fd versus sd

[rslifkin]

You got that all backwards, SD hulls are almost never more efficient than a FD hull. SD requires more hp, more wetted surface, less efficient hull shape at FD speeds and in consequence need to carry more fuel. Sure they're faster but at what cost? More expensive machinery, more fuel and a generally less seaworthy boat.

SD is definitely a less efficient shape for operating at low speeds. But I'm not sure why they'd have more wetted surface, as they're typically a flatter, lower deadrise, lower draft hull and typically lighter weight too (both of which should reduce wetted surface). In a practical sense, the reduced weight, etc. often makes up a significant part of the shape disadvantage.

So FD will still be a bit more efficient, but in many cases, not as much as one might expect. The heavy FD will have an easier time carrying a huge, heavy fuel load for a long trip, however.


As far as seaworthiness for SD hulls, it varies a lot. Some are good, capable designs. Other "SD" boats are just a mediocre planing hull without enough power to actually plane. Those are the ones that tend to be less capable, as they don't have a great design for low speed work and don't have enough power to run at a speed where the design works well. Of course, hulls that rely on speed/power to handle some conditions are range limited by nature, as you can no longer count on going slow 100% of the time to save fuel.

 

[Fish53]

SD is definitely a less efficient shape for operating at low speeds. But I'm not sure why they'd have more wetted surface, as they're typically a flatter, lower deadrise, lower draft hull and typically lighter weight too (both of which should reduce wetted surface). In a practical sense, the reduced weight, etc. often makes up a significant part of the shape disadvantage.

So FD will still be a bit more efficient, but in many cases, not as much as one might expect. The heavy FD will have an easier time carrying a huge, heavy fuel load for a long trip, however.


As far as seaworthiness for SD hulls, it varies a lot. Some are good, capable designs. Other "SD" boats are just a mediocre planing hull without enough power to actually plane. Those are the ones that tend to be less capable, as they don't have a great design for low speed work and don't have enough power to run at a speed where the design works well. Of course, hulls that rely on speed/power to handle some conditions are range limited by nature, as you can no longer count on going slow 100% of the time to save fuel.

As with anything this complex there's a wide range of factors from which to chose. When I think of SD boats I usually think of our local lobster boats, all with extremely rare exception are SD hulls having skegs and a long keel. My FD boat is 26' and has less wetted surface than a typical 26' lobster boat with a built down keel. I once owned a 26' Sisu lobster boat of just that configuration and with the same beam it had about 15% more wetted surface. Certainly there are many variations and this may not be true in all comparisons. As to weight two boats of the same length and beam built with similar scantlings would have somewhat similar hull weight. The bigger variation would be in superstructure, machinery weight and tankage. As an SD hull requires significantly more hp to drive it to it's designed speed a larger and heavier engine would be required with the attendant fuel capacity necessary.
Seaworthiness is another complex subject but having spent the majority of my working life offshore I've observed very few SD boats still out when the weather is less cooperative. That's not saying that given plenty of money and the single stated goal of building an offshore capable SD boat wouldn't be fairly easy but that's not very realistic in this forum. The facts are that many trawler type pleasure boats have many more design issues that limit seaworthiness than simply hull type.

 

[rslifkin]

As with anything this complex there's a wide range of factors from which to chose. When I think of SD boats I usually think of our local lobster boats, all with extremely rare exception are SD hulls having skegs and a long keel. My FD boat is 26' and has less wetted surface than a typical 26' lobster boat with a built down keel. I once owned a 26' Sisu lobster boat of just that configuration and with the same beam it had about 15% more wetted surface. Certainly there are many variations and this may not be true in all comparisons. As to weight two boats of the same length and beam built with similar scantlings would have somewhat similar hull weight. The bigger variation would be in superstructure, machinery weight and tankage. As an SD hull requires significantly more hp to drive it to it's designed speed a larger and heavier engine would be required with the attendant fuel capacity necessary.
Seaworthiness is another complex subject but having spent the majority of my working life offshore I've observed very few SD boats still out when the weather is less cooperative. That's not saying that given plenty of money and the single stated goal of building an offshore capable SD boat wouldn't be fairly easy but that's not very realistic in this forum. The facts are that many trawler type pleasure boats have many more design issues that limit seaworthiness than simply hull type.

