fd versus sd

[phillippeterson]

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..

 

[ScottC]

You are correct. There are many threads tossing this around. I did a quick search and this one came up:
https://www.trawlerforum.com/forums/s3/trawlers-cruise-bit-faster-62109-2.html

 

[phillippeterson]

You are correct. There are many threads tossing this around. I did a quick search and this one came up:
https://www.trawlerforum.com/forums/s3/trawlers-cruise-bit-faster-62109-2.html

Thank you. It will save others from having to spend their time on this.

 

[kapnd]

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

 

[twistedtree]

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

That's a good one.

 

[phillippeterson]

We are communicating with a dealer for full displacement boat and he somewhat dissed a well regarded semi-displacement brand. Although he noted that Mr. TF (sic) had taken his design across big water.

We're wondering just why a semi-displacement boat might be relegated to "coastal" status.

 

[grahamdouglass]

The main difference between the two is that the aft third of the ships bottom will be flat with a semi displacement hull and the full displacement will be fully rounded. Think Grand Banks (semi displacement) versus a Nordhavn (full displacement). This means that underway that when the boat "squats down" underpower, the semi displacement will ride on her flat bottom. This will provide more lift and raise the bow resulting in less drag. At least this is my understanding.

 

[Lepke]

We're wondering just why a semi-displacement boat might be relegated to "coastal" status.

Most SD hulls are usually high speed and don't carry the fuel needed for long ocean voyages. Non military SD hulls aren't really built to survive bad weather. Light structure and too many widows. Fishermen carry heavy plywood sheets slightly bigger than their front windows, I wonder why?

 

[phillippeterson]

Most SD hulls are usually high speed and don't carry the fuel needed for long ocean voyages. Non military SD hulls aren't really built to survive bad weather. Light structure and too many widows. Fishermen carry heavy plywood sheets slightly bigger than their front windows, I wonder why?

Doesn't a semi-displacement get good range if it goes 7-8 knots just like a full-displacement?

 

[rslifkin]

Doesn't a semi-displacement get good range if it goes 7-8 knots just like a full-displacement?

Yes, but performance of an SD design suffers more with increased weight than FD does. So most SD boats carry less fuel than a similarly sized FD and they're often not build as sturdy either. It's a case of once you build an SD boat to do what a really heavy duty FD boat can, you've lost most of the advantages of the SD boat anyway.

 

[Hippocampus]

Think there’s two totally separate concerns.
FD v SD
Voyaging boat v coastal near coastal.

There’s so much more that goes into making a boat suitable for blue water than displacement. There’s multihulls and long narrow Al boats that displace less than many coastal cruisers and definitely suitable for voyaging. There’s even ultralights with more favorable Gz curves than many tried and true FD blue water trawlers. No one feature defines the suitability of a vessel for a given purpose. A boat designed for range, ride, down flooding risk, redundancy, and self sufficiency in an open ocean setting is not determined by displacement. Been on FD boats I wouldn’t takeout of sight of land. BWBs whether sail or power are usually more expensive than near coastal or coastal because the design requirements require a greater safety margin and therefore the components are more expensive. Like race cars it’s even more expensive to eliminate weight in either a FD or SD hull while maintaining strength and durability. Which is “stronger”? Prepreg and CF or Fe. Former does quite well in the southern ocean. In either eliminating weight means more useful payload. Yes there’s a range of cost in both and those ranges over lap. But boats are like people. None do everything well.
We recently went SD after first wanting FD. Had gotten use to passage making and enjoyed it. Initial hunt was for N43 or a mark 2 N40 but ended up with a NT42. Issue is in FD regardless of how much you want to climb your bow wave there’s a fixed upper limit to speed which is low enough to be limiting until you go to a quite large lwl. Outside of passage making, where FD comes into its own, that impacts on use and planning.I know my current boat will go 17-18kts. I know except on rare occasion she’ll be run at 8-10kts. However I needn’t be concerned about timing East river/hellsgate perfectly. I have more flexibility in getting to landfalls in daylight. I choose to go at the upper end of the spectrum for coastal so know I’ll be comfortable and safe in fresh breeze. But I definitely gave up passage making. My design has gone Panama Canal to Florida but near coastal.
So appeal to folks to not only think about FD and SD but rather the totality of the whole design and it execution. Much of what is said and thought is generically true but false in the specific. To say a Coastalcraft , N35, many patrol boat derived designs and many other coastal boats are not strongly made doesn’t seem true to me.

