Boat speed under sail, what should I expect?

[PeterWright]

Performance multihulls are very light for their waterline length and their beam enables them to carry a large sail area for their weght. The result is easy planing, which is the precondition of exceeding hull speed. In my student days, we used 1.4 x sqrt(LWL) as the hull speed, if I remember fluid dynamics correctly and my memorties of 45 years ago are still pretty good - just have trouble remembering what happened yesterday.

Peter

 

[TimBennet]

How can one explain the speed of the MacGregor 65 and 70? Double figured multi-hull speeds are reported all the time with research on said model. :ambivalence:

Ultralight displacement, narrow beam gives them a very narrow angle of entry and a flat run to their buttocks. They therefore don't obey the rules of pure, displacement boats.

The behaviour of boats between pure displacement and pure planing are some of the most complex and least understood hull types.

 

[Spyro]

How can one explain the speed of the MacGregor 65 and 70? Double figured multi-hull speeds are reported all the time with research on said model. :ambivalence:

I would think using 1.34 in the formula would give those hulls a max speed into double figs. Add a bit on for surfing and Bob's your uncle.

 

[grumpy_o_g]

Ultralight displacement, narrow beam gives them a very narrow angle of entry and a flat run to their buttocks. They therefore don't obey the rules of pure, displacement boats.

The behaviour of boats between pure displacement and pure planing are some of the most complex and least understood hull types.

Edit: Just read the post and realised I'd dropped into pompous lecture mode - not aimed at yourself or meant to be patronising but it took ages to type so I'm leaving it there...

I'll modify that statement slightly and say they don't obey the rules of the shorter and wider hulls on almost all monohulls. I know a lot of it is about the fineness ratio but each hull on a multihull behaves just like the hull on a monohull. I do agree about the lack of understanding of the transition from displacement to planing though. Coming from an aerodynamics background I was amazed how little research has been done into it and the gap in knowledge.

A narrow entry and a flat run aft sounds more like a planing hull in some ways and the big trick of a multihull is that it can reach high speed displacement speeds and still use "displacement" levels of power to drive it - it doesn't need to get on the plane and the designs for high displacement speeds and for planing tend to be contradictory. Put that flat planing area aft and you won't get the fineness ratio that is one of the most important things nor the lowest wetted area possible. That doesn't matter up on the plane as some of the hull is out of the water anyway.

For displacement the longer and thinner the hull the better, right up to the point when lack of buoyancy, too high a wetted area and a couple of other factors take over. A typical displacement hull has to go onto the plane after the wave it generates exceeds the hull length - this is the "climbing onto the bow wave" often wrongly referred to - it's not that you're trying to force the boat uphill at all - the rules of physics mean that a fluid is fairly easy to move around within certain constraints. An analogy is perhaps like trying to make something vibrate outside it's natural resonant frequency - you're making matter behave in a way nature doesn't want you to. It takes huge amounts of energy to make it move outside those constraints because you're now trying to compress the fluid as well as move it (okay, simplistic but effectively the faster you drive a displacement hull the more you compress the fluid it is travelling through).

A long thin hull can actually travel through water at a far greater speed without generating a wavelength less than the length of hull - it's sort of intuitive that the hull will be faster but not for the reasons a lot of people think. The form drag and low frontal area are factors and possibly the the biggest ones on a lot of cruising multihulls (because they never actually go fast enough for the fineness ratio to be the biggest factor) but I remember asking the designer of Icarus ( a foil-equipped Tornado cat around in the '70's) why a catamaran was so much faster. His reply was simply that you'd have to shove a couple of tons of lead on a monohull that thin to stop it falling over on dry land, never mind with a sail on it. What he was saying I think is that much of the speed of multihulls is simply down to weight (or lack of it) for the size of rig. The owner of a fast cruising cat will know full well how much weight can affect their cruising speed. Less weight puts less hull in the water and allows the boat to accelerate much faster.

 

[Buck Turgidson]

Displacement hulls suffer a huge increase in drag when they hit hull speed. Hull speed is defined by the waterline length and is derived from the formula for waves:
L = 2*Pi*V^2/g
Where:
Pi = 3.14159
V = wave speed in feet/second
g = acceleration of gravity = 32.174 feet/second^2

if you do the maths it equates to 1.34 *sqr lwl

However, this is the speed above which the hull has to climb it's bow wave. That just requires power.

