Low-speed sailboat hull

For those who don't know, my Duo dinghy is a 10ft 2 sheet ply hard chine dinghy. So similar to what laukejas is drawing. One difference the Duo has is that the CL chine is only 1.2m (4ft) long and is at the aft end. That means shorter glasstape joints and a far simpler build; less setting up and a easy daggerboard box installation

Anyway, it is a small boat, so not particularly stable. So I decided to put the buoyancy on the outside, which frees up space inside, is more efficient for stability, and acts as a huge fender when coming alongside.

They are out of the water when rowing and should be when sailing. A "sort of" RIB if you like.The videos and photos show basic cheap beach rollers (about Euro 12 each) but I now have "Dinghy Dogs" which look better, but of course were far more expensive and cannot be used as beach rollers.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

The lowest part of the keel does not seem to correspond with the lowest part of the side panel? Maybe this is an optical illusion?

If you don't like the look I would try moving the chine and keel line around a bit to improve looks, you don't have to have constant deadrise.

 

the boat that is drawing laukejas just swerves 5° increases the wet surface, and resistance to friction and residual.
is the opposite of how it should be a boat for light winds.
The section of my design allows to obtain the same wet surface around at all angles of heel. Also considering the whole section as radius chine can built it round with ease, so you can even enlarge the beam.

 

there's one thing I can not understand: why many Americans when they draw a small boat have the mindset of the third world?
Boats flat, square, to be built with a hammer and nails, as if were ten years old even when they are forty?
It almost seems to have the religion of the ugly, poorly functioning.
They have also famous architects specialize in junk.
Draw a small boat nice and efficient, and build it with skill, instead of a bath box, should not this be the first pleasure of the adventure?

 

Stick and glue.
Stick and glue is the modern version of the hammer and nails. It is not true that it is cheap and fast.
First needs to build the jigs to lose, then use expensive materials such as strips and epoxy glue, then the boat needs the same operation inside and the realization of all the reinforcement structures that have not been done before.
It is less expensive to make a hull in a semicircle, starting from the internal structures, and using the normal red glue. The material is the same, or rather, to be more precise a bit 'less.
Maybe it takes a little 'more time, but for a small boat the difference is insignificant.
In return, this will be a real boat.

 

Since you plan on sailing in areas where the wind is usually less than 10 kts, I think you will be sitting in the boat more than on the side decks.

You will probably find that, with two people aboard, one will face forward and the other (the passenger) will face aft.

One or both will have their legs under the bench.

As the wind builds, they will slide Their butts toward the windward side.

If the wind gets stronger, they will move to sitting on the side decks.

So you really may not need as much Beam as you think you do.

 

I have just come in from sailing my Duo in the Sea of Cortez for the first time. There was about 8-10 knots of wind, its been 25 knots onshore until, now making it challenging to get off the beach.

But while I was out I was trying to visulise two people sailing it with a bigger rig. I think both will be pretty active balancing the boat, not sure how the crew would turn round when you tack.

My analogy would be that it is like riding a bike with someone sitting on the handlebars. The faster you go the easier it is.

Upshot is I think it would be difficult for two adults to lazily sail a small 10ft dinghy unless it was heavy, wide and stable

Richard Woods

 

Richard, some time ago, you mentioned that you plan to work on a slightly larger version of Duo. Maybe you have started already?

True. I actually tried re-designing my boat in a way like yours - 5 panels instead of 4. It didn't work out well. It had 15% lower wetted surface area, but a lot less interior space, no way to sit on the bottom of the boat. Also, had to make it very beamy or with lots of rocker to have sufficient buoyancy - my boat is a lot smaller than yours. Also it would be somewhat more difficult to build. There were also problems with vertical bow. I can show it if you like.
So, I agree that purely from low-wind sailing viewpoint your design is much better. But it complicates other requirements too much.

I am not sure why are you so against stitch-and-glue. It is popular for a reason, no? Lots of experienced boatbuilders have used this method to produce fine boats with relatively less work and less expense. For example, there is this boat, Nutshell Pram. I found a tutorial video made by designers who show how to build it traditional way, on a strongback. Even though this boat is just 2 or 2.5m long (I don't remember), the bill is in thousands of dollars. I have seen boats made with stitch and glue that cost just a fraction of that to build, require no advanced machinery (like table saws), and in the end, perform and last nearly as fine as this Nutshell Pram.

You know, I actually redesigned my boat. Made a backup of my current design, and tried another approach you suggested earlier - instead of side tanks, go with bow and stern tanks, with double gunwales for structural support. It results in far lighter boat, requires less materials, and in the end, is still within original requirements.


The n-th re-design

Compared to previous design, the side tanks were removed and replaced with bow tank (~42 liters) and stern tank (~45 liters), which acts as a seat too. To compensate for structural integrity loss, I added a double gunwale (inwale and outwale with spacer blocks) along the sheer line. Getting rid of these tanks will give also give more space inside for crew, who can move their weight to the side more, which increases righting moment. The downside is that sitting on gunwales might be not as comfortable as sitting on deck, but by the idea, I won't be sailing in high winds very often, and there is always an option to reef.
Because this resulted in lighter boat, I could increase LOA to 3.3m.

