Skinny Class 40

I claim no expertise, but would it not have been worthwhile to extend the aft end so the bottom just clears the water? That stern will drag water, no?
I like it.

 

Yes, the transom drags water here. It obviously increases drag at lower speeds, in this case below 7 knots, but reduces the rapid growth of the drag above this speed. As you can see in the theoretical polars, the boat is intended to sail faster than 7 knots.

However, the trim problem remains. The hydrodynamic list will rise up the bow and pull the stern down at speed, so maybe the transom should not be immersed at rest. On the other hand, there is the diving moment from the force on the sails that rises the stern up.
So, I am not sure about this.

thanks

krzys

 

Beginning to look more windsurfer-esque, at least In plan view

 

Add to that how much Open 40 races are sailed in lighter winds, would the advantage of gains made in the light or upwind to bigger wind systems hold up? Amati, for example, is a light wind / upwind demon with a lot less sail (like blade and main only- with the code zero she’s much better ), but still can manage low 20’s in the heavier stuff- how long that would hold against a hull that can do 30 knots, if transit across a light wind patch was faster? Sail changes in the light might be less onerous for the crew? And sail inventories might be lighter?

notice Amati’s stern, for example, post #31 (for some reason I can’t get the lines to post here). 1998 design, with pointy bow.

 

Lower and lighter mast for the skinny hull? Same height mast but lighter for the skinny hull? (Which might be better for light wind when gradient is a bigger deal?) Lower CG would interact differently with the stability test- and given how much effort is going into limiting standing rigging mast support and cabin shape to gain stability test numbers on present boats, might be a way to go…

 

As a matter of fact, I'm just working on two boats, one being a class40 being assessed to World Sailing regulation, the other being a 30' IRC serie build (built in serie by a serious boatyard). The two boats share the same philosophy, as described by skaraborgcraft. I cannot give you much details about them, but here are two snapshots :

First ,the floatation as you can see, there is quite an obvious tendency to mitigate the approach "the longer the better", regarding the waterline length. Clearly, planning mode is enhanced downwind, while keeping the important propertie for these racing boat to increase their waterline length when heeling and sailing close to the wind. You can also notice that the hull section is divided into three regions : a bottom one, looking like a skinny hull, a top one, reaching the limit of the box rule, where the ballast are located, and an intermediate third one, somehow flat, authorizing also a planning mode.


Second, the scow type bow. I find amazing that the bottom part exhibits the shape of a windsurf board, reinforcing the "skinny hull" effect. As a matter of fact, the wetted surface area is minimized, making these kind of shapes a good compromise between skinny hull and traditionnal hull. Also being fully compliant with the rules of the class.

 

Generally, I aimed at trans-Atlantic races (both upwind and downwind), so 6-8m/s wind speed. The 40's are relativelylightweight and over-canvassed, hence the proportion between the drag and righting moment is so important under these conditions.

I compared two similar hulls with and without the immersed transom. According to Michlet,

As you can expect, the hull with immersed transom (red line) produces more drag at lower speeds and more drag at higher speeds. With the same rig, typical for Class 40 boats, this leads to:

For the wind speed above 4m/s, the hull with its transom in the water performs better - the more wind, the better.

***
Now, the trim problem.
Generally, scows produce bigger hydrodynamic moment rising the bow than the pointy hulls with more triangular waterlines. I tried to estimate the moment treating the hull like a wing, which is a very rough estimate. Surprisingly, the trimming moments from the hull and the sails seem to cancel in a wide range of speeds. Up to 7 knots, the moment is very close to zero, then falls slightly below zero (i.e. bow down), then above 15 knots becomes positive (bow up). This corresponds to a shift of the center of displacement by only 0.1m, which is negligible.

***
About the mast height and stability.
All hulls I analysed maintain the same (only rotated) shape of the immersed body when heeled up to 30deg. I like this principle, as it allows to easily make hulls good enough when heeled at any angle. For a maximum width allowed, i.e. 4.5m, such a shape needs a very high bilge, so the righting moment at 90deg heel becomes exceptionally high. The 90deg righting moment is limited by class rules, which limits the position of the center of mass.
The narrow hull can have lower bilge and more vertical sides, which produces the righting arm at 90deg heel smaller by 20cm. So, although the wide hull has its metacentre much higher above the waterline, the eventual righting arm differs only by 1/3.

