Scow boats / Hydrodinamics

Hello boat lovers,

I have a doubt about the prismatic coefficient of a scow boat. It is said that the Cp should be between 0,5 and 0,6. But it is the same for scow boats?

I'm designing a scow boat (LOA: 8,75 m , Beam : 3 m , Draft: 1,9 m). And the Cp is 0,62. Is it too high? Or it is possible because of the round shape of the bow?

Hope you can send me some help. I'd really appreciated

 

Hello Aral - welcome to the Forum.
I am wondering - who is saying that the Cp 'should' be between 0.5 and 0.6?
I would have thought that a Cp of 0.62 should be ok generally - but how fast are you planning on going with your scow?
Will it be displacement speeds or fast planing (can a scow plane? Is your scow similar to a garvey type?)
Can you post a sketch or drawing to show what your scow looks like please?

 

Cp in the region of 0.6 is not unusual for a typical scow. What is the purpose of the scow? Is it to be a sailboat, work boat, power boat? If it is to be a sailing boat, the beam is wider than usual when compared to length. If it is to be a power boat the Cp may be considerably higher than 0.6

 

Aral, I just noticed that you mention a draft of 1.9 m. - is this correct?
If it is, then I guess it is a sailing scow, and that this includes the keel (?)

 

Many first posters display an extreme economy of words, for some reason. It leads to misunderstandings and misapprehensions. Like when a journalist, curious about the "real" age of the actor, sent a telegram to Cary Grant's agent, asking " How old Cary Grant ?" The telegram sent in reply...…... "Old Grant fine, how are you ? "

 

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Aral, I just noticed that you mention a draft of 1.9 m. - is this correct?
If it is, then I guess it is a sailing scow, and that this includes the keel (?)
--------
Thanks for your answer.

It will be a planning boat. Similar to a mini transat 6,50 but bigger.

Yes, 1,90m with bulb keel.

In a few days I'll post a sketch or drawing and you can have a look.

 

Thanks for the answer.

Is a sailing boat. Exactly, the beam is wider than usual especially on the bow. So do you think that for a sailing boat cp = 0,62 is it correct?

 

Scows are sailed heeled when going upwind to increase length/beam ratio. Downwind, on a strong breeze, they behave like planing hulls where the length /beam ration reduction can benefit.

 

To your last question, I can propose you an answer based on the investigation that I did previously within the thread :
From classic bow to scow bow https://www.boatdesign.net/threads/from-classic-bow-to-scow-bow.59871/
From an initial hull shape with a plumb bow and pointy fore waterlines, having a Cp of 0,572 , I introduce progressively a scow bow influence in the hull front lines, with some slight adjustements concerning the waterline length (a bit reduced), the bow line (a bit more rounded) and the hull draft (a bit reduced) in order to have an adequat roundness of the bow within the same displacement (2400 kg for this Loa 8,65 m sailing boat) and same LCB location for the comparison. As an output data of this process, the Cp evolves from 0,572 (no scow bow) to 0,630 (a « full »scow bow). So in my opinion it is a natural trend due to the geometrical adaptation of the hull, and if Cp 0,57 is good for the initial hull, Cp = 0,63 can be good for the scow bow version of this hull. Attached the revision of my previous study with including the Cp evolution.

 

Dolfiman, the heeled waterline on your full scow appears to be much more symmetrical fore and aft than that on the pointy bow - I don't know much about sailing yacht design, but I can only assume that this must be a good positive factor in favour of the scow, as she should then be less likely to round up or spin out by having a more 'balanced' heeled waterline?

 

That's a really helpful study. Thanks for sharing it!

 

Yes, the full scow concept aims to have as much as possible such heeled waterline symmetrical, decentered parallel to the boat axis as if it was the waterline of the leeward hull of a cata, giving more righting moment while reducing the wetted surface. The more this trend, the less the obliquity of the floatation shape and the trim variation with heel, so leading to a more natural balance.

You are probably aware of the various interviews of David Raison (the naval architect) and of Ian Lipinsky (the successful skipper on Mini 650 circuit, and this year on Class 40 with Credit Mutuel, winner of the Jacques Vabre 2019), who are the main actors of the scow formula for an oceanic race program. David Raison also designed the Maxi 650 for IDB Marine, for the Serie class of the Mini650 circuit :
Présentation du Maxi 650, le nouveau Mini de série du chantier de construction navale Idb Marée Haute en Finistère https://idbmarine.com/fr/maxi/650/maxi650.php
, and cruising versions for Afep Marine : Revolution 18, Revolution 29 and the new Revolution 24 under construction :
Accueil - Afep Marine https://www.afep-marine.com/
Another parameter more important probably is the Displacement Length Ratio DLR : for the racing version Maxi 650, it is ~ 100, for the cruising versions Rev24 and Rev29 , it is ~ 160 . To note that for these ratios, I took the Loa instead of the usual Lwl because the fore overhang due to the roundness for scow adaptation, reducing of course the Lwl, is not relevant (in my opinion) to compute this ratio and to compare with pointy equivalent ships. And the displacement here taken is the ship light weight. In my numerical examples above, I took DLR 130 for the initial pointy version, leading to 2400 kg, then keeping this displacement constant for the scow versions.

 

This foiling scow is the american boat for the 36th America's cup.
First regatta against the other teams will be in Sardinia 23 untill 26 of April 2020.

This their site with photo's and information.
https://americanmagic.americascup.com/

 

If you go in this direction, i.e. the foiling scows, you should also consider the Pogo foiler Cerfrance, a Mini650 designed by Guillaume Verdier. She already crossed the Atlantic Ocean in race, being 4th at the last Mini transat 2019 despite a broken bow sprit in the second leg which prevented it to sail harder and to contest the domination of the 1st, the non foiler scow Team BFR designed by David Raison.
https://voilesetvoiliers.ouest-fran...-skipper-3508863e-da38-11e9-9cb2-2490234e5e74
https://www.minitransat.fr/sites/default/files/atoms/files/resultats_proto_gen.pdf

 

If the boat is up on foils, how much does the hull shape matter? I assume only that it needs to be able to reach the minimum speed to lift.