Lifeboat Hulls

Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

Displacement, full length deep keel?

There are two very nice museums for the Mary Rose and the Vasa (Stockholm) both of which are huge, and both of which are now in museums, having been raised after sinking in slight to moderate conditions. My 21' high performance planing hull would have had no problems in the conditions they both sank in /forums/images/graemlins/grin.gif
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

Well the Mary Rose sank because she was totally unstable and poorly designed... it amazing thatyou have to get such point out of the dregs of the barrel!!
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

Just goes to show that heavy displacement hull and long keel don't equate to sea worthiness. Bad design of one displacement hull and good design of one planing hull, would mean that particular planing hull would equate to better seaworthiness than the displacement hull. Too many generalisations, as mentioned previously
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

Brendan ... at that time the ducked witches in water and they were only innocent if they drowned ... they burnt witches ... and they did not know how to design a boat!!!! It would not have passed the stability test!

You are a bit of rogue having to go back to the flippin Mary Rose as your defences crumble before the weight of razor sharp logic. Have you no shame?

Anyway, that area does not boast any real sailors, its just a pond protected by the IOW .. Mary Rose after all was a raggie in the Solent and we all know how many times they need to be rescued ... even now.
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

I do not argue that the bad design of a d hull could produce a less seaworthy boat than a good design of a planing hull ...... I agree.
All I am saying is that a good design of both would mean the d hull being a better sea boat.
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

No shame at all. Just proving once and for all that heavy displacement and deep long keel do not equate to seaworthiness. Do you agree or disagree?, just answer the question
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

Brendan -

A well designed dispacement boat I beleive to be more seaworthy than a well designed planing boat. The reason is that the stability of the planing boat is dependent on its forward speed and it gets its lift from its forward speed whereas the displacment boat sit in, rather than on the water and gets its lift from the water it displaces. This is the reason you will never see the displacment lifeboat replaced by a planing version - if such a a boat could plane it would be fantastic because it would get there much faster and in rough weathe, when not planing, it would get there as fast..... there woukd be no disadvanatge but that can never be because as soon as a planing boat loses its speed and comes of the plane it is less stable than a displacment boat with a keel - the latter is sitting in the water, the formaer on top of it.

Of course its possible to design a terrible displacment boat that has poor stability but that would mean throwing away its intrinsic advantages.

Just to be clear - I am assuming the same good helming for all boats.

Also for the record, I would not put too much weight on the RCD Class A,B stuff ...

So my basic point throughout the thread I still maintain to be true and I think the above statements are a balanced reflection of my views on the subject of seawothiness.
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

Is this a private fight or can a raggie join in?

I have done about 18 hours on a mobo,and know nothing about the hydrodynamics vis a vis D/SD/Planing,however I have never allowed ignorance of a subject to prevent me from expressing an opinion!

If I went over to the dark side I would want a hull based on the same dynamics and profile as a European commercial Pilot Boat.(I presume this is SD)

Why. Because I have observed these out in filthy sea states proceeding at good speeds and watched them manoeuvring at low speeds with astonishing ease,when other MOBOs were having difficulties.(I also think they look nice in a workmanlike sort of way)
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

I happen to totally agree with you on this.
The Cara Marine boat was based on a pilot boat hull and these boats offer great sea worthiness.

There are no planing pilot boats around because the planing hull relies for its stability on speed, lose the speed and you lose its stability.
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

There used to be 3 Pilot boats in the Humber area that could plane at arround 40 Knots. I think one even had a lady driver in charge ?
I heard one got wrecked after a bump, one caught fire and the other one was only on loan( they were running out of boats) but I don't know if it survived, I doubt it the way it used to tear arround.
I knew it as the Yellow Peril, after it aimed at us one day at 35 Knots.

I don't think these boats survived a year.
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

I actually thought of Virgin Atalantic Challenger - certainly a planing boat but not very stable - hence, highly subject to weather and it is failed to break the record!

You cannot get around the simple fact that a planing hull when not planing is sitting more on the water with nothing dug into the water - a displacment hull is in the water with a keel. Turning the planing hull over requires less energy than turning over the displacment hull - this assumes both are well designed, same length etc.

