BALLAST, where and how much

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Oryen

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Hi

I am refitting a boat. The new equipment is heavier than than the old equipment. I am wondering whether I should add ballast to the bow.

Specs:
Mono Hull Motor yacht with flybridge
LOA: 43’
Max Beam: 11’
Previous displacement: 8.5 ton
Centre of Buoyancy: 3.5m from transom

I am adding an additional 700 kg (+-) of weight to the engine room (additional to the previous equipment’s weight). How do I calculate the position and amount (weight) of ballast to add? Is there a formula to calculate this?

The weight is added as the engine size has increased along with some additional equipment.

See attached for drawing of vessel.

I would appreciate any assistance regarding ballasts.

Regards
Oryen
 
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I think that this is going to fit into the trial and error folder. Without properly understanding the buoyancy of the boat, I think it very difficult to predict how she will settle in the water.

Good luck.
 
It's a bit difficult to work out the TPI effect of the new engines, but the volumetric profile seems to show the majority of the flotation towards the stern. The new mass is +0.8% of the quoted displacement, and might indeed be even less when the boat is fully loaded with stores, fuel and personal belongings.

Instead of adding ballast (which is unuseable deadweight) can you move one of the fuel tanks further for'd ? Or a water tank ?


EDIT

It looks as if the CofG of the new engines is about 2.7m aft of the CofB. So to balance up, a mass of 350kg needs to be placed 5.4m ahead - up in the for'd cabin area. This is going to increase the moments of polar inertia a lot, and might make the boat porpoise significantly. How does the boat handle with the water and fuel tanks nearly empty ? Impossibly nose up ?

I'd try anything except adding ballast.

You may have to think of trim tabs if not already fitted.
 
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Oryen said:
Hi

I am refitting a boat. The new equipment is heavier than than the old equipment. I am wondering whether I should add ballast to the bow.

Specs:
Mono Hull Motor yacht with flybridge
LOA: 43’
Max Beam: 11’
Previous displacement: 8.5 ton
Centre of Buoyancy: 3.5m from transom

I am adding an additional 700 kg (+-) of weight to the engine room (additional to the previous equipment’s weight). How do I calculate the position and amount (weight) of ballast to add? Is there a formula to calculate this?

The weight is added as the engine size has increased along with some additional equipment.

See attached for drawing of vessel.

I would appreciate any assistance regarding ballasts.

Regards
Oryen
Click to expand...

posting this from my tablet so may be some mistakes.

If the extra weight is being added where the centre of bouyancy is located, then you may not require any more ballast at the bow. I had this situation years ago when i cha ged the engine in my boat and added the difference in weight to the bow but found that it pulled the boat down too much so dumped about half of the extra balast. One point is that if you add ballast, you should make sure it is secure and not free to move arround.




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With only 700kg, I'd do nothing and just try it. It might be fine. If you feel somethig is needed then bigger anchor/chain might do it (though that will increase polar inertia/angular momentum, but that may not matter)

Those drawings are odd. Are they from the builder? The waterline is implausibly high (ie boat very sunken) in top drawing, compared with middle 2. And the left hand drawing of the middle 2, "transom view", implausibly shows more deadrise amidships than at transom which is inconsistent with the top drawing. So if you are doing CoB calcs from those drawings take care cos the answers are bound to be wrong
 
If new engines are bigger, are they also more powerful? If so, you may want more weight aft at the higher planing speeds.

I agree with others, try it and see. It's easy to add the ballast later, so no need to second guess it now.
 
Thank you all for the responses. They have all helped.

From what I deduced, 700kg in the stern (near the CoB) is not a significant change on this scale. Possibly the only option is trial and error as mentioned a couple of times.

Sarabande: you mentioned " a mass of 350kg needs to be placed 5.4m ahead " is there a formula for how you reached these figures? Can you share it please?

The boat was bought on the blocks. So no sea trials have been done by me (i.e with empty tanks or full). Unfortunately moving tanks is not possible at this stage. We are indeed fitting Trim tabs (not present before) the Volvo's QS vertical blade trim tabs. But I am more worried about the boats static position, not sitting level at a stand still

As Nicki_H has mentioned the engines are indeed 130HP more powerful (twin installation, so total 260 HP additional). Possibly the extra weight is necessary. Is there a way to determine this?

Bandit: It is a GRP mono hull boat with fly-bridge. We are removing everything and replacing it all with new. The new engines and the generator are bigger than the old ones. It is a twin installation sterndrive (2 x Volvo Penta D6 370HP) and a 13.3KVA Coelmo generator. We are adding a water heater/calorifier, in the engine room, and changing the position of the Air conditioner (from cabin compartment to a sealed box in the engine room).

I will try and avoid adding ballast and test it once we put it back in the water. I will post up results once I have launched.

Thanks
Oryen
 
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