Semi-Displacement and Planing Hulls, Question?

Nautical, I would agree with you that podded propulsors do seem to be the way forward - pretty much all of the new cruise ships built nowadays have azimuthing pods - and if they can design systems for the Queen Mary II, then systems for little motor boats should be a doddle in comparison!

Gludy, I see your point - I did also make a comment in the next sentence re how they could be considered to be semi-displacement (or semi-planing? Terminology can be confusing!).
And note Nautical's comment above re the Tamar being modified planing.
 
Fair point, however there is an element of planing hull design, more so on the Tamar, to be true SD there is little hydro dynamic lift, it is the sheer power of the propulsion that literally forces the hull through the water, however over power a SD hull and it starts to become unstable and erratic in handling. With the Tarmar there is a proportion of lift at over 18 knts so one could ligitamately say at 25 knts she is semi planing rather than semi displacement.
 
I would say that the Tamar is an SD hull. It does not plane - the bow lifts a bit.

The Trader I have is basically almost a planin hull with a keel and because of that it cannot plane although the bow does lift a bit. Like a planing boat as regards fuel consumption I might as well do 19 knots as do 12 knots as consumption is very flat between those two.

A Fleming on the other hand is more towards displacement whilst still being called an SD hull. I would be happy to call my hull a semi-planing except for the fact that I think that confuses the issue.

Maybe we are just getting involved in the semantics of the issue. In truth there are planning hulls – displacement hulls and a huge variety of SD hulls.

I recently sea trialed a 72 foot Marlow – an SD hull that had one of those New Zealand wave bulge things sticking out the front – it was doing 23 knots with hardly any change of angle – maybe lifted 4 or 5 degrees. Again that is basically a planning type hull with twin keels and stabilisers. However overall the boat is an SD boat. So I think a planning boat is a planning boat, a displacement boat a displacement boat – SD boats are all those in between and that includes the lifeboats.
 
Well perhaps thats the point I am making with SD there are two ends of the scale, however I think we are past the traditional pigeon hole three forms hence my thoughts on a fourth element of Modified planing or if you are a die hard SD Modified semi displacement, my own view is that planing hulls are becoming so good that the role for true semi displacement has probably met its day, for some such as pilots boats and lifeboats the form will remain in some element. Twenty years ago if you showed a Tamar hull to coxswain he'd probably shoot you on the spot and I can remember when the first ply built Arun came along (think it went to Guernsey) there was an uproar amoungst the die hards, now you if offered a Watson or Solent class to a crew they'd laugh you out of the place.
 
Gludy, I am glad you made this post, because I was about to disagree with the earlier comments.
There are many types of hull design and SD covers almost any form that is not full displacement or planing. That is why it is called SD, it is neither one or the other!
I have been out in pilot boats, Flemings, Traders and each is very,very different. I guess I would rate them in the order I have listed for sea keeping.
 
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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.

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Alistair, I just found your comment above.

With a 115hp engine on the back of an MF 625 at 30 knots, I would think it would be more or less up on the plane /forums/images/graemlins/grin.gif

Mine, with only a 90 hp, does 25/7 knots.

I think this pic shows an MF 625 on the plane.

gp243a.jpg


Def not SD.
 
I would agree with that and the order you, placed them in.

Semi-planing is to me just another form of SD in the multitude of SD's.

Without a keel and stablisers I reckon the Trader would plane - so some would call it semi-planing. Same for a Marlow which even fits a bulge on the front and can manage over 30 knots with the right engines.

I think the balance of SD boats is moving nearer to the planing hull than the displacement hull. Hover they are still SD hulls - adding new terminology would only complicate more.

I agree that some sailors of old would reject the new Tamar hull - in fact I agree with most of what has been said except I think the terminolgy, if messed about with can confuse even further.
 
Also fair comment, the point I am making is that for decades semi displacement took one or two forms and very little changed over the years, now of course recent technology has pushed the boundaries further than ever certainly far nearer to planing than displacement in many cases.

Perhaps an analogy would be IPS, technically it is a form of outdrive but no one would class it as an O/D system nor would you call it shaft drive, it is a new breed of drive system, robbing ideas from both camps so has become known as pod drive or azimuth drive.

One can't consider a 1970's SD Nelson in the same way as a SD Trent or Severn, they are totally different hull forms with completely different charactoristics.
 
I agree with that - the SD hulls have moved nearer to tyhe planing hulls.

I remember the review of the Aquastar 65 - it was good in a following sea because it had stablisers - so I think stablisers in themselves have extended the felxibility of hull forms.
 
