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post #1 of 23 Old 09-19-2014 Thread Starter
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Bigger is better & faster?

This is probably a dumb question.
I was doing a little research on my probable hull speed and I was wondering...

So, you've got a 90' monohull.. say 80' at the waterline. That puts your hull speed at about 12 knots. On points of sail other than a run, do larger boats need comparatively stronger winds to hit max speed, or is it all relative.

I seem able to top out in 12-14 kts of wind on a reach in my 30'er... barely besting 6kts. Can someone in a boat 3x as long barrel by me at nearly twice the speed in the same wind, or would they need a stiffer breeze to top out?

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post #2 of 23 Old 09-19-2014
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Re: Bigger is better & faster?

Yes they could easily pass you at twice your speed with the same wind. Read about Steve Dashew's Deerfoot designs or the Maltese Falcon.

I crewed on a Santa Cruz 70' and recorded nearly 14kts beating into the wind and pointing quite high.

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post #3 of 23 Old 09-19-2014
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Re: Bigger is better & faster?

Horsepower (sail area) vs weight and wetted surface (drag) and overall length are all factors that dictate ultimate speed, along with the ability (or lack thereof) to break out of displacement mode and actually 'plane'.

Boats like the Volvo RTW boats are 70 feet long but routinely break onto a plane and can do well over 30 knots, clearly in excess of 'theoretical hull speed'.

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post #4 of 23 Old 09-19-2014
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Re: Bigger is better & faster?

Faster has you covered...
A smaller boat might hit your top hull speed with less wind... if they have a different shape, and their wetted surface gets bigger with heel, and their rig is powered up... Now if they are also much lighter and can plane, they might well be able to sail much faster than you as well.

Hull speed on my Capri 25 was somewhere south of 6 knots. I considered it a good day sailing if I could spend the whole day out above 6, and managed to do it a few times.

I'm by far NOT the best sailor and I saw double digits a few times with the boat (mostly broad reaching with some waves, or under the kite). The Capri isn't exactly a boat that planes either (fin keel, drag monster that it is).

Watch this video in HD... 22 footer (J/70) you'll be able to read his knotmeter.
https://www.youtube.com/watch?v=Mad3H-27xTY

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post #5 of 23 Old 09-19-2014
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Re: Bigger is better & faster?

As a rule... longer waterline means it's scientifically capable of a higher speed. But then the variables come into play: sail area, boat hull design, crew capapbility and even the sails themselves. I have a 31' boat with a 27' waterline, but know a Catalina 27 can go scooting by me. I have a heavy "cruising" hull, so in my case... the longer, faster thing doesn't apply.

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Re: Bigger is better & faster?

Thanks all.

I'm not asking in a racing sense. I'm just talking about, all things being otherwise equal, two displacement hulls of differing lengths. My boat is 40yo, so I'm pretty sure it won't be planing, or that that was even a goal of the designers.

Even with my very limited abilities though, it pretty easily surpasses its theoretical hull speed. I thought that was impossible.

So, another trick question. I'm sitting pretty high in the water this year. No water in the on-board tanks, very little gear on board. Am I faster this way, or would I do better to fill the water tanks, install the stove, add a few spare anchors... basically get heavy enough to buy myself another 4" of LWL?

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post #7 of 23 Old 09-19-2014
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Re: Bigger is better & faster?

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Originally Posted by BeejDeC View Post
So, another trick question. I'm sitting pretty high in the water this year. No water in the on-board tanks, very little gear on board. Am I faster this way, or would I do better to fill the water tanks, install the stove, add a few spare anchors... basically get heavy enough to buy myself another 4" of LWL?
I'm the first to admit that I'm not an expert here so this is just my understanding.

The hull speed determined by waterline length is a top speed that under normal conditions will not be exceeded. I believe even if you're under-loaded and sitting higher in the water, you still won't exceed hull speed.

So being lighter will allow the vessel to be more responsive and reach upper levels of speed quicker, be lighter to handle but theoretically will still not exceed hull speed.

Loading it down to the waterline makes the boat less responsive but will (theoretically) not make the boat faster in terms of top speed than a lighter-trimmed boat.

Just think about how much waterline length is lost by loading lighter - 4 inches is about 1% of your LWL - would you notice 1% loss in boat speed? Probably not. (Maybe this is not a linear relationship - just saying).

My boat has a theoretical hull speed of 8.7kn but I have seen it do 14 knots - down a 30 foot wave in a storm!!! Not for the faint-hearted (including me).


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Re: Bigger is better & faster?

The fallacy of trying to determine maximum actual speed possible and that supposedly possible by waterline length, is that as speed increases, the boat sits deeper in the water.
I was caught in a Med storm with way too much sail up and the boat was literally decks awash as she hurled herself before the wind. At times only the cabin top was visible from the helm, the hull and deck completely submerged. Being before electronic navigation and common installed speedometers, I have no idea what our speeds were, but to say I was in control most of the time, would be stretching the truth some. After that experience, I could see how it was certainly possible for the clippers to sail themselves under, with a loss of all hands.

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Re: Bigger is better & faster?

Quote:
Originally Posted by BeejDeC View Post
So, another trick question. I'm sitting pretty high in the water this year. No water in the on-board tanks, very little gear on board. Am I faster this way, or would I do better to fill the water tanks, install the stove, add a few spare anchors... basically get heavy enough to buy myself another 4" of LWL?
I think esp in lighter air you're better off 'light' and 'short', I think the tradeoff of wetted surface will do you more good than the couple of inches of WL you might gain by loading her down.

Work out the theoretical hull speed of WLs 4 inches different and I suspect there's not a lot in it - never mind how irrelevant that calculation may be ultimately in the 3 or 4D world we sail in.

Ron

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Re: Bigger is better & faster?

'was certainly possible for the clippers to sail themselves under' Back when stuff broke and saved the ship . Now dacron sails and modern materials can easily drive a vessel to trouble The Bluenose off Burmuda caught a 90mph burst and would have made the big dive before anybody could react but the main sail came out of its bolt ropes . Less displacement and the right shape would appear to allow the hull to yield, plane or just get dragged without depending on prudent seaman ship or gear failure for survival. At that point are you calculating the square root of your WL?
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