. From what I understand he took a few "tens-of-thousands" ocean waves too hard standing in his racers. So many that his liver and other organs eventually dislodged and were extremely damaged. But, that’s not what killed him! This video is very interesting...

http://espn.go.com/30for30/film?page=collisioncourse-the-murder-of-donaronow

 

[psneeld]

racers push no matter the conditions...not what I said....

 

[healhustler]

Larry wev'ed talked about the SRD before .. On of my favorites.

Woops...sorry bout that. I found it through the "search" option.....somehow I missed that input.

 

[Art]

racers push no matter the conditions...not what I said....

Pardon, but:

Who said, you said, not me! I was just adding some more flavor to this boat cwazy thread.

Interesting video though, on Aronow - tain't it??!!

 

[Nomad Willy]

TAD sums up the SD/FD difference very well here;

"designed to ease the return of water from under the hull to the "at rest" level somewhere astern. At the correct speed this stern leaves no wake at all. The deeply immersed transom of DeFever's later designs is far more "draggy", but does allow greater speed without squatting. It costs money to drag that around though..."

The water passing along the bottom needs to return to the surface w/o tumbling up from the bottom of a submerged transom.

In a way I've taken TAD's comment out of context as he was talking about a stern very common on fish boats in the PNW. The "Horshshoe" stern. But his words convey very well why most trawlers do not have full displacement hulls and are considerably less fuel efficient. IMO.

I just learned how to copy and paste on my i-pad. Like design. Often the need leads us to solutions.

 

[hfoster]

Talking about taking a wave-pounding in go fast boats...

Don Aronow, my hero when I used to race around in some fasties (Donzi) during 60's in my teens, was world famous speed boat designer and a renowned off shore speed boat racer through the 60's, 70’s, 80’s. From what I understand he took a few "tens-of-thousands" ocean waves too hard standing in his racers. So many that his liver and other organs eventually dislodged and were extremely damaged. But, that’s not what killed him! This video is very interesting...

http://espn.go.com/30for30/film?page=collisioncourse-the-murder-of-donaronow

Good post Art. The video as something to watch.

Thanks

Happy Cruising

H. Foster

 

[sunchaser]

The water passing along the bottom needs to return to the surface w/o tumbling up from the bottom of a submerged transom.

Without intending to steal TAD's thunder, there are many very efficient vessels with submerged transoms out there. It would appear his own PL designs, Dashew's, Garcia and Shannon's vessels are examples. Or how about a Fleming's decent NMPG at displacement speeds?

Having spent time on various newer Nordhavn models, they have submerged transoms very different than the older N46 or N62 designs. Why is this?

Not to be overlooked and important for blue water work is the vessel's ability to safely surf - at say 14 to 17 knots. Which is one of the advantages of the frigate design TAD refers to. The frigate design allows an easy 9 to 11 knots in most conditions, dependent upon WLL, while exceeding the 1.34 "rule" to not only yield efficiency but also the ability to avoid and outrun the bad weather.

 

[Nomad Willy]

Tom

Having spent time on various newer Nordhavn models, they have submerged transoms very different than the older N46 or N62 designs. Why is this.

Tom I've been worried someone would bring that up for a long time. The submerged transom is an easy way for antibody to identify the SD hull from FD. So I was very reluctant to say anything about it. But since you've let the cat out I feel the need to respond. The answer is in the QBBL angle. Full D sterns w some submerged transom all have a steep QBBL angle so the returning water is racing toward the surface and the transom has little turbulence drag. The high drag comes from sterns that have basically horizontal bottoms aft hence very low QBBL angles. So the water under the stern is moving straight aft (or nearly so) and it is forced to make an abrupt turn of almost 90 degrees. This creates a lot of turbulence and water tumbling desperately clawing it's way back to the surface. At least several very heavy Seaton boats that are no-doubt FD boats have such sterns. FD hulls w a bit of submerged transom aren't terribly unusual so I'm surprised it took this long for someone to blast a hole in my slightly loose rule of thumb that FD boats will not have submerged transoms.

AND .. You make a very good point about the FD boat not having the ability to surf. There are times that that ability adds seaworthyness to a boat. Negative raked transoms, swim steps and other such design features having at least some loss of seaworthyness can redeem themselves w the ability to plane and outrun the rushing sea from astern.

I just re-read page 1 and am still amazed at the really insightful input that came out on that page.

