Low-speed sailboat hull

With my pdracer design, both leeboards are supposed to be down at the same time, so the shown area is actually doubled. This is for two reasons. One, to reduce sailing draft. Two, to make the strain on the axles less.

The only thing wrong with enclosed spaces is long term lack of ventilation. Otherwise, you are correct, they do serve as buoyancy, as long as they stay intact.

I usually don't draw them in my designs, as I am not confident they will stay intact (Air tight).

A pdracer once sank with great embarrassment for its builder, when all three air of its boxes failed. The boat was originally designed to be built with epoxy taped seams and epoxy coated plywood. This one was built using a different kind of glue and just paint. The dagger board hit the bottom, while sailing at speed, rupturing not only its case, but the three air boxes as well.

I use construction foam in my designs, or full height bulkheads, that are at each end, and open at the top, with a hatch.

 

Please don't take me wrong. I highly respect your Duo design, and admire how cleverly you managed to make such low weight sailing vessel from minimum amount of materials. This dinghy is one of the most elegant solutions I've ever seen it it's niche.

It's just that seeing videos of Duo sailing, it's hard to me to imagine how two people could comfortably fit for whole day cruise in sailing mode. My budget and weight considerations allow a slightly larger and heavier boat, that's why I'm trying to make a design inspired by yours. I hope that's okay with you, and if not, please do tell me. If I go with this design and somehow it proves to be lake-worthy, I will post plans on the internet for free.

You're right that my design looks different from Duo. But not too different, I believe. I made following changes:

Increased LOA from 3m to 3.75m (+25%);
Increased beam from 1m to 1.25m (+25%);
Substituted single piece half sliced bottom plywood panel with 2 separate panels;
Changed transom from V-shaped to flat one (maybe I shouldn't have, I don't know);
Transformed rear flotation tanks into seats by lowering them, and extending up to center seat (for more space to sit, as well as adding structural integrity);
Changed bow cap from 2 piece, V shaped to one piece, flat one (maybe shouldn't have done that too, still considering).

As for weight, I am not sure of the final figure. I'm using this Solidworks software, which is very good at calculating exact weight, but you have to know the densities of the materials. Since I'll be building from MDO plywood, and I have found on the internet that it weights approximately 450kg/m^3, as of now, the software tells me that my boat will weight 23.3kg (including timber). +3kg of epoxy, +1kg of hardware, +3kg of paint. It seems that it should come close to 30kg in total. If it exceeds, I'll scale down boat a little bit.

Naturally, my expenses will be far more than USD150. But maybe it can be worth it.

Richard, do you feel my design is flawed? If so, please tell me. I have all this software and everything, yet I have little experience, and I know it is far more valuable thing to have.

Thank you for the link to these tubes. Maybe I should invest in them.

Also, I'd be very interested to hear more about your 4m dinghy you mentioned. Do you have a figure on weight?

Sharpii2, I'm sorry to hear about your accident with pdracer. But, well, you said it yourself - it was built with not-as-good glue and paint, and it hit the bottom...
So, do you have more sketches of this boat?

BTW, this design of mine has 5 buoyancy tanks in total, I can actually make 7 by separating two of them (the ones in the side seats).

In my previous post, I provided more information and screenshots of my design. Could you please comment on it? Do you see any flaws I should take care of? I'd appreciate your input

 

I think the flat transom bottom is a mistake.

19th century American "Flatties" descended from "Flat Iron Skiffs" of that period. Flat Iron Skiffs were similar to sharpies, except that they were usually wider, in proportion to Length. They didn't row quite as well on account of their wider Beam. Someone discovered that, by putting a slight "V" in the bottom, which started at the widest section, and got steeper, as it reached the transom, the rowing qualities could be improved. The light wind sailing qualities also improved.

What you propose to do is the opposite.

Have the "V" start at the bow, then taper to flat at the transom.

It would probably be better to run the "V" from bow to stern. It will gain you some extra displacement at the expense of a slightly deeper hull draft and a very minimal increase in Hull Surface Area. For your original requirements, this should work quite well. Doing this will also make the boat easier to build than what you have in mind.

Another weakness I see in the design is the dagger board trunk. It is housed inside an air box, and, IMHO, is quite vulnerable to a bottom strike. Isolating it from the other air boxes would be the least I'd do.
To do this, just eliminated the center air box and simply build two benches, with the trunk in between.

