From classic bow to scow bow

[Dolfiman]

I propose you a specific thread on this trend which seems to influence the oceanic racers new designs and may be some future cruisers. The 2017 races has confirmed the pertinence of the scow bow designs for the Mini650 class in a great variety of conditions. David Raison, who introduced this design (well known since long on lakes and sheltered seas) on the oceanic scene in 2011 with his successful Magnum, is now proposed the « Maxi » for the Mini 650 series class : Présentation du Maxi 650, le nouveau Mini de série du chantier de construction navale idbmarine en Finistère https://idbmarine.com/fr/maxi/650/maxi650.php

This show that, within the frame of class rules with a beam restriction but otherwise open, this design can give a slight but determinant bonus of performance. In the pro's are, with heel, a floatation surface more leeward and more parallel to the boat axis, leading to more RM and sooner/faster planning when downwind (as long as the boat is light enough). In the con's are more wetted surface in light winds and a bumping bow in short waves when upwind.

Imoca 60 class, being reluctant to evolve in this direction, has added an anti scow bow rule : the beam at 1 m aft of the bow end is limited to 1,12 m. But nevertheless, the naval architects try to put more roundness in the front waterlines in their last generation designs, for the same purpose (as I understand) of favor a fast planning configuration.

Class 40 rule remains opened on that aspect and the last design of Marc Lombard, Carac, shows also a scow bow which seems in a search of an intermediate option between a Mini full scow bow and an Imoca rounded bow (See photos here attached).

I have tried to demonstrate that these trends can be captured in a same continuum in hull generation. With Gene-Hull, by adding a specific hull transformation piloted by one coefficient (that I named « Scow » , = 0 to 1), one can show a graduation of the scow bow influence on the front waterlines, allowing comparison/evaluation of the hydrostatics data when heel at 20°. Here attached this investigation, with a reference boat with Lwl 8 m, from Scow = 0 (classic bow) to Scow = 0,95 (full "rectangular" size scow bow) + 2 versions of an Imoca 60 typical hull, one at the limit of the rules (Scow = 0,037), one out of the rules (Scow = 0,6).

Of course, it lacks the comparison with more sophisticated CFD tools including dynamics forces, but I think these first hydrostatics considerations can give a first trend for an early stage design. This video of the Mini 650 Griffon / Skipper Ian Lipinski, in upwind to downwind sailing conditions, can help appreciate these dynamic forces in action :
https://www.youtube.com/watch?v=1tmASmJblRE

 

[myszek]

Here attached this investigation, with a reference boat with Lwl 8 m, from Scow = 0 (classic bow) to Scow = 0,95 (full "rectangular" size scow bow) + 2 versions of an Imoca 60 typical hull, one at the limit of the rules (Scow = 0,037), one out of the rules (Scow = 0,6).

What about the "oversquare" bow, for example =1.1? This could be beneficial for boats with an ordinary, single keel, as the AoA of the keel fin will be positive at heel.

regards

krzys

 

[Dolfiman]

What about the "oversquare" bow, for example =1.1? This could be beneficial for boats with an ordinary, single keel, as the AoA of the keel fin will be positive at heel.

regards

krzys

Thanks for your reply, and yes it works with 1,1 (here attached) and even more, mathematically speaking, with scow bow 1,5, 2,0 ... of course leading to irrationnal hulls.

 

[Doug Lord]

Dolfiman, thanks for your efforts-most interesting!

 

[CT249]

Dolfiman, aren't the scow bows largely a rule beating device? They essentially create a larger boat for a given LOA, and allow that boat to carry more power which normally costs money. A 21 foot scow is probably a slower boat than exactly the same boat with the bow drawn out to 24 ft, isn't it?

In that case, the scow bow is in the same category as an IOR boats's rating bumps.

 

[gggGuest]

Rule beating is maybe a bit harsh. It seems to me that the issue is that a very wide flat boat with a conventional bow which sails heeled upwind will have a much shorter effective waterline length than a thin boat of the same length. By having the scow bow the water length is similar to that of a skinny boat. If rules permit an obvious thing to try would be tunnel sections round the bow, which ought to make for less slamming. If you follow a logical line of development on from there, of course, you end up with a catamaran.

 

[TANSL]

Scow bows get the boat to adopt a lower trim when it heels. Can this be one of the advantages of this type of bows?

 

[Doug Lord]

No, they are not "largely a rule beating device". The Mini scow has a bit shorter heeled waterline than a conventional Mini but what it doesn't do is pitch down as it heels and that allows somewhat greater RM upwind. The design is basically similar to the Inland Lake type scows that reduce wetted surface as they heel.
It's a new direction in Mini design but inspired by a design type with a long history.

Conventional Mini:

Scow Mini:

"A" Class Inland Lake Scow:

 

[CT249]

Rule beating is maybe a bit harsh. It seems to me that the issue is that a very wide flat boat with a conventional bow which sails heeled upwind will have a much shorter effective waterline length than a thin boat of the same length. By having the scow bow the water length is similar to that of a skinny boat. If rules permit an obvious thing to try would be tunnel sections round the bow, which ought to make for less slamming. If you follow a logical line of development on from there, of course, you end up with a catamaran.

