Ride comfort - displacement vs Semi-Displacement

[mvweebles]

In the 40-foot trawler size range, the difference in stability between displacement and semi displacement is minor (assuming similar center of gravity for both). Motion is a bit different - displacement hull is initially tender then stiffens so has more of a gentle roll. But both would benefit from some form of at-rest stabilization in a rolly anchorage.

There are some solid reasons to select a displacement hull boat. A displacement boat is optimized for that use-case. The hull is designed to be efficient at displacement speed and the engine is matched to deliver sufficient power. If TF users are any indication, vast majority of users run at displacement speeds so you'd think there'd be more displacement boats out there. If TF is an indicator, buyers make a decision based on exceptions: someday they may want to go faster. Sounds nuts to me but the market has spoken.....for decades.

In the 40-ish foot range, I just don't think there are meaningful differences in comfort/motion between the two hull forms (assuming similar center of gravity).

Peter

 

[Jklotz]

To complicate matters further, in my experience, comfort is subjective. I'm comfortable with far more motion than I was when I first got my boat.

My hull is SD with a keel and is quite heavy. I don't go fast, but I get the same fuel economy and range as many FD hulls. Would I notice a difference in the rough stuff if I had a FD hull? Maybe, but stabilizers would make a far bigger difference.

 

[ranger58sb]

I don't think I'd be able to tell the difference between FD and SD, probably both OK in almost all sea states.

Probably both can suck in some seas unless stabilized.

-Chris

 

[Don L]

Sounds people here on TF don't have an answer to the original question. One thing that seems to have been posted a few times is that sometimes for the SD hull you have to go faster to get comfort.

I have friends right now in south Bahamas looking to do a 150 mile open crossing today/tonight. Seas forecast will be 3-7 feet and wind 4-20 during the passage. This is a semi-common hop and it type on my mind for the question.

Another note - I most likely will be getting a 1980s boat, whether the hull is a displacement or SD may turn out to just be a "thats what the boat has" thing. If you look up the hull difference is always list the displacement has "more comfortable". But that is like saying "better" with no definition.

 

[PierreR]

Don, you have always been in full displacement boats. This thread makes it seem like there are a lot of full displacement 1980's trawlers out there but they are few and far between. You will likely find 20 semi displacement boats for every full displacement boat.
Full displacement trawlers cruise right down there at 6.5-7 knots along with their sailing brethren. The bulk of posters here tend to travel around 7.5-8.5 knots and think that is displacement speeds. Semi displacement doesn't save fuel as big at 6.5-7 as a displacement boat and they ride harsher so they cruise a bit faster where the ride is better.
Now you mention 3-7' waves and 4-20 knots of wind and I assume in the Gulf Stream. Are we talking significant wave heights (predicted) because every 100th wave, or every couple of minutes, you could expect something 10-14' high and steeper. Trawler weather is 1-2' predicted max and a southerly component to the wind to cross the Gulf Stream. You are sitting or standing higher above the MetaCenter in a trawler and the motion is greater than on a sailboat. Sails also considerably reduce the rolling rate.
For ease of handling and geriatric creature comforts, you are giving up ride comfort. Without stabilizers, old folk don't belong out in that crap. There are a lot of single men in trawlers like sail boats for a reason. You get out there in 3-7' chop and we will make you write sentences like you were back in grade school. "I, Don L am not 20 years old anymore and don't belong in those conditions." In cursive not double spaced.
Benefit, the trawler is not healed over unless rolling.

 

[Don L]

wonder why this is about me and not about boats (no need to write up some reason)

I asked trawler owners their experienced real life opinion. I probably need to track down blogs of cruisers and ask them.

 

[boomerang]

Don L, ever since you joined this forum I've watched your posts with bemusement. You never seem to be happy with the replies received. I'm not a huge poster or follower of too many forums but to me, I find this one a good forum to learn from others opinions and experience, not be offended by it. If the heat in the kitchen is too much, you either leave or endure it.

