Sd/fd

Here is a FD hull. My 12 foot rowboat.
You can see the WL on the keel. This was unloaded. But w a full load the transom would still be out of the water.
But more importantly the aft part of the hull rises up as the hull goes aft.
Quarter beam is 1/2 of the half beam. In-between the keel and the outer edge of the boat. 1/4 beam is in between the keel and the outside of the hull.
The QBBL is a fore and aft line on the bottom of the boat from about amidships all the way aft.
Planing hulls have the quarter beam line as horizontal. FD hulls are like my rowboat below. SD hulls are in-between. Shallow QBBL. Some FD hulls have a very small amount of transom submerged (like 5-10%) but they have a somewhat steep QBBL. SD hulls have on average 1/2 of their transom submerged at rest. There are SD hulls in trawler form that have a O degree QBBL but have so much weight and keel drag they are called SD. They can't plane. And they surely aren't FD.
There are numbers that can probably be found on boat design.com. Numbers identifing what angles apply to FD and what numbers apply to SD. I don't know what they are.

DCDC wrote;
“Not sure whether displacement or semi-displacement. Best I can tell from photos the QBBL angle is around 6 degrees. Regardless, there will be no semi-displacement speeds for us. With 50' LWL and 300 hp to push 45 tons we cruise 6-7 knots, topping out around 10 knots.”


Is this the photo on post #70?
I see no QBBL angle at all. 0 degrees.
Please see what a FD stern looks like in post #1.

CaptainSEA wrote;
“Hi,
Your answer regarding not following Nordhavn tells me all....

Thank you anyway for your contribution.”

Cap., no I don’t follow Nordhavns. I like the old 46 but they are mostly too pretentious for my taste.

He also posted;
“Thanks for your post, in fact just making a quick surch on the net and it will become clear that Selene are FD, unless the articles and reviews from Passagemaker and the like are all fake!!! I am sure that Howard Shen, the world renowned Naval Architect and owner of Jet Tern Marine (Selene Brand) would be happy to learn that according to a TF member, his designs are not what he thinks, I will send him a quick note today to let him know...”

Nomad Willy said:

DCDC wrote;
“Not sure whether displacement or semi-displacement. Best I can tell from photos the QBBL angle is around 6 degrees. Regardless, there will be no semi-displacement speeds for us. With 50' LWL and 300 hp to push 45 tons we cruise 6-7 knots, topping out around 10 knots.”


Is this the photo on post #70?
I see no QBBL angle at all. 0 degrees.
Please see what a FD stern looks like in post #1.

Click to expand...

I don't have the hull design but re-estimated from photos at around 4 degrees.

Eric, I don’t know anything about hull design. I am confused though because it appears that Selene claims a max speed of 11 knots for their 49’ Explorer. The LWL is 46’ 10”. The classic “hull speed” formula would come out to 9.17 knots. So how is that boat going to achieve 11 knots with 330hp unless it isn’t acting as a full displacement hull at that speed?

dhays said:

Eric, I don’t know anything about hull design. I am confused though because it appears that Selene claims a max speed of 11 knots for their 49’ Explorer. The LWL is 46’ 10”. The classic “hull speed” formula would come out to 9.17 knots. So how is that boat going to achieve 11 knots with 330hp unless it isn’t acting as a full displacement hull at that speed?

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I was under the impression that the hull speed was not an absolute top speed, but the speed at which the hull was still efficient.
Greater than that speed, the stern is falling into the trough the hull produces so additional power goes to trying to overcome that wave, which is impossible.

So, as a guess the Selene may need only 100 hp to get to 9 knots, but 200 more to get another 2 knots.

If I'm off base here, I'd appreciate understanding it better.

DCDC said:

I don't have the hull design but re-estimated from photos at around 4 degrees.

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DCDC,
It actually looks like there’s a bit less than one degree. One could say the bottom has 1 or so degrees negative buttock line at the chine. That would be called a “hook” bottom. Usually used to lower the angle of attack of planing hulls.

