Heavy vs Light Displacement

[Cheechako]

The wave train was running straight up the Chessy... this was just after I headed out that morning, eventually I got the Staysail up with the main (Water running on the foredeck at my knees in the process!), cracked off a little and motorsailed to point up as much as possible and started moving like a freight train! Never did catch the Hunter who had no sail up though.... wind was blowing about 30

[Dockhead]

Originally Posted by Don Lucas

I think we are getting displacement and weight confused. Weight is weight and has to do with a material density and it's strength as to use. Displacement is more about shape were the boat needs to displace more than it's total weight. And the dimensions for a cruiser are about living space and storage. There is nothing that says that a carbon fiber boat using the same dimensions is going to displace much less than a fiberglass one. Yes it is going to be a little less due to the overall weight of the hull, but probably not all that much. Then you would have to go back and adjust the sails/ballast etc to get the boat ride back to want you wanted. The reason carbon boats can weigh less is because it is stronger and can use less material so a thin hull can displace a lot and carry the needed extra ballast for it's sails before it breaks in half. But is this what a cruiser wants?

Poppycock. Displacement = weight, by definition. The weight of the water "displaced" -- pushed away -- by the hull, put into the water. Shape has nothing to do with it.

[S&S]

Originally Posted by Dockhead

Shape has nothing to do with it.

Until you try to move it.

[Cheechako]

yep, weight = displacement really. Take a Westsail 32 empty hull without deck and put it in the water, it displaces a lot less water than a fully found Westsail 32! I guess this thread started with the Carbon Fiber thought, but for me, I'd want a light/moderate weight boat, but dont want radical skimping on the hull layup, just good engineering and structural support and not a lot of weight penalty for eye candy.... Why save weight below the waterline? Offset that weight with less ballast. Save weight above the waterline with proper structural engineering. Bulkheads and floors make a boat strong... sadly missing in many designs. Take an empty peapod and hold it on each end. Now bend it up on the ends... what happens? the "beam" at the center pouches out (making it wider) . When you tension your rig this is what happens to your boat. Bulkheads prevent this from happening. One thing to be said for most designs that are built heavier is that the bottom is more "Vee'd" (for want of a better term) as opposed to a flat bottomed canoe. It'll be much harder to flex the V bottom than the flat bottom. Having said that my favorite mono was a 44 ft canoe essentially, but it was heavily laid on the centerline and had large stringers running fore and aft to stiffen it....

[Dockhead]

Originally Posted by S&S

Until you try to move it.

Nothing to do with displacement whatsoever.

But shape has everything to do with wetted surface. That is, resistance to driving force. But again, carbon or glass or kevlar -- no big relationship to wetted surface. That's all design.

[S&S]

Originally Posted by Dockhead

Nothing to do with displacement whatsoever.

But shape has everything to do with wetted surface. That is, resistance to driving force. But again, carbon or glass or kevlar -- no big relationship to wetted surface. That's all design.

Not nothing: imagine a skinny pointed plank and a box both with the same displacement. which is easier to move thorugh the water? Shape counts.

[Dockhead]

Originally Posted by S&S

Not nothing: imagine a skinny pointed plank and a box both with the same displacement. which is easier to move thorugh the water? Shape counts.

Sorry, no one said that shape doesn't matter in any respect. Just that shape has nothing to do with displacement. Nothing, whatsoever.

Of course shape counts, and of course a better shape will perform better. But it's a different question from displacement, that is, the mass of the boat.

[S&S]

Originally Posted by Dockhead

Sorry, no one said that shape doesn't matter in any respect. Just that shape has nothing to do with displacement. Nothing, whatsoever.

Of course shape counts, and of course a better shape will perform better. But it's a different question from displacement, that is, the mass of the boat.

I'm thinking about the general gist of the thread that displacement somehow relates to performance (it does) but just not as negatively that some think- rather than the definition of displacement (which is just as you say- not shape determinative).

[barnakiel]

Originally Posted by Joli

...
I think you will agree that the J37 in race trim is faster around a W/L course but when loaded with cruising gear the Centurion is going to be faster.
...

Well, then your J is too small for this load, isn't she?

Go for a bigger J or take less stuff. And sail the J.

I think you will agree that J is a sailing boat and Centurion is a cruising tub.

b.

[delmarrey]

Here lets use this sample to show the difference in hull shape vs. performance.

If you take a balloon and fill it with air (mass) then place it on the water. The wind will carry it quite fast away from its spot.

Now take the same balloon and crush it into a small ball and place it in the water. if it doesn't sink the wind will most likely not even affect its position, and it still has the same displacement just a smaller contact area with a lot less free board (less mass).

