Factors to Consider & More
Imtra thruster expert Peter Nolet explains the most important factors to consider when planning to install a new bow thruster on your boat.
If you’ve been looking for a bow thruster for your boat, you may have noticed there are a lot of options. Bow thrusters come in many styles and sizes. They are generally located in a tunnel that’s set right through the hull, although sometimes they are mounted externally or installed so they can be retracted when not in use. Before choosing a thruster for your boat, many factors should be considered with guidance from knowledgeable dealers and installers.
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Consider these factors when choosing a bow thruster:
- Strength of winds and currents – The more the wind and water move in your environment, the more power you’ll need from your thruster
- The boat’s profile – The higher and longer your boat’s superstructure, the more pressure a thruster will need to push against a cross wind
- Bow shape, interior space – The deeper your bow is in the water, and the more interior space is available forward, the more easily a bow thruster can be fitted.
- Power options – Typically, smaller boats use DC power, and larger ones use AC or hydraulic power. But two 60-footers of the same make and model may or may not already have an AC generator or a hydraulic system.
- Budget – The size, power and sophistication of thruster available to you will vary depending on price and installation requirements.
In the last 25 years, since thrusters began to take hold in the recreational marine market, manufacturers have created a variety of configurations for most types of boats from 20 feet long to superyacht. Sleipner, for example, offers over 160 different thruster models for boats of all sizes and configurations.
What a bow thruster is and how it works is the subject for a different article, but even when you know the exact make, model, and size of boat, the bow thruster you select will often be different depending on who the owner is and where they plan to run it.
3 Factors to Consider When Choosing a Boat Bow Thruster
1. What’s your boating environment and ambition?
The best place to start your search for the right bow thruster has nothing to do with your boat or your budget, and everything to do with where you’ll use your boat and in what conditions you expect your bow thruster to perform.
Does your boating area have calm harbors or is it more of a “rough and tumble” marine environment? In particular, what is the average wind speed in your area? Or do you often need to maneuver at slow speeds in strong currents, say, when waiting for a drawbridge to open? Above all, what are the docking situations you’ll commonly encounter, especially your primary berth, fuel dock, and favorite lunch spot?
Now layer on an evaluation of your ambition as a boater. How far afield will you go and what unusual harbors and inlets will you enter? What type of weather suits you and your crew? Do you leave the boat in its slip at the first sign of white-capped waves or do you expect to get your boat off a dock in 30 knots of breeze?
As an example, let’s consider a well-known boat, a Sea Ray 360 Sundancer. Boater No. 1 may not leave their dock if the wind is blowing over 10 knots. This owner needs something to help maneuver around the dock with relatively low performance. He/she could put in a smaller thruster with on/off control and spend less money.
Boater No. 2, on the other hand, will take their Sundancer out on a 30-knot day and will need a thruster with more power. This captain will be more likely choose a proportional DC thruster with more power at the top end.
The thruster tunnel could be the same size on both boats. For Boater No. 1, you could install a 4hp (60 kilogram force thrust) model, which is quite adequate in light conditions. For Boater No. 2, we would recommend a higher power thruster model in the same 7-inch (inner diameter) tunnel size, like the 6hp (80 kgf thrust) or even the 8hp (100kgf thrust). However, with this much power, we would recommend only Proportional control for comfortable operation.
2. Boat profile
What’s the profile of your boat? As with a person’s profile, the boat profile is literally the view from the side, and it makes a big difference when you’re ready to choose the right bow thruster.
A bow thruster is often called a side thruster for a reason—the goal is to thrust the boat sideways in one direction or the other. That’s why plotting the windage presented by a side profile is an absolute requirement to choosing your thruster. Don’t worry, you don’t have to figure this out yourself. All companies selling bow thrusters have a system for doing this. Here’s what we do at IMTRA and why:
Essentially, we obtain the drawings of every boat model that’s a candidate to receive a bow thruster and we send the side-profile view to our engineers at Sleipner in Norway. To determine how much power is required to push a boat sideways against the wind, the engineers plot the area of the profile and add a calculation based on the position of the superstructure on your boat—that includes any hard top, soft top, or bimini you may have.
