ROLL Control pleasure boat

FF

A post elsewhere in this forum directed me to http://www.loganboats.co.nz/index.html

Jib sail and bilge keels on a very good looking vessel. Worth a look. Read the log of the Cadenza as she crossed the Irish Sea from Wales to Ireland.

http://www.vetus.nl/en/index.php on pages 26 & 27 have their own hydraulic stabilisers.

Pericles

 

Roll center ?

How is determined (or approximated) this point ?

Stabilizer fins should should be aligned with that point to have the best efficiency.

But from what I have understood, small move of boat (roll or pitch) are around CF CoG of Waterplane (of course variable point). And inertial forces are around CG CoG of boat ?
Not speaking of added mass effect / hydrodynamic damping.

 

Using tabs has far more appeal than active fins that can be damaged,and cause extra drag.

This boat will have trim tabs , as the reports of using high speed on these hulls suggests too much stern lift and bow steering.

The question then becomes one of response time, electric , hydraulic or pneumatic.

AS the boat will have a Hyd system for on board tasks , I would assume the solenoid controls used on autopilots on boats with full time pressurized hyd steering would be simplest to adapt.????

FF

 

No one for roll center ?

 

It would seem that a small trim tab bump to raise one tab , or lower the other ,should be able to keep the boat more stable in a beam sea.

The effort to effect a light weight ( under 8000lb) very narrow (7.5ft) boat at speed should be far less than a 3-1 L-B boat weighing many tons.

What does an "interceptor" look like ?

Since electronic control seems to be a huge portion of the cost. I would have NO problem with an antiroll "Joy stick" or old fashioned aircraft rudder pedals.

The lowest cost version would not be trim tab based, but could be a direct swivel of a balanced "center board" , which could be retracted for beaching or non rolly courses.This could be built for a few hundred bucks , rather than thousands.

How would the location of the center board be optimized to reduce roll?

FF

 

Fast fred rolling along

Years ago I was at a commercial boat show in New England somewhere. I was checking out this builders boat models when I came across his answer to rolling. He had a box about 16 high 24 wide and the lenght was as wide as the cabin roof. The box lenght went from side to side acrossthe cabin roof.
Inside the box\tank was three baffles with drain holes. The box\tank had enough water in it, about half full to soften the roll when the water was tring to drain to the other side during the roll.
What do you think.... never saw anything like it again

 

Beware I am not engineer at all, even less marine.

Seems that a anti roll device should be able to heel a boat 4° to 5°.

So the effort on fin like device should be righting moment at 4-5 ° heel / lever arm of the fin.

Righting moment is GM*sin(heel)*displ. for low heel angle.
lever arm of the fin is approx middle span of the fin to somewhere between CF and CG (Section view).

So for normal fins, for same righting moment, the narrower the boat, the greater the effort on the fin.


Now, for lighter boats, there is minimum righting moment to reach (heel during turns, offset loads, wind + vawes). So lights boats increase GM to get some stability. (Form stability (GM term) for light boats, weight stability (Displ term) for heavier ones).

So the effort on a fin on a narrow light boat may not be that smaller than the effort on a heavy beamy boat.


Next, instead of speaking of effort, it would be better to speak of power = effort / time.

As BMcF said, fin period (one stop to the other, then back to first stop) should be 1/5 of boat natural roll period.

T (roll period) = 0.44 * B / sqrt(GM).

So the narrower the boat, the higher the GM, the quickest the roll period. Again a light narrow boat will need much more power to move fins, because even if the effort is lighter, he will have much less time to do it.

Some numerical application:
Disp = 8000 lbs. Bwl = 7.5 B=8 GM=3

4*200 lbs people on the same catwalk will make heel the boat 8°.

Roll period = 2 seconds.

That means fin period = 0.4 s : one stop, to the other end, back to the first one.
Expressed other way : fin middle between the two stop : should reach requested postion within 0.1 seconds.
For comparison, an electric autopilot is 4s for full stroke : 8 sec for full period. A antiroll device for that application need to be 20 times faster.


Electronic is not the cost. as said, mems gyro is 30$, and a 5$ microcontroller will fit. The software programming for a PID control is an exercise for first year automation student. What I have not checked is the finding of the natural heel of the boat. Maybe the natural heel is where the average displacement of fin is centered on the neutral position of fins. Also there is an issue with very long period of rolling, such as quartering seas, when the boat is slighly faster or slower than the waves.

