Boat balance?????

[mallo]

Hi all
I am in the process of re-building/re-fitting a one off 50ft 1939 steel Ketch, we have stripped the inside out (to sand blast and re-paint) due to a sinking and re-fitted it including some modifications (it has been a huge job, never again…..) one of the modifications was to plate over the deep bilge (c/w Lead ballast) and have a 2nd fuel tank for longer range and to save cleaning and painting this difficult area (always been a nightmare due to the nearly 4ft depth), we have now re-launched her to find her down by the bow a bit, I would be interested to know if I should remove some of the lead, or if this will affect the stability/amount of heeling. The tank is currently empty, will hold 140 gallons when full, the boat is bilge keel and centre keel, has water and fuel in the centre keel (2 tanks), has about 3.5 tonnes of lead in the tanks, and weights about 20 tonnes.
Sorry it’s a bit of an open ended question but I would appreciate any comments.
Should I just paint up the waterline and leave things as they are or remove some of the lead.
Yours faithfully
Michael Allo

[jehardiman]

First, I would get all her gear aboard and stowed and then see how she sits her lines.

[Tim B]

How much is she down by the bow? one or two inches on a 50 ft waterline is unlikely to pose a problem. It may be that it performs better at certain trims. If you're worried, put a few bags of gravel on once you've got all the stores aboard and see if the trim makes a big difference.

The problem is that the mass moment of inertia (particularly in pitch), makes a BIG difference to the handling characteristics through waves. MMI represents the distribution of mass on the boat and affects the angular accelleration term in the differential equation governing angular response. Typically, larger MMIs will reduce the natural frequency in pitch, which may or may not be a good thing depending where you sail. The MMI is increased by moving masses towards the ends, and reduced by moving them closer to the CG.

Hope this helps, search for simple harmonic motion for more details,

Tim B.

[mallo]

Hi Jehardiman & Tim B
Thanks for your post’s, we have all the gear onboard, and stowed, and this was the results, the bow is down by about an inch or two as suggested by Tim I have got a bit back by taken some of the lead out from the new fuel tank to try and lift the bow a bit, I am planning to keep this tank with some fuel in(extra weight and a reserve) having plated over the bilge it has added some weight, although not as low down as the original lead its still under the floor(only about 2-3ft higher than the original lead). I have also added a watertight bulkhead and door between the fore section and the main cabin(thinking about floating rubbish/containers etc, as an “insurance policy”) this is going to of added weight to the problem of lowering the bow in the water but this is higher in the hull, the original bulkhead was 4mm hardboard,(non structural) and were done for lightness, I have put back 6mm coated aluminium to make it watertight and strong enough to take the water behind it (it also supported with a channel.
My original question was when the waterline is drawn in the plans how is it worked out? And how accurate is it? When the boat was originally built in the 1930’s things were very basic from what I can understand, we have added a certain amount of comfort, and the original centre punch waterline marks have been moved up for obvious reasons, I was wondering if this is detrimental to the performance, she still sails fairly fast and keeps up with a lot of the modern boats in the right conditions, if it goes light she slows, but in heavy weather off she goes and is fairly sea kindly.
She was one of the original bilge keel boats.
Thanks for your replies, I will include some photos and see what you think?
Best regards
Michael

[jehardiman]

The original waterline was probabliy in the correct place the the designed loading (which may or may not be recorded).

Is she is down by the bow by an inch or two and up on her lines midships? Or is she on her lines midships and additionally down by the bow?

Try taking her lines when she has two people in the cockpit. If still down by the bow and up on her lines, add lead aft. If still down by the bow and down on her lines, the only thing left is to shift weight aft.

[mallo]

Hi Jehardiman
Thanks for that, the original waterline was with a different engine (only a two cylinder) basic electrics, one battery and basic interior, the water line is generally down along the hull and was painted up about 1”, but since the repairs she is down but more so in the bow, as I mentioned before I have taken some ballast out to try and bring the bow up, my main worry is that she will heel over too much by bringing the centre of gravity up higher, will this make much different (I don’t want her to roll in a big sea, I know it sounds stupid if I have taken too much ballast out)?
Standing in the cockpit doesn’t make any difference, I am currently fitting an Onan generator under the cockpit (160 Kgs) that will probably bring the bow up (or the stern down).
The boat is probably more loaded than she ever has been, (more gear + extra fuel tank etc) hence why she is lower in the water.
I can’t move too much weight aft as when she is sailing hard she scoops up the stern quarter wave (not all the time but when punching into a sea) also when motoring hard she buries the stern (this is due to canoe stern, running short of buoyancy also a larger engine than the design) it’s a lovely sensation sitting on the afterdeck very close to the water watching it fly past, a number of people who have their own sailing boats have commented on this.
My main worry is affecting the “righting moment” I think its called, but I don’t know if this would happen by lifting the weight up in the boat, and by taking some of the ballast out, but if she is full of fuel its in a tank in the bottom of the boat and wont move. Their might have been a “righting moment curve” at some time but I haven’t seen one and it might not have been done in the 30’s.
Please correct me if I am incorrect with what I am saying, I am not a boat designer…..
Thanks for your help
Michael