Yes, "rule beating" may be a bit harsh; perhaps I should have asked whether it's the result of a rule that allows a very high sail area and high RM on a short overall length, which therefore encourages high RM and great beam, and where reducing wave impact and wsa may be of less importance due to the high power. In part, I'm also musing about the modern Anglo habit of classing boats by LOA, which is arguably quite typeforming.

It's just that in many boats with a big rig and high RM for their length (ie Minis, Int 14s, 12 Foot Skiffs, Moths and Cherubs) the design becomes dominated by the effects of the comparatively short waterline and the need to harness the power and maximise effective length. These boats are dramatically larger in almost all ways than Bob Salmon probably envisaged when he created the Mini Transat under a simple length restriction.

Could a scow Mini become significantly "better" (which is of course a loaded term) by adding 3ft of cheap empty pointy bow and leaving everything else the same? If so what does than mean about the class rule and the class impact on the wider world of sailing, to the extent that's seen as important?

 

[Dolfiman]

Dear all, many thanks for your interest and your comments. My answers here below :

Dolfiman, aren't the scow bows largely a rule beating device? They essentially create a larger boat for a given LOA, and allow that boat to carry more power which normally costs money. A 21 foot scow is probably a slower boat than exactly the same boat with the bow drawn out to 24 ft, isn't it?

In that case, the scow bow is in the same category as an IOR boats's rating bumps.

I would say that the scow bow introduction in the Mini 650 class was logic and pertinent because 1) both Loa (6,5 m) and Beam (3m) are restricted 2) Sails surface are not restricted, just the mast height (12 m), 3) Races are mostly oceanic with a high percentage of downwind and 4) Boat can be built light enough >>> i.e. sailing conditions for which scow bow boats are deemed surfing sooner and faster than with a classic bow. Currently, scow Mini are (at rest) about Lwl 21-22 ft although faster than 24 ft Lwl with classic vertical bow. Scow bow is just a natural option especially adapted for some sailing conditions, not an artifice to take advantage of rating rule bump's as we as we could see in the metric rating rules.

It seems to me that the issue is that a very wide flat boat with a conventional bow which sails heeled upwind will have a much shorter effective waterline length than a thin boat of the same length. By having the scow bow the water length is similar to that of a skinny boat.

I am in line with Doug answer : when heeled, it is not the length issue as such that makes the difference, but more the larger RM, a less oblique floatation surface (so less draggy), less pitch-down trim and rounded/flat front waterlines which favor dynamic lift and so planning.

Could a scow Mini become significantly "better" (which is of course a loaded term) by adding 3ft of cheap empty pointy bow and leaving everything else the same? If so what does than mean about the class rule and the class impact on the wider world of sailing, to the extent that's seen as important?

I think it is to preserve the existing fleet that Imoca 60 class rulers have banned (up to now) scow bow introduction, despite the high adequation of the Vendée Globe race conditions with this design option.
On the other hand, scow bow is a non sense for displacement boats and coastal programmes with more upwind and short waves conditions.
The question is open for some cruisers not very light but which put forward front cabin volumes and large righting moment instead of speed, this market slot is currently explored by Afep Marine with 2 models : Revolution 22 and 29 :
Accueil - Afep Marine https://www.afep-marine.com/revolution22-bateau-aluminium-presentation.php
Accueil - Afep Marine https://www.afep-marine.com/revolution29-bateau-aluminium-presentation.php

 

[Dolfiman]

..... and videos of Revolution 22 and 29 :
Accueil - Afep Marine https://www.afep-marine.com/revolution29-bateau-aluminium-videos.php
Accueil - Afep Marine https://www.afep-marine.com/revolution22-bateau-aluminium-videos.php

 

[tspeer]

Thanks for your reply, and yes it works with 1,1 (here attached) and even more, mathematically speaking, with scow bow 1,5, 2,0 ... of course leading to irrationnal hulls.

The first two are starting to look very much like the Inland Lake Yachting Association scows. I've aways viewed a scow as a narrow monohull being morphed into a catamaran, but stopping half way. The heeled lines of the 1.1 scow hull show very well how the waterlines become straight and narrow when the boat is heeled, and the wetted surface is greatly reduced.

For offshore, though, scows have one big disadvantage - they pound. I'm talking jar-the-teeth-out-of-your-mouth pounding, as I discovered when sailing my M16 on Chocktawhatchee Bay.

 

[gggGuest]

a narrow monohull being morphed into a catamaran, but stopping half way

 

[Dolfiman]

There was a dinghy version of this idea developed in the sixties by Roland Tiercelin, the Wassmer 535, not a great commercial success but becoming famous thanks to a comic movie named "Le Petit baigneur" . The drawback of this concept is the tunnel itself : exposed to slamming in all conditions including downwind, so many occurrences of braking :
Brochure : http://historique.wassmer.free.fr/rev_1_en_cours/wassmer_535_brochure.pdf
Photos : https://photos.google.com/share/AF1...?key=OTRibFp1N0hsTVd6b2lPc0lNT0Rmel9hTFJmV2Nn
Video (the teaser of the movie "Le petit baigneur") >>> go directly to 1:58 to 2:48 :

 

[tspeer]

Yes, that was the idea behind Dominion and the M20/I20 scows.