The real cost is the actuation mechanism:
Say the fin move between -10 + 10° 0° is neutral.

the fin is neutral:
the microcontrol will say:
go to +8° : 0.1s to do it.
go to +6° : 0.1s to do it.
go to -7° : 0.15s to do it.
back to neutral : 0.1 s to do it.




NB For interceptors, see http://www.maritimedynamics.com/interceptor.pdf

 

You can also go a bit further in stability preliminary estimation:

Say Lwl = 36 ft. Bwl = 7 ft, disp 8000 lbs.

You can have BM = Cwp^2 * Lwl * Bwl^3 / 12 * disp

Cwp Waterplane coefficient around .75 (rather full)


That should give you BM aroud 3.5 ft. (optimistic)

B = estimated 1/3 D (D canoe draft).

D = disp / Cp * Cm * Lwl * Bwl

Cp = hi speed powerboad = 0.64.
Cm = Midship coefficient = somewhere around 0.6 = between triangle(.5) and ellipse. (.78)

D = 1.5 ft.

B = 1/2 ft below waterplane.

BM = 3.5 ft, GM = 3ft (previous post).

G = around waterplane = well ballasted sailboat, not powerboat.

Preliminary estimation of G for a powerboat would be around mid freeboard.


Assume 3 ft freeboard = rather low freeboard.

G = 1.5 ft above waterplane.


Now GM = 1.5 ft.

Recompute the heel

Now the heel with 4 people on the catwalk around 15° . GM approximation may be out of range.
Heel angle with 2 people = 8°.
Rather tender boat, you should seriously analyze stability if you want some moderate offshore capabilities.

4 ft freeboard would give GM = 1ft : serious analysis of sailing uprigth capability to do.


NB virtually ALL powerboats have engines and heavy weigths in the bilge. Mid freeboard CG estimation is for theses conditions. You may think of adding lead ballast in the very bottom bilge or in a long keel like older sailboats. But you will have to build VERY ligth to have a displacement of 8000 lbs including engine weigth, and still have allowance for ballast. Heavy engine wont fit : Typical engine 3 ft heigh will have its CG around or slightly above waterplane (D Canoe draft = 1.5 ft). Not low enough.


Another possibility is increasing waterplane Beam. you gain stability, but you loose containerability, decrease powering efficiency and motion comfort ration (bigger waterplane for same displacement).



Roll period = 2.7 second.

You fin should now move in 135 millisecond, instead of 100 millisecond. Not a big change.

 

Thanks for all the help.

I wonder if with a roll period of only 2 seconds! weather the hull would simply "surge (rise) rather than have time to roll?

If so the simple nonpowered centerboard might give enough roll resistance for the time it takes for the hull to lift , rather than roll.

Yes the boat will seem really tender dockside , but that concerns me far less than the ride quality.

The hull will be based ob one of the Atkin box keel, reverse deadrise hull styles , which seem to have a good reputation for a seaworthy, seakindly ride while underway.

Inside the box\tank was three baffles with drain holes. The box\tank had enough water in it, about half full to soften the roll when the water was tring to drain to the other side during the roll."

I think these are known as Fram or surge tanks.
I know of the concept , but the interior volume is so tiny with a 7.5ft beam I would be reluctant to loose much room, and the water wont have much time to get from side to side , if the roll period is 2 seconds.

FF

 

You are confusing things. The natural roll period is something inherent to the boat. It is not the rolling period of the boat in waves. The rolling period in waves is of course the period at which the boat encounter waves.

If the boat is hit abeam by waves with a 10 s period, the boat will roll with a 10 sec period. As the wave period is very different than the boat rolling period, the boat will gently follow the wave slope at twice the angle. For normal waves 1/13 heigh / wavelength, it will heel around 10 ° each side.

If the wave period comes nearer the boat natural roll period, things becomes nasty. The heel will be much more than twice the wave slope. up to 4x or 6x. because it will enter in resonant rolling.

And if your boat, 7 ft Bwl, 2s natural roll period meet abeam a chop 2 sec period, 1 ft heigh, 13 ft wavelength, it will ROLL nastily, probably throwing things inside due to the roll speed, and the helmsman will probably change heading within minutes for more comfort.

Offset heeling hangle is used by ISO as a way to assess seaworthiness of small boats in ligth conditions (Category C and D less than 40 ft). Before speaking of ride quality, you will be more concerned by floating uprigth in most wheater conditions you intent.

