Adding buoyancy roll reduction to monohull

[trapskiff]

I am planning on adding floats(amas) to my 36' monohull for roll dampening. Kurt Hughes has one version on a 38' hull
http://www.multihulldesigns.com/designs_stock/38tri.html

I was thinking of making the amas like the ones on the old flying boats with vertical metal struts straight down to the amas from a wing deck that extended out six feet on either side.

Placement would be from around midships (12' beam) back. Michael Kasten has a power trimaran design where he recommends putting the amas where the weight is which in my case is a 100 hp Yanmar diesel and two 50 gallon fuel tanks in the stern.

http://www.kastenmarine.com/PowerTrimaran.htm

What is the calculation of sufficent amas buoyancy?

John Shuttleworth says 100%-150% of the main hull displacement for amas
http://www.john-shuttleworth.com/Articles/NESTalk.html

Do I need 100% + displacement or could less serve the roll dampening aim considering that my hull is fine except for the rolling?

I noticed that Banka boats from the Phillippines have quite small amas.

I had a 36' boat previously that had a flopper stopper system or stabilizers as we call them in Nfld but it was a pain. OK for offshore voyages but I am coastal cruising and I had enough with the last boat.
Any thoughts would be welcome.

[Ad Hoc]

How much roll dampening do you want?
What do you currently have and what would you eventually like afterwards?

Yes, the banca's work very well...been on many

[trapskiff]

Right now we have no roll reducing equipment. We could easily add a stabilizing system with booms and fish as we have hydraulics but it is a big nuisance which we already went through. It is on most larger fishing boats here in Nfld.

We'd like to reduce roll to a minimum and get a couple of small wing decks in the process but it is not like our boat is unstable. It is a great seaboat but does roll a lot sideon to wind and swell. Running down in twenty five knots it is steady as a rock and steers straight as an arrow. But at anchor it rolls a lot.
This style has been built in Newfoundland for a long time. Ours was constructed by two fishermen without a plan. They've built a lot over the years as they needed them. We did all the wiring, hydraulics etc and helped with the woodwork and fiberglass. I have attached an old picture of a typical inshore boat similar to ours. They were called Bully Boats.
I recently found a picture of a Swedish boat that has amas like we're thinking about. Picture attached.

[trapskiff]

Here's the Swedish boat

[Ad Hoc]

So you just want to minimise roll at anchor?..Is this at sea or when moored in 'harbour'?

If you're very happy with the roll characteristics when at sea and in 'normal' duties, you need to be careful with additions. These will affect your GM, which will effect your roll period. (Do you have a stability book and weight breakdown of the boat?)

So, you really need to think about the proposed changes, is it a "nice to have" or a "I really need it because I'm not happy" type of issue.

[trapskiff]

Heading in a sea or running down is fine. Steaming side on to a swell there is a lot of roll. Not unsafe but everything gets bounced around. At anchor in a swell similarly. This is a comfort issue.
This is a traditional design with a long history of successful experience. and the builders figured it will carry 50,000lbs of fish so we don't have anything like that amount of weight. A single engine, 2 x 50 gallon aluminum fuel tanks and the simple house you see. I don't think stability is the issue. Comfort very much is.
Thanks

[ancient kayaker]

I assume 3 of the pictures in your first post are of your current boat. If it's like most work boats, more thought was given to capacity and survivability than to comfort. You are probably operating with the equivalent of empty holds so the CoG is rather high.

The rolling behavior should provide clues what to do. It may be unusually slow for this class of boat (lower the CoG) or it may just keep building up (add damping). Damping can be supplied with longitudinal edges. That is a very smooth looking hull which doesn't seem to have much damping. If the deck angle tends to follow the surface of the waves then primary stability may already be too high and adding amas may have little effect or even worsen the problem; that's not likely however.

I would be inclined to buy an old fisherman a couple of drinks, praise the traditional design to get him into an even better mood then describe the problem before he's too drunk to think of the solution.

[trapskiff]

Good thoughts. You are right about the current weight not being anything close to the weight this thing can carry . Apart from us, and a couple that our builders constructed for themselves, nobody has built a boat like this in Nfld in twenty years. Everything in the inshore is 27 foot fibreglass boats with twin 90hp outboards. After that the jump is to 35-40 with 220hp diesels which are 18 feet wide and need stabilizer systems with booms and fish etc if you want to stay upright.
Whatever I decide to do I'll send you a picture.
You'd like our kayak camp
www.coastalsafari.com
Thanks again

[Tcubed]

It is very important to understand and distinguish between roll damping and roll righting moments.

In the case of motorboats there is typically very little heeling moment so righting moments should not be very large or it can in certain circumstances be too much and create other undesirable motion characteristics. What is paramount is damping.

There are many ways to increase damping.

The rule of thumb about ama buoyancy is derived from the need to counter large heeling moments, which are not present in a motor vessel. Even minimal buoyancy ama's will provide massive damping along with other cons which must be taken into consideration for the given objective.

I can delve deeper for you if you want (if i see interest).

[trapskiff]

Thanks for your reply. It confirms my thought that the 150% amas recommended by Shuttleworth is for a vessel where the main hull is sometimes flying and totally supported by amas.
When I looked at the Ocean Alchemist ( pic attached) obviously they are not 150% of the main hull.
A slightly different arrangement is found on Michael Kasten's site:
http://www.kastenmarine.com/PowerTrimaran.htm#Image%20Links

http://www.kastenyachtdesign.com/maxsurf/images/pennywise_above_fwd.jpg

Roughly I have a 900 cu foot displacement on my main hull.
Any thoughts on this idea would be great.
When it comes to the downside it can not be any worse than a mechanical stabilization system. I recall a few years ago on our previous boat being at the entrance of a small harbour having to get the fish up before we could go in and having winch problems as we rolled out in a big swell a couple of hundred feet from a cliff. No fun!
Thanks again.