Full Displacement or Semi Displacement

[psneeld]

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.

About the only thing I can say about QBBL (and it sure depends on who you think knows what on boatdesign.net)...is that yes the angle is of some importance to displacement efficiency...but by NO means is a big factor in displacement determination...only efficient displacement design (go back to my barge example)....and again there are other very important factors that weigh into efficiency on displacement boats that equal or exceed QBBL...

The only real acceptance of QBBL as a major player from my research is going in one direction.... in that a larger angle at some point will prevent a boat from attaining decent semi-xxxx performance (whichever one you wish to label it). But you most certainly can have zero QBBL angle and still be designed for full displacement. Which is exactly what many here are trying to say about displacement hulls with lessened QBBL and immersed transoms...they are NOT semi anything to either the NAs that designed them or the people who operate them.

 

[Jobe]

There is a lot to be said about those "Carolina" boats and the yards that produce them. Mention 'Cold Molded' or heaven forbid 'plywood' and people run for the hills! Fact is, it not only produces a lighter hull than glass, it produces a much stronger hull than glass. With modern day epoxy it ain't Gramps wood boat anymore and the few builders utilizing these techniques deserve serious consideration for their attributes.

 

[psneeld]

There is a lot to be said about those "Carolina" boats and the yards that produce them. Mention 'Cold Molded' or heaven forbid 'plywood' and people run for the hills! Fact is, it not only produces a lighter hull than glass, it produces a much stronger hull than glass. With modern day epoxy it ain't Gramps wood boat anymore and the few builders utilizing these techniques deserve serious consideration for their attributes.

Unfortunately a lot of the cold molded interiors rot because of cost overruns I guess so many are not finished on the inside as well as they should be. Many are..but I have worked on a few that 10 years later were really showing premature deterioration.

 

[Ski in NC]

Cost overruns are a big deal in Carolina boats. Cold molded hulls are IMHO very superior, but they are very labor intensive to build. Builders are known to underbid to get the contract, then try to make a profit through some other means. Hard to build these and make money. Impossible in this economy. Labor and materials cost more than a similar late model used boat.

 

[kulas44]

Possible (probable?) thread drift here, but, I was in a boatyard in GooseCreek S.C. when a 63 foot (IIRC) coldmolded sporty came in for some minor things. The Captain was an older fellow, Bob I think was his name. He told me the boat was built locally and at the time was the fastest sportfisher, at 60 knots. Boys, that is fast. The cabinets and all of the interior was 1/4 in ply. Bob was not impressed. He said they used plywood everywhere possible, even in the hull in flatter sections, and laminated 1/4 luan in places with moderate curves. I fully believe in 20 years we will be avoiding these $$$$$ boats like we do ferrocement now.

 

[Underway]

About the only thing I can say about QBBL (and it sure depends on who you think knows what on boatdesign.net)...is that yes the angle is of some importance to displacement efficiency...but by NO means is a big factor in displacement determination...only efficient displacement design (go back to my barge example)....and again there are other very important factors that weigh into efficiency on displacement boats that equal or exceed QBBL...

The only real acceptance of QBBL as a major player from my research is going in one direction.... in that a larger angle at some point will prevent a boat from attaining decent semi-xxxx performance (whichever one you wish to label it). But you most certainly can have zero QBBL angle and still be designed for full displacement. Which is exactly what many here are trying to say about displacement hulls with lessened QBBL and immersed transoms...they are NOT semi anything to either the NAs that designed them or the people who operate them.

psneeld,

Take a look at the third paragraph of post #33. Using a flat bottom canoe shaped hull as an example, he describes exactly what you're saying....a displacement hull with no buttock lines....and then promptly dismisses it by alleging that even psneeld realizes that there are exceptions that can't be covered by engineering methods. In case you missed it...

 

[Jobe]

....

 

[psneeld]

psneeld,

Take a look at the third paragraph of post #33. Using a canoe in lieu of a barge as an example, he clearly acknowledges exactly what you're saying....and then promptly dismisses it by alleging that even psneeld realizes that there are exceptions that can't be covered by engineering methods. In case you missed it...

