Deck sweeping sails and effective aspect ratio

I values everyone's input, but you really should bother to first read and investigate deeper before coming with these shoot-from-the-hip replies.

The wingsail on the ferry in question is autonomous and the drive it produces is used to reduce the fuel consumption of the ferry.

Here is the report that was compiled after a period of operational monitoring -

http://www.arb.ca.gov/msprog/aqip/demo/demo final reports/wind_assist_demo.pdf

 

Is your AoA the 20 deg from true wind? The spreadsheet will be quite inaccurate for an AoA of 20 deg.

 

The 20 degrees is the angle between true wind and boat heading.

Apparent wind is 21.7kts, 13.7deg off the bow.

Sail AoA is 11.9deg to apparent wind for the sealed gap, 12.1deg without the seal. This is because the seal provides a 5.5% steeper lift curve slope.

- for the sake of simplicity I ignored leeway. The fact that the vessel only uses the sail in an auxiliary capacity and keeps its speed at a constant 7knots provided a nice "all-other-things-being-equal" scenario to point out the effect of the gap.

I should mention something important about the leeway force and in particular the heeling moment it creates:

The leeway force also goes up by 4.7% when the gap is sealed. This is partly offset by the fact that the entire sail is assumed to have been lowered by 2inches to close the gap. BUT THE VERTICAL CENTER OF EFFORT DROPS BY 11 inches, not just 2. As a result, the heeling moment is 0.5% less than that for the unsealed case.

This big drop in vertical CoE is due to the substantially altered spanwise loading once the gap is sealed.

Keep in mind that this is for a 2in gap on a 40ft luff.
I would be happy to model specific real world sails -with much bigger gaps and much lower aspect ratios - found on pure sailing vessels if anyone has a particular case study in mind.

 

Gonzo has a point here. Because of the sheerline rising close to the boom level, many real world boats exhibit their maximum drive at 10-15 degrees (or more) of heel. The vortex from the gunwhale is pushing airflow over the boom, partially blocking the flow between the boom and the deck.

Daiquiris point was a good one too: The genoa already seals the flow to the deck.

 

All valid points. So let us put some numbers or percentages to all this conceptual speculation.

I have the hull lines and a rough sailplan for Ragtime (aka Infidel), a particularly slender yacht that would have heeled a lot when beating.
I will give it a go with cfd (there goes Easter weekend...) - the spreadsheet will be no use trying to model the effect of flow over the gunwhale. Sail camber and twist will have to be thumb-suck values unless someone can come with suggestions.

 

I do not have time to investigate deeper, it was just an observation made from similar studies I have read in the past. Are you saying that those automated sails come with no maintenance nor replacement costs? Have they accounted for those costs in their projected operating budget? I do not know and do not really care.

lifecycle costs of a commercial vessel are what determines if it worth doing a new design or not. It will be interesting to see if the total lifecycle cost, vs. total payload transported is actually lower than conventional designs. If so than it is likely the first time it has been done. That would be great, more power to them, I am just skeptical of the claim.

 

I am not saying anyting - go read the report if you want to know. The only relevance the commercial viability has to this discussion is that by sealing the gap it might be possible to recover the cost of the additional capital outlay in a shorter time due to improved efficiency and its resultant benefit on fuel consumption.

As the OP I intend to keep this thread from degenerating into pages and pages of talking in circles. It is hard enough to find concrete data on this subject, hence my reason for posting it here in what ought to be a more intellectual and academic sub-forum.

Random opinions based on assumptions simply add no value and throws the discussion off-topic - no hard feelings.
If you think that sealing the sail gaps could have changed the outcomes of the other studies you refer to, that would be a most relevant contribution.

 

I would expect you'll start to see noticeable error at these AoA too. The spreadsheet is a simple tool, and should only be used within its intended envelope.

 

The spreadsheet was developed for sailing, not flying, so I would assume its envelope would specifically cater for high lift coefficients and strong downwash. If you know something about the spreadsheet that I don't, please share.

Remember that AoA can be quite high for 3D flow while the foil section still operates in its linear part of the lift curve slope - even more so for low aspect ratio sails.

I reckon the lift coefficient is a better indicator of how close you are to stall. For that reason I actually agree with you that Cl of 1.2 is a tad optimistic for a symmetric foil at the Reynolds numbers concerned. However, as I mentioned earlier, the percentage difference in induced drag seems to remain constant at Cl = 0.4, 0.8 and 1.2.

 

She's a plank double ender built in 1958. I read that ketchs don't point well so I thought this theory you have posted may help a bit. . Should be a good experiment.

 

Any idea of the luff length on the main and total sail area?

It seems as if the biggest gains - percentage wise - tend to be for the lower aspect ratio sail plans. The ketches, yawls and schooners certainly stand to gain more than a tall sloop.

 

But first you should sail her quite a lots, to measure her actual capabilities.
Then you can modify the rig and check how much the things have changed.

 

I haven't measured the rig yet but I will put the measurements on here when I do. To busy with the woodwork at the moment.

 

Good advice.. thanks d.

 

It seems that the reduction in induced drag from closing the hull-sail gap is small compared to the total drag during actual sailing.

Additionally it is only in beating windward that the induced drag becomes most detrimental. At apparent winds greater than 90 degrees from the bow, induced drag is actually beneficial.

So once we venture away from the situation of racers beating to windward, the benefit of closing the hull-sail gap becomes not worth the trouble.