It’s all about the waves

[Mike OReilly]

Springing from the recent ‘heavy boats are slow’ discussion, the issue of boat behaviour in waves came up. My experience is that speed over water is mostly about sea state, and much less about wind.

I’ve not been very methodical about this, but my heavy, full keel, 32’ LWL boat seems to bash when going into a certain wave hight (amplitude) combined with a a certain frequency. Below or above this I either cut through them, or just follow the curves. But when they get into a certain range we’re severely affected.

For example, we just went through a day of average 2 meter seas on the forward quarter, with a frequency of around 4 seconds. This got us into a pattern of coming up and then down one wave, only to smash head-on into the next. The bow didn’t have time to rise before hitting the next, so we’d loose all speed.

But when these waves are smaller, or more spaced out, or even tighter together, then we’re not nearly as affected.

All boats I’ve sailed have this kind of pattern, although each is different (and my sampling is small). What is the relationship to LWL, wave size (amplitude), wave frequency (wavelength) that makes one ride easy and another a slog?

BTW, I’m speaking about displacement hulls. Planing boats and multihulls will have completely different physics.

[boatman61]

I dont know Mike, a Catalac I was on had its underbody cracked open and came close to sinking in 3-4 metre seas such as you describe, up over and slam down violently to have the next crash over the top.. could not risk turning and only keep head to wind by steering with the engines.
Nature rules all..

[Dockhead]

All boats are affected by this. Even big ships.


There are a lot of different factors at play:


1. Weight of the boat and thus momentum (heavier is better)
2. Hull form -- fineness of the entry (finer is better)
3. Hull form -- forefoot shape (flatter is worse)
4. Polar moment of inertia ("heaviness of the ends") -- less is better

5. And obviously LWL -- longer is better


So a long boat with light ends, a fine entry, and a deep forefoot will be more pleasant bashing into head seas, than a short boat with heavy ends and flat forward sections. But all boats bash in some conditions or another, and as you observed, there are harmonics involved which can be especially unpleasant.


Small, light boats are particularly affected by this. Large, powerful boats less so, and they can afford to be lighter. I sometimes motor against those short steep head seas like we get in the Baltic; it's unpleasant but sometimes you have to do it. I find it's better to take them at some angle and keep mainsail up and drawing, and if necessary use a fair amount of engine power to keep momentum up.

[GordMay]

Boat-Wave encounter frequency depends (in part) upon:
ship’s heading, ship speed, and wave length.
The boat’s tendency to pitch is magnified, if the frequency of encounter with the waves is close to the natural pitching frequency of the boat. The boat’s natural pitch frequency is determined, in large part, by the moment of inertia (MOI).
Those things that we can control are:
ship’s speed, moment of inertia (MOI), and (to a lessor extent) ship’s heading.
So we can:
Slow down, or speed up.
Move weight into the ends (reduce pitch frequency, increase MOI & amplitude), or out of the ends (increase frequency, decrease MOI).
Sometimes, change heading.


There are three stages to a man’s life:
Stud, Dud, Thud.

[john61ct]

In my research for narrowing down the prospect list of boats to look at once shopping

"seaworthy for safe offshore passage making" quickly became the top "less objectively defined" criterion.

AKA, structural integrity, *designed for* survivability in eventual/occasional blue water passage making, best possible safety for its occupants when the sea gets rough.

Hoves to well, and can jog along self-steering in 35kn without lots of input.

Also safety around reefs.

Among the bargain older plastic classic choices that seems to inherently mean limited pointing ability, or at least lower speeds going upwind.

_____
The qualities you're highlighting here, are apparently covered by the term

"sea-kindly" meaning least tiring motion.

Also "motion comfort".

Now, naval architects have their ratio-formulas for hull shape, vertical CoG vs metacentre, distribution of the displacement fore vs aft.

These factors being 3-dimensional and infinitely variable, my guess is a lot depends on designer "intuition" perhaps trial & error experimentation, likely improving with experience over time.

I'd be surprised if there aren't some designs that don't have "sea comfort" gaps, certain wave height and frequency, swell period where they slam like that. But I bet they are very rare, and can't imagine how the factors behind those phenomena can be easily laid out objectively, explained for lay sailors to grok.

Even just collecting measurements as you've written - systematically for various common boats - would I think be a huge challenge.

But even generalized anecdotal reports are valuable, and help build up the community knowledge base.

[john61ct]

Originally Posted by GordMay

There are three stages to a man’s life: Stud, Dud, Thud.

Funny? not. but deep 8-)

[liquidice]

I am into this thread. I attached this picture because I feel it illustrates the importance of amplitude and frequency, as was brought up by OP. I wish for a 2nd sailboat in the picture, with a significantly different length of waterline, to highlight how much that can matter as well.

[Mike OReilly]

Thanks guys. I guess what I want to better understand is the natural pitching frequency, as Gord aptly described it. For my boat there is a sweet spot (or rather, the opposite of sweet), where we just get into a nasty pounding rhythm. Seems less about absolute wave size and more about wavelength or frequency.

Frequency will be affected by boat speed. And as DH aptly stresses, LWL is a huge factor. Inertia certainly matters. This is where displacement helps I assume.

What I’m really trying to get at is a planning tool to help make decisions about when to stay and when to go. On my boat it seems less than wave size and more about wave frequency/wavelength.

[hpeer]

I think some are missing Mikes point, or perhaps his experience.

