Definition Of Full Displacement

Fish wrote;
“Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull.”

Those are actually planing hulls. The soft/rounded chine causes more drag but those boats have enough power to overcome it. If they had hard chines the’yd be faster but the’re planing hulls. Reason being the majority ofthe bottom is flat and there’s a clean break at the transom/bottom edge. Given a little speed the water breaks free at the end of the transom. And as you go faster the water comming up around the chine curve has lesstime to rise up so the soft chine restricts speed even less.

Fish53 said:

There are several sentences that I take issue with but some may be me not getting your point exactly. But this one "The bastard child is the SD hull, not as efficient at either mode, typically has characteristics of planing hull but is over weight and underpowered for that mission. Nevertheless, it is very popular for other reasons.". Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull. I 've had a couple one was a BHM 32 with a 215hp Isuzu that did 18kts WOT and the other a Sisu 26 with a 200hp TAMD41a Volvo that did 25kts.

Click to expand...

In other words, you know of planing boats that you prefer calling semi-displacement.

Already posted, but a good description of the thtee types of hulls is here:.

https://www.outerreefyachts.com/hull-types

IMO......
Does She love a following sea.....
If so... full displacement....

Nomad Willy said:

Fish wrote;
“Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull.”

Those are actually planing hulls. The soft/rounded chine causes more drag but those boats have enough power to overcome it. If they had hard chines the’yd be faster but the’re planing hulls. Reason being the majority ofthe bottom is flat and there’s a clean break at the transom/bottom edge. Given a little speed the water breaks free at the end of the transom. And as you go faster the water comming up around the chine curve has lesstime to rise up so the soft chine restricts speed even less.

Click to expand...

. I think it's fair to say that a semi displacement boat that lacks the power to plane becomes a planing hull if you increase the power sufficiently. Unlike a displacement hull, which, as Outer Reef notes, "can never plane."

Fish writes the SD hull is the bastard child in hull design. Indeed it is. Kind of a “morfidite” but important as it fills the gap between planing and FD. A planing hull does SD speeds but very inefficiently and w/o enough speed to be in solid control with her small rudders and even smaller keel. The SD hull offers some speed w significantly better economy than the planing hull. Without the SD boat rec trawlers would be stuck with 6-8 knots or a planing hull. Rec Trawlers probably owe their existance to the SD hull. Note that SD boats designed for the upper range of SD speed are very close to planing hulls and frequently called the wrong hull type. Often on TF (not so much as in the past) people would call SD boats FD. If we tried to label some boats half of us would say it’s FD and the other half would say SD. And since there’s no clear decisive description of both types the boat is just in a grey zone. Probably 20% (or more) of the trawlers here are in this grey zone. All one can do is hear and evaluate some differing opinions on the same hull. And then draw you’re own conclusion.

Delfin wrote;
“I think it’s fair to say that a semidisplacement boat that lacks power to plane becones a planing hull if you increase power sufficiently”

Not so at all. The hull type is entirely due to it’s form or shape. For example many have tried to define the three hull types by tieing wave length to group them in their various types. But the definition falls appart in argument.
Other attempts have been made but one can’t apply them to some hull types, especially very unusual designs.

The best (IMO) way to separate the three hull types is to assign approprate QBBL angles ... basically the angle that the bottom of the hull is to horizontal. Horizontal would be planing. A slight angle to a bit more = SD and anything more is FD. Of course arbitrairly assigning numbers makes grey zones and one could argue it’s up to sombody’s whim.
This has been done though with a group like a bunch of advanced students at a well recognized school of yacht design. As an example with a source representing so much accumilative knowledge one could assume some fairly high degree of credibility and/or accuracy.

Nomad Willy said:

Delfin wrote;
“I think it’s fair to say that a semidisplacement boat that lacks power to plane becones a planing hull if you increase power sufficiently”

Not so at all. The hull type is entirely due to it’s form or shape. For example many have tried to define the three hull types by tieing wave length to group them in their various types. But the definition falls appart in argument.
Other attempts have been made but one can’t apply them to some hull types, especially very unusual designs.

