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 Post subject: Boat speed calculations
PostPosted: Tue Nov 11, 2008 2:43 pm 
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Location: Scotland east coast
Just in case my legs develop into those of an athlete and not wanting to waste energy or damage the hull peddling too fast when the wind drops, I tried to calculate the maximum boat speed of the AI. The methods of calculations I found on the www did not seem to compute correctly with the speeds that I have read the AI is capable of being peddled which is probably due to me feeding in the wrong information. One of them did show about the maximum speed I managed to peddle with the sail fully wound in. I am not going to embarrass myself by saying what that figure was but was wondering if any experts (self affirmed is OK) have made any calculations. Sailing wise I have been out twice (only took delivery two weeks ago) and my top speed has been 9.3 mph. I was too exited to remember to change the gps to knots or unclip the rudder shock cord or put my gloves on, the weather forcast said the wind was 17mps, it was probably gusting 20.


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PostPosted: Tue Nov 11, 2008 3:01 pm 
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Location: Florida
Northsea

I posted some time back on some speed numbers using trip logs, GPS and a spreadsheet

http://www.hobiecat.com/community/viewtopic.php?t=7197


The Island's max top speed seems to be about 60% of the max wind speed as a rough guide.

So 9.2 mph in 17mph is certainly in that ballpark. Those bursts of speed may be infrequent and your average overall trip speed will be less. On a 2 hour trip an average speed of 6 -7 mph in 12-15mph wind is doable.


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 Post subject: boat speed
PostPosted: Thu Nov 13, 2008 10:29 am 
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Thanks for that Yakaholic, I did read that post earlier on in the year. What I am interested in is the mathematical calculation that is done to measure maximum boat speed. With my Trailer Sailer I had to input the length on the waterline the breadth and the weight, it calculated to 6.75 knots which was quite accurate when using the outboard.
I could get to 6.75 knots on half throttle, full throttle just pulled the stern down without any more forward movement. When sailing I was able to hit about 8.5 knots (I never got to go out in anything over 25).
We've got 30 mph winds predicted this weekend, I'm looking forward to sailing the AI in it. I will be on a small loch though so the chop should not be too bad.


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PostPosted: Thu Nov 13, 2008 3:57 pm 
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Location: Wilmington, North Carolina
Sweet. Take some pics or video and tell us how she does

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 Post subject:
PostPosted: Thu Nov 13, 2008 6:20 pm 
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Location: Punta Gorda, FL
Yakaholic wrote:
On a 2 hour trip an average speed of 6 -7 mph in 12-15mph wind is doable.


That's true as long as your destination is not directly downwind, or worse, directly upwind.

In 12-15 downwind I've found that my AI goes about 4-5 knots. Turn to a bit of a reach, it will go 6 or 7. I got to sail downwind in some very strong winds from hurricane Fay last summer, and it's hard to say how fast I was going because my speed varied a lot as I crashed through waves. I'd guess an average of 7, only because some nosedives through waves really slowed me down.

If I really want to make progress upwind, I'll sail as close as possible to the wind and also pedal. Boat speed will be around 4, but speed made good toward the actual destination is a little under 3.

I'd say 12-15 is the very worst wind speed range if you want to go directly upwind. If it is any lighter than 12, you can get there faster by just furling the sail and pedaling directly upwind. If the winds stay above 15, you can pedal/sail upwind at 4-5 knots with pretty good tacking angles. I furl the sail as needed to keep from completely burying an ama.


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PostPosted: Thu Nov 13, 2008 8:09 pm 
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If you have a 'trailer sailer' or any boat which does not get up on plane...the calculations are for 'displacement hull speed'. The boat hull pushes its way through the water generating waves. So the boat is restricted to not being able to travel faster than the waves it is shedding.
The calculations thus are for predicting the speed of the waves being generated.
However, if the boat is light enough/has a plaining hull/has enough power, the hull will rise upward and skim the water surface. The hull essentially rides on top of the water surface and thus is not restricted to the displacement hull maximum speed. As long as the hull is planing...there is no real top speed limit (except the boat's structural strength).
You might notice on a speed boat, that as you first accelerate, you have to keep opening the throttle...once the hull is on plane you can usually throttle back and still keep your speed up. The same phenomenon will occur when you are sailing an AI. The hull at lower speeds will have to push water out of the way...but once the wind speed is high enough to accelerate the kayak onto plane...you will experience a 'spurt' of additional speed because less energy is then needed to keep the hull moving.
My 26 ft trailer sailer will plane at about 12 knots...but the acceleration then scares the pants off of me as she shoots forward...since the hull and steering are not really designed for planing.


