Will reducing weight of Sea Ray 42, affect stability?

[Magnetar]

A Sea Ray 390 Motor Yacht I own was damaged in Hurricane Sandy.
Performing the repairs required the removal of much of the interior.
I would very much like to abandon the heavy plywood interior elements and opt to refit the interior with newer lightweight composites for all the interior structural elements. The dry weight of the vessel is 28000 lbs.
I would be reducing the weight by about 3000 lbs. Bringing the dryweight to around 25000 lbs. With the substantial, heavy, upper structure of the vessel, will all this weight reduction below deck lead to a boat that is too top heavy?
It already has a large profile to catch the wind. I am wondering if all this excessively heavy furniture and cabinetry is thought of as Balast in a sense.

It is so heavy and unnecessarily so. I am thinking that such a weight savings will lead to a lowering of RPMs required to remain on plane, an increase in fuel savings and faster speeds - or- is this heavy weight, a design necessity, and by lightening it I risk having a top heavy boat?

 

[michael pierzga]

I doubt it. When you operate with empty water, fuel tanks you boat is not unstable .

Your boat was probably substantaly over design displacement to begin with.

Minus a ton of junk above the waterline sound s good to me.

 

[TANSL]

From the point of view of stability, if the c. of g. resulting is higher than at present, can be harmful. But it is possible that despite that the boat remains stable enough. We can not say anything without making a study of weights, new c. of g. position and study of the resulting stability curves, GZ and DN, for normal load conditions.
An 8% reduction in weight is a lot. Should be treated with care and, of course, not give opinions that are not backed by a calculation.

 

[michael pierzga]

Also check your weight saving calc.

3000 sounds a bit optimistic. Thats equal to removing 75 sheets of half inch marine ply from your interior

 

[Magnetar]

Thank You for your responses

I will be moving forward with the lightening of the vessel. I'm not sure how easy it will be to calculate the new center of gravity - would love some details on how I can calculate this.

 

[Ike]

Well I would suggest hiring a naval architect. You will need the original parameters for the boat, Displacement, CG, Center of buoyancy, metacentric height. You could do an inclining before and after removing weights. But an NA would know how to do all that.

 

[TANSL]

I totally agree with Ike. It is essential to carry out an inclining test of the ship before the reform, to determine her actual weight and the position of cog. After obtaining these data will be easy to determine whether changes you want to introduce are feasible or not.
In my country, for example, when the lightship weight variation is greater than 3%, it is mandatory to carry out the inclining test.

 

[groper]

My friends owns a searay... It's the most overweight, stern heavy, fuel hungry, poor performing boat of this size I've ever had the misfortune of travelling on... Lightening the boat is a splendid idea and should have been done by the factory designers (******) when they built them... But the others are correct, you won't know the stability effect until its analysed and tested... My friends boat is stern heavy, so if th interior is lightened in the forward cabin, it's just going to get even more unbalanced riding more on its transom.... Ain't much you can do besides sell the damn thing IMHO....

 

[Tad]

Guys if you re-read the first two lines you'll see it's too late at this point for an inclining or even to calculate actual displacement. Possibly one could find another identical model and check flotation and even incline her. Real stability of these was not great and almost the entire interior (except the aft cabin sole) is above waterline. Heaviest items on the boat are tanks, engines, and generator, they aren't moving.....

Lightening the interior will be a good move but someone should look at the stability once she's re-launched. That idea usually falls on deaf ears.

 

[TANSL]

Guys if you re-read the first two lines you'll see it's too late .

You are quite right. The solution is:
1. - carry out the desired changes.
2. - make the inclining test once the boat is finished.
3. - add fixed ballast (the minimum and as low as possible), if necessary to meet the stability criteria.

 

[Paul Kotzebue]

Lightening the interior might raise the VCG and reduce stability, but probably not by much. You can measure metacentric height with an inclining which may be enough information to determine if the boat is stable enough. You won't know displacement or location of the center of gravity from the inclining unless you have access to the hull lines or hydrostatics (doubtful).

 

[TANSL]

You won't know displacement or location of the center of gravity from the inclining unless you have access to the hull lines or hydrostatics (doubtful).

You're right, of course, but in these cases you have to take several dimensions to draw the lines plan of the ship. I've done it many times and there are procedures that allow you to do very quickly and relatively low prices.