Hard vs Round Chine

[RedRascal]

Trying to get some hull characteristics straight in my head between a hard and round chine semi-displacement boats. Is my thinking on track with general boating conditions where winds are less than 20kts and seas are less than 5 feet and anchoring in less than a 1 foot chop.

I believe a hard chine boat will generally be less rolly at anchor but will get pushed around more in an aft quartering sea.

I believe a rounded chine boat will more rolly at anchor but will get pushed around less in an aft quartering sea.

Going into a head sea the rounded chine hull may pound less.

Trying to figure out if there is a big advantage of a hull like below vs a hard chine Taiwanese trawler.

 

[psneeld]

I think the amount of curve makes a world of difference.... a rounded off hard chine is probably barely noticeably different than a hard chine when it comes to roll... when you get into wineglass shapes is where the world of difference comes to play.

 

[rslifkin]

In the conditions where hard chines tend to roll less they'll tend to have a harsher return at the roll reversal, so it's not necessarily more comfortable.

Getting pushed around in a following sea has a lot to do with how much beam is carried aft at the waterline and how fine the bow is. The finer the bow and the broader the stern, the more the stern gets picked up, bow digs in, and steering troubles develop. Make the bow more full and handling will improve notably in a following sea, but the ride will be worse in a head sea (either harsher impacts into waves, more pitching, or both).

There's no 1 defining characteristic of a hull design, the details all work together.

 

[O C Diver]

For me, the comfort of a hard chined boat depends on how far below the surface of the water the chine is. On my boat, there is definitely some bang in beam seas with near empty fuel and water tanks. A very different ride with full tanks (7,000 pound increase).

If I were going to spend most of my time in the ocean, I would be more concerned about the difference. For most displacement cruisers, the rest of the boat layout is more important.

Ted

 

[FWT]

I'd emphasize what's been said above: its a lot of things working together.

But what bears more comment is sheer weight.

While a full displacement is easily defined (can't do more than hull speed), and a planning hull is easily defined (designed to get on top of its bow wave and plane), semi-displacement isn't so easy. Some are designed to turn a good bit of speed, and move well into the teens or more. And others lean closer to the full displacement end, where a small handful of knots above hull speed is about all one should push for.

Sharp / hard chines help somehow with the higher speeds. And such boats are designed to keep weight down to help with speed.

And then you can get into how top heavy it is or isn't.

Stability at rest isn't purely about hull shape. Step aboard an ocean liner and it won't notice you have done so. Mass and weight. At the other end, look at dingy hulls. West Marine sells a flat bottom plastic dingy that is very light. There are videos out there where folks test them for stability. Hard chine, flat bottom boat. Its very unstable, because it is so light weight. I have a plastic dingy that empty is about 240 lbs, and it is much more stable.

Its not directly to your question, but if the interest is comfort at anchor, there is another element to the hard chines. Many hulls designed for speed carry the hard chine well forward and what gets created is a sort of "notch" in the bow as the chine carries above the waterline. SOME, of those designs get complaints that this sets up hull slap noise in the forward berth as waves slap under the notch. It apparently impacts some designs more than others.

 

[Nomad Willy]

Hard chine = snap roll.
Can be violent and make one feel kinda sick.
More efficient at higher speeds.
More stable at dockside .. float.
Not likely but can trip and capsize sliding sideways on chine edge.

Rounded chines have less drag at low speeds .. lower wetted surface.
More tender at dockside or float. Usually a pleasant small rolling motion.
Better hull form for trawlers at their normal speeds of 7 to 10 knots.

Skippers that seek to be able to outrun sudden stormy weather best buy hard chine.

Skippers here are quick to seek speed but talk a lot about fuel consumption .. a lot.
Most rec trawler skippers have boats too large to gracefully keep up the fuel demands.