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 Post subject:
PostPosted: Thu Nov 13, 2008 8:39 pm 
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Location: Florida
Tom Ray wrote:
Yakaholic wrote:
On a 2 hour trip an average speed of 6 -7 mph in 12-15mph wind is doable.


That's true as long as your destination is not directly downwind, or worse, directly upwind.

.


Yes, correct.

You need favorable conditions. Sailing in the Florida ICW is mostly a north or south sail in the "ditch". With a steady 12-15mph east wind you get to make a medium reach and have little to no chop to slow you down. Add to that a favorable tide and your golden.

My best average, for longest duration, with those conditions was a 3.5 hour trip avrg. 6.2mph; and that includes going under a bridge and dealing with occasional boat traffic.

I managed a 40mile sail in one day and had 3 hours of daylight still left. Could easily have pushed it another 15 miles but was tired and hungry :wink:

Pointing upwind in the Island is a real art, especially in higher wind and with chop. Definitely points higher/better with a flatter sail, which means full sail, otherwise the battens curve the sail out as they wind onto the mast.

Sail trim, daggar board angle & use, pedal use, weight and weight distribution plus all the tide, water & weather factors all affect speed.


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 Post subject:
PostPosted: Fri Nov 14, 2008 6:01 am 
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This is a very interesting topic. So when do you think the AI starts to plane then?? And do you feel the acceleration difference?

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PostPosted: Fri Nov 14, 2008 7:35 am 
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Location: Scotland east coast
TIDALWAVE wrote:
If you have a 'trailer sailer' or any boat which does not get up on plane...the calculations are for 'displacement hull speed'. The boat hull pushes its way through the water generating waves. So the boat is restricted to not being able to travel faster than the waves it is shedding.
The calculations thus are for predicting the speed of the waves being generated.
However, if the boat is light enough/has a plaining hull/has enough power, the hull will rise upward and skim the water surface. The hull essentially rides on top of the water surface and thus is not restricted to the displacement hull maximum speed. As long as the hull is planing...there is no real top speed limit (except the boat's structural strength).
You might notice on a speed boat, that as you first accelerate, you have to keep opening the throttle...once the hull is on plane you can usually throttle back and still keep your speed up. The same phenomenon will occur when you are sailing an AI. The hull at lower speeds will have to push water out of the way...but once the wind speed is high enough to accelerate the kayak onto plane...you will experience a 'spurt' of additional speed because less energy is then needed to keep the hull moving.
My 26 ft trailer sailer will plane at about 12 knots...but the acceleration then scares the pants off of me as she shoots forward...since the hull and steering are not really designed for planing.

That makes sense and gives a good reason to ease of the sheet or reduce sail when the amas goes deep.


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 Post subject:
PostPosted: Sun Nov 16, 2008 6:03 pm 
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Location: Punta Gorda, FL
An AI running at the top end non-surfing speeds of 8 to 10 knots is not really planing, at least all of it is not. By "planing" I mean a boat riding over its own bow wave and skimming across the top of the water. That's as opposed to a boat that is down in the water and is driving through it, not over it.

The downwind ama is going through the water, not over it, when the AI is running at speed in flat water. It is going significantly faster than the standard hull speed formula would indicate as hull speed.

I have only felt that the main hull and one ama "planed" very briefly, when surfing. Actually, I felt more like I fell into the next wave. The waves were pretty large, steep, and close together, with cresting tops. Really just extreme chop. It was fun, but I'd like to try surfing the AI on large, smooth ocean swells.


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 Post subject: Planing or not?
PostPosted: Sun Nov 16, 2008 9:07 pm 
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Location: CLEARWATER, MN
If the AI is traveling faster than a displacement hull/wave calculation would predict, the boat is probably planing even if it doesn't appear to be.
The displacement calculations have been proven quite accurate for many years.
I think what makes it very confusing, at higher wind speeds, is that one hull (the lee ama) may be submerged enough to act as a smaller displacement hull while the windward ama is usually suspended (not generating any wave resistance) and the kayak hull is probably trying to ride on top of its bow wave.

I wonder if Hobie ever tried to use different cross-sections for the amas.
A horizontally flattened (oval) ama would generate more lift and would
keep the ama from digging as deep. This would allow the ama to 'plane' more easily at higher speeds. It would also produce more bouyancy on the ama at a more gentle heeling angle. But this shape would produce more bending of the ama as it rode over short waves and also not provide as much lateral resistance (as the ama acts as an additional keel)

The original amas were tree trunks of circular cross-section...but with modern materials, Hobie could design any cross section they wanted.


