Max Props on Saildrives Astern of the Rudder

[UFO]

I have a Lagoon 440 where the saildrive is astern of the rudder.


I also have Maxprops on there which give a massive amount of wash in reverse over the rudder - So much so that it can rip the wheel out of your hands.


I have tried centering the wheel with wheel lock and a plastic wheel stop, which is OK, but have broken one plastic wheel stop and the wheel/rudder slammed around.


So since I have just been putting over to port when reversing and that has been OK, until the other day when springing off a dock in a 15KT wind I had to give full reverse when a very large gust decided to put me back too close for comfort - This resulted in shearing off the rudder backstop and jamming the rudder, which I had to eventually dive on to free.


I have fixed the rudder backstop so this can not happen again, but I am interested in what other people do to stabilise the rudder when reversing with a similar setup with Maxprops?

[tkeithlu]

From a design standpoint the only thought I have is a balanced prop.

Thinking about the overall situation, I'm inclined to suspect that your maxiprops have sucked you into using too much throttle/thrust. Any boat with an unbalanced rudder has some degree of the problem, and those with chain/cable steering definitely get the force on the rudder transmitted to the wheel. But, fixed props just aren't that efficient in reverse. If you learned your boat before adding the maxiprops, or learned on another boat with conventional props, I can see a problem.

If I was having these sorts of nasty experiences, I think I'd take my boat out in the bay somewhere and determine just how much throttle can be applied without breaking your thumbs on the wheel. Then I'd practice differential steering with the engines, since yours are widely spaced and could help you. Then I'd do approaches to and away from a piling.

You might find that given your considerable windage there are limits to you ability to get out of a slip on a brisk day. Sad, but knowing that is better than an insurance claim from using too much power suddenly in a crowded marina. You might also change your mooring line routine - turning out of the slip on lines, then use the engines.

I hope I'm not being pedantic. It just seems that the answer lies in getting that beautiful big boat under as much control as possible. Our situation is worse - one prop, no thrusters, and 65,000 pounds. We don't even attempt narrow slips and channels between slips. If that's not enough comfort, in the words of my late Panama Canal pilot father-in-law, the only ship he was ever able to steer in reverse was a three prop destroyer.

[Tupaia]

There are 2 issues here; folding/feathering props and rudder in front of the props.


All folding/feathering props have a delay when going from forward to reverse. Imagine the boat is moving forward under power, the moment you engage neutral the props will feather / fold due to the forward momentum through the water. You can blip the throttle in reverse to reopen the prop just sufficient to stop the boat but without applying full thrust then return to neutral. Once the boat is stationary then gently apply reverse power while solidly gripping the wheel.



It sounds as though you are trying to go from forward to reverse without stopping and not allowing for the props to re-open. When they do re-open they already have too much power and wash.


The whole idea of putting the rudders in front of the props is a poor design compromise to give more internal space. This is a problem even with fixed props.

[UFO]

Originally Posted by tkeithlu

From a design standpoint the only thought I have is a balanced prop.

Thinking about the overall situation, I'm inclined to suspect that your maxiprops have sucked you into using too much throttle/thrust. Any boat with an unbalanced rudder has some degree of the problem, and those with chain/cable steering definitely get the force on the rudder transmitted to the wheel. But, fixed props just aren't that efficient in reverse. If you learned your boat before adding the maxiprops, or learned on another boat with conventional props, I can see a problem.

If I was having these sorts of nasty experiences, I think I'd take my boat out in the bay somewhere and determine just how much throttle can be applied without breaking your thumbs on the wheel. Then I'd practice differential steering with the engines, since yours are widely spaced and could help you. Then I'd do approaches to and away from a piling.

You might find that given your considerable windage there are limits to you ability to get out of a slip on a brisk day. Sad, but knowing that is better than an insurance claim from using too much power suddenly in a crowded marina. You might also change your mooring line routine - turning out of the slip on lines, then use the engines.