 

[Don L]

hey I am not offended, just annoyed

no one has to read my posts etc., I have not attacked anyone, and have in fact read every respond to my question

The conditions I posted on post 34 are the expected conditions next couple days heading south from Long Island Bahamas going toward T&Cs. Posters are free to look it up ad decide if that is a coastal cruise to you or a passage and how you boat would be during it.

 

[mvweebles]

I have friends right now in south Bahamas looking to do a 150 mile open crossing today/tonight. Seas forecast will be 3-7 feet and wind 4-20 during the passage.

........ If you look up the hull difference is always list the displacement has "more comfortable". But that is like saying "better" with no definition.

3-7 feet at what period and at what direction? If it was sustained, short/square chop on the nose, I'd come up with a Plan B.

I've read all the comments on displacement vs semi-displacement. Have been reading them for 30-years. I've spent a lot of time on both. At some point it starts to make a difference in comfort - displacement boats are heavy and have a lot of capacity to carry stores. Somehwere in the 60+ foot range, they really come into their own (as long as they don't go full double-decker - Hatt 58LRC, Cheoy Lee 60-65, etc.). But in the 40-ish foot range, it just isn't a big difference in comfort between them. Now, if you toss in planing hulls (such as this Tiara Convertible), you start to divide the field a bit and the differences become more pronounced. The Tiara is wide and capable of some speed even in heavy chop.

I'm a bit perplexed by the comment that no one answered your question. This is the third time I've said there isn't a difference. May not be the oft-repeated wisdom, but it's an honest answer.

Peter

 

[Don L]

I accept you dont feel there is a difference

 

[ksanders]

hey I am not offended, just annoyed

My apologies if I have annoyed you with my constant "the boat is not that important" postings. I really do not want to offend anyone.

We get so many folks here that focus their cruising dreams on the boat, only to realize it's not the boat holding them back, it's them. I often post about lifestyle to try to help them.

I remember a guy on a Nordhavn in Seward Alaska several years ago. He told me he was waiting on weather to go across the Gulf of Alaska. I was surprised he was waiting as I was leaving on that day in my Bayliner. He explained that he was retired and had the time to wait for perfect weather, and I explained that I had a week of vacation, and needed to go now, or not go. That day I understood.

Real cruisers can wait for weather, so the boat is not as important. Weekenders need to go to work on Monday.

 

[mvweebles]

hey I am not offended, just annoyed

no one has to read my posts etc., I have not attacked anyone, and have in fact read every respond to my question

The conditions I posted on post 34 are the expected conditions next couple days heading south from Long Island Bahamas going toward T&Cs. Posters are free to look it up ad decide if that is a coastal cruise to you or a passage and how you boat would be during it.

I don't know the area but I did a PredictWind from "Clarence Town" to someplace called "Mayaguana." It's 150 nms. I wouldn't do the open water passage given the forecast which is essentially on the nose with 4-7 footers at 7-secs. Now, there are many islands so perhaps its possible to stay in the lee for much of the trip.

But open water in those conditions? I wouldn't do it voluntarily. It's damn uncomfortable - not great on a sailboat either I might add, essentially because the wind is only 15-20 degrees off the port bow so you'd be tacking and hard-on the wind.

I don't know how long I'd have to wait for a weather window to do that run - but I wouldn't do it as shown.

Peter

 

[Don L]

I have been a cruiser and get it. I am looking at factors I know are important to us based on experience and those include of mistakes.

I am trying to convince my retired friends who just left LI Bahamas above to turn around because they are allowing a schedule to determine the trip. So in post 1 question that answer could always be "pick your weather better" and it will never make any difference. But I have lots of experience with the lies of the werather forecast and how an OK forecast can turn to ****. Once spent 3 days out in F6-7 conditions that weren't forecasted as such. It SUCKED!

 

[Greg QS]

I purposely south out the SD platform for the simple reason that it gives you more options when things get dicey. Such as running from weather, overpowering currents, matching swells, and power to get out of the way.

 

[mvweebles]

I am trying to convince my retired friends who just left LI Bahamas above to turn around because they are allowing a schedule to determine the trip.

