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Go Back   SailNet Community > On Board > Boat Review and Purchase Forum > Sailboat Design and Construction
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  #1  
Old 10-30-2007
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I am considering a liveaboard for myself in Texas later next fall. I have received two sets of information about a 37 footer and a 33 footer, but I have no idea how to compare them. this boat is similar to one I owned before, but I never had the chance to really get it into storms, nor sail it in saltwater.

Here's what I will be doing - I am singlehanding or shorthanding, living aboard, but also with frequent blue water passages to Cozumel or Grand Cayman from Texas. When I live aboard, I go places. I don't buy slips and sit for more than a week unless I'm looking like I will head towards a storm.

Could one of our local experts check this out and tell me if this is a suitable data sheet for a bluewater boat? If you could break it down "sesame street" style for me, that'd be wonderful too! the cabin is exactly what I need for a cruiser, water and fuel is ample, and it even has a shower, which is one thing I don't have now.

The keel can move up or down to suit shoals or reefs.

How do I figure righting ability of this boat? The one I am on now has a data sheet that says that over a factor of "2" is not desireable for ocean crossings, and it scored a 1.65. Is there a way to take this data and compare it with some sort of software to get another figure like this to see if having a fixed keel and no shower might be a better idea?

Thanks tons!

What does this sheet mean:

Best Boatspeeds (kt)
4 6 8 10 12 14 16 20 25 30
32.0 1.93 2.92 3.81 4.43 4.77 4.96 5.06 5.13 5.00 4.44
36.0 2.22 3.34 4.31 4.94 5.26 5.45 5.57 5.70 5.69 5.48
40.0 2.46 3.70 4.71 5.34 5.66 5.84 5.97 6.12 6.17 6.08
45.0 2.73 4.05 5.11 5.76 6.04 6.21 6.33 6.48 6.56 6.55
52.0 3.02 4.44 5.53 6.17 6.41 6.57 6.68 6.83 6.93 6.96
60.0 3.25 4.74 5.85 6.46 6.70 6.85 6.96 7.11 7.23 7.28
70.0 3.41 4.93 6.04 6.65 6.95 7.10 7.21 7.39 7.53 7.63
80.0 3.87 5.46 6.36 6.69 7.06 7.29 7.42 7.62 7.81 7.93
90.0 4.04 5.66 6.54 6.87 7.08 7.33 7.56 7.82 8.03 8.16
100.0 4.03 5.65 6.59 7.00 7.22 7.39 7.54 7.89 8.15 8.29
110.0 3.80 5.39 6.45 7.00 7.32 7.53 7.71 8.03 8.35 8.64
120.0 3.42 4.98 6.16 6.84 7.30 7.65 7.90 8.28 8.72 9.15
135.0 2.80 4.20 5.41 6.35 6.93 7.37 7.80 8.58 9.43 10.18
150.0 2.25 3.41 4.51 5.50 6.32 6.88 7.33 8.18 9.50 11.25
160.0 2.04 3.09 4.11 5.07 5.93 6.59 7.08 7.92 9.11 10.94
170.0 1.89 2.86 3.82 4.73 5.58 6.30 6.83 7.66 8.72 10.30
180.0 1.77 2.68 3.59 4.46 5.27 6.02 6.60 7.42 8.38 9.68

And these measurements?

Up.Vs 2.73 3.99 4.92 5.52 5.72 5.85 5.95 6.11 6.22 6.28
Up.Bt 45.0 44.0 42.5 41.9 40.7 40.0 39.7 39.9 40.5 41.8
Up.Vmg 1.93 2.87 3.63 4.11 4.34 4.48 4.57 4.69 4.73 4.68

Dn.Vs 2.65 3.89 4.82 5.16 5.90 6.43 6.87 7.67 8.90 10.84
Dn.Bt 138.7 140.5 144.4 157.5 160.8 165.5 168.3 169.2 165.3 161.7
Dn.Vmg 1.99 3.00 3.92 4.76 5.57 6.23 6.73 7.54 8.61 10.29

Here are some other specs.

LOA : 32' 10"
LWL : 29' 2"
Beam : 10' 9"
Mast length (over water) : 48' 7"
Draft : 31"-7' 0"
Ballast :3,064 lbs.
Displacement : 10,325 lbs. (approx.)
Engine : 22 hp.
Sail Square feet: 572
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Old 10-30-2007
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Robert,

The first table appears to be what is referred to as the "polars" for the boat in question. It shows the the boat speed you can expect to achieve at a given windspeed (listed across the top) and a given relative true wind angle (the first column on the left.)