It sounds like you probably have a lighter FD hull (relative to size) than many and a design that was aimed at minimizing wetted surface. That should make for a particularly efficient boat. Many FD boats pile on so much weight in the build that they lose a good chunk of the potential efficiency advantage and just get the nicer motion in heavy weather and ability to carry more weight. And as you pointed out, the SD boat either needs to be limited to below hull speed on a long crossing (with the resulting limitations in hull performance) or it has to carry even more fuel...

As far as the weather, yeah, many SD boats aren't designed to be as sturdy as many FD designs in heavy weather. But of course, the owners of the boats are a big factor. Outside of the big sport fishes, many SD and planing boats are owned by people who have no reason or desire to be out in heavy weather, so they just don't (even if the boat can handle it).

There have been a couple of times locally (on Lake Ontario) where we determined that the weather was acceptable for a run to our intended destination based on direction and how long we'd be out there getting beaten up, so we went. And once we were out there, there was not another powerboat to be seen, just a couple of the more hardy sailors looking at us like "what the heck are you doing out here!?". But we didn't find the conditions concerning, just not necessarily comfortable or reasonable to go in certain directions and sometimes not conditions I'd want to run in for hours, but no problem for a shorter hop.

Or the time the weather turned much uglier than expected and we got whacked with a nasty thunderstorm about 20 minutes out from our destination. Didn't see another boat out there, made it in and everyone (including sailors with sturdier boats) were shocked that we ran through that storm without issue. Mind you, the ride through that thunderstorm absolutely sucked and we weren't comfortable at all, but we were still making 15 kts and the boat was doing just fine and not struggling. I've never been so happy to turn downwind though, the last 2 minutes heading into the channel straight downwind was a huge change from wet and miserable to a perfectly comfy, smooth ride even though it looked and sounded like chaos around us.

 

[phillippeterson]

What kind of boating do you intend? Ocean crossings? Coastal cruising and island hopping? Other?

-Chris

Maine to Belize to USVI through the Panama then Alaska. Also want to throw in a revisit to Kapingamarangi.

 

[ranger58sb]

Maine to Belize to USVI through the Panama then Alaska. Also want to throw in a revisit to Kapingamarangi.

So mostly coastal, sorta-kinda. Excepting that Micronesia thing.

If the latter isn't the main driver, or if an airplane ride will do... could be either SD or FD could meet your requirements... and it might be more about other features of the boat that could drive your eventual selection.

??

-Chris

 

[RCook]

Semi Displacement [Semi Planing] Hull:

- Unlikely to have prop and rudder protection by deep keel with full skeg for single screw or twin screws. Nakedly exposed props and rudders require boat's pilot to NOT hit bottom... or many thousands of dollar damage may happen... not to mention circumstances that could sink the boat.

??? Here's my NT, capable of 16 knots WOT, single screw, well protected.

 

[Hippocampus]

RC you’re absolutely right. After buying our Nordic Tug 42 we took her from Connecticut to Rhode Island but then from there to Virginia. During that trip there was a lot of junk in the water. Especially in the C&D canal. We transited the canal in the middle of the night with not much moon and a lot of overcast. Not 5 minutes went by without a clunk. Apparently when holding pools are emptied the canal collects junk. Boat was pulled shortly after arrival to be put in a shed for refitting. Absolutely no damage to anything.
BTW the trip was done without any stops. So we arrived (again at a moonless night) quite tired. Never been in Deltaville before. Wandered around trying to figure out where to tie up and ran aground. Fortunately soft mud. So with a large rudder and 540hp got off in a few seconds. Again. No damage.
So FD/SD doesn’t tell the whole story. A boat is a totality of its many design features. Change one thing it impacts on everything else. The genius of a great N.A. is in achieving a balanced design optimized for the expected use profile.

Sea trial showed 17-18kts at WOT in 2-3’. Nice.