Hull shape or design has a whole lot to do with it! It's the balance between shape and displacement that creates performance.

e.g. My boat is an IOR design. When it sets dead in the water it has a 32' LWL but when it heals over at 20º it increases to 36' LWL due to the shape of the transom, which gives it more speed in a stronger wind ratio. So yes! Shape does matter! And mass matters! But a hull will displace the same amount of water no matter what shape it has, until it sinks, then it'll displace its volume (mass).

[zeehag]

my boat is a formosa 41. i like the heavy and big cruisers---i figger the light and fast ones can cool the wine/beer for me --i leave first and get there last---no problem...in a hurry?? why???
isnt sailing fun yet??
after this summer i really appreciate my heavy girl more than i did prior to sailing a performance cruiser--these are very fast under conditions in which slower is safer --yet they get out of lightning storms faster than mine will......i still like the heavies best......maybe because i learned to sail on a heavy and therefore appreciate them better than someone learning to sail a light boat......makes a difference as far as what is comfortable-----

[dcstrng]

Originally Posted by Joli

A planning boat is great but do you really think the cruising sailor is gonna push a boat to do that... go... with a minimum amount of sail that lets you recover when (not if) the auto crashes. So you may as well buy the heavier boat and slow down.

Yep… I think this hits the nail on the head… our little chunk has an amazing turn of speed when she gets down on her lines (hull-speed plus just enough we sometimes think our GPS might be fibbing a little…), but for actual cruising we seldom plan on more the 80 miles a day (19ft-LWL), reef early, enjoy the ride and rest contentedly…

[barnakiel]

My boat is little and mid-disp. But if I could afford a bigger boat she would be in the lighter disp range. And why?

It is easier to use the speed potential in the lighter boat. Off course, in small boats, we do not have the option - we simply need the heavier disp to safely and comfortably carry our stuff - stores, provisions, water, some diesel and fave books, etc.. But once the cruisers' chest can be comfortably carried by the boat (say 1000 Pounds per crew) then any excess of displacement will simply slow us down, call for more sail are and muscle to attain the same speed. In result a potential for safety coming from extra speed will be lost.

So, for a long range, small boat cruiser the heavier boat may be the safer way to go. But once a boat is beyond some size, we can go for the lighter displacement and reap the benefits in handling, speed and safety.

What wrong with such an attitude?

Example:
- my boat 26ft, 7000 pounds,
- my 'light' boat, 36ft, 9000 pounds

Both will carry my 1000 Pounds of stuff with ease. But the bigger boat will be easier to drive, faster, safer.

b.

[sneuman]

Originally Posted by barnakiel

My boat is little and mid-disp. But if I could afford a bigger boat she would be in the lighter disp range. And why?

It is easier to use the speed potential in the lighter boat. Off course, in small boats, we do not have the option - we simply need the heavier disp to safely and comfortably carry our stuff - stores, provisions, water, some diesel and fave books, etc.. But once the cruisers' chest can be comfortably carried by the boat (say 1000 Pounds per crew) then any excess of displacement will simply slow us down, call for more sail are and muscle to attain the same speed. In result a potential for safety coming from extra speed will be lost.

So, for a long range, small boat cruiser the heavier boat may be the safer way to go. But once a boat is beyond some size, we can go for the lighter displacement and reap the benefits in handling, speed and safety.

What wrong with such an attitude?

Example:
- my boat 26ft, 7000 pounds,
- my 'light' boat, 36ft, 9000 pounds

Both will carry my 1000 Pounds of stuff with ease. But the bigger boat will be easier to drive, faster, safer.

b.

There's much said about "speed = safety", but little evidence that anyone is able to put it into practice offshore. We had a thread a year or so ago to discuss those who had "outrun" a storm but there were virtually no real-life experiences to talk about, as I recall.

I wouldn't want to go to sea in a boat who's primary heavy-weather tactic was based on winning sprints against bad weather. Even if you could do it 50 percent of the time (which almost certainly you could not), how many of us would rely on a heavy-weather tactic that was only reliable 50 percent of the time?

Of course, as waterline increases, the odds of beating a weather system do too. If I had a 60-something footer, I might be more inclined to the outrunning bad weather argument.

[dcstrng]

Originally Posted by barnakiel

once a boat is beyond some size, we can go for the lighter displacement and reap the benefits in handling, speed and safety.

What wrong with such an attitude?

I’m certainly not immune to this line of thinking and have looked longingly at some of the Dix vessels that seem almost directly comparable to what you propose… I’m content with a Spartan existence (the Admiral is reluctant, but I am fine living out of a rucksac…), so don’t need a lot more displacement; however, would most likely see the increased speed potential of a lighter D/L as more in the exhilaration and thrill category than outright safety… indeed, for me at least, it would probably be a waste cuz my customary sail-handling is basically indolent carelessness and the fastest we’ve ever observed on our B24 was when we were all preoccupied rigging some trolling lines and the boat was tending to herself…