Confused by semi-displacement

[FWT]

It depends on the planing hull. Personally, I split them into 2 types. Fast and slow planing, as in design cruise over/under 20 kts.

Fast planing hulls tend to be pretty miserable at low speed, and unless a deep V with lots of power are often a nasty pounding affair in a head sea. If you can run over the tops a following sea isn't bad, just go fast.

Slower planing hulls may have a bit of keel, bigger rudders, etc. so running slower is much more manageable. Especially if you slow down to the low end of planing speed, the ride in a head sea is often decent. Below hull speed pitching may be excessive due to the more buoyant bow. Following seas will vary, but as long as you can at least keep up with the waves they can be a very comfy ride.

Good points.

It just goes to show there are many variations within each hull type.

Its hard to generalize, but also hard to discuss without generalizations.

 

[Arc]

I just watched ‘ I am second ‘ . They went across the Atlantic in a 18’ flats boat. All things are possible if you leave judgment and wisdom behind.

 

[Art]

The reason we currently enjoy owning a planing hull boat with a twins having relatively substantial horsepower... is... because, it offers fun - yet - individually unique cruising capabilities.

Can go really slow 4 knots on one engine... for super relaxation at 3 nmpg

Can go 7 knots [just under 7.58 hull speed]... for mellow sightseeing at 2 nmpg

Can go 17 knots... for doing a pretty quick run to locations at 1 nmpg

Can go 23 knots WOT... for emergency at God knows what nmpg

I really do like P and SD hulls!! Not too much in favor of D hulls.

Happy "Boat-Speed" Daze - Art

 

[Baker]

Personally, when the weather picks up I don't enjoy being on a planning boat. If you are trying to keep the speed up (where they do best, and are quite stable and comfortable in flat seas), bigger seas can beat the snot out of you. If you slow down, its not where they are at their best and you can pitch and roll more than in a D or SD boat.

Is that what you mean?

I would mostly disagree with this. The only time I would agree with this is if it is a head sea. And even then, it can be managed depending on the period. But a planing boat in a following sea is simple. You have two choices....go the exact same speed as the wave train and never hit a swell. Or go fast and ride over the tops. As long as it is not too steep, I will go faster. It is usually more of a mushing affair at the top. A beam sea is not bad either. The dynamic lift of the hull keeps you from rolling and you simply go up and down with the swell. There is a little roll, but not much. Now a short period head sea would be the worst senario.

In a slow SD boat....pretty much all of the above scenarios suck EXCEPT the head sea. They roll like crazy and are unable to keep up with the wave train so the big fat buoyant ass end gets pushed around by the waves.

An FD boat will roll. But has a tendency to go throu a head sea and does not have a lot of buoancy in the stern to get pushed around. They usually track straight in a following sea unless very steep and short which could cause broaching.

A SD boat with power to plane is not an efficient planing boat as only part of the hull is generating lift. BUT, it does provide better seakeeping as the fron portion of the boat is not providing lift and is more likely to through the weather instead of over it. Yes, on smaller boats this will provide a wetter ride. But it is still a better ride and does not pound.

Interstingly, a planing boat will bank into a tun on plane. An SD boat remains flat in a turn on plane because there is usually no deadrise. They need as much lift as they can get because the sacrificed it in the forward portions. Most planing boats have deadrise that allows a boat to bank into a turn.

The most efficent best performing planing hull is a completely flat hull. Obviously you suffer comfort and seakeeping so if you must:

You have a spectrum. A sailboat style hull on one end. And a flat bottom boat on the other. Seakeeping ability(and efficiency) on the FD end. And performance(speed) on the other. Put a slider on that spectrum and slide it up and down that spectrum and design your hull based on your personal criteria. Move it one way, potential speed increases(on plane) but seakeeping ability decreases. Slide it the other, and the reverse happens.

As an aside, the big sportfish boats have so much deadrise it is hard to call them a planing hull. the only reason they is that they are generating lift throughout most of the wetted surface. But they require tremendous amounts of power to achieve their desired speeds. they sacrifice planing performance for seakeeping ability by deep deadrise.

That's all I got.

 

[Art]

Professor Baker... That's A Lot!

 

[rsn48]

Here's a good article: https://improvesailing.com/guides/s...is also called the,up the boat using buoyancy.

 

[Slowmo]

As another poster (correctly) pointed out, SD hulls are planning hulls. As another pointed out in order to plane the hull must have a straight run from midships aft to function as a lifting surface, unlike the FD which has a no straight run. Chines can be hard or soft in either case. The fact that the vast majority of 'trawlers' are SD hulls, mostly with hard chines is to provide speed flexibility beyond what is possible with FD. The broad square shape without much taper to the transom (which you do have in FD) provide lots of accommodation room.

