Boat Shop On Powerboats Boat handling: the boat’s role
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Boat handling: the boat’s role

In Part II, our powerboat guy looks at such elements as steering, prop pockets, hull form and thrusters

Last month we looked at the role propulsion plays in boat handling, including pod drive (IPS and Zeus), outboard and sterndrive systems. Of special note was the advent of joystick controls, which points to the ability of modern technology to meet an obvious need for improved handling capabilities.

Never have such systems been more necessary, as an evening at your local marina, when the boats are coming back in, will indicate. (This can be every bit as entertaining at the launch ramp.)

In addition to propulsion, a boat's overall design affects how it handles around the dock. This captain and crew back in smartly.This demand is due, for starters, to the number of relatively inexperienced people out on the water. And — stand clear — they’re driving larger boats. Second, many of these boats are seagoing condos with lots of cross section above the waterline and precious little below, which naturally puts them at the mercy of the wind — like a sailboat, only without a keel. Third, wives and kids on deck and handling lines are no doubt getting tired of being yelled at by clueless boat handlers (husbands) who keep screwing up the approach. With pod joysticks in particular, you have to really work to screw up a landing.

Boatbuilders love joystick controls, too, because it helps them sell bigger boats. Pods make dad look like a pro, he stops screaming at mom and, since mama is happy, everyone is happy (and those moms who frequently take the helm will appreciate joystick control, as well).

Now let’s have a look at few of the factors that also impact how your boat handles as well as an explanation of the final propulsion option: waterjets.

Steering responsiveness

Steering responsiveness depends on three things, fundamentally: the number of turns lock-to-lock, with three being ideal, in my humble opinion; the amount of effort it takes to turn the wheel — in a perfect world you get fingertip control, like driving a car with power steering; and what happens (how many degrees per second turning rate) when you put the rudder over a given amount at a given hull speed. With the typical express cruiser’s undersized axe-head rudders, not much happens anytime soon when you turn the wheel a half-turn below 20 knots.

My steering-responsiveness benchmarks would be the pods, or MerCruiser or Volvo counter-rotating sterndrives. They all have power steering that a baby could operate, and the twin-prop lower units have more surface area to act as a larger rudder with more (sideways) lift. IPS or Zeus pods have similar steering ease and responsiveness, as well as variable-rate steering that adjusts automatically depending on boat speed. Maximum rudder angle and turn rate (selectable by the boatbuilder) at high speed is less than it is at low speed, so unsuspecting passengers don’t become projectiles in a hard turn.

And remember, predictability and repeatability are key when it comes to skillful docking.

Prop size plays a big role when it comes to low-speed traction and responsiveness.Prop size

Another thing that helps with any boat is the size of the propeller. Compare twin 300-hp diesels turning a pair of 22-inch props through 1.5-to-1 reduction gears with the same engines turning, say, 28-inch props through 2.5-to-1 gears. The smaller props have 380 square inches of area compared to the larger props’ 616 square inches — that’s 62 percent more area and 62 percent more water for the prop to get a grip on. Guess which props will bring the boat to a full stop from 4 knots quicker with the engines at idle speed?

It’s this responsiveness at idle speed that makes such a big difference. It’s so much easier to jockey a twin-screw inboard around a dock — or a fish — without having to mess around with the throttles, and you pretty much eliminate the chance of mistaking a throttle for a shift in the middle of a maneuver. It’s not just horsepower; it’s how the power is delivered to the water that matters. Bigger, slower-turning props move a much larger column of water, resulting in tremendous traction at low speed. It’s the only way to go when it comes to low-speed traction and responsiveness. Big props and, as we discussed last month, big rudders equal happy boaters.

Prop pockets

I was driving a well-known builder’s 40-foot express into its slip a few years ago. This boat had a pair of 350-hp Caterpillars with 1.68-to-1 gears turning 22-inch props mounted in deep, slab-sided propeller pockets, or tunnels. (Smaller shafts, struts, props and rudders cost the builder less.) When I backed down on the starboard engine to pull the bow over to starboard, it actually fell off to port because — I’m guessing here — of the non-hydrodynamic shape of the prop pockets, whose vertical walls prevented smooth water flow to those little girly-man props. Who could make this stuff up?

Now, take the same hull, modify the pockets so they’re radiused in cross section, and install deeper gears with larger props whose blade tips follow the pocket curvature. The boat will behave like it’s supposed to — for example, falling off to starboard when backing the starboard engine.

Lesson learned: Don’t expect the express cruiser with pint-sized running gear and ill-shaped prop pockets to handle like the sportfisherman with deeper gears, larger props and rudders, and relatively shallow, radiused pockets. The only reason I can think of why people don’t complain about how their production cruisers flail around dockside is they have never run a boat that handles well.

Above- to below-water area

The best close-quarters-handling single-screw inboard I know of is the sailboat. Many of these boats turn pretty much in their own length, thanks to a full keel or centerboard moving the pivot point much farther aft than it is on a powerboat. And with their large keels, they hold their own well against the wind.

