# Thread: A displacement question for an N. A. or other

1. smart person.
So I have this old boat , Waione and I'm curious about her displacement. I have a pretty reasonable idea based on empirical information but I wonder if with the following data some confirmation might be able to be gleaned.
The boat is a 3 skin kauri hull of 41 ft on deck x 8'6" beam x 6'3" draft, 31 ft waterline ( overhangs in spoon bow and counter)

My guess comes from published information on similar sized types and shapes, plus asking experienced haulout men their opinion. She's never been on load cells and the travel lifts are notorious for their innaccuracy ( not that she's been on one of those for a while either)

Here's what I have and its obviously only best estimates but......

waterline plane of the boat ( graph paper drawing and assessment)of 17.5 sq M( 188.37 sq ft)
When I removed 310 kg from the boat I measured what effect it had on the waterline. The effect of removing 310 kg (683 lds) was to lift the boat an average of 20mm ( .787 " . Probably use 3/4 inch would do.)

In other words, .. I removed 683 pounds from a 188 sq ft w/l plane boat and it lifted the waterline out of the water 3/4 inch.

Can that be extrapolated into a guideline displacement figure?

[ 10-10-2005, 05:23 PM: Message edited by: John B ]

2. Hi John,

Ya good question, The quick answer is yes, kinda, sorta.

Your PPI (pounds per inch immersion) figure is dependent on the salinity of the water and the area of waterplane. What you want is to guess the volume of your boat underwater. One way to do this might be to multiply your waterplane by the draft and come up with a partly boat shaped prism, and then multiply the prism volume by some factor and arrive at a displacement volume.

But that's not the usual way and therefore I don't have a figure in my head for that (waterplane x draft * volume coefficient). Someone else may have though.

There a couple of ways to guess your displacement. The simplest would be to guess the displacement length ratio and multiply by the waterline length.

ie (31 * .01^3) * say 375 as a guess. This gives you 11.17 long tons or about 25,000 pounds. Is that close? I've got nothing in my hands here John, no research!! Just a guess.

The second way to guess displacement which is perhaps somewhat more accurate is to guess the midship area, multiply that by the waterline length, and multiply that by the prismatic coefficient.

We'll guess the waterline beam midships is 8'3" by draft 6'3" is 51.56 sq. ft. A guess at CM (midsection coefficient) might be .52, as your boat is very fine. 51.56 * .52 = 26.81 sq. ft.

26.8 * 31' = 831 cubic feet. Guess the CP at .55

831 * .55 = 457 cubic feet * 64 (salt water) = 29,200 pounds. Again it's a guess and with more study and some references we could get lots closer.

And I hope MMD (or someone smart) will come by and correct the above.

[ 10-10-2005, 07:12 PM: Message edited by: TR ]

3. "...I hope MMD (or someone smart...) " - TR
Hey!! I resemble that remark!

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I don't think so, although the 29000 might be good it seems too high to me. There are too many different underwater shapes/too many different displacements that would have that waterline plane. If you had a lines plan of WAIONE, the lines could be scanned into a program that would calculate displacement to a waterline. Alternatively you could take measurements of the hull shape under water at a few stations and calculate their area, doing a rough differetial analysis that would be more accurate the more stations you use. Or, you could find a crane with a scale that could weigh her.

5. Yes.. not looking like there is enough info there eh. I just wondered if there was one of those black art things going on......
Last year a man took her lines but I've yet to see them. Because he was doing it as an un asked for favour, I'm reluctant to push on it.

The reality is that she's around the 7 ton mark. Whether thats 6.9 or 7.5 ,or more,I don't know. I'd be quite surprised if she was 8.
This is a combination of ' wellll the back wheels on the tractor start to spin at 7 so..." and as I said, comparable other local designs with known displacement.

6. of ' wellll the back wheels on the tractor start to spin at 7 so..
so you guys do have rednecks down there, eh?

7. That were back in the good ole days when it were a 2 wheel drive machine. Its a 4 wheel drive now.

here, see those little bitty iron stakes sticking out of the water. Thats the target( cradle arms, place boat 'here') and thats the tractorrrr tied to it sitting there on the ramp.
helps if I actually attach a pic eh.

