Subject: [harryproa] Re: sailing BD with leeboard
From: Mike Crawford
Date: 11/3/2009, 9:30 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

<<How far apart do they need to be to avoid interference?>>

  That's a good question. 

  Tandem keels, an option on Etap and Bavaria monohulls, have been very well received -- reports are that they develop significant lift.  I've not seen one in person, but from photos, it appears that each keel might be 750 mm, with the same space in between.

    http://www.yachtworld.com/core/listing/displayPhoto.jsp?ybw=&boat_id=2064544&photo=10
    http://www.yachtworld.com/core/listing/displayPhoto.jsp?ybw=&boat_id=2064544&photo=11

  If one were to build a tandem keel for a proa, real dimensions wouldn't be too difficult to find.  The section would obviously have to be symmetric.

  The big question for me would be how this would interact with a thruster or sail drive.  I have no idea how that would affect the spacing.
 
  Perhaps it would still be angled slightly to windward in order to minimize interference.

  A question for anyone skilled in fluid mechanics: would two higher-aspect keels or mini foils be better than a single lower-aspect keel?  I'm told the twins would generate more lift, but don't know if that's true or not.

  Also, how deep?  My guess is that 300-450 mm would be enough to help handling without being too obtrusive. 

       - Mike

 
robert wrote:

 


Possibly the time has not come for electric drive on these boats but as batteries get lighter and more efficient and liquid fuels get more expensive the time should not be too far off.
Good point about twin keels not bringing CLR as far forward. How far apart do they need to be to avoid interference?
--- In harryproa@yahoogroups.com.au, Arto Hakkarainen <ahakkara@...> wrote:
>
> I have also thought about this kind of arrangement for the auxiliary. The idea I like most is twin keel with saildrive unit between the keels or even faired in to one of the keels. Twin keels should move the CoE less forward and they would give great protection for the drive unit. Retractable thruster would of course be nice but also complex and expensive. Steerable electric saildrive would provide great manouverability in all conditions, especially if combined with electric outboard close to ww hull. Would the complexitiy, weight and price of electric be worth it... well let the boat buyer decide. Inboard and saildrive would be enough for most situations.
>
> Arto
>
>
>
> From: Mike Crawford <jmichael@...>
> Subject: Re: [harryproa] Re: sailing BD with leeboard
> To: harryproa@yahoogroups.com.au
> Date: Monday, November 2, 2009, 5:38 PM
>
>
>  
>
>
>
>
>   One thing I have debated for a while is going with a propulsion-rated retractable thruster in the lee hull for the boat's motor.
>
>   On one hand, I realize that this carries a $10,000+ penalty when compared to outboards, as well as some weight (once you figure in a genset and any propulsion batteries). 
>
>   On the other, it would have a number of benefits:
>
>   - Convenient collapsing without having to worry about 000 cables, outboards, or other apparatus.  Everything could be in one hull, with just house power going to the windward hull.
>
>   - A highly cavitation-resistan t drive system that's unlikely to get beaten up or swamped by waves or chop.
>
>   - The ability to drive forward or reverse without issues.
>
>   - The option of charging batteries, either propulsion or house, while under sail.
>
>   - The option of using diesel, or fuel cells in the future, instead of gasoline.
>
>
>   But retractable thrusters are expensive, and they likely won't take very well to grounding or running into logs.
>
>   A mini keel could be a nice way of making sure there's something sturdy to take impacts that would wipe the  thruster out.  There could be either two mini-keels in tandem, each with its own foil shape, with the thruster in the middle, or the keel could be one piece with the thruster angling out to windward. 
>
>   There would be less need for a keel if the boat had a deeper draft and the thruster could poke out above the hull bottom on the side, but that's not likely to happen with a proa that has any reasonable performance.
>
>        - Mike
>
>  
>  
>
> rpvdb@ymail. com wrote:
>  
>
> Hi,
>
> We sailed BD again with the repaired leeboard.
> I fixed it to the lw hull with a Dyna One line through the hull side to the mast frame. There were a lower and upper guard to keep it in position, at least that was the intention. It appeared very hard to obtain enuogh tension on the line to keep the board thightly next to the hull. A bolt would be better.
> Results of the sail pretty much confirm the earlier ones.
> I asked somebody new to the boat to come along for the test.
> When we where less concentrated on sailing the boat (too busy talking)the board would come off the lower guard and after that it was hard to bear away and the boat luffed into the wind a couple of times.
> Next day I sailed with the board of the foundation owning the boat and
> After a while the line connecting the leeboard to the hull broke.
> Now they could see with their own eyes the difference between having the leeboard and not having it. The boat would luff into the wind when steering higher that a close reach.
> For simplicity they prefer a keel as I expected.
> The extra hassle of uphauls and downhauls and keeping an eye on the leeboard is more that they care for.
> So I will try to get a keel under the boat before winter really kicks in.
> Regards,
> Rudolf
>

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