Stabilizers

We have Naiad Zero Speeds on a 78 ft Dutch Built, 10 knot, ocean going boat. Installed them three years ago. Best thing I ever did. My wife & I do not get seasick. Neither does our crew. Looked at gyro's. Seemed to me they are only about 60% effective. The old Naiad's that we had (not zero speeds) were worthless in a big following sea. Had to hand steer the boat, autopilot could not keep up. Old fins weighted 180 pounds each. New fins weigh 1800 pounds each!!!. Following sea now no problem. We anchor all the time and cruise extensively. The boat does NOT roll at anchor. Also on regular cruising they are amazing. No idea the maintenance on Gyro's but Naiad Zero speeds not a big problem. I have NO connection, in any way, to Naiad.
Two years ago we were going down the St Laurence Seaway. Anchored in a beautiful very large lake. Flat calm. Went to sleep and about 4 am it starts to blow 45 Kts and 5 ft waves on the beam. Boat rolling like crazy. Crew already up setting a second anchor. I walked up forward but holding on. Takes about 3 minutes for the pressure to come up in the Zero Speeds. Captain throws the switch and the boat STOPS rolling. Go back to bed and my wife says " guess the storms is over" Waves crashing on the beam and winds 45 its.

The spool up time is a non issue for sea keepers. All of the yachts I ran where they were a factory install had the shore power to run them on shore power along with the a/cs and most everything else. I also have run Them at the dock. I also have started them after starting generator during normal start up and they were ready by the time I hit the inlet.

With all due respect, I run 100-150 different yachts per year in all conditions and types. I do 15,000 nm's a year, each and every year since 2003 and the least amount of miles I've done in a year in the last 14 years is 10,000. I manage a 2013 Chinese built yacht. The quality of it's basic components are still the same poor quality raw material American copies with crap materials they've always used. Seacocks, sea strainers, fluorescent light housings that the bulbs have to be ordered from china, stainless, wire bound wet exhaust hose, exhausts, exhaust insulation, wiring, and on and on. They also handle a sea not nearly as well as most American or European built motor yachts.

There have only been 3 chinese/Taiwanese built boats that impressed me with their ride and handling, the 72' hunt, a 105' Johnson, and a 43' Charles sf. But the quality fell short in a lot of places in the last 2. All of these are late builds. I've run several hundred Taiwanese built boats. I'm on one now taking it from NY TO FL.

Current effects semi displacement and planing hulls much much less because they don't have a set hull speed. A 4 knot current is going to slow down most semi-displacement .5-1 knot (maximum). A displacement hull it's going to slow down darn near all 4 knots......maybe 3 at best. If a displacement hull has a displacement speed of 8.5 knots, that is it's maximum speed through the water PERIOD, no matter how much power you give it. If the current is 4 knots against you, well you almost deduct all 4 knots from your SOG.

Also, when a displacement hull gets in shallow water, with only a few feet under the keel (1-5') they veer all over the place because they can't displace the water underneath them and you have to slow way down to maintain safe steerage. It's the nature of a displacement hull.

on a planing hull or semi displacement, wave height doesn't have that much effect on either speed or fuel efficiency, unless it's so big you shouldn't be out there in the first place.

No not at all, they just continue on from where they left off. The time is getting the gyro to it's rpm.

Our experience is the same as Capt J. We start them up on shore power and the switch to the generator does not impact them at all. In fact, you can turn the Seakeeper off for a while and realize you prefer it on and it spools up from the speed it is at. This makes it ready much quicker.

And to answer the question, we use it at the dock or on the hook when conditions are tough. Sleep makes cruising lots better.

Your text that I've highlighted is a false statement. Current affects displacement, semi-displacement, and planing hulls in exactly the same way as far as speed goes. If any of the hull types is head-on against a 4-knot current at a given rpm, their speed through the water will not change from that of a no-current condition at that rpm. However, all hull types will lose 4 knots in their speed over ground as compared with a no-current condition. As Sea Gull said in post #7, "it's just physics."

