timjet wrote:I fly twin engine aircraft for a living, so I understand the concept of assymetrical thrust.
Your twin engine aircraft experience won't be of great value in operating a twin engine boat.* In a boat, asymetrical thrust in the sense that you think of it in a plane is not something you really use since you have the rudders to provide a turning force far greater than speeding up or retarding an engine.* You're correct, the principle is the same and adding power on one side and retarding it (or not adding it) on the other side will swing the stern to the side with the most power. But it will be pretty slow swing.* So rather than using asymetrical thrust to generate yaw you simply use the rudders and leave the power where it is.
And remember, anything you do with the props and rudders moves the STERN of the boat, not the bow.* Same as in your sailboat, although in your sailboat you probably had a fair size keel so putting in rudder probably*resulted in a bit more* of a pivot than a swing.* But in the typical semi-planing or planing powerboat there's not much keel down there.* So the stern will slide sideways a lot when you put in rudder.
So as your entering a marina fairway, for example, and need to turn into a slip, don't get right over against the boats on the other side of the fairway because when you put in rudder to turn, what's really going to happen is your stern is going to slide over, not the bow.* If you haven't left enough room between you and the boats next to you, you'll swing your stern right into them.* The only way to make the bow move one way or the other without moving the stern is with a bow thruster.* I find it easiest to think of maneuvering a trawer-type boat (without a thruster) as being the same as driving a car around in reverse.
Now what you do have that your plane doesn't is a really effective reverse.* I know many planes have reverse thrust but you don't use it the same way as in a boat.* Where the twin really shines is in opposing the prop thrust.* By putting one transmission in forward and the other in reverse you can walk the stern pretty smartly to one side or the other.* And if you want to pivot the boat even faster, put the rudder hard over in the direction you want the boat to pivot.* The propwash from the prop in forward will push against the rudder and pivot the boat around that much faster.
I had never operated a twin engine boat before we bought our GB although I'd operated a number of singles of various types and sizes.* When I mentioned this at the start of our pre-purchase sea trial the selling broker asked me if I'd ever run a bulldozer.* I said yes.* He said to think of the boat the same way.* If you want to go forward, but both transmissions (caterpillar tracks) in forward.* If you want to back up, put both transmissions in reverse.* If you want to pivot left, put the right transmission in forward and the left one in reverse.* If you want to pivot right, left transmission in forward, right in reverse.
Now, twelve years later, even though I make these thrust changes almost automatically the mental image that sometimes comes to mind is that bulldozer.* My wife, on the other hand, has a different "crutch" to help her visualize what to do with the shifters.* In her mind, pivoting the boat left is the same as swiveling her hips counterclockwise.* The left side goes back, the right side goes forward.
As you get the feel for the boat and its response you'll start seeing you you can modify the "rules" to make the boat do exactly---- well, almost exactly--- what you want it to do.* For example when entering our slip, which is almost always a crosswind, cross-current situation there are times when I want to move the stern to one side but not as much as the one-forward-the-other-reverse will.* So I'll use the "wrong" engine in forward, the one on the inside of the turn* This keeps me moving forward but having the "wrong" engine in forward (with the other one in neutral) actually makes the boat slide a bit sideways which is what I want to have happen in this case.
Don't bother trying to figure out how what I just told you works--- at this point it will just be confusing.* But it's the sort of thing that you begin to figure out as you get better and better at maneuvering your boat.
The last piece of advice on maneuvering I can offer is follow the advice in the two commercial lobster boat names below.* Unlike a car or a plane, things actually happen relatively slowly with a boat.* So don't get yanking the shifters back and forth in a frantic attempt to correct something that probably wouldn't have happened if you hadn't started yanking the shifters back and forth.* For one thing, it's VERY hard on your transmissions.* Always pause in neutral a couple of seconds at least to let the shafts and transmissions come to a stop before engaging the other gear.* If you want an interesting experience open the engine room hatch so you can see the prop shafts.* Then put one in forward at idle speed and then pull it into neutral and remember how long it takes to stop.* That's how long you need to pause in neutral when going from forward to reverse or the other way.