 

[Art]

??? Here's my NT, capable of 16 knots WOT, single screw, well protected.

RC - I said unlikely, not impossible.

For years my dad had a simply beautiful 38' SD, single screw with full keel and full skeg. She also [such as yours] had a top end of 16 +/- knots. Was simply a greatly designed raised deck woodie "one off" boat whose keel was laid in 1950. Built by and originally owned by a master shipwright at Brooklyn Navy Yard... I hope she's been kept alive. Wish I had photos of her. Did a lot of work on her... year after year. Pop sold her in early 70's...

 

[bowball]

Hi,


what about Fleming and SD hull and they have crossed the Atlantic. Elling from Holland SD also crossed the Atlantic. There is still a planing boat No limits in Holland, which they drove on their own keel to the US boat show.

https://youtu.be/7UenHrUmt20

This video is No limits trip Holland -US

NBs

Here is the hull on a Fleming.

 

[phillippeterson]

So mostly coastal, sorta-kinda. Excepting that Micronesia thing.

If the latter isn't the main driver, or if an airplane ride will do... could be either SD or FD could meet your requirements... and it might be more about other features of the boat that could drive your eventual selection.

??

-Chris

No airplanes to Kapinga. Only reachable by boat.

 

[kthoennes]

Here’s a graphic example:
The first four are full displacement, the fifth is planing, and the sixth is semi displacement.

One of the funniest posts I've ever seen on TF. And true! That one had me laughing all afternoon.

 

[mvweebles]

Full diplacement is a lifestyle choice. Our boat carries a lot of stores despite it's small size. And we happen to like life at jogging speed. Our plans take it into account be it bridge openings, change of tides/currents, or weather systems. It's just how we roll. It's a classic cruising pattern that suits us well. If you want/need an escape hatch of speed, then full displacement isn't your best choice.

Peter

 

[kapnd]

“It’s just how we roll”
Me too, even the roll is good!
People that don’t know how to roll with it should roll in golf carts!

 

[Jmarsh203]

I currently own a full displacement boat and semi displacement boat and will be selling the full displacement boat. I’d agree for the most part with what everyone has said so far. Although my semi displacement boat is the much more capable boat in every respect than my full displacement boat, outliers to the general rule of full displacement boats are needed for serious offshore cruising or passagmaking do exist. While one of the sister ships of my Willard 36 (full displacement) pilot hours has crossed the pacific my semi displacement boat has 1/2 inch windows all the way around, 1/4 inch thick aluminum hull with a 2” thick by 3”wide aluminum keel that terminates 3/4 of the way down the keel of my boat and is stabilized with paravanes, also carries more fuel than any boat I’ve ever heard of for a 42 footer at 1200 gallons with a range of over 4000 nm at 7kts and is twin engine, the boat can also do about 12kts with current power plants and I’ll prob upgrade sometime in the future to be able to get around 15-16kts. I plan on doing some serious passagmaking after a remodel of the boat interior and systems and very much considering crossing oceans with her. The downside is it’s a 42 foot boat and it has the interior space of my 36 foot boat lol not surprising they didn’t sell very well when it has such a small interior and cost over twice as much as a Grand Banks 42 when she was new in 1972.

 

[ranger58sb]

No airplanes to Kapinga. Only reachable by boat.

Charter from nearest mainland?

Ship your own boat to somewhere nearby?



-Chris

 

[long-cours.62]

Some border

put between FD and SD are so caricatural ...


For example :
-superstructure of Sd aren't strong enough,
-even at low speed SD consumption will be higher than FD
-SD can't carry enough diesel to do blue water cruising...
-etc


Ok for you this hull line are what FD or SD ?
Probably you will say SD ( if what I read on this thread will apply : flat, hard chine, no big draft, etc )


It mean :

- higher consumption even at "low speed" , really ?