Not being a physicist but it's my understanding that because of the difference in density of air and water, the boat on plane will be less impacted than a full displacement boat sitting deep in the water. It's the same principle that says a boat with a high profile and fly bridge will be impacted more by wind than a low profile boat. Every boat is partially in the water and partially in air. The impact is especially true in cross currents. To quote Chapman, "Hull type has the most impact on how a boat reacts to the current. Displacement type boats with considerable draft are affected by current to a greater extent than shallower-draft, lighter, planing type hulls." He goes on to make it clear both will be affected. I know this gets complicated by water speed vs. speed over ground, but do believe there is a different impact based on the amount of the boat in the water.

I may be wrong as I definitely do not claim to be expert which is one reason I went looking for other sources. However, my experience says that in a Riva doing 35 knots into a 4 knot current, I am not slowed to 31 knots over land or bottom. Also, in doing two way runs I haven't found that.

Hull shape also has a little impact in how the water with or against you affects you. Certainly a squared surface would seem to me to be more impacted than a sharp angled surface.

As my only physics was over 30 years ago and we never once discussed current or boats as I would have definitely been interested in that section, I'm not refuting you but just expressing that my understanding is different.

That is not true. A displacement hull has a set speed through the water in which it can travel, it is based on length, beam and other factors. No matter how much hp you give that hull it will not exceed it's displacement speed, period. If you get into a 4 knot current, you're going to lose almost all 4 knots SOG, period as you hit your displacement speed. The boat cannot displace water any faster than its hull design.

A semi displacement and planing hull both exceed displacement speed, so current has a lesser effect on their sog at the same given rpm as they are either cutting through the water or riding on top of it, NOT DISPLACING IT. For example a 35 knot riva going into a 4 knot current will lose .5 knots maximum usually at the same rpm.

That's simply untrue as it depends on many factors. The faster the vessel the less hull in the water and the less drag that it has and the lesser speed reduction from current.

For some of us this is intuitive, others are confused by speed over ground (SOG) and speed through the water (STW).

Imagine walking the wrong way on a moving sidewalk, like they gave in some airports, or the wrong way on an escalator. If the sidewalk moves at 2mph, you have to walk 2 mph just to keep up. I walk at about 4mph STW, so walking backwards on the moving sidewalk my SOG is 2 (4-2=2). Now Usain Bolt is dashing for his plane, he whizzes by me at speed of 20mph STW. He's 16 mph faster than me STW, but he still is affected by that moving sidewalk and only realizes a speed of 18mph SOG. No matter how fast he goes, or how little he touches the sidewalk, he is still affected by the moving surface.

Getting back to boats, maybe what Capt J and others are saying is that a planing hull boat can increase her STW by goosing her engines to overcome a head current, while the STW of the hull speed boat is maxed out? That would be a good argument, and an entirely different discussion regarding fuel burn efficiency and engine life.

A displacement hull has a set speed through the water and will not exceed that speed. It is based on a formula and the hull designer can tell you that exact speed before the vessel is even built. So current effects it almost exactly the current is. A hull with a 8 knot displacement speed through the water, will not exceed 8 knots through the water no matter how much hp you give it. So 3 knots of current will knock down your SOG (speed over ground) 3 knots......

Semi displacement and planing hulls don't have a set displacement speed. They cut through the water or ride on top of it not just displace it and have less drag so less current effect. They have less drag a lot of times, so current has a much lesser effect even at the same rpm. For example a center console doing 60 mph, hardly any of the hull is even in the water, so current has a minimal if even noticeable effect because the hull isn't in the water

When I'm running 500 knots into a 200 knot headwind, my airplane slows down. It is still going 500 knots through the medium that is causing drag and providing thrust, does not know there is a headwind, it is still doing what it does. A boat on a plane is primarily affected by water for drag and thrust/propulsion. Speed through the air or over the bottom is irrelevant to a boat on a plane or in displacement. It does not know there is a current, it just knows speed through/across the water. At 60mph, a car is using only a few hp, while a boat at 60 is using huge hp. Wind drag is not a significant part of drag on most boats.