Low-speed sailboat hull

Hi Laukegas.

You can move the maximum Beam aft and make the Transom wider. This will be quite an effective way to move the LCB aft. This is a major reason modern sailing dinghies look the way they do.

 

Yes, max beam can come aft.

Also, your helmsman needs to move forward. Tiller extension!

There is no rule saying you need to have a thwart along your daggerboard box. For example the thwart could run behind the box (see pic). There are other solutions as well (knees etc), look around.

On the boat in the attached pic, we would sail it in light winds with the crew against the sidestay, and the skipper right next to them. Or both sitting on the thwart going downwind. Thats pretty normal on a boat designed to plane.

Why have side benches? I would keep the interior simple.

 

Thanks. But there lies a problem. After a lot of modeling, I determined that having V-shaped transom AND LCB back aft are incompatible goals. To have LCB as far as 12% aft of the midships would require a hull that looks just like hulls designed to plane. But since mine is a displacement hull, I find it impossible to place so much buoyancy aft without submergining transom.

Do you know any dinghies with displacement hulls that are designed to have crew very far aft? Maybe I could compare and get some ideas... Because for now, I'm stuck.

Yes, but the thwart is required when rowing. Also, it provides structural integrity, giving longer lever arm to negate sideway forces on the daggerboard box. If I made upper support lower, I'd need stronger/more materials, all adding weight.

As for side benches, they serve following purposes:
1. Comfortable seating when hiking out without getting too wet;
2. Buoyancy tanks in case of capsize;
3. They add structural integrity, especially for twisting (remember, I use 4mm plywood for bottom and side panels);

In the screenshot you provided, it seems that this boat has thick hull, so it doesn't need buoyancy boxes or additional support. It also has a lot of space to sit on, yet it seems to me that crew would get a lot of spray, meaning wet pants. Okay when racing, but not so well when cruising...
And the center thwart - can't imagine sitting on it, it being so low. Definitely not a rowing boat, I think!

 

Take a look at some of the old American fishing skiffs. They often carried a considerable weight of nets aft, and had a deep run to get the LCB back where they wanted it. This limits maximum speed, but shouldn't be an issue for a light weather boat where wave drag will be negligible.

ETA: Good example here...

 

Seating Arrangement

Hi Laukegas.

I've given your seating arrangement a second look.

In the second arrangement, you have the person in front facing forward.

You could just as well have this person facing aft, with her/his feet aft the bench. This will actually move his/her CG forward.

Such would also allow face to face conversations.

Even more likely, the person in front will be sitting on the bottom instead of the bench.

The reasons for this are two fold.

1.) So his/her head will be better able to clear the Boom, or the foot of the sail, and
2.) to lower the CG of the boat.

With both people seated so high, the boat will have a very limited range of stability. Its righting moment could easily diminish before the capsize moment of the sail does.

By putting the second person on the bottom, or on a grate of some kind, just above the bottom, will lower the total CG considerably.

Having both people sitting on the bottom might be even better, on a 4 ft (1.25 m) wide boat.

I suggest you consider replacing the side air box/seats with bow and stern air boxes.

The bow air box can extend aft to the front of the dagger board case, and support it.

The much shorter stern air box can start about 1.8 m aft the aft end of the bow air box. You might have to add a web frame (see attachment) in between. A spreader bar, like on a canoe, can replace this web frame, if necessary.

Doing it this way will accomplish three things:

1.) open up the widest part of the boat for seating,
2.) allow much more flexibility in the seating arrangement, so the heel of the boat can be more precisely controlled. (On very light wind days, you might actually want to have the boat heeled to leeward), and
3.) greatly lower the CG of the boat and crew.

Some form of false bottom, or grating will, will be necessary to make such seating more comfortable. This false bottom, or grating, can be removable, for when the boat gets car topped.

I suggest you try sketching it out to see for yourself.

Each air box can have a useable hatch on top. (See second attachment.)

 

NoEyeDeer, that seems a heavy duty boat. However, I can't tell from sketches whenever it has LCB very far back. I'd think 5-7%, not more...

Sharpii2, I'm sorry, I have quite a hard time visualizing your ideas and your sketch.

When you suggested removing side seats, did you meant removing center seat also? Because it is what limits seating options the most, yet it must be there for rowing.

If people were seated on grates, do you think it would be comfortable enough for many hour passages? I believe it might end up in back pain. But I might be wrong here.

As for position of CG, I thought that having crew sit on side seats would allow them to hike out a bit, which would be impossible if they were seated on the bottom of the boat. Is having CG low more important than having it far out to the side?

Boom clearance makes sense. I can still remember my first hit.

Could you clarify this, please? I can't visualize it. The mast is between the box air box and daggerboard case. It would be extremely difficult to make watertight step and partner for mast inside an air box.