I hope I'm not being pedantic. It just seems that the answer lies in getting that beautiful big boat under as much control as possible. Our situation is worse - one prop, no thrusters, and 65,000 pounds. We don't even attempt narrow slips and channels between slips. If that's not enough comfort, in the words of my late Panama Canal pilot father-in-law, the only ship he was ever able to steer in reverse was a three prop destroyer.

The pressure on the rudders starts to build after idle - Its a long way from Max revs (even before 1800) and you really have to hang onto it or its gone and if slightly off center then its really hard to hold on to - Hence just turning the rudder over to Port and using engines only to maneuver.

[tkeithlu]

Does that work? Can you maneuver on engines alone with the rudder hard over one way or the other? If so, go with it.

!800 RPM sounds like a lot of power for work around a dock.

[tkeithlu]

UFO, I think you have every reason to be frustrated. Putting the props behind the rudders does not impress me as a user-friendly design. I didn't bring it up because I couldn't quite believe that a firm as well known and respected as Lagoon would do that.

[UFO]

Originally Posted by tkeithlu

Does that work? Can you maneuver on engines alone with the rudder hard over one way or the other? If so, go with it.

!800 RPM sounds like a lot of power for work around a dock.

Yes it does work OK - Except when my backstop decided to give way That is now fixed and will not happen again.


I was just after some other people to pipe in with a simialr setup to see what they do, but alas it is not to be.



Normally as you say around the dock, revs are low, but sometimes a bit of grunt is needed in Gusts and in strong winds - As you said earlier there is a lot of windage and in a big gust its amazing how quickly things can start to go pear shaped.


The positioning of the saildrives was for a long time a deal breaker when looking at buying cats, but after viewing many 44 cats which you could not swing a cat inside of and the space of the 440 in comparison, it was a compromise that I made - Room for a family of 4 sometimes 5 is very important to long term cruising plans in my opinion.

[MicHughV]

you don't say if your maxprop is the three blade or the two blade variety....just curious...

a max prop, because it has flat blades, basically provides the same thrust in reverse as going forward......if you are used to a standard fixed blade prop, reverse invariably requires a hefty dose of throttle...as the blades do not lend themselves for reversing prowess.

I went from a fixed blade to a maxprop, and had to come up with new backing up procedures, as the max prop provided far more thrust in reverse as did the old fixed blade.

As above posters have suggested, limit rpm going in reverse, as your maxprops will be way more efficient in reverse than that of a fixed blade..might take a bit of practice...

[UFO]

Originally Posted by MicHughV

you don't say if your maxprop is the three blade or the two blade variety....just curious...

a max prop, because it has flat blades, basically provides the same thrust in reverse as going forward......if you are used to a standard fixed blade prop, reverse invariably requires a hefty dose of throttle...as the blades do not lend themselves for reversing prowess.

I went from a fixed blade to a maxprop, and had to come up with new backing up procedures, as the max prop provided far more thrust in reverse as did the old fixed blade.

As above posters have suggested, limit rpm going in reverse, as your maxprops will be way more efficient in reverse than that of a fixed blade..might take a bit of practice...

3 Blade version. Even limited power gives enough over the rudders to make it interesting

[Thumbs Up]

Thumbs Up has giant stern hung rudders. I have a similar problem but it is not from propwash. When moving astern my rudders want to slam hard over. It happens with such violent force that the spokes of the helm wheel are a danger. I always lock my rudders in the straight ahead position before any astern maneuvers. With your setup (though I don't know a solution to the problem) there are times when you should be able to use the prop wash that happens in reverse to your advantage. If I understand correctly, a burst of throttle in reverse should push the stern in the direction that the wheel is turned even if you are stationary or still moving forward.

