Semi-Displacement and Planing Hulls, Question?

[Alistairr]

I know the 3 different types of hulls, Displacement, Semi-displacement and Planing hulls.

I know that Displacement hulls go through the water, and Planing hulls go over the top of the water, so what the best description for a Semi-Displacement hull???

Also what the advantages/disadvantages of a Semi-Displacement hull over a Planing hull??

Does the hull and the engine of a Semi-displacement take more abuse? Does the engine need to work harder as its got more boat in the water than a planing boat? Does that also mean higher fuel consuption then?

Can someone please explain all to me.

Many thanks,

Al.

 

[lanason]

In the absence of BS, I'll post a good google link

Hull types

 

[hlb]

Well, it's said, that a Nelson can do a comfortable 18 knots through a F 8. Most ships cant do that..... It will use alot of fuel. The Nelson.

Having said that, good planing boats are not that far behind and use less fuel at speed. But not many want to be out in a F8 anyway.

 

[Alistairr]

So does a Semi-displacement cope with rough conditions better than planing then?

Al.

 

[hlb]

Well it's said that they do. But it's more complicated than that. A moddern planing boat is better with a following sea and will just zoom along in a f7, maybe 8. But not good in a head sea. Then maybe have to zig zag or down to maybe 6 knots, even that could be a problem, so back to zig zag. It depends much on the sea. I've happily planed over waves higher than the fly bridge. Just hung on to the rails, letting the auto pilot do the work. Going over at crazy angles, but smooth. Over 20 knots.

But a semi, or displacement boat, just goes straight through the waves, loads of water and spray over the boat. It lifts from the middle, rather than with planing lifting from the bow.

 

[KCook]

Err, I could be wrong .... but I believe the semi-displacement types are usually heavier than a planing boat the same size. Whether heavier is better or worse is a whole other argument. But it is a difference to consider.

Kelly

 

[Renegade_Master]

the jury is out on that one IMHO

 

[oldgit]

More of a comment than an answer.All of the pilot boats round here are of the Nelson type of boat but appear to be blooming fast anyway but presumably also need to be very stable in any weather in order to get the pilots on and off the ship,Not much choice about staying in harbour during inclement weather.Our local lifeboat also appears to be a semi displacement craft,so if you want all weather capabilty that seems to settle the case.Cannot think of any example of marine authorities using fast planeing craft for serious all weather work.
A certain much vaunted fast planing craft used frequently by the police is nearly always only to be found doing light duties well upriver of any serious wavelets.

 

[Sultans_Kapitein]

quoting a couple of snippets from Bruce Roberts;
[ QUOTE ]
Generally speaking for vessels with 30 ft 9.1 M to 60 ft / 18.3 M waterline length, you should only consider Semi-displacement hulls if your speed requirements do not exceed 12 to 18 knots.

[/ QUOTE ]

and;
[ QUOTE ]
this is the type I would personally choose when planning to undertake extended cruising, that is cruising that regularly involves cruising distances of over 100 miles from home base.

[/ QUOTE ]
For my sins Sultan is a semi-dsiplacement hull so sometime I should be able to know the worth of it.

 

[Gludy]

I have had both types of hull.

1. Planing boat is normally better in a following sea but a semi-displacement with stabilsers also does that very well. RTge SD in a head sea is much better.

2. The SD is a more comfortable ride because it cuts through the water rather than bouncing on top like a stone. This was a major factor for us in our choice. SWMBO now enjoys getting there.

3. The SD hull can habdle low speeds very well - my boat gives me well over 2 mpg at displacement speeds and it is comfortable at those speeds. A planing boat relies on speed for its stability and is not comfortable at low speeds in any sort of sea.

4. An SD hull at the higher crusing speeds uses more fuel. I get the same on the SD hull at 19 knost as I doid on my same size planing hull at 25 knots - so at high speed the planiong hull is about 33% more fuel efficient.

5. An SD boat is always a wetter boayt because it cuts through rather than over the top.

All commercial boats like RNLI and Pilot boats are SD boats because they can handle weather better.

We changed over because we wanted long dustance economic crusing in comfort. If we had one just weekends out as far as possible in fine weather then a planing boat would have been better.

The stablisers on SD hulls really have changed a lot basically roll is a thing of the past.

SD hulls go from almost displacment to almost planing - there are wide variations.

That is my two penny worth. I shall now put my steel helmet on.

 

[Alistairr]

Thanks for all the answer.