Yep. That's what @ksanders in #41 observed.

I don't mind a bit of slop. But the forecast I saw is pretty much non-stop and on-the-nose (or dang close to it). I didn't mind it too much when I was delivering. Biggest issue was the windows and ports leak in heavy spray. And the cabinets aren't well secured. Can be pretty hard on the boat if it's not well setup. But now that I'm a normal cruiser guy who enjoys the company of my other-half, I just don't see the need to do that type of weather. If I did, I would buy the biggest boat I could afford. Size matters in heavy weather.

Peter

 

[O C Diver]

Ok I understand the difference of the hull designs and the pros/cons of each compared to the other. But what is the ride comfort difference of a displacement versus a semi-displacement (semi-planning) trawler at hull speed or a little less?

I had a semi displacement hull. For me, hard chines and vertical sides made the ride substantially less comfortable in beam seas. A more rounded bottom and a wine glass stern was more comfortable. In most cases, I wasn't willing to burn the fuel in semi displacement to improve going into seas and down wind.

Lets calls say a displacement trawler at 6 knots in seas of 5-7 feet with 20 knots of wind and cal that ride a 7 on the comfort 1-10 scale.

What would you call the same conditions in a SD on a 1-10 scale?

Depends on size of the boat. 20 knots doesn't generate 5 to 7 foot seas unless the fetch is hundreds of miles. Depends on your angle relative to seas. Down wind (10), my boat had a huge rudder, so reasonably comfortable with maybe course corrections of 10 to 15 degrees. Going up wind (3 to 7 based on wave height), miserable, unless you're willing to punch through the waves with speed. Beam seas (2 to 5 depending on actual seas) was also miserable from the slap shudder of the up wind chine.

I read 3 pages of google search results and for he most just got paper design blanket answers

One other point. Being in a sailboat lowers your helm station to the near roll center and you're typically steering from the last 25% of the boat. A pilothouse trawler or flybridge substantially elevates the helm station and moves it to the forward 40% of the boat. It's far more comfortable to be in the stern at near the roll center than forward and substantially higher. The speed of beam roll and snap reverse is miserable. That's what stabilizers and masts with sail up are for.

Ted

 

[JRZ1]

Sorry but your formula lacks dividing by 0,65, so your numbers are are fairly low.

Ah yes, you're correct. Not sure how I missed that. Updated:

	CR= D/(.65x (.7LWL +.3 LOA)x Beam ^1.33)
		D	LWL	LOA	Beam
COR 60	70.31	124,000.00	57.10	63.40	17.79
N51	57.77	71,575.00	48.33	50.75	15.67
S60	69.61	123,000.00	56.25	65.10	17.83
N60	77.25	138,000.00	57.25	62.50	18.00

 

[rslifkin]

FD hulls will tend to have a different feeling motion than SD hulls, at least in roll. Usually slower and smoother for FD (compared to a faster and snappier roll for SD). Whether that's better or worse is a matter of preference.

Different people are bothered by different kinds of motion. I've known people that would be perfectly fine on one boat on a given day, but put them on a significantly different boat on the same day and they'd be bright green.

In general, SD (or planing) hulls tend to be lighter weight than a similar size FD hull, but not always. Weight (and how it's distributed throughout the hull) can be a significant factor in how the boat rides. The finer details of hull shape can also make a big difference. A finer bow will tend to ride better upwind, but may be a wetter ride and more likely to stuff the bow through a wave. A more full bow will be less likely to dig in going downwind, so will likely behave better in that situation (particularly if the stern is broad and will be lifted significantly by a wave).

With SD (or planing) hulls, size of rudders and amount of keel can vary significantly. Some of these hulls steer very well at low speeds, meaning you only have to speed up if you want to take advantage of dynamic lift to smooth rolling motion a bit or if you just want to go faster. Others have undersized rudders and can lack authority at low speeds, so some conditions may force you to speed up for better control.