All my opinion

 

[RedRascal]

Thanks for the explanations, I think I was putting too much emphasis on the chine shape vs the overall hull design. So if we were to compare a grand banks 36(hard chine) and an eagle 36(round chine) in the pictures below the expected ride and at anchor behavior would probably be similar, correct? I don't hear anyone saying that rounded chine boat(eagle 36) is an unicorn and I wish I owned it.

 

[O C Diver]

Thanks for the explanations, I think I was putting too much emphasis on the chine shape vs the overall hull design. So if we were to compare a grand banks 36(hard chine) and an eagle 36(round chine) in the pictures below the expected ride and at anchor behavior would probably be similar, correct? I don't hear anyone saying that rounded chine boat(eagle 36) is an unicorn and I wish I owned it.

While there may be some difference, the eagle appears to have a large keel. At anchor, a large keel slows roll. At a point where it becomes uncomfortable, the Grand Banks will likely be uncomfortable also. My Bruno & Stillman (Downeaster) had rounded chines, but a large keel, and was very reasonable at anchor.

Ted

 

[Mambo42]

My experience is that on anchor on in the dock both chines have horrible characteristics. My Defever 49 has a hard chine and could roll like a roller coaster in the dock (no matter how much fuel you had). Sometimes I was holding on to dear life, while our neigbors were just lying quiet.
Recently I was on a 60' motor sailor with a rounded chine and that boat was rolling all the time in port, it was not fun anymore. We were wondering how it would be on anchor, luckily we don't need to find out.
While weight for us does make a difference when we are underway, we found out that the rolling in the marina (caused by waves) did not make a big difference.
Now that we have zero speed stabilizers we don't roll anymore. When we have the stabilizers 'off' the roll is limited by the large fins and when it then still gets too much we simply switch them on and the whole marina may be rocking and rolling, we will lie dead in the water.
At anchor it is the same deal, the wind would always put us beam on to the swell and that would become uncomfortable. Now with the stabilizers that is over. If the wind is strong enough we just put an extra snubber and pull the bow into the swell, problem solved.
So, if I have to make a choice between rounded and hard chine I think I would want to have stabilizers in both options. In fact, I don't think I would ever buy a boat without stabilizers anymore.

 

[Hippocampus]

It’s the totality of the design not any one feature that matters. Several here have pointed this out. Spend some time on a dismasted sailboat. Roll is miserable. Speed, magnitude and recovery from a roll depends upon inertia, righting arm, % of form stability among other factors.
Appropriately it’s been pointed out slack bilges on a hard chine will behave differently than a soft chine with a flat bottom. Deep V differently than wineglass. Many boat have a combination as well. Deep V entrance becoming radius chine becoming soft chine, becoming hard chine or other combinations.
Been on slice of pizza hard chine sail with much better behavior than traditional redingskoite Archer hulls.
Only statement I can support is light hulls mainly or totally dependent on form stability (ultralights, racing multihulls) in general have a quicker motion and recovery. Also more sensitive to smaller waves and chop. Chine is usually irrelevant. Type of chine is commonly associated with other features which do impact on behavior.

 

[rslifkin]

Thinking about it, even within the realm of hard chine, there's a big difference in roll behavior between a hull like the Grand Banks that has no meaningful chine flats and a hull that has big chine flats carried all the way aft.

My own hull is in between, with a small/moderate reverse chine up forward, but it narrows and twists out as you head aft, with no chine flat by the time you're about 40% forward of the transom. In terms of small angle rolls, it's actually a pretty tender hull (it'll move quite a bit form stepping on from a dock), but it stiffens up dramatically past about 5* of roll.

 

[Hippocampus]

Yup, perfect example. The chine is a small surface area. The rest of the immersed hull a large area. Which impacts more? Especially when the chine isn’t the only or even a major contributor.

 

[Shrew]

Does anyone have an example of a soft chine on a semi-displacement hull?

I would think that the hard chine contributes significantly to semi-displacement.

With a round chine, lift would seem to get lost at the edges of the hull.

 

[psneeld]

Quite a few lobster boats have rounded chine and are semi-displacement....

