R27OB Bow thruster

[ixlr8]

I was wondering what the thruster force was for the bow thruster on a RT-27OB.

 

[baz]

Jim:

This https://www.lewmar.com/node/11697 provides some info on the Thrust (Kg/cm and lb/in). It seems to range from 82 to 93 lbs of thrust force.

I suspect the Moment for our R-27/OB using a 15-foot moment arm is around 1200 lbs of force.

When I use my bow thruster with maybe a 3 sec burst when it's in still water and no wind at my marina the boat rotates quickly. Why or why does the motor make so much noise.... It scares my grandkids much like the horn does. :lol:

 

[ixlr8]

Our problem is that we have afternoon onshore sea breezes, 10-15 mph, directly abeam on the port side and they over whelm the bow thruster making it very difficult to get to the dock. When we pulled our boat out of the water the last time, there was a guy there who had been boating out of that location for 30 years. He took one look at our bow thruster and pronounced it too small for use at that dock with the afternoon breezes. I was wondering where we were at for thruster power and what, if any options, I might have.

 

[baz]

A 10 to 15 mph wind on your beam will present a sizable force on the boat. My calculation for a 15 mph wind will create around 0.03 PSI or some 3.7 pounds per sq foot. Let's say your bow stem to midships hull's exposed area is some 90 sq feet (6 x 15), then the force would be some 350 lbs. So if the thruster is providing some 90 lbs of thrust it's easy to see it will be overwhelmed and insufficient to turn the boat against the wind. The hull's bow side area is a lot more than the aft section so more force will be pushing on the bow even with the aft section providing some countering force.

Is there any way for you to dock on the dock's windward side thus allowing the boat to be pushed to the dock side without really needing the thruster at all ? I presume your docking is at a boat ramp so is it a finger pier dock allowing either side to be used ?

I've been in 20 mph winds when docking and knowing its typical direction at a Marina I will always request a windward side of the dock when making a reservation. In a 20 mph wind and docking on the lee side of a dock is very stressful for me as I'm usually operating the boat solo.

 

[Matty]

well I have none of the technical knowledge displayed here but I have a timely experience with the comments. Last weekend I was out with the wife. Weather was good but then the FL afternoon storm kicked up. I thought we made it back in time to dock but results proved otherwise. I probably approached at a less than perfect angle not accounting enough for the wind and thinking the thruster would help out more than it did. Thruster was of little if any help and I ended up in a bad position but was now committed. Ended up brushing up pretty hard against my son's boat that was on the lift. Part of that was because in reaching out the window to push off I hit the shifter and lurched forward. At that time I received plenty of guidance from my wife who was aft. Did not know she knew some of the words that come out.
So to correct I had to go in reverse a little hard. end result was that my son's prop took out my sliding window. By then neighbors arrived to help and we got it in.
I suppose one option was to stay out until it settled down some. I will have to think about my approach. It is a fairly tight area that I have managed to master in decent conditions. It is like a parallel parking maneuver. But without the thruster it is difficult in bad weather.
Thanks to quick response from parts @ RT the window is ordered and I will replace. Lessons learned the hard way.

 

[ixlr8]

Barry, Thank you for the link to Lewmar. I finally found the manual that came with my boat, it shows it being the 110TT Thruster with 62 lbs (28kg) of thrust. https://www.lewmar.com/node/12176

 

[ixlr8]

A 10 to 15 mph wind on your beam will present a sizable force on the boat.
Is there any way for you to dock on the dock's windward side thus allowing the boat to be pushed to the dock side without really needing the thruster at all ?

Unfortunately the windward side of the dock is not acceptable. It is a little shallower, rocky and the dock posts are in the way. The rocks are not an issue so much with the boat, but they raise he.. with the trailer and the dock posts are 20ft apart.

 

[baz]

Jim: yep.... with just 62 lbs of thrust any wind velocity over 5 mph will present a problem for the bow thruster giving you some bow rotation when needed.

I've seen some big boats in my marina which have massive thrusters. When they're operated the swirling water that the thruster makes is almost like the wake my F300 makes at speed. I'd like to see how well they dock if the wind was at 20 mph.

 

[baz]

Jim: It seems you have no option but to practice docking without the use of the thruster. It's almost like you will need to nudge up to the dock and have 1st Mate jump off onto the dock with a line in hand. If that line is attached to the bow section then use the powerful F300 to push the stern against the dock side and hold it there after 1st Mate has tied her bow line and can get a stern line tied down as well. Practice, practice and more practice..... 1st Mate will get many brownie points for her efforts, right ? :lol:

 

[baz]

Jim: How about employing a 4-prong grappling hook attached to a long line that can be thrown at and over the dock such that when it 'grabs' the lee side of the dock you can haul the boat to the windward side of the dock and then make good ? 😉 You need some form of innovative solution to make docking in those 15 mph winds. Just throwing out a wild idea.... but I've seen worse things used for docking under bad weather conditions.

 

[Matty]

I have not thought of a grappling hook yet or a crossbow with line attached for that matter but... I was thinking about installing whips on the dock that would hang out 4-5 foot. Place a line on it when you leave and you can grab it before you get to the dock or alternatively use it to secure the boat temporarily while you finish maneuvering. That way the boat should not be able to get away from you --as it did me.

 

[BB marine]

Honestly with a 300 HP outboard on the back of a 27' boat the thruster should only be relied on to make slight adjustments. Heavy winds against the beam is always difficult with a single screw. A midship cleat is your best friend. Having a line on a midship cleat and getting a wrap around a dock cleat will do far more then a thruster. Keep you bow slightly into the wind and approach the dock have the line ready, land the dock and wrap the line or throw the line if someone is available. Give a command to wrap the line to a piling or cleat. You can now use your engine to bring you in square to the dock. It may not be pretty but it works.

 

[ixlr8]

A couple of items I didn't mention which will shed more light on my situation. The dock I am using is a public dock at a public boat ramp. I trailer my boat. So 'whips' are not an option. It is hit or miss if somebody is on the dock to help out. 1st mate has a bad knee and is very limited as to what she can do to help with docking. I am almost docking solo. To add to my difficulties, the largest boat I have had in the past is a 21' diesel skiff. So I am not used to handling a boat this size. It will be back to practice, practice, practice after I get the boat back from service. Hopefully it will get easier with reverse working properly. Presently, in reverse only, I have idle and then it jumps to 1/2 throttle or higher.
Thank you for your suggestions, I will give them a try next time we are out.

 

[baz]

Honestly with a 300 HP outboard on the back of a 27' boat the thruster should only be relied on to make slight adjustments. Heavy winds against the beam is always difficult with a single screw. A midship cleat is your best friend. Having a line on a midship cleat and getting a wrap around a dock cleat will do far more then a thruster. Keep you bow slightly into the wind and approach the dock have the line ready, land the dock and wrap the line or throw the line if someone is available. Give a command to wrap the line to a piling or cleat. You can now use your engine to bring you in square to the dock. It may not be pretty but it works.

Yes, the midship cleat technique is a good one. I've had moorage slips that don't have a cleat at the boat's midship location and I've had to installed one there for myself. Once you're close enough to the dock it's relatively easy to lasso a dockside cleat from the helm window location and then to hold/steady the boat and even tie the bitter end down to the boat's cleat or railing to allow time to get out onto the dock with another line in hand to finish the tie up.. ...again, it's practice and more practice to get the procedure easily done.

One point I would make here is that with a strong wind and wanting to dock there's a tendency to over speed the boat toward the dock to counter the wind resistance. It's a tough call to make; too little speed and you just get blown off, and too much speed means a collision with the dock side.