I'd call our boat a slow planing hull (it planes cleanly, but has a keel, big rudders, full bow, convex forward sections, low transom deadrise, and by looks you might question if it would plane). We run 95% or more of the time around 6.5 kts (this year we ran at 17 kts for about 4 hours out of 195 total). Beam seas are about the only time it rides better on plane. This boat isn't a great ride in steep head seas at any speed, but it loves following seas.

Counterintuitively, I've found a few times where the best ride in following seas was actually slower than our normal cruise (last time I was all the way down to 1000 RPM instead of ~1280). Having the waves pass under the boat faster helped, as the boat wasn't sitting on top for as long, so it didn't mush around rolling and yawing as much (and we still had plenty of rudder authority at the lower speed). Of course, running on plane in following seas works too, but it's tiring, a little bit sketchy, and needs a careful helmsman to let the boat surf down a wave at 20 kts and not stuff the bow into the next one too hard. And if you really want to work the throttles, there's always the "pace the waves" method where you just sit in the trough right behind a wave, but that's typically at an inefficient speed on top of the effort required.

As others have mentioned, where you sit on the boat is also a big factor in how it feels. Higher up and further forward will generally feel worse.

I'd agree with the others that have said there's no way to really pin a big difference in ride to just SD vs FD for a given size boat. Too many other factors play into it. The spectrum of SD hulls is also pretty broad, ranging from slightly tweaked FD designs that can get a bit above hull speed to a planing hull that's optimized a bit for lower speeds (and may not actually have enough power installed to plane).

 

[cfr]

very interesting for me because i just bought a SD coming from a heavy sailboat....and i can confirm that even with the sailboat we allways wait for the right wind or tide. i will test next season on atlantic coast of europe and i assume in winter i install some stabilizers too, but we will see. the last owner put 3000 hrs on the boat and a lot on the sea without stabs....

 

[Mac2]

Surprised this hasn’t been mentioned yet, but what about following seas? Im talking about being in conditions you can’t go fast, but instead have to slow down. These are the conditions I have heard the flat stern sections of SD hulls loose control in following seas. I’ve never owned a SD hull, so I cant comment. I got caught in 10+ foot breaking waves in my last FD boat. I got the crap kicked out of me for a couple hours heading into the waves (could only go about 4 knots). When I was finally able to turn around and head with the waves, I was amazed at how comfortable the ride was in those following seas. From pure hell to calm surfing in seconds. This was 20 years ago when I was extremely ignorant. No sane person would have gone out in those conditions. I only mention this in case the OP needs to consider conditions that force you to slow down and you can’t head directly into head seas.

 

[PierreR]

I have been a cruiser and get it. I am looking at factors I know are important to us based on experience and those include of mistakes.

I am trying to convince my retired friends who just left LI Bahamas above to turn around because they are allowing a schedule to determine the trip. So in post 1 question that answer could always be "pick your weather better" and it will never make any difference. But I have lots of experience with the lies of the werather forecast and how an OK forecast can turn to ****. Once spent 3 days out in F6-7 conditions that weren't forecasted as such. It SUCKED!

Your struggles with the transition from sail to power makes me reflect on my own transition and to realize although I accept many things I have not really fully made that transition. A part of me would still like to sail and a bigger part of me is starting to realize far more possibilities in power boating that I had realized.
I struggled with the same exact thing in your first sentence and ended up in a trawler that was weighted more towards the seaworthiness end than the floating condo end of the trawler spectrum and I suspect that is where you will end up also.
What I did not grasp or appreciate was the weather part of my dilemma. In sailing you are always trying to balance the difference between too little wind and too much wind and waves. Since just the right amount exists only about 20 % of the time you are always in some sort of compromise from what you would like and often get your nose clipped.
I knew the natural domain of the power boat was calmer weather but also knew (from mistakes) that you could not always count on the forecast. I motored a lot with sail boats in calmer weather but never really wanted to. What I completely missed was how much easier it is to predict calmer weather and how much more prolific it really is. When wind is expected, the strength of that wind is far more unpredictable than calm. The result with modern weather forecasting is that after thousands of miles, I have not been moderately hammered once.
Slower more seaworthy boats seem to subject me more to getting stiff sitting too long and not being able to get off the boat. A more seaworthy boat also feels more restrictive where I can gunkhole and also waiting for bridges to open. Well I have come to the conclusion that I may not be in the best boat for where new found knowledge is taking our cruising.
Going down to something like a 36' Down East design with a more sailboat like interior has it's charm and provides much faster dispatch and smaller weather windows at nearly the same fuel burn per mile.
Coastal cruising, more open and airy, getting off the boat more to explore locally would be cheaper and less boring. The only thing I would be giving up is some creature comforts of a larger boat but I would not spend as much time fixing things either.
It's getting more tempting Think Sabre 40 sail to Sabre 36 power. Same room, different lifestyle. Way more off ocean cruising with some seriously charming rural cruising areas.
My transition remains incomplete but I am learning that I don't have to endure weather or outrun it. Calms are much more predictable and shorter the travel window even more so. Being able to choose between slow or fast and giving up some seaworthiness and ride comfort needed for heavier weather looks to be much more doable than I imagined.
Just food for thought.