 

[Lepke]

Doesn't a semi-displacement get good range if it goes 7-8 knots just like a full-displacement?

A SD hull gets better range than a FD hull. It's not pushing as much water out of the way. At reasonable speed they get great economy. My 83' SD hull does 10kts @ 10 gph and 5.5kts @ 4 gph. But I like 10. Rethinking with the new fuel prices.

A FD gets a better ride, but some roll badly.

 

[phillippeterson]

A SD hull gets better range than a FD hull. It's not pushing as much water out of the way. At reasonable speed they get great economy. My 83' SD hull does 10kts @ 10 gph and 5.5kts @ 4 gph. But I like 10. Rethinking with the new fuel prices.

A FD gets a better ride, but some roll badly.

This is exactly the kind of input we are hoping to get. Thang Kew.

 

[markpierce]

If you don't ever anticipate going more than six to eight knots, go full displacement.

 

[FWT]

The tough thing about SD conversations is how much each SD boat can differ. Some tend closer to planing designs, and others closer to FD designs. Its so hard to generalize.

I have a Helmsman 38 on order. The hull is solid glass from keel to rail and about an inch thick. That's not light. Some SD's are light. It is SD but leans closer to FD on the spectrum, while some SD's lean closer to planing.

The range at 6 knots is about 2100 nautical miles. About 1100 at 7 knots. About 750 at 8. At 10-12 its 250. So what's the range? You pick. Some have powered up with larger engines that will deliver speeds in the mid teens, with trim tabs. No clue what the fuel burn on that setup and speed might be, but I'd think its there for the sprint when needed more than for the extended cruise.

But yes, as others have said it too is a coastal cruiser. It isn't "hardened" for blue water passage making. The windows are not designed withstand something serious. It doesn't have the systems redundancies of a Nordhavn. And so forth.

A friend was talking about having spent some days as a guest aboard a Prestige 50. Planing design, with twin Volvo IPS drives. So I looked it up. Max speed 29, fast cruise at 24. 340 gal of fuel, and range at fast cruise of 250 miles. So it will go twice as fast as mine at fast cruise with the same range. But I'd rather be on mine, I think, at the 6-7 displacement speeds especially in stinky weather. Its just a different boat with very different strengths and weaknesses.

 

[rslifkin]

The tough thing about SD conversations is how much each SD boat can differ. Some tend closer to planing designs, and others closer to FD designs. Its so hard to generalize.

I have a Helmsman 38 on order. The hull is solid glass from keel to rail and about an inch thick. That's not light. Some SD's are light. It is SD but leans closer to FD on the spectrum, while some SD's lean closer to planing..

Out of curiosity, I looked up the weight of the H38. Looks like dry it's about 8000 lbs heavier than my boat as a point of comparison. My boat is 2 inches longer in LOD and an inch wider, so for practical purposes, they're the same size, but mine is a planing hull with more power (2x 340hp) and has a fast cruise of about 17 kts fully loaded (but with enough rudder and keel to handle fine at 7 kts). Weight difference loaded would be similar, as my boat holds slightly more fuel and waste, but a little less water. Being so close in size, I'd expect "stuff" weight to be about equal.

 

[FWT]

Out of curiosity, I looked up the weight of the H38. Looks like dry it's about 8000 lbs heavier than my boat as a point of comparison. My boat is 2 inches longer in LOD and an inch wider, so for practical purposes, they're the same size, but mine is a planing hull with more power (2x 340hp) and has a fast cruise of about 17 kts fully loaded (but with enough rudder and keel to handle fine at 7 kts). Weight difference loaded would be similar, as my boat holds slightly more fuel and waste, but a little less water. Being so close in size, I'd expect "stuff" weight to be about equal.

Fuel burn and range?

How does she handle at the slower speeds in weather?

 

[rslifkin]

Fuel burn and range?

How does she handle at the slower speeds in weather?

My boat has gassers, so it's about 5 gph at 6.5 - 7 kts, 30 gph at 17 kts. Fuel is 420 gallons total, I figure it as 300 plus reserve, so good for about 360nm at slow cruise (conservatively and using 300 gallons before refueling). About 160 - 170nm at 17 kts. Modern diesels would burn about 60 percent less at slow cruise, 30-40 percent less on plane in the same hull. Technically the planing shape is less efficient at low speed due to dragging a flat transom through the water (many SD designs suffer from this too) but the lighter weight offsets a lot of that compared to FD hulls.