@6.6 kts 4 liter per hour
@7.4 kts 6.7 liter per hour
@8.3 kts 10.8 liter per hour
@9.8 kts 17.4 liter per hour


- SD can't carry enough diesel to do "blue water" (suppose we could use all the original tankage 8.5m3)

@6.6kts 14025 nm
@7.4 kts 9388 nm
@8.3 kts 6532 nm
@9.8 kts 4787 nm


It means we can't make so strict "limit" between two type of hull, you could have hard chine,"flat" bottom" and have enough tankage and very small consumption. In another hand you could have full round hull, deep keel and not enough tankage to do ocean crossing and even higher consumption.


Because in my point of view the main "problem " is : weight, LWL, beam.
Only the problem is to notto be too narrow and too light to keep enough comfort at sea.


Even a former "patrol boat" could have smaller consumption and more autonomy than a, for example, Sélène trawler ( source sea trial Yacht by Neptune):


Sélène 72' 72 t 11.3 cubic meter diesel

@9.5 kts 53 liter per hour range (theory) 2025 nm

@12 kts 125 liter per hour range (theory) 1084 nm


Patrol boat 95' 72t 12 cm diesel ( source sea trial made by French administration/builder/engine maker )

@9 kts 36 liter per hour range (theory) 3000 nm
@10kts 48 liter per hour range (theory) 2505 nm
@12 kts 84 liter per hour range (theory) 1718 nm


The consumption of the patrol boat could be optimized because this consumption was made with two big engine, old generation 32 liter each...

 

[phillippeterson]

put between FD and SD are so caricatural ...


For example :
-superstructure of Sd aren't strong enough,
-even at low speed SD consumption will be higher than FD
-SD can't carry enough diesel to do blue water cruising...
-etc


Ok for you this hull line are what FD or SD ?
Probably you will say SD ( if what I read on this thread will apply : flat, hard chine, no big draft, etc )


It mean :

- higher consumption even at "low speed" , really ?

@6.6 kts 4 liter per hour
@7.4 kts 6.7 liter per hour
@8.3 kts 10.8 liter per hour
@9.8 kts 17.4 liter per hour


- SD can't carry enough diesel to do "blue water" (suppose we could use all the original tankage 8.5m3)

@6.6kts 14025 nm
@7.4 kts 9388 nm
@8.3 kts 6532 nm
@9.8 kts 4787 nm


It means we can't make so strict "limit" between two type of hull, you could have hard chine,"flat" bottom" and have enough tankage and very small consumption. In another hand you could have full round hull, deep keel and not enough tankage to do ocean crossing and even higher consumption.


Because in my point of view the main "problem " is : weight, LWL, beam.
Only the problem is to notto be too narrow and too light to keep enough comfort at sea.


Even a former "patrol boat" could have smaller consumption and more autonomy than a, for example, Sélène trawler ( source sea trial Yacht by Neptune):


Sélène 72' 72 t 11.3 cubic meter diesel

@9.5 kts 53 liter per hour range (theory) 2025 nm

@12 kts 125 liter per hour range (theory) 1084 nm


Patrol boat 95' 72t 12 cm diesel ( source sea trial made by French administration/builder/engine maker )

@9 kts 36 liter per hour range (theory) 3000 nm
@10kts 48 liter per hour range (theory) 2505 nm
@12 kts 84 liter per hour range (theory) 1718 nm


The consumption of the patrol boat could be optimized because this consumption was made with two big engine, old generation 32 liter each...

Thank you for this post. This thread gets better and better and more informative. All of the contributions are very helpful.

 

[mvweebles]

Let's not make exceptions the rule. Of course there are semi-displacement vessels that are incredibly seaworthy and can carry enough fuel to cross oceans. There are incredibly seaworthy planing and semi-displacement vessels in use for applications such as crew-boats taking workers to oil rigs.

Just because such anomolies exist doesn't make them best suited to our usage. For understanding, look to Prismatic Coefficient (PE) is a design parameter used by naval architects to match the hull shape to the anticipated use-case of the vessel.

HERE IS Kasten's white-paper on topic of hull design parameters - see Page 3 for PE

The prismatic coefficient is the ratio of actual underbody volume to the volume of a prism having a length equal to the DWL, and a section equal to the boat's maximum sectional area.

