Design refinement in small sailboats that aren't raced?

[misanthropicexplore]

By small boats I a sort of full displacement, human powered motorsailer, a 50/50 row sail boat, like a Solway Dory sailing canoe or one of Atkins small row/sail skiffs, with the following restrictions:

Total displacement at full load: < 500 lbs (227 kg)
LoA: 8 > 16' (2.5 > 5 m)
Max beam: 2.5 > 4' (0.8 > 1.25 m)
Average human powered: 75W continuous
SA/D: 10>15

Over a half hour race, 10 seconds can be the difference between placing in the winner's circle or not at all, even though it's 0.5% of the total time raced (1800 seconds). So, I understand the need for a high level of design refinement in racing sailboats. However, for camp cruising in lazy rivers and lakes, 0.5% is an extra 500' (150 m) at the end of 20 mile (32 km) day. I could just row for another 5 minutes and be fine.

For this sort of boat, in these sorts of waters, sailed by a sailor of low to average skill, and low to average athletic ability, what matters? What will make a difference that you can actually feel in the seat of your pants or at the end of the day, and what factors simply disappears into the "noise" of low speeds and low skill?

I'm planning on building Hannu's 12' skiff (A 12 ft skiff | Free Boat Plans https://hvartial.kapsi.fi/skiff12p/skif12p.htm). Does the skiff form instead of a round bilge form make any real difference here? It would be simple to use leeboards. Does the inefficiency of surface piercing foils matter at all here? Will it make any difference if the leeboards are carved into NACA 0010 airfoil, or will a slightly rounded plate be good enough? Does mast profile make any difference?

In *this* application, what's the effort to payoff ratio of all the little mods that matter so much in dinghy racing?

 

[DCockey]

The psychological effects can be much more important for some folks than observable, measurable quantitative effects.

What do you want to do with the boat? How to you important is the perception of performance? How important to you is how others perceive your boat?

Does the skiff form instead of a round bilge form make any real difference here?

A performance comparison would depend on the details of the boats being compared, who is in the boats, the conditions they are being sailed in, and the skills of who is sailing the boats.

How the boats are perceived depends on the observers, their knowledge, experiences and prejudices.

Will it make any difference if the leeboards are carved into NACA 0010 airfoil, or will a slightly rounded plate be good enough?

An experience sailor may notice the difference because the slightly rounded plate may be more prone to stall. Some sailors may be able to largely compensate for the differences.

Does the inefficiency of surface piercing foils matter at all here?

Perhaps you have answered your question:

Over a half hour race, 10 seconds can be the difference between placing in the winner's circle or not at all, even though it's 0.5% of the total time raced (1800 seconds).

Or are you asking for an estimate of how much slower the boat will be with leeboards rather than a centerboard or leeboard?

 

[messabout]

Good questions Misanthrope.

First of all, a 12-footer is not the best approach to sailboat or rowing races. If the rules of the game restricts you to a LOA of 12 feet, then we must address other issues. If not go for a 16 footer. It will be faster in almost every condition.

I am a fan of flat bottomed skiffs such as the one pictured. BUT the flattie is not the best of designs except in very brisk winds. In the case of strong winds, the little flat-bottomed boat can plane (if properly designed for such conditions) and leave the others in their wake. In moderate to light winds the flattie has the disadvantage of larger wetted surface and a tendency to make eddies at the chines which absorbs energy. Flatties also pound dramatically when sailed upright. So pound away if your only aim is to go fast. Aaaah well. Boat design for a particular application is a set of compromises that are to be aimed toward result within a particular application.

If you encounter strong winds, where the flattie will excel, then you will need some athletics ability and a good sense of tactical competence.
On the whole, a good Trapeze bottom will both plane in high winds and give little advantage to round chine types. Trapeze bottoms are much easier to build than full round chine types. They can easily be built in plywood, whereas the rounded bottoms will not be so easily done. Make the boat longer if your rules allow. Longer and narrower boats are better for low power inputs, such as when rowing or sailing in light airs.

 

[wet feet]

I think it would be short sighted to ignore the developments made in racing classes.The principles of getting through the water more efficiently are equally applicable to any boat but the purely recreational sailor might not wish to push them to extremes.Having a bit more performance on tap makes it possible to travel a bit further on each trip or maybe to make it back before the tide turns adverse.Not too many miles from me there are several tidal harbours where one must launch into the early flood and then head out for three or four hours maximum before sailing back against the ebb.In other words you venture out and return against the tide every time.Being able to squeeze a bit more performance out of the boat extends the time in open water.