 

Anti-Roll Tanks

These work but only on large vessels. The tanks are tuned to vessels roll frequency, the neat thing about then is that they work at anchor. The calculation on these is complex and they require a strong structure to hold weight. Do you remember boat builder?

 

"you will be more concerned by floating uprigth in most wheater conditions you intent."

Indeed the boat will be expected to make a few offshore , or long shore passages to be able to visit the Med and beyond.

My readings suggested the long narrow shape would help the seaworthyness and sea kindlyness of the boat , and I was planning on no basic restrictions on travel.

No the North sea in winter wasn't on the plan, but being caught out in a 50K thunderstorm , or making (uncomfortable) progress an 8ft sea should be doable.

Can these hull dimensions be used for these conditions? With the 1300lb engine in the box keel I would have assumed the CG would be low enough.

I do expect the boat to be tender when in port , and to move a good deal when walking on the deck.
IF it proves too uncomfortable simply securing , mechanically and firmly the dink alongside should solve much od the motion.

FF

 

I fear you do not uderstand what your figures mean for the boat. You should really try something like Freeship, or other design tools, to draw hull lines and check some basic figures with your expectations.

FYI, a 40 ft 10ft beam 8000 lbs looks like : http://www.classicsolution.nl/frameset.html
http://www.stadtdesign.com/images/products/608-7-0.jpg

You see how low profile is the boat, and the very limited surface where you can stand up (around the engine only). And even then the headroom is limited to barely above 6 ft. No mast, no antenna, no radar, nothing on top.

It has a waterline beam around 9 ft. Trying 7 ft would mean narrwing the boat 2ft. Just imagine the photo without the catwalks. In previous post, I said initial stability is proportional to Bwl^3. 7^3 = 343 9^3 = 729. So narrowing by 2ft, you just cut initial stability by more than two. Very significant.

Another with the right beam, but a bit shorter and lighter is: http://www.parker-marine.com/interiorcom36.htm
http://www.parker-marine.com/descom36.htm
You will note the designer said of his boat : "It should be born in mind that standing headroom is limited to 6' in the cabin (as designed), and cannot be increased more than 1". "

Imagine now lengthening it 10%. You have now a 39ft boat weighting 6600. Add now 20% in depth to the underwater part of hull. Your boat will now weight 7900 lbs and will have a draft of 12 inches + 20% = 14.4 inches of draft.

You now have something like 40ft long, 8ft beam, 8000 lbs boat. Are these boats seaworthy ?

 

Trawler twin screw Vs. Single

I am looking at Used trawlers like Island Gypsy, Marine Trader, and Albin. I am wondering whether what would be more stable in rough seas? A twin screw hull or a single screw hull design?

Could someone please send me an Email at: baskew9859@aol.com if you have a reply to this question.

Thanks!

 

Perhaps I have been a bit rude in the previous post, but I really suggest to play a bit with a design tool. Current computer design tool can give you in real time the displacement and BM. They may lack features for designing real things, but are very good for a preliminary design.

You will see how shallow draft is a hull when you target Lwl 40 ft , Bwl 7 ft and displacement 8000 lbs. This shallow draft forbid to put the engine deep down. We are speaking there of canoe draft around 1 ft, 1,5 ft. The engine will be above bottom, at least to access oil drain. The bottom and keel themself will have some thickness. So the engine CoG will likely be ABOVE waterplane.That a bit hight to be useful for righting moment.

So the only way to give these boats some seaworthiness, is not to put weigth down ( no way, no deep enough ), but it is to remove the maximum of weigth on top. So limited headroom, limited roof size, very ligth building method. No extra weight on top. Up to the point where seaworthiness is in competion with liveability.

For your size, these boats cannot match category A: They will lack by far the required righting moment of 25 kN.m. That would mean a GZ of 0.70m , on hull where half beam is 1.00 m. No go.
They will probaly pass category C. but C is under what you request. Top limit Force 6 wind (27kts) and 7 ft waves.
Remain category B up to Force 8 wind (40 kts) and 13 ft waves. But then it need extensive calculation and cannot be told with just a few figures.

PS : I remind that CE / ISO norms are not a classification society. It is just the bare legal minimum a boat must technically meet to be sold on the EU market. Real classification society have rules much stricter than CE / ISO.