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 Post subject:
PostPosted: Mon Nov 17, 2008 3:35 am 
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In reply to speeds reached by a adventure island.
It was a cold morning I had planned for my maiden voyage.I had picked up my new toy on thursday at Mona Vale. Northern beaches Sydney.. or gods country as the locals know it by. Saturday had come and it was time to wet my new red Hobie Island.
Sydney harbour had a grey green look , the high winds warning for enclosed waters, added to the heady mix of fear and there was no turning back as I wheeled my machine towards the abiss of the local wharf.
Before I knew what was happing I was pulling out of the sheltered cove soon reaching along in front of a building sountherly forecast to exceed 30 knots.
I reefed the main and refected on my reading of the manual.
I had been on these waters before in similar conditions , but never this wet. I was disappointed with the GPS at 7-8 knots max, with gritted teeth
I went for broke and hammered my turbo fins, as the bow rose the GPS hit 13 something and I did not want to go any faster.I made it back cold tired and safe, and saw no other boat under sail that morning on Sydney Harbour............. Thankyou Hobie.
[/img]


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PostPosted: Mon Nov 17, 2008 8:58 am 
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I ran some additional calculations for the 'planing speed' for the AI:
Hull Speed = 1.34xsquare root of Length at Waterline
If you could keep both amas out of the water (improbable)...the max hull speed for the AI is about 5.2-5.4 knots (6-6.2mph).
So any faster and the AI main hull is planing!
If the amas are 8.5 ft long their 'hull speed' is only 3.9 knots (4.5mph)
The width of the hull determines only how high the 'bow wave' will be produced over which the planing hull has to climb (more power required), not a change in velocity.
So thanks to Yakaholic (see his wind speed/boat speed determinations above): for wind speeds above above 7 mph the amas should be planing and for wind speeds above 9 mph the main hull should be planing). One caveat to Yakaholics calculations is the total weight including the operator. The heavier the total boat weight the taller the bow wave and thus a faster wind would be required to get the AI up onto plane.

Assuming a 200lb person is aboard, some other interesting results: Displacement to LWL (42); Sail area to displacement (19.9); LWL/Beam (7.5); Motion Comfort (12.6) [very 'bouncy']; Capsize Ratio (1.18) not calculated for the support from the amas and suprisingly good for just
the main hull, values >2 are bad; Pounds Per Inch (104 lbs/inch) how far the hull will settle down with additional weight.

Most marine hull calculations are for monohulls or cats where the hulls
tend to be equal in dimensions. Trimarans with smaller amas are more difficult to calculate. Most designers would prefer to construct a model of the Tri and run it in a wave tank to extrapolate real numbers for the vessel.

So my conclusions are that whenever the AI is running above 6 mph, you are probably planing, even tho' it doesn't look like it!


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 Post subject:
PostPosted: Mon Nov 17, 2008 4:28 pm 
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Exceeding hull speed and planing are not the same thing. Many sailboats and low-speed trawlers routinely exceed hull speed. Displacement boats require lots of power to exceed hull speed. Planing boats require lots more power to actually ride up over the bow wave, instead of just climbing it.

My 15' Boston Whaler probably has a hull speed of 5 knots, and above that it is squatting down and shoving its bow wave aside, making a huge wake, requiring lots of power.

It doesn't actually plane until almost 15 knots, and at that point the bow comes down and it overruns the bow wave and starts skimming the surface. The resulting wake is much smaller.

I have only felt the AI do something like that on the one occasion I mentioned. I have felt one other kayak do it: my Klepper foldable, in the surf. I think surf-sailing the AI in big, smooth ocean swells could result in planing at speeds in the teens. Maybe reconion can tell us.... ;)


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 Post subject:
PostPosted: Mon Nov 17, 2008 11:24 pm 
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I'm certainly no expert in such matters, but it is my understanding that this speed to length ratio (commonly 1.34 x the square root of the LWL) is attributable to the very talented William Froude for his work with the British Navy and their heavy displacement steam powered warships during the 19th century. This was not for the purpose of calculating "maximum" speeds, but determining efficient operating speeds.

Froude's Law does not apply to light or semi-displacement hulls, narrow catamaran hulls, outriggers, etc. As we've seen first hand, that would include most kayaks, including the AI.

Other important speed determining ratios are displacement to length, beam to length, power to displacement, among others. There is no one ratio that adequately predicts "hull speed" for all or even most boats.

Some boats will begin to plane at a speed to length ratio of 2 or higher, but again, displacement and hull shape matter.

IMO, the AI is occasionally a surfer, but very rarely a planer and Mr. Froude's law is greatly misunderstood. 8)


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