ROLL Control pleasure boat

I know this section is usually a discussion of ultimate stability , "how far can she roll and still come up?"

I would like some help on concepts of roll stability for comfort.

The hull is a NJ "Seabright Skiff" which is claimed to be a good offshore performer, the design a modified Atkin with box keel and reverse deadrise.

The hoped for cruise speed of 15 -20K precludes "flopper stoppers" or paravanes from too much drag.

Std fin roll control as well as the new gyroscopic are just to pri$y for me.

A few power boats have used dagger boards as a roll control, I would be delighted to install a center board ,even with an active trim tab , if that would work.Grounding would cause no hassles.

The negative deadrise could use individual stern trim tabs for roll control, if they could take the work input , and would have enough force.

An article I recently read claimed the US Navy used rudder input to control some roll, but it was unclear if the vessel had a single or more than one rudder.

Any references to this would be appreciated.

Comments?

FF

 

Older boats/ships had bilge keels. The newer CG 110' Cutters have fins that are contolled by a computor.

 

Roll control

FF,I like your daggerboard idea. If you could try this on a model it might be a worthwhile experiment: how about a daggerboard in a trunk that functions like a gybing board. When the force of the roll pushes the front of the board over it would counteract the roll! It would require experimenting to see if the thing would just oscillate or not. You might have to experimentally vary the pivot point which could just be a slightly raised portion torward the back of the board(looking down from the top). It might require some sort of centering spring? I made a gybing board one with a "V" block in the front end of the trunk: the "V" would fit over the leading edge of the board to prevent it from gybing or it could be slid forward to vary the range to which the board could gybe each side. Seems to me it might be able to be made to work....What do you think?

 

Hi Fred,

Anything you stick into the water is going to create more drag and therefore reduce efficiency. I would think that because of this, you might want to focus on the use of a steadying sail or auxiliary sail rather than any additional appendages that will run in the water.

Sails work great for roll control, and they increase rather than decrease your fuel efficiency. They are "low-tech" so they won't require high cost computer controls either.

If designed innovatively with a special mast, you could have a sail that rotates into position as a big sun shade or rain tarp when you're moored rather than cruising. A tabernacle would be a 'good thing' for passing under bridges and such too of course.

It seems you may be interested in a more high-tech solution than a sail for roll control. On the other hand, anything you try OTHER THAN a sail is only going to reduce your fuel efficiency -- and isn't fuel efficiency one of your primary goals in this boat?

 

"On the other hand, anything you try OTHER THAN a sail is only going to reduce your fuel efficiency -- and isn't fuel efficiency one of your primary goals in this boat?

True , but at 18K the wind is almost always forward so far that nothing but aerodynamic drag would be created , along with the drag from the weight of mast , rigging et all.Plus it hardly fits with a Commuter hull shape , and is more junk to assemble , disassemble and ship.

A gybing center board would be far easier to use , esp if it self tended.The form drag of a few sq ft is worth .1K to keep the crew happy.

Probably a control of some sort would be required , even manual would be fine as a rudder bar hooked to the control point would be OK. The boat will be operated offshore (as needed) however long overnight passages are not contemplated for cruising.

If the speed can be maintained , our 50 to 100 mile days should be quickly done in daylight.

So 3 questions remain on this design , the fore and aft location of the proposed board to stabelize her.

Weather a more efficient shape of box keel than the simple one used by Atkin , better shaped leading or trailing edge? outside to NCAA 0010 or ?

And the final question of weather an aluminum hull & deck (not my first choice ) will protect a modern electric controlled engine from a lightning strike?

FF

 

For the daggerboard, I do not understand what will bring the daggerboard centered when the rolling correction will be enougth. As for me, a spring wont do it, because the force will be dependant of the speed of the boat, not of the roll angle.

For using rudder instaed of fins, I am note sure. Because I fear main of the cost of a stabilizer system is the control logic and the actuators. The fins by themselves are not the expansive part.

And the first problem I see is when you will want to use an autopilot. Coupling roll control + yaw control on a single gouvern will lead to very pricey controls.

The only thing I see that MAY lower costs are electronic advances. Mems gyros : http://www.analog.com/en/subCat/0,2879,764%5F801%5F0%5F%5F0%5F,00.html may lower the cost of control logic. And electric driven fins may be cheaper than hydraulic actuated ones. See http://en.wikipedia.org/wiki/Electric_power_steering. Hi power low voltage electric engines do exist for the later.

 

Using trim tabs for active roll and pitch stabilization is becoming quite common and is very effective (and efficient..since trim tabs actually improve the L/D of most planning hull designs at low-to-mid cruise speeds). Cost is an issue..the smallest vessel that we have provided this type of system for is 57' LOa and was a rather expensive 57-footer at that..roughly 3.5 million total cost. (The owner likes his bells and whistles..;-) We dis install a set of active tabs that worked very well on a 37' Deep-V demonstrator craft, but the installation was anything but sanitary or permanent.

