Gardner

You have limited to 1ft deep and it stays fixed. 

Some basic information that might help in you calculations:

1.  Adding a non-cambered stub keel will increase drag over rudders.  You may get a small benefit in control because the CLR will not shift as much with pitching.  For calculation purposes a 2ft long by 1ft deep keel will have a maximum Cl of around 0.1 at 5 degrees.  It will need leeway to generate lift.  The Cd for this stub at zero AoA is 0.006.  So added drag downwind is very small. 

2.  You could use a cambered keel of similar proportions.  An example of a cambered keel with 10% thickness and 5% camber will have L/D of 10 at zero AoA and Cl of 0.2.  There may be a size that will do better than rudders overall.

A cambered stub keel 2ft by 1ft will produce 500N lift at 10kts.  This is probably not a lot relative to sail loads.  Extra drag will be 50N and this will persist off the wind as there will be some tendency to crab toward the ww hull.  Increasing length reduces efficiency and there is no net benefit.  There is a net benefit in keeping it short but going to thicker section.

So a stub say 300 deep, 600 long on the base, 120 wide at the base, 900 long at the top and 180 wide at the top - roughly 0.2sq.m.  Cl at zero AoA is 0.5 and L/D is 7 - a bit worse than ventilating high aspect rudder.  The lift generated at 10kts is 1250N.  I expect this will be significant.  Extra drag is roughly 200N.  This will likely be significant detractor off the wind.

I can do polars to compare these options but it takes a lot of time to set up the boat model - it is a weekend job.  I need hull shapes, displacements and sail plan to do these.  They do not have to be precise because you want to compare one change from existing.  The absolute values may be in error but the relative performance should be close.  

Rick

On 17/05/2011, at 11:16 AM, Gardner Pomper wrote:

 

Rick,

I should have added a couple more pieces of information. I am not interested in a centerboard or any other break in the lw hull. I like the idea of not busting that up if you hit a centerboard on coral (I have done that). My main concern with Rob's idea of just big rudders is the force on them gets really big, especially as the mount point is 2-3 feet above the waterline. Even if they can be built strong enough, I would always worry about them. I would much prefer smaller rudders that are mainly for steering. 

I also don't care for the idea of having to fiddle around with rudders, like raising the front one each time I shunt. I am out to relax and explore; not for excitement or (heaven forbid!) work.

- Gardner

On Mon, May 16, 2011 at 9:06 PM, Gardner Pomper <gardner@networknow.org> wrote:

Rick,

I am trying to keep the draft to a minimum. The ww hull will draw 1'. I guess that the rudders probably need to draw 18" to work right (bad assumption?). So, I figure I could add a foot to the lw hull without increasing the effective draft. I am just looking for coasting along (5 kts max) in really shoal water.

The idea I have is something like the stub keel on the Maine Cat 30 (http://mecat.com/MC30specs.htm). It is built seperately, then glued to the hull with 5200, with the idea that if it gets hit hard enough, it will come off instead of ripping open the hull. The leading edge is tapered to help rise up over the obstruction if possible.

If I try to make the aspect ratio > 1, then my surface area for leeway prevention is < 1 sq ft, which seems pretty pointless. So, it sounds like making it any kind of foil is useless, but what about having it as just a barrier?  I thought I had read that the water just flows around a semicircular bottom and that if you put a barrier, it will "bite" better. I think that was in a discussion of a hard chine dory.

Anyway, I know that increasing the wetted surface adds drag, and I also know (i think) that a stub keel will prevent leeway. My problem is that I don't know how to get even a ballpark feel on how much performance I am giving up and how much leeway prevention I am gaining.

I also haven't explicitly stated, but I am a "performance oriented cruiser", which to me means that I will never enter a race, but I hate outboards so I want to sail as fast as possible to shorten passages and to be able to sail from spot to spot instead of motoring. Also, it is just cool to be faster than everybody else <grin>.

So, if you can give me a simple guide on how to figure this, or (if you are willing), a list of the information you would need, I would appreciate it.

- Gardner

On Mon, May 16, 2011 at 6:15 PM, Rick Willoughby <rickwill@bigpond.net.au> wrote:

 

Gardner

A stubby keel under the hull will work more efficiently than using ventilating rudders or the hull if it has an aspect ratio of about 1.5 or greater.  