It doesnt - he needs a planning boat. Full stop.

All this talk about Displacements and hull speeds - nonsence from his point of view but very interesting none the less.

less than 25 foot has to be planning IMHO, unless you are going to be a commercial mackeral fisherman in Lyme Regis and Ali is certainly not that !!!
 
Alastair, I'm as guilty as anyone of using the word 'semi-displacement' but actually I think the term is confusing because, in reality, there are only 2 types of hull, those that are designed to exceed their maximum displacement hull speed and those that are not. Any hull that exceeds it's maximum hull speed, by definition, has to generate sufficient lift for it's hull to rise out of the water, overtake it's own bow wave and plane. When a boat is planing, the amount of hull in contact with the water is much reduced which reduces water drag on the hull and the hull displaces far less water, hence a planing hull can go much faster than it's max hull displacement speed
So, generally speaking, SD hulls are simply hulls that do not lift out of the water as much as what is termed a full planing hull but even that is a bit misleading because how do you define a full planing hull? Something like your Sealine certainly planes but it does'nt lift out of the water as much as a racing power boat.
The vast majority of what we call planing hulls are similar in design. The most efficient kind of planing hull is completely flat but obviously a boat with a completely flat bottomed hull would be very uncomfortable at sea so some angle or V has to be incorporated in the hull bottom to cut through the waves. The difference between planing hulls tends to be how much of a V angle the hull has and where. Performance planing hulls tend have a deep V hull which has a constant angle of V along the whole hull but a lot of power is needed to drive such a hull so designers developed the modified deep V hull which has a sharp V angle at the bow to cut through the waves and a flatter V angle aft to generate more lift and reduce the amount of power needed to drive the hull. Virtually all planing hulled pleasure boats have a type of modified deep V hull
One of the characteristics of such a planing hull is that they have what is known as a hard chine ie. there is a sharp angle between the bottom of the hull and it's sides. This is one area where there can be a difference between a planing hull and whats known as a SD hull. Some SD hulls are developments of full displacement hulls or incorporate elements of full displacement hull design and this is particularly the case in the area where the bottom of the hull meets the sides. A lot of displacement hulls have curved bottoms and sides (known as round bilged), the idea being that a round surface makes for an easier motion through the waves compared to flat surfaces. Some older SD hulls like the Nelson and older Grand Banks hulls are round bilged but have a sufficiently flat surface of hull aft to lift out of the water and exceed their hull speed. So this is one possible area of difference between a SD hull and a planing hull but only a possible one as there are some so called SD boats eg Traders and even some full displacement boats eg Dutch steel displacement cruisers which have hard chine hulls. Another possible area of difference is that most if not all SD hulls have a keel under the hull designed to give directional stability and most planing hulls do not have a keel, the idea being that the V angle of the hull provides the directional stability. However some planing hulls eg Broom OS hulls do still have a small keel but this is because Brooms are also designed to run at slow speed on inland waterways and a keel is deemed to be required for directional stability at slow speeds. There are a couple of problems with keels though. Some earlier planing boats with keels actually started to plane on their keels rather than their hulls if you drove them fast enough which is obviously an unstable condition. The other problem with keels is that when the sea is from behind or particularly from a rear quarter, the waves tend to push the keel to one side or another. This is why some keeled SD boats do not track as straight as a non keeled planing boat in a following sea
Another factor that tends to differentiate SD boats and planing boats is weight. Obviously, the heavier a boat is, the less it will lift out of the water and the more power is needed to drive it. SD boats tend to be heavier than planing boats because they're not designed to lift as much as a planing boat and the max speed requirement is lower. It is true that you can make a SD hulled boat go nearly as fast as a planing hulled boat but you need a lot more power and you will use a lot more fuel. So weight is another possible area of difference between a SD boat and a planing hulled one. But there are advantages to having a heavier hull which lifts less out of the water. The length of hull in contact with the water is longer and, because the hull lifts less, the sharper forward part of the hull forward tends to impact the waves rather than the flatter part aft. This is why SD hulls tend to be better in a head sea than planing hulls. The sharper forward part of the hull cuts better through the waves, the greater waterline length bridges gaps between waves better and the additional weight helps to displace the water rather than bounce over the top.
As I say it is difficult to be specific about the differences between SD hulls and planing hulls. In one sense they're both the same in that both types of hull exceed their max displacement hull speed ie. plane and it certainly would'nt be true to say that SD boats are better sea boats than planing boats. I do agree with nautical in that, because of the inherent greater efficiency of boats which plane better, the future of fast boat hull design is probably with planing boats rather than SD boats
 
I would go along with most of that Mike.