[ 10-10-2005, 10:19 PM: Message edited by: John B ]

8. Thanks Tad, Michael and Thad for looking at this ( John Welsford has looked at it too) My data is too sketchy for a conclusion obviously.

Heres a bit more . on Michaels suggestion I'm looking for some good end on and profile photos.. I might try and take a couple more while she's out.

So deep in my boat file I've found the local S and S which just happens to be 41 ft x 8'10"x 6ft and is of the general look .Its in there because of its similarity in general size to Waione. So its shorter on the w/l but 4" more beam.. has more freeboard so there's some give and take there.

It's design displacement is 17344 lds and it has 8484 lds ballast. 48.91% ballast ratio

I spent quite some time calculating the ballast on Waione a few years ago and came up with 8119 lds.
The original ballast estimate was 6785 but 1334 lds were added as pads on the keel in 1930. ( hot rodders)

Knowing how stiff the boat is, she could easily be in that 48 to 50 % ballast ratio figure. that would make her heavier than I ever thought. So reverse engineering it off a 48% ballast ratio....16900 lds x 48% = 8112...

7.66 metric tonnes.

[ 10-10-2005, 11:56 PM: Message edited by: John B ]

9. You could get a good idea at haul out. Most lifts and cranes have strain guages anyway.

Failing that and hauled on a railway, perhaps you could improvise a pressure guage tapped in where the relief valve goes on a 10 ton hydrolic jack (or 2 5T) and after some experiment to find near enough to center lift her up just barely off the blocks and braces, maybe with some buddies bow and stern, port and starboard, to ease her weight entirely onto the jack.

Then calculate from the small chamber psi to large chamber psi to lift weight.

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As a member of Eastport Wooden Boat Program in Annapolis I sailed on a 40' Alden sloop with about 6' draft - very similar in size to your boat. She had a wineglass cross section. I'm told her displacement was about 19000 lbs.

Will.

11. I'd forgotten about the lines being taken off. You promissed a set to MMD as i remember. So there is your excuse for pressuring him.

I'll sleep on the problem, must be possible.

I'm gonna ring him. " gimme what you got", "please".

13. OK, so here's how I did the "quick & dirty":

John e-mailed me pictures of the boat in profile & end view. I imported them into AutoCAD and scaled the profile up until the visible waterline matched John's stated waterline length (31 feet). I scaled the end-view picture up until the beam was as John stated (8'-6"). I traced the outline of the underbody in the end-view and presumed that this was the midship section. I placed this at the midpoint of the LWL in profile. I then extrapolated (guesstimated? fudged? faked?) the LWL in plan view from the midship section. From this I could determine the LWL beam at two stations half-way from midships to each end, giving me five stations. I struck a polyline - based on the shadow line on the hull - to represent the fair body line, and sketched in the shape of the two new sections. Then it was simple to use the AutoCAD area utility to measure the sections. Here comes Mr. Simpson and his confounded rules. I plugged the numbers into a little Excel utility program I created, and came up with a number.

How does 20,200 lbs (9.16 metric tonnes) sound?

Actually, I think that it is too high. I believe that my method got fooled by the quick changes in the profile where the fin meets the backbone (and too few stations to compensate). I think I'll take another crack at it tomorrow (it's 2:00 am and I'm sleepy), and calculate the fairbody volume and fin volume seperately and sum them.

Nighty-night.

****************************

Next day...

I ran the calcs for the fairbody and fin as described above and got pretty much the same answer: 20,175 lbs. So, until the lines plan arrives, that is what I'll stay with. Let's call it 20,000 lbs (9.15 tonnes) just to make it a nice, round number.

[ 10-12-2005, 01:38 PM: Message edited by: mmd ]

14. Wow. thanks Michael.

I'm in a bit of shock mind you 9 tonnes!!

I'm a gonna ring about those lines.

