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Old 11-23-2008, 03:09 PM   #1
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Why semi-displacement (or semi-planing)?

Given past discussions on the advantages and disadvantages of displacement hulls vs. semi-displacement (I prefer the term semi-planing but it's just semantics), I thought this portion of an article on trawler-yachts in a past edition of Power and Motoryacht magazine might prove interesting. It contains what I think is a simple but pretty good explanation of why most trawler-yachts use a semi-planing hull as opposed to a full displacement hull.* Sorry about the type size in the article.* I made it larger in the "wrte a post" window but when I submit it it always comes out small.


<table style="border-collapse:collapse;border-left-width:0pt;border-right-width:0pt;border-bottom-width:0pt;" border="1" cellpadding="0" cellspacing="0" width="100%"><tbody><tr><td style="border-style:solid none none;border-width:1px medium medium;" valign="top" width="34%"> Up, Up, And Away
The popularity of trawler yachts is largely related to their ability to carry a fair load under a wide variety of sea conditions. And that is a direct result of hull form. All hulls are in a displacement mode at rest. Planing hulls attain speed by being able to get up out of the water to run literally on its surface rather than through it. They do so via a combination of horsepower, comparative light weight, and hull lines which create lift.
Lift both requires and contributes to hydrodynamic (moving) stability, and is usually dependent on speed. This often put planning hulls in the Catch-22 situation of needing to go fast to have proper stability. When sea conditions arent conducive to speed, the boat becomes less stable and thus less comfortable and less efficient. Even under ideal conditions, a planning hulls need for lightness runs counter to carrying the amount of gear, fuel and supplies dictated by the needs of long-range cruising.
At the risk of slightly over-simplifying, let me say that displacement hulls essentially rely on hydrostatic (not moving) stability under way. This means they cant move as fast as a planning hull since they are moving through the water, rather than over it but can often maintain a steady speed even under sea conditions which would require planning hulls to slow down anyway.
And since displacement hulls maintain stability and efficiency while moving slowly (neither of which can usually by said for planning hulls), they are ideal for carrying the load of fuel and stores you need to travel long distances. Add the fact that it usually takes relatively little horse power to achieve displacement hulls can go a long way on little fuel.
The low horsepower requirement for displacement speeds is a big reason many trawlers and trawler yachts are driven by a fairly small single engine.
</td> <td style="border-style:solid none none;border-width:1px medium medium;" width="1%">
</td> <td style="border-style:solid none none;border-width:1px medium medium;" valign="top" width="33%"> Indeed, the only reason for using twin screws in most trawler yachts is to gain close quarters maneuverability and the supposed safety factor of a back-up engine.
Even with a full cruising payload of fuel, stores, gear and guests, most trawler yachts these days can be light enough that, with a slight modification of underbody form and a modest increase in horsepower, they can move from pure displacement in the semi-displacement mode in which they lift part way up, reducing the amount of hull moving through the water. This is, in many ways, the best of both worlds: nearly the same seakeeping qualities, slightly faster speeds, and only slightly reduced economy. For many its an ideal combination for pleasure cruising.
</td></tr></tbody></table>



-- Edited by Marin at 16:11, 2008-11-23
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Old 11-23-2008, 03:44 PM   #2
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RE: Why semi-displacement (or semi-planing)?

"Slightly reduced economy" - hmmm, not so sure I'll buy into that statement. Speed up 50% and the fuel consumption will most likely double.
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Old 11-23-2008, 04:05 PM   #3
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RE: Why semi-displacement (or semi-planing)?

British Seagull had a write up regarding the fuel consumption required to exceed hull speed.*If I can find it I'll post it.

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Old 11-23-2008, 08:05 PM   #4
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RE: Why semi-displacement (or semi-planing)?

Hiya,
*** Good post Mr. Marin.* The article states many trawlers are driven by a fairly small engine.* A lot of the boats I've seen under 40' or so are powered by a single Lehman or Perkins in the 120HP range and they burn about 2GPH.* Now 2GPH equates to about 40HP.* I've often wondered what the extra 80HP is there for.* Surely you don't need all of the extra HP to deal with wind, weather , or currents.* Couldn't these boats be run with say.....60HP engines thus increasing their economy even more and still have enough reserve?
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Old 11-24-2008, 04:54 AM   #5
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RE: Why semi-displacement (or semi-planing)?

" Now 2GPH equates to about 40HP. I've often wondered what the extra 80HP is there for."

Its only "there" for very short term use.

These are taxicab or tractor engines and the rating is a peak, like your 400hp Buick ,, not a realistic rating for 24/7 trawler needs.

That is GOOD! because the taxi/tractor is built for lots of ideling , and light loads , so takes the 2gph fuel burn in stride.

No loss of compression, wet stacking or lube oil dilution , from under loading thank goodness!

