Maximum hull speed

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BlueSkyNick

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I am having a run in with some others on another forum who claim their Moody 33/34/346's will do 9+ knots on a close reach. ie Speed through the water on accurate instruments, no surfing or other gravitational impact..

I reckon they are talking poop given the theoretical maximum speed in knots for a displacement hull is (the square root of 2 x LWL in feet) or (1.4 x square root of LWL). Which ever version you prefer is irrlevant as they're both the same.

So is if a boat has a 32ft waterline length times 2 is 64, square root is 8Knots.

Is it really possible to sail faster than the formula ?
 
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Is it really possible to sail faster than the formula ?

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Yes. I have sailed 6.2 knots on a 17 foot waterline several times. No current, no surfing.
 
Yes it is possible but you are effectively sailing uphill. Mobos do it all the time. A spinnaker helps because it gives a bit of vertical lift as well as push which helps climb up the bow wave.
 
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With my 9m trimaran I've been tacking regularily 13 - 14 kn !

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Ah, but you are cheating, yours is not truely a displacement boat!
 
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I am having a run in with some others on another forum who claim their Moody 33/34/346's will do 9+ knots on a close reach. ie Speed through the water on accurate instruments, no surfing or other gravitational impact..

I reckon they are talking poop given the theoretical maximum speed in knots for a displacement hull is (the square root of 2 x LWL in feet) or (1.4 x square root of LWL). Which ever version you prefer is irrlevant as they're both the same.

So is if a boat has a 32ft waterline length times 2 is 64, square root is 8Knots.

Is it really possible to sail faster than the formula ?

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Certainly it is ...
 
I agree with you. It is possible to exceed theoretical hull speed for short periods of time, particularly on light displacement boats, but getting over your bow wave takes power, far more than the boats you mention can ever generate.
 
who cares !
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"Hull speed" is not necessarily a maximum: it is just the speed at which the "secondary pressure wave" from the bow aligns with the wave created at the stern. This speed may be exceeded, but as the vessel is effectively beginning to climb up the slope of her own bow-wave, she will require increasing amounts of power to do so. For what it's worth, my old cutter (21ft waterline) often does 6.5 knots on a on a close reach, up to 8 downwind - but then she does have a big sailplan (450 sqft), and a leadmine down below to keep her upright.
 
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I am having a run in with some others on another forum who claim their Moody 33/34/346's will do 9+ knots on a close reach. ie Speed through the water on accurate instruments, no surfing or other gravitational impact..


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It is quite possible to exceed the formula speed given enough power in the sails and the constant of 1.34 isn't set in stone and given enough power (and the boat's ability to handle it) 1.4 even 1.5 is possible.

BUT before certain Moody owners get too excited, 9+kts on a close reach in Moody 33/34/346s is either bullshit or very optimistic instrumentation! They might (rarely) achieve that on a downwind surf for a few seconds, but no more than that and indeed IF that. For one thing none of these boats have a LWL anywhere near 32ft. more like 29ft maybe so the theoretical hullspeed is say 7.2kts. We, at 41ft LOA, have a dynamic LWL of 36ft and yes we can make the 9+kts claims they have, but that is with a racy hull and a big sail area it is still rare except when surfing, when we have a confirmed 14.7kts and a momentary 19.3kts with a log averaged over 10 secs.

But you'll never sell that to Moody owners. /forums/images/graemlins/laugh.gif
 
I'm sure that in the right conditions I could drive a Moody 33/34 hull to 9 knots in flat water, with no surfing, but it would need plenty of wind and be on a very broad reach or run, steering to keep the hull under the rig. On a close reach you just can't generate enough raw power before the boat heels and spills the wind, and helm action to correct acts as a brake.

Your best chance of sustaining 9+ knots in a boat this size/weight/type (apart from surfing) is probably a very broad reach on genoa or spinnaker only (to give neutral helm) in 30+ knots of wind.

You can push over the theoretical 'hullspeed' but it takes massively more power to get every little bit of extra speed.
 
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, 9+kts on a close reach in Moody 33/34/346s is either bullshit or very optimistic instrumentation! They might (rarely) achieve that on a downwind surf for a few seconds, but no more than that and indeed IF that.

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But you'll never sell that to Moody owners. /forums/images/graemlins/laugh.gif

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Fully agreed on both points, and I'm one of them !
 
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They might (rarely) achieve that on a downwind surf for a few seconds, but no more than that and indeed IF that.

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But you'll never sell that to Moody owners. /forums/images/graemlins/laugh.gif

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I think you assume that Moody owners are all very conservative. Its not the case.

Donald
 
The principles stated here are good but leave you with a slight misconception, namely that it requires HUGE power to lift the boat above waterline hullspeed. This might be true of many old tubs not so with today's more modern design. My own 35 footer starts to plane/surf (whatever you want to call it) in flat water as I exceed around 8-8.5 kts. The stern wave just gently brakes away from the transom and the transition from displacement to surfing is without drama - in fact I often don't notice I'm planning until I glance down at the log/gps and see that I'm doing double figures (and yes that's without swell or tide). And in fact once the pop has lifted slightly the loads on the gear actually reduces as the boat is now doing what the wind wants it to do (going faster) not fighting against it!

And I've been on countelss other boats where the same is true, even more so the racier the design - they literally start surfing as easily as a well, surfboard, cos they have the right underwater profile to achieve it. But once you drop the anchor you find yourself in a regualr cruising boat not a stripped out racer.

Whilst I love the look of traditional design (like classic cars) in my mind there is no going back from today's modern designs. Bring em on.
 
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I think you assume that Moody owners are all very conservative. Its not the case.


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I'm not sure what you mean, but if it is that some Moody owners drive their boats very hard and don't just bumble around then I agree, but that wasn't what I was saying.

My point was that in order to break out of the hullspeed limitations requires a lot more power because you are now in the 'hockey stick' part of the power curve. Moodys and most other cruising boats would find it very hard to find enough useable sailpower to beat the limits and yet still be controllable. Firstly you would also need very flat water, not generally the case if the wind is enough to do the deed. Secondly the probability is that you would need to apply a lot of helm in order to keep pointing in the right direction, more rudder = more brake and slows the boat. Off the wind with less heeling force enough sail and enough wind then maybe - occasionally.

That it could very occasionally happen, in very flat water and for very short times is of course possible, but to claim 2kts plus over normal hullspeed in normal conditions and on a close reach is not credible, otherwise the race boys might consider trading boats!
 
The Volvo boats are all planing hulls so hull speed doesn't come into it. The same is true for multihulls and very fine monos. If you don't generate the bow/stern waves they can't slow you down.

I'm always sceptical of claims of very high speeds in non-racing boats. If all the stories I've heard were true I'd have been passed by every boat afloat but I haven't seen it happen yet!
 
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