Bottom shapes

[lumphammer]

Wandering around the boatyard yesterday and came across a Westerley Oceanlord. From bows-on viewpoint I was impressed by how wide and flat the hull shape was, especially compared to a Halberg-Rassy next door which was much more of the traditional wine glass shape.

What are the main differences between these two hull types when it comes to movement whilst sailing or anchoring? Is one or other more stable?

 

[pvb]

The more traditional shape which you saw on the Hallberg Rassy tends to give a much gentler motion at sea.

 

[Stork_III]

The more traditional shape which you saw on the Hallberg Rassy tends to give a much gentler motion at sea.

And spreads the impact load better when you say hello to a scandinavian rock.

 

[V1701]

Wider & flatter hull supposed to give more form stability...

 

[prv]

I'm just amazed that we haven't had any illustrated posts from Lakesailor in a thread about bottom shapes!



Pete

 

[Lakesailor]

So am I.

 

[Tranona]

So am I.

C'mon! - you can titillate better than that.

 

[William_H]

The flatter bottom at the bow tends to give easier lift up to planing but on the other hand gives harsher slamming into waves. The deeper vee shape at the front giving a gentler entry into a wave.
Down the back the flatter bottom again gives easier lift up on to plane for power boats but does cause more drag at low speeds but more stability (resistance to roll). It still rolls of course with wave sides that are sloping.
For a low speed boat a round bottom gives less wetted surface area so min drag at low speeds. The round bottom however gives less form stability. In side profile the hull line that rises to near water surface level gives minimum drag at the stern and the deeper vee at the bow gives easier motion into waves.
So it is all a compromise depending on what speed you want. So a power boat capable of planing is a much different shape to a sailing boat wanting minimum drag. (unless it is a planing dinghy). olewill

 

[onesea]

To over simplify things to the extreem..

Most "Traditional" style boats are kept upright or gain there righting leaver by the lead on the bottom of them (Halberg Ratsey).
Like a fishing float, if the lead was attached to the bottom of the float or a hydrometer.

Where if you remove the lead it will lie on its side, but as long as the lead is still attached and it has not sunk it will try and right itself. LEading to boats like this

Where as more modern boats rely on stability by form, more in the manner of an Ice Cream tub, where there is no lead ballast required.

Leading to boats like this..

Obviously htere is lots of space in the middle for arguments about which shape is best ..

 

[Lakesailor]

This looks pretty stable

 

[Rossynant]

To over simplify things to the extreem..
Obviously htere is lots of space in the middle for arguments about which shape is best ..

Especially which shape is best for which boat.
Flat is for light boat. Heavy boat would be hopelessly bad in hydrodynamics with flat bottom.
So reason for flat bottom is: there is not much boat in the water. Flow of water can go unrestricted underneath. Simple.

Heavy boat has lots of displacement, some volume. This volume must be put into water, but not causing much disturbance in water flow around it. So it has V shape, and easy to imagine water parts gently to both sides of the V. On sleek hull this V can be carried whole length and gives best flow. This the other extreme, a V bottom heavy craft.

Yachts are in the middle, as not many are so heavy to be V bottomed. Usually the boat is widening much amidships (very common now) and cannot retain this V there - then flow would detach after this wide section; so rounded section is usual in midship and farther aft.

fast V bottom to illustrate http://www.angelfire.com/fl4/mft/Schooner_America.JPG

 

[ghostlymoron]

I've just been reading Heavy Weather Cruising by the wise Tom Cunnliffe. He compares three types of underwater profile (I hesitate to say bottom shape for fear of the discussion becoming distracted)
A. Long keel, wineglass cross section.
B. Moderate fin and skeg
C. Flat bottomed fin and spade
He compares them on several criteria; directional stability - boat A wins, working to windward - boat C wins, ability to remain in one piece - boat B wins, motion - boat A wins, pumpability - boat A wins
Overall he is in favour of traditional boat A but admits that the other types have their advantages.

 

[Tranona]

Overall he is in favour of traditional boat A but admits that the other types have their advantages.

Well, he would not want to alienate all his fans!

 

[johnalison]

Just to confuse things, my HR 34 is semicircular in section and is thus "flat-bottomed". The entry is a V that merges into a curve after a few feet and the stern also becomes a shallow V. The shapes referred to as Hallberg-Rassys probably mean the older Enderlein designs which haven't been built for over twenty years. Later Frers designs have been broadly in line with those of other cruisers but with more ballast than some - mine is close to 40%.

In the end, it is cost that determines hull shape in cruisers as a rounder hull will give more space for the same money.