I wouldn't recommend a NACA 0010 leeboard in any case as the best shape for leeboards is asymmetric in any case which still leaves lots to choose from.

It is well established that at low speeds skin friction is the biggest proportion of total hull drag so a good bottom finish has to be a high priority and will be an advantage at all times.

 

[misanthropicexplore]

Thank you all.

Sorry, if I was unclear. What I'm asking for is some sort of quantifiable amount to help me consider the issues.

The speeds here are very low 2.5 to 5 mph (1.1-2.2 m/s).
The power is very low: 75W continuous (1 man power)
The water is mirror flat with no current to occasional shoals with 5 mph (2.2 m/s) currents.
The sail, is for occasional use only when easy to do so, and is very small for the displacement: 40 ft2 (3.7 m2)
The rocker of the flat bottom is is a full displacement design. It would never plane at the sort of power available. I want a solid flat bottom (rather than the rounded hull of SoF canoe, which is something else I can build at my skill level) because rivers around her are turbid and full of pointy garbage

Hannu's skiff is 12' long. I want it to be as short as possible because I want it to be as small in all dimensions as possible, for both transportation, storage and weight (all other things being equal, smaller is lighter) The reason for not using something like PDR is because no matter how I sketch it out, I can't seem to get a comfortable sleeping space laid out in a boat 7'10" (2.39 m). It's important to me to have a comfortable sleeping space because this is going to be a camp cruiser for one.

The number I've chosen for being worth the effort is "measurably greater improvement than 10%". So if pointing ability is 55° with a flat plate leeboard, and 55.6° with a NACA 23012 leeboard, the hassle of making the proper airfoil simply isn't worth the payoff to me. Maybe a better way to ask would be "what's the low hanging fruit" of low effort to high payoff in a boat like this?

 

[wet feet]

The obvious answer is a good bottom finish and a much bigger sail area,40 ft^2 is miniscule and will need a stiff breeze to do any good.Leeboards that are asymmetric and vaguely foil shaped will be better than any flat plate with square corners.Longer helps with speed as the waterline length is advantageous but it seems you will have to decide where the optimal trade off is between all up weight and useful length.Good luck.

 

[Steve Clark]

We can say unequivocally :
Longer is better than shorter.
Lighter is better than heavier.
Smooth is better than rough.
You can tell the difference.
Some things are trade offs:
Narrower is better than wider but wider is more stable.

Crude works, but refined works better.
You build it once but use it many times, so it is probably worth the taking the trouble to do it nicely.

And just because apparently I can't say it too many times
No small boat needs more than one leeboard!
SHC

 

[sharpii2]

I think that one must weigh performance efficiency against operational efficiency. In racing, the two are nearly the same.

When one is no longer racing, the two can go off on different tacks.

For instance, the most efficient leeway preventor is a deep fin keel.
After that, comes a dagger board. After that comes a centerboard. And, finally, after that comes a leeboard.

The deep fin keel gets ruled out immediately, because it is not beachable. This leaves the last three options. Two of which require a case and a slot in the bottom of the hull. This slot need not be on the center line, but wherever you put it, it is going to take up space.

The center board can kick up if it hits an obstacle. The dagger board can't. But the center board needs a longer case and a much longer slot in the bottom.

The lee board eliminates the need for a case, but it probably needs a guard to hold it vertical when it's used. On this boat, you may be able to get away without one and tolerate the board not being vertical.

From the original post, I get the impression that this is to be primarily a rowboat. If this is the case, the lee board is probably your best option. The point is not whether or not it is the best anymore, but whether it is sufficient to do the job.

The lee board is the biggest PIA in light winds. In such conditions it may simply refuse to stay in place. But in stronger winds, it's far more likely to stay pinned to the hull and do its duty.

I once thought of a boat in which the leeboard doubles as a rowing thwart. The boat would have a slightly elevated platform under the thwart to sit on when sailing.
This eliminated the need to stow the lee board when rowing.

An airfoil cross section is best but probably not practical, because it would require the lee board to be much thicker. Thicker means heavier, and even worse, more buoyant. If it is more buoyant, it will be much more difficult to keep down. And ballasting it would make it much harder to deal with when it's not being used.

