Love the reversed bow vaka but curious as why you need / want a reverse bow ama

Funny, I’m about to say exactly the opposite. 😉

I’m no expert by any means, but I’m personally pretty skeptical about using reverse bows on the vaka. It’s a well known fact that it’s not difficult to bury any kind of wave-piercing bow on catamarans and trimarans in a good seaway, potentially leading to a pitchpole. On a proa, I personally think the situation is worse, because you have a bow on each end. Unlike the catamarans and trimarans you can’t move all the heavy stuff in the boat to the comparatively wide stern, to act as a counterweight against a potential pitchpole, but instead have the weight near the middle of the boat, where it’s got much less leverage in that regard. Inverted bows are nice to have for that extra little bit of speed, but when the shit really hits the fan, I at least think I’d feel a lot safer with a vaka bow that has substantial volume.

That said, I think a wave-piercing bow on the ama is a very good idea. I think a good ama should have low drag in all directions and essentially do whatever the vaka tells it to do, to keep loads to a minimum and avoid any akward see-sawing motions if the waves are coming in at an angle. If the vaka says “go through that wave”, then it should go through that wave, if the vaka says, “slide sideways over this wavecrest with me”, then that’s what the ama should do. I personally think the best ama is a very fine (e.g. 20:1 at WL) ama which has a wave-piercing (potentially inverted) bow, a round bottom for low drag and low lateral resistance (while hopefully not slamming too much), a strongly arced deck so it sheds water very quickly, and has a very large volume so that it has plenty of reserve buoyancy even if lots of weight, whether that be people/gear or water ballast, is moved to windward.

That’s just my 2 cents though.

Marco

I love your Vaka! I think those bows will slice through waves with ease and controll.
The only thing I would like to change is the depth of the rocker and the center of gravity. To avoid the potential for pitchpooling you need to have a very low center of gravity and a low aspect light-weight rig. A deeper rocker will help trying to lift the forward bow when sailing at speed. The flat bottom will start to surf slightly on the water it pushes aside, and I think that is an important thing to use in resisting pitchpooling.

Cheers,
Johannes

Hi guys,
Michel is working on this design for me.  I have had this proa floating around in my head for a long time now, and Michel is making it a reality.  The main aim is for high performance day sailing/inshore club racing, with the ability to be used as a simple camp cruiser for 2.
The initial rig will be from a Hobie 16, most probably with a light wind code 0.  Our intention is use the H16 rig and sail plan until any bugs are sorted from the design, and then maybe move onto a bigger rig in the future.
I’d like to thank Michel for all his work, and his great ideas as we continue with this proa.
Cheers, Dave

Michel is working on this design for me.  I have had this proa floating around in my head for a long time now, and Michel is making it a reality.

Just out of curiosity I must ask who “Michel” is, that is helping you with this?!?

Cheers,
Johannes

Johannes - 31 August 2013 07:16 AM

Michel is working on this design for me.  I have had this proa floating around in my head for a long time now, and Michel is making it a reality.

Just out of curiosity I must ask who “Michel” is, that is helping you with this?!?

Cheers,
Johannes

Michel is Luckystrike118, the guy who started this thread.

Manik - 31 August 2013 12:07 AM

Love the reversed bow vaka but curious as why you need / want a reverse bow ama

...  but I’m personally pretty skeptical about using reverse bows on the vaka. It’s a well known fact that it’s not difficult to bury any kind of wave-piercing bow on catamarans and trimarans in a good seaway, potentially leading to a pitchpole. On a proa, I personally think the situation is worse, because you have a bow on each end. Unlike the catamarans and trimarans you can’t move all the heavy stuff in the boat to the comparatively wide stern, to act as a counterweight against a potential pitchpole, but instead have the weight near the middle of the boat, where it’s got much less leverage in that regard. Inverted bows are nice to have for that extra little bit of speed, but when the shit really hits the fan, I at least think I’d feel a lot safer with a vaka bow that has substantial volume.
Marco

HI Tink, Manik and Johannes and of course Hello all,

its nice that a discussion started about the reverse bow that I designed into this hull. There are a few things to say from my point of view…

First of all this is a cruiser racer and and the rig will have a lot of horsepower so that it can be sailed successfully on local club races. It will have no cockpit openings and Dave will sail it with two crew on trapezes. There will be a screecher for downwind and lightwind conditiones.

