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Old 13-12-2020, 04:10   #1
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Shallow draft vs deep draft performance

Hello,


I'd like to get an insight into (in particular upwind) performance differences between shallow draft and deep draft versions of a yacht. I'm currently looking at a Beneteau First 51. The shallow draft version has a draft of 1.8m vs 2.8m of the deep draft version, a mast height of 22.5m vs 23.7m and is a ton heavier than the deep draft version (why? not more ballast).


Shallow draft would actually be good, but I am wondering what kind of performance penalty you'll take.


Thanks for any experiences, numbers, anything!


Regards.
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Old 13-12-2020, 04:45   #2
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Re: Shallow draft vs deep draft performance

The shallower one needs to be heavier because the righting arm is shorter. If you're a gung-ho racer who needs to point high more than anything else, get the deep draft. If you're cruising, get the shoal draft. Unless you're already a gung-ho racer, you probably won't notice the difference.
If you get the deep draft and load it up for cruising, don't expect it to sail quite as well as if it was stripped down for racing.
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Old 13-12-2020, 05:00   #3
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Re: Shallow draft vs deep draft performance

@Benz.Thanks for you reply! If I were a gung-ho racer, I would probably not be looking at a 30 year old boat . On the other hand I do have "a feel" for boat performance. Boats that feel heavy, make too much leeway, or are noticably slowed by growth for example, "feel" annoying, if that makes sense.
We rented a Jeanneau Sun Odyssey 43 once with what I felt was a short mast and ugly mast-furled mainsail and it just wouldn't go. We barely ever made more than 6 knots. That I found annoying, so I am trying to avoid that.
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Old 13-12-2020, 05:38   #4
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Re: Shallow draft vs deep draft performance

For 2 exact boats with the different drafts: if fin keeled the performance will be pretty much the same because the shape and weight of the keel will account for the difference. If not a fin keel with wing the shallow draft probably has performance loss.

I have had both and to the noticeable difference is how the boat reacts to wind gusts. My deep drat both tends to just dig in a power up on a gust, when my shallow draft one would heel over a lot. But these are different boats with 12 years of engineering design difference so who knows.
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Old 13-12-2020, 06:47   #5
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Re: Shallow draft vs deep draft performance

Slash, your response suggests to me that performance is higher on your priority list than your original post indicates. So does finding 6 knots to be sluggish.

Have you compared the 51 with catamarans of similar interior space? I don't know your other priorities, but the biggest loss in shifting to a catamaran is ultimate stability in situations most cruisers avoid, and the gain is much better performance, lighter rig, and interior space per length.
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Old 13-12-2020, 06:57   #6
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Re: Shallow draft vs deep draft performance

A catamaran of similar interior space (and cost) to the OPs boat will be a 40-45 footer and will not be at all faster and likely slower especially upwind.

My advice back on topic is assess cruising grounds. If draft not a limiter where you plan to go, deep draft will always be noticeably better performing. This particular model you cite has both shallower draft, higher displacement, and shorter mast, all of which will lead to worse performing boat. But, I personally could never own a boat with 2.8 meter draft where I sail as many harbors I really enjoy have less than 2 meters depth in the channel
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Old 13-12-2020, 07:03   #7
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Re: Shallow draft vs deep draft performance

No single answer. Varies from boat to boat and the keel designs. I have a shoal draft Cal 33-2. Most regions rate the shoal draft version about 6 secs a mile slower under PHRF, but here on western LIS it's the same as the deep keel version. I am very happy with my Cals upwind performance and don't find it tender at all. It's actually less tender than my former deep keel 30 foot race boat.
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Old 13-12-2020, 07:42   #8
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Re: Shallow draft vs deep draft performance

Quote:
Originally Posted by SlashV View Post
Hello,

I'd like to get an insight into (in particular upwind) performance differences between shallow draft and deep draft versions of a yacht. I'm currently looking at a Beneteau First 51. The shallow draft version has a draft of 1.8m vs 2.8m of the deep draft version, a mast height of 22.5m vs 23.7m and is a ton heavier than the deep draft version (why? not more ballast).

Shallow draft would actually be good, but I am wondering what kind of performance penalty you'll take.

Thanks for any experiences, numbers, anything!

Regards.
There will be a certain performance hit on all points of sail which results from the tonne of extra ballast required to produce the required righting moment off the shorter righting arm of the shoal draft version.

But 1 meter (!) difference in draft will make a very big difference in upwind performance. Depending on the shape of the hull, that would roughly cut the length of the keel in half.

For sailing upwind, there is no substitute for draft, or rather, length of the keel. The keel and rudder are wings just like the sails are, the keel "sails" in the water to produce lift to counteract leeway and get you upwind. This lift is the force which opposes the drag force on the sails and is no less necessary than the lift generated by the sails, in getting you upwind.

The higher the aspect ratio of any foil, the greater the efficiency. So if you cut nearly half of the length of that foil, you have just slaughtered the aspect ratio and potential lift.

I would not have a Beneteau First -- a performance oriented boat -- with 50' LOA and only 1.8m draft. Maybe a tubby cruising boat which you don't plan to sail much upwind, but not a Bene First.

And if you don't care much about sailing upwind, likewise -- no reason to choose a performance oriented boat. Just go for a normal cruising boat with less sacrifices of comfort for performance.
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Old 13-12-2020, 07:57   #9
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Re: Shallow draft vs deep draft performance



Neither one looks that high aspect. Maybe the winged keel gives it some extra boost?
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Old 13-12-2020, 08:04   #10
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Re: Shallow draft vs deep draft performance

My gut feel is that with a full meter of difference in draft, there will be a performance hit no matter what fanciness they applied to the shorter keel design-wise. Having an extra ton of ballast and the designers still feeling it appropriate to shorten the mast points in this direction as well.
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Old 13-12-2020, 08:18   #11
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Re: Shallow draft vs deep draft performance

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Neither one looks that high aspect. Maybe the winged keel gives it some extra boost?

