How to decrease the inclining oscillation and increase the stability on the boat

Hello all,

We are a boat builder in Viet Nam.
Last month, we built a hardtop speedboat for a customer. The building process is good, but when we testing it, we realised it's not enough stability, the inclining oscillation is too big when we operating on the sea, it's seems like the boat is nearly capsized down (small waves (<0.5m height) and light wind weather (~5m/s). I think the reason is our cabin is too high and the hull shape is not good.

So, are there any method help to increase the stability on the boat and what we should do on the next boat to get rid of this problem? We don't want to delivery to customer a boat like this.

You can see my pictures for your reference:











Hope to receive your feedback soon.

Thank you very much.

 

Likely multiple factors at work, the high structure, the high (seemingly) level of the cabin floor, insufficient waterline beam, a fair degree of deadrise in the bottom of the boat. All you can do is add some kind of buoyancy bands to both sides. Something like this, but scaled up to suit your boat, is a possible remedy:

Kapten Boat Collars https://www.facebook.com/KaptenBoatCollar/

Looking at the lines drawing, I would venture this would be a quite tippy boat, even without the block of flats sitting atop it. You will have to add a lot of buoyancy at the waterline along each side, or have a very scary boat. The waterline beam particularly, seems quite inadequate.

 

How does the lighthship weight compare to your predictions? You can add permanent ballast to reduce VCG but only if there is enough deadweight capacity on this boat to do this

 

The shapes of the hood appear to be correct but, indeed, the superstructure is very high and the surface exposed to the wind is very large.
The freeboard mark is seen on the side. You should recalculate the minimum freeboard required to try to reduce it. That would allow you to place ballast on the bottom, on the keel, to increase stability. Well-sized balance keels, both sides, could reduce the balance of the boat.
Perhaps, in addition, you could try to lower the height of the roof of the superstructure.

 

It is a narrow-gutted nightmare, looking at the lines drawing and the dimensions, quite unsuitable to the task it has been assigned. Sand-bagging won't save it, it cries out for more waterline beam, which obviously can't be done with the existing mould. The beam across the chine seems to be not much more than 5 feet, just an accident waiting to happen. Needs to be about 18" more, at the very least, then do the hydrostatics for that, it would still be questionable. The planing performance (or lack thereof) of this boat also seems highly questionable.

 

I have not done any calculation and, therefore, I do not dare to say if the sleeve is very big or if it is very small. Of course the feeling is that, probably due to the great height of the superstructure, the beam is small. But maybe it's an optical effect.
What is very clear is that in the boat that is already built, increasing the sleeve will not be easy. It is necessary to give a solution that is viable and, of course, what always increases the stability is to reduce the vertical position of the center of gravity and to diminish the surface exposed to the wind.
If fixed ballast is placed, better lead than sand, and perfectly fixed to the hull.
But, as I always say, it is only my opinion.

 

You can't do much about the COG without getting rid of the superstructure, and/or adding an amount of ballast that would render the thing a total slug in terms of planing performance. Take a look at the overhang at the sides, it is extreme. ANd it is a "skinny" boat to start with.

 

You really need to provide the GZ curve...and also...what is the vesels natural period of roll too.

 

Yes, it is a "skinny" boat, there is no doubt, but from there to say that you have to increase the beam at least 18 " there is an abyss.
On the other hand, a ship of that size, with such a high superstructure, looks like anything but a glider. I also assume that Nickname123 will need to increase stability, and reduce rolling, prior to planing.
Talking about whether the ballast will influence plan performance, when we do not even know if the OP wants the ship to reach the planing situation, is a bit risky. But of course, knowing so little about the ship and what the ship owner wants to do with it, it is not easy to comment. All we know is that the OP wants to increase stability (what does that mean for him?) and reduce the rolling. To do this, without changing the mold, you can only lower the CoG and decrease the surface exposed to the wind (In the pictures you can see a great distance between the passenger's head and the roof. It is necessary?). Undoubtedly, those changes will cause some other performance of the boat to change. But, as always, it is a compromise between opposing variables.

 

It is, of course a planing hull, with a big banger on the back. THE OP wants to avoid a "glug, glug, glug" experience, and that is why he wants a solution to prevent it happening. I really think the moulded shape is a lost cause, but he might be able to modify what has already been built.

 

I agree, it's a hull planing. What I do not know is if the OP intends to take the boat to the planing regime. In any case, when the boat enters this regime, the conditions of stability and balance are totally different because other forces come into play, which do not act when the boat is stationary.

 

He would not have an outboard that size unless there was a desire for speed. Of course it has to operate successfully both on plane, and off. I doubt that boat would be much chop at either, in fact it scares me !

 

It would all become academic when someone in the cabin spotted a "mermaid", and everyone moved to that side for a closer look......whoops ! This is probably the most frightening thing in Vietnam since a B-52 raid.

 

The GZ curve?, yes and the dynamic stability curve and several other things, but if what you want is to increase the initial stability (initial GMt), that curve is useless.

 

If you ignore the armchair background chatter, as they do not understand facts or data...yet pontificate as if they do reasoning "common sense".....yawn!

Your GZ curve will tell you how much, if possible, changes are required to obtain a better degree of statical stability. Since it is all about moments and waterplane inertia.