Perhaps I didn’t explain the QBBL clearly enough.
Zero degrees would be a straight bottom .. fore and aft wise. It looks like the chine (in your pic) is below the top of the AF bottom paint more so all the way aft. And further fwd there is less distance from the chine to the chine.
One cannot see the keel line to estimate the quarter beam line. We’d need a better pic to do good guessing of the 1/4 beam line. But unless your hull is really strange your buttock line is probably near zero. Seeing the pic of the transom gives me confidence in saying that. My best guess w what I have to go on.

dhays said:

Eric, I don’t know anything about hull design. I am confused though because it appears that Selene claims a max speed of 11 knots for their 49’ Explorer. The LWL is 46’ 10”. The classic “hull speed” formula would come out to 9.17 knots. So how is that boat going to achieve 11 knots with 330hp unless it isn’t acting as a full displacement hull at that speed?

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Dave,
The boat I’ve been seeing pics of would easily achieve 11 knots w just some more power. It would require more power if it was a FD hull. SD and it would also not require much more power. Depending on the QBBL of the SD hull it may require less power for 11 knots .. definitely less power that it would if the BL angle was 0 as basically this boat has. That’s the purpose of a SD hull. That is to be driven easily above but still fairly close to hull speed.
With a FD hull you’d probably only need one engine (450hp) to cruise at 8 knots. That’s just a guess though.

Richard,
Hull speed is not efficient. Unless you’ve got a SD hull w an appropriate QBBL.
With a FD hull (I think that’s what you’re talk-in) I wouldn’t start talk-in efficiency until you got down to one knot below hull speed. If I wanted to save any significant amount of fuel over a given distance I’d drop 1.5 knots below my hs of 7 knots .. 5.5 knots. But 1/2 a knot below and up is not efficient. The stern starts falling at close to one knot below hs. Maybe even lower.
You wrote;
“So, as a guess the Selene may need only 100 hp to get to 9 knots, but 200 more to get another 2 knots.” Probably more than 300 to to get 11 knots. Maybe 400-450. I’m guessing of course.

Any hull can be over driven with enough power. The stern goes down , as the bow attempts to climb its own wave .
The easiest to watch is a working tug on its way home.

A 4 or 5+ ft wake can be created , just ask the folks at the NYC 79st marina about their wake adventures.

Those that travel in crowded waters see the effect from faster boats that think they have slowed enough for a good pass. They drag massive wakes as they fall to displacement speeds .
The power for the massive wake is the boats momentum.

Hard to measure but the most efficient speed for a displacement boat is where the surface area drag and the wave making drag match.

Go faster and you make a big bow wave , slower and the wave climbing is not the handicap , but the skin friction remains .

A speed/RPM log will easily indicate the operating speed where adding RPM does not add to the speed much at all.

ON a well set up boat the cruise RPM will be low noise and low vibration.

HaHa FF your last no-tech sentence says it all for most pleasure boaters. Sweet spot as everybody says. Vibration and noise in that order I’m guessing. And sweet spot has all to do w the skipper and crew. The engine could care less whether it’s shaking something awful or as smooth as it ever gets.

Many talk about the “most efficient” speed and don’t realize how slow that would be. I assume that would be mpg. What speed would you say had equal drag from wave making and surface area drag for a 36 foot trawler .. typical like a CHB or a GB. It’s probably just a spot on a scale that means little or nothing kinda like hull speed. Hull speed has mostly to do w waves .. not efficiency. And it means different things for different boats. For example hull speed on my little Willard is something that almost never needs to be considered. I’m not even positive hs can be achieved w a 30’ Willard w stock power. We talk about hs almost constantly but for FD boats it means little.
Hull speed for a SD boat has alot more meaning and SD boats tend to cruise at hs for hours and days at a time. And some SD boats w a hull more like a planing hull rarely go that slow.
SD and FD boats are really more different than most think most of the time.

While I agreed about the Selene issue..... I can't accept that SD and FD have a massive gap between them.

With tweaks n a hull shape, I see the transition more gradual.

Now why that's done? There are good and bad reasons to do it... But as it is said, every boat is a compromise.

psneeld said:

While I agreed about the Selene issue..... I can't accept that SD and FD have a massive gap between them.

With tweaks n a hull shape, I see the transition more gradual.

Now why that's done? There are good and bad reasons to do it... But as it is said, every boat is a compromise.

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Since I really don’t understand this stuff, I think I agree with you. With the use of overhangs where the waterline length increases with increased speed, a FD hull can achieve a better than 1.34 S/L ratio. Not a lot better, but a little. To do better than that, a hull needs to have some form that will generate lift above a certain speed and be light enough to allow it.