[Tim B]

To be honest, as long as you're keeping the heavy stuff down low, you're not likely to cause yourself any headaches. The main question, as I've said before, is does it sail better as it is? few people know the exact effect of pitch on resistance and manoevring, though there have been a number of papers written on the subject.

Tim B.

[mallo]

Hi Tim
Thanks for the last reply, with regard to the sailing performance, she is sailing as well now as she has ever done, however I don’t think this much to do with the balance, we now have a larger sail area than before and in light airs she was a bit sluggish, when she was built I think her sail area was a bit conservative side, when we moored her along side a slightly shorter modern boat there is a considerable difference in mast heights, however I know a ketch rig would have a shorter main mast. I will leave things as they are and see how we get on over the next few seasons.
My original question was how is the water line drawn in on the plans and how important to stick to this line. I am sure we have more gear onboard now than ever.

[Tim B]

In a fully dynamic situation (ie. not at a pontoon) the adherance to the static DWL may be a little questionable. The water certainly isn't flat. Obviously you don't want to be hugely bow up or down, but a small change probably won't make much noticable difference.

If anyone has any test results on the effect of trim on resistance I'd love to see them.

Tim B.

[tri - star]

To mallow,
speaking on the behalf of the original designer:
( Proberly long since departed.)
YES ! It does matter if a vessel is floating on it's lines.
The most common sin commited by owners is OVERLOAD
their boats.
You say she sails as well as ever.
However, as you indicate - her waterline was re-painted
higher, so you presumably - have never sailed the boat
on her designed lines.
Your description of "....scooping up the quarter wave..."
supports this. As - if she was lighter on her feet - there
will be less tendency to do this.
If it was my boat I would take out some of the " essential
stores and other stuff " and remove ballast untill she floats,
if anything, a little ABOVE the ORIGINAL design waterline.
You do not need to know anything about the rest of her lines,
righting moment or center of gravity.
Designers do all that - long into the night.
They define all the above
- by establishing the DESIGN WATERLINE.

Later when a new owner puts in a heavier engine - yes,
the c. of g. goes up. The righting moment becomes worse
........and so on.
However, the new owner - does not have to do any calculations,
or search out the original drawings.
The boat will tell you.
She will sink below her DESIGN waterline.

The answer to all your questions is simply:
Reduce the overall weight, take some weight off forward
- a or simply sail at a slower speed.

[LP]

Here is a stab at your question.

In the beginning of a design, the waterline is the first line on the paper. In this respect, the waterline is an arbitrary line. However, as the design progresses, this line is "the line" that most other lines are based on. Ultimately, the displacement of the vessel is determined based on this waterline. With knowledge of the displacement, rigging requirements (loads) are determined. That would more of my concern than the physical aspect of the vessel floating below it's design waterline.

As an exersize, take your vessels LWL' X BWL' X (average sinkage, in feet, below design waterline) X .70. (.7 is a scaling factor and is based on the fullness of your hull. Could be higher or lower.) Multiply the result by 64 and you'll have a rough estimate of how much heavier (in pounds) your hull is now than when it was first built. A heavier vessel requires heavier rigging. The design of your rig may be well within the loads imposed by the heavier hull or it might not. I'm not in a position to say. An NA could do an evaluation and give you a more precise answer. Increasing your sail area to compensate for the heavier hull might also be a concern. If just compensating for light wind performance, it shouldn't be a problem. Another one of those NA problems.

There are just so many aspects in the design of the vessel that are tied to it's displacement. Where the boat floats is aribitrary. The reasons for where it floats are not. Remove ballast with caution. Relocate ballast and look for non-essentials that can be removed. IMHO. Looks like a great vessel.