Unlike most fast planing hulls, I've got decent size rudders (many SD boats with twins are similar) and it's got a keel, so it tracks well at low speeds and following seas are manageable at slow cruise (but speeding up does improve the ride). It rolls more at slow cruise, but an unballasted SD hull with hard chines will be similar. Draft on my boat is only slightly less than the H38 (measures 3'9" loaded in fresh water to the props, keel is at about 3'4" so the props are a bit exposed).

The biggest ride quality issue especially as low speed is the very full bow shape. It carries weight well and is good in following seas, but it hurts the ride in a head area. On plane, it leads to lots of spray and more pitching than some hulls have. At slow cruise into a head sea the pitching can be significant. It pretty much goes up and over no matter what rather than going through a wave. Long period stuff is better, as is extremely short, close chop where you span multiple waves. It's the stuff that's short and steep, but not short enough that can get unpleasant.

 

[FWT]

rslifkin:

That's a really interesting and good analysis. Thanks for that.

I don't think anyone has much fun with the stuff that's short and steep.

When I was in the shopping / decision mode I combed the net for clips of everything in snotty weather. Some just looked like they would beat you to death. That doesn't sound like your experience. The only clip of the H38 I could find in rough weather was a promo clip discussing hull design. If you go to the 2:12 mark you can see that. And its a good discussion of SD designs too.

 

[rslifkin]

Looking at that video, the Helmsman seems to pitch a bit more than I expected. But it doesn't appear to be as fine up forward as some SD designs. I'd say at low speeds my boat may pitch a hair worse, but it's pretty dry. I've got enough hull flare that getting the decks wet at slow cruise is rare. On plane, all bets are off. It pushes enough water out of the way that even with the flare it's not hard to get water 20 feet in the air in steep 3-4 footers.

For comparison, here are a couple photos of my hull shape. It's a bit more rounded than most planing hulls, but the big thing is that it's a slow planing hull. No lifting strakes as the more rounded shape and big trim tabs provide enough lift. Between that and the keel, it handles well up to the high teens, but if you open it up, 20+ kts isn't great. Full rudder at 20+ is a bad idea. Plus the ride isn't particularly good or planted feeling at those speeds unless it's dead flat. So more power would be pretty pointless. Some would almost call it SD because of that, but unlike an SD hull it doesn't start to hit a drag wall, it'll go faster, you just don't want to.

 

[FWT]

Well for starters, I really like the look of your boat. I've always been pulled to a well drawn line along the rail.

Seems to me, the slow speed roll you describe is in the amount of deadrise aft. Mine has virtually none. I am left wondering why it begins to misbehave at higher speeds? Not enough V / deadrise to handle well above the teens? Just guessing.

 

[phillippeterson]

If you don't ever anticipate going more than six to eight knots, go full displacement.

We anticipate going seven knots - mostly.

We also anticipate being fifteen miles from a marina that closes in an hour. And will probably wind up fifteen miles out from an anchorage with sunset in an hour. And will likely to be fifteen miles from a necessary crossing with tidal slack in an hour. Not to mention missing happy hour by ten minutes - heaven forbid.

We're leaning towards a SD that we'll cruise mostly at seven knots.

 

[FWT]

Helmsman bought the design for the Camano, which is now the H31. It has an unusual feature in the hull design that gives it a smooth speed / fuel curve up to about 15 knots. No bow wave to climb over. But Scott basically won't power it higher than it takes to get to that point, or at the least will argue convincingly against it. Apparently above about 15 knots it begins to act squirrely. It sounds like what you describe. It sort of sounds like too much lift amidships above 15.

Here's an old Camano clip talking about the design.

 

[rslifkin]

Well for starters, I really like the look of your boat. I've always been pulled to a well drawn line along the rail.

Seems to me, the slow speed roll you describe is in the amount of deadrise aft. Mine has virtually none. I am left wondering why it begins to misbehave at higher speeds? Not enough V / deadrise to handle well above the teens? Just guessing.

Less deadrise plus a bit more keel will reduce roll. We've only got 10 degrees of deadrise aft, but the rounded forward sections and narrow chine flats that twist out pretty early (they're gone a little aft of midship) don't help. We also have tanks, etc. close to the centerline, so there's not much weight outboard to resist roll. In general, I wouldn't say it's excessively rolly. Something like an H38 will roll a bit less, but both will have a point where you'll want stabilizers.

For the high speed behavior, I think the hull starts to lift too high. And then in a turn at higher speeds, it starts to plane on the side of the keel and cause issues. At 17 kts, full rudder produces a tight, banked turn. But if you try it on a WOT test at 25 kts, it'll pull the outboard chine out of the water, jump and slide sideways a bit, then catch (not good).