Planing boats get their stability from their speed - when planing they do not roll and track straight because of the speed and hull shape - not just the hull shape.

However when my boat is doing 25 knots or the Marloe for that matter - it does not lify much at all - in fact with the Marlow the wave piercing bow bulge is still in the water.

I think that the future is not in planing aty all - I think the future of boating is in either slow speed SD albeit at the planing end of the spectrum or fuel efficient displacment cats. In fact I think within a few years you will see the end of large planing boats in any number in the UK.

Its all just my guess if course.
 
Gludy, I meant directional stability ie. the ability of the hull to track in a straight line, not lateral stability which, as you say, is provided by the hydrodynamic lift generated by the hull itself. Also, I said the future of fast motor boats was in planing hulls rather than SD. What I meant by that is that, where a craft is designed to exceed it's hull speed, it will use some form of planing hull rather than SD hull. I agree with you that, with fossil fuels getting scarcer and more expensive, where speed is not a necessity such as in pleasure craft, there will be more displacement craft but never underestimate the ingenuity of humankind. There is a need for speed in most of us and humankind will find a way of making boats go fast without using fossil fuels
As to your last point, we are going to disagree there for the simple reason that in the rest of high fuel cost Europe, the fuel guzzling planing motor boat is very much alive and kicking so I cannot see that they will die out in the UK although I accept that marine fuel in the UK will end up being the most expensive in Europe. The fact is that there are plenty of people about with money to burn on this particular pastime
 
I would just add that although not relevant to used sales I don't see new larger planing boats slowing up sales wise. (A) the majority sold to UK buyers go to the med anyway, (B) If you are spending £600k - £1m on a boat fuel cost even at a higher rate is not going to put you off it will still be the smaller element of cost in running the boat (C) many new buyers from my experience assume that diesel is the same for boats as cars and in fact are quite shocked to find that we actually use red!, they thought red was for fishermen, farmers and commercial operators. (D) new technology will provide cleaner, more economical means of pushing your gin palace at 30 knts.

Even at the moment you can save yourself a third of your fuel cost by specifying IPS in your new boat yet still gain an extra 20% in speed and acceleration, in another five years we will probably be powering the same size hull with half the horsepower of a conventional shaft drive.
 
>>>
Even at the moment you can save yourself a third of your fuel cost by specifying IPS in your new boat yet still gain an extra 20% in speed and acceleration, in another five years we will probably be powering the same size hull with half the horsepower of a conventional shaft drive.
>>>

Therein lieth the rub. "New". There are quite a lot of "old" boats around that won't benefit from IPS, retrofit is basically a no-no. If Yellowfin becomes a realistic retrofit for large numbers of shaft drive boats that's another matter.

As for SD vs planing I recall, from years back while learning about such things, a remark that SD made for a "roomy slower boat" and planing made for a "cramped faster one". Any hull form will go fast if power is not an issue. I would not describe the various >30knot warships over the years as having anything other than displacement hulls, but forms optimised for differing speeds. Indeed one of the noteable failings in British destroiyer design pre 1945 was a hull form optimised for 30+ knots in North Sea conditions rather than the eventual escort duties of sub 12 knots winter North Atlantic. The discussions on hull form relate more the the advantages of transom sterns over spoon ones rather than the exisitence of hard chines and suitable deadrise. Its a bit more sophisticated now of course, but there is still very much the all weather requirement that is more Nelson than Sealine.
 
"Therein lieth the rub. "New". There are quite a lot of "old" boats around that won't benefit from IPS, retrofit is basically a no-no. If Yellowfin becomes a realistic retrofit for large numbers of shaft drive boats that's another matter."

Exactly what I said.... /forums/images/graemlins/grin.gif better get into an IPS powered boat quick then /forums/images/graemlins/grin.gif

I think the topic was more to do with sea keeping SD V's Planing and my point is that planing can be just as good in the rough stuff as Semi D given the right power plant and hull design.

Warships..., thats a whole different ball game, the numbers involved are collossal compared to small leisure craft and the same dynamics do not come into play.
 
Many thanks to you and all the others for the very lengthy detailed replies.

There is a lot of mention about boats that i don't even know about, but it has all been a very interesting read, even if some of it was above me!!

I understood the little Jeanneau, i was concidering was a SD hull, but it appears put a big enough engine on the back of it, and it will plane! So maybe it was designed as a planing boat, and not a SD boat!?!?

Fuel cost slightly higher on a SD hull too.

Thanks again for all the info.

Cheers

Al.
 
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