That’s why I would much rather be graded on docking a large Maine-style full-keel single-engine inboard in a lot of wind and current than a conventional twin-engine outboard or sterndrive. The Maine-style hull’s underwater area holds its own against a crosswind much better, and that makes the boat much more predictable around the dock. Predictability and repeatability are what it’s all about when it comes to docking a boat with finesse.

Comparing twin-screw inboards, you can’t expect a typical condo-proportioned boat, with its large superstructure creating so much sail area and shallow hull creating so little underwater area, to handle well around the dock. You may love the boat’s king-size forward berth and his-and-her hanging lockers, but when you choose a boat primarily for its accommodations volume, you can’t reasonably expect it to handle well dockside or run efficiently.

A full-keel commercial Maine-style boat will be at the other end of the spectrum, with a low sheer, small deckhouse and lots of hull under water. An express sportfisherman, like a Viking or Hatteras, with a comparatively small above-water profile and more underwater area will be the next best thing after the Maine boat dockside, handling much better in close quarters than a condo boat. Here’s a good test; Set a 50-foot condo boat and a 50-foot Viking adrift in the middle of a marina at slack water in a stiff breeze and see which reaches the lee side of the basin first.

Hull form

So what else makes a difference in the way a boat handles? Lots of deadrise. Twenty degrees or more all the way to the transom is like having a shallow keel. If you have two boats without keels, the one with the flatter sections aft will blow around more readily and will wander around its heading when running downsea offshore. Deadrise aft adds lateral resistance, so it makes a boat less susceptible to crosswinds around the dock, and it will track better downsea.

In a waterjet boat in particular, deadrise aft is key to the hull’s course keeping, since there is no underwater gear or keel aft to lend directional stability. I’ve even seen waterjets fitted to Maine-style hulls, with their flat sections aft, which I would avoid like the plague for this reason without a thorough rough-water sea trial. Also, the stern’s shallow immersion would make it more difficult for the waterjets to stay primed, providing an uninterrupted flow of water to and from the impeller.

Waterjets

Twin waterjets with an integrated bow thruster is a pretty impressive setup when it comes to fine motor control (so to speak). However, largely because the waterjet’s reverse bucket is trying to neutralize fore-and-aft thrust with the engine control in neutral, rather than stopping it altogether like a propeller system, the waterjet doesn’t have quite the finesse and predictability of pods. It’s very hard to precisely balance the forward thrust of the always-turning impeller with just the right amount of deflector bucket so that there is no residual fore or aft thrust. When you put a propeller in neutral, it stops turning, which in my experience is a pretty effective way of eliminating thrust.

You can really jockey waterjet boats around in close quarters, but since there is no underwater gear to add lateral resistance, they are more susceptible to the vagaries of the wind. It also takes some getting used to having to turn the wheel to port in order to back to starboard, because of the bucket design. But an experienced operator can make a twin-waterjet boat do some amazing things at the dock; it just takes more time to get to that level.

And nothing stops like a jet. Just drop the reversing bucket without touching the throttle or changing gears, putting all that horsepower to work nearly instantly, deflecting thrust forward and stopping the boat.

Thrusters

A boat with a lot of windage can be challenging to maneuver around the dock, though thrusters can help.There are other options if you want to make your boat actually go where you visualize it going. Vendors like Control Engineering (www.joystickcontrol.com) will back-fit bow and stern thrusters with waterjet or inboard propulsion to create an integrated propulsion system that makes boat handling a whole lot easier. You might think of it as a sort of Vulcan mind-meld with your Tiara; you both visualize and agree on the same outcome.

Backing into a slip is so much easier when you have a thruster to line up the bow, especially when maneuvering a boat that’s susceptible to wind. Getting under way from a dock is also much easier. Just push a button, watch the bow point out into the channel, put the engines in gear, and off you go — no spring lines or throttle jockeying required. When less power is needed dockside to dock and undock, the maneuvering becomes intrinsically safer. The downside, of course, is since thrusters make such a racket, everyone within a city block knows you need them to get in or out of the slip. Now if they would only come up with a stealth version …

Conclusion

One thing’s for sure: How you’re viewed as a boat handler by others might depend more than you think on your boat’s design, especially its propulsion type and its above- to below-water proportions. Think twice before buying that flat-bottom, keel-less, top-heavy barge, or at least make sure it has pod power. If you own an inboard, an articulated rudder may be the ticket and, if you have a choice in the matter, deeper gears with big props and large rudders make life a lot easier for everyone. And if you have unresponsive manual hydraulic steering, adding power-assist may get you the agility you didn’t know you’ve been looking for, whether dockside or at 30 knots.

An excellent resource for information on steering systems is Dave Gerr’s “Boat Mechanical Systems Handbook,” and my book has a chapter on the subject as well. Dave’s book is well-written and illustrated, and offers excellent comparisons between different steering options.


Eric Sorensen was founding director of the J.D. Power and Associates marine practice and is the author of “Sorensen’s Guide to Powerboats: How to Evaluate Design, Construction and Performance.” A longtime licensed captain, he can be reached at This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

This article originally appeared in the November 2009 issue.

 


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