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Old 11-24-2008, 06:28 AM   #6
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RE: Why semi-displacement (or semi-planing)?

Hiya,
** Thanks FF, but how much extra HP is too much extra HP?* Hypotheticaly, if I have a 34' or 36' boat with a single Lehman (120HP) and had reason to re-power, what is the smallest replacement engine I could realistically put in it's place?* Hull speed on one of these would be in the 6.5kn to 7.5kn range.** Similarily if I had the same vessel with twin Lehmans*(120HP) how much smaller could I go again?*
** Along the same lines, take two vessels.* One 36'X14' and the second 36'X10' They both would have the same theoretical hull speed but the skinnier boat would be more efficient.* If re-powering either, would not the boat with the 10' beam be able to install a smaller engine?
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Old 11-24-2008, 08:17 AM   #7
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RE: Why semi-displacement (or semi-planing)?

Quote:
RT Firefly wrote:

Hiya,
** Thanks FF, but how much extra HP is too much extra HP?* Hypotheticaly, if I have a 34' or 36' boat with a single Lehman (120HP) and had reason to re-power, what is the smallest replacement engine I could realistically put in it's place?* Hull speed on one of these would be in the 6.5kn to 7.5kn range.** Similarily if I had the same vessel with twin Lehmans*(120HP) how much smaller could I go again?*
** Along the same lines, take two vessels.* One 36'X14' and the second 36'X10' They both would have the same theoretical hull speed but the skinnier boat would be more efficient.* If re-powering either, would not the boat with the 10' beam be able to install a smaller engine?
Not to bust your balls, RT, and in keeping with Walt's other post reference trawler forums in general, this is off the topic and shuld be discussed in another thread.* This is one reason why threads on this forum are very few but go for many many pages....it is because they go off on so many tangents.* SO maybe you oughtta start a thread discussing the "proper power" for a particular type/size of boat.
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Old 11-24-2008, 09:52 AM   #8
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RE: Why semi-displacement (or semi-planing)?

Hiya,
** No balls busted Mr. Baker but doesn't power plant choice flow directly from hull form and efficiency*to the best mating of the two?* I have had both semi-displacement and displacement boats and at the speed I travel, have not found very much difference in performance/economy between the 2.* Now, if a semi-displacement boat has enough power, it should be able to plane.* Albeit at the expense of more fuel consumed.* I HAVE found that slowing down a bit (1 or 2 knots) my fuel burn has dropped signifigantly but I'm sure this would be true whatever the hull form.
** Why have the marine architects of the world designed boats that are NOT displacament to be operated at displcaemnt speeds either through owner choice or HP limitations?* Do they ride any better?
* I will start a new thread....
***
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Old 11-24-2008, 11:18 AM   #9
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RE: Why semi-displacement (or semi-planing)?

Quote:
krogenguy wrote:

"Slightly reduced economy" - hmmm, not so sure I'll buy into that statement. Speed up 50% and the fuel consumption will most likely double.
I don't know when the article I posted was written, so I don't' know the context in which the author said "slightlyi reduced economy."* I know in older generations of GBs, the ones powered with FL120s and the like, the typical cruise speeds are only a knot or two above the boat's hull speed.* The boats can do this without much of a fuel consumption penalty because of the semi-planing nature of the hull.* Now on newer GBs where the boats have things like one or two 450hp engines, the same hull can be shoved along in a semi-planing state at 14 or 15 knots, but the fuel consumption is pretty staggering.

But in its original concept, the GB benefited from the semi-planing hull in that fairly low-powered engines like the FL120 could cruise them comfortably at a relatively low fuel consumption at speeds a little bit faster than the hull's displacement speed.


-- Edited by Marin at 12:20, 2008-11-24
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Old 11-24-2008, 06:06 PM   #10
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RE: Why semi-displacement (or semi-planing)?

In answering your question RTF and the OP's question, I think there are 2 main reasons boat builders use semi planing hulls. One of them is space. A nice square wide stern offers a lot more space than a rounded turned in bilge. One could then argue, what about Krogen? I think Krogen sacrafices draft in order to offer space. IOW, the bilge does not turn until deck level is met and the beam is carried thru the boat with the bilge turning after the floor.

The other reason is stability. Someone used "hydrostatic" stability above. I will just call it static stability. Static stability is the boats INITIAL resistance to being upset. The fact that you have a wide boat with hard chines means that it will resist roll.....INITIALLY!!! I will use "dynamic" stability also...this is the tendency of the boat AFTER the boat has been upset. A wide square assed boat with hard chines has a tendency to "snap back" once a roll moment has occured. At some point it will dampen out but if the stimulus is continuous, that snapping will continue. A full displacement boat likely will not have as much "static" stability but will have more dynamic stability since it does not have the bouyancy moment far away from the longitudinal axis of the boat.