 

[Don L]

I long learned that when the forecast is nice that it MAY be nice. But if the forecast is bad it WILL be bad.

I know the boat I want is a 40-42' boat. But I also I still look at it with eyes of being on it full time. Now I am trying to look at it as "only" being on boat 3-4 months at a time and never doing a real long passage (but still eyeing Bermuda) So I am still trying to iron it all out, but do have boats down to 34' saved . I also understand that 10 years from now the old boat I get will be almost worthless far as resale. This effects money decision some and I want to be sure I can at least afford good salsa to go with my dog food if needed.

 

[Mambo42]

Ok I understand the difference of the hull designs and the pros/cons of each compared to the other. But what is the ride comfort difference of a displacement versus a semi-displacement (semi-planning) trawler at hull speed or a little less?

Lets calls say a displacement trawler at 6 knots in seas of 5-7 feet with knots of wind and cal that ride a 7 on the comfort 1-10 scale.

What would you call the same conditions in a SD on a 1-10 scale?

I read 3 pages of google search results and for he most just got paper design blanket answers

If you take a look at the channel of In Too Deep you will hear what the difference is based on boat length and I think that this is where the real difference comes into play. As most of you know Gus has experience in boats ranging from 30' to 50' to now 70'. His first one was I think full displacement, the Hatteras was semi displacement and the Broward is mostly big.
You can hear him make the comparison between the hatteras and his broward when he is in a storm. The broward just goes straight through the waves, even at 8 - 10 kts, while the hatteras was pitching violently up and down plus rolling heavily.
I have heard it from many people now, saying that a 70' boat does not dive in between the waves, it rides the waves, especially when the waves are high with a short period.
When the waves come on the beam the stabilizers will make the difference in the ride. The difference between stabilization and no stabilization is immense and I know that from experience. We used to be thrown around in the boat in a bit of a beam sea and especially on anchor. Now that we have zero speed stabilization we are rock steady. If I come in a heavy head on sea I just put the bow 45 degrees into the waves and ride it out like that. My boat will pitch heavily and violently from one wave to the other, a 10' head on sea is not fun in my boat, but if I put it at 45 degrees it is much smoother (still uncomfortable though) and the stabilizers will stop the rolling.

When I look at larger (and thus heavier) boats when we are on anchor, my boat is much more stable then they are, but again, that is because I have zero speed stabilization. If I would have to compare my boat, without stabilizers, to the bigger heavier boats it is a world of difference, they are much more stable.
If I have to compare my boat, without stabilizers, to semi displacement hulls then my boat is more stable. Reason is that I have a large keel and they don't.
So I guess the answer to what is more comfortable is a larger boat, I don't think it is a matter of speed. I have seen the semi displacement boats of my size slam into waves and roll like a drunk duck in heavy seas, even when they try to go fast. In fact, the majority of them don't go out when we can still go out. Not that we like it, but sometimes it is necessary.

 

[PierreR]

I long learned that when the forecast is nice that it MAY be nice. But if the forecast is bad it WILL be bad.

