CD36 waterline question

[M. R. Bober]

I turned green with envy when I read that John D had not had to paint hbis bottom since the mid 90's. Makes you want to move to fresh water !

I've heard that you can even drink the stuff.

Mitchell Bober
Sunny Lancaster (where I think our local waters are about to become a bit fresher, or that big black cloud is just kidding), VA

[John Vigor]

"Moment to trim 1 inch = 1400 foot pounds"

Not sure what that means.

John, I believe you measure fore or aft from the center of buoyancy.

For example, if you place a 100-pound weight 14 feet aft of the CB, the stern will trim (sink) by 1 inch. And if you place a 200-pound weight 7 feet forward of the CB, the bow will trim by1 inch.

John V.

Are you sure it isnt refering to port/starboard trim? I am shy of 200lbs and I am pretty sure standing on my bow doesnt sink the boat an entire inch, much less 2 (that would be 13 feet). However standing on all the way outboard on either side at the widest point certainly does (200lbs at 6 feet from center).

No Russell, it does not refer to port/starboard trim. This is from Ted Brewer's primer on yacht design:

"MOMENT TO TRIM 1 INCH (MTI): The MTI is the moment, expressed in foot-pounds, that will change the fore and aft trim of the yacht one inch. For a displacement hull, the MTI is, roughly (but close enough for all practical purposes), .35 times the square of the waterline area divided by the WL beam, or:

"For example; a boat has a LWL Area of 165 sq ft and a Beam WL of 8 feet. The MTI is .35 x 165 x 165 /8 = 1191 ft-lbs., say 1200 for rough figuring. Now you hang a 100 pound dink 18 feet abaft the CB. You've added 1800 ft-lbs of aft moment so the boat will trim 1800/1200 = 1.5 inches down by the stern. However, the boat does trim about its CF and, as that is usually abaft amidships, the stern will move less than the bow. You might find that she trims 5/8" down by the stern, and 7/8" up by the bow, making a total trim change of 1.5 inches."

John V.

[John Danicic]

I turned green with envy when I read that John D had not had to paint his bottom since the mid 90's. Makes you want to move to fresh water !

It needs to be COLD fresh water.


Troy: An interesting discussion. Will do. I am also thinking, it might be good to get the bow and stern measurements as well. I am not sure what the year of the third CD36 is but will find out.

I do think that both sides should be even, even if they list and Mariah does list to port when both starboard water tanks are near empty.



On Mariah, the line is pretty even around the stern. I can't see it deviate too much from side to side, nor would I want it to.

[WaywardWind]

Isn't Morningside "Nordeast Edina"?

A former Wayzata boy.

[John Danicic]

Isn't Morningside "Nordeast Edina"?

A former Wayzata boy.

You are correct but I like to think it is West of Minneapolis and East of Edina. It no longer exists as a town and only emerges from the mists of time during sailing season.

[2tocruise]

To somewhat answer the question asked earlier about calculating weight:

Designers normally assume a "light-load" displacement that includes tanks half full (water, fuel, etc) and may or may not include some gear (extra sails, ground tackle, etc) when they perform the performance calculations. "Live-loads" (people and their stuff, including provisions) are not normally included.

Most of the time the sales dept only provides the actual empty weight of the boat (tanks empty, no equipment) in sales brochures and owner's manuals. It makes the boat look like it performs better. So expect that your boat will be significantly heavier than what the literature says.

And boats are built in the real world, so no two are going to weight the same or exactly match the design weight. Not in any production boat anyway. (The America's Cup boats are probably pretty darn close).

Bottom line, as Lem mentioned, is that the actual weight of the boat is not important. Only how you are planning on altering that weight.

[John Stone]

Troy,
I have been through this same nut-roll. On my boat the scribed lines were not accurate. They were different on each side. I checked mine about nine different ways and never could get a consistent reading. The deck went one way, the floor beams another, the scribed lines another, and so on, etc. Once I decided how to deconflict all those differences I decided to raise my waterline 4 1/2" above the scribed marks which were several inches below the old bottom paint. Since my boat has not yet been relaunched I can't offer any more than that.