And what did you mean by webbing? Sorry, I looked at your sketches, but with little experience, they mean little to me. Can't visualize it in 3D.

I'll try modeling what you suggested, but if you could clear up these questions, it would save lots of time!

 

Seating Arrangment 2

First:

Forget what I said in my previous post about getting rid of the air-box/seats. It probably wouldn't work for your boat because the dagger board case is too far aft.

Second:

Look at the first picture of your "Second Arrangement" drawing. In it you have the forward person with his feet forward of the bench.

If you were to redraw that, with that person's feet aft the bench, that person would have his back to the mast.

Due to the width of the bench, his CG would actually move forward a few inches. This is the simplest fix.

Other than that, you can consider moving the dagger board case to the edge of one of the air-box/seats, and simply eliminate the fixed bench.

Now the dagger board case is out of the center of the hull. This will reduce the efficiency of the boat somewhat, but by such a small degree that it will be hardly noticeable.

Now you are free to extend the air-box/seats further forward, giving your bow person a comfortable place to sit.

The bench can be replaced with one or more set-in-place thwarts, that simply span the space between the air-box/seats.

Since these are not fixed, they can be slid fore or aft, to precisely adjust the rowing trim of your boat.

I don't have time right now to do any sketches.

I'll try to have them by Tuesday.

 

All right. It was getting weird in my drawings anyway

That's very logical. I shall redraw it soon.

I believe you mentioned something like this in this discussion some time ago. "Bildge boards", or something. I could actually make two of them, on each side. It would allow to ride two daggerboards, or have each shaped with specific, unsymmetrical camber to allow ultra performance on each tack.

Anyway, this is a very good idea. I have forgotten it, thank you for reminding. It would also save some weight from building separate daggerboard box and reinforcements for it. It would also free up lots of space in the cockpit, more freedom on crew positioning. This actually sounds like a great idea!

Are there any disadvantages to this system? Seems too good to be true.

Also, if thwart were to be removable, and removed while in sailing mode, doesn't the hull need any spreaders instead? Maybe I should design some plywood/timber ribs by the daggerboard to give the hull more rigidity?

You don't need to bother now! This time, all is perfectly clear

And thank you again for this great idea.

 

Spent the better half of the day re-designing. I have eliminated center seat, extended side seats, made them follow curvature of side panels, added plywood rib and moved centerboard box to the left side.

It looks somewhat awkward. Here are some screenshots:

http://static.dyp.im/LOESlId2Fo/13e04b3aaa09f489ca597af467b26806.JPG
http://static.dyp.im/Ie9TJ3reEE/502bfba8a2aa7b40dfc971989ef907ef.JPG
http://static.dyp.im/5OJYuhjqOs/cc3adfacf71fa8e9ba162f23c42fda87.JPG
http://static.dyp.im/kqsL7gLIJd/4f48055c02ee1b1c249a2bd570610e20.JPG
http://static.dyp.im/rOZ8X8U4F3/8c1b5d15d2524fb4a46744f96984124c.JPG
http://static.dyp.im/Z578Z45GHZ/ec50afbd4eb3bf9b86d36f63e76eec13.JPG

With this configuration, LCB is only 3.5% of LOA aft of midships. So I might need to modify the hullshape slightly, but it's obvious that with such arrangement the crew could easily balance the boat without sacrificing comfort and safety.

It's just that it looks very awkward. I have never seen such a thing. Must be a reason why boats aren't made like this. What am I missing?

 

That's looking very good. The layout seems logical and well proportioned. I can't see anything wrong with it, apart from one tiny detail.

While the offset daggerboard makes sense, it would be a very good idea to build the thing so that it was perfectly flush with the seat when fully down. This would enable the crew to sit anywhere comfortably.

 

Actually, I just had another thought. I think it would be best to raise the seats to the sheer, and just have continuous side decks all the way. This would make hiking out far more comfortable and effective, and would give more bouyancy, without complicating construction or noticeably increasing weight.*

This is basically how the International Finn is set up and it works very well. If you put drainage flaps in the transom, you'll be able to right it from a capsize and sail off immediately. This can be very handy if pushing it in gusty conditions.

*ETA: This would also increase the strength and stiffness of the structure.

 

Looks good! Actually this is used on some boats. A good example is SCAMP by John Welsford. He calls it an "off-centreboard".

 

NoEyeDeer, I've tried raising seats almost up to the sheer.

Here's a screenshot:

http://static.dyp.im/Rg6shoPJk9/55157bdf5ca4d56cd2ede07260fca831.JPG

I left some minimum distance up to the sheer to prevent slipping out of the boat.