I’ve had this happen a couple of times on our 33’er. In the one event where I counted on 7th wave she would face-plant and come to a complete stop. COMPLEAT STOP. I was motoring in light wind with the wave a coming from a bit off the port bow. She would then speed up and start to gain momentum until the 7th wave and face-plant again. I was getting nowhere fast or slow for that matter.

I eventually changed course and raised the sails. I wasn’t going where I wanted but I was making a lot better headway. I essentially tacked because of the wage period. Changing the angle changed the apparent period and broke the harmonic.

It only happens in very specific instances, right period and right wage height. I’ve had something similar on the big boat but not nearly to the same extent.

Now the small boat is 8 tons with 13hp motor and the big boat is 20 tons with a 72hp motor, although I never use over about 50hp. But it’s still a significantly better ratio. Might have something to do with it.

[Dockhead]

Originally Posted by Mike OReilly

. . What I’m really trying to get at is a planning tool to help make decisions about when to stay and when to go. On my boat it seems less than wave size and more about wave frequency/wavelength.

I think you're overthinking it.


If you get into a resonance situation, just speed up, slow down, fall off, or head up, until it stops. Often a very slight change is enough. Simples.

[CatNewBee]

Its about resonance frequency. If the wave length gets in resonance with the boat pitching and phase, they synchronise and anplify the motion, if they are out of sync or at different frequencies, you plow through barely notice them because your down motion is stopped by the up motion of the wave and vice versa, at different frequencies not harmonical, means not a multiple times the base frequency, you experiance a phase shift, means some waves in sequence are just pass through, and than there are some 1..3 waves that thrash the bow into and stop you a little.

Changing course helps because it changes the frequency / wave length and makes it more comfortable. The longer the wavelength the better, this you see when you turn downwind with following seas, same waves, same hight, just a different frequency. If you can adjust your speed in synch, you can surf the wave for a very long time.

[barnakiel]

Waves have paramount influence on all sea craft, even on SAR helicopters (some have been downed by waves). In case of small boats, waves are the major, dominant factor for speed,comfort and safety.


How a hull is designed will depend on when it was built and what it was aiming to achieve (cruising vs. racing) and at times designs take a twist (and return to earlier abandoned forms!) - as can be seen in recent designs of e.g. Mini boats (with round, spoony bows).


If I were to make any generalizations, these would be:


- double-enders are crap downwind, they roll like tubs,
- beamy, voluminous hulls are boor upwind IF you try going fast,
- beamy, flat hulls are good downwind,
- narrow entry is good upwind, light or heavy,


etc.


Where I use upwind the meaning is up-waves.


Our own boat is a mixture - some displacement (+/-), very narrow entry (+++ upwind), double-ended (- - - downwind).


In this respect, it is not true that all boats are compromises. There are hulls that are great up and downwind. Just these are not normally found in most common cruising choices.


b.

[mikedefieslife]

Originally Posted by boatman61

I dont know Mike, a Catalac I was on had its underbody cracked open and came close to sinking in 3-4 metre seas such as you describe, up over and slam down violently to have the next crash over the top.. could not risk turning and only keep head to wind by steering with the engines.
Nature rules all..

My boat slams so violently in sort steep seas in the Med that is tears fixtures and fittings off inside. There's not a lot you can do about it. Still, I've never heard of a Prout cracking open. That must have been seriously scary on that Catalac, I thought they were built tougher than that.

[MJH]

Originally Posted by Mike OReilly

Springing from the recent ‘heavy boats are slow’ discussion, the issue of boat behaviour in waves came up. My experience is that speed over water is mostly about sea state, and much less about wind.

I’ve not been very methodical about this, but my heavy, full keel, 32’ LWL boat seems to bash when going into a certain wave hight (amplitude) combined with a a certain frequency. Below or above this I either cut through them, or just follow the curves. But when they get into a certain range we’re severely affected.

For example, we just went through a day of average 2 meter seas on the forward quarter, with a frequency of around 4 seconds. This got us into a pattern of coming up and then down one wave, only to smash head-on into the next. The bow didn’t have time to rise before hitting the next, so we’d loose all speed.

But when these waves are smaller, or more spaced out, or even tighter together, then we’re not nearly as affected.

All boats I’ve sailed have this kind of pattern, although each is different (and my sampling is small). What is the relationship to LWL, wave size (amplitude), wave frequency (wavelength) that makes one ride easy and another a slog?

BTW, I’m speaking about displacement hulls. Planing boats and multihulls will have completely different physics.

On our voyage to Alaska this summer we entered British Columbia's Johnstone Strait and found ourselves in 2 meter waves on the port bow spaced about three seconds apart. We only had six miles to go to our anchorage but it was the roughest six miles of the entire trip, far worse wave action than during my round trip to Hawaii three years prior. The autopilot couldn't handle it and I had both hands full trying to maintain some assemblance of our course. For me the spacing of such waves is the key.

~ ~ _/) ~ ~ MJH

[George DuBose]

Skylark, a 1973 Pearson 36-1 just flat out won't motor directly to windward in seas more than 1 meter. If I am facing an upwind destination in windy conditions, all she will do is sail 35-45° off the wind.

We sail frequently on the IJsselmeer which due to the low depths, often gets seas 1 meter with 1 meter period. Like sailing on cobblestones. Trying to motor against that brings the speed down to 2 kts. I just don't even try to motor.