Volvo open 70

249,

Back in the old IOR days one famous designer was interviewed and was quoted to say the U-shaped bow was not only a better shape for rating, but also it proved to be faster. So maybe the IOR left us with one design element that was useful.

I think even the 18s don't have hollow bows, do they? Check the attached pic.


Woody,

One thing you might consider would be to make the transom vertical up to the point where the stern wave would interact at useful heel angles. By going with the full reverse shown you are being robbed of some waterline while heeled.

 

Sorry for the link, can't seem to make it work from the website. I uploaded here my dissertation without 2d drawings (too big).

I had to go in the tank early so my hull design wasnt finalised, i designed the tested hull at the middle of beam, and this ended up being a bit too wide (compared to the final design).
I compared a twin daggerboard with twin small rudders aft (Open 60 style) and the CBTF system. After putting everyting in the VPP and a race model (weather model inshores + offshore), the results were very close but with the CBTF coming on top, so this was the configuration chosen.
With another go at towing tank with my final hull design, i would have tested a twin daggerboard system with a single rudder aft. I think it would be better than both systems for a Volvo70.

 

I think it is very difficult to see if there are hollows on waterlines or not from pictures. The following picture is the bow of a well known mini 650, and actually there is no hollow on the waterline.

 

===>ok? www.fwall.blx.pl

 

Nico,

I copied the link into my browser address line, then eliminated all the info back to your root domain. I was then able to navigate through your site to the Abstract and Dissertation, and even download with the dwgs. Cable modem makes it pretty quick. I sure wouldn't expect to do it in a timely manner with a dial up modem.

I think some of the other design houses have come to a different conclusion than you. Maybe not due to speed but to reliability? On the other hand, I have talked to one designer who thinks the forward rudder IS a detriment in the mainly downwind conditions. But so far he has no horse in this race either, so all theoretical at this point.

 

Maybe this has no hollow in the DWL, but what about at DWL+250mm or +500mm? Is that WL hollow? Minis are a bit different from larger boats since the B/LOA is much greater. This might necessitate some hollows in WLs. I've only seen one Mini ever, tilted on side on a trailer in Lausanne, so I don't have a feel for these boats.

 

249,

I believe you are an ex-14er, or have some affiliation/working knowledge of them. I have just been looking at my book on 14s 1928-1989. There are a lot of nice plates in there with linesplans from early Giles boats, through Fox, and onto the Benedict era.

Seems Fox bounced back and forth between hollow at DWL and not. Alarm 1935 No, Thunder 1937 Yes, Thunder and Lightning 1938 No, Fairy 1949 Yes (slight). I once owned a Fox 14, US191, perhaps originally a Souter build in bent ply?

Proctor's boats from '50 to '60 seemed to progress to less hollow, then came Kirby. K3 was very straight, K4 maybe a slight hollow, then the K5 came along and obliterated all comers. The K5 looks to be dead straight back to station 4.

The raised chine boats then began and Morrison's Snoggledog looks dead straight to 4, and the K VII maybe to 3. The Opus looks straight to 3, then comes out, so the hollow is quite long (but shallow) on that. I always liked the look of that boat. Then the Benedict (to 3) and Cross (to 4) boats were very straight.

These later 14s did have U-shaped bows, as do the current boats (correct?)

What does this all prove? Nothing I suppose.

 

yes it s strange if i download from the website or from here (link above) i get that the pdf is corrupted, but if i use the same link in an email and download from there, there are no problems and i get a good pdf. Anyway i'll post the drawings in two parts here later.

yes i think they need it for buoyancy forward downwind.

I agree with you, the dissertation is a big part of our academic assessment and therefore my choice was straight from performance results. Moreover i found that the CBTF had more opportunities to be optimised (hydro + structures) using simple tools (no CFD).
As i said before if i had more time in the tank with a better hull, i think it could have been shown that the twin daggerboard - one large rudder aft would have been a better choice on a Volvo both in terms of performance and reliability.

 

Here are the 2d drawings

 

Nico,

Two questions that might help Woody, as well as satisfy my curiousity.

First, why go with a J dimension greater than E? Was this an attempt to get the mast back to center the weight for pitching?

Next, why use 15% for the rudder foils? Seems a bit fat. Was this due to the required moments (therefore OD) of the shafts?

Finally, Seahorse Magazine this month has a photo of Pyewacket with the canting keel caught on the wrong side after a tack. After seeing the photos of the V60s in the last race lying flat, with their fixed keels, it is going to be real interesting for the V70s if they get caught out with keel position in a big breeze.

 

Canting Keel Nonsense

Paul B maybe you should contact Volvo-there may still be time for you to save the fleet and stop this canting keel nonsense once and for all!
Nico, does the Volvo rule have a static heel limit with max cant ?

 

I didnt spent much time on rig design, the dimensions chosen are the largest possible from the rule, a larger J will give a bigger sail area downwind. Bring the mast closer to centre of gravity.
15% is not that fat, and from a structural point of view it s important. obviously there is a compromise between wetted area, aspect ratio, and tc ratio both from an hydrodynamic and structural point of view, i think it is explained at a large enough length in my dissertation (see rudder and keel chapter)
i dont see a real problem with the keel being on the wrong side, it will happen, crew should expect it, i dont think it is so different from bad gybe, or other things
There is no limit on static heel angle, only the canting angle is limited at 40°.

 

It is just strange to see a race boat with these proportions.

I doubt you'll find many modern raceboats with a constant 15% chord thickness. I didn't see any structural calc showing the need, and I didn't see any performance comparison to a thinner design.

I think the reason the cant limit is set at 40° is due to pressure from some well known design offices to limit the downside of possible wrong-side excursions. There is concern from within the design community about this. The Southern Ocean is not a forgiving place.

 

rig proportions are actually between open 60 and volvo 60, longitudinal position is more forward than an open, and rig proportions are close to a volvo.

the rudders havent a constant t/c, t/c is varying from 16% at the root to 10% at the tip. The structural calculations (page 80 and 81) and the rudder stock drawings show clearly that the rudder stock needs this t/c for structural reasons.
The idea was to get the highest aspect ratio with a reasonable t/c ratio; taking hydrodynamic and structural requirements into account.
For a well designed section, drag is not affected mainly by t/c ratio, mainly because a thicker section will be able to carry a higher proportion of laminar flow. A 16% ratio was seen as the highest reasonnable ratio.
I expect CBTF boats to have similar t/c ratio

 

Final open 70 designs

Hi

Since posting my first initial ideas, my designs have changed hugely, i have taken on board a lot of feedback, and i understand the design a lot more. The max beam has moved further forward, and the LCB is about 0.013m aft of amidships. I've flared to bow slightly to keep spray off the foredeck, and steepened the top sides aft, introducing a knuckle which will help with stability. she has no hollows, and the shape is fully developable, and a 1.25 meter model is currently under construction. I am currently developing the keel design. The rig and keel shown in the attached pictures are not final designs. I am very interseted to hear your views on the final hull design, and any tips on the keel design would be very much welcomed. Will try and add some more pics later.

Cheers
Paul