Principal dimensions:

LWL: 3.3m
Beam: 1.28m
Displacement: 210kg
Draft: 16.3cm
Prismatic coefficient: .52
Wetted surface area: 2.73m^2
Longitudinal center of buoyancy: midships
Maximum heel angle: 24.5°
Freeboard: 25.6cm
Dry hull weight: 24.5kg
Finished hull weight: ~29.5kg
Sailing weight: ~42-45kg

Sail area: 7.31m^2
CE height: 2.4m
Daggerboard area: .23m^2 (3.2% of sail area)
Rudder area: .12m^2 (50% of sail area)
Lead: 0


Righting moment:

Single handed:
0° heel: 255nm
24.5° heel: 383nm

With both crew:
0° heel: 405nm
24.5° heel: 648nm

Equilibrium windspeed:

Single handed:
0° heel: 4.84m/s (9.4kt)
24.5° heel: 6.22m/s (12.09kt)

With both crew:
0° heel: 6.09m/s (11.83kt)
24.5° heel: 8.09m/s (15.72kt)


Linespan:



Screenshots:












Obviously, the oar fittings on the hull are missing, I'll work on it. As for gunwales, I did some research, and right now, I've decided to go with 3cm wide, 1cm thick inwale and outwale, with 1cm slot in between. The spacers are 1x4cm, spaced 15cm apart (tell me if I need more of them). All of this will be made out of spruce or pine, because no other wood is available here.

In these screenshots, crew is in positions for zero trim, but as you can see, there is quite a lot of space for both of them to move around if needed, and enough space for stern crew member to trim the boat right if he's sailing single-handed.

Please let me know what you think about this latest update. Personally, I think it is a better solution than my previous one - lighter, yet longer boat, more space inside, easier to build, better hullshape (less wetted area, high deadrise), narrower beam yet more righting moment, smaller daggerboard, also more comfortable when rowing.

Looking forwards to any comments. And again, I really appreciate you all helping me out. This thread has become big, I hope it doesn't get dull. I'm waiting for the moment when everybody says "it's finally right, go build it", and then I can give something in return.

 

You still have to work out how the crew get under the boom. The position you have the crew is for winds over 15 knots, maybe more. In less wind they will be sitting on opposite sides to balance the boat. And what about off wind?

I still think you have drawn a singlehander, not a 2 person boat

How do you plan to get rid of water that comes on board?

I am drawing two new 14ft hard chine dinghies. One a 2man cruising dinghy called Spectrum, the other is a singlehanded racing boat called Zest which should be easier to build than my Stealth design yet hopefully just as fast

Richard Woods

 

Well, since crew are already as low on the boat as possible, I don't see a lot of solutions. One may be raising the boom, but that will diminish sail area, unless I make even taller mast, which currently is 4m, and I'd really like not to go over that length. You're right that in low winds, one crew member will have to sit on leeward side, and he'll get in the way of the boom.

What solution would you propose?

I am not sure why it seems so to you. After all, your Duo is also a two person boat (at least according to this description), yet it is smaller by both length and beam. Where do your crew fit when sailing together, and how did you solve boom problem? I haven't been able to locate any video or photo of Duo with two crew members.

Duo aside, how would you have me make this boat more suited for 2 persons? I can't make it any larger without increasing length (which goes over weight limit) or beam (which is not good for light wind sailing). I can move center seat further back to make more room for bow crew, but that would require re-designing the sail too.

What would you advise, considering these limitations?

If you mean spray, then it should collect midships, since that is the lowest part of the boat. If you mean capsize... Well, a bucket then. With current gunwale design, I could actually add inflatables like in your Duo, although I hope they won't be mandatory - boat has sufficient buoyancy in it's tanks. Or so I think.

 

The Duo is a sail/row boat and was primarily designed as a yacht tender that would be fun to sail

So yes it takes 2 when rowing, maybe 3 when motoring, but I only planned it to be a one person boat when sailing - unless the second was a small child

I need someone to take photos while Jetti and I row/motor it but that won't happen until next spring as I don't have the oars or engine with me right now

Richard Woods

 

I see... Well, anyways, what would you suggest I should do to make my boat more suitable for two persons? I can't make it any bigger without going over weight limit. And what about the boom problem you mentioned when one crew member is on the leeward side? Any advice?

 

You're planning on using 1x4 cm gunwales and inwales with 1 cm spacers? For the same weight you could use 2x2 cm and have stacks more strength and stiffness, and a wider surface to sit on.

Duck. Seriously. You don't have to sit bolt upright all the time. You can slouch and lounge and whatever. You can even sit on the leeward side deck while the other person sits out to windward. All of these are common tricks in dinghies.

Also, what about when you need to heel the boat slightly to leeward in light airs while sailing solo? This is common. You'll run into boom problems there too, and figure out ways around it. Just make sure the boom is high enough that you can easily duck under it.

Oh, and you stern tank would makes more sense IMO if it was just done like the bow tank. Simpler and stronger.

 

1x3 actually, but okay, I can make them 2x2, that makes sense. Space between should also be 2cm, or 1cm?

Jeez, guys, you're not making this easy for me Richard says this is a real problem, that I should do something about it or it will be 1 person boat, while you say it's common thing, not to worry about. Could you comment on what other said? With very little experience, I find myself in a position of confused intermediary...

What exactly do you mean? Simple seat/box instead of U shape?

P.S. From the screenshots, does it look like the boom is high enough to duck under?

 

Again, full size mockup! The problem I see is not in sitting. Just in tacking. The forward crew member will have to scrunch up to fit under the boom and then somehow turn around in the small space. It will be possible, but may not be very nice!