Modified John Lewis Petrel design

[Gareth]

I am currently building a planked hull using the John Lewis Petrel design drawing as a basis.  I am not intending it to be a serious racing model, just something different for social sailing. I am planning to make it dual control. radio or vane so I will modify the rudder arrangement to have a skeg and rudder rather than a balanced rudder as the design.  My main questions relate to the keel and ballast.  I plan to use a Sails etc wide delta planform ballast weight which seems to be available at about the right weight of 9 kg.  Should I keep the design keel profile, which is quite 'fat' at the bottom or would there be any advantage in making the keel thinner but retaining the same chord length?  The 6M website suggests a keel bolted through the hull to deck level but I am considering fitting an internal keel box instead - any views on which is best?

[stevevw]

Gareth

I only had this conversation with Henry Farley a couple of weeks ago although it was about a Sylth that I am recomisioning.

I am going to reverse the fin head to foot and use a NACA 0008 profile. \_/

The Petrel will make a nice light wind boat as I hope will the Sylth My Ballast will be around 6.4kg and also have the wide delta wing.

[Shaun Holbeche]

Hi Gareth,

I would suggest making the fin so that it is a uniform naca profile top to bottom. That is the norm (unless you were being exceptionally clever). The thickness will then depend on the chord length at different fin depths.

I believe the latest Bantock designs have a keel box rather than the keel bolt running up to deck level. The later is more user friendly for dismantling to transport, if that is required. The former is going to be slightly lighter. 

[Dave Kent]

Carbon tube to deck, bolted through. Use locating pins fore and aft to stop twist on lead. Saves the weight and faff of a keelbox - ends up with more fin in the boat than in the water because of the hull depth. You could locate some of the fin as a stub through the hull for stiffness. 

Edited February 28, 2023 by Dave Kent

[Richard98]

A reply to Stevew 

I have also built a fin for a 6M project; see "not really a 6M".   I note your selection of a Naca 0008 section which would have to be in a fin box. I selected a higher 0000 series number because the fin was external with a carbon tube up the centre; as Dave above.  Also I wanted to utilise the extra lift in association with a trim tab. These sections are quite easy to fabricate in this configuration, but will require a more advanced / expensive moulded fin for the slimmer alternative.

The boat did indeed sail higher as a result, but because I never managed to sail against another 6M I could not quantify the payoff in speed.  All the same it was an interesting little project with a huge number of potential variables. 

Richard

[Gareth]

Thanks everyone for the advice.  I will abandon the idea of a fin box and go for a bolted fin through a carbon tube to deck level.  I plan to incorporate two 6 mm diameter carbon rods extending into the keelson to stiffen the fin and aid location.  I will keep the fin elevation the same as the plan, although I am intrigued why stevevw reversed it head to foot.

I measured the sections on the John Lewis Petrel drawing and they are around 13% thickness to chord ratio, slightly more at the bottom, less at the top.   I will try and make the fin thinner and constant section but I think it will end up nearer NACA 0010 than NACA 0008

[stevevw]

Gareth

My reasoning on flipping the fin is that the centering pins are further apart making the twisting force on them less.

If you need a 10% aerofoil at the top then an 8% at the bottom may work. I am currently 3D printing my fins with a 1mm wall and only 15% infill. This is then vac bagged with two layers of 200gms carbon. The carbon tube is glued in to the fin. I have the option to use a threaded stainless stud and locating pins to make the ballast removable or making the fin and ballast one piece removable from the hull. On my Ravenna (not built by me) the fin and ballast are all non removable which is a shame, making any experiments difficult to say the least.

Keep posting pictures of your build please.

[Gareth]

After an 8 month hiatus where I was banned from my workshop in order to prepare for a possible house move, (which has now been abandoned), I have started work again on my modified Petrel 6M.  The next question for the 6M experts out there is what incidence should I aim for on the ballast weight and what on the ballast do I take as a horizontal reference axis?  Its a Sails etc wide delta planform unit and looks about right just resting on its lower surface.  However it need quite a bit of smoothing and finishing yet.

[Bill Green]

Centre line of the ballast and between 1-2 degrees nose up. You may need a drawing from Graham to establish its true C/L or advice. The ballast  chosen is the one in common use on both 6 or A but be prepared to drill holes to achieve the right displacement. Do not be tempted to take weight off the surface. if you do you will have reduced its volume and therefore displacement and you may have a problem achieving the design waterline. 

[Bill Green]

Correction

 

“Be prepared etc to achieve right weight” not displacement 

[Gareth]

Thanks for that Bill, I have had some advice from Graham and should be OK in fitting the ballast.  I am also taking some other advice from Graham to make the skeg detachable so that when sailing with radio control it will have a balanced rudder as the original John Lewis design.  The skeg and rudder shown in the photos above will be used when vane sailing.

[stevevw]

Gareth

What was the advice on ballast set up please.

[Gareth]

Steve, 

The advice from Graham was as follows, paraphrased somewhat.  Imagine the ballast weight lying on its underside on a flat plane.  When installed on the yacht, that flat plane should be parallel to the waterplane.  I have taken a couple of photos showing the rough ballast to illustrate the issue. 

I placed a steel rule alongside the ballast, propped up at one end to what looks like the angle of the ballast horizontal axis. I then worked out the tangent of the angle and thus the angle itself. I think it gives an attitude of about 2 degrees nose up, but its difficult to be precise because there is no clear datum line along the edge of the ballast.  The casting split line gives some indication where the horizontal axis is.