The prismatic coefficient provides an indication of the distribution of displacement. It is an indication of the fineness of the ends relative to the midsection of the hull. A low prismatic means there are fine ends and large mid‐body. A high prismatic means there is more displacement distributed toward the ends.

Since the fullness or fineness of the ends has a large effect on wave making resistance, for any given speed to length ratio there is an ideal prismatic coefficient. As speed increases and the bow and stern waves rise, additional buoyancy in the ends becomes more favorable. Since racing yachts are more often driven at higher speeds, they benefit from a higher prismatic.

Optimum prismatic coefficient depends on the expected speed range.

• Up to SLR of 1 the least resistance is with a prismatic of .53.
• At SLR of 1.2 the ideal prismatic is .58.
• At SLR of 1.35 the ideal prismatic is .62.
• At SLR of 1.8 the ideal prismatic is .70.

 

[Art]

Wow - M/V

prism /ˈprizəm / noun

GEOMETRY

a solid geometric figure whose two end faces are similar, equal, and parallel rectilinear figures, and whose sides are parallelograms.

 

[long-cours.62]

The coefficient

[QUOTE

The prismatic coefficient is the ratio of actual underbody volume to the volume of a prism having a length equal to the DWL, and a section equal to the boat's maximum sectional area.

The prismatic coefficient provides an indication of the distribution of displacement. It is an indication of the fineness of the ends relative to the midsection of the hull. A low prismatic means there are fine ends and large mid‐body. A high prismatic means there is more displacement distributed toward the ends.

Since the fullness or fineness of the ends has a large effect on wave making resistance, for any given speed to length ratio there is an ideal prismatic coefficient. As speed increases and the bow and stern waves rise, additional buoyancy in the ends becomes more favorable. Since racing yachts are more often driven at higher speeds, they benefit from a higher prismatic.

Optimum prismatic coefficient depends on the expected speed range.

• Up to SLR of 1 the least resistance is with a prismatic of .53.
• At SLR of 1.2 the ideal prismatic is .58.
• At SLR of 1.35 the ideal prismatic is .62.
• At SLR of 1.8 the ideal prismatic is .70.

[/QUOTE]
prismatic of the hull line I put above is 0.609...

 

[mvweebles]

[QUOTE

The prismatic coefficient is the ratio of actual underbody volume to the volume of a prism having a length equal to the DWL, and a section equal to the boat's maximum sectional area.

The prismatic coefficient provides an indication of the distribution of displacement. It is an indication of the fineness of the ends relative to the midsection of the hull. A low prismatic means there are fine ends and large mid‐body. A high prismatic means there is more displacement distributed toward the ends.

Since the fullness or fineness of the ends has a large effect on wave making resistance, for any given speed to length ratio there is an ideal prismatic coefficient. As speed increases and the bow and stern waves rise, additional buoyancy in the ends becomes more favorable. Since racing yachts are more often driven at higher speeds, they benefit from a higher prismatic.

Optimum prismatic coefficient depends on the expected speed range.

• Up to SLR of 1 the least resistance is with a prismatic of .53.
• At SLR of 1.2 the ideal prismatic is .58.
• At SLR of 1.35 the ideal prismatic is .62.
• At SLR of 1.8 the ideal prismatic is .70.

prismatic of the hull line I put above is 0.609...[/QUOTE]The transom on that wire frame drawing is pretty small. Beam is not carried aft. Would take a lot of power to get that hull on plane.

 

[Hippocampus]

