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-   -   3 blade prop question. (http://www.sailnet.com/forums/gear-maintenance/49284-3-blade-prop-question.html)

baboon 11-28-2008 11:13 PM

3 blade prop question.
 
The facts (let's keep SD happy)
Early 70's Tartan 30 with 3 cyl 18 hp diesel. Fin keel, skeg hung rudder, properly sized and pitched 2 blade prop in place. The engine on this boat sits on the keel, the shaft exits aft the keel and the prop is RIGHT there. No strut, no exposed shaft, just prop with the zinc aft of the prop. Forward is fine, sailing is good as the shaft is marked in the bilge and the prop can therefore be aligned with the keel verticaly.

The problem. Reverse is a mess. The boat will go where it wants, period. In an adverse wind or current the boat is almost uncontrollable. You can push the tiller any way you want, until you get a good speed up it does not matter.

The question. Would a good folding 3 blade prop help. The issue is NOT speed or efficeincy, it is control in reverse at low speed.

Thanks.

Faster 11-28-2008 11:34 PM

Quote:

Originally Posted by baboon (Post 408734)
....The problem. Reverse is a mess. The boat will go where it wants, period. .....You can push the tiller any way you want, until you get a good speed up it does not matter...

While many boats do not handle well in reverse, many are in fact quite good. None of them, however, will steer well until you get good speed up. In order for the rudder to have any effect there must be flow past it.

The configuration of the Tartan is not one I'd expect to be particularly bad in reverse, (compared to a typical full keeler, for example) but I've not driven one.

A three blade prop may get you going a bit sooner, and allow attached flow more quickly, but you may also suffer more prop walk and of course more drag under sail.

It's important to not try to steer by tiller until you have speed, if you try to turn the boat then you are really just putting on the brakes and probably making things worse.

Find some open water, and practice backing up with the rudder amidships initially, see how she walks and at what point you have enough speed to think about steering effectively with the rudder.

All that said, however, there are boats that simply won't go along with any plans to go where they are pointed when backing... and that's tough if you have one of those.

paulk 11-28-2008 11:38 PM

Going backwards
 
A folding 3-bladed prop might help. Then again, the rudder was designed to work with the boat going forward, so could increasing reverse thrust simply make it worse, with the rudder stalling out sooner, because you're going faster and losing contact flow before you used to? Lots of guys get into plenty of trouble in reverse, even when they have good steering control; your lack of control may not be as bad as you think. If you can experiment without actually buying the prop, it mght be worthwhile. Dave Gerr has an interesting book - "The Nature of Boats" - that goes into some of the issues involved with propellers and the surprisingly big results - both positive and negative - that apparently minuscule changes can have.

sailingfool 11-29-2008 06:12 PM

A three blade prop should have no effect on your backing, only on your pocketbook. If you bought a prop like a two blade MaxProp, you might get some improvement just from the better drive in reverse, allowing you to steer sooner.

btrayfors 11-29-2008 07:03 PM

A 3-blade Maxprop will almost certainly help you in reverse. Mine does. I've had it for 18 years and tens of thousands of miles of sailing. Made a very BIG difference when I replaced the fixed prop. The blades set themselves to pull as well in reverse as in forward. It did NOT significantly increase drag in forward (no change in boat speed vs. a 2-blade fixed prop; maybe even a bit of a gain).

It did not help in forward; same thrust as fixed prop. But in reverse, it made all the difference. You have to learn to work with the prop walk, but the ability to put on some speed (or brakes) in reverse quickly makes a big difference in boat handling.

Bill

Rockter 11-29-2008 08:43 PM

My own ship takes a while to gain steerage going astern. If there is an meaningful wind, it really is difficult, particularly when reversing into a pontoon finger arrangement. I am happy to have moved to a longer dock for that reason.

In reverse, you can normally tell when you are gaining directional control as the rudder begins feeling "heavy", in my case through the wheel.

If the rudder is feeling "easy", it ain't steering.

If the ship you have is moving forward ok with the prop you have, I would be inclined to leave it as is.

Changing props doesn't always improve things.

brak 11-29-2008 09:16 PM

My boat does not steer in reverse at all. She does have a full keel, which explains a lot - but the prop is a properly sized 3 blade. I can certainly gain a lot of speed in reverse fairly quickly, but steering just does not happen. So, there is no guarantee that a 3 blade prop will help you steer - this may simply be an issue with your boat's underwater configuration.

sailingdog 11-30-2008 08:10 AM

I don't think a folding prop would help much, as folding props generally aren't much better than fixed props in reverse, and many are worse. What you probably want is a feathering prop, which usually increases efficiency of the boat in reverse, as the blades feather to re-orient themselves in reverse. A good one that is fairly reasonably priced is the Kiwi Prop.

Also, please do remember that until the boat is actually making way in reverse, you're not going to have any steering.
Quote:

Originally Posted by baboon (Post 408734)
The facts (let's keep SD happy)
Early 70's Tartan 30 with 3 cyl 18 hp diesel. Fin keel, skeg hung rudder, properly sized and pitched 2 blade prop in place. The engine on this boat sits on the keel, the shaft exits aft the keel and the prop is RIGHT there. No strut, no exposed shaft, just prop with the zinc aft of the prop. Forward is fine, sailing is good as the shaft is marked in the bilge and the prop can therefore be aligned with the keel verticaly.

The problem. Reverse is a mess. The boat will go where it wants, period. In an adverse wind or current the boat is almost uncontrollable. You can push the tiller any way you want, until you get a good speed up it does not matter.

The question. Would a good folding 3 blade prop help. The issue is NOT speed or efficeincy, it is control in reverse at low speed.

Thanks.


SEMIJim 11-30-2008 01:42 PM

Um, correct me if I'm wrong, but ISTM no sailboat maneuvers worth a damn in reverse until she gets some way on--other than perhaps smaller boats, with outboard engines, where the prop itself can be turned.

Jim

baboon 11-30-2008 02:22 PM

Thanks for the input. Looks like I will probably stick with the prop I have for now. I understand that no sailboat will back with any control until you have some movement in reverse. Mine might be a bit worse than average given the midship location of my prop. My pet theory is that a boat with a prop near the stern will be "pulled' along in reverse, making it a little more likely to back strait. With a midship prop against the keel my boat seems prone to pivit on its prop/keel in whatever direction it is pushed by the wind or current. The current set up is fine for quick acceleration and stopping.


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