Might be a hull material thing more than a design issue...but not an absolute.

 

[PierreR]

The Buttock angle matters more than the Chine shape for the hull type.
As per Gerr, Quarter beam buttock angle is the angle of the bottom slope towards the stern at the midpoint of the water line length and 1/4 of the beam from the midpoint of the keel. Measured out of the water with a leveled water line and a 4'+ level from the point described above back towards the stern. anything over 7 deg is displacement, anything between 3-7 degrees is semi displacement and 2 and under is planning. The above explanation is good reference and generally holds regardless of the chine shape.

A round chine is more efficient at lower speeds than a hard chine and vis versa, but still does not determine the hull type.

I have a full displacement hull that is round forward and hardens up near the stern. I am quite satisfied with it 60% of the time, 35% indifferent and 5% of the time I cuss it. A couple less degrees and I could go a knot or two faster with the same engines and the extra fuel it would burn at slower speeds would be okay with me. Still a good boat for me.

 

[Jeff F]

Quite a few lobster boats have rounded chine and are semi-displacement....

That's a navy utility boat, originally. Classic design ?

 

[Nomad Willy]

PierreR,
Very very good point.
QBBL has even been used to identify FD, SD and plaining hulls via angle.
IMO clearly the most significant element of speed in hull design. But if the rocker is mostly fwd an otherwise SD hull could be a planing hull .. or at least getting into the bottom of a planing hull speed range. Very influential element of hull-form.

But chine talk is much better understood here on TF.

 

[Nomad Willy]

Quite a few lobster boats have rounded chine and are semi-displacement....

Might be a hull material thing more than a design issue...but not an absolute.

This one not a Navy personnel boat me thinks. Ust’a work at Uniflite as a draftsman and woulda recognized it. Probably a local boat.
There’s a big difference between a large radius round chine and a short rad. This fishboat appears to have a fairly short rad chine … stiff for rounded chine.

Oh Jeff F I see you have a 50’ Navy boat. Is it a personnel boat carried aboard a ship or other. They made a lot of 36’ utility boats that were wood and had a FD hull w a fairly small transom. Prolly 50 footers too…?

Another significant thing about round/hard chines is that hard chines tend to direct the spray away from the boat but rounded chines (especially long rad.) produce slop instead of spray. Heavier water lower down that usually dosn’t come over the rail.

 

[Jeff F]

Oh Jeff F I see you have a 50’ Navy boat. Is it a personnel boat carried aboard a ship or other. They made a lot of 36’ utility boats that were wood and had a FD hull w a fairly small transom. Prolly 50 footers too…?

Mine is a utility boat that was carried aboard aircraft carriers. Similar to the 40 ft. Psneed changed the photo after my comment. Maybe he'll share it again.

Here's mine.

 

[Nomad Willy]

Interesting your relatively short keel. Must be very maneuverable.

Re the identification of the “Shelby 1” I think you are right. The Shelby 1 seems to be a USN 50’ Utility/launch. You find it easily driven I assume. The original power was a 160hp GM 6-71. I wanted to buy one a few years ago but she was in Eastern Canada.

In a quartering/following sea do you find yourself rather busy? When I worked at Uniflite I had my eyes on the 33’ version of this hull. The 33 is probably hard to find now. QBBL should be close to 0. If kept light they should be fast … given more power and staying light.

Did you “convert” your boat?

 

[Jeff F]

Someone else converted it. It's still light, at about 35k pounds on a 49' waterline. Very easily driven. I can cruise at ten knots with the 135 hp JD4045 if I choose to. Very maneuverable, as the stern swings easily.

I find the boat fine in quartering seas. It accelerates and surfs rather than wallowing, and the big rudder keeps things under control. But rolling in a beam sea can be unpleasant.

 

[Fletcher500]

Psneed changed the photo after my comment. Maybe he'll share it again.
.

Funny, I noticed that as well.

