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Boat Forum / Cruising / February 2006Bow thruster drag
I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read any studies or done any research on what this hole and the subsequent tunnel might do to its fuel economy? Capt. Jeff If the tube is properly installed and faired, it should be negligible > I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read > any studies or done any research on what this hole and the subsequent > tunnel might do to its fuel economy? > > Capt. Jeff > I'm thinking of putting a bow thruster in a 36" [sic] trawler ... A bow thruster on a 36 inch trawler seems like overkill to me. Heck, I'd say the same about a 36 _foot_ trawler.  SignatureGood luck and good sailing. s/v Kerry Deare of Barnegat http://kerrydeare.comcast.net > I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read > any studies or done any research on what this hole and the subsequent > tunnel might do to its fuel economy? > > Capt. Jeff Jeff Bow thrusters are supposed to be mounted such that, at crusing speed, the unit is out of the water. This may not be practical on some very slow moving boats that do not sit bow high at cruse. John Come on. To be effective, a tunnel thruster needs to be 2 - 4 diameters below the surface. A lot of boats are less but their thrusters are usually toys that don't have enough thrust when you really need the help. There's only so much thrust you can generate with a certain diameter. Put these factors together with a boat shallow enough that it's going to be able to go fast enough to lift its bow very much and lifting the thruster clear of the water to reduce drag becomes petty improbable. How do you like these thrusters? http://home.maine.rr.com/rlma/D124-7.PDF SignatureRoger Long >> I'm thinking of putting a bow thruster in a 36" trawler. Has anyone >> read [quoted text clipped - 12 lines] > > John > Put these factors together with a boat shallow enough that it's going to > be able to go fast enough to lift its bow very much and lifting the [quoted text clipped - 3 lines] > > http://home.maine.rr.com/rlma/D124-7.PDF All retractable? >>Put these factors together with a boat shallow enough that it's going to >>be able to go fast enough to lift its bow very much and lifting the [quoted text clipped - 5 lines] > > All retractable? Nope. Probably Schottel or Gill Pump Jets. 360 degree thrust in shallow water. Rather poor efficiency though. Evan Gatehouse They are actually a Schottel type that is now out of production. If the project ever gets funded, we'll have to go back to Schottel's. Efficiency is low but the operational advantages outweigh that, even for a ship like this that used them for long periods on scientific station. This is a diesel electric vessel so can divert an enormous amount of power the thrusters. In this case, they also double as propulsion devices in areas where there are marine mammals, divers, or sensitive bottom habitat. SignatureRoger Long >>>Put these factors together with a boat shallow enough that it's >>>going to be able to go fast enough to lift its bow very much and [quoted text clipped - 13 lines] > > Evan Gatehouse Roger I spent a week on a boat with this kind of setup once, and it worked very well, easily moving the bow . The boat manufacturer, Viking (58), was very specific about the mounting location, out of the water at crusing speed. I haven't seen a factory mounted bow thruster on a power boat yet that didn't sit out of the water at cruse, where it should be. Obviously, all thrusters do have their limitations with respect to their ability to counter the effects of wind and tide. And I do agree that they are probably over kill on smaller boats (less than 55 feet or so), but they often help a captian get in and out of places that they might not normally be able to. John In the commercial world, I mostly deal with people who can dock fine in light conditions. They need the bow thruster for those tough days that many yachtsmen probably wouldn't be out in anyway. They want the full potential power that the manufacture says they can get. The thruster on the Viking probably wasn't putting out but a fraction of what it could have if properly immersed. With proper tunnel fairing there would have been no need to have it so high and it would have been much more effective with the identical unit. SignatureRoger Long > Roger > [quoted text clipped - 16 lines] > > John >Tamaroak wrote: > I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read > any studies or done any research on what this hole and the subsequent > tunnel might do to its fuel economy? We're preparing for the same project. If it's faired & flared correctly, the aperture should add very little drag at our operating speeds (below 8 knots). > To be effective, a tunnel thruster needs to be 2 - 4 diameters below > the surface. A lot of boats are less but their thrusters are usually > toys that don't have enough thrust when you really need the help. > There's only so much thrust you can generate with a certain diameter. Correct. Although I'm wondering about design of the tunnel to incorporate a nozzle to improve flow and reject air. A problem with that idea is that the most effective converging/diverging nozzle designs are not symmetrical for input & output... you need two tunnels, one for each direction... or a steerable unit... or a less efficient symmetrical design. Compromises compromises! The problem with our boat is that while it does have a relatively deep forefoot, there is a limit and it doesn't include an option to put the tunnel 4 diameters below the LWL. And I don't want to give up the interior room to put in multiple tunnels, either. > Put these factors together with a boat shallow enough that it's going > to be able to go fast enough to lift its bow very much and lifting the [quoted text clipped - 3 lines] > > http://home.maine.rr.com/rlma/D124-7.PDF Not as much as I like the daggerboard! Fresh Breezes- Doug King >> I'm thinking of putting a bow thruster in a 36" trawler. Has anyone read >> any studies or done any research on what this hole and the subsequent [quoted text clipped - 9 lines] > > John Maybe on some boats, but most I've seen are on cruising type hulls that don't lift the bow enough to get the thruster tube out of the water (other than in higher sea states). As someone else mentioned, a proper installation will include a flair on the tube opening (both sides) that diverts the water away from the tube while underway at speed. I have a Navigator 4800 Classic (52' LOA) and had a bow (and stern) thruster installed by the dealer. Made absolutely no difference in the boat's top speed (23 knots) or cruise performance. RCE |
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