Second, this is no offshore boat, sailing will mainly be in sheltered waters with no more than 0.75m waves.

With this two points in mind I began the design.

Reverse bow and its lack of volume:
A “normal” hull would be 2 or 3 feet shorter if this boat was meant for cruising alone. The needed volume for the desired payload could be displaced in a 6.8 m hull without problems. What I did is nothing else what VPLP does with there new record breaker.  Take a 60 Foot Tri and stretch out the mainmull so that it gaines volume at the bow, sharpens the lines for a better cut though the waves and lower wavemaking resitance. The result looks like this. http://www.boatdesign.net/forums/attachments/multihulls/73979d1346845878-80-trimaran-route-du-rhum-skippered-lionel-lemanchois-prince-de-bretagne.png

Manik you are definitely wrong on two points.
A Wavepiercer hull in a catamaran does not mean that it can pitchpole easier than a cat hull in the standard configuration. It also does not mean that it lacks volume overall or is unstable. Its exactly the opposite. 
If you have ever sailed a Hobie Wildcat (Formula 18 wavepiercer) against a Hobie Tiger (Formula 18, older standard cat) you will know that the wavepiercer is streets ahead in terms of controlability against a pitchpole situation. The flatter run of the rockerline and the flat bottom gives a lot of longitudual stabilty and lift forward. The bow glides up the bow wave and stabilizes in this position. It may cut through the (small) waves, but it does not bury the hulls complete for a pitcpole. What is not yet shown on the scetch is that the final design will have the water shedding deck that works together with this kind of hull.

“All the heavy stuff” you are carrying on races is the crew alone, nothing else. And the crew will trim the boat “nose up and tail down” on a fast reach, simply by moving aft on the trapezes. This is again supported by the flat bottom which generates lift. The “pin- tail” is pressed more easier into the water than the fat ass of a fast cat or trimaran.

On a cruise with 100kg camping gear you will depower the boat until you feel that the boat sails safe, that should be normal.for every cruising boat. I expect that the long vaka will be very forgiving andl should provide a lot of longitudual stability, even with the crew sitting on the trampoline.

And if it really capsises one day this will be no problem. It can be rerighted by the crew alone like every beach cat. The watertight mast prevents the boat to go further than 90° and a rigged boom under the trampoline increases the righting leverarm of the crew.

The ama has the same characteristics and looks as the vaka. It has a good angle of entry, high volume and low area. For me its fine. Iam not 100% sure if a v-bottom would be better than the actual flat bottombut I think it will be ok. In every way a little clapping under the bottom from small waves is normal in a cruiser racer.

Johannes,
as I wrote the rockerline is not finished and will become a little rounder, but it will be flatter than your taste and experiences from your work with models. I have some different experiences and Iam sure that my hull will work fine.

Best Regards, Michel

Thanks for that post. I definitely learned something again! Where the wave-piercing bow is concerned, you may well be right for the design brief of this boat. Where the ama is concerned, I agree with you that the flat bottom will probably be ok in terms of vertical accelerations, as long as the crew is diligent about only scimming the ama. If they are constantly flying it half a meter over the water to impress their friends though, then that would be a different matter. 😉

Looking at the stats again I noticed you have 280l volume planned in for the ama. I recall hearing in an interview with John Harris (designer of Mbuli and Madness) that he had issues with Mbuli where the volume of his ama was concerned. Although the ama was designed to theoretically be able to carry 2 people with a bit of volume to spare, in practice it was never enough because whenever both guys were hiking and the wind suddenly dropped a bit, the ama would go into submarine mode. Now I don’t know how much reserve bouyancy he had there, but for your boat, with 2 guys I’m thinking 160kg + 20kg or more for the ama, which gives you about 35% reserve buoyancy in the ama. Do you think that’s enough?

