Hull Balance

[henrikb]

The boat is 11*3.2m so the hull is quite narrow compared to the mainstream 36ft boat. I want the wider aft sections to get the off-wind performance. As the hull is quite narrow, my idea is that the rudder will not be lifted out of the water. That the hull will go windward when heeled must be a good thing, as long as it is not TURNIG to windward.
Comments?

Henrik,
I am not convinced fat sterns are fast. They seem to add to nosediving, poor balance and extra wetted area. If you get the centre of bouyancy aft with deeper sections instead of width, you will have much better all-round design.

[henrikb]

Thanks for your input!
When making the aft stations deeper, will not the angle at which the hull goes trough the WL will become too steep?
What do you meen by nose diving, is that longitudal trim when heeled or what might happen when sailing downwind in heavy air with a spinacker...?

/Henrik

I'm going to disagree with the first guest on the fat sterns. I think that it is very well proven that the narrow entry and wide stern is faster. Open 60s have a very wide stern. Not only does it provide the ability to surf off wind but provides more form stability over a deep narrow stern. That allows the boat to sail more upright which allows the sails to function more efficiently. It also pushes the stern wave back further creating a longer effective waterline. I will agree that wide stern creates some handling problems but I think if you look at anything that is raced, whether it be, boats, cars, airplanes, bicycles, what makes them fast often makes them uncomfortable and difficult to control.

[Stephen Ditmore]

I mostly agree with the last post. I also like Henrik's hull, and I think it's closer to a modern ILC or Transpac 52 hull than to an "IOR type". But I do think trim by the bow can promote weather helm. I'd be interested to know what the shift in LCF is between upright and 25 degrees heel if the boat is not allowed to trim (so we can compare the statistic with Cy Hamlin's recommendation). I'd also be curious to know how far forward the upright LCB is of the upright LCF. I'm inclined to think a difference between these of more than 1.5% of DWL might be excessive.

The phenomenon under discussion can be an issue upwind or down. A contributing factor downwind can be a quartering wave lifting the stern and driving the bow down. The boat can start to turn, heel increases, the bow trims down more, etc. until you have a broach. Only very quick compensation by the helmsman and sail trimmer can avert disaster once this cycle gets going. Dual rudders help, especially if they have a bit of toe-in so that the compensation occurs as the boat heels even before there's any response by the helmsman.

In the December '03/January '04 Sailing World on page 70 there's a photo of the Bakewell-White Transpac 52 "Baveheart" broaching. It has a hull very close to Henrik's to my eye. In fairness the boat sports an enormous rig, which is clearly a contributing factor, so I'm not sure the issue is hull shape. I think it was a mistake for the authors of the Transpac 52 rule to ban dual rudders.

I questioned Chuck Paine once about whether he uses a method akin to Cy Hamlin's. He said no, but that he's inclined to think that a boat should not be flatter or have less deadrise aft than it does amidships. I found this interesting. As a method of countering the potential problems with broad sterns and fine bows I'm now drawing hull shapes with U shaped bow sections and flattish bottoms fairing into broad sterns with deadrise (a shallow V section).

I see from another thread that Ian ("Wardy") is experienced in the Moth class. I'm hoping to test my hull shape by building a "Classic Moth." (In the U.S. the "Modern Moths" are akin to current International Moths, but have recently gone to allowing assymetric kites. The "Classic Moth" rule is essencially the older Moth rule that produced the Europe Dinghy. See www.mothboat.com for more on U.S. Moths.) I'm less interested in hydrofoils, Ian (a reference to the other thread), than in what hull shapes seem to be working there in Australia.

Concerning your analysis, Ian, it's not clear to me why you think straight, symmetric heeled waterlines not parallel with the centerline would impart a turning moment ....?

To answer your question, I'm glad you raised the subject, and I think it's worth pursuing, but I'm not sure I'd want to be a complete slave to a method if I saw other approaches producing good results. I'm interested, but the final test is on the water, as you know well as a Moth sailor.

[dougfrolich]

When designing the hull ALLWAYS allow the hull to trim freely as it heels, as part of the lines development process. Centers should only migrate enough to bury the knuckle for normal sailing angles. If it is a wide hull be mindful of when the rudder starts to interact with the surface.

[dougfrolich]

Note the float mark at the bow.

[Paul B]

The "mark" is actually the lower end of the 2:1 pole downhaul. It is spliced through a hole in the bow near the waterline.

http://boatdesign.net/forums/attachm...p?postid=14862


Regardless, your point is correct. Pyewacket is not trimming much through this useful heel angle.

[SailDesign]

PaulB,
Look at the photo above (the "profile" shot) and tell me that boat is not trimming much. The forefoot is maybe just above the water, while the centreline aft is a god 24" or so above water. I realise we are talking about a whole lot of length here, but ...
;-)
Steve

[dougfrolich]

Woops, That is the downhaul.
Steve,
I wouldn't concider that the hull is trimming +,-, because of no change (or very little change) along the pitch axis, although the hull is "rolling out" quite abit. However, the angle shown is very large 30 degrees or so I would guess.

[Paul B]

Here's a better picture of the foreguy attachment.

[Paul B]

Originally posted by SailDesign
PaulB,
Look at the photo above (the "profile" shot) and tell me that boat is not trimming much. The forefoot is maybe just above the water, while the centreline aft is a god 24" or so above water. I realise we are talking about a whole lot of length here, but ...
;-)
Steve

I guess our definitions of "trim" are as different as our definitions of "planing upwind".

I watched the testing for more than a half hour. The boat would go from upright to canted in less than 10 seconds. It was very smooth, and the bow didn't trim down at all.

If you think a boat this fine forward with an aft LCB can trim "up in the stern by 24 inches" without trimming down in the bow, well that's pretty interesting.

Here's the boat upright.

[Paul B]

There is no significant trim by the bow visible.

[Paul B]

You can guesstimate the OHA from this pic, and compare to the pic you cited.

I actually was initially discussing "useful angles of heel", and responding to the trim in the bow-on sailing picture. It seems my posts remain a red cape to you.


Back to the sailing photos: I think the most interesting thing is the 20 guys stacked on the rail in flat water with no whitecaps, with a good bit of heel on. Should be interesting in a bigger blow.

[SailDesign]

Paul,
"red cape?" - don't flatter yourself.
Go look up "trim" and "heave", and imagine the consequences of both happening at the same time to a heeled boat (it's not that hard...)
If you think you can guesstimate trim from a nearly head-on photo, then your definition is way off.
Steve