Stability Issues on my TUG - pls help!

Not totally useless. An estimate of the k (by aproximating distribution of mass) and a bit of measurements to estimate the displacement. Some leeway to both directions and you could say if there's possibly issues in stability or not. Very long or short period also gives some insight of the possible problems.

 

jfrech- Don't worry about the freeboard yet. Tugboats are designed like that. You can almost touch the water in the aft deck, the work area. It is the superstructure that is sealed with a watertight door, sometimes two. The door sill is high, 350 mm. minimum. Sometimes you need a step to go over the door.

 

The formula in the attached figure allows an estimate , inexact , of the GM value of a boat. Right.
But a value of GM, no other data, does not allow us to deduce anything about the stability of a boat.
From one point of view, a very short period of balance (which is short or long ? ) Can give an idea of ​​the boat is recovered easily from heeling induced by some external element . But that can do the boat very uncomfortable for passage .

Regarding my comments concerning the freeboard, just want to add that too low a value of freeboard will make the boat does not meet any of the standards that the boat, probably, must to fulfill. Regardless of the coaming height of doors and hatches.

The windage area , someone has grown a lot, can also be a problem .
I agree that it is best to put the boat in the hands of a naval architect for study .

As I said above , the boat does not have to have any problems but you need to check. Is now, not later, when you need to worry about potential problems. Now it is much cheaper fix.

 

To address prior comments. You can determine the GM by means of an inclining experiment [without doing a thorough hydrostatics calculation from the Lines Plan, etc], but that is all you get.

GMt= Wt. Moved x Dist. Moved x pendulum length(ft) / Disp.(Lbs) x Pendulum movement(ft)

This is the first but indicative number. Once you have that you need the hydrostatics data or drawing to work out the values for KM, KB, etc and eventually the KG.
The reason I brought this up on an earlier post is that if you do not have the hull "Lines Plan", developing one can be expensive. In most cases I have worked on (boats 35' to 130', whether lifting the lines manually or by laser scanning the costs for a small boat can run several thousand dollars for the scan and the stability assessment in various loading conditions.
A recent case: for an initial 'look' at a suspect GM on a 26' boat, we got it weighed (displacement) and did an abbreviated basic incline experiment. And then we did a roll period test (you don't need the vessel weight) to verify. The answers pretty much agreed- it was a low GM, as the boat's motion indicated. The cost for the work was about $ 1800.

Note:
the weight - use proper scales - Travelifts can be out as much as 15%- not good enough).
Fresh water is less dense that salt, your boat will sit a bit 'lower'. You 'scum line' is probably from the lake so you should be okay.

If you don't want to hire a professional, there is some general reading in non technical language on some technical boat stuff, Dave Gerr's 'Nature of Boats' is a good start.

 

JSL, sorry if I insist but there are times that I closed the brain and am not able to understand what is obvious to others. Your magic formula: "GMT = Wt. Moved Moved x Distance x pendulum length (ft) / Av. (Lbs) x Pendulum movement (ft)" has a term called Disp. (Lbs) you can not know without more.
When an inclining experiment is done, the first thing to be measured are the draft bow and stern, then, with the corresponding mean draft and trim, go to tables of hydrostatics values and calculate displacement, CoB position, CoG of waterplane and several other things.
Moreover, to calculate the weights to move, you have to perform a simple operation that must take into acount the value of displacement.
For all that I have to confess that I am not able to follow the procedure designed for you. And I would get it because there is no doubt that greatly simplifies things.
By the way, another thing that should be measured is the density of water at the time of the test.

 

If you've ever been on a Tub that went rolling to the left, and eventually the stern swung around to the left, and the dang thing is still rolling to the left, and the water us up over Rails and coming into the boat, and it's still rolling....and turning.....and wont stop, or come back....You'll know what roll rate is.

I was on a troop ship, that rolled over...over....over..till a Bell rang up in the wheel house.
Soon it started back. And it took forever to get back up, and only went a little bit in the correction way, and started back over again.
"Criminy, when's this gonna stop?" and the Sailor said: "How far is it to Japan?"

 

If she had a professional history to her, you might be able to find her stability book from a previous owner.

 

Years ago I went all round the Ijselmeer (Zuider Zee) and out in the North Sea to a couple of the Dutch Isands in a converted all steel Tug. However she was mainly stripped of her working 'stuff' and the large rope locker converted to accomodation. She was well behaved even in F8-9 off Hoorn but that was not open sea. The Ijselmeer is generally about 12 -16' deep and gets very choppy, but no real swell.
Worst thing was preparing food in her bow galley.....at least for some members of the party!. There was enough roll to engender a few green gills

She did not feel at all unstable though, so I think it would be wise to get your boat looked at as sugested by others. If anything the vessel I spent a couple of weeks on, was floating higher than her LWL by virtue of being stripped down yet she remained quite seaworthy IMHO. As she was owned by 3 Dutch Sea Scout troops, I think they would have been responsible enough to ensure a reasonable level of safety after her conversion.

 

A hull of that type is made of developable flat panels. You only need to take a few measurements to be able to pull a set of lines. They don't need to be super-accurate. If you are within an inch or so it should be fine. The waterline (actual scum line) will give you what the displacement is.

 

I can not match your opimnión, gonzo. Looking at the photograph of the two boats, I would not dare to say that the hull panels are developable. The plates of the sides have double curvature, light, but double curvature. The bottom, at its junction with the keel, is certainly not developable. The stern, which in these vessels is usually complicated, we can not see it. Therefore, although they are simple forms, I am convinced that it is not taking 4 measurements and draw a plan, but that must be treated with a little more care and seriousness. Saying that something is easy when it is not, does not increase my wisdom.

 

Could it be similar to a "Five sided dory" ( a small wood home made boat)

 

Plan B

Thank you all for your responces.

Was hoping there would be more of a difinitive answer, yet looks like I have to get a line plan drawn up at a minimum and then possilby a stabilty test. No really wanting to drop a few thousand on this yet I may have too. Would rather spend that on a new GPS display etc.

If there is an issue with stabilty, roll rate is bad and easily disrupted by small to med size waves. What can be done?
a) increase weight - by means of lead on the bottom centre of hull?
b) add a small wing with balastes on either side of hull mid point to lower centre of gravity and increase displacement yet also add to beam?
c) ??? any thing else ????

 

Surely there is a solution and does not have to be expensive. It is essential, imo, to know the CoG situation.

 

In my experience, very few inland tugs have any stability documentation and very few plans of any kind. In the US, inland tugs and towboats have traditionally been uninspected vessels, although the USCG is currently writing regulations to cover inland tugs. This change has come about because tow boat operators do not wish to be regulated by OSHA, which has jurisdiction over uninspected vessels according to recent court decisions.

Large ocean-going tugs usually are better documented, since many of these vessels have to carry a load line in order to go offshore or on an international voyage.

I can't comment on what other countries, like Canada, do with respect to regulating small tugs.

 

Subject vessel underway