I’m a big believer in a balanced boat as discussed on Attainable Adventures in contrast to the slice of pizza shaped boats which took over as de rigor for RTW and open ocean racing sailboats. The prior thinking of light ends with optimal gyradius, low frontal plane and optional prismatic coefficients was entirely turned on its ends as these designs had much less parasitic drag and demonstrated in use to be equally seaworthy. With the utilization of bow bulbs, wave piercing bows and revisiting of Herreshoffs thinking by Dashew and others the shape of recreational power was given a different paradigm then that reviewed by michael kasten above. Kasten is brilliant and realizes there are different ways to skin the cat so is aware and has designed to this paradigm as well. Beyond length/beam ratios these boats also have low rocker both fore and aft. Are of low displacement. Are able to efficiently operate above hull speed. As said by me and several others repetitively throughout this thread FD/SD doesn’t tell the whole story. Are the following FD or SD?
Ironheart
Functional power boats
Tenant designs
Artnautica
Arksen
LM
Deep Water
The list goes on.
Both the military and oil industry have been designing to this understanding of hydrodynamics for many decades with boats not fitting the prior paradigm whereby the equation of FD=blue water and SD=coastal. I agree with Peter prismatic coefficient is very useful to think about when judging a hull. But it’s independent of displacement as well as FD/SD.

 

[long-cours.62]

prismatic of the hull line I put above is 0.609...

The transom on that wire frame drawing is pretty small. Beam is not carried aft. Would take a lot of power to get that hull on plane.[/QUOTE]


I never wrote it was a SD , I wrote : "

Probably you will say SD ( if what I read on this thread will apply : flat, hard chine, no big draft, etc ).
This line was the lines of our former boat and I know she was not able to plane


I use that only to illustrate the "border" between SD and FD, the efficiency SD FD, the range SD FD is not so easy to definite because we can't only say :
" flat, hard chine, no big draft," SD
" full deep molded hull, heavy displacement, deep draft " FD


You have some boat who stay somewhere ON the "border"

 

[angus99]

To the point of there being many shades of SD, here are the contours of a DeFever 44 hull. This boat is sometimes described as FD . . . incorrectly, I believe, because it clearly has elements of an SD hull form, but perhaps more subtle than some SDs. It has some relative “flattening” aft and a soft chine along the aft quarter of its length. But it’s also deep draft, very rounded for most of it’s length, prone to roll without stabilizers and very fuel-efficient below hull speeds. It also wouldn’t plane if it were strapped to a rocket.

 

[rslifkin]

To the point of there being many shades of SD, here are the contours of a DeFever 44 hull. This boat is sometimes described as FD . . . incorrectly, I believe, because it clearly has elements of an SD hull form, but perhaps more subtle than some SDs. It has some relative “flattening” aft and a soft chine along the aft quarter of its length. But it’s also deep draft, very rounded for most of it’s length, prone to roll without stabilizers and very fuel-efficient below hull speeds. It also wouldn’t plane if it were strapped to a rocket.

The big question would be whether it would exceed its hull speed by any measurable amount given enough power. If it does, then I'd call it SD. If not, it's either FD or an underpowered SD. In my mind, SD boats don't necessarily plane, although some can sorta-kinda plane.

 

[angus99]

The big question would be whether it would exceed its hull speed by any measurable amount given enough power. If it does, then I'd call it SD. If not, it's either FD or an underpowered SD. In my mind, SD boats don't necessarily plane, although some can sorta-kinda plane.

By that measure, then, it may be closer to SD. Top speed is perhaps 9.5 to 10 Kts; hull speed is calculated to be 8.5.

 

[NEtrawler58]

With the cost of fuel this season, I suspect even SD boats will mostly become FD boats in terms of cruising speeds.

 

[AnnapolisJoy]

The big differences are:

Hi, We want to learn about full displacement versus semi displacement. This has surely been discussed but I can't find any threads. I don't want to annoy anybody with a rehash, so if you have a link about previous threads please post it.

I know enough about the hull shapes and speeds but want to learn more about pros and cons, re: how is the ride, how is the roll at anchor, what is maneuverability, etc..

Cruising speed: FD are faster in my experience.

Slapping of water on the hull when anchored or moored: some people love it, I personally hate it as the noise drives me nuts.

Spend an overnight on a FD at a typical anchorage. The smallest wake sends noise through the berth.

 

[Fish53]

The one sure measurement to determine an SD hull from a FD hull is the buttock angle. That said the boat in my avatar has a flat bottom as do container ships and VLCC's the idea of them planning or reaching SD speeds is absurd.

 

[long-cours.62]

With the cost of fuel this season, I suspect even SD boats will mostly become FD boats in terms of cruising speeds.