And if you really want an eye opener, get the boat up to cruise speed, pull the power to idle, pull the transmissions into* neutral, and watch how long it takes for the shafts to stop rotating.* Depending on your transmissions and how tight your cutless bearings are the shafts will likely keep turning until the boat is almost stopped.* That's the way it is on our boat, anyway, with our Borg Warner transmissions.* So what it tells me is that if I pull the transmissions into neutral at cruising speed, don't go putting them into reverse until the boat's almost stopped unless I want to put a hell of a shock on the driveline.
Have you ever flown a Cessna 310?* The ones with the wingtip tanks.* When I was learning to fly one I had a hell of a time with overcorrecting to keep the wings level during final approach (at Honolulu International where a 20 knot wind was a no-wind day to us).* A wing would dip, I'd pick it up, use too much aileron, and the inertia of those big tip tanks would roll the plane right on through where I wanted it to be and the other wing would drop, I'd use too much aileron to pick IT up,* the inertial would roll me back the other way, and all the while the runway was getting closer.* It took a fair amout of time in the plane to learn to stop chasing the roll and making things worse and learn just how much aileron to use, and more importantly, to learn the timing for putting it in and taking it out.* It's the same deal in your boat.* And as Skipperdude said, the only way to get the feel is to operate the boat.
If you're worried about hitting things, have someone on deck with a big-ass fender on a line that they can drop down between you and whatever you're going to crunch into.* This, by the way, is a great way to get off a dock when you're between two boats and the wind is blowing you hard onto the dock.* But we'll save that for another time.....
I can't say much about heavy weather boating because we haven't done any what I would consider heavy weather boating here.* What I will say is that the best thing is to ease into it.* Skipperdude said that a boat will take more than the owner (usually) and that is correct.* But knowing that won't make you any more confident when you see the bay whipped into whitcaps and big waves.* What we did was set limits (same rule as in flying--- know your limitations and don't exceed them).* When we started this kind of boating, Bellingham Bay in 15 knots of wind looked pretty spooky.* So that was our limit.* The couple of times we got caught out in 15 knot winds convinced us that this was a pretty good limit for us at the time.
After year or so of this and of getting caught a few times in 15 to 20 knot winds and waves, particularly when the wind was opposing the current which, with the current strengths around here can make some pretty nasty conditions, we decided that it wasn't all that bad.* It wasn't pleasant but after awhile I thought it was kind of neat to be heading into or quartering waves that our fairly straight-sided bow kicked high into the air and the wind smashed back into the boat* I could pretend to be a destroyer or PT boat skipper and the salt water pouring down our decks was good for the teak.* And our boat has good wipers.
So we upped our limit to 20 knots, which is about where it is today.* There are plenty of bodies of water around here where we would not want to be out on them at 20 knots, like the Strait of Georgia, but at least we know it's doable.* And we know we could handle 25 or 30 if we had to although we don't deliberately go out in these conditions.* And, just like in a plane, when the weather gets nasty, forget about being on time or getting somewhere on a schedule.* Operate the boat safely even if it means ending up somewhere other than where you wanted to go, or it takes you twice as long to get to where you wanted to go.
And, just like in flying, there is no shame in deciding it's too nasty to go.* Thinking that you're losing face in front of experienced boaters and so going anyway is dumb, in my book.* My wife and I have turned around or not gone in the floatplane plenty of times when we were not confident in our abilities to deal with the weather conditions.* This was far preferable to ending up smeared across a rock face somewhere.* You don't want to be so timid as to never gain the experience necessary to deal with what the water can throw at you, but you don't want to be stupid about it, either.* If you don't want to go, don't go.
And above all--- when maneuvering into your slip, an unfamiliar marina or anchorage, or slogging your way through less than ideal water conditions--- follow the advice so kindly provided by the commercial lobster fishermen of Prince Edward Island
-- Edited by Marin on Thursday 22nd of July 2010 10:19:52 PM