There are two obvious advantages of higher masts: smaller induced drag and ability to catch stronger, upper winds. There are also obvious disadvantages: more weight, more complicated rigging, more heeling arm, more parasitic drag. For 4.5m wide hulls, the maximum allowed mast height, i.e. 19m, may be optimal. But I guess, that for a skinny hull a lower mast could be better. Say, 16m - unless you are going to sail only under very light conditions.

The basic idea of the presented boat is simplicity and low costs. The narrow hull can be lightweight enough regardless the materials, hence hard-chined plywood structure. The 16m mast can be made of aluminium instead of carbon, still maintaining the biggest permitted righting moment. And the performance should be comparable with more expensive boats.

regards

krzys

 

@myszek
Your approach totally makes sense as long as you do not intend to take part to ORC races against "full beamed" boats. Vertical walls with narrow hulls have been heavily sailed before the scow type come to the scene. Just looking at the granpa monohull ACC give you an hint where such boats perform at the max : very light wind.

One remark. There is a point that should also be covered, for your comparison to be complete : the angle of vanishing stability. The inclining test @90deg is required by the class as to demonstrate the capability of the boat to recover from a 90deg heel (at least in calm waters), but one should not forget that any class40 should comply with the 12217 AVS requirement (design category A). Some additionnal weight may be required for a narrow hull to fullfill this requirement.

 

How much of this agrees with Gutelle’s experimental findings that shallower hulls are faster, even for heavy boats? Are formula windsurfer hull shapes coming to get us?

 

The narrow hull has generally bigger AVS. It is mush more resistant to turning the turtle. For the presented hull, AVS is about 140deg, which is enough.

regards

krzys

 

That kinda looks similar to the modified bow Pips team added to her IMOCA Medallia. More lift, less bury. Always compromises......

 

This is quite a question, Paul. My work on class40 and IRC boat mainly concern their structural configuration. I work with D.Raison, who is known ( at least in France ) for his MINI6.50 scow, and has drawn, since two, class40 in the same spirit, and is currently working also on an IMOCA. I also work with B. Nivelt, who has done a lot of experiment on one tonners, and has drawn numerous successfull IRC. Having exchanged a lot with them during the whole time, it seems that it is also quite a surprise to them to see how good are performing the shallow hulls.

That said, I've had a look at what the Gutelle says about this subject. Page 109 on his volume I, he gives some indications regarding the influence of the master section on the total drag :
"There is little data on their influence on the wave resistance. However, it seems that the increase of B/T leads to an increase in resistance"


Gutelle give some master section of half tonner (down below) as well as the results of a testing campaign made by Delft and the MIT, in the scope of the project PRATT from H.Irving (down down below).




About the B/T effect, Gutelle compare the results for model [2] and model [3] :
"Models 2 and 3 show little influence of the B/Tc ratio with however a very slight advantage for the wider and less hollow boat at the corresponding speeds weighing at R > 0.9 "(FN > 0.29)

So... What conclusion ? I also came through the thread Draft effect on Resistance https://www.boatdesign.net/threads/draft-effect-on-resistance.54713/ which shows that if the subject seems quite clear to top contributors, some rightfull comments are clearly in contradiction with their assertion.

From the work on the structure of shallow boats, I remark that the slamming effects on scows do not respect the traditionnal rules of panel designs, relative to the bounding strength of stiffeners and bulkheads. It could be due to a non linear effect of the boat velocity. So clearly, it indicates that a shallow class40 is faster than a deeper one.

Finally, being very curious, I ask ChatGPT3.5. Nothing stupid here, but nothing new under the sun also.



Personnaly, I have always been amazed by the windsurf board, and the way they are sailed. I have test many many different boards, many many different sails, and I always dream of a boat that could have similar performances. Planning is so amazing.

 

And the planing that can result from good planning is even better!

 

Oupsy....

 

Didnt that boat do far better than expected in the rough upwind conditions in a Fastnet race some years back? I imagine the ride was absolute hellish, but it was out performing many pointy bow boats, much to the astonishment of many. Faster is seems to be on top of the water (like a surfboard), than trying to plough through it, if you can handle the ride, and if the boat is constructed to take it. Any idea how much carbon was in that boat?