TO summarize, a common semi displacement boat will ride more comfortably and offer more space in "milder" conditions. A full displacement boat will offer a more "predictable"(rhythmic) motion when in heavy conditions. Personally, I think efficiency is a wash.

Am I making sense???
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Old 11-24-2008, 06:22 PM   #11
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RE: Why semi-displacement (or semi-planing)?

I think the efficiency might be a wash if the semi-planing hulled*boat is driven at speeds more or less the same as the displacement-hulled boat.* But if the semi-planing boat is driven at actual semi-planing speeds--- say twelve to fifteen knots--- then its efficiency will go the direction of today's economy.

I think the point made in the article, that the semi-planing hull offers "nearly" the same stability as a displacement hull in rougher water but allows the hull to be driven faster than a displacement hull in water conditions that allow this, is the main advantage.* A true planing hull, operated at lower speeds, will really wallow around in rougher water because it's not developing the hydrodynamic forces needed to lift the boat which in turn provides a lot of the hull's*stability.

The typical trawler-yacht's semi-planing hull is definitely a compromise, so as such it's not as efficient in moving*through the water*as a displacement hull*or over the water as a planing hull.* But in practice it offers characteristics and advantages that*have*more in common with a displacement hull than a true planing hull.
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Old 11-24-2008, 09:18 PM   #12
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RE: Why semi-displacement (or semi-planing)?

I will disagree in the fact that a semi hull is not as stable as a displacement hull(at displacement speeds)*due to that snapping action in a short beam sea. Now put the semi planing hull on "plane" and it is a more stable boat and the absolute main reason the hullform was "invented" in the first place. My point above was in regards to operating the baot at displacement speeds. You operate a semiplaing boat at planing speeds and you have the most stable boat of the bunch. It does not lift too much to cause pounding. ANd it gets stability from the lift in the aft sections of the hull. Obviously, if you run it hard it is not as efficient.

-- Edited by Baker at 22:18, 2008-11-24
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Old 11-25-2008, 04:28 AM   #13
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RE: Why semi-displacement (or semi-planing)?

"I think efficiency is a wash."

Dragging a huge transom thru the water costs.

My guestimate is about 15% more fuel burn at SL 1.1 than a displacement vessel.

For folks that need the dockside stability for boarding lubbers that are shocked a boat MOVES! , wide aft is probably a reasonable trade off .

For the folks that do cruise , the better sea motion and fuel efficency would probably win.

Unfortuniatly I believe the choice for most mfg is made by the advertising dept , not the NA.

Get a Brit boat mag , with higher use and fewer thousands of miles of inshore waters , the boats are far more rational, for sea use, although they still build "gin palaces".

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Old 11-25-2008, 07:31 AM   #14
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RE: Why semi-displacement (or semi-planing)?

FF, what if the beam of the disp boat was equal to the beam of the semi boat? Krogen, Nordhavn all have quite beamy rear ends although they are full displacement.
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Old 11-25-2008, 07:38 AM   #15
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RE: Why semi-displacement (or semi-planing)?

My Krogen narrows in quite a bit at the stern. While not a canoe stern, it certainly tapers back at the waterline. Take a look.
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Old 11-25-2008, 12:11 PM   #16
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RE: Why semi-displacement (or semi-planing)?

Quote:
Baker wrote:

I will disagree in the fact that a semi hull is not as stable as a displacement hull(at displacement speeds)*due to that snapping action in a short beam sea.
I think the operative word is "nearly."* The point made in the article seems to be that the semi-planing hull when driven at displacement or near-displacement speeds is "nearly" as stable as a full-displacement hull.* "Nearly" is obviously open to interpretation, and it will depend on the design of the semi-planing hull.* The hull Smith designed for American Marine's Spray that became the hull form for the GB line that followed has a very deep forefoot, a very deep keel (for a powerboat) but the flat-ish afterbody and hard chines that give it the ability to be driven at semi-planing speeds. (To get the boat onto a full plane you'd need to strap a GE-90 to it.)*

This configuration gives the hull stability at slow speeds*that's "nearly" as good as a displacement boat.**The fact the GB hull*has the shallower roll with the "snap back" at the end is not, I think, an indication of signignificantly*less stability,*it's just the*characteristic of the roll.** Now there may be other semi-planing hull forms like Bayliners and whatnot, that are not nearly*as stable at slow speeds as a GB and are more stable at higher speeds than a GB.

Of course, it all depends on how you define "stable" too.* If you mean from a safety standpoint, then the GB hull is "nearly" as stable as a displacement hull.* If you mean from a ride comfort point of view, then I would agree that the GB hull does not give as gentle a ride as a displacement hull (if you don't mind longer albeit slower rolls).* Some people prefer the shorter but snappier roll of a GB-type hull to the "longer, sickening" roll you get with a full displacement boat.* Other people like it the other way round.


