Calculations

[Gary G]

When performing the usual boat design calculations, sail area/displacement, tonnage and such, that use displacement, should the displacement include ballast? In a few places I've seen reference to a "total displacement" number that appears to be the displacement that is usually listed in the boat's specifications plus the ballast. In some online design calculators, especially those driven by their own database of boats, it's not always clear what they're using to come up with some of the numbers. Thanks.

Gary

 

[u079721]

Ballast as in that 6500 lb. lead keel hanging down there? Ballast would for sure be included. But one possible source of confusion is that some designers use displacement figures for calculations that specify, for example, half full water and fuel tanks. Not sure whether there is any accepted standard for that point or not, but I know I've seen some calculations done both ways.

 

[Dan Morehouse]

Gary,
My impression is that published figures for displacement tend to be "light ship" figures...that is, unloaded. I put this question to Bruce Roberts, the guy who designs for amateur builders, regarding one of his designs I was looking at, and he confirmed that the published displacement was the unloaded boat. Some of Bob Perry's comments I've read suggest the same thing. And to give an idea of the potential difference between the two, the designed displacement of my boat is 15000-16000 lbs.; and yet, the actual weight (according to the yard in San Diego who hauled the boat for survey) was more like 21,000 lbs.! The difference between those figures is close to the weight of ballast for the boat, and so could generate confusion. Designers can't possibly predict the weight that will be piled on a boat by owners after it leaves the factory, so they necessarily use light ship figures, or half load figures based upon tank capacities and modest loading of stores...something that IS relatively predictable.
I'm unfamiliar with how "tonnage" is calculated, but "displacement" simply means the weight of the water pushed out of the way by the boat...and therefore, the exact weight of the boat itself. And this weight MUST include the weight of the ballast, for the same reason it includes the weight of everything from the rig to the bottom paint. I am unfamiliar with the term "Total Displacement", but it seems more likely to describe the difference between a loaded and unloaded boat than the difference between a boat with or without ballast.

Dan Morehouse
1981 E-38 "Next Exit"

 

[CaptDan]

Gary,
I'm unfamiliar with how "tonnage" is calculated,

Here's how it's measured when a vessel is federally documented:

http://www.uscg.mil/hq/msc/docs/CG-5397.pdf

Even if your vessel is state registered, it's fun to fill in the form's measurments to find out how 'heavy' your boat really is.:nerd:

Capt Dan G>E35II "Kunu"

 

[Loren Beach]

This is an interesting thread.
The "brochure displacement" of my model is 10600#. I figure that this must be the "Light Ship" displacement, sorta kinda.
:nerd:
The interesting thing is that several years ago, I had stripped out most all of the removable interior parts like personal items and cushions and gear, and took it to the yard for some upgrade work inside. (Removing all that stuff makes their work go lots faster and lowers the labor cost to me. The yard manager once told me that he could *never* understand why some boat owners wanted to pay his crew $70./hour to have their personal stuff moved out and into compartments while work was done inside!)


The Travelift operator there has several decades of experience. He and I were conversing about the weight of our boat and whether he paid a lot of attention to the built-in load cells.
He told me that, based on his experience and his interpretation of the read-outs, that it was around 11K total. I told him the advertised weight when new. He smiled and said that the number seemed about right!
This was fully rigged, but with no sails on board.

As equipped for regular use, there must be at least another 500 to 800 # of stuff... maybe more... plus a bottle of rum...


Loren

 

[Dan Morehouse]

My first reaction to the news that my boat weighed five or six thousand pounds over its design displacement was to discount it. It just didn't seem believable. I know a lot of head butting goes on between designers and builders over this issue, but I don't know how good Ericson actually was at hitting Bruce King's displacement numbers with the boats as built. If they were sending them out the factory door reasonably close to the weight they were designed to be, then I suspect my lift operator was in error with his weight. Is it usual for a yard to accurately report the weight, or is it more of a guessing game?

Dan Morehouse
1981 E-38 "Next Exit"

 

[dpddj]

overweight

There are several things that could be astray. First, the published weight may not be an accurate reflection of the build weight. Published weight, as mentioned, is seldom reflective of full tanks, gear, beer and Ketel One stowed aboard. There is also the possibility that the hull and/or deck may have taken on water over the years, How does the boat float compared to its original WL?
Then there is always the possibility that the hydraulic load cells on the boat lift are not accurate, especially if the yard does not have a regular calibration performed.
The weight you mention does seem excessive and it may be a combination of all of the above, however I suspect it is more likely and erroneous reading on the boat lift.

My two cents.