All that said, we have had under development for about 2 years, a transom stabilizer solution based on interceptors instead of trim tabs and using integrated electro-hydraulic actuators that only require DC power (existing systems all require hydraulic PTO or electric-driven pumps, reservoirs, servo valves, etc). We have been installing large active interceptor packages for many years (USN X-Craft, for example) , but recent advances in servo technology are allowing us to head down-size with the concept, making it affordable for even boats i the 35-40 foot size range.

But we are still about a year of so away from fielding the systems commercially. And....interceptors do not work as well as trim tabs at lower speeds..below, say, about 25 knots..and so are more suited to go-faster boats. An all-electric trim tab solution is not in the cards right now..trim tabs require too much force relative to interceptors and thus still require a decent hydraulic power installation to be active for stabilization. The set of trim tabs we are currently installing on a 63' planning MY require about 14 HP, as an example, and get that from a pump mounted on one ofthe tranny PTO pads.

 

What kind of roll control would fit on that style of boat ?

http://www.cnsco-silure.com/Francais/Mer/LovStar40/Lovstar_40.htm
http://www.rangeboat.com/index.php
http://www.classic-boats.com/en/fiches_tech/Andr12en.pdf

Cruise in the 11-15 kts, top around 20 kts.

What is the break even point between trim tabs and fins ? (Although a hydraulic system is a nogo on this kind of boats).

And whatabout pneumatic systems, like http://www.gyrogalestabilizers.com/index.html if electric only are not powerfull enougth ?

 

If you cannot use hydraulics..then how could you use a pneumatic system? Both require some form of PTO or electric motor to operate the pump. The gyro-gale system is not really any different than Wesmar and the others except for the pneumatic drive..and the benefits of that are all marketing hype for the most part. A rate-based fin stabilizer system is a rate-based fin stabilizer system and there are a number of good options out there. Vetus recently introduced a novel and comparatively inexpensive fin stabilizer package for smaller vessels that was actually developed by Koop (formerly KoopNautic).

The boats you pointed links too would all be compatible with an active transom trim tab solution as a better alternative than trying to fit 2 or four fins, even at the lower curise speeds..but the requirement for a hydraulic power source to 'wiggle' them remains. There is not a breakeven point, per se..its more a matter of lifting area and it is often easier to install oversized trim tabs than it is oversized fins, to deal with lower speed control requirements.

 

I checked Vetus price list. The inexpensive system (without the hydraulic pump) is more expensive that the main engine + gearbox for above boats. (volvo D3 130 or yanmar 4BY150).

It was my understanding that fins were better than stern flaps. Fins (if submerged) work either side, but only the down flap is working. (a single fin can make the boat roll both side, but a single flap will only make the boat roll one side). Fins are also further of roll center than transom flaps. So better efficiency. Also fins are somewhat compensated. So less power to activate, or faster move.

But there are no fins systems for "small" boats, so there may be problems somewhere.

 

Agree there are really not any 'cheap' solutions for smaller boats. Some of the key system elements just don't 'scale down' in price.

Trim tab systems are often more effective than fin systems, despite their 'one-sided' force-producing nature, because we can often fit a larger lifting area tab than can be obtained with a fin. Fin areas are limited by their span exceeding rub rail or keel in most cases. It is true that fins require less power to operate than do trim tabs.

 

What would the the order of magnitude for trim tabs "wiggle" power for boats above ?

When you look at autopilot for same weigth boats, (with tiller), they do not use more than 150 - 200 watts.

 

Order of magnitude?..at least 10 HP. There is no comparison betweeno the autopilot requirements and those of an active trim tab stabilzation system. To achieve good stabilization performance means having real-time tab position response be a minimum of 5 times faster than peak unstabilized roll rate of the vessel. Thus 25 degrees/second or better is not unusual for fin or tab deflection rates..which takes horsepower.

 

10 hp

That 7500 Watts.

Even with a 30 % mech efficiency : 2500 watts.

As fas as I remember the work of a force of 1 N moved 1 meter in one second is 1 watt.

That you need to move 25000 N over 0.1 M in 1 second.

That you move 5600 lbs over 4 inches in 1 sec.
Or 2800 lbs over 4inches in 0.5 seconds.

That's really huge. You are speaking of forces around a quarter / half the weigth of the boat.

 

Another misunderstanding from my part.

When a boat roll, there will be a moment between Center of Boyancy and Center of Gravity. All forces are vertical.

But with a roll induced by a fin. One force will be perpendicular to the fin. But what is the other force, and with what application point ?