[mallo]

Hi tri-star
Thanks for your reply I can understand what you are saying regarding everyone “overloading” their boats, most people are guilty of this, in one way or another.
I mentioned “that she sails as well as ever” what I mean is that I don’t notice any significant change; perhaps we are pushing her harder than before…..
The boat was originally designed by Robin Riverdale in 1939, he died only in the last 5 Years or so. She won’t have been sailed on her original lines for a long time as my father painted the waterline up 30 years ago!!!
I understand what you are saying regarding taking some of the weight out, but I am worried about moving the centre of gravity too far up and the righting moment becoming worse… she is bilge keel and I don’t know if this will affect it.
I am not really worried about scooping up the quarter wave, it doesn’t cause a problem and it only does it in a big confused sea (wind against tide) that she does it.
The main thing was I didn’t want her “falling over” which I know sounds stupid but I am trying to find the best answer.
As she was designed a long time ago I am not sure how accurate the original waterline is/was to the plans this is why my interest. Robin was not a professional designer so although he probably took advice and was a very clever person I wonder how the original line was come to…..
She is a solid boat, very much over engineered as she was built in a small tug yard in the UK, she was riveted at about 2” centres and also welded; I think this is one of the reasons she has stood the test of time.
I am not a slow sailor, if I see a boat ahead I like to catch them, (I am quite a completive person) but not over the top.
I am just trying to do things right, I cant take much more off without taking out ballast and I am reluctant to do so.
Thanks for your input
Michael Allo

[mallo]

Hi LP
Thanks for your comments, I am thinking along the same lines as you regarding the ballast, in the earlier picture when she was stropped on the crane they weighed her in at 19 Tonnes the original registration paper work has 18 tonnes (I think). When she was in the strops and weighed she didn’t have the masts in or the water or fuel tanks weren’t full ( I don’t know if this should count).
Thanks for idea to work out the sinkage, but I don’t really know by how much, I suspect it’s only about an inch (don’t have the original plans unfortunately) and I cant tell where the original waterline is compared to the current waterline.
As in the other E-Mails I am not too worried about her being lower in the water and scooping up the quarter wave, my main worry is that we don’t make her too tender, if that makes sense.
As far as the rigging goes w have tended to over spec the rig, in fact we are currently in the process of re-rigging the main mast ( I will go a size up if I can)….. I have to be a bit careful with loading the rig as she has the original 1939 mast (probably a bit smaller in diameter as it has been scraped an re-varnished a few times).
Thanks for the interest in the boat, she is very much a one-off, I have more photos if you are interested.
Thanks again for your input
Michael Allo

[jehardiman]

You know Michael, I'm beginning to think that you should start to worry about her being tender. Everything you said so far points not only to an increase in displacement (new bulkhead, down on her lines, more hook weight), but also to a rise in CG (new bulkhead, rasied ballast, more gear, heavier rigging which is weight high).

Running up a set of lines, an inclining, and a set of righting moment curves may not be out of line for such an old vessel comming out of what looks like a major refit.

EDIT. BTW, a tank low is not the same a fixed ballast. The free surface of the tank actually decreases the righting moment. Do not be fooled into thinking that just because the tank has a lot of water/fuel in it that it makes you more stable

[mallo]

Hi jehardiman
Thanks for your post and concerns, I agree with you regarding the tender issue however I haven’t noticed any difference in her sailing in a breeze and we aren’t reefing any earlier due to her being over pressed in an increasing breeze, however I would like to but my mind to rest on this subject.
What is involved to do a set of righting moment curves? What would be the cost and how do they do them (from plans or the actual hull)?
Sorry for being so stupid but it’s not my area…..
I was giving a bit of thought and there was a sister ship (well not really a sister ship as she isn’t exactly the same) and she only has bilge keels and a more current (common) underwater profile (she doesn’t have a centre knuckle that she sits on as well as the bilge keel) otherwise she is similar length and breath. I can try and find out what the keel arrangement is (what material it is) and if she carries any ballast and where it is as she doesn’t have the centre keel.
I understand what you are saying regarding the tank not being the same as fixed ballast, the contents will only “run” to the lowest part, i.e. up the sides onto the top as she heels over. On another boat I have seen fuel running out of the deck vents when she was heeled, the owner fitted valves on these lines and a small vent along the centre line and up into the cockpit, opening the valve lines during filling the tanks.
I was a bit hesitant when Dad decided to remove a small proportion of the ballast.
I feel fairly sure there isn’t a lot wrong with the tenderness as if I step on her compared to a similar sized modern boat the modern boat sinks a lot more than our, if this makes sense.
I wish I knew a friendly designer up the road who could take a look and advise, unfortunately as a one off I don’t have anyone to compare notes….
Thanks again for your help
Michael Allo