If the 49’ Selene does have a max speed of 10.9 knots, that would be a S/L ratio of 1.6 which is pretty clearly outside the FD range and into a SD design. Now, I’m not doubting that there is some hull design magic that allows Selene to have a very efficient hull. I’m just saying I doubt that really nice hull design is still a FD hull.

Not that it matters. They give a cruise speed of 8.5 knots which is a very nice efficient cruise speed for a 49’ boat.

The Selene is a odd hull and if I were designing it I would have given it a stern sloping upwards to the transom or almost.
It really has the basic hull form of a planing hull. Numerous large Hatteras flush deck boats or yachts (as they are often called) have the same stern w a straight run to the transom. A considerable amount of efficiency is lost from this rectangular shape but there is IMO a possible reason. Load carrying ability. The old Chris Craft sixty some feet “Motor Yachts” had the same chopped off stern. Or perhaps higher speeds. Whatever the reason it dosn’t change the fact that the boat (Selene) has the same SD stern for much the same reason. They are all slow boats and as a result mostly marketed as FD. Not correct but the people buying these boats on the average know little about hull design. So it’s better communication to call them, however wrong .. FD. Sounds big and heavy and slow ... and they are.

But you’re right the differences are very widespread and using your word .. “gradual”.

psneeld wrote;
“While I agreed about the Selene issue..... I can't accept that SD and FD have a massive gap between them.”

There are three classifications.
FD takes many forms. Barge, tug, ocean liner, canoe, some trawlers, most all sailboats, even an aircraft carrier .. compute it’s hs w it’s water line length.

There’s SD and then there’s SD. One is hard to tell from a FD and others are hard to tell from planing boats. Some go 20 knots and others operate best and are designed for hs or a bit above.

Planing.
Some have design features common to FD hulls like a Bartender’s very pointed stern. Some are a bit like SD with a little rocker on the bottom. May of these features and differences have nothing to do w speed but all to do w how a boat turns and other handling issues.

But there is a wide range of boats w many great differences.

Soo-Valley said:

Willy, can you explain why post 69 is FD. I read the brochure on it and it says FD, but any description of FD says rounded bottom.
Looking to learn something.

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Nomad Willy said:

Yes,
The water line rises in the stern to the end of the boat (transom). Go to the thread FD/Sd and see the pic of the rowboat w a FD stern. The rise in the stern is obvious and even profound. Agreed
This FD/SD chat should be on the new thread specifically for that to quit the off topic content.

Click to expand...

Nomad Willy said:

The Selene is a odd hull and if I were designing it I would have given it a stern sloping upwards to the transom or almost.
It really has the basic hull form of a planing hull. Numerous large Hatteras flush deck boats or yachts (as they are often called) have the same stern w a straight run to the transom. A considerable amount of efficiency is lost from this rectangular shape but there is IMO a possible reason. Load carrying ability. The old Chris Craft sixty some feet “Motor Yachts” had the same chopped off stern. Or perhaps higher speeds. Whatever the reason it doesn’t change the fact that the boat (Selene) has the same SD stern for much the same reason. They are all slow boats and as a result mostly marketed as FD. Not correct but the people buying these boats on the average know little about hull design. So it’s better communication to call them, however wrong .. FD. Sounds big and heavy and slow ... and they are.

But you’re right the differences are very widespread and using your word .. “gradual”.

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Eric,
My original question wondered why you said definitely FD. In this thread you refer to it as a typical SD or even a planning hull design.
I am still unsure what it is SD or planning, but since the rules for hull speed are ignored the Selene should not be referred to as a FD without an explanation how it can achieve greater than HS.

We once towed a broken down sailboat and almost sunk it by exceeding the 5 knot hull speed. I wood expect the same to occur trying to go faster than hull speed with a FD under its own power.
But I am here to learn how this or other FD boats may be an exception to the HS calculations
Steve

Soo-Valley said:

We once towed a broken down sailboat and almost sunk it by exceeding the 5 knot hull speed. I wood expect the same to occur trying to go faster than hull speed with a FD under its own power.
But I am here to learn how this or other FD boats may be an exception to the HS calculations

Click to expand...