For calculation purposes I considered a 12% section with a 6% camber.  This has a Cl of 0.6 at zero degrees AoA and a corresponding L/D of 15. So better than the ventilating rudder which has a best L/D of around 9.

It would not need to be as big as you have suggested but likely deeper.  Based on the Cl my first approximation would be about 1/200th of the sail area to prevent leeway. It does not need to be constant chord.  It could be shorter at the bottom than top such that the edges are raked to provide lift if struck.

If it is rigidly fixed it will create lift as long as the boat is moving.  So there will be extra drag off the wind.  A non-cambered shape is no better than using the hull as leeway preventer.  If it can lift then the varying chord creates sealing problems.  It could be a combination of constant chord and tapering. 

Another advantage of a centrally located board that prevents leeway is that the point of lateral resistance is not moving longitudinally with changes in trim as the boat pitches under various loads.  So it is more inclined to stay balanced.  

Using the hull as the leeway preventer will cause the CLR to be forward of midship even in level trim.  So this is already creating turning moment that needs to be countered with the helm.  

Rick

On 17/05/2011, at 12:01 AM, Gardner Pomper wrote:

 

This seems like a thread that might be able to answer a related question that has been nagging at me for a while. I am mostly concerned with how shallow the water can be for practical sailing (this would be, for example, cruising off the south tip of florida, inside the florida keys).

Each of the harryproa configurations I have come up with ends up with a very shallow draft on the leeward hull (usually 6-9" when sailiing flat). My impression is that you will probably want to allow at least 18" of rudder in the water for steering. So, it would seem like I could put a stub keel on the bottom of the leeward hull which is about 12" high and maybe 10' long to help with leeway prevention.

My question is, what does the additional wetted surface area do to the overall performance of the boat? To put some numbers on it for my wooden boat competition entry, the wetted surface area for my boat are:

lw hull: 80 sq ft (1.5' hull beam with semicircular underwater profile)
ww hull: 70 sq ft (2' hull beam with 1' flat bottom and 6" radius curved edge, with a 1' draft)

if I added a 1' stub keel, 10' long to the lw hull, that would add 20 sq ft of wetted surface, an increase of 13%.

So, does this mean I would lose 1 kt of boat speed at 12 kts (i.e. would I go 13 kts without the keel)?

Also, what effect would this have on leeway prevention?

- Gardner

On Mon, May 16, 2011 at 8:46 AM, Rick Willoughby <rickwill@bigpond.net.au> wrote:

 

Rudolf

There is performance merit in resisting leeway with a non-ventillating foil or foils.  Once this is achieved it does not matter much how the boat is steered unless the rig is poorly balanced.  I am now thinking dipping rudders a little more sophisticated than what Tarawa is using would be my starting point. 

The other conclusion is that a slab sided or very deep V (> 45 degrees) hull will be as effective at resisting leeway as ventilating rudders.  So a reasonably elegant solution is exactly what Tarawa has in concept.  It could be engineered with a bit more precision.

Rick

On 16/05/2011, at 11:16 PM, Rudolf vd Brug wrote:

 



 

 

Arto,

 

That's exactly what I was wondering:

to keep thing as simple as possible could a keel with two small retractable rudders work well?

And I mean a keel put onto the same hull you would use having boards, not a deep hull like Tarawa as that would have more wetted area.

 

Can you say anything about that Rick?

 

Rudolf

----- Original Message -----

From: Arto Hakkarainen

To: harryproa@yahoogroups.com.au

Sent: Monday, May 16, 2011 2:04 PM

Subject: Re: [harryproa] Re: Design your proa

 

3) A comment, not a question: what you have said here seems to make sense to the traditional proa lee hull design with almost flat leeside and curved windward side like this Gary Dierking's Tarawa design: http://homepages.paradise.net.nz/garyd/tarawa.html .When working with no foils it must have been the most effective hull form.     Arto 0419 104 821

Rick Willoughby

rickwill@bigpond.net.au

03 9796 2415

0419 104 821

Rick Willoughby

rickwill@bigpond.net.au

03 9796 2415

0419 104 821

Rick Willoughby

rickwill@bigpond.net.au

03 9796 2415

0419 104 821