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Old 29-08-2011, 17:42   #1
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Reaching Hull Speed . . .

So what are pro/con of reaching hull speed at reasonable wind speeds? I am thinking Valiant 47 versus Baba 40 for example... Hull speeds are 8.5 kts and 7.87 kts respectively... If I assume the V47 can reach hull speed at ~16 knots(close reach) wind speed while the Baba reaches hull speed at ~19.2 knots wind speed... True? don't know...estimating based on SA/Disp. It would follow that if I want to sail from Culebra to St. Croix... ~51 miles. And if the wind were 12 kts i would estimate the V47 would make 6.4 kts (12/16*8.5) while the Baba would make 4.9 kts(12/19.2*7.87). Over eight hours, the V47 would make St. Croix while the Baba would require 10.4 hrs. This seems to be an important aspect of a cruising boat - reaching hull speed at reasonable wind velocity since it would yield better port-to-port distances. What I am concluding is that the V47 should travel 30% faster over the bulk of the "common" wind range... until hull speed is reached and that controls the problem. Is this on track, or am I missing something?
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Old 29-08-2011, 17:46   #2
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Re: Reaching hull speed...

I think a Valiant 47 in 12 knots wind would only get to 6.4kts boat speed if you dropped it off a bridge.
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Old 29-08-2011, 17:58   #3
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Re: Reaching hull speed...

Your basing your results on an assumption that may or may not be valid. Do you know that there is such a speed differential at those windspeeds? There are so many variables that determine achievable boat speed for a given windspeed that you probably can't come up with a valid constant.
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Old 29-08-2011, 18:28   #4
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Re: Reaching hull speed...

Quote:
Originally Posted by MarkJ View Post
I think a Valiant 47 in 12 knots wind would only get to 6.4kts boat speed if you dropped it off a bridge.
a very tall bridge, a very ,very tall bridge................
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Old 29-08-2011, 18:28   #5
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Re: Reaching hull speed...

I think that you could safely say that a boat with a longer LWL and a higher SA to D ratio will both have a higher hull speed and be better able to reach it than the "lesser" vessel.

Trying to fine tune such things requires detailed knowledge of design parameters and some sophisticated computer skills. Sort of thing that the IMS handicap system is based upon, in fact.

One fairly useful comparison is to check the PHRA ratings for the subject boats. They are not absolutely correct in a cruising environment but do give reasonable basis for speed comparisons (for rated boats, which includes most recent production boats).

Cheers,

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Old 29-08-2011, 18:41   #6
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Re: Reaching hull speed...

This helped me to understand it,,,




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Old 29-08-2011, 18:45   #7
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Re: Reaching hull speed...

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Originally Posted by roverhi View Post
Your basing your results on an assumption that may or may not be valid. Do you know that there is such a speed differential at those windspeeds? There are so many variables that determine achievable boat speed for a given windspeed that you probably can't come up with a valid constant.

It's very unlikely that the speed gradient is a straight line. Bigger boats are going to slow you down in other ways. There's a lot more to take care of, and a lot more that can break and screw your sailing up. You'll have to have a larger crew, and if the water gets rough enough that could just mean that more people are so seasick they're worse than useless -- on deck they're a safety hazard to themselves and others. You have a jammed sail, you're going to slow down or stop no matter what the size of the boat is.

Your speed is going to depend on whether the sea is fighting you or working against you, how steady the winds are (if they're variable enough you'll have to be constantly tweaking them to maintain your top speed), whether there's a current, etc. Do you know enough about all of this to be absolutely certain that the sails on the boat actually fit that boat? One headsail that came with my first boat was ridiculous, with the luff WAY too short and the foot WAY too long. If I'd tried to sail with that sail, the boat would have under-performed.

"It's the journey as well as the destination" is rolling through my head. Buy the boat you love and then learn to make it go fast.
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Old 29-08-2011, 18:51   #8
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Re: Reaching hull speed...

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I think a Valiant 47 in 12 knots wind would only get to 6.4kts boat speed if you dropped it off a bridge.
MarkJ you have a wicked sense of humor.
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Old 29-08-2011, 18:54   #9
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Re: Reaching hull speed...

Hull speed?
Why?
Are you in a hurry?
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Old 29-08-2011, 19:50   #10
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Re: Reaching hull speed...

Thanks gang, I like the PHRF reference... this will help compare the various boats. not a straight line... would it be straight or not ? maybe asymptotic.
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Old 29-08-2011, 20:25   #11
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Re: Reaching Hull Speed . . .

PHRF... V47 = 84... Panda 40 = 162(per SanFran listing). This translates into 5.7 Kts for the V47 in "average wind conditions" and 4.4 kts for the Baba. Using eqn (3600/(550+PHRF)) Yep, SA/Disp matters. So... what does this mean to me? I think it means if I wanted to sail to the next anchorage vs. motor, I would seek a healthy SA/Disp. Otherwise for marginal trips I would be motoring.
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Old 29-08-2011, 20:39   #12
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Re: Reaching Hull Speed . . .

that area is soo gorgeouis,. why ruin it by speeding thru it without regard for the scenery. why are you in such a hurry. i dont wanna sail at hull speed thru life. too much to miss by hurrying past it. you racing or cruising.
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Old 29-08-2011, 20:44   #13
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Re: Reaching Hull Speed . . .

we already know, from another thread, that you've fallen in love with the Baba 40. At that point hull speed becomes irrelevant.

You want hull speed, don't be poking about those Bob Perry designs. They will frustrate you.

You want to be in love with your boat, remember the basic rule that you don't pick your boat, it picks you.
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Old 29-08-2011, 20:47   #14
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Re: Reaching Hull Speed . . .

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Originally Posted by zeehag View Post
that area is soo gorgeouis,. why ruin it by speeding thru it without regard for the scenery. why are you in such a hurry. i dont wanna sail at hull speed thru life. too much to miss by hurrying past it. you racing or cruising.

Ohhhh I'm new enough to sailing that hull speed is a real "Yee HAA!" moment for me.
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Old 29-08-2011, 20:50   #15
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Re: Reaching Hull Speed . . .

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Originally Posted by Rakuflames View Post
Ohhhh I'm new enough to sailing that hull speed is a real "Yee HAA!" moment for me.
Me too. (And I've been sailing actively since the 60's)
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