Is there a way to make my trawler boat more stable?

[Teemu Visakko]

Hi everyone!

I have a ~27 feet trawler fiberglass boat and I really love it but the only thing that really bothers me with it is the heeling from side to side when there is either more people sitting on the other side of the center line or when someone is walking on the side decks etc. One good example is that if I'm driving the boat alone and going to the bathroom, the boat heels between 20cm (8 inches) and 30cm (12 inches) on that side.

Specs on the boat:
- 28hp Volvo Penta engine with cruising speed of 6kts and max speed of 7kts
- Draft of 60cm (~2 feet)
- Length overall 8,5 meters (~28 feet) - including swim platform
- Length at the waterline 7,5 meters (~24,5 feet)
- Max width at the middle 2,55 meters (~8,45 feet)
- Weight approx. 2,5 tons

I have been trying to think ways to reduce the heel because of weight distribution (this also is happening when driving through side waves but this is less of an issue). I have put about 200kg (440lbs) of lead weights to the bilge of the boat to make it more bottom heavy and this has helped a little but not enough.

You guys have way more knowledge than I do when it comes to boat designs so I would like to get your input on if there is something that could be done.

My different ideas so far on how to mitigate this issue:
- Building pontoons on the side of the boat that are attached to the hull so the pontoons are floating on top of the water and take the load when boat starts to heel to reduce this. Basically making the boat as a small trimaran. In my mind, this is the most effective way to reduce the heel but it's also the "ugliest".
- Building some solid "trim tabs" just below the waterline from the middle of the hull all the way to the back that are on level with the water to take the load when the boat starts to heel to one side or another to reduce/eliminate the heeling
- Increasing the keel size and draft but I think this is not that effective unless I increase the draft by a lot
- Installing adjustable trim tabs to the back of the hull

Below, you can find couple of photos from the boat and if necessary, I can try and create some 3D models of the hull to better visualize what I have come up with so far.

 

[seasquirt]

Don't put so much water in the bath !

 

[TANSL]

The simplest and most effective way to increase stability is to lower the ship's CoG.
For a boat 8.5 m long, a weight of 2.5 tons fully loaded is very little. This means that you can increase the ballast quite a bit to get the Cog down. Growing the keel on the outside with steel or lead strips is the best solution I can think of.

 

[Ad Hoc]

......My different ideas so far on how to mitigate this issue:
- Building pontoons on the side of the boat that are attached to the hull so the pontoons are floating on top of the water and take the load when boat starts to heel to reduce this. Basically making the boat as a small trimaran. In my mind, this is ..

The stability is dictated by the location of the KG and the metacentric height in relation to the KG.

Thus lowering the KG will help. But adding too much weight, in an attempt to lower the KG may back fire. Since the displacement will increase.
Why is this important?

The location of the metacentric height, M, is governed by the BM= I/V
In simple terms if the V (volume - but think displacement) in creases, the value of BM will reduce. Because the 'I' remains the same. Ergo the means of restoring itself, the righting lever GZ, will also reduces, since the GM reduces.

Therefore, there is only so much of lowering the KG can assist, with all things being equal.

Thus, increasing the value of 'I' will be the other part of the solution, to which you have touched upon.
So if you added blisters, or basically widening the hull just above the waterline, like so:

So the added width of the blister increases the I to a value that you need, that would do it.
Rather than adding amas or outriggers.
It wont be a pretty boat, but it will achieve your objective.

These blisters can also be foam filled, so they act as additional buoyancy if punctured or damaged.

 

[bajansailor]

It might also help if you can remove that strange section of superstructure between the wheelhouse and the aft cabin (does it serve a useful purpose?) - this is a lot of weight (relatively) quite high up, and removing it should help to reduce your KG a bit (and thus increase your GM and the righting lever GZ).

 

[Will Gilmore]

So if you added blisters, or basically widening the hull just above the waterline, like so:

Why just above the waterline? If the added floatation is not in the water, the hull must necessarily heel before that added flotation will stabilize it. Let some of the displacement be carried by the wider "blisters" to begin with. Perhapse those foam blisters can be molded into a hull form that looks like it is part of the naturally designed hull. Basically, use foam and glass to build a second outer hull around the chine area of the original boat.

Take the deep vee and fill in to make it more like the flat bottom or tri-hull. Glass to cover.

-Will

 

[TANSL]

The best solution is to place ballast as low as possible. The boat perfectly allows adding 600 kg to its mLDC. That will greatly lower your CoG while increasing the draft and, therefore, the inertia of the waterplane. The wet surface, and thus the resistance, does not increase as much as adding flotation volume. Don't get involved in doing very expensive works that don't improve the situation any more than the solution I'm telling you. Others don't care but you sure do care about your money.

 

[Milehog]

First, keep it simple. Adding sponsons will kill the resale value of the hull and cause negative unintended consequences.
First, learn to live with the boat. Really.
How important is the tall, aft part of the superstructure? Can it be replaced with a sailboat style companionway?
Some ballast may help but you still need to accept a boat your size will act the way it does.

 

[Ad Hoc]

Why just above the waterline? ....

Because it doesn't effect the resistance and hence the powering. All things remain the same, except once the vessel rolls.
Once it does roll, it gains the increase in righting moment required.

 

[Teemu Visakko]

It might also help if you can remove that strange section of superstructure between the wheelhouse and the aft cabin (does it serve a useful purpose?) - this is a lot of weight (relatively) quite high up, and removing it should help to reduce your KG a bit (and thus increase your GM and the righting lever GZ).

This structure houses the head so it cannot be removed. From the factory, this boat didn't have any head on it and the previous owner build this "upgrade" to get the head installed. I would say that all in all, this additional structure (including the solar panel), weighs about 100kg so it shouldn't make that big of a difference.

 

[DCockey]

I would say that all in all, this additional structure (including the solar panel), weighs about 100kg so it shouldn't make that big of a difference.

That may be wishful thinking. The CG of the addition is a considerable height above the waterline.

 

[TANSL]

Moving 100 kg, 1.5 meter higher than where it is, for a 2500 kg boat, could mean something like raising the total CoG by 6 cm. It doesn't seem very worrying. More worrying is the increase in the surface area exposed to the wind, which, on this boat, does seem to be greater than normal.

 

[Milehog]

Do you really need to stand up to pee? Really?
Before you add more unnecessary stuff, remove other unnecessary stuff.