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raritan
03-24-2000, 07:16 PM
This is probably an elementary question for the donzi gurus (ask me something about VW's) but when did Donzi change to right hand steering. Also is it a SEVERE problem to drive an early left hand boat with a rt turning mercruiser????? Again you guys are super with advice and its greatly appreciated in the Ozark Mountains

AVickers
03-25-2000, 10:19 AM
Without trim tabs, the boat will list to the left when running with a right-rotating prop (when viewed from the rear of the craft). The rotating prop creates a vector of force perpendicular to the theoretical plane of the blade -- part of this is translated back; part of it is sideways -- all the way around the arc of the rotating prop. Problems occur because there's more force created at the bottom of the prop sweep than at the top. This is because the prop has more clean water at the bottom of its sweep (less hull-induced turbulance and no foot in the way). Thus it takes a better bite in the water and pushes sideways more at the bottom of the sweep in the tengential direction of rotation -- in your case, right rotating prop pushes the whole thing to the left. The foot acts like a crank arm to try to rotate the whole boat around its own axis of force in a direction couter to the rotation of the prop. Since water can be pushed out of the way by the hull; with enough energy, the whole boat tends to rotate (in your case) to the left. The more energy applied to the prop, the more the twisting moment...and the closer to the water the left-sitting pilot will be.

Trim tabs would allow you to counteract this force. Essentially, you'd set the left tab a bit deeper in the water and it would counteract the force of the prop. One problem with tabs is that this "force" to rotate and counteract the rotation would take away somewhat from forward force, thereby reducing the ultimate top speed of the boat. Of course what good is ultimate speed if you can't control it?

I'm not at all sure, but there may also be some slight "twist" built into the hull to counteract the torque of the prop as well. A sort of balance that is greater than the weight of the driver. I'm thinking this because the torque produced by the engine would make the 200# weight of a driver seem insignificant when considering the mechanical advantage (gear ratios and force radii) that the engine would have to move things around. But, that's just speculation...

I have a Volvo on my boat and was trying out different props -- one of which was a right hander. Changing rotation on a Volvo is cake, so I would run one prop, pull in, change stuff and immediately go out and run another. The list with the right hander was quite disconcerting.

What makes me wonder about the hull having a twist of some kind is that when the boat is loaded with people (weight more-or-less equalized right-to-left) I don't notice that it rides more one way or the other. It still seems "balanced." So, my thought is that there might be some counterbalance built into the hull or into how the engine/foot are placed or canted...

A left rotating system would be the best to have in an old right-helm Donzi. You could get a left-rotating foot or a clockwise engine (change cam, water pumps, and starter...at least). OR you could install tabs. From what I understand about Alphas, tabs might be the way to go -- cheaper and simpler. Plus you'd have more control over the attitude of the boat in wind or waves...

Does that create any more confusion than necessary????

AVickers
03-25-2000, 10:48 AM
Other than a spelling error or two, there is a significant error in my next-to-last paragraph. The ALL CAPS SEGMENTS should read as follows:

A left rotating system would be the best to have in an old LEFT-helm Donzi. You could get a left-rotating foot or a clockwise engine (change cam, water pumps, and starter...at least). OR you could install tabs. From what I understand about Alphas AND THE DIFFICULTY OF CHANGING ROTATION IN THEM, tabs might be the way to go -- cheaper and simpler. Plus you'd have more control over the attitude of the boat in wind or waves...

http://206.150.187.82/ubb/eek.gif

Tony
03-25-2000, 05:43 PM
Avickers, that gets my vote for one of the most thorough hypotheses proposed in these pages! I have a '67 16 with a LH Volvo and left side helm and, when at rest, it lists to port. Probably due to the location of the battery, captain's chair and steering wheel, right? Is the engine itself pretty balanced side to side, weightwise?

Also, a recent thread highlighted a couple of items that were supposed to do a real nice job of reducing steering torque. Anybody have experience with these?

I'll use the awesome "search" feature and hunt that thread up.

AVickers
03-26-2000, 09:17 AM
Torque steer is another of the difficulties encountered with manual (cable) steering Volvos. And others too, I suppose. Although caused by the same forces, they are different in their actions on the boat (and its occupants).

As near as I can tell, torque steer is a result of two things: 1) "Paddle-wheeling" and 2) vector force around the gimble axis.

Paddle-wheeling is the the prop's attempt to simply push the boat sideways. A left-hand prop would try to push the stern of the boat to the left -- some of the force tries to rotate the whole boat (causing the helm-location issues), while some of the force results in movement of the boat sideways.

The other problem is probably more noticeable and dangerous. And can be controlled by other means... As you look down on the boat, the sideways force of the prop occurs behind the axis of rotation of the foot on its mounting bracket. On a left-hand prop, this force tries, literally, to push the foot to the left, thus steer the boat to the left. The little fin on the bottom of the cavitation plate is there to counterbalance this force. And it does quite a good job of it, once a steady state is reached -- force of the prop is counteracted by the opposite and equal force of the trim tab. Problems occur when things aren't in a steady state...

Like when you're on plane and then get on the throttle hard. At this point, the prop acts visiously and the tab can't do anything. You, as the driver, are left to counteract the steering forces created. Thus the term "handful" when referring to high-power manual drive boats. You gotta grab a handful of steering wheel and hope the #7 pucker you got on the seat don't break loose! (Perhaps they should call it an "assful.")

Anyway, there are some things you can do with prop selection that can help with this -- but they'll all kick the foot around. Especially if things are a bit loose (wear in the cable and housing, wear in the bushings, etc.)

I installed a Teleflex Hydraulic System and it seems to have cured the "handful" part of the equation. I can still feel the boat trying to pull sideways under hard throttle, but the zero-feedback nature of the system isolates me from having to wressle with it. It has totally transformed my boat! Later model MerCruisers have power steering, so guys with these rigs don't know...

Like I say, it's the one thing I've done to the boat that has made the biggest difference in how it handles. The other thing you could do is install a Duo-Prop foot. I've looked into that and they come in somewhere around $3K for a foot and you'll have to spend another grand on props...

Makes the $700 I spent on the hydraulic system seems like a bargain!

Enjoy!