I know the boat I want is a 40-42' boat. But I also I still look at it with eyes of being on it full time. Now I am trying to look at it as "only" being on boat 3-4 months at a time and never doing a real long passage (but still eyeing Bermuda) So I am still trying to iron it all out, but do have boats down to 34' saved . I also understand that 10 years from now the old boat I get will be almost worthless far as resale. This effects money decision some and I want to be sure I can at least afford good salsa to go with my dog food if needed.

One thing I learned pretty early on is that I could throw very doable things into a sentence and then finish it with a deal breaker. Statements like " I just want a nice little boat that I can take on the ICW, go the Bahamas and live on for a month or two and once in a while go to the Caribbean". I found out that any time you want to do true blue water, with a power boat, the budget triples but the pleasure doesn't A bit more research and magically charter and cruise ships look cheap for those end of sentence locations. My wife still takes a cruise to the Caribbean at least once a year even though we spend months on our own boat.
In coastal cruising I sometime just marvel at the fact that I am out there and on the right side of the dirt. I just soak it up.
My wife probably only has a few more years of this and then we move on to something else.

 

[klee wyck]

Different folks are susceptible to different motion types, but going for comfort likely means thinking about mitigation not just of roll, but also pitch, heave, and yaw.
In the small stuff for sea states, there is likely not a strong argument that either FD or SD are superior overall. Both can be fitted for roll dampening if desired and the flatter, lighter hulls can use speed to mitigate heave and pitch in conditions under 3’.
But, any plan to spend significant time in conditions greater than that likely favors FD in my view. Deeper, heavier hulls with good freeboard, balanced fore and aft both in hull form and trim (likely using ballast) will result in greater comfort, and likely safety, when going further to sea for longer periods where the 5-7’ stuff you describe can be expected.
It seems clear from many threads on this forum that both can work if you are prepared to wait it out and stay within range of a three day forecast. Beyond that, maybe FD is your ticket.

 

[SeaDogAK]

I now have a 49’ displacement hull boat (Defever 49 RPH). Most recent previous boat was a 37’ semi displacement boat (Tollycraft 37). The Tolly would wallow uncomfortably in even 2’ seas at 7-8 knots, but would ride much better at 10 or 11 knots.

My Defever, which is a very heavy boat, rides much better, and I think not just because of its size and not just because of the stabilizers. Of course 8 knots is its top speed.

 

[RedRascal]

What would you call the same conditions in a SD on a 1-10 scale?

3

 

[ABfish]

As another posted pointed out, seasickness is a personal condition. What makes one miserable may not be an issue for another.

My wife often says "If you don't speed up, I'm going to be sick." She just can handle a slow roll.

For the purpose of this discussion, what's the definition of semi-displacement? A 12 knot cruise? A 15 knot cruise?

 

[North Baltic sea]

wonder why this is about me and not about boats (no need to write up some reason)

I asked trawler owners their experienced real life opinion. I probably need to track down blogs of cruisers and ask them.

Both hull types can handle waves of 5-7 feet, but you need to take into account the direction they are coming from and find the right angle to meet them as comfortably as possible.

The SD hull and sufficient engine power make the boat much more stable when you exceed the hull speed by about 1 knot or more.

In addition to wave height, wave cycle speed is also an important factor.

Furthermore, we sailors differ in our ability to withstand conditions in rough seas, so there is no right answer to this question.

 

[psneeld]

Hull shape and superstructure (weight aloft) strongly determine how vessels handle both wind and waves.

It's really hard to generalize as weight distribution withing a boat can vary wildly.

2 Boats off the production line with different options - say single versus twin...but even then a mounted much lower in the bilge single of heavier weight can cause either a higher or lower CG over the twins. Then there are years of alterations by owners. Even storing a dink on an upper deck versus the swim platform can vary roll charachteristics.

I am sure anyone here with experience knows all this but that's why probably no one here can answer the simplistic answer of what boat or what type hull is "more comfortable".... let alone people's opinions on what is comfortable versus uncomfortable.