 

[kulas44]

My 48 sporty is VERY smooth running at 25 knots, even in 6 foot seas if they are fairly close. It is also very stabile at that speed and does not pound at all. It sprays water to the sides and fuel to the rear that is impressive. I dont know what the fuel burn is at that speed, cant calculate numbers that large in my tiny brain. I dont run much over 10 knots these days, even that speed uses a lot of fuel. In the rough I might step it up to 15 knots but using fuel to flatten the water is really not the best idea, it does work though. My trawler is a totally different animal, more speed is not helpfull, even if it had enough power to do it. Mostly due to the fact that it is a "true FD" hull identical to the KK42 but bigger, the same as the KK54 without the canoe stern. The designs are totally different, polar almost between the sporty and the Trawler. Each achieves its design goals in different ways. For coastal cruising either would work great.

 

[psneeld]

http://www.trawlerforum.com/forums/s3/our-next-boat-lrc-full-displacement-13601-4.html

"And I have posted MANY TIMES WITH LINKS to professional NAs that say the QBBL is only part of the equation...the prismatic coefficient is huge too.

A perfectly flat bottom barge with square front is certainly a displacement object...yet the QBBL angle is zero...meaning it's a planning hull (to some).

Sure if you lightly load it, tip the bow up and apply lots of horsepower...it can plane...hell it can fly through the air like an F-4 Phantom.

But the flat bottom barge has a normal use as full displacement.

So, lets seriously discuss QBBL and what it really means."


Above is my post from another thread several days ago...right after that thread was not going in the right direction I guess...this one was started.

Last YEAR I linked NA articles showing how the other design factors were just as if not more important than QBBL and immersed transoms mean...what they mean....they are less efficient than some FD transoms but they do NOT disqualify a boat from being well down in the FD design criteria....just less efficient.

 

[Nomad Willy]

I agree psneeld that there are many other factors of design. The reason I chose to use QBBL is that it was discussed on BoatDesign.net and the angle was offered to define the division point for FD to SD. These people know a whole lot more than I do about this. That's what they came up with and general calls can be made. More precise calls can be made w the number of degrees of the angle. But as far as I know the angle was generated within that discussion.

And all are right that the line cannot be actually numerically known or stated as a fact that is unalterable or 100% definitive.

And I believe the QBBL is the best way to make the call. It surely will hold up re most if not all the trawlers on this forum.

When we get through w this I have another design question to launch.

 

[Ski in NC]

I do a lot of pre-purchase engine inspections, and through that have sea trialed many boats. It's given me much insight into what works in higher speed boats.

Conclusion: Weight is God when it comes to speed.

Here in the Carolinas, we have many locally built boats using either wood plank-on-frame or plywood cold-molded construction. It turns out that both these methods result in hulls much lighter than solid fiberglass layup. The performance of these boats can be fantastic. I'm lucky that I've been able to test run many of each, and log the numbers.

Case in point: late 1970's sportfish 52' x abt 15', plank on frame. Two 320hp 8v71n, 1.5gears. Top speed about 23kts!! Cruised super nice at 18kt, about 18gph (est). Hull shape modified vee. Boat very spartan as far as interior clutter, no accurate weight known. This general design was copied by many Carolina builders, but later boats got heavier and faster, but through weight lost the efficiency.

I decided to give it a try and built a 38x12 using cold mold construction. Tried to keep the weight out of it, but budget and construction fiascos got in the way somewhat.

Ended up 37.5 x 11.5, Cummins 450 6CTA, 5kW gennie. Weighed boat on truck scale, tanks nearly empty, minimal loose equipment: 13,000lb. (how many of us really know the true weight!!!). Figure cruising weight abt 14-15000lb.

Performance:

7.5kt 950rpm 1.9gph 3.9nmpg (verified over abt 7000miles and tank dipstick)

18kt 1850rpm 9.5gph 1.9nmpg

20kt 2000rpm 11.5gph 1.7nmpg

top sped abt 29.5kt 2700rpm

So it works well at trawler speed with good nmpg. But has the capability to run reasonably efficient at 18-20kt.

In hindsight, I could have gone with the 6BTA engine and saved abt 600lb and still got the 18-20kt cruise. Parasitic drag on that 8.3l at low power is a good bit more than the 5.9l. Oh well.

One trip I started with an abnormally heavy load (about 3000lb additional) and the hull speed performance was much worse. I did not run long enough to get burn rate data, but speed at same rpm (950) was at least half kt slower. Had to go to 1000-1050 to get the same speed. Again, weight is key.

When I sea trial solid glass boats it amazes me how much hp it takes to push those things. There are boats of similar size with TWO 6CTA's that get similar performance!!

I'm no Naval Architect, (but am a ME) and do not understand much of NA tech. But I saw a hull shape that worked (modified vee) and learned how important weight was, and went with it. It worked.

Another story on boat weight:

A local builder built two 58' sportfish back to back. First one had MTU v12 at about 1500hp ea. Second 58' had Cat c18 at 1050hp ea. Top speed difference: 3kts.