 

[FWT]

"For the high speed behavior, I think the hull starts to lift too high. And then in a turn at higher speeds, it starts to plane on the side of the keel and cause issues. At 17 kts, full rudder produces a tight, banked turn. But if you try it on a WOT test at 25 kts, it'll pull the outboard chine out of the water, jump and slide sideways a bit, then catch (not good)."

Ah ha. I get it.

For what its worth, I won't be getting stabilizers. I want to keep things simple and shopped for a hull that didn't "need" it to behave reasonably well. I think I have that, but of course time will tell.

Good thing too. Fin stabilizers seem not to fit well on the boat. I think its "possible" to go with the gyros but I'm guessing that should be placed where the generator is located, and if its one or the other I'll take a generator. At least, that's my guess since I didn't pursue it with Scott.

 

[Art]

With my early understandings of boat characteristics running alongside my young start of piloting and working on boats that began in the latter 1950's [I've been on boats starting 1952] I want to say how I feel regarding boat hull designs' capabilities and reactions to sea conditions. This is not to get too detailed, but rather an overview of how I see things.

Displacement Hull:
- Best fuel efficiency at or just below hull speed. No faster speeds available; basically no matter how much power.
- Good handling in most sea conditions... stabilizers recommended.
- Heavy roll at anchor in broadside wakes and from waves during some wind conditions.
- Usually single screw with good prop and rudder protection by deep keel with full skeg. Occasionally twin screws with twin keels and skegs. Rarely single screw or twin screws without keel and skeg protections for prop and rudder. Much prop and rudder safety/protection when mistakenly hitting bottom with boat moving underpower or otherwise.
- Due to fuel efficiency... most often chosen for long distance sea crossings.

Semi Displacement [Semi Planing] Hull:
- Pretty good fuel efficiency at or just below hull speed. Somewhat faster speeds are available with enough power; considerably increased fuel usage.
- Good handling in most sea conditions... stabilizers owner's choice.
- Medium roll at anchor in broadside wakes and from waves during some wind conditions.
- 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.
- Often chosen for river, lake other in shore and for coastal cruising.

Full Planing Hull:
- Pretty good fuel efficiency at or just below hull speed. Many levels of faster speeds are available with enough power; considerably increased fuel usage... depending on just how fast you want to go!
- Good handling in most sea conditions... IMO, stabilizers not needed.
- Least roll at anchor in broadside wakes and from waves during some wind conditions.
- VERY 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.
- Often chosen for river, lake other in shore and for coastal cruising.

My boat use preference and hull design choice:
- I do not want to cross oceans nor brave big ocean storms during cruising.
- I do like off shore, in shore, river and lake cruising.
- Full displacement hull is nice... but for reasons... not my choice to own and use due to some of its drawbacks.
- Have had semi displacement hulls and appreciated their design as well as their cruising usability conditions.
- Planing hull design is my choice due to its many capabilities and widest span of usability conditions... that is... in regard to my "boat use preference".

So, it seems to me; before even looking for a boat:

Most important first consideration for any boat purchaser to first decide upon is their "Boat Use Preference". Then the many other important portions of boat design can be well contemplated upon [written down] to develop the actual premise of type boat that will satisfy your boating desires and needs.

Happy Boat-Search Daze!

 

[Fish53]

A SD hull gets better range than a FD hull. It's not pushing as much water out of the way. At reasonable speed they get great economy. My 83' SD hull does 10kts @ 10 gph and 5.5kts @ 4 gph. But I like 10. Rethinking with the new fuel prices.

A FD gets a better ride, but some roll badly.

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.

 

[North Baltic sea]

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

 

[Fish53]

Hi,


what about Fleming who is SD 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.

NBs

It's not that it can't be done but as you showed it's a notable thing rather than an accepted norm as it is with a FD hull. My FD boat which is quite small would easily carry the fuel and provisions for an Atlantic crossing. As it is now I can go from Maine to Florida without refueling, it's much more my age than my boats capability that prevents my crossing the pond.

 

[ranger58sb]

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..

We are communicating with a dealer for full displacement boat and he somewhat dissed a well regarded semi-displacement brand. Although he noted that Mr. TF (sic) had taken his design across big water.

We're wondering just why a semi-displacement boat might be relegated to "coastal" status.

Doesn't a semi-displacement get good range if it goes 7-8 knots just like a full-displacement?

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

-Chris