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.
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</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.
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-- Edited by Marin at 16:11, 2008-11-23

"Slightly reduced economy" - hmmm, not so sure I'll buy into that statement. Speed up 50% and the fuel consumption will most likely double.

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

El Sea,
1976' Thompson Trawler
Suite 44

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?

" 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!

FF

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?

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?

Click to expand...

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.

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....
***

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.

Click to expand...

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

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???

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.

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

"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".

FF

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.

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.

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.

Click to expand...

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

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.

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.

Libertarian wrote:

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.

Click to expand...

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.

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?

FF, can you find a set at one of your flea markets? Where would you go to snag a used set?

They sometimes show up at flea markets , but the boards are better.

I purchased a small set ,with SS poles from RY on the PM board , but I have to move the dry exhaust to get them on C/L .

Found telescoping SS pipe , 10ft lower for 12ft upper and will mount them "in time".

There out there, keep looking.

AS noted above EGO and the add dept are the source of all these overpowered "trawlers".
Throw in a huge lack of understanding about the costs of this compromise and you get 35 -45 ft "trawlers" with 200-300+HP.

FF

-- Edited by FF at 05:09, 2008-11-27

nomadwilly wrote:
How fast would your GB go Marin with 4hp per ton?

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Well, in our case that would be about 52 horsepower.* I suspect that at our normal cruise rpm of about 1600, that's not much lower than what our FL120s are putting out.* Of course we have two of them so that makes something of a difference.* But I would suspect that a single engine GB36 being driven by 52 horsepower would reach or be just below hull speed.* Somewhere between 7 and 8 knots.* We cruise at about 7 knots on one engine at 1500 rpm unless the shaft is locked off (which we have to do if we're going to run more than about 10 minutes on one engine).

Marin wrote:
nomadwilly wrote:
How fast would your GB go Marin with 4hp per ton?

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* I suspect that at our normal cruise rpm of about 1600, that's not much lower than what our FL120s are putting out.(Marin)
Marin,
Aren't you eating turkey? What does that mean " I suspect .. putting out"? Are you saying your engines are making 52 hp at 1600 rpm? If so how do you know that? Do you have BW gears and do they suffer from free wheeling?

Eric Henning

Sloboat,

Very good copy from Krogen. The rest of you should read this and note that the semi-displacement in the example burns over ( at a higher speed though )*4 times as much fuel. I know most of you don't agree but I think the SD hull gererally burns about twice the fuel at the speed of a FD boat at typical cruise. Eric, I couldn't pull up the Shannon 38 site. Could be my favorite boat and I haven't seen anything about them for a long time. I'm very glad to see they are alive and hopefully well. Thanks for the post.

Eric Henning

nomadwilly wrote:Are you saying your engines are making 52 hp at 1600 rpm? If so how do you know that?


*

Click to expand...

That's what the power curves in the FL120 operating manual indicate.* It's actually more like about 60 or 65 hp.** More than your 4hp/ton but not that much more.

For old farm tractor engines like the Leman- Econo Power marinizations , operated at low speeds I would figure 16 hp /gal of fuel burned would be close., or a bit optomistic.

Most folks interested in economy never run hull speed as a slight reduction pays so well.

Reducing the speed from SL 1.34 down to SL 1.1 will usually pull fuel burn down 50% at little cost in speed.

FF

sloboat wrote:

Libertarian, have you considered adding stabilizers?*

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I have, especially those fancy-schmancy gyroscopic ones.* But $50k is a hefty investment for a boat that might only need stabilization once in a blue moon.* I suppose if we were interested in regular long distance cruising in the ocean or great lakes, it might make more sense.* But we're at the dead end of the Potomac and even if we have the Roamer until we die, >95% of the time this will be a river boat and 95% of that time we'll be living aboard at the dock.

Sloboat,

I GOT THE*SRD 38 this time .. thanks. As to the disp/SD compairison I see the numbers .. had me scrachin my head for awhile but now I think I see the light. I think they wanted to slant the comp so they listed the fuel burn of the FD hull at it's hull speed. FD boats don't cruise at hull speed. Almost all cruise about one knot slower or very close to it. My own Willard*I cruise at 6.15 knots and 7 is hull speed. Nine knots is over hull speed for the Krogen 42 and cruising at " normal " cruise will burn far less fuel. Notice at the end of the next to last paragraph they claim the fuel burn for the K42 is 1.7*gph. The SD boat should shed about one gph from loosing a knot and that should produce more realistic numbers. That is .. a*SD boat should burn about 2 to 2.5 times as much fuel as a FD boat at the FD boats cruise speed.*Ops .. I see I compared a 48 SD to a 42 FD. That should change the numbers to about 1.8 to 2 times as much fuel. Still the FD boat should burn about half as much fuel at a speed/length ratio of about 1 to 1.1.

Eric Henning


-- Edited by nomadwilly at 22:56, 2008-11-28

"The Shannon design (http://www.shannonyachts.com/38faq.html is interesting in that he is making a serious engineering attempt to manage flow over the hull. Along those lines, I have to believe there are many other techniques to improve SD design efficiency that have not been fully developed because of cost. "

The Shannon is a simple but partial take off of the pioneering work by John Atkin.

His concept of reverse deadrise and a "box keel" pioneered the speed of SL 2.5 or so at the same fuel burn of a SL 1.1 snail boat.

Google RESCUE MINOR or Rob White , and in the small launch size , with light construction and a homebrew but efficient engine , over 30nmpg at 12-18K was reported.

Yes 1/2 gph at speed!

We have been working with a NA for a while now to incorperate the Atkin hull with light weight construction (under 8800lbs for the Euro reduced rules) and about 39 ft (just under 10 meters) with the deck beam of 7ft 6 inches and a bwl of about 5.5 ft.

This gives a Needle boat as Geere would call it , and if the computer can be believed incredable low fuel burn at speed.

Think 10K at 1 gph (yes about 20hp) and 18 at 3,5 gph!.

With the slender shape it will be a cruising boat that carries 8 , feeds 6, sleeps 4 (5 in a pinch) and cruises 2.

The scale model tests are yet to be run , to see how far off the computer is.

The box keel is great as the prop is totally protected (similar to the New Jersey Seabright style skiff) and take the ground naturally.
The box keel also equals loading/discharging* with simple pipe rollers , no equippment but a big push to load into a Sea Land box.


-- Edited by FF at 05:49, 2008-11-29

Slowboat,

As I said nobody runs at hull speed. At a knot below much much less*fuel is burned. FF IS a fan of SD. His Navy personel boat IS one.

FF
Is that boat of yours like a Hickman sea sled ?.

Eric Henning

-- Edited by nomadwilly at 22:28, 2008-11-29