I think i understand it a bit more now.

SD crafts cut throught the water and cope with some waves better as they usually come over the top, and Planing is more trying to ride the top of the waves.

Question was cause i understand the Small Merry Fisher boat i was talking about on anothe thread, i believe is a Semi-displacement hull, it being a much smaller boat that Nelson or Traders, does the same still apply.

I've only ever had 3 planing boat, vwould i fell a difference, will i enjoy the difference, and do i need to learn to drive differently?

Cheers All.

Al.

 

[Gludy]

The same will apply.

Its a personal thing - hard to say if you will enjoy it more or less. Try it first.

I would ask how far I go for how long? Is it always weekends? Do I want to go slower and have a more comforstble journey?

 

[Hurricane]

Here's the view from a planing boat in a F8 with a big following sea.

Some of the cushions are still on the seat - so not all that bad!

 

[Nautical]

Hi Al

Not such an easy question to answer these days, ten years ago the answer was simple, want the best heavy weather hull no question it would be your traditional round bilge semi displacement, aka, Nelson, Aquastar or commercially... Tyne class lifeboat.

Now however the line is now somewhat blurred, developments in new design, tank testing and more importantly CAD/CAM where everything can be designed and tested before even a mould is produced have created some fabulous all weather planing hulls such a the Botnia Targa range from Scandinavia. In really bad conditions I would choose the the Targa 35 over some semi displacements such as the older generation of Nelsons which while fantastic up wind were a pig down the weather and beam on rolled like a barrel.

Now we have boats coming along which are hybrids such as fast patrol boats and military stuff and more recently the Interceptor range from Safehaven Marine. One could suggest that there should now be fourth catagory to go along with displacement, SD and Planing.......'Modified Planing'. These hulls are more planing than SD but have certain features carried over from SD like a small keel and reverse flare to the after quarters additionally reverse chines to literally 'grab' the water as she turns and buffets in big seas.

A brilliant example is the Safehaven 42, generally a deep V planing hull with very fine sections forward which from memory is about 50 degrees which literally slice through the water like a knife, down side of course is turn around and go with the weather and with such a fine entry she will tend to steer by the bows in a following sea as she digs into the back of a big'un', they have corrected this tendancy by adding very full and flared upper bow sections so that past the waterline point the bows fill out dramatically giving tremendous bouyancy and better down wind helm. The midship V section is also very deep around 25 degrees which means if you do fling yourself off a big one she won't slam like a normal planing hull.

Probably one of the driving forces in new Modified Planing hulls is the cost of fuel, SD hulls take alot more power to attain the same speed and hence much bigger fuel burn whereas 'MP' requires about 25% less power to attain mid 20 knts. Weight also used to be a factor, basically build it like a tank and whop in monster motors and bobs your uncle most SD hulls built with enough weight and power could literally bash the way through the water, thats not acceptable anymore so something like the Safehaven 42 weighs in with twin derated D12's about 16/17 tons whereas a commercially built 42 pilot SD from say 15 years ago was around 20 tons.

So in answer to your question, SD has very specific qualities which you can value depending on what you are doing but quite a few down sides also, a MP hull is probably where it is at for the future along with new propulsion systems be they IPS, Zeus or Yellowfin. A MP hull of say 45' with twin IPS600 could probably achieve an easy 30 knots yet burn 40% less fuel of a traditional SD at 25 knts never mind be much much quieter and less vibration.

Just looked up Savehaven and this should show you what a MP hull can do........... SH 42 sea trials

Have to say also that current crop of well designed production planing hulls from the leisure market can if correctly trimmed and driven attain quite respectable speeds in some fairly nasty weather. We did an Irish Sea crossing in the last couple of weeks in some fairly horible seas and although not by choice would one want to do it was actually reasonably comfortable, trick being to keep the hull on the plane at the lowest possible speed which on an out drive boat means trimming the legs right in and giving it full tab, we were fortunate in that the boat we were on the manufacturer fits jocky model tabs so you can literally force the hull onto the plane at around 16 knts and still attain a fairly level angle of trim to cut the mustard in a head sea.

Well there's an hour wasted! /forums/images/graemlins/grin.gif I await the trads to pummel the bejesus out of me now /forums/images/graemlins/frown.gif

 

[Nautical]

Hi, if I remember your post on the trip you had a video too, could we see it again please.

Thanks

 

[DAKA]

[ QUOTE ]

All commercial boats like RNLI and Pilot boats are SD boats because they can handle weather better.