I'd even consider moving it to one side and turning it into a Center Board, with the side of one air box serving as half the trunk. This way, a bottom strike will merely push the board back into its trunk, like the blade of a jackknife.

 

Thank you for your comments, Sharpii2. I have adjusted the hull shape - added a V that starts at transom, and flattens towards bow. Also increased displacement to 210kg, just in case. Here is the linespan:

http://static.dyp.im/jDOjwBYVtg/05ca19cd3e8cc8c85c0d1ef8ccb3a84a.jpg

And some screenshots:

http://static.dyp.im/aOZj7nTXUK/7c7fb411a92b0985661fc457a266d442.JPG

http://static.dyp.im/DnKjEzEBRa/65e851bd71d16f5f0aba334b4b651237.JPG

http://static.dyp.im/9dKH1ZwIHx/f46154716c9d4b6918bb21ac4d12f5cf.JPG

http://static.dyp.im/DUaRhQbFtM/089ee38b59d2e7b1f58b61494e34b35f.JPG

Now, I haven't made changes to the daggerboard case yet. I am not sure if I explained correctly how buoyancy tanks and case are designed here. So, here is a screenshot with center seat removed, so you can see what's inside:

http://static.dyp.im/zw32FLL7bg/b93fb5bd463972618e84ea54419ed5d8.JPG

As you can see, it is isolated from all other tanks, however, I am not sure if that's what you meant.

Now, I have strongly considered centerboard instead of daggerboard, but since low weight, space in cockpit and simplicity of the build is on top of priority list, I have to dismiss this idea.

You sea, our bathymetry is very different from yours. The lakes, where majority of sailing is taking place, are usually 2-15 meters deep just a few meters away from shore, and shoals of anything but packs of weed and clay are unheard of. Even at beaches, you'd have to work hard to find a shore which could damage your hull. The worst you could encounter is soft gravel mixed with mud. I have never heard of a sailboat damaged by grounding in our country. Usually it's capsize from unpredictable winds Had 2 cruising yachts capsize in the past few weeks.

What I mean to say, is that unpredicted grounding is very unlikely with our deep and soft-floor lakes. So I guess that is not reason enough to add weight and complexity to the boat...

Nevertheless, I can add some bracing to the daggerboard case, if you think it's necessary. I thought that making fiberglass filets would be enough.

 

P.S. I took liberty to model daggerboard, rudder, and rigging as well.

Screenshots:

http://static.dyp.im/OC1R86Fb8v/c9fc0002c8561f94e5ebf823739a70ab.JPG

http://static.dyp.im/C2LZk7fTGc/14c4630107545df3c2f6a68db744cca2.JPG

Principal data:

Sail area: 7.9m^2
Daggerboard area: .3m^2 (3.8% of sail area)
Rudder area: .14m^2 (48% of daggerboard area)
Lead: 5%

I know sail is rather large for this boat, and most would suggest not to go beyond 6m^2, but as I mentioned, I will often have to deal with no-wind scenario, and having an large and effective sail will help to catch the most of it. For any other conditions, I can reef to more conventional sail area, like 5-6m^2.
Daggerboard is two 8mm plywood sheets epoxied together with usual leading edge rounding and trailing edge tapering. Rudder is one sheet of 8mm. I know you'd say that plywood is not the best material for boards because of the cross-grain. I am still considering a number of planks epoxied together at 17° angle, but that would add some complications and cost.

I have a question about lugsail. Even though I had some experience with balanced lug this summer, I still don't grasp the essence of balancing the yard. With some experimenting, it's possible to find the sweet spot for attaching the halyard, as well as mast height to provide proper angle, but how do I do it on paper? I can't exceed mast height, and I want to use as much of it as possible to have a really large sail. What are ways to calculate the balance of the yard?

P.S. I estimated that rigged boat weight would be close to 40kg. Hull itself won't exceed 30kg.

 

Hi Laukegas.

I like your new hull lines. Big improvement. A further improvement might be to raise the fore foot out of the water. This can be done by slightly increasing the rocker.

This will accomplish two things:

1.) It will make the boat slightly easier to row, as the bow will be making less of a bow wave, and the hull will have slightly less Whetted Surface Area (WSA)
2.) It will decrease the prismatic coefficient, making the hull better for light winds.

Your dagger board arrangement is probably sufficient, as the loads will be spread by the two bulkheads. I would just increase the radius of the taped joints there and along the bottom of the trunk and the bottom of the bulkheads. and use more fiberglass tape. Maybe two to three times as much as I would with other hull joints. Next, I would put hatches on either side of the trunk, on top.