Believe me.... on some topics posted by some...like a hawk!!!

me and a couple others have tried to but have lost track of all the backpeddling some have become masters at.....

 

[Art]

Speaking of ferrocement's poor reputaion - due to lack of know-how for building thin wall products with cement (see post #65) ... I have plans to greatly reduce hull construction costs via monel expanded metal reinforced super-cement whose mixture is polymer assisted, fiber filled, and applied into pressure injection molds that will provide full saturation into all voids via sonic vibration techniques and vertical lift male mold forms. I developed this years ago... still in the wings for my business endeavors. If/when time to further research this technique presents itself I just might proceed. Would be very interesting to see how much cost could be stripped off hull construction while providing low weight characteristics and very strong long lasting material outcome.

Any hull shape could work in this process.

 

[THD]

Art-at the risk of going completely off-topic-I would be interested in a bit more of a description. I cannot envision an injection molding system that could be used for hull construction. Mold costs alone, not to mention issues with the physics of injection molding itself, would be astronomical compared with current mold techniques.

 

[psneeld]

From my limited experience...ferrocement has a bad rep because 90% of the people who tried it couldn't build a birdhouse in high school woodshop thinking they had found the magic formula for easy cruisin' to the south seas.

The hulls that were build professionally by good yards or persons who did it correctly wound up with certifiable hulls that were exactly what the method was supposed to produce...an inexpensively made one off hull to high standards that will outlast many other conventional materials.

 

[kulas44]

I completely agree, and coldmolding is a great way to build a boat. But when cutting corners and trying to build as light weight as possible the end product suffers. Even one of the best builders of coldmolded boats uses lots of fir and okumee plywood in the hull and bulkheads. What has been most folks experience with plywood in boats after 20 years. Rot comes to mind. And yes, epoxy encapsulation does work , but, it adds weight and expense so it is the first thing to go. After all, the old rich boy wants the fastest boat now not 20 years from now.

 

[Tad Roberts]

To return to the thread subject for just a sec....bear with me

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

Real Frigates are really long, thin, and light....we can't manage these proportions with smallish pleasure boats because they become unusable. The Halifax Class in the picture below is 440' long and only 53' wide, that's a beam/length ratio of over 8. In a forty footer of similar proportions the beam would be 5'.....not workable.

The buttock lines in these hulls; Frigates, PL's, FBP's, LDL's, etc, are very straight and flat. There is a slight rise in most of them from midships to the transom. There is some transom immersion, more with higher speed expectations and/or heavier displacement (longer range). You have to pay if you want that speed capability. The only way to get away from it is to create a variable geometry stern (trim tabs plus).

Note that the Ottawa (below) is running at close to her published max speed of 29 knots (slightly above "hull" speed) with almost no bow rise and no big hollow midships in the wake. While she is running at a semi-displacement speed (above that of FD), she is not getting any lift or trim, thus the "High-speed Displacement" Form. Compare this wake to that of Marda further back in the thread.

 

[markpierce]

The following was copied from a webpost by Seahorse Marine. It exemplifies full versus semi displacement hull shapes:

http://ducktalk.net/eve/forums/a/tpc/f/5621093711/m/4611082241

Two boats, Martin's Diesel Duck 462 and a used Seahorse FRP 52, were shipped from the Doumen port, near our factory, today. They are heading for Seattle hopefully arriving in time for the boat show.

Note the difference between the 52's semi-displacement and the 462's full displacement hull shapes. You can see why we call the SH-52 a "coastal cruiser".

The important looking guy walking towards the camera in picture two is Number One Son "Fido".

 

[Nomad Willy]

Mark that DD is clearly a FD but the "52" is very close to FD. The biggest difference in the two boats is displacement not hull form. The "52" is probably closer to FD than the DeFever 49. That's close.

With a low displacement and low wetted surface for a boat w a keel she (the 52) could be a very efficient cruiser.

 

[healhustler]

Mark: Thanks for posting those two examples. Although I would have guessed it, seeing them side by side really does help.

 

[Moonstruck]

That SH52 looks if it would take a huge engine to get her up on plane, and if you did she would be especially tender. Looks very close to displacement to me.