15. here's the s and s I was referring to above( for comparison). Both 41 ft hulls, similar beam and draft.
She does have a lot more room in her than us.
much deeper and slacker through the bilge.
Sapphire

Us

but Then I suppose ,who's to say sapphire actually did displace what she was designed to displace( heresy. intake of breath... Olin Stephens).. never the less she may be heavier now after 46 years I guess.

[ 10-12-2005, 04:58 PM: Message edited by: John B ]

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Fun looking at those two profiles. SS, more waterline/less overhang, drag in the ballast keel, full bilge amidship; WAIONE, clean lines everywhere with the almost fin keel, ballast foreward, with that bow sprit to carry her rig. The clean lines are a direct product of her structure.

17. S&S/Lawley: Voyager Class
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Hello John, I periodically review postings that look interesting and found your displacement question. Using a scaling formula in a two part series (WB 175/176) I compared Waione size to Sapphire and my S&S Voyager
Using the following info
Waione Lod 41.0 ft,LWL 31.0,Beam 8.5
draft 6.25
Sapphire Lod 41.0,Lwl 27.42 Beam 8.75 and draft 5.84 (taken from S&S Association year book)
Voyager Lod 44.25, Lwl 30.08,Beam 10.58 and draft 6.25
The formula was LxBxD where
L= (Lod+LWL/2), so here's what I got Lw/Ls)x(Bw/Bs)x(Dw/Ds)= 1.094
Sapphires displacement is 17344 lbs
x 1.094 puts Waione at 18,974 lbs, now scaling down from VoyagerI got :
(Lv/Lw)x(Bv/Bw)x(Dv/Dw)= 0.778
Voyager displaces 23,106 x 0.778=
17,976lbs, I also scaled
Sapphire against Voyager and vice a versa to see the error for known values (i.e. 17344 vs 23106) which gave me a high number going up and a low number coming down so the average is(18974+17976/2) +18,475 lbs or 8.25 long tons. As far as your waterline plane I figured (310kg x 2.24lbs/Kg)= 694.4 lbs
divided by 0.787 = 882.34 lbs/inch of immersion givng you a waterline plane of 165.44 sq ft ( 882.34 x 12.0)/64lbs/cuft of sea water.
Your ballast to Displacement ratio is 8119/18475 = 0.44 or 44% which is very reasonable, my Voyager is 41% and was built in 1939 and was tank tested at Davidson Laboratory.
I'm confident beyond any doubt that Olin Stephens displacments for my Voyager and Sapphire are right on the mark and not off by the suggested 15%.
I too would love to see the lines of Waione when you get them. I do have some question though such as when and where she was built? If you know who designed her? Has she always had a bow sprit, and what is the area of sail she carries, with and with out the bow sprit?
you cna repond here or send me an email : moralest@hotail.com Take car and Happy New Year!

18. S&S/Lawley: Voyager Class
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John I must have had a freudian slip, that e-mail is
moralest@hotmail.com

19. Hi Tony... a bit of a surprise to see this old thread emerge again. Thanks for the effort you've been to.
I'll take a while to digest it . No I haven't got the lines yet. I'm not too hopeful so I may have to learn how to do it myself.
Olin Stephens was here last year. I didn't get any closer than about 10 ft but he was sitting on Sapphire with Ed Dubois and a group of my friends( who own S and S boats). He went out for a motor on Sapphire and I was invited out on Rawene and then very generously given the helm ,so I buzzed them.LOL.
Waione is a Charles Bailey jnr design. The Baileys and the Logans were our Watson/Fife/ Herreshoffs and no doubt influenced by them.
She's 1907 so always a gaff design with sprit.
sail areas now

but thats the rig I put in her in 99. The original would have been more like Rawenes...longer boom ,shorter luffs,lower gaff angle, probably a bit less area.

We're off tomorrow hopefully.( this isn't thread drift ... she's all loaded up and the displacement has increased substantially!!) Gale going through today and clearing after that with a bit of luck.
Thanks again for the data. I'm reprinting the whole thread and taking it with me

[ 01-03-2006, 04:08 PM: Message edited by: John B ]

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