-- Edited by Marin at 13:13, 2008-11-25
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Old 11-26-2008, 04:16 AM   #17
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RE: Why semi-displacement (or semi-planing)?

FF," what if the beam of the disp boat was equal to the beam of the semi boat?"

A simple "rule of thumb" for 40 50 ft boats is the stern in Ft is close to the hull design speed.

Our Uniflite launch was created for .17c Navy fuel , so has a 10ft transom and a full load design run speed of 10 -11K.

Running at 1/2 the expected weight load does reduce the rear drag at 1/2 the design speed , so low speed economy isn't too bad , just 2.5 gph at 7K.

A true displacement hull probably would have a 7ft transom , but would probably be 300% heavier so would get about the same fuel burn.

A real price +200% or + 300% comes when one attempts to push the real displacement hull to semi plaining speeds SL 2.3- 2.5 or so.
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Old 11-26-2008, 06:18 AM   #18
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RE: Why semi-displacement (or semi-planing)?

This question of stability, hull form, and how much power is enough is something I've been pondering for a while. The 46' Roamer we're revamping is a pure planing hull with almost no keel. and a very flat bottom. But because of the way we tend to operate our Connie, it just didn't make sense to repower with 300hp x 2, though that would certainly get her up on plane. The features we've installed leave me wondering how this planing hull will ultimately perform at the displacement speeds we run at--

* Lehman 120s w/TD502 gears (1250# or so each) vs 534SeaMasters (1850# ea) or the other powerplant that came on these boats, DD 871s w/Allisons (3500# ea)--The Lehmans offer huge weight and fuel consumption savings, but with 1/2 the speed potential and ??? effect on stability in all cases.
* One centerline 420 gallon aluminum tank vs two 200 gallon steel fuel tanks outboard in the aft stateroom under the Ozzie and Harriet bunks--The one aluminum tank weighs less than one of the steel tanks, and since it's centerline vs outboard, I have to wonder about roll stability.
* 12.5kw Panda genset (350#) vs 6.5kw Kohler gasser (1200#)--again, that's pulling a lot of outboard weight out of the boat, which I suspect will have notable influences on economy and roll stability.
* Then there are the forward fuel tanks, 128gal forward of the ER under the galley and 98gal in the ER on the starboard side to be used mostly for fuel storage and to counterbalance the genset & port-mounted house battery bank.

Unfortunately, I have no experience with this boat on the water in its original configuration. It will be interesting, to say the least, to finally get it out there and see how this lightweight, planing hull performs at consistently low speeds.
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Old 11-26-2008, 10:53 AM   #19
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RE: Why semi-displacement (or semi-planing)?

Quote:
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It will be interesting, to say the least, to finally get it out there and see how this lightweight, planing hull performs at consistently low speeds.
While this does not mean your boat will behave this way, from what I have observed planing boats do in choppy or rough water, from our own 17' Arima fishing boat to large, planing-hull cruisers and sportfishermen, when they are at rest or running slowly they rock and roll something fierce with a very jerky motion.
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Old 11-26-2008, 10:01 PM   #20
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RE: Why semi-displacement (or semi-planing)?

Marin,

I think I'm going to answer your question .. at least I'm going to try .. because no one else has.
People want a boat that is viewed as a " Trawler ". It has masculine implications ( people ( guys)) that have Trawlers have more hair on thier chest. People want a boat with some range but don't want to be locked into* hull speed .. viewed as dreadfully slow. With a semi-planing hull it is possible to go faster with enough power. Power is cheap. Not many people go very far so fuel burn is even less important. People embrace the idea of going slow* .. IF .. they can go faster. This all sorta explains people buying new boats but as the boats *get older ( and cheaper ) people buy the boats that may not be as able to pay for the fuel. The purchase of new boats is what drives the market and determines what type of boats come to pass. So .. people want large roomy boats that are slow ( in a vouge way ) and can go faster and of course people love stability. So .. the NT GB type emerges. A planing hull thats wide ( BIG ) heavy and with a large keel ( that seems to make people think it's seaworthy ). Install an engine thats at least twice as powerful as a displacement hull would need and there you have it. Thats my take on why most " Trawlers " are semi- displacement. In my opnion a semi-displacement hull 35' long will gracefully go 12 to 14 knots but not 25 or 30. A good example is the " Handy Billy " skiffs. Look up " Southport Marine ". When the fuel prices go up again NT may offer a full displacement version of the 26 ( it's back ) and 32' boat. Unlike many that have said there's not much difference ( in efficency ) between semi-displacement and full displacement boats try this on .. a good FD boat ( Nordhavn 46 ) has 4hp per ton of displacement. How fast would your GB go Marin with 4hp per ton?
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