I know you didn't ask but if you have not decided how to deconflict some of the inconsistencies you may be seeing I used a method that Tim Lackey explained on his site. Woodenboat magazine also ran an article describing the same method. I captured some of the info on my site http://www.farreachvoyages.com/projects ... rline.html

Best of luck.

[Tom Keevil]

I understand the need to have the bottom paint cover the hull that is under water, but the desire to have a "level" boat seems rather quixotic.

On our boat we have two fresh water tanks and a fuel tank on the starboard side. That's a lot of weight, and it changes continually as the tanks empty and fill. If Jean and I are on the same settee, that's quite a difference from one person on each side. We load cans of food and bottles of wine into lockers, and then we eat and drink the food. We fill our 5 gal jerry can with diesel, store it in the lazarette, and then we pour it into the fuel tank. We put our hard dinghy on the bow, or we tow it. Our dog moves around to lie in shady, windward (uphill) spots. A graph of trim vs time for Rover would never approach a straight line, except when we're not there. Also it's not all that easy to see both sides of your boat simultaneously, and when we are underway we are heeled over and tossing about on waves. What's the deal with a "level" boat?

[Larry DeMers]

I would add an extra 2-3 inches of bottom paint to the waterline, once you settle on it's location.

The reason for this is that the ports outside of the US are fairly dirty with scum, oil, debris etc., and if that extra stripe of protection isn't there, you will have a "Bathtub Ring" around the boat just above the waterline, consisting of a nice sample of everything that has slopped up against the hull since the last bottom job :^)

Over ther years, many of our friends have broken loose and headed from Lk. Superior to the Carrib. and even Venezuela, for 5 years. They all practiced this extra protection at the waterline.

Have a great time, wherever you are headed. Gotta be exciting.

Cheers,

Larry DeMers
s/v DeLaMer
CD30c sailing Lake Superior, full time

[Troy Scott]

Thanks, Len!

First, let me say: I like your dodger. I'd love to have one like it.

Second, you wrote: "You already know where your boat floats." But that's just the point: I don't. I've never been aboard my boat with it floating. I did record how she apparently floated before, but she certainly won't float like she used to after this refit. These are why I'm collecting information about how other older CD36s float. If the vast majority of older CD36s are floating lower than the design waterline, and if there is some majority experience WRT how much lower, then I may as well adjust my waterline accordingly. I would feel foolish not utilizing the collective knowledge and experience of this board. Plus, I want to "help" my boat float at least parallel to the planned waterline when loaded as she will be most of the time. I just think it looks nicer that way..., and she just may sail better as well.

[David van den Burgh]

Troy,

I raised Ariel's bootstripe and bottom paint 2" a few years ago to compensate for added gear. I probably should have gone with 3". When she's on her lines, there's about an 1.5" of bottom paint showing. Good news is that we don't get a lot of growth.

PS - Will there be a grand unveiling when you're done with your project? I sure hope so 'cause I've been dying to see pictures for a couple years now.

[Troy Scott]

David,
I apologize for not posting more photos. I really should be sharing more. I suppose the excuse is that posting time could be boat work time. I'll try to do better.

[mahalocd36]

Troy : some measurements for you:

Our newly raised waterline is 29" down from the bottom of the rubrail at the midship cleat. At that same point (midship cleat) it is approximately 68" to the bottom of the keel (from the waterline).

We expect, given where the 'scum line' was, this to leave 3-4" of bottom paint exposed. This was recommended with Awlgrip (not to have it submerged 24/7). We always drew more than 5' with our boat, and it's not even loaded for cruising (though, we do have a fair amount of stuff on there, and all-chain, etc). We use 5 1/2 if a marina is asking for our draft.

[Russell]

Thanks John, very interesting, good stuff to know.

[David van den Burgh]

Just measured Ariel at 65".