[MicHughV]

I should add here....the maxprop has flat blades, ie, no twist like a fixed blade. This reduces the efficiency of the maxprop somewhat as compared to a fixed blade.
Max prop compensates for this, by making the blades a bit wider, the end result is backing up in reverse with a max prop provides enormous low end thrust as compared to a fixed blade prop. You have to recognize this when backing up.
I had similar problem as you after I fitted a maxprop. I had to hold the wheel firmly with both hands going in reverse, and if I did want to turn the wheel a bit, I would do so only in small increments of wheel movement, all the while holding the wheel in a death grip. Too much movement would turn the wheel out of my grasp. It took some trial and error, but it all came together eventually.

[Jim Cate]

I had to hold the wheel firmly with both hands going in reverse, and if I did want to turn the wheel a bit, I would do so only in small increments of wheel movement, all the while holding the wheel in a death grip. Too much movement would turn the wheel out of my grasp. It took some trial and error, but it all came together eventually.

Yes, but that was due to sternway rather than propwash from the aft mounted saildrives... a rather different situation.

And IMO that configuration is a poor choice!

Jim

[BillKny]

Wow... such a lot of bad info in these answers...

Somebody said it was because your rudders were not “balanced”. Baloney. Your rudders ARE balanced, but they are balanced for moving FORWARD. You can not balance a rudder for water flow both forward AND reverse.

It has nothing to do with the “slow response” of your MaxProp. A MaxProp will fully unfeather in less than one rotation of the prop shaft, so that is obviously not the issue. It has NOTHING else to do with the fact you have MaxProps, and their flat blades, or anything else.

The issue you have is fully, and simply, a function of the design having the prop abaft the rudder. I am sure there were good reasons that Lagoon chose to do it this way (cough, cough) that made the difficulties worth the compromises. But, in reality, it was done strictly to gain a couple extra feet of space for an aft cabin. That’s it. As others have said, it’s just bad design, but it is the boat you have got...

Driving the boat in reverse with the rudder hard over to one side to avoid torque on the wheel is just a bad idea in so many ways. It GREATLY loads the rudder, steering gear and the rudder stops. It makes your maneuverability messy and unpredictable. Just DO NOT do it this way.

When confronted with catamarans with this design “feature” (and yours is not unique) what I have always done is to drive in reverse standing at the helm looking toward the stern of the boat (where else would you look while going in reverse?) with my butt firmly pushed against the wheel, and the wheel as close to centered as possible. At any reasonable engine speed, this keeps the wheel centered. Steering is only done with the differential thrust of the engines. When ever you are in close quarters, forward or reverse, this is how you turn the boat.

There is no need to be doing this at much more than idle throttle. If you are running your engines at 1800 RPM in reverse, you do simply not have good control of your boat. SLOW DOWN.

Just so you know, I taught the US Sailing cruising catamaran class for years, so I do know of what I speak.

[MicHughV]

I often back my sailboat into my slip.....it took me a while to perfect this maneuver.....neither the prop nor the rudder are very good tools for this procedure...the boat's momentum plays heavily into this process......like a symphony, a lot of different parts must work together....I don't have a bow thruster either....it require practice and patience...

I also run a twin engined fishing boat, with counter rotating props....having two engines is helpful in docking in backwards, but still requires some forethought......what I learned here, is that an outboard engine in neutral does not provide any steering ability either in forward or reverse, the prop must be engaged. As with the sailboat, there was a learning curve involved here. I am fortunate that I know some very experienced powerboaters, that back very large boats into their slips. The rudders on those boats are so small as to have little affect so they are left amidships and the whole procedure is done under engine power alone..

prop thrust is not directly behind the prop.....the angle of the blades pushes the water to one side or another, this is also a feature that can be used in backing the boat up...

as the above poster has stated, on a cat, use the engines as the rudder...it's that simple.....requires some practice..

[EWOL Props]

WOW ! 1800 RPM while docking is a lot !
Are you sure you have the right pitch ? You should not need so many RPM because feathering props are already very efficient in reverse.
If you need so many RPM you might have the wrong pitch or diameter