And my final set of questions; what are your thoughts on the rig? If you intend to show those poor monohullers, and anything non-proa for that matter, what the meaning of speed is in a race around the cans, then your shunts will have to be fast. Do you intend to have dual roller furling jibs for that, or do you have something else in mind? And what about downwind laundry; you intend to take a spinnaker / gennaker along as well, or will that will you leave that out? I personally kind of got scared away from the bermuda rig once I realized just how much running rigging would be involved if you want the normal trimming and downwind sail options because you need almost everything twice, so I’m kind of curious where do you intend to draw the line there.

Cheers,
Marco

Manik - 03 September 2013 01:09 PM

1. ... as long as the crew is diligent about only scimming the ama. If they are constantly flying it half a meter over the water to impress their friends though, then that would be a different matter. 😉

2. ...  I noticed you have 280l volume planned in for the ama. I recall hearing in an interview with John Harris that he had issues with Mbuli where the volume of his ama was concerned. Although the ama was designed to theoretically be able to carry 2 people with a bit of volume to spare, in practice it was never enough because whenever both guys were hiking and the wind suddenly dropped a bit, the ama would go into submarine mode. Now I don’t know how much reserve bouyancy he had there, but for your boat, with 2 guys I’m thinking 160kg + 20kg or more for the ama, which gives you about 35% reserve buoyancy in the ama. Do you think that’s enough?

3. ... If you intend to show those poor monohullers, and anything non-proa for that matter, what the meaning of speed is in a race around the cans, then your shunts will have to be fast. Do you intend to have dual roller furling jibs for that, or do you have something else in mind? And what about downwind laundry; you intend to take a spinnaker / gennaker along as well, or will that will you leave that out? I personally kind of got scared away from the bermuda rig once I realized just how much running rigging would be involved if you want the normal trimming and downwind sail options because you need almost everything twice, so I’m kind of curious where do you intend to draw the line there.

Cheers

1. Dave, the builder of this boat, is currently sailing a Hobie 16, so he is used to live on the deck of a submarine, but it will no problem for him to fly the ama constantly.

2. Checked the ama and made a mistake, the current volume is at 370 litres, not 280l. I will sharpen the lines a little to bring it down to 330 to 350. I think this is enough, Ama wheight will be in the 30kg range, 20kg is not realistic.
Mbuli started its live with that small ama and it behaved as you discribed it. They built a bigger ama for it and its ok now. On early photos you can see the original ama sometimes. Its a big difference between then and now.

3. Shunting on a racecourse is a buisy time for a two man crew, thats right. I don’t know if Dave will have two jibs on roller furlers or just one. H16 normally have fully battened jibs, that would make furling imposssible. To be honest, I have no idea how Dave wants to have it. There are a few possibilities:
- two jibs on furlers (preferably)
- two jibs with halyards (oh my god)
- aero rig with a balestron (fast alternative, but more complicated to build)

Downwind: Dave will have a screecher on a roller furler. A Screecher is a very flat gennaker, like a genoa. This will be furled and brought from bow to bow.

Handling a lot of lines to trim the boat is a matter of training. Racers are used to work with this. Compared to the trimming possibilities on a flying dutchman or 505, the two jibs and rudders of a proa are childs play.

But all that is in the future. At the beginning is the design of the platform and beginning is now.