All of that goes out of the window once you consider semi planing and planing hulls.

As someone already posted, those little 21' mini 650's which have displacement length ratios around 110 and plane at the drop of a hat get up to mid teens speed regularly.

 

[lpdsn]

I guess the simple answer is that it depends upon the 27 footer.

I find this website handy for looking for polars. http://www.blur.se/boats/

 

[Doineann]

Is it correct that the standard formula for hull speed applies only on flat water, a situation in practice that would not happen?

What is the effect of sea state? On the face of it, waves coming at you should reduce the speed, but could they disrupt the bow wave build up, or do they have no effect at all?

 

[Sailfree]

The true speed a boat sails at is the one used for passage planning!

On my 36 Dufour it was 5.5 kts but only 2kts against a headwind.

I then bought a 38 Dufour and learnt to sail only with the wind ( gentlemen don't beat) and passage planning was 6 kts.

On the 43 Jeanneau it is about 6.5kts.

For these speeds I am assuming the 60ml to Cherbourg etc not the spot speed you get for 15 min with favourable conditions but then I am probably a **** sailor in spite of 20 yrs racing a Wayfarer.

 

[BrianH]

I have hit 9.1 knots through-the-water speed in a 27.3' Trapper 500. Once. With small jib only and 40 knots of wind and surfing on a wave-crest.

I was beating north-west against 18 knots of north-easterly wind, which rapidly ramped up to double that with over 40 knot gusts. I dropped the main and my planned destination, went onto a reciprocal course and broad-reached downwind for shelter in the Limski Kanal, Istria, some 5nm distant.

 

[Birdseye]

I sail a awb of 40 feet. I usually get her up to seven knots quite easily say in a F4/5 and I'm chuffed when I get over eight knots, top speed so far is 8.7 knots, this is boat speed using the log not gps.
I was recently talking with someone who sails a 27 ft boat and said that he regularly gets over nine knots. I suggested that that must include tide, but he said no, that was from his log.
Now either I'm doing something wrong or he needs to recalibrate his log?
What sort of speeds should I expect?

Dont know about what speeds you could expect but I do know you can expect a lot of wishfull thinking in replies to your post.

 

[doug748]

The true speed a boat sails at is the one used for passage planning!

On my 36 Dufour it was 5.5 kts but only 2kts against a headwind.

I then bought a 38 Dufour and learnt to sail only with the wind ( gentlemen don't beat) and passage planning was 6 kts.

On the 43 Jeanneau it is about 6.5kts.

For these speeds I am assuming the 60ml to Cherbourg etc not the spot speed you get for 15 min with favourable conditions but then I am probably a **** sailor in spite of 20 yrs racing a Wayfarer.

I like the point, I was mulling things over in a similar manner.

When I shifted from a 26foot boat to over 30 my average speed increased from something under 5kts to something over 5kts. I always think it rather depends on how willing you are to start the engine.

No matter how eye catching the best speed is, the average always seems a bit deflating.

Running, in waves in open sea conditions, you can clock up some momentary eye openers. 11.5kts is my best, scored twice, corrected I guess it may have been towards 10. Wind assisted gravity is the explanation, a great relief when it stops.

 

[Salty John]

I sail a awb of 40 feet. I usually get her up to seven knots quite easily say in a F4/5 and I'm chuffed when I get over eight knots, top speed so far is 8.7 knots, this is boat speed using the log not gps.
I was recently talking with someone who sails a 27 ft boat and said that he regularly gets over nine knots. I suggested that that must include tide, but he said no, that was from his log.
Now either I'm doing something wrong or he needs to recalibrate his log?
What sort of speeds should I expect?

Your figures seem pretty well spot on for your boat.

I blogged on this subject here:http://www.saltyjohntheblog.com/2015/02/displacement-hull-speed-2.html

(I defer to Grumpy o.g's hugely superior knowledge of hydrodynamics and apologise for using 'climb its own bow wave' to describe the moment the displacement hull hits the wall).

 

[BrianH]

From my earlier sailing days - a picture of my 27 ft Trapper running at 7 knots before a strong sirocco in the Adriatic, which is what I would expect as a maximum speed. I know the main is furled but all of the big furling genoa is out and pulling hard.