The best (IMO) way to separate the three hull types is to assign approprate QBBL angles ... basically the angle that the bottom of the hull is to horizontal. Horizontal would be planing. A slight angle to a bit more = SD and anything more is FD. Of course arbitrairly assigning numbers makes grey zones and one could argue it’s up to sombody’s whim.
This has been done though with a group like a bunch of advanced students at a well recognized school of yacht design. As an example with a source representing so much accumilative knowledge one could assume some fairly high degree of credibility and/or accuracy.

Click to expand...

Not so sure about that, Eric. You could drop 10,000 hp in Delfin and she still wouldn't plane, but if you increased the hp on a Fleming to, say 2,000, I am pretty sure she would because while not horizontal in her stern sections like a navy destroyer, it's not far off. In my mind, you really only have two hull types - full displacement where drag increases faster than lift and everything else where lift can exceed drag if the propulsive power is sufficient. Under that scheme, a semi-displacement boat is just an underpowered planing boat.

I do like the description of the three hull types in the Outer Reef link above.

Well Mr Vizier,
OK good. We have an opinion.
But you’re assigning a type by it’s speed capability. Perhaps you’re trying to explain to a better degree common trawlers like the IG. Take the keel mostly off and put it on a serious diet .. weight wise. Then nobody would doubt it was a planing hull/boat type.
But if that’s your yardstick put my 37hp engine in an IG32 and call it a FD boat.

I put in the link as listed and got everything outer reef but hull types. ???

I actually like four hull types.
1. Planing
2. Semi displacement
3. Semi planing
4. Displacement

1. The planing hull operates at 2 to 3 times it ‘s hull speed retaining good seakeeping abilities as in control and the ability to carry medium to light loads.
2. Semi displacement hulls operate at slower planing speeds. They displace more water therefore not “on top” to the degree the full planing hull does. Thus they sit a little bit down in the water.
3. Semi planing hulls plane to some limited degree. Usually at a higher angle of attack that all the other types. Not much faster than a FD boat and with not much speed advantage. But some degree of dynamic stability at their speed at and above displacement speed. Good examples are older NT and GB boats.
4. Full displacement boats are the most efficient and seaworthy of boats. Sensible speeds .75 to 1 knot below hull speed deliver great fuel economy, easy motion and excellent low speed handling abilities.

Nomad Willy said:

4. Full displacement boats are the most efficient and seaworthy of boats. Sensible speeds .75 to 1 knot below hull speed deliver great fuel economy, easy motion and excellent low speed handling abilities.

Click to expand...

So, by that definition, regardless of speed, a SD or even planing boat, becomes a FD when is is run hull down 0.75 or 1 knot below where is would try to start coming out of the hole?

No the def is assuming a boat on her lines.
Running hull down is not part of it. Challenging thought though.

So maybe that boat is "running at full displacement?"

Since the hull is fully in the water it is displacing at its maximum displacement value.

Seventy posts to answer the question so far....you guys are making this hard.


Full Displacement: You know it when you see it!

klee wyck said:

Seventy posts to answer the question so far....you guys are making this hard.


Full Displacement: You know it when you see it!

Click to expand...

How many hp do you need to waterski behind her?

Delfin said:

How many hp do you need to waterski behind her?

Click to expand...

Depends upon how flat bottomed the ski is.....

menzies,
Play on words there I’m think’in.
Not an NA but I think you just described a boat “on her lines”.
Assumably that would be at max working displacement.
And of course would not require any motion or speed.

Nomad Willy said:

Well Mr Vizier,
OK good. We have an opinion.
But you’re assigning a type by it’s speed capability. Perhaps you’re trying to explain to a better degree common trawlers like the IG. Take the keel mostly off and put it on a serious diet .. weight wise. Then nobody would doubt it was a planing hull/boat type.
But if that’s your yardstick put my 37hp engine in an IG32 and call it a FD boat.

