Saturday, April 26, 2008

Cleaning up. Abstract on mast discussions

It was over due for a thorough clean up in the workshop today (see pictures from the few previous posts). Eivind picked up the centerboard yesterday evening and will probably mount it this weekend. I'm anxious to try it out, I expect a noticeable difference in upwind performance. I also discovered a major defect in the melamine table surface, probably due to the vacuum lifting it off the table when bagging the centerboard and not being able to let the bag follow the table all the way. I have not decided on how to address this problem yet.




I placed a question regarding the effects of choosing a larger wing mast profile (large aerodynamic section, no spreaders) on the discussion forum of the Norwegian Multihull Association. I got a few extensive inputs from sailors and designers of former world championship campaigning boats. Summary:

Topics to be considered:
- Total weight
- Aerodynamic qualities
- Parasitic drag
- Functionality

Possible mast choices:
- True wing profile
- Small regular "wing" profile rotating mast with diamond spreaders
- Non rotating mast

Advantages on Wing mast design (e.g. Lege Cap Ferret, Mirage)
- Superior aerodynamic properties, a lot more power (reaching effectiveness)
- Dramatically less parasitic drag (upwind effectiveness)
- Functionality. It will withstand the sail forces very well including reefed main and large masthead spinnaker. Less drag on trailer. No spreader/headsail interference.

Disadvantages:
-Weight. Probably marginal differences in total weight (including running and staying rigging) assuming correct engineering (with a shear web), high quality fibers and quality workmanship.
- Cost and work required.

Advantages on a non rotating rig:
- Weight. Lightest of the three.
- Functionality. Standing rigging giving the reqired support, safe reefing.
- Low cost, easy availabe.

Comments on the smaller (standard) rotating profiles:
- Too small profile to be really aerodynamic efficient, better than non rotating though.
- Difficulties due to spreader/headsail interference, prohibiting sufficient rotation, leading to extensive side forces on the mast top.
- Not enough stiffness when reefing, this can easier be overcome by a babystay on a non rotating mast.
- Heavier than you like to believe

8 comments:

Dag said...

Hei Tor

Verkstedet ditt har virkelig blitt flott! Det samme har sideskrogene blitt. Blir spennende å følge med på byggingen av hovedskroget, og ikke minst mast. Blir det for mye jobb kan du alltids sette inn en god bensin V8. Det gir bra fart det også.

Hilsen motorbåtbror.

Tor Rabe said...

For all foreign readers: This is my kid brother entering a comment in Norwegian. In the summer season he is often observed in his 30 kind of feet, V8 petrol propelled Bayliner. He wishes me good luck with the main hull and the mast. And he points out that if I run into too much trouble a V8 would also give good speed!

Meanwhile, the discussion on mast design continue on the Norwegian multihull forum, and it seems the big section mast even outweighs a standard carbon wing profile with spreaders when everything but standing rigging and halyards are included in the equation even though the spar itself is almost twice as heavy (these observations based on a 15m (49,2') spar)

silas said...

Hi Tor,

I'm not too clear on the above comment - is the spreaderless mast heavier or lighter than the standard carbon rotating mast?

Another thought that occurred to me recently is anchoring. I know that in Norway you often end up mooring to one of the small islands, but for cases where you have to anchor my experience with wing masts is that they like to power up and sail around on the anchor. The larger and more aerodynamic the mast, the worse this is. A bridle helps a lot, but doesn't eliminate the problem. I don't know if techniques have been developed to address this issue, or if you have considered it and are not concerned but I thought it might be another factor to weigh up.

Silas

Tor Rabe said...

Silas!

If I remember correct, the spreaderless mast mentioned weighed about 78 kg, standing rigging and halyards not included.
The standard wing section 42kg or something close to that, but with spreaders and wires, fasteners for these (in the discussion referred to as scrap-iron) except halyards and standing rigging it ended up at 94kg. So, the section is heavier, the complete mast is lighter when spreaderless (correct engineering, high quality fibers, good workmanship).

Regarding drag at anchor, it has been considered and I have tried to find some information on the subject. Mirage had her home harbour in Oslofjorden, not very rough conditions, but they claim they never experienced any problems with this. I am willing to take the risk. I do not know of any techniques to minimize or eliminate the problem.

Best regards,

Tor

Silas said...

Cool!

If you can get better performance AND less weight that is pretty sweet.

Regarding anchoring, it is not drag that is the issue. The problem is that the mast actually sails. The boat will turn off the wind slightly, say to port, the mast powers up and the boat sails way off the the port until the anchor rode comes up tight and jerks the bow around. Then the boat is angled off to starboard, the mast powers up again and the boat sails along until the anchor rode jerks it back to port. Depending on wind strength and mast area this process can range from gentle swaying back and forth to violent, jerking motions.

One sure way to avoid this is to anchor stern to wind, but this has some draw backs if the anchorage is not very well sheltered (waves breaking in the cockpit). Some people have had luck angling the mast off the centerline and then using a bridle with arms of unequal length to point the mast into the wind (with the boat pointing somewhat off the wind).

In short, there are things that can be done but none of them (that I am aware of) are 100% effective.

Silas said...

Just to be clear, the standard rotating mast would have similar problems to the spreaderless mast, but they would be less severe as the standard rotating mast is a less efficient airfoil on its own.

silas said...

Me Again,

Someone else said earlier 4 m^2 of mast area is quite a a lot so I've been doing some rough calculations:

boat weight = 650kg
distance from center of buoyancy to center of gravity = 2.7m
center of effort of the sail is 6m above underwater center of effort

I used a lift coefficient of 1.12 as this is about the maximum a standard NACA 0012 airfoil is capable of at the kinds of Reynolds Numbers the mast would have in a storm (about 1 million). If the mast is thicker, more like maybe a NACA 0016, then the maximum lift coefficient goes up by quite a bit.

Result - wind capsize speed of about 64 knots.

On its own this doesn't seem to bad, but reductions to this stability estimate have to be made due to the kind of sea state that is likely to be encountered in 60 knots of wind. At a 30% reduction, that gives a capsize windspeed of just 45 knots.

Like I said this is pretty rough but it's enough to have some cause for concern, I think.

silas

Tor Rabe said...

I do not have the necessary knowledge to comment on your calculations, Silas, I'm just a country surgeon. But I found a copy of CJ Marchaj's Aero- hydrodynamics of sailing on Amazon and I'm bringing it on the holidays. I will have to look further on the subjects you discuss.

Best regards, looking forward to seeing you in Trondheim