This is so the area can easily be inspected. That air box would also make a handy place to stow loose gear, such as life jackets, and maybe even picnic lunches.

I see no problem with the increased Sail Area. The largest sail used on a pdracer was around 10 sm. Sails the size you're thinking of are routinely used in the pdracer fleet. Given the expectation of chronic light wind conditions, the choice is well justified.

I think the mast is too far forward of the dagger board to use a standing lug rig. IMHO, the yard does not go far enough past the mast for the sail to stand well. There will be a huge load on the luff of the sail, which will cause it to stretch excessively. When that happens, the leech will sag, and the sail will go out of shape.

Moving the mast further aft, by making the forward air box longer might be one solution. Moving the dagger board box further forward, might be another.

A third choice might be to change the rig into a gaff one. This can be done by simply turning the yard into a gaff, by adding jaws to its forward end. Then you will need a peak halyard as well as a throat one, but the shape of the sail will be much easier to control.

A fourth choice might be to have a check line, that has one end attached to the lower end of the mast and the other end attached to the front of the yard. This line, not the luff of the sail, will then take the load of holding the aft portion of the yard up.

If it were my boat, I would want the yard to have at least one fourth its length in front of the mast.

The halyard attachment point, on the yard, might need some experimenting to get right. It will probably be best to design it so it can be adjusted (moved forward or aft). Typically, it's attached at about 40% of the length of the yard aft.

BTW-having the yard pitched 45%, gives you the maximum amount of SA, per yard length, and hoist distance.

 

sharpii2, thank you for such quick reply and a ton of ideas. You are helping me a lot

I can easily make the changes you suggested about the hull, but are you sure about it? I thought that partially immersed bow is desired, that it's better if it acts like a wedge to slice the waterflow (and waves), instead of passing it underneath. As for prismatic coefficient, with the hull I've shown in previous post, it is 53.6 - I'd say pretty low.

So, you're sure that it's worth keeping bow above waterline? I'm kind of afraid that doing so will create this bow wave, which is perpendicular to the direction of movement. Like prams often do at higher speeds.

Then again, I am not experienced in hydrodynamics. It's just that I thought that immersed bow helps creating proper waterflow entry, like I said, as a wedge.

Correct me where I'm wrong, please!



As for the lug, I've corrected it - extended yard 1/4 of it's length forward of the mast, and estimated an attachment point for halyard by 40% you suggested. Well... It seems the mast is too short, then:

http://static.dyp.im/qkXvc5tdBe/ced5c7727b596e01fbf22b28b70dbd22.JPG

(zoom in, there are blue points for CoE, CLR, and for halyard attachment point).

If my gut feeling is right, then the sail can't be raised up like that by the laws of physics. The mast is as 3.75m long now. I'd really like not to exceed the boat length (which is also 3.75m). So, I guess my options are to either lower the sail (which will traumatize area), extend boom (which will give low aspect ratio), or to go with gaff.

I'll go with the gaff if I must, but I'd really prefer lug. It seems much more simple (no throats needed, no lashing to the mast which may jam, one less halyard).

Do you see any ways to keep this sail area without extending the mast?

As for the helm balance, I can easily move mast position and/or daggerboard, but as it is right now, CoE and CLR are pretty much on the vertical line (few centimeters within each other).

I know I ask a lot of questions. But if I will build this boat, I will gladly credit you as a major adviser!

 

You sure it's 40% aft of the yard? I googled some, and it looks like 40% from front end (halyard closer to the front end of the yard). And it seems a bit more logical - halyard raises the yard until the luff tensions, and then angles until leech is tensioned too.

I didn't see your advice about 45% angle of the yard. Re-did my sail.

Looks like this now:

http://static.dyp.im/F8DHymFpc7/8cc74359c93b96ed8bedeb01b5fb2189.JPG

Take a look at halyard attachment points. 1 is 40% from the fore end of the yard, and 2 is 40% from aft end of the yard. 1 looks more logical to me. Is that what you meant, or did I misunderstand?

BTW, with this edition of the sail, the area is 7.9m^2. I dare not raising it a bit more, for there may be insufficient room and angle for halyard from the top of the mast. How about that one? Is the halyard angle okay (if we assume it runs from the top of the mast to the 1st attachment point)?

 

I like your final sail plan drawing. I actually did mean the attachment point was to be 40% of the yard length aft the front end of the yard.