I put in the link as listed and got everything outer reef but hull types. ???

I actually like four hull types.
1. Planing
2. Semi displacement
3. Semi planing
4. Displacement

1. The planing hull operates at 2 to 3 times it ‘s hull speed retaining good seakeeping abilities as in control and the ability to carry medium to light loads.
2. Semi displacement hulls operate at slower planing speeds. They displace more water therefore not “on top” to the degree the full planing hull does. Thus they sit a little bit down in the water.
3. Semi planing hulls plane to some limited degree. Usually at a higher angle of attack that all the other types. Not much faster than a FD boat and with not much speed advantage. But some degree of dynamic stability at their speed at and above displacement speed. Good examples are older NT and GB boats.
4. Full displacement boats are the most efficient and seaworthy of boats. Sensible speeds .75 to 1 knot below hull speed deliver great fuel economy, easy motion and excellent low speed handling abilities.

Click to expand...

Agreed, Eric! That said... I like having planing hull capabilities for accomplishing inner waterway and close-coastal cruising. Such as the majority of pleasure powerboat owners enjoy doing.

A displacement boat under 45-50 foot in length (or more?) doesn't need more than an 80 horsepower engine.

markpierce said:

A displacement boat under 45-50 foot in length (or more?) doesn't need more than an 80 horsepower engine.

Click to expand...

That is true.

However, quite slow.

Fairly inexpensive... comparatively.

Art said:

That is true.

However, quite slow.

Fairly inexpensive... comparatively.

Click to expand...

Not slow at all.
It should make hull speed as designed.

Nomad Willy said:

Fish wrote;
“Up here in Maine there's tons of SD hulled lobsterboats that do quite well in the planning department, I have one friend with a 42' lobster boat that does over 40kts and race boats do up to 70kts with an SD hull.”

Those are actually planing hulls. The soft/rounded chine causes more drag but those boats have enough power to overcome it. If they had hard chines the’yd be faster but the’re planing hulls. Reason being the majority ofthe bottom is flat and there’s a clean break at the transom/bottom edge. Given a little speed the water breaks free at the end of the transom. And as you go faster the water comming up around the chine curve has lesstime to rise up so the soft chine restricts speed even less.

Click to expand...

The builders and designers don't think they're planning hulls...….https://ellisboat.com/ellis-downeast-semi-displacement-hull/

Art said:

That is true.

However, quite slow.

Fairly inexpensive... comparatively.

Click to expand...

Interestingly, the greater the distance you travel, the more that speed is the norm.

Ted

So I think we're all clear now, right?

Simi 60 said:

Not slow at all.
It should make hull speed as designed.

Click to expand...

Err - "Hull Speed" is quite slow... compared to a planing hull boat that easily does 2X to 3X increased mph [some even reaching into the 4 and 5 X category].

Regarding our comfortably equipped, quite sea kindly, made for inner-waters/close-coastal cruising... sea worthy, planing hull Tolly tri cabin:

I'm not saying that I don't appreciate the relaxation and economy in going slow at 7 knots, which is just below its hull speed [i.e. 7.58]; instead being on a still rather relaxing full plane doing 16 to 17 knots... or a bit faster if for some reason encountering a need for a real rush, with 22 knts available at WOT.

What I am saying is that I really like having the alternative of going slow or fast... such as a full planning hull offers every time out and about [sea conditions accounted for regarding speed used].

When we take a trip of 100 miles it is often 17 knots at which we travel = 5.88 hrs travel time. If we stayed at 7 knts, [just below hull speed] = 14.28 hrs.


Fuel use/cost for our boat is double at the 5.88 hr full plane travel time compared to 14.28 hr at hull speed travel time... however... leaving at 6AM and arriving at 11:50AM leaves much of the day for play. Whereas, leaving at 6AM and arriving at 8:15PM only leaves time to make ready for bed!


In the boating world there is surely a trade off for everything. I like options... therefore I very much appreciate owning well built planing hull pleasure cruisers!


