How To Decrease the Rolling of an Existing Vessel

Hi,
I have come across a nonclass Tug Boat that is already built. The vessel is experiencing too much rolling and pitching. The principal particulars of the vessels are LOA 12.2m, Beam 4.68m, Depth 1.8M Draught 1.4M. The GA and other information are attached herewith. Now at this stage, how would you like to suggest steps to reduce excessive rolling and pitching?

I was thinking about adding bilge keels and solid ballasting. I assume the CG has gone too high that is causing this excessive rolling. What do you think? How to solve this problem?

 

You would need to have degree and frequency info on the roll or pitch cycles to garner some idea if it was "excessive". Otherwise it is a bit subjective. It doesn't "look" top-heavy, nor does it "look" like the form would pitch or roll madly

 

So can you define "too much".
What roll period/angle does she currently have... what would you like it to be?
And then the same for pitching.

Without knowing this it is hard to state what may or may not be done.

 

That is a very neat little tug - with some impressive power and bollard pull.
Was she built in Bangladesh?
Re bilge keels, there initially appears to be a fairly substantial bilge keel shown in the photo of the boat out of the water - but when I saved the photo and enlarged it, it appears to actually be the side of the hull in way of the bilge.
Where are you proposing to add a bilge keel?

I am also interested to find out how much is 'too much' re rolling and pitching.
What is the typical sea state that the vessel encounters when things become 'too much'?

And what is her stability like - do you have a stability booklet?

Re the 2 tonne crane, does this only lift over the bow, or can it slew to the side as well?
If it can lift 2 tonnes over the side at 2.5 m. outreach, does she heel a lot?

I am reminded of @DogCavalry and his quest to design / find a suitable crane for his boat.

 

The picture shows it trimming well down at the stern, how that is affecting matters regarding pitching might be pertinent.

 

It looks like she rolls because she's deep in the middle *with a slightly rounded bottom* (second look, not rounded), but she has a lot of buoyancy near the surface and out at her full width. While the deep hull lifts her, the canister-like bilges react to the waves, like rocking her on a fulcrum.
Put a mast and crane on her to get some weight high, away from the CG to provide inertial dampening like on a sailboat.

BTW. I know not of what I speak. I'm a sailor, and a cabinet maker, not a NA.

She's real cute. I like her.

 

If the superstructure is built of steel, and the crane as well, that's a lot of mass high up. Pitch and roll moments of inertia are high, with a short wide but rounded hull. Natural roll and pitch frequencies evidently correspond to local conditions.

How extensive is it feasable to modify the vessel? Changing natural frequencies not be easy.

 

Lighten the superstructure, extend the bow. That bow area looks like the side of a bowl. With that combination, amazing if she didn't pitch and roll.

 

I think, before you can proceed, it must be understood if she makes her lines. That is what is the weight and available loading, if any. Did I miss that? It appears to be below her lines aft and above them for'd?

I do not have a c.v., but am wondering if you have established how much ballast can be added. Ballast tanks at waterline could be tested. If the boat is pitching, then the shallow forefoot along with the crane forward is a rather easy suspect. But, that does not agree with a simple view of the lines of we can take them from a picture.

I'd say the boat needs a much more comprehensive weight study than we can offer here.

My gut hunch, and nothing more, is that she is stern low, or bows high and with a shallow forefoot, she is getting tossed about like a fishing cork, not really as much pitching and rolling as haphazard. The push to power and a big steel house and the lack of a good final effort to balance her is the cause. But I cpuld be wrong. When she has a crane load on, if she makes her lines; you may need some variable ballasting, etc.

 

I think your gut may be right. The hull as designed is a wave follower (i.e. high waterplane inertia compared to mass) and the dimensions are poorly selected with the beam being right at the 3 second wave period which is the mean period for SS 2. I doubt if rolling can be effectively controlled.

 

Can you expound a bit? What formula(s) are you reflecting on? Thanks.

 

This is a perfect opportunity to point to Dynamics of Marine Vehicles by Bhattacharya with specific attention to Chapter 4 (Uncoupled Heaving, Pitching, and Rolling Motions) and Table 5.1 (Typical Sea State data). Basically, when the wave period and therefore wave length are near unity with the beam, forced rolling occurs (i.e. when one side of the boat is on the crest, the other side is in the trough). If GM is low (i.e. the vessel has a long roll period) this is not a problem. However, GM is Ixx/volume +KB-KG...so a large rectangular waterplane like this vessel has coupled with small submerged volume leads to a large GM and consequently a fast roll period. I can't do the the numbers with the data provided, but I bet the natural period of roll is very close to the mean period of what would be considered a low Sea State. It is an easy mistake to make, thinking that high GM is good for a crane vessel, but forgetting how short the wavelengths are at low Sea State.

 

That's not correct...
With a nominal T =4.5s for SS2 wave the wavelength = 1.56 x 4.5^2 = 31.6m.
This tug does not have a 31.6m beam!

Based upon the data proposed, we can estimate the Ixx with a nominal value with Ixx=12x4.6^3/12 = 102.5m^4
The volume...well, looking at the shape we can 'discount' the aft end 1-2m, so vol = 10x4.6x1.4x0.8 = 51.5m^3
The 0.8 as a 'shape' factor.

So BM = 102.5/51.5 = 1.99 = 2.0m

The KB..we can estimate as roughly 1/2 draft as the hull shape lends itself to such. = 0.7m

The KG...looking at the GA..will be above the engines, but not by much. Since there is a large wide deck which will be as heavy as the engines - roughly. So with a hull depth of 1.8m, let's say KG=1.7m

So, GM = 0.7 + 2.0 - 1.7 = 1.0m

Roll period can be estimated = 2.Pi. (0.45x4.6)/sqrt(9.81x1.0+ = 4.15 = 4.2s

So, yes, the natural roll period is very very closely aligned with the wave period.
So this is correct

 

Tomato, Potato...Nominal, Mean. Assuming a random seaway with Rayleigh distribution and Pierson-Moskowitz Sea Spectrum, yes I would concur that the period of maximum energy is closer to 4.5, but the mean period is closer to 3.0.
Anyway, we agree that it is a poor selection of dimensions.