The reason for the extra calculation is because a bow thruster is up front; as an example, if you own a sportfish—with the cabin-house and flying bridge set aft—less force is required to move the bow than if it were a motor-yacht, which has more windage closer to the bow, with the cabin in the center of the boat.
When the engineers have run the numbers, they will normally send three options for your boat based on the top wind strength in which you wish to operate: Economy (18-20 knots), Normal (20.5-22.5 knots) and High Power (23 knots plus). For a Sea Ray 350 Sundancer, the recommended models are Economy SE40/125S, Normal SE60/185S, and High Power SEP80/185T delivering 40, 60 and 80 kilogram force thrust, respectively.
3. Potential Limitations: Sizing & More
If you have a boat that you’re looking to refit with a new thruster, let’s make the assumption that you’re doing so because you have an insufficient thruster or because the boat has no thruster at all. Once your boat’s profile has been plotted, there are still several potential limitations you’ll need to consider with your dealer or other advisor.
The first limitation may be a physical one—the designed shape of the bow and what’s already there, inside and out. For example, your boat may already have a bow thruster that operates inside a 7-inch-diameter tunnel in which it can’t generate adequate thrust. One solution may be a more efficient, higher-powered thruster such as a Side-Power model with twin 5-blade propellers. It might make more sense to install a larger diameter tunnel, in which case you could specify a thruster with higher top-end performance and longer-running capabilities. If your boat has no tunnel currently, you may face the same sort of limitation if the boat’s design only has space for a small tunnel and thruster.
Physical space is even more critical on a boat with a shallow, narrow forefoot, such as a modern sailboat. The tunnel also may need to be installed farther aft so that it has adequate length; if a thruster tunnel is too short, the water flow may not have time to straighten out before it hits the thruster propeller and cavitation may then render the thruster less effective and excessively noisy. Moving the tunnel aft may create other challenges, however, perhaps interrupting tankage or other equipment inside the boat that must be relocated.
Thruster choices will also be dictated by the required location of a thruster; for example, some thrusters must be ignition-protected due to their immediate environment inside the boat. The motor and switchgear on some Side-Power models have a hermetically sealed cover, which is critical in areas that may get wet, such as bow locations with adjacent anchor lockers. This type is even more important in proximity to a gasoline engine, which can often happen when installing a stern thruster.
Another limiting factor for some owners is the drag created by the turbulence at each end of the tunnel, although this has proven to be minimal for most boats with a properly faired tunnel opening. Even so, performance-minded sailors may prefer to install a retractable thruster, which opens out of a compartment in the hull only when needed for maneuvering at slow speeds. Hylas and Dufour are two manufacturers currently offering this option.
The power available to drive your thruster may also direct your choices. Owners of smaller boats will often choose DC-powered thrusters because the boats aren’t equipped with a generator to provide AC power, nor with a hydraulic system to run a windlass or joystick controllers. For larger boats, owners will consider adding a generator or hydraulic setup, but here again, options are often narrowed by a lack of space for a generator and/or the difficulty of running hydraulic hoses in some pre-existing hulls.
Budget is almost always a limitation, as well. Choosing an on/off thruster instead of one with proportional control is one way to add a thruster for less money. If you have a 50-foot boat, an on/off thruster might save you between $2,500 to $3,000 versus a proportional thruster on the purchase.
Budget could also be a reason to choose a slightly more power-hungry thruster with the tunnel that’s currently in place rather than enlarging the tunnel and installing a more efficient unit. On a Tunnel-thruster system, the install of the bow tunnel is often a large component of the overall job, so reusing an existing tunnel can represent a major savings.
Bottom Line - Consult the Boat Bow Thruster Experts
These are just a few of many factors that should be considered when choosing the correct thruster for your boat. However, it’s always a good idea to speak with the product experts, as there are often multiple thrusters that would provide the results you’re looking for, and someone who specializes in the product line can walk you through the options.
Every boat is unique, and no two thruster installations are the same. That’s why it’s also important to have an experienced installer aboard your vessel to evaluate the potential obstacles, as well as weigh the options as to what size, style and location of the thruster will be best for your boat.
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