A way to increase the SA without lengthening the mast is to lengthen the boom. The boom is generally of lighter section than the mast, and a longer boom raises the Center of Gravity far less than a longer mast.
Also, the tall triangle of sail rising from the aft end of the lengthened boom, to the aft end of the yard, will counteract the bigger triangle (than in your original drawing) of SA in front of the mast, caused by having more of the boom in front of the mast.

The American sharpies, flat iron skiffs, and flatties all tended to have their stem heels clear of the waterline.

These boats were in the oyster trade and work year round, on the east coast of the USA. Summer winds tended to be fluky and light. These boats often had to be rowed great distances in near flat calm water.

Phil Bolger, a modern sharpie designer, believed that the upward curve of the bottom, at the bow, should come close to matching the curve of the side, at the bow. Having the Stem heel out of the water doesn't accomplish this, but it comes closer to doing so than having the stem heel immersed.

The sharpie of the 19th century were designed by trial an error and often sailed against one another on Sunday races. The stem heel above the waterline rule probably came from experience.

 

Thanks, sharpii2. Well, you see, I could lengthen the boom, but the aspect ratio of the sail is low enough already. Not very good for reaching, especially in low winds. And I have heard many people say that lug rig isn't the best for reaching as well. So I am not sure if it's worth adding more sail which won't contribute to reaching ability.

In my case, I want a weatherly rig which doesn't require a long mast. But since weatherliness depends heavily on aspect ratio, I'm starting to consider different rigs. Initially, I didn't give your suggestion about gaff rig a lot of consideration. But the more I think about it, the better it seems.

So, just to have an alternative, I sketched gaff sail plan, highly peaked (10°), although not a Gunter yet, because I'd rig I wouldn't hoop-lash the yard, but raise it in gaff-like way.

http://static.dyp.im/Xroq2MKusi/98ccd9078dcb7ebf0d2295e3261d40e2.JPG

The problem I can't solve is the jaws. I sketched two options, one for boom, influenced by traditional jaws, and one for gaff, influenced by this non-traditional solution.

By the way, how to they make jaws like this? Is this steam-bent or something?

What do you think of this sail plan? It is 8.85m^2 (compared to 8.4m^2 for lug), and has much higher aspect ratio. I added roach in the leech to further enhance sail area, it may be supported by battens.

I'll start working on the hull to raise stem heel out of the water. In the meantime, could you comment on gaff rig option?

P.S. Maybe it's possible to build a pivoting jaw like this one, but using timber and plywood instead (I don't have access to such fancy hardware)?

 

The curved ply jaws would have been made by making one plane bend ply first not steaming. In fact you can get a bit of 2D curvature if you are careful, and of course if you needed it.

Thin enough veneer or stock will bend pretty well. Remember that Ash is a good timber for shock loads, hence a common tiller timber. It was also used for wooden full length battens, before f/glass. So a combination of spruce and Ash would be very strong and pretty light for a jaw.

 

Hi Laukegas.

I looked at your new gaff rig design.

I don't think it will work very well, because the little stub of mast, above the jaws, has very little leverage to hold the long gaff nearly vertical.

I think high Aspect Ratio (AR) sails have become a bit of an obsession with you.

It is true that they win races and they can sail up wind better than their low AR cousins. But this is only true if the high AR sail sets well. A low AR sail, with a good set will beat a high AR one with a bad set any day. It seems to me that a low AR sail tolerates a bad set better than a high AR one.

For sailing across the wind and sailing down wind, the low AR sail may actually be better. This is because it is less likely to stall, and it has a lower Center of Area (CA). The lower CA heels the boat less, per given amount of drive.

If you are sure you want a high AR sail, let me suggest you use a two piece mast.

A two piece mast has a scarf joint that is not glued or bolted. Instead, it has a tube, or pipe that that surrounds the joint.

The beauty of this is it makes your life a whole lot easier. Now you can forget about yards and gaffs, which need fantastic amounts of tension to stand well. Now you have a bottom mast piece, which the Boom attaches to, and a top piece, which only has the halyard. You simply attach the top piece to the bottom piece, before raising the mast, and you're done. You just raise the sail with a single halyard and go sailing.

I can see a two part mast that is 6.0 m, when joined together. This would give you 1.0 m between the butt of the mast and the boom, and still allow for a 5.0 m hoist.

To get the 8.0 sm SA, you would need a 3.2 m Boom. This would allow you to dispense with battens, which are a huge hassle on a small boat, especially when they are not parallel to the Boom.