[/ QUOTE ]

Just for completeness

The Humber Pilots got sacked a few years back and ABP got one fast 40 knot planning boat to replace the fleet.

Known locally as the yellow peril this thing would spend most of its time out of the water (either bouncing off waves or on the hard under repair).

There was a female in charge of it sometimes which caused most fear among local mariners.
She smacked one into a jetty, one caught fire and one then had to be put on loan as they ran out of boats for her to prang.

If you imagine a badly behaved jet ski, paint it Yellow and expand it to 40ft, thats what we had to put up with !

The H&S incident book was nearly full so it had to go.

Pleased to report the Humber is now populated with SD pilots who are not much better mannered but at least they only do 18 knts. /forums/images/graemlins/smile.gif

 

[Bajansailor]

Most definitely not a wasted Nautical hour!
That is a most interesting and informative dissertation on hull types you have posted, thank you!

If I may add my tuppence worth - re the RNLI offshore boats, vessels like the Tyne, the Waveney and the Arun would probably be considered as semi-displacement, and are wonderful sea boats - the Arun is quite legendary in fact.

The 'new' generation of fast offshore boats (the Severn, Trent and Tamar) are faster (design speed is 25 knots) hence I guess they would be considered to be planing boats. Although one could say that they are still more like semi-displacement craft re hull shape, and re how they have tunnels aft for the props.

They could be regarded as 'blood and thunder' machines - the Severn (17m) has twin Cat 1,200 hp engines, while the Trent (14m, same hull shape but scaled down a bit) has twin MAN 800 hp motors.

And their seakeeping abilities have been proven to be as legendary as the Arun and other predecessors - they are quite capable of sustaining 25 knots into a F 7 or 8, with accompanying nasty sea conditions. In fact they have got to the stage where the boats can take a lot more than the crew can!

It will be interesting to see what future developments happen, especially with the introduction of podded propulsors like the IPS units from Volvo. Exciting development times ahead!

 

[whisper]

That's because the plod always get sea-sick - load of wimps /forums/images/graemlins/wink.gif

 

[Gludy]

[ QUOTE ]
The 'new' generation of fast offshore boats (the Severn, Trent and Tamar) are faster (design speed is 25 knots) hence I guess they would be considered to be planing boats.

[/ QUOTE ]

Sorry - cannot agree.
A planing boat planes and relies for its stability on speed. The lifeboats are SD hulls that have the power to go fast - my boat does 25 knots but it is still an SD hull. As full speed, it uses more fuel at that speed than the same size/speed planing hull.

 

[Nautical]

Thanks, one does try! /forums/images/graemlins/grin.gif

Wholeheartly agree with the comments re RNLI lifeboat designs.

The Tyne was an excellent sea boat, into the weather she was tremendous but a bit of a handful down the weather on the early models untill they increased the size of the rudders. I can remember a few nasty ones surfing down a whopper, full helm and knowt happening, luckily that monster iron keel helped keep her upright when broached, sort of got to a point almost on her beam ends and then the force of gravity took over and the three tons of keel rolled her back again.

Interestingly the new tamar could almost be discribed as Modified planing with the addition of two big tunnels, certainly the round SD bilge of old has gone on all the new boats, they are hard chine SD verging on planing. The Tamar particularly has a planing hull form and having been lucky enough to have a play she is the dogs thingys much more responsive than that of old and much easier to keep a good track with the weather, partly to do with the joystick helm which is very fast to react, lock to lock in a couple of seconds. Future developments have to be with propulsion though and once in VP's case D9's and D12's are brought on line we should be seeing far more sub 18 metre commercial stuff adopting pod propulsion.

It is one of those strange situations where the leisure market is driving commercial market developments rather than the other way around. Pod propulsion has certainly taken off in the leisure market (although the UK has been very slow to adopt it, nothing different there then!) but simple economics dictate that it has to be the way forward, three years ago the knockers had it dead before it even got off the ground, two years ago, tis a bit of a gimmick, a year ago well maybe it will work for techno heads, now, well yes it is selling in considerable numbers but still has the potential to flop, a year or two from now, oh bu88er it is outselling trads and in five, why didn't I switch to it, now my shafty thing is outdated, smelly, noisey, expensive and drinks for England and dropped like a stone SH.

Now come on guys that has to be worth a pop or two or have I beaten you into submission yet /forums/images/graemlins/grin.gif