So, going with a more or less flat board is likely the most rational choice, under these circumstances.

Tapering or rounding the forward and aft edges is almost certainly well worth the trouble of doing. This will allow it to cause considerably less drag, and even more importantly, it will make it behave better.

The boat I am presently building will have such a board.

 

[gggGuest]

> An airfoil cross section is best but probably not practical, because it would require the lee board to be much thicker.
Might you be being a little simplistic? Isn't the thickness of any board, centre or lee, primarily governed by the strength requirement? And because of the critical importance of thickness in strength might an aerofoil section of the same total cross sectional area and thus weight be a fair bit stronger than a flat one? The shaped board can also be smaller for the same degree of effectiveness, which reduces weight again, and also should have less drag, which is more speed. Because you're accepting such a large efficiency penalty with a leeboard isn't it worth paying extra attention to the other details?

I idly wonder about a false floor like modern racing craft for a sleep aboard dinghy. Perhaps a foot well for comfortable sailing, and then put boards across the foot well for sleeping... Stream of consciousness here... With clever design maybe centreboard and rudder could double as the boards across the footwell. The centreboard case would then disappear into the false floor, and the floor could be completely flat for comfort sleeping.

 

[sharpii2]

>...
Might you be being a little simplistic? Isn't the thickness of any board, centre or lee, primarily governed by the strength requirement? And because of the critical importance of thickness in strength might an aerofoil section of the same total cross sectional area and thus weight be a fair bit stronger than a flat one? The shaped board can also be smaller for the same degree of effectiveness, which reduces weight again, and also should have less drag, which is more speed. Because you're accepting such a large efficiency penalty with a leeboard isn't it worth paying extra attention to the other details?

I idly wonder about a false floor like modern racing craft for a sleep aboard dinghy. Perhaps a foot well for comfortable sailing, and then put boards across the foot well for sleeping... Stream of consciousness here... With clever design maybe centreboard and rudder could double as the boards across the footwell. The centreboard case would then disappear into the false floor, and the floor could be completely flat for comfort sleeping.

1.) An airfoil cross section leeboard is likely half again as effective as a flat one, so you can reduce its area by about 1/3rd. But this by no means makes up for the much thicker section needed. The minimum section thickness, to avoid failing under bending loads, is much thinner than the thickness required for an effective airfoil.

Back in the days I was involved in the pdracer forums, Mike Storer had an aeronautical engineer design him a foil for his dagger board an leeboard. It was designed so it could have a flat section, to make it easier to make.

The dagger board was jammed down in its case with shock cord pinning it against the ends of the case. It had more than enough friction to keep it from popping up.

The rudder blade was held down with a similar system with the rudder stock design.

But this dagger board was bothe heavier and more buoyant than a few layers of flat plywood, despite its modest chord length.

I might add it was deep as well.

As a side-mounted dagger board, or even a fore-and-aft only, pivoting leeboard, such could be made to work.

But for a leeboard that must be shifted from one side to the other, with every tack, or be able to pivot out, as well pivot fore and aft, it would most likely be both too heavy and too buoyant to be practical.

The keeboard I designed for my 10 ft scow is to be shifted from one side to the other. This is to save weight and material (see my JoeBoat Alana thread). It has a relatively heavy handle on top, that is designed to keep it from pivoting fore and aft from buoyancy. It can pivot outward much more easily, but even the slightest pressure against the hull will most likely keep it down, so there is no need for ballast.

Being heavier near the top should make it much easier to handle, when shifting it to the other side.

2.) I have drawn four boats with slatted, pallet like soles, with this thought in mind. Being able to lay down in the bottom of a very small boat, comfortably, makes a lot of sense to me. This can not only expand the posible uses of the boat, but can make it arguably more seaworthy as well.

The boat I'm building now will not have this feature, because it must have a hull weight that is as light as possible, considering th materials it is made of, and because it will only be used a few hours at a time.

A bigger sister, if I ever build one, will have this feature.

 

[Paul Scott]

Viola 14 Sailing Canoe Plan - plywood sailing dinghy https://www.storerboatplans.com/boatplans/viola-sailing-canoe-plan-cartop-sailing-dinghy/

 

[gonzo]

5 mph in a 12' boat is approaching planing speed. That is not low speed.