Help with designing a new rudder for a fisher 34 motorsailer

Discussion in 'Boat Design' started by dm567, Feb 10, 2012.

  1. dm567
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    dm567 Junior Member

    Just got back from a trip. Sorry for the delay in getting back to you. Thanks for the posts.

    I looked up crevice corrosion and found this definition: Corrosive degradation of metal parts at the crevices left at rolled joints or from other forming procedures; common in stainless steel heat exchangers in contact with chloride-containing fluids or other dissolved corrosives. Also known as contact corrosion.

    Fast Fred. Are you saying that the stainless will corrode from the inside? Can't that be stopped by attaching zincs? Please explain.


    I like the idea of the 00 foil. Will adding end plates really cause that much more drag?

    I'm really looking for increased maneuverability in the harbor at the slowest speeds. Currently if I I use full rudder in reverse it doesn't steer at all. Perhaps one of you could explain what is going on.
  2. PAR
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    PAR Yacht Designer/Builder

    The proper grades of stainless hold up exceptionally well underwater. Countless tens of thousands of boats, have stainless props, shafts, struts and rudders.

    Your boat suffers from what all boats of this general shape do. It'll travel in a straight line really well, but doesn't like to turn. Going forward, maneuverability isn't so bad (it's not good, just not that bad), assuming you have a reasonably amount of way on, but in reverse, you're pretty much screwed (I feel your pain).

    A foil shape on your rudder will improve it's efficiency a small amount. So small, that you'll likely not notice it. In reverse, you're still screwed and steering with prop walk and blast deflection, is the only real way to get her butt to swing one way or the other. Learning this technique takes an afternoon of practice, once someone shows you how it's done. After you've mastered this skill, you'll have a much easier time in reverse.

    Unfortunately, hulls that are shaped like this suffer from this and there's not a lot that can be done. Some improvements can be applied, but we're taking about very slight differences here, particularly in reverse. It's a function of the shape of your boat and not the rudder.

    Simply put, that big hunk of keel straightens out the water flowing past it (regardless of boat direction). It's really good at doing this, so the helm is steady and easy, which is a good thing. When going forward, the flow is deflected to one side, when the helm is moved and with nothing behind the rudder, this flow continues in that direction, levering the boat around it's center of buoyancy. Picture the "stream lines" of water flow, traveling along the keel, then being deflected to one side. Much like an airplane's wing. Now in reverse the rudder is the leading edge and still deflects the flow, but this flow travels the length of that massive, straight keel, which straightens it out and the boat refuses to turn. I have a 40'er that doesn't turn in reverse either, so I know your pain, but then again, I can "walk" my boat up to any wharf or dock with a combination of helm and throttle inputs. This is what you need to learn or you could install a thruster.
  3. dm567
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    dm567 Junior Member

    Sounds like there's no cure for reverse; not even a meaningful improvement.

    I've spent some time practicing and will do more. My new slip required me to reverse back and turn to the opposite direction of the prop walk and I'm having a difficult time in the confined area. Can you describe what you mean by blast deflection? I'm not familiar with that concept.

    Also, I've tried getting moving and then going to neutral and using the rudder in reverse. This seems to help a bit. Any thoughts on this procedure?

    Lastly, does it help not to turn the rudder hard left or right? I've read that at the extremes you might negate the rudder's usefulness completely.


  4. michael pierzga
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    michael pierzga Senior Member

    Sure..stopping the prop from rotating in reverse helps tame prop walk.

    A big propeller that is efficient in reverse, to stop the boat rapidly, and give it way in reverse is the only solution. The MAX type props are good for this because they give stopping power with less prop walk.

    Modern sail handling systems have made sailboats very hard to maneuver at slow speed. Too much windage forward with the roller gear and this easily overpowers the rudder at slow speed. . This is why with roller furling , a bow thruster is standard equipment these days.

    I drive very many sailing yachts, for a very long time.

    Without a bow thruster I hang plenty of fenders over the side then perform a controlled CRASH each time I dock.

    See if you can lower your windage aloft , buy plenty of fenders , practice and keep your wits about you.

    I avoid docking during the afternoon when the thermal wind, seabreeze, is strong, preferring to berth late when the wind has gone home.... then people say...WHOA, that guy knows how to dock boats !!!!!!!!!
  5. dm567
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    dm567 Junior Member

    I installed the autoprop a few years back and it helped. I found it on ebay unused for 1/4 of the price and it was the perfect size.

    Yes, the windage is a problem especially trying to back out of my slip with some wind present and trying to turn the boat in the opposite direction it wants to go. I try to leave earlier in the morning because of that.

    Thanks for the help
  6. PAR
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    PAR Yacht Designer/Builder

    The process of steering a boat with the prop is more difficult to explain then to show. A powerboater with a single straight shaft also will know how to do this.

    Michael, it's not a function of his modern design or handling systems:

    It's a function of his (the OP) well founded hull and appendage design approach, that favors tracking to maneuverability and has little to do with windage. He could have a surfboard, with that underwater configuration and wouldn't be able to back it without learning how to steer with the prop. This technique is commonly called "back and fill" and several pieces are written about the process, but it's just one of those things that has to be practiced, because each boat is different and reacts differently to the inputs.

    To address Michael's comments, modern sailing vessels have relatively small, low wetted surface appendages, which at slow speeds don't preform well. Coupled with the trend toward taller freeboards in shallow bellied modern craft, these types of boats (much unlike what the OP has) require a reasonable flow over the appendages to actually work. Without sufficient flow they'll wander all over the place and be affected by the slightest winds or wakes. This is a completely different set of variables then the full bellied, full keeled craft we're talking about here.
  7. viking north
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    viking north VINLAND

    Also don't underestimate the helping factor of mooring lines(retrieviable) at stratigic locations for both departing and docking. In certail tight locations with wind and tide situations it's the only way you're going to be able to handle her.

  8. Yobarnacle
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    Yobarnacle Senior Member holding true course

    Trying to steer astern won't work with more than 5 to 10 degrees of rudder on any vessel. Hardover is futile, just brakes. It requires a fair amount of stern way speed to get any rudder benefit.
    Now if you installed flanking rudders forward of your prop, you could steer pretty good going astern.
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