Now, this increased weight somewhat, but what bothers me is that with seats this high, parts no longer fit on three scarf-joined 4mm and one 9mm plywood sheets:

http://static.dyp.im/yWEnr2e3jU/71b588a52d3dd36f63c615e097991876.JPG

I've tried everything, and I'm pretty sure there is no way to fit them.

Now I've been thinking. If I could extend seats up to the bow compartment, the boat would become more rigid, and I could save some weight. However, plywood sheet dimensions doesn't allow seats so long. The distance between bow compartment and end of the seats is 45cm. Boat LOA is 3.75m.

What if I shorten the LOA by 45cm, to 3.3m? Then I could space out parts on sheets more freely, have assurance that they will fit, save some weight on the boat, and have a stronger structure. I could increase the rocker to compensate.

What do you think about this idea?

P.S. tdem, I've looked up SCAMP. Very nice boat. Heavy, but amazing. Saw the capsize test. It looked like poor sailor just couldn't tip it over! I've searched a lot on off-centerboard, and there's little information. What change of hydrodynamics does it make? How does it affect performance? Is it very different on different tacks?

 

IMO, the last part is exactly what you shouldn't do. If sailing this boat in a breeze, especially solo, you will probably want to sit out to windward. This is far more comfortable if there are no ridges cutting into the backs of your thighs. Boats that cater to hiking never have the decks set down from the sheer. Boats that have decks set down from the sheer are not comfortable for hiking. Anyone who says otherwise does not understand the meaning of the word "comfortable".

Now admittedly this is based on the way I have always sailed small boats, which is that I always find myself wanting to sit on them rather than in them. Small boats have little stability anyway, and shifting 50% of the displacement an extra 0.5 metres to windward dramatically improves the sail carrying power. In light winds I just sit in the bottom of the boat, or if two people are aboard one can sit on the windward deck and the other on the leeward deck. I have to say though that lounging in the bottom of the boat with a suitable backrest can be very comfortable.

The other advantage of having the decks at the sheer is structural. You don't need a separate piece of timber to support the seat, since it just sits on the sheer clamp.

Ok, but your one sheet of 9mm appears to include the side decks/seats, and your 3 sheets of 4mm appear to include the bottom. 9mm is crazy heavy for seats/decks. 6mm would be perfectly adequate, even for bulkheads. It's also lighter and cheaper. With adequate support, you could even use 4mm for these, perhaps double-layered for the bulkhead at the mast and the transom. Maybe think about buying 2 sheets of 4mm instead of one sheet of 9mm, and seeing how you can work that.

I like that. Makes a lot of sense.

My 2c is that the current length is a nice length and 3.3 is getting a bit on the small side. I'd think about getting two sheets of 4mm instead of one sheet of 9mm, and playing around with the structural side of it on that basis.

 

Seeting Arrangment sketches

Hi Laukegas.

I drew this sketch before I got your message.

As You can see, my idea differs from yours a bit.

Instead of making the air-box/seats longer, I simply moved them forward to where the crew will actually be sitting.

To stiffen the last 60 cm or so of hull, I added an aft deck.

This could be very useful when launching the boat, down a steep embankment. The aft deck will keep the stern from filling with water.

This deck can be attached flat across the gunwales and follow the vertical sweep of them. It would also provide a nice dry place to stow things.

NoEye's idea of having the air-box/seat tops even with sheer top is an interesting one, deserving some thought.

I do have a few reservations, however:

1.) this boat was originally intended to sail in predominately light wind conditions, so hiking out should be a rare treat.

2.) the air-box/seats might end up having too much volume. This could make getting back into the boat, after a capsize, difficult.

This is because the boat will end up nearly empty of water, once it's righted. Pulling yourself over the side of a boat, that is lighter than you, will tend to capsize it on top of you. This is one of the reasons I favor bow and stern flotation, rather than the side version. This way, once the boat is righted, it will still have a good deal of water left in it. This water will lower the effective free board, making it much easier to board, over the side. If proportioned correctly, you can simply push the gunwale under water and swim back in.

Once back in, you have to bail it out, however.

For this reason, this sort of solution is not popular amongst the dinghy racing crowd. They have a very interesting but effective way of getting back aboard. They grab the gunwale, push themselves under water, then with a quick jerk, they launch themselves over the side, to flop into the cockpit, like a landed fish. This technique takes advantage of the boat's inertia. There was a youtube demonstration of this on the pdracer web site. Obviously it requires a certain amount athletic ability and physical prowess.

3.) the rowing bench will have to be lower than the air-box/seat tops. In my sketch, these tops are probably already too high. I have them about 10 cm below the gunwales. They night need to be at least 20 cm below.

If you choose to take NoEye's suggestion, you can have vertical faces of the air-box/seats straight, as I have sketched them, and have a timber track on them, below their tops, for the rowing seat to sit on.