Clearly FD is more efficient, a soft bilged FD can cruise very economically at hull speed, but don't try to go much faster. Crossing an ocean isn't a design criteria for most boats. Frankly I were to cross an ocean I'd choose a full keel sailboat...go by Boeing.

 

[rslifkin]

I very much agree with Baker's assessment. At least with my boat, there's still roll in a beam sea up on plane, but it's a very tightly damped roll of somewhat lower magnitude rather than sloppy roll with momentum behind it of a boat going slowly. So the constant back and forth rolling changes to more of a "tilt, pause, tilt back, pause, tilt" cycle.

On the following sea thing, I agree that running over the tops is a fun ride. In a really fast boat, it's almost like a head sea. But in a slower planing boat, it's just a cycle of "slow down and climb, mush through the top, accelerate and surf down". In a good hull, it's all done with just a very light touch on the steering.

At lower speeds in a following sea, the faster planing hulls start to suffer from small rudders, while the slower ones tend to have a deeper forefoot, so you either have to keep the bow up high enough (if going fast enough), or you'll start to get pushed around like an SD hull due to the forefoot digging in and making it easier to push the transom around. Unlike an SD, it won't dig in and broach quite so easily unless the rudders are tiny, as the planing bow tends to be a bit more full and buoyant, so even with a deep forefoot, it won't stick into the wave as hard.

Here are a couple of pictures of a slower (high teens planing cruise) planing hull (my boat). My hull is pretty simple and old school in design (10* transom deadrise, no lifting strakes, giant trim tabs, has a small keel, chine flats only carried about half-way back before becoming simple hard chines with no flat). The forward and sections of my hull are pretty rounded compared to a lot of planing hulls. The aft side photo shows the lack of chine flats and the rudder size (they're decent size, but fairly low aspect).

 

[Baker]

The fact that the vast majority of 'trawlers' are SD hulls, mostly with hard chines is to provide speed flexibility beyond what is possible with FD. .

I will mostly disagree here. I will ask a rhetorical question to clarify. If this were true, why are the vast majority of SD trawler power so modestly as to never be able to achieve planing speeds??? And I’ll answer my rhetorical question. The reason is initial or static stability. These boats are more stable at rest or in mild to moderate conditions due to buoyancy being carried all the way out to the edge of the hull. To put it simply, their initial tendency is to resist roll....to a point. Once that point is exceeded that advantage goes away and becomes a disadvantage as the roll has a tendency to snap back due to all of that aforementioned buoyancy out at the edges. A FD hull has none of that. Therefor it’s initial roll stability is less. But it never snaps back at you for the same reason an SD hull does.

 

[rslifkin]

I will mostly disagree here. I will ask a rhetorical question to clarify. If this were true, why are the vast majority of SD trawler power so modestly as to never be able to achieve planing speeds??? And I’ll answer my rhetorical question. The reason is initial or static stability. These boats are more stable at rest or in mild to moderate conditions due to buoyancy being carried all the way out to the edge of the hull. To put it simply, their initial tendency is to resist roll....to a point. Once that point is exceeded that advantage goes away and becomes a disadvantage as the roll has a tendency to snap back due to all of that aforementioned buoyancy out at the edges. A FD hull has none of that. Therefor it’s initial roll stability is less. But it never snaps back at you for the same reason an SD hull does.

Interestingly, not all planing and SD hulls have the same snap back. When I compare how my boat rolls vs others around, my boat rolls more like the sailboats than the other planing powerboats. It's only a 3 second roll period, so a little faster than most of the sailboats (and a little snappier at the ends), but it takes just as little to get the roll started. Most of the other planing hulls with big chine flats, lifting strakes, etc. roll far less, but are far snappier once something does get them rolling. With my lack of big chine flats, etc. it has to roll further before things stiffen up a lot, as the simple hard chines provide some roll resistance, but not nearly as much as chine flats would.

 

[Art]

Another good, descriptive article regarding D, SD and P hull designs [this is a link off an article already linked in previous post #36]

https://www.soundingsonline.com/boats/how-different-hull-types-react-in-rough-water

This is aft bottom view to my planing hull Tollycraft.

 

[Nomad Willy]

As Baker points out there are huge differences within the SD range.
Low speed SD boats are mostly like a FD boat and for example won’t bank in a turn. Faster SD boats will and probably most mid-range SD will bank. My favorite boat hull is one in this category .. the slowest boats that bank in a turn.

Boats that don’t bank like my Willard are, at any speed they are capable of, just kinda wallowing around in the water. Like a helicopter .. whereas an airplane that banks in a turn is truly flying. Give me a boat that flies in this respect. This analogy isn’t perfect as helicopters bank. But only w some fwd speed on.

OK OK I don’t have a boat as described above. But I don’t own my favorite car either.