Dinghy Cruiser /Racer Design

[Ian]

Originally Posted by FoveauxSailor

Hello Ian
Have you sailed a standing or balance lug before ??

Hi, no, the only experience I have of a standing lug was one I made myself some years ago as an experimental canoe sail. I used a tan-bark coloured polyester cloth--not sail-cloth-- and cut it using techniques I vaguely remembered from a very old book I had once read on how to cut your own cotton sails. I cut a curve into the luff and head and sewed the sail stretched out losely to a tightly stretched boltrope. It set wonderfully. The cloth I used was just stretchy enough for this technique to work perfectly.
As you have no doubt read, the lugsail is much less prone to twist than other sails (which is pretty helpfull if you are not using an boom and kicker).
From photographs I have seen in many books and magazines, the dipping lug sets particularly well. More of a hassle to handle of course. I get the impression from my reading that a standing lug should not have an extremely highly peaked yard--but a high peaked yard works fine on a dipping lug. That might be incorrect. I will be using a boomless sail so I have drawn a sail with a low tack and high clew--trying to balance the sail area around the extended line of the sheet. The idea is to get a sail that--like a high cut yankee jib--can be sheeted from one fore and aft position. I will use a two part sheet. The lower sheet block will be on the rail and the dead end position will be movable. It will be possible to move the dead-end from its usual position on the rail near the sheet-block to the centerline for windward sailing--thus moving the sheeting angle inboard. Experience will probably tell me that I need a second dead-end position further forward along the rail to control twist on a broad reach and run. I will find out. A friend who has raced double-ended boomless standing-lug rigged boats in a local informal class, tells me that they have only a single sheet and move it from side to side as they tack, hooking it in different positions on different courses. They also have no rudder and steer and tack by moving their weight around. I believe that there is a-- Saint laurence skiff?--that steers in this way. I would like to see these boats myself.

[spoedvraat]

I have been busy doing important work on a CNC machine, which will make my life a lot easier when building of the boat starts, and was pleasantly surprised to see the new thread has seen some traffic.
The first images of my boat is posted in the gallery (sailboats/monohull), so please have a look, critisize, rip apart etc.

[spoedvraat]

HI Foster,

Here is an intriguing write-up of "rules:"

The centreboard should be about 4 per cent of the sail area. Marchaj
There should be about 1sq ft of centreboard area to 40sq ft of sail area; in light craft the rudder area should be half this, while in heavier craft it should be still smaller. John F Sutton Comment: Your guess is as good as mine...
A rudder area of between 8 and 10 per cent of the total lateral plane or underwater profile is the desirable size for a sailboat. F S Kinney
(The hyperlink is http://home.clara.net/gmatkin/therules.htm)

I went larger on the centerboard and rudder than this on my first little dinghy, and she sails beautifully balanced and points very well for a gunter-rigged boat. (3.5 metres long, marine ply, weighing approx 50-60 kg rigged.) As soon as I can find the drawings again, I'll post the exact sizes of sail area to centerboard and rudder areas.

[Tim B]

Interesting thread guys, and an interesting project

You'll be pleased to know that this is not the first time this question has arisen, and for a 5m daysailer/cruiser, this is what I came up with...

Length 5m
Beam 1.5m
Loaded Displacement 600Kg
Unloaded weight 220Kg
Keel bulb - 100Kg

The use of a keel adds the necessary safety factor for cruising, which a centre-board may not, but it can always be redrawn (in fact I would redraw it before I started) but just so you have some idea, here is a rendering from the old drawings.

Cheers,

Tim Brocklehurst

www.marinedesign.tk

Interesting design Tim! I like the keel idea as well : I'm slowly putting the ideas for my boat together.At this point she'll be 18'LOA(5.48m),3.5'(1.06m) Beam with a 55 degree retractable, lockable canting keel using a retractable "wing"(kFOIL) on the bulb for the extra lateral resistance.
Still working on further details except to say the canting keel will probably be manually controlled or at least backed up manually. She'll have a high power to weight ratio and an asy spin.....

[Tim B]

Keep it simple, Lorsail. 18ft sounds very small to do the Canting keel reliably. I guess you could use a block and tackle system of some kind, but check out the loads that are present just from moving the weight... and triple it for safety. I reckon that it would be difficult to do it the hydraulic way on anything less than 30 foot. That said, rope and a winch would work. it just wouldn't be as compact.

Anyway, I'd be interested to see what you come up with.

Cheers,

Tim B.

Thanks, Tim. An 18 seems perfect to me to take advantage of the power of a canting keel. I have an engineer working with me on it and we think it can work exceptionally well. We're looking at electro-mechanical ways to move the keel with manual backup and a locking method.Also the keel must retract 100% except for the bulb/kFOIL combo.
So there are interesting design challenges -I wish I could work on it full time!

