How to determine a 'heavy boat'

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BlueSkyNick

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Time to get back to basic principles of physics and boat design.

how can one tell from a boats spec such as length, beam, displacement, ballast, etc. if a boat will be heavy in terms of its seakeeping capabilities. ie

- stability in a breeze
- maintain a steady heading
- minimal slamming
- any other criteria
 
MoodyNick said:
Time to get back to basic principles of physics and boat design.

how can one tell from a boats spec such as length, beam, displacement, ballast, etc. if a boat will be heavy in terms of its seakeeping capabilities. ie

- stability in a breeze
- maintain a steady heading
- minimal slamming
- any other criteria
Click to expand...

Suprisingly, I don't include marine architecture amongst my accomplishments so I can't give you a technical answer, but I'd have thought a google of a few of the more conventional models might give you a starting point to put together an idea of the kind of proportions you're looking for.

As a f'rinstance, these are the vital statistics of one of a range of well-known Swedish models:

Hull length
11.32 m / 37' 2"

Waterline at rest
10.20 m / 33' 6"

Beam
3.55 m / 11' 8"

Draft
(Also available as a shallow draft version)
1.90 m / 6' 3"

Displacement
7 500 kg / 16 500 lbs

Lead keel
3 200 kg / 7 100 lbs

Sail area with working jib
69,1 m² / 744 sq ft

Sail area with furling genoa
77,4 m² / 833 sq ft

Mast over water, ex Windex
16.75 m / 54’11"

Headroom saloon
1.92 m / 6’ 4 ’’

STIX 49

(I note that Saloon headroom might make this one a bit marginal.....)

Of course "long keel" and "inability to steer in reverse" are also useful pointers :)
 
Some thoughts but not directly related to length, beam displacement etc:

Slamming:
Slamming is exaggerated by designers pushing the waterline length further forward. Not for performance but to increase the accommodation up front. The consequence is a flatter hull. To avoid slamming look for bows that are angled back. A lot of modern boats have a vertical or almost vertical bow.

Maintain a steady heading:
For performance stick a big weight on the end of a very thin keel. All the weight is then as far as it can be for performance. However, the stick between the bulb and hull is now so thin it doesn't add any stability to the steerage and so the helm is constantly at work. A long keel with a bigger wetted area is more stable. Your Moody can easily be trimmed to require no input from the helmsman.

One thing I like about our boat is the encapsulated keel. Expensive to do for the manufacturer but great in terms of less worries about grounding. A number of modern boats will distort significantly if sitting on their own keel.
Keels that are bolted on after the boat is manufactured make boat manufacturing much cheaper. The hull is at walking height rather than everything having to be done from a gantry. However this keel makes for a slower boat.
 
Slamming is caused by flattish bottom forward of the keel. More 'vee' shaped is best, but some can still slam when heeled, as the are hitting the water is flat. On my dinghy, in extreme chop, we move right forward to keep the bow well immersed, this helps stop slamming, but is wet!
Directional stability is helped by having more depth towards the ends of the hull, rather than a flat body with a 'stuck on fin keel'.

Stability more weight in the keel is good.

Most sea keeping qualities are improved by having a bigger boat! Length is good.

There are also subleties, such as the natural frequency of rolling, things like trawlers are designed not to be too stable as the motion on them would be too harsh in really rough weather.
Lots of good books on these subjects but Marcaj and Larsen and Eliasson (sp) are the classics IMHO.
 
MoodyNick said:
Time to get back to basic principles of physics and boat design.

how can one tell from a boats spec such as length, beam, displacement, ballast, etc. if a boat will be heavy in terms of its seakeeping capabilities. ie

- stability in a breeze
- maintain a steady heading
- minimal slamming
- any other criteria
Click to expand...

Nick,

The easiest indicator is the cloud of flies over boat, the second is that unique smell below coming from years of leaking bilges, the third is the dark interiors that would look amiss in Stalag 17.

At least that's my experience.

Chris
 
In Nigel Calders technical book he has a number of formulae for the very things you are looking at. Can't remember them, and I am not close to my copy, but I do recall that you could plot the boat based on various things like sail area to displacement ratio and ballast ration. I think it came up with an overall comfort factor he called it.

Worth a look if you can get hold of a copy. It is the technical boat owners book, not the practical one.
 
ChrisE said:
Nick,

The easiest indicator is the cloud of flies over boat, the second is that unique smell below coming from years of leaking bilges, the third is the dark interiors that would look amiss in Stalag 17.

At least that's my experience.

Chris
Click to expand...

as helpful as ever - i guess I'll give the R38 a miss now, even though it was on my long shortlist.
 
MoodyNick said:
Time to get back to basic principles of physics and boat design.

how can one tell from a boats spec such as length, beam, displacement, ballast, etc. if a boat will be heavy in terms of its seakeeping capabilities. ie

- any other criteria
Click to expand...

Is it made of ferro-cement or steel? If yes, then heavy.

But - heavy boat equals slow boat unless big rig. If big rig, big loads, big winches, big muscles unless electric, expense in rigging and sails, problems sail handling (maybe), etc.
 
Fergus said:
What are you looking for Nick?
Click to expand...
That;s the question I am trying to answer !!!

Because of my job (which I want to continue for quite a while), we are just not going to be able to use FD to her full potential, so we want to downsize and save some cash.

SWMBO has stipulated that she wants a boat which is sturdy and robust, not flimsy, tippy or skittish. I want something that sails well, easy for two of us to handle, with comfortable accommodation.

Something Swedish would be ideal, except it fails on the 'saving some cash' criterion. :(

All ideas gratefully received !
 
MoodyNick said:
SWMBO has stipulated that she wants a boat which is sturdy and robust, not flimsy, tippy or skittish. I want something that sails well, easy for two of us to handle, with comfortable accommodation.
Click to expand...

I had the same issues. Couldnt bring myself to cough up the money involved in a Swede but didnt fancy the French / German boats that I looked at. In the end I chose a Bowman built Starlight 35 which very definitely fills the " Sails well, sturdy construction, not flimsy tippy etc" bit of your requirements. The downside, particularly for you coming from a 44 footer, will be the accommodation which is OK but not palatial. Thats the price of having a good hull shape as someone has already pointed out. IRC weight 7.2 tonnes, lead keel on a grp sump, mast head rig
 
Main problem will be your headroom requirement - I would recommend Bowman, Rival or Nic but they are all narrow(er) than your current boat and with less headroom - otherwise Catalina maybe? I would seriously suggest looking at a '80s Swedish boat though or an Oyster of similar vintage! Alternatively, heaven forbid, a cat?!
 
Twister_Ken said:
But - heavy boat equals slow boat unless big rig. If big rig, big loads, big winches, big muscles unless electric, expense in rigging and sails, problems sail handling (maybe), etc.
Click to expand...

Supposing you have two boats with the same water line length, same beam and both with a similiar reasonable hull shape.

If one is 50% heavier than the other, does that mean it needs 50% more sail area to go at the same speed?
 
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