 

[Art]

TY for picts Mark. Nice boats. Interesting bottoms! - Art

 

[Nomad Willy]

On the pics Mark posted the 52 and the DD both have about the same area of submerged transom but the angle of the buttock line is way different. The buttock line is steep .. possibly 7 degrees or so on the Duck while the buttock line on the 52 is no more than 2.5 degrees. Here again the BL tells the story.

 

[Peter B]

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

And here's another useful tip Eric - only just discovered it myself recently. Not having cursor arrows to move just a letter or so one way or another to correct a typo used to drive me mad, as the tap with finger-tip often would not place the cursor where you want it. Son showed me the other day, when I was cursing away, that if you just rest your fingertip lightly over the offending word, a wee magnifying glass comes up which allows you to move the cursor to exactly where you want it. It was a function there all along, but how to find out, if you don't know the right question to ask..? Sort of a Rumsfeldt thing, when you don't know what you don't know...

 

[markpierce]

The big difference for my untrained eyes is that the DD hull bottom (not keel) has a convex curve from bow to stern while on the FRP bottom it is almost flat.

 

[FF]

>The Halifax Class in the picture below is 440' long and only 53' wide, that's a beam/length ratio of over 8. In a forty footer of similar proportions the beam would be 5'.....not workable.<

To get most of the speed at low power only 6-1 LWL x BWL is required .

A 6+ beam at floor level with the usual flair of hull sides would make plenty of space for engine ,noisemaker and big tankage.

With an aluminum or foam core deck house a DL of under 100 could be had , and a fast cruiser created.

I have done a number of drawings keeping the beam over all under 7.5 ft , to fit inside a std shipping container and the concept works,

BUT ,

I dont think it would sell as the cabin space is mostly below and only the pilot house is up on deck where most marine motorists seem to insist on living.It works for us as most cruising time is spent in the PH , we go below to cook, eat and use the head , and as long as the sleeping cabin has excellent all weather ventilation , its fine .

If I came across a spare $100,000 , I would have metal cut tomorrow!

 

[Underway]

About the only thing I can say about QBBL (and it sure depends on who you think knows what on boatdesign.net)...is that yes the angle is of some importance to displacement efficiency...but by NO means is a big factor in displacement determination...only efficient displacement design (go back to my barge example)....and again there are other very important factors that weigh into efficiency on displacement boats that equal or exceed QBBL...

The only real acceptance of QBBL as a major player from my research is going in one direction.... in that a larger angle at some point will prevent a boat from attaining decent semi-xxxx performance (whichever one you wish to label it). But you most certainly can have zero QBBL angle and still be designed for full displacement. Which is exactly what many here are trying to say about displacement hulls with lessened QBBL and immersed transoms...they are NOT semi anything to either the NAs that designed them or the people who operate them.

Ditto.

 

[Nomad Willy]

The big difference for my untrained eyes is that the DD hull bottom (not keel) has a convex curve from bow to stern while on the FRP bottom it is almost flat.

Exactly.

 

[psneeld]

Mark's eye ball is correct about the shape's and I would say probably one is full displacement...but the other isn't necessarily SD/SP...just by QBBL....takes more than that.

 

[Underway]

Displacement hull with horseshoe stern.

 

[Nomad Willy]

Interesting boat.

It's the opposite of my Willard in that most of it's displacement is fwd whereas the W30 is light fwd and heavy aft. Relatively ... The W30 is heavy overall for a 30' boat. The other obvious difference is of course the hard chine.

But horseshoe stern it has.

Actually the WL will determine if this boat I'd SD or FD. Riding high enough it would be FD. In a way it's a bit like the Bartender and it's a full blown planing hull.

Skidgear I'm considering your PM very carefully. Thanks.

 

[Underway]

Actually the WL will determine if this boat I'd SD or FD.

Real world testing...not a photograph...has established that it is displacement. (Confirming what the manufacturer says it is).

 

[Nomad Willy]

A trap.

So what was this real world testing?

 

[Underway]

A trap.

So what was this real world testing?

What, the displacement guru doesn't understand how to gather performance data on a hull/engine combination?