 

[Dan Morehouse]

I like the conclusion that the weight reading was erroneous. The only fly in the ointment is that the boat does sit low on its waterline...to the point that the top edge of the bottom paint is over an inch below the actual waterline. Of course, this permitted grass to grow like a hula skirt on that unprotected inch or so. This chapped me, and I had the bottom painters in Ensenada move the top edge of the paint higher by several inches. That solved the symptom, but the cause is still up for debate. The boat is not heavily equipped or loaded, so it doesn't seem likely that the cause, whatever it is, will be correctable. Pretty unhappy about that, especially since adding gear & etc. is inevitable...and only going to make matters worse.

Dan Morehouse
1981 E-38 "Next Exit"

 

[dpddj]

overweight

Hmmm, sounds ominous. Do you haul for the season? I used to live in Vancouver, WA so know the area. Next time she is out of the water you could have the hull checked with a moisture meter (be sure to get someone who knows what they are doing). Exploratory surgery may be an alternative by cutting a hole into the hull somewhere that can be repaired. I am not sure if your hull is cored and if the whole thing is cored or just the bottom or just the topsides. If it is cored with balsa you may have a hull shaped sponge going on. In that case, you will probably opt for the heavy boat.

 

[Loren Beach]

An observation, if I might, on Dan's description of his waterline...
it appears that his hull is an early number (45?). This hull design, with several changes in deck, interior, and keels, had a long production history.
My "SWAG" is that by the time they reached #100, give or take, they simply adjusted the gel-coated waterline color band. Calculating an as-produced waterline stripe on a hull with a lot of newer engineering concepts like the TAFG must have been taxing for the designer, at that time.

On paper, some features were calculated to lower displacement, like the TAFG (with accompanying changes in hull layup schedule), while other trademark Ericson features like the wood interior, had a longer track record for weight calculation. And do not forget the late 70's change in fashion from mahogany to teak, with it's different density.

Back to Dan's purported 21K pound E-38.... I seriously doubt that number. Seriously.
(Unless he carries a cargo of anvils.)



Best,
Loren

ps: in 1983, Ericson ran two-page ads bragging about how their E-33 and E-36 exactly (!) hit the designer's displacement calculations...

 

[Dan Morehouse]

I'm glad to hear that Ericsons' conscientious effort to control build weight gave them an issue to make advertising hay out of.

My boat sat in the water as part of a sailing club in San Diego for about 18 years under the PO. And, according to the listing info, it had a blister job done around 2000-2001. Having just had it out of the water, I know there are no blisters on it now...but if there was enough water absorption to make a blister job necessary, there is probably some weight gain due to water absorption too.

The hull is a solid laminate; I had a redundant transducer removed and the hole plugged and glassed over, and that hole, on centerline under the forefoot, showed about an inch and an eighth of solid, dry appearing laminate. The good news is that it doesn't sail like it's wearing a 6000# spare tire. It moves well in light air with old second hand sails. It may well be that no excess bottom paint was carried above the original design waterline, and it took only modest weight gain over 27 years to immerse that line. 21K is a cartoonish figure that it WOULD take anvils to explain. Taking moisture readings the next time it's out of the water may be too tempting for my curiousity to resist.

Regardless, there is another design calculation that would be interesting to know...the pounds per inch immersion figure. This would be a constructively sobering value for everyone who loads stuff on their sailboat, whether it's already overweight or not. Anyone know where we could get that number?

Dan Morehouse
1981 E-38 "Next Exit"

 

[footrope]

Dan,

My E38 is out of the water and I have stripped the bootstripe paint, revealing a gelcoat waterline about two inches above the current bottom paint waterline. It would be interesting to compare our waterline to rub-rail measurements to figure out if your waterline is painted a bit low as compared to my current paint line and the gelcoat line. Not the easiest thing to do while you're in the water, but I'll post my measurements in any case.

The tape line is the current waterline, which is about an inch or so above where the water comes when the boat is afloat and level. I can't offer a weight, but I'll see about a sling weight when I go back in the water (be patient).

 

[Dan Morehouse]

Craig,

Excellent idea. I won't be back down to mine until mid-August, but I will measure it then. And may I suggest measurements in at least three different spots along the run of the rubrail, from spots easy to describe (such as certain stanchions, or scuppers,etc.)? Also let me know if you measured plumb from the rubrail to the water, or along the hull to the water instead. The measurement at the front of the boat at least would be affected by that consideration. It would also be interesting to know the distance from the rubrail to the top of your gelcoat waterline. What Loren mentioned about the gelcoat waterline band has me wondering if those are different.

Dan Morehouse
1981 E-38 "Next Exit"