A planing hull when planing is stable because of its speed. Its when you have to reduce that speed and come off the plane that it is not as good. You cannot plane in very rough seas.
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

I thought she did break the record, just the Yanks chose not to give the Blue Riband back as she was not a liner.

FWIW I had the chance of a trip on the first challenger, the cat. My best mates parents at the time owned a Hotel in Hoylake. The maintenance crew were staying there while she was doing engine trials etc. on the Mersey. The crew had seen my mate and I cleaning and taking our sailing dinghy out over the summer and spoke to his mum, offering a trip out.

She answered saying oh they wont be interested as they are out sailing everyday or something similar. I will never forgive her for that.....
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

still not happy with this continued persistence that a planning hull relies on it's speed for it's stability - we are talking about sea worthiness in difficult conditions here not the comfort argument. It's about centres of gravity / mass, righting moments etc which are affected by many things to a far greater degree than the hull shape - as highlighted by the picture of the SQ58/Trader the differences are not big in this context. I bet the Trader has a significantly lower C of G than the SQ though - and the Sealine range higher still (not knocking it's how they get so much usable space out of a given hull lenght (marina bill).
As an extreme example of a sea worthy craft you would have a craft that was inherently unstable when capsised - ie self righting, sealed (to prevent water ingress) and of course with a fridge for chilled mineral water... /forums/images/graemlins/cool.gif. Well padded etc and with a reliable propulsion unit capable of making way in extreme conditions. Some early lifeboats were designed this way! Generally very uncomfortable boats 'cos they aren't at all stiff upright either!
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

A planing hull does rely on speed for its stability - read Dag pike in rough weather or any technical book on the subject.

"we are talking about sea worthiness in difficult conditions here not the comfort argument. "

I agree.

"It's about centres of gravity / mass, righting moments etc which are affected by many things to a far greater degree than the hull shape "

I again agree.

The STIX number is made up from a number of different safety/stability related features. These are: Length, Dynamic Stability, Inversion Recovery, Knockdown Recovery, Displacement Length, Beam Displacement, Wind Moment, Downflooding, and Reserve Buoyancy.

"I bet the Trader has a significantly lower C of G than the SQ though"

I yet again agree.

Basically the planing hull is stable when its up planing and the speed it generates gives it the lift. If you manage to take a boat onto the plane that is not designed for it, I would think it can become unstable on the bends etc.

If the planing hull was as seaworthy or as stable ast the d or semi-d at diplsacment speeds then thats ll we would have - planing hulls. D and semi-d hulls would not exist.

All hull shapes are a compromise, a full dispacment hull would be the best for just displacment crusing and there is no doubt a planing hull is best for high speed cruising.

If the counter argument was true - and many seem to be advocating it - that there was no difference between a planing hull and a D or semi-d at displamcent speeds, then the whole market would have shifted over to the planing hull. There would no point in having any other type. .... they have not and they will not because each hull is designed as best for its partiulcar purpose.
 
Re: Lifeboat Hulls, Prop Tunnels, The Bristol Channel and Tea

Thak you for your considered response.

I believe that much of the issue under discussion rests with the last paragraphs an dteh small, but significant, changes that are made in the design of a hull once it's operational objectives are made - and it is these that will combine to give you the practical differences in operation between, say, the 2 similar craft you have brought to this discussion.
As examples, and not a definitive list, SD will benefit from a slightly finer entry (which tend to go hand in hand with greater flare higher up to get the same deck proportions), narrower beam (to length), keel and engine further forwards (to get the weight lower in that narrow hull) or if 2 closer together for the same reason.
Given that you would cofortably drive a displacement boat of a similar size to the 2 you are discussing with 100 diesel HP the reason for the 2 types to have evolved is rather more obvious than the sea worthiness. In the SD/planning interface it's really comes down to engines again - not just economics but size, weight and fuel considerations. Generally they dominated the comercial angling sector until recently - now most new craft are cats!
 
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