 

[psneeld]

I was in a rush and hit the utility boat pic that was mixed in with generic lobster boat pics...but I wanted a lobster boat as the example... based on many posts on TF a tiny slipup like that sets the wrong stage....so I changed it to what I originally wanted.

No explanation really needed.

 

[ksanders]

Does anyone have an example of a soft chine on a semi-displacement hull?

I would think that the hard chine contributes significantly to semi-displacement.

With a round chine, lift would seem to get lost at the edges of the hull.

Yes my Bayliner 4788 has a rounded or soft chine and the hull form was marketed as semi displacement.

Here are some photos of the hull as the rolling chocks were being installed this last summer.

The rounded chines made for a rolly boat, and the rolling chocks reduced that rolling considerably.

 

[Jeff F]

No explanation really needed.

No foul... people tell me all the time that my boat is lobster style. The transom and stern shape looks a lot like the lobster boats. I'm not sure of the design provenance. Built in wood originally, then GRP from (I think) the same design.

 

[rgano]

My hard chine GB42 lived in a slip alongside a rounded chine Bristol 42. I cannot speak as an expert about the overall shape of the Bristol's hull vis-a-vis wineglass shape etc. etc. While my GB would take a roll or two if a wave somehow got into the marina, that Bristol would roll deeply back and forth forever. Out in the Gulf of Mexico with two to three foot waves (I was smart enough to never go out there in larger waves) my GB would snap roll unmercifully. I traveled on that same Bristol through the Gulf down to south Florida from Panama City and also through Lakes Huron and Michigan. The Bristol with its sailboat heritage was an easier riding sea boat for sure and drier too due to its more flared bow. In calmer waters like the ICW and rivers I mostly traveled in my 29 years owning the GB, give me the GB. If you are a coastal waters traveler, the Bristol was a better choice. These two boats had identical propulsion of twin FL 120s.

 

[Southern Boater]

My experience is that on anchor on in the dock both chines have horrible characteristics. My Defever 49 has a hard chine and could roll like a roller coaster in the dock (no matter how much fuel you had). Sometimes I was holding on to dear life, while our neigbors were just lying quiet.
Recently I was on a 60' motor sailor with a rounded chine and that boat was rolling all the time in port, it was not fun anymore. We were wondering how it would be on anchor, luckily we don't need to find out.
While weight for us does make a difference when we are underway, we found out that the rolling in the marina (caused by waves) did not make a big difference.
Now that we have zero speed stabilizers we don't roll anymore. When we have the stabilizers 'off' the roll is limited by the large fins and when it then still gets too much we simply switch them on and the whole marina may be rocking and rolling, we will lie dead in the water.
At anchor it is the same deal, the wind would always put us beam on to the swell and that would become uncomfortable. Now with the stabilizers that is over. If the wind is strong enough we just put an extra snubber and pull the bow into the swell, problem solved.
So, if I have to make a choice between rounded and hard chine I think I would want to have stabilizers in both options. In fact, I don't think I would ever buy a boat without stabilizers anymore.

Hi Mambo, your comment about your neighbour in the marina not rolling and your boat is reminds me our boat does the same, sometimes, and not other times.
It may have something to do with the wave frequency, we can be rolling and the neighbours are hardly moving, then other times we are virtually still and they are rolling?

 

[Hippocampus]

https://the-bosun.com/the-importance-of-the-sailboat-comfort-ratio/

This references sail but it also applies to power. Note no, soft or hard chine is not considered a major factor. It’s an epi phenomenon as the totality of the hull shape is usually correlated to the presence or absence of chine. Note comfortable at rest may mean uncomfortable in motion. Comfortable in benign conditions may mean uncomfortable in a seaway. The reverse true as well.
For motion sickness there’s three groups. Those more sensitive to low frequency motion ( heavy displacement boat or ship). Those more sensitive to high frequency motion (ultralight, ocean multihull). Those sensitive to both. Have had crew with no problems on a multi but sick on a heavy mono and the reverse. Comfort does vary with the boat but also the person.