The bottom piece of this mast would be about 3.5 m long, and the top about 2.75 m long, allowing a 0.25 m joint between them.

The tube could be made of fiberglass, using the mast itself as a mandrel.

To keep things simple, the tube would be permanently attached to the top piece. This way the two slide onto the larger diameter bottom piece as a single unit, as the mast will probably be tapered. Gravity and halyard tension will hold the joint together, from that point on.

The sail will have to be held to the mast with lacing or hoops, which can slide over the bulge created by the tube.

If it were my boat, I'd just go with a lower AR standing lug, balanced lug, or spritsail, and have a boat far less likely to tip over.

 

SukiSolo, thank you for explanation.

sharpii2, you told me a lot I didn't know here. I have considered two piece mast, but the complexities of making it, and fearing all the time if it would break, and the added complications of raising and lowering the sail through that joint has set me aback.

Anyways, if I were to go with tall mast, then the logical sailplan would be Bermuda. I heard it is very hard to set right without stays and shrouds or very stiff mast, and it is also a very tricky sail to make, for any imprecision in shape would ruin all the advantages it has over lower AR rigs.

Thank you for your detailed suggestion, but you convinced me to go back to lug! It seems far less complicated and much more lovely. I hope that with other features of the boat made right (generous board area, low displacement) it will beat to windward good enough. I'm thinking of saving up to buy a marine halyard block for the mast top. When I was sailing this summer, none of the blocks from hardware store worked good enough, as they tended to twist and get stuck. In the end, I used a simple carbine, but the friction never allowed me to tension the luff enough. So, a proper marine block, combined with low stretch line for halyard and downhaul should provide rig weatherly enough, right?



By the way, I fixed the hull by raising stem heel out of the water. Now, with 210kg displacement (which is the maximum I designed for), with boat trimmed just right, waterline will coincident with lowest point of bow and transom. Please take a look at the linespan:

http://static.dyp.im/4PyG3JeYOD/15574a937a783662e56c354debf1a9b9.jpg

However, I doubt I will sail at full displacement often. So, with lesser displacement, the bow will be even higher above waterline.



Nearly all problems are solved for this design. But there is one more thing I'm not sure about. It's lateral resistance. I red quite a lot about it, and yet I'm still at loss on how to calculate it. Some people say that I should take underwater shape of the hull and boards, and the centroid of that area will be center of lateral resistance. Some say hull shape is to be neglected, because it presents a slope to lateral waterflow, therefore, not contributing a lot to resistance. Some say that CLR is located at the centroid of board area. Some say it's on the leading edge, while others swear it is located on the axis of maximum camber. It gets better and better with the rudder: some say that it should be neglected at all, while others say that I should include rudder area as well, but take only 1/3 of it's area into calculations.

This is pretty confusing, if you ask me.

Now, my centerboard will be two 8mm plywood pieces glued together, with leading edge rounded and trailing edge tapered. The profile looks like this:

http://static.dyp.im/kWTw3Iv9mO/fd1c82dd5c3070587061cff42f02a402.jpg

Since this is not a NACA profile, then I guess I should just take the centroid of board area, and call it CLR. Is that correct?

But then look again at the linespan I provided. While the bottom panels present a lateral slope of only several degrees, the side panels area nearly angled vertically, that is, nearly 90° to the lateral waterflow. Correct me if I'm wrong, but I believe they would provide quite considerable lateral resistance.
So, should I include that area into calculations? If yes, with what ratio (since they are not exactly vertical)?

Also, what about rudder? My common sense tells me that if I want neutral helm, then rudder shouldn't be included when calculating CLR, but if I want weather helm, it should be included somewhat.

Could you please comment on this, please? This is, I believe, the last major issue I haven't yet decided on. And thank you for taking such an effort to help me once again

 

Have you considered weight placement? The centre of buoyancy looks quite far forward. You might find the transom will submerge with you sitting aft even a little of midship.

I think I would take your midship profile and move it back a bit, stretching everything with it. Put it near where you want to be sitting. If the rocker at the transom increases a little, well it is a low speed hull right?

 

Tdem, yes, I have considered it, and yes, you're right that I might need to move rocker back a somewhat. I just wanted to finish the boat in Solidworks so that I can see where is the center of gravity of the boat, and where the sailors will sit. Afterwards, I will modify the hull in Delftship again to make center of gravity and center of buoyancy coincident on vertical axis.

So yes, I'll probably move rocker around, but I need to know where the weight will be located.