True, but the shoal keel is extremely low aspect. The wings should help. I don't know how much.


The deep keel appears to lack a bulb. That is a missed design opportunity.
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Old 13-12-2020, 08:33   #12
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Re: Shallow draft vs deep draft performance

Quote:
Originally Posted by SlashV View Post
Hello,


I'd like to get an insight into (in particular upwind) performance differences between shallow draft and deep draft versions of a yacht. I'm currently looking at a Beneteau First 51. The shallow draft version has a draft of 1.8m vs 2.8m of the deep draft version, a mast height of 22.5m vs 23.7m and is a ton heavier than the deep draft version (why? not more ballast).


Shallow draft would actually be good, but I am wondering what kind of performance penalty you'll take.


Thanks for any experiences, numbers, anything!


Regards.
Solid, comparative, numbers are hard to come by. Perhaps a set of polar diagrams for the two boats would tell you a lot about absolute speeds. I skimmed through the Southern California PHRF data base (3000 boats) and I didn't find many direct comparisons. I did find one set though which was enlightening: Two Beneteau 45F's, one with a 1.5 mt draft, the other with 2.4 mt draft. The PHRF difference was 48 seconds per mile (around a buoy course). It would be greater going strictly upwind.

But the difference in "feel" while sailing will be greater than the absolute speed difference. The boat with the shallow draft and shorter rig will lack "sparkle". It will not come alive in a 8 knot breeze, and it won't be as solid on a 25 knot day. That difference will prevail over all sailing angles, in my opinion.

If accessing shallow harbors is a requirement you have to go with shallow draft. If having fun sailing is your priority then the deeper draft boat will likely deliver more of it.

Note: We have a 2.4 mt draft keel. In 34 years of visiting ports, including a circumnavigation, we have not felt constrained by our draft (we have not visited the East Coast of N. A.) There have been a few places, very few, where we had to avoid some marinas or beautiful anchorages because they were shallow. The boat's performance more than made up for that.
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Old 13-12-2020, 08:44   #13
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Re: Shallow draft vs deep draft performance

My second boat was a C&C Landfall 42....it had a lot of go-fast goodies....rod rigging....adjustable hydraulic backstay, maxprop, etc.....but one thing it did not have, was a deep draft.....5'6" was all it had.....
I often entered that boat in area races....and at the start, where everybody is usually always hard on the wind, my boat's lack of pointing ability was a pain in the arse for other boats on the starting line....as depending on where I was in the line, boats to my leeward side had to fall off to avoid hitting me.....often accompanied with some choice language....
No question, that draft is an issue when racing....but in the same breath.....cruising in shallow waters like the Bahamas, a shoal draft is your friend....
The Beneteau also has a shoal draft (5'6"), but has a more of a planing type hull, which also makes a big difference and is a good sailing boat, but has no bilge to talk off.
The bilge is actually a shoebox size indentation in the keel, but if you are heeled over, and you get water inside the boat, the water runs to the lower side of the boat, not that indentation....you can get several 100 gallons of water inside the boat, before the bilge pump goes off....ask me, I know....
Hard to pinpoint what is important to you. You won't always be racing, planing hulls are fast, but....see above...
Mast height is only important if you are traversing the east coast USA ICW.....which has a 65' clearance on area bridges...about 20 m.....so a Ben 51 will not make it here...
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Old 13-12-2020, 09:44   #14
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Re: Shallow draft vs deep draft performance

Quote:
Originally Posted by SlashV View Post
@Benz.Thanks for you reply! If I were a gung-ho racer, I would probably not be looking at a 30 year old boat . On the other hand I do have "a feel" for boat performance. Boats that feel heavy, make too much leeway, or are noticably slowed by growth for example, "feel" annoying, if that makes sense.
We rented a Jeanneau Sun Odyssey 43 once with what I felt was a short mast and ugly mast-furled mainsail and it just wouldn't go. We barely ever made more than 6 knots. That I found annoying, so I am trying to avoid that.
Quoting Bob Perry from my memory, “Nothing goes to windward like draft.” Anything else, including the Scheel keel on my Tartan is a compromise.I traded a deep fin for not going aground. If I lived in NY or on the west coast of the US, I would own a T-33R with a fin keel.


Regarding in-mast and a chartered Jeanneau 43- as a previous charter Capt, I ran 34-53’s and found they were quite capable. As a delivery guy I have moved a few in the 40’s. I have also moved many in-mast systems.

While the in mast can never give the sail shape of a conventional rig, it can move if: 1)the sail is not blown out- happens on well used boats in charter fleets; 2) the sail abused by not reefing in heavy winds, or reefed with folds in it- once again between chartered vessels and lazy captains, this can happen; and 3) the sail has a few leach telltales so one can read the trim.
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Old 13-12-2020, 10:01   #15
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Re: Shallow draft vs deep draft performance

I previously had a production boat with a wing keel and furling main. Tried everything and got rid of the boat after 3 years. My new boat ( 10 years ago) was the current Catalina 375 with over sized sails ( Full batten main 150 Genoa) and a deep keel of 6'10". I suspect with all the beer etc on board it draws 7'. No comparison in sailing. On Lake Ontario, we get big swells and this boat leans over and goes steadily, loving 20 knots with full sails. She also points very well. I know the feeling, when my boat is going less than 6 knots, I feel like it is stopped. I only get concerned about depth after Labor Day when the St Lawrence Seaway pulls the plug on the lake and the water levels quickly drop. If I desired, I would take the boat down the ditch of the east coast. I looked at it once and one just has to be a master of tides in some areas.
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