Depending on the pitching moment imparted by prop thrust a FD hull might not stuff the bow quite as readily when over-powered as when being towed too fast. Depending on length vs beam and other factors it might be possible to push a little past theoretical hull speed, but likely not much. And at some point you'll reach the point where doubling your horsepower will only give you another couple tenths of a knot at best.

Not quite sure why some think when you exceed hull speed in a full displacement boat it magically falls over Niagara Falls, the engines magically explode, or need airliner turbine engines to get it to go another tenth of a knot.

There are variations on both full and semi displacement hulls where there's an overlap.... In other words there's a smoother transition than just this abrupt gap. The only trick is is who determines exactly what and where that magic point where the label SD vs FD comes into play.

psneeld said:

Not quite sure why some think when you exceed hull speed in a full displacement boat it magically falls over Niagara Falls, the engines magically explode, or magically need airliner turbine engines to get it to go another tenth of a knot.

There are variations on both full and semi displacement hulls where there's an overlap.... In other words there's a smoother transition than just this abrupt gap. The only trick is is who determines exactly what and where that magic point where the label SD vs FD comes into play.

Click to expand...

LOL, fixed it for you.

Steve (Soo-Valley) wrote;
“Eric,
My original question wondered why you said definitely FD. In this thread you refer to it as a typical SD or even a planning hull design.
I am still unsure what it is SD or planning, but since the rules for hull speed are ignored the Selene should not be referred to as a FD without an explanation how it can achieve greater than HS.”


Steve,
I think you are referring to the Selene and I was referring to the GH.
Re the GH look at his avatar and notice how his draft goes from full deep amidships to zero at the transom. The water travels along the bottom and is gracefully discharged (or one might say “released”) at the transom edge.
That’s why FD is so efficient. With a submerged transom the water running aft gets released at the transom/bottom junction (like the Selene) whereas it wants to leap up to the surface. If you look over the stern the water looks like water in a washing machine. Very turbulent like thrashing about. It takes considerable energy to make the water that agitated. Also the boat assumes a higher angle of attack requiring more additional power to drive the boat fwd. Lots of inefficiencies to require more fuel and energy.
But the water will only flow gracefully requiring very little energy at low speeds. Hence the limitations of FD boating.

psneeld wrote;
“There are variations on both full and semi displacement hulls where there's an overlap.... In other words there's a smoother transition than just this abrupt gap. The only trick is is who determines exactly what and where that magic point where the label SD vs FD comes into play.”

Yes there is an overlap the way we talk. But if you use the QBBL and establish an “either or” number that makes the question black and white. In that regard there is no “overlap”. But in a friendly conversation we can say “it looks like a lot of both to me” and have “overlap”. But most (say about 75%) are clearly this or that. Your Albin is SD and my Willard is FD. If there’s any argument in that it would be re your Albin. And about that there would be no argument either.

But there are other ways of identifying hull types.
I think the speed route has merit. One could say if a boat goes at a SLR (speed length ratio) of 1 or less it’s FD. And SD is from 1 to 2.25 2.25 and up is planing. Haven’t worked w SLR’s for some time so those numbers are most likely not good ones but they work as examples.
I don’t care for this method as there are too many variables. How do you establish the power in the boats, gear ratios, loads and what hulls are used as examples. A Sea Ray and a Nordhav’n engineer may come up w very different numbers.
I think there are other methods of establishing break off points for the hull types. I think the QBBL is the most useful but that is just an opinion.
If anyone else knows of other standards or rules of thumb please submit same.

For a long time I used the percentage of transom submerged at rest to separate FD from SD and planing. But that only identified FD. And over time I kept encountering boats that I considered unquestionably FD but they had some submerged transom. Seaton designs frequently do. But look at sailboats w transoms. Very very few are not out of the water at rest.

Nomad Willy said:

I think the speed route has merit. One could say if a boat goes at a SLR (speed length ratio) of 1 or less it’s FD. And SD is from 1 to 2.25 2.25 and up is planing. Haven’t worked w SLR’s for some time so those numbers are most likely not good ones but they work as examples.

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Most of the metrics for boat design simply go over my head. S/L ratio is the only one that I can wrap my head around. Max speed/sq rt of LWL = S/L. My guess is that you have enough power in your Willard, its S/L would be about 1.34.