The second and third tables appear to show optimal upwind and downwind sailing angles for maximizing Velocity Made Good (VMG) for a given windspeed.

Compare these to the similar tables for your current boat to see how their performance differs.

I vote for a shower.
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Old 10-30-2007
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This does not look like an ideal offshore boat to me She appears to be overweight for her length and so would have pretty high drag, and to offset that you need a bit more drive, and that means lots of stability. This is a low ballast ratio carried in a shoal draft, which is not a good recipe for seaworthiness.
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Old 10-30-2007
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I just got a confirmation that the 31" is with the retracted keel for hunting around the keys. It will extend down to 7' in deep water. with 3000 lbs hanging off of a 7' arm, won't that be good for righting and stability?

What type of ballast ratio should I be looking at?
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Old 10-31-2007
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Umm...the 3000lbs ain't on the board unless it is a very unusual design...it is in the keel. What type of boat are you looking at?
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Old 11-01-2007
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I'd have to agree with Cam...most Keel/centerboard boats don't have much weight in the centerboard, since the primary purpose of the centerboard is to help with windward performance. If it is a lifting keel design, with a bulb keel, then it is possible that the majority of the weight is concentrated at the end of the keel, but lifting keel design boats with bulb keels aren't all that common.

IIRC, the Tide28, which a 14 year-old boy sailed across the Atlantic earlier this year is a lifting keel design with a bulb. It was modified somewhat from stock for the trans-atlantic voyage. It has 500 lbs of ballast in a bulb at the bottom of the lifting keel.
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Old 11-04-2007
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I did a detailed response a couple of days ago which appears to have got lost. This will be briefer.
You can ignore the polars except as general interest and to show the big difference the board down makes to windward ability vmg.
The capsize ratio is came in after the Fastnet disaster and is a rough guide. It essentially looks at beam in relation to displacement. Beam gives you initial stability ie resistance to healing from hull shape, but this then falls away. Ballast insofar as it puts the centre of gravity a distance d below the centre of buoyancy creates a righting moment. The further a mass eg keel weight is down the greater the righting moment as the two are multiplied. This righting moment as the boat goes over initially increasing then decreasing. You need to find the stability curves to see the angle of vanishing stability, which should be roughly 120-. standards vary on use, but essentially when the mast goes under you want it to self right.
The capsize ratio as a method is inadequate because it takes no account of where the weight is.

In your case I doubt that of the 3000 lb ballast it is all in the centreboard, and certainly not at the end of it. If it were then it would be impossible to retract any bulb and the draft would be deeper, secondly it would be very difficult to raise and would require some specialised hydraulic system. The whole structure would also have to be very strong to have that weight swinging around.

The ballast/displacement ratio is a little light at 29%. A blue water boat such as the Allied mk 2 would usually be around 35%.

Jeff H has argued in the past for lighter modern designs and against heavier traditional blue water boats. Certainly the former will have better light airs performance, but there remains disagreement on motion comfort in heavy weather.

However you are not seeking either a race boat, or an ocean cruiser, rather it seems a liveaboard, capable of short ocean trips where presumably you will take account of the weather ie short term as long term doesn't apply.

In that regard the displacement is some 4000 lb lighter than the Allied and 10,000 lb lighter than the Westsail of the same length. That doesn't seem excessively heavy. Further it is not always clear whether full tankage is included in the design weight. I have seen boats stated by the designer to be say 4 tons yet the owners say they are 6 and that is the registered weight which I have also seem miscalculated.

The sail area is not excessive and gives a sail area/displ of 19% which means she should be ok in light winds but is higher than the usual cruiser maximum.

For other boat comparisons and further help understanding the figures see http://www.image-ination.com/sailcalc.html.

In essence a centreboard has the advantage of shoal draft but to get a decent righting moment has to have greater ballast as this is higher. Yours is limited. I agree it is not a blue water boat, but don't feel the overall displacement is too high.

It, on the figures, as always, is not perfect as a liveaboard, racer, or blue water cruiser. It is also likely to be quite different from the 37'. How well it suits your needs depends on the specifics of the design and build and intended use. Others can guide you on this.
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Old 11-05-2007
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Hmm. This would not be the Beneteau 323 by any chance?
The specifications match the lift keel version exactly except for the dispacement being about 9975 on that version which has increased ballast over standard. It also shows sail area as 540.
In this case as with others all or the bulk of the ballast is probably in the lifting keel which would change my remarks as the righting moment would be better.
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