[spoedvraat]

Originally Posted by Tim B

Interesting thread guys, and an interesting project

You'll be pleased to know that this is not the first time this question has arisen, and for a 5m daysailer/cruiser, this is what I came up with...

Length 5m
Beam 1.5m
Loaded Displacement 600Kg
Unloaded weight 220Kg
Keel bulb - 100Kg

The use of a keel adds the necessary safety factor for cruising, which a centre-board may not, but it can always be redrawn (in fact I would redraw it before I started) but just so you have some idea, here is a rendering from the old drawings.

Cheers,

Tim Brocklehurst

www.marinedesign.tk

Hi Tim,

Nice looking boat.

What was the sail areas, and how did she go ?? At 1.5m beam she seems slender by today's skiffy standards, but with the ballast below it makes sense.
I am a bit worried that I'm going overboard ;-) by taking my boat to 5 m length, beam of 1.9 m and the addition of a bulb of approx 80 kg. It looks great on the stability curve though.......

The questions do arise thick and fast however: Is there a standard for how high the boom must be above the cockpit floor, so the average guy can get under it when tacking. Too high is obviously bad.

And on the subject of asking silly questions, how does one really determine the shape of a sail mold during sail design. The curve in the luff adds draft when the luff is pulled straight, but how much and how is this determined.
Or are we left at the mercy of the big sailmaking guys with the expensive software?
Is there anything written somewhere about sail design, especially asymmetric spinnakers? I'd be willing to pay good money to get my hands on material like this, but I can't seem to find it.

block and tackle canting keels
chris sayers 'Navman' first minitransat had just this system controling the canting keel. that said crossing ceans doesnt really require repeated rapid movements. structually it shoulnt be too dificult.

how about loading it onto a reversible sheet winch with a continious loop of rope holding it. that way there is no tail and spaghetti problems. and the keel is held rigid on both sides so that it can not swing further to windward if things get airborner.

really if you are just after a cruising dinght grab and old FD,505 or light weight sharpie and reduce the rig by 30%. give it a little keet if you wish then roberts your fathers brother.

[Tim B]

The suggestion I posted was just a very quick model of the hull (which I am not particularly happy with). It has no sail plan as yet, and I'm not entirely sure of the stability curve (because I'm not happy with... etc.) basically, it is just a quick mock up which I produced the last time someone asked a similar question, and I haven't had much time to do anything to it. If you are interested in using a similar hull then I may be able to have a more serious look at it in a few weeks, when I have less work on.

The canting keel: Yes, the reversible sheet winch would work well, and could, potentially allow a greater angular movement. the attachment to the hull or bulkhead should be damn strong though.

Booms: Somebody deep in a lab doing ergonomics has probably thought of this one. certainly too large a gap reduces the sail's loading at the root, thus losing power, but then, losing the helmsman doesn't help either. some boats have high booms, some low, I would have thought that between 3' (for a racing boat) and 4' (for a cruiser) would be about right.

Sails: Not my best area of expertise really, there is another designer, Steve Baker, alias SailDesign, who could probably help you.

Good luck,

Tim B.

[SailDesign]

Sails: Not my best area of expertise really, there is another designer, Steve Baker, alias SailDesign, who could probably help you.

Sadly, I always defer to a sailmaker. When owners discuss sails with me, it's usually the "Send the sailplan to Shore" variety of conversation, and I like that. One less thing this jack-of-all-trades has to remember.

STeve

[spoedvraat]

I suppose it's back to Larsson and Eliasson then, to see what I can glean from those learned pages. There must be someone somewhere who has done the math and will give me the formula........

For some up to date info on molded sails and asy spins check out the book with the title above by Brian Hancock published by Nomad Press ISBN number 0-9722026-0-9 The book can be ordered thru The Independent Publishers Group 814 N. Franklin St..Chicago,Il. 60610
or call Nomad Press at: 802-649-1995 or ,of course(probably) Amazon..
For boom height on your own boat I would go with the lowest you personally feel comfortable ducking under- you can set it up any number of ways to test before deciding.

[Stephen Ditmore]

What if instead of a canting keel you had an (upside down) T shaped keel where the endplate could be rotated. At one end of the endplate could be a ballast disk - round in planform, perhaps ogival in section. The whole thing could be raised and lowered vertically. The ballast could be moved using a lever at the top to rotate it to the windward side via a shaft inside the main foil.

Stephen, interesting. I'm not sure the ballast could be moved as far to windward as a 55 degree canting keel. And I think the wetted surface would be less using the kFOIL™(retractable bulb mounted wing). This system would be completely retractable.
I think I get your idea but could you post a hand drawn sketch?