Michel

Hi All,
Just a quick reply on the actual sailing of this proa.  As Michel mentions above, we are still in the design stage so any of this is subject to change.
For short upwing tacking (shunting) I would probably use the mainsail alone, and only where I can do longer shunts would I use the jibs, depending on how quick I can raise and furl/unfurl the jibs.  I have a roller furler setup on my Hobie 16 currently, Sail shape needs a small modification, and it is very quick to furl and drop/raise this sail, so I dont forsee any issues with this on the proa. 
I forsee this to be my upwind shunting process: 1 - sailing close hauled on STBD tack, 2 - prepare to shunt by raising other bows (currently the stern) furled jib (PORT tack) ready to be unfurled, 3 - furl current jib (STBD Tack), 4 - Shunt proa onto PORT tack and unfurl new jib, 5 - lower old furled jib (STBD tack).  Sounds confusing but should be relatively simple I think.
As Michel has stated, for downwind legs I will have a roller furling Screecher which will be moved from end to end as required.  As funds allow I may buy a second screecher so as not to have to worry about changing ends.
Where I sail we mainly have passage type races with longer legs, I expect to compete OK in these events, however, I suspect the slower shunts (slower than tacking or gybing anyway) would be a problem on Windward Leeward races.
I guess a mainsail only rig would be a good option to speed upwind legs, and I may investigate this option once sailing.
Cheers, DAVE

I think your 5-step shunting process makes a lot of sense. For the upwind legs though, if the alternatives are to either keep both jibs up (with one furled) or keep both jibs down, then I would definitely recommend the former. If you roller-furl the jibs, and roll them tightly, then the extra drag of the furled jib at the back will easily be made up for by the fact you have a jib at all in the front.

You’d have to do the calculation in more detail (find some precise values for the drag of a furled jib, or maybe use XFOIL to simulate a cylinder at that length, angle, and reynolds number), but if the flow doesn’t separate completely, and you can assume a C_D of 0.5, then the drag of a furled jib would only be on the order of 5-10N, which is maybe 1% of the total drag of the hull & rig, which is not so bad.

For the longer (reaching) legs, I would definitely take the furled jib down though, even 1% is more or less enough to make up for the time you spend shunting at the end if the legs are a couple minutes each. 😊

There’s the issue of how to get a furled jib down though: does that mean someone has to leave his post at the trapeze to go walk onto the vaka, to lay the jib on deck so it doesn’t go over the side? Or could that be done from the trapeze? When I was thinking about putting a bermuda rig on my boat, I was considering using the jibsheet or a downhaul at the jib head run to a roll at the opposite bow, to be able to get the jib down cleanly without having to leave the ama; that increases complexity a little though.

Marco

Manik - 09 September 2013 02:54 AM

You’d have to do the calculation in more detail (find some precise values for the drag of a furled jib, or maybe use XFOIL to simulate a cylinder at that length, angle, and reynolds number), but if the flow doesn’t separate completely, and you can assume a C_D of 0.5, then the drag of a furled jib would only be on the order of 5-10N, which is maybe 1% of the total drag of the hull & rig, which is not so bad.

Circular and elliptical cylinders have been pretty well researched over the years, and wind tunnel derived values of Cd vs Re are publicly available for them.  The ellipse described below would be the equivalent of a furled jib inclined at 45 degrees with the apparent wind at 0 degrees and a furled jib will be circular to the apparent wind at 90 degrees.  So in reality it would be somewhere between the two depending on the angle of the jibstay from vertical and your point of sail.

Thanks for posting that!

Re = v * L / kinamtic_viscosity

If the length of the furled jib is around 0.03m, and we’re doing 14 knots (7m/s) at 20C, then we get a reynolds number of around 14000. Depending on the values we assume, it varies a bit obviously, but it’s on the order of magnitude of 10^4, so we’re looking at a C_D between ~0.4 and ~1.0, which gives us around about 8m * 0.03 * 7^2 * 0.5 * C_D = 2.4 - 5.9N of drag. If the diameter of the furled jib is larger, then so is the drag, but I doubt it’s going to be more than 5cm, which would gives us about 10N.