Point is, unless the 27 ft boat has some sort of planing hull, anything above this sort of speed is very rare in a displacement 27 footer.

 

[Trop Cher]

I have hit 9.1 knots through-the-water speed in a 27.3' Trapper 500. Once. With small jib only and 40 knots of wind and surfing on a wave-crest.

I was beating north-west against 18 knots of north-easterly wind, which rapidly ramped up to double that with over 40 knot gusts. I dropped the main and my planned destination, went onto a reciprocal course and broad-reached downwind for shelter in the Limski Kanal, Istria, some 5nm distant.

Have we met! The other boat in question was a Trapper 500, however he said that he regularly got over nine knots!

Again thanks for replies, it's always reassuring that I'm getting things about right.

 

[BrianH]

Have we met! The other boat in question was a Trapper 500, however he said that he regularly got over nine knots!

No, wasn't me, Guv, honest. I would never make such a silly claim. Even the 7 knots shown in the photo was on a wave-front and surging forward (I timed the photo to catch that point). Once passed and in the trough the speed fell back to just under 6 knots, which must be about the theoretical maximum speed for the waterline length (23ft - 7m).

 

[sailorman]

From my earlier sailing days - a picture of my 27 ft Trapper running at 7 knots before a strong sirocco in the Adriatic, which is what I would expect as a maximum speed. I know the main is furled but all of the big furling genoa is out and pulling hard.

Point is, unless the 27 ft boat has some sort of planing hull, anything above this sort of speed is very rare in a displacement 27 footer.

The T27 is a fine boat & will out sail a 500 on all points of sail

 

[RupertW]

I can't see being able to plan on 9 knots on a 27 footer except for a couple of hundred yards if it surfs but I do know that my 24 footer sails faster than my 42 footer, despite the little one being 35 years old and built like a brick.

We bought the bigger boat for comfort and stability rather than speed and knew that at the time (vital piece of negotiation with partner after some frisky boats which she didn't enjoy). But we are talking about 4-8 knots (once 10) through the water in the 24 footer compared to 3.5-6.5 knots in similar conditions in the bigger boat (once 9 knots).

A slippy little boat can bash upwind faster and closer and surf downwind much more easily than a big tub, so it's the type of boat rather than the size that counts for me. Also it's a doddle getting spinnakers up and down and adjusting reefing for speed not stability in a small boat. It might be that I get the bigger one moving faster in the next year or two but I'm far more likely to just bung the engine on as the wind dies than get up the asymmetric.

 

[jimi]

Interestingly, my previous boat was a Bennie 331 with LWL of 30'6" my current is a Moody S38 with LWL of 32'6".

I passage plan at the same speed (6.5Knts) , the Beneteau off the wind and not bashing waves would comfortably exceed that, hard on the wind and bashing a sea state forward progress could easily slip to 3 to 4 knots.

The Moody, whilst faster is not enormously faster in flat water , but will maintain its speed and comfort much longer whilst bashing a sea state.

 

[markhomer]

Performance multihulls are very light for their waterline length and their beam enables them to carry a large sail area for their weght. The result is easy planing, which is the precondition of exceeding hull speed. In my student days, we used 1.4 x sqrt(LWL) as the hull speed, if I remember fluid dynamics correctly and my memorties of 45 years ago are still pretty good - just have trouble remembering what happened yesterday.

Peter

sorry . most high performance cats don't plane , even ones designed to , like my stealth , however normal rules do not apply when the beam to length ratio gets very small as in a cat , performance cats usually sail on one hull , even with two in the water the overall beam is not a factor other than for sail carrying power , rather the narrow ness of each hull throws the 1.3 or whatever straight out the window .

My ole classic tornado , would actually get pressed down into water even further when doing 20 knots plus .

 

[Bobc]

I sail a awb of 40 feet. I usually get her up to seven knots quite easily say in a F4/5 and I'm chuffed when I get over eight knots, top speed so far is 8.7 knots, this is boat speed using the log not gps.
I was recently talking with someone who sails a 27 ft boat and said that he regularly gets over nine knots. I suggested that that must include tide, but he said no, that was from his log.

Sounds about right to me. You mate's talking shite