Sailboat performance and seakeeping evaluation when sailing upwind on irregular seas

This evaluation is performed within the frame of the Dolfi 37 daysailer project (brochure in : Dolfi 37 classic daysailer https://www.boatdesign.net/threads/dolfi-37-classic-daysailer.61001/). To evaluate its speed potential, we try to go beyond just the traditionnal estimation on calm water providing by a VPP, by adding a tentative evaluation of both the speed and the behaviour when sailing upwind on irregular seas.

Remind of the main results on calm water (as already showed in the Dolfi 37 brochure) :

• Upwind : expected maximum speed is 6,2 Knts (Fn = 0,35), reaches for a wind speed of 14 to 16 knts.
  
• Downwind : speed can reach ~ 9 Knts with 16 Knts of wind, and ~ 10 Knts with 20 Knts of wind.

*** Method :

• To set a framework for this evaluation, typical « usual maximum » sea states for a daysailing programme in coastal waters are adopted : wind range from 6 to 16 knots is considered and for each wind force, a significant wave height Hs is set within a linear relation (for simplification), i.e. from Hs=0,2 m for wind 6 Knots to Hs=1,2 m for wind 16 knots.
  
• For each doublet [Wind, Hs] so defined, a Jonswap spectrum is built.
  
• From hull + appendages geometry, boat mass, CdG and inertias : computation of the RAO's and of the drift forces transfert functions.
  
• The drift forces on waves with 45° of incidence from forward (in particular FD1 i.e. the added drag due to waves) are introduced in the VPP, and so boat speed on such waves can be computed.
  >>> doublets [Wind, Hs] become triplets [Wind speed, wave height Hs, Boat speed Vb].
  
• For each triplet so determined, the RAO's functions are crossed with the Jonswap spectra to compute the various boat behaviour parameters, e.g. heave, pitch , speed, acceleration significant amplitudes at various reference points of the boat (CdG, bow, sailplan centre, cockpit).

*** Main results :

• The speed loss when sailing upwind on irregular seas at 45° incidence is 0,5 to 0,7 Knots / calm water, in percentage it is a speed loss of 8% to 12%.​
• At the max, significative amplitudes, when wind is 14 -15 Knots and Hs is 1,0 – 1,1 m, are :
  • - pitch amplitude : ± 7°
    - bow vertical acceleration amplitude : ± 0,65 g
• Speed amplitude at the sailplan center is ± 15% to 20 % the apparent wind force for quite all the range of Hs (Hs 0,3 to 1,1 m)
  
• Speed amplitude at the CdG is up to ± 16% the boat speed, e.g. ± 0,9 Knots when the boat speed is 5,5 Knots.

These non negligeable fluctuations of both the apparent wind speed and boat speed should affect themselves the average speed loss of the boat (as estimated above with only the effect of drift forces due to the waves), but how can we introduce these effects for a second iteration loop with the VPP ?

Details of the assumptions, computations and results in the document attached.

 

Hi Dolphiman, I'm not a sail aerodynamicist, but I do race a lot. Some observations from your performance doc: I noticed the jib clew in your model is well aft of the bow (perhaps a meter or so). Would you not get better upwind performance with a larger jib that is moved forward to the tip of the bow? Cruiser/racers also keep that jib pretty close to the deck so you might try lowering it a bit as well to see if you get better upwind numbers.

Also, does your analysis software account for various sail trim positions? e.g.

• Moving the jib cars forward/aft to control the angle & twist of the jib?
  
• Adjusting the main angle & twist?
• Adjusting backstay tension?
• What about a larger mainsail?

My first instinct it to trim the sails and max out the VMG as much as possible. That boat has fast + fun written all over it. I would zip all over the coast with it. Very nice work.

 

Hi Joseph, thanks for your questions, here below my answers :
*** The sail surface considered for this numerical study is mainsail + jib (total 42 m2) , consistent with a upwind sailing for wind ~ 10 to 15 Knots, leading to the peak speed 6,2 Knots (Froude 0,35) on calm water. For lower wind speeds < 10, I kept this sail configuration for simplification, but on the real boat, yes it is possible to put up a light genoa on a removable forestay ahead of about 1 m of the jib fixed one as you suggest , and as mentioned myself and sketched in the brochure, page 12, leading to total surface of 50 m2 : likely that extra surface can improve the boat speed for light winds. I can simulate this and evaluate the speed increase with the VPP (I wind do that in the next days when more time available).
*** yes, a larger jib thanks to its closest position with the deck can improve the performance (a bit more sail surface together with a bit lower centre of effort), but in my opinion it is of second order of magnitude (by comparison with jig/genoa option or calm water /on waves performance) for racing purpose mostly.
*** adjustments of the sails : in a VPP there is a factor called Flat which is supposed to represent the sails adjustment that an experienced sailor usually did to optimise the efficiency, especially on the sails camber, so the name. In the VPP simulation (at least mine), this Flat factor acts on sails thrust and side forces, reducing them from a max to a min as long as the apparent wind increase from light to strong. I must confess that I did it more by imitation than by complete understanding on how professional VPP use and trigger this factor, I can change my mind on that if I can collect more info.
*** Alongside the flat factor, for each wind force input, when upwind sailing I try various true wind angle (twa) from 45 to 38 deg and keep the one which gives the best VMG : typically, for the Dolfi 37, that means an evolution from 45 deg for light winds to around 40 deg for winds 13 to 16 Knots. In the presented results I just give the boat optimal speed without indicating for which twa exactly they were obtained, neither the VMG, but I can give these info too. To add that , for simplification and better comparison, I took the same twa's when simulating upwind on waves that the ones so obtained from the calm water simulation.
*** Adjusting backstay tension : not simulated in the model
*** About a larger mainsail : ok as long as the Lead remains in the norm, especially when sailing upwind to avoid the strong weather helm, I detailed on that in thread quote#6 :
Sail Plan update https://www.boatdesign.net/threads/sail-plan-update.60706/

 

Hi Dolphiman, thank you for the additional details on your analysis. Yes, overall there is only so much you can approximate with simulations and a fixed sail configuration. It's good for a benchmark. Hopefully the Dolfi 37 becomes a reality and sails away from a harbor some day.