My boat is definitely a SD hull. The NP 43 with the 330hp engine will have a top speed of about 12 knots (unless it is loaded down with a HUGE amount of stuff like my boat is). The 38.25 LWL gives this boat a S/L of ~1.94. Sea trials of a NP43 with a 425hp engine (and not loaded down with all kinds of crap like a typical cruising boat) was able to hit 14 knots. Bigger engine will give fast speed (unless you weigh it down too much). A FD hull will hit a point where a bigger engine really won't give you a faster speed.

So the bottom line is that a boat’s hull speed is not necessarily its actual maximum potential speed. Probably it is more accurate to say that hull speed represents a minimum maximum figure. It still provides a useful rough estimate of how fast you can reasonably expect a boat to go, particularly if you bear in mind that sailboats, particularly cruising sailboats, are rarely traveling at top speed in any event.

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This article explains it in an understandable way.
(BTW, I noticed he is talking about sailboats)

https://www.boats.com/reviews/crunching-numbers-hull-speed-boat-length/

Always good to hear your input Dave.

OK 14 knots w 425hp.
How fast w 550hp? 650?
There comes a point where you hafta admit she’s a planing hull. Or she goes so fast there are handling and seaworthyness problems. That’s what usually happens w an overdriven SD hull. But if you’ve got a straight run aft that shouldn’t happen. Porpoising and rolling at the same frequency or corkscrewing can result from an overdriven SD boat.
Does your NP have some rocker? If not she’ll probably behave like a planing hull. I think you’ve mentioned lots of weight before. If the aft run is straight it could be just a heavy planing hull underpowered. Nothing wrong w that but it’s hard to classify. But the definition is dependent on hullform .. shape. The speed capabilities are just an indication of the shape below. That’s why I’m so keen on seeing hull pics on “Interesting Boats”. Andso often ther’e missing.

SV,
Hull speed is a numerical thing but the numbers represent something physical. Physical properties of wave action. That’s all it is but boaters think it’s “magic” like psneeld says. But there’s no magic. It’s just physics and arithmetic.

Haven’t got time to read the article but will later. Thanks for posting it. I feel out on a limb trying to do this myself.

I am not saying that math is magical ......what I'm saying is is picking a number a decimal point above or below the result of some mathematical equation and an inventing a name to it is magical......

To me it's like picking temperatures 1° above and below the boiling point and calling them two different things instead of hot. True that they are above and below the boiling point, but they are both still hot.

Exactly and I agree.

What what is this? I’m saying SD without question.

This is the overlap that psneeld was talking about .. I think.
I would be more comfortable saying she is a planing hull.
Certainly capable of planing speeds. Can you envision her doing 40 knots? I can relate to that easily and very likely w/o an ungodly amount of power.
And her QBBL is almost 0.
Yup .. planing.

uh oh .....
Looked a little closer at the first picture. Especially the right side where she shows more girth and the displacement that represents. So at 50 knots and maybe less she’d poke her bow up pretty high. Still a very low QBBL but she’s really not a planing hull. Capable of 25 knots .. probably gracefully. But probably not much more. Call her a very fast SD hull.

This raises another possible route to definitions. If you measured the cross sectional area .. the area below the WL amidships and then very close to the transom one could have a very good assessment of the boats displacement amidship and how much less there is at the stern. With 0 to 100% possible this boat shows about 40. Just a guess. So we could have a QBBL of 1.5 to 2 degrees. And one could use the percentage of displacement loss much like the buttock line.

FD/SD is in the eye of the beholder or the company sales manager. Nobody follows the basic description. Some FD hulls have a hard chine and in fiberglass or metal, small flat plane areas running as far forward as midships that give the boat a SD effect. Also, the length to width ratio greatly effects the hull speed. Sharp boats like 1920s classics usually can beat their hull speed. I had a boat built in 1922 as a rum runner. Owned at the time by the mayor of Sausalito. Stories said the CG couldn't catch it, but finally ran it out of gas by stationing a series of CG cutters. The CG auctioned it, the mayor bought it back and put it back in service. The next time they caught it, it became a CG cutter. Years later when I had it, it still did 25 knots, with twin diesels. It was about 55' x 12' and a 3' draft. Round chine but almost flat bottom near the stern. I was told in its' hey day with gas engines could do 40+ knots.
Sausalito musta been a fun town in those days.