Happy Cruising-Speed/Cost Daze! - Art

Greetings,
All this "discussion" about full displacement and NOT one comment about what anchor is best for said, as yet undefined, vessel...Just sayin'...





Somebody is asleep at the switch.

Fish53 said:

The builders and designers don't think they're planning hulls...….https://ellisboat.com/ellis-downeast-semi-displacement-hull/

Click to expand...

Them lobster persons put BIG, BIG HP engines in their boats.

I know, for years [back when - i.e. early 70's] I was often on Lobster Boats pulling traps with my buddies in the Penobscot Bay, Maine. Engines were powerful even back then. From what I hear now - the engines are crazy crazy powerful monsters!

O C Diver said:

Interestingly, the greater the distance you travel, the more that speed is the norm.

Ted

Click to expand...

That is for sure!

Simi 60 said:

Not slow at all.
It should make hull speed as designed.

Click to expand...

You guys are still stuck on hull speed.
Some designers may be too. At least in their add copy because they know many boaters know what hs is and place some importance on it.
“Most all” designers design FD boats to be FD boats. Whether or not they reach hull speed or not is almost irrelevant. As someone eairler said hs is just a number or a formula and the boat may or may not go that fast. My boat has a hs of 7 knots. I’m not sure I have ever gone 7 knots .. through the water. Gone 11 several times (otg) on my chart plotter going through rapids and gone 7 knots with a .85 knot current (my usual speed is 6.15 knots) probably hundreds of times. I’ve never been in a lake to run the boat up to her top speed. And I have no need to know my top speed.

But no .. FD boats are not designed to “make” hull speed.

Fish53 said:

The builders and designers don't think they're planning hulls...….https://ellisboat.com/ellis-downeast-semi-displacement-hull/

Click to expand...

Ellis lists 26 knots as top speed, not the 70 knots you referenced. At 26 knots it would still be pushing some water. At 70 knots, it's a hydroplane. This illustrates the point that if you put enough power in a semi displacement hull, it will plane, and at that point it's s bit hard to argue the vessel is still semi displacement.

Delfin said:

Ellis lists 26 knots as top speed, not the 70 knots you referenced. At 26 knots it would still be pushing some water. At 70 knots, it's a hydroplane. This illustrates the point that if you put enough power in a semi displacement hull, it will plane, and at that point it's s bit hard to argue the vessel is still semi displacement.

Click to expand...

If you'd read correctly I clearly stated "race" lobsterboats do 70kts. I thought you also claimed FD hulls could plane so to you FD, SD and planing hulls are all one in the same only segregated by the HP employed? This makes it hard to argue that you have any position for us to consider on this subject. I'll stick with the builders and designers concepts thanks.

Fish wrote;
“The builders and designers don't think they're planning hulls”

Pardon me for saying so but this is add copy. And these designers say it often enough that they may actually belive it. They are selling boats and SD boats carry some mystique that draws people in thinking SD hulls are something special. But it is harder to design a SD hull than a planing hull. Haha they should take the credit.

But in their/your defense these planing hulls carry some design features and well known shape types that identify w SD .. not planing hulls. The most obvious is the soft chines. But like hs people make too much of these features. In the case of the Ellis boat the size of the chine is so small it is hard to say it makes it a SD hull.

But they do say it and I’ll give you credit for research .... but the Ellis boats I saw in the link are planing hulls.
Perhaps you remember me saying when you get in the middle of two types of hulls, since we’re dealing w shape, it’s hard to call many boats. The Ellis when it comes to the call they use some of the shape features and ignore more important shape features and the speed performance. It planes out over the water. Since it’s in the grey area though they feel they can call it what they want .. and they do. SD sounds a lot more salty and special than planing.

I’m of a mind that thinks the obvious high speeds they attain and even more importantly the perfectly straight run aft on the hull bottom far outweighs the very small soft chine that is so small it should be called hard.

Planing hull.