your boat, and how fast she goes

[Tranona]

The best way to compare practical performance is to look at racing handicaps, particularly the PY handicaps on the Byron Software site since these cover a wide range of mostly non racing boats. This gets away from the maximum speed arguments into a much more realistic world of average speeds in a mix of weather over all points of sail.

For example your boat has a handicap of 1050. The HR34 mentioned above has a handicap of 926 which means that the HR will on average sail 1050/926 = 13% faster. Interestingly the HR 36 at 976 is slower than the 34.

While PY figures are useful as they are empirically derived, they are a product of the methodology. Their "accuracy" relies on the volume and quality of the data used, the effectiveness of the power producing equipment on the boat and the ability of the crews sailing the boats.

I would guess that the number of HRs for which data is available is very small, so the figure only represents them, not necessarilythe class as a whole, and probably not the real difference between the 34 and 36.

Actual performance of a boat is affected by so many variables which is why people spend lots of money on sails, fancy props, expensive bottom coatings etc. none of which affect the "theoretical" maximum hull speed, but do affect the ability to make use of the potential.

 

[mocruising]

HR 46 22 Tonne LWL 40'

Sail 9.3 FB MAIN 135% GENOA
Engine 100 HP 8.6
Spin 10-10.5

 

[saltwater_gypsy]

These spot speeds are meaningless for cruising.
In good conditions we can do 150miles in 24 hours and that's just fine for most passagemaking.

 

[VO5]

While PY figures are useful as they are empirically derived, they are a product of the methodology. Their "accuracy" relies on the volume and quality of the data used, the effectiveness of the power producing equipment on the boat and the ability of the crews sailing the boats.

I would guess that the number of HRs for which data is available is very small, so the figure only represents them, not necessarilythe class as a whole, and probably not the real difference between the 34 and 36.

Actual performance of a boat is affected by so many variables which is why people spend lots of money on sails, fancy props, expensive bottom coatings etc. none of which affect the "theoretical" maximum hull speed, but do affect the ability to make use of the potential.

Yes, just like baking a cake, isn't it ? (SWIMBO comments.....).

The better the ingredients and dependent also on the skill of the baker the better the result ?

 

[Avocet]

Avocet's a Cutlass 27 (20' LWL) with a traditional long keel.

On a beam reach the theoretical hull speed is about the best I've seen with a clean bottom and no "surfing". That was on Windermere so nice flat water and no tide. About 6 knots on the log, which matched the GPS SOG quite nicely.

Not sure what's the best I've seen on the wind, (but she does point nicely)!

Under power (also on Windermere) about 5.5 knots from a very tired 12hp diesel.

 

[ianj99]

Tranona, I have always heard the same formula, with 1.3 to 1.4 being cited as the constant where the hull has a waterline beam of approx 1/3 of the waterline length. It certainly does seem to be reliable with that configuration.

Do you know please how the constant changes with different waterline beam ratios. I have an old 26ft Heavenly Twin catamaran. The hulls are portly by modern standards, but still a good deal less than 1/3 LWL. I have no idea what speed I should expect. There are many wild stories and people claiming 15kt without surfing. I don't believe them. I have never had a reliable log. But I seem to get 6kt sailing or motoring. (I can ignore close hauled as I sheet very wide to go round a centre doghouse)

6kt ish seems very reasonable to me. However I would like to know the theory for multi-hulls.

Any ideas please.

Mike.

Does not the hull shape at the stern have much to do with it as well?
Eg transom v cruiser, v double ended etc

Those with the least flat area aft having the lowest hull speed, all other factors being equal.
The old ship lifeboats my father has owned (double ended) were always slow and produced a massive bow wave if pushed even a little too hard.

 

[haydude]

What make and model is your boat, and what’s her fastest speed underway both by sail, and by motor?
For Example
Shipman 28
beam reach, 7-8kts
motor, max speed 7-8kts (nanni diesel 21hp)

If that is a 28ft, your figures are wrong. Which begs the question how many of us actually do calibrate their log and maintain it calibrated?

Which is quite difficult around the UK unless behind a lock, because otherwise you would need a verifiable slack tide and a flat sea.

 

[Robin]

If that is a 28ft, your figures are wrong. Which begs the question how many of us actually do calibrate their log and maintain it calibrated?

Which is quite difficult around the UK unless behind a lock, because otherwise you would need a verifiable slack tide and a flat sea.

Quite easy to do using a measured mile or a known distance between two points, with two timed runs, one each direction one with and one against the tide at a time when the tide is not rapidly changing. There is one such measured mile between Anvil Point and S Albans Head on the South Coast. Otherwise check at slack water neaps in a very sheltered bay out of the tide and again take the average of two GPS SOG readings one into one against the tide.

 

[pagoda]

What make and model is your boat, and what’s her fastest speed underway both by sail, and by motor?

For under sail, assume no run of tide, a flat sea, and quote your max speed before you need to take in your first reef.

For Example
Shipman 28
beam reach, 7-8kts
motor, max speed 7-8kts (nanni diesel 21hp)


I can go faster under spinnaker, but lets leave spinnaker out of it for now to keep it simple.
This isn’t a thread for people to boast on who’s got the fastest boat
I’m just interested to see what boats do what.............

Ok.... most of us max out at 5-8 Kts (prior to things getting exciting/scary), off the wind with some surfing sometimes 10-12 Kts SOG - definitely exciting...

Injecting some realism..driving home after a week only on water.... 30 mph in traffic is absolutely terrifying

Graeme

 

[reeac]

calibrating your log

Quite easy to do using a measured mile or a known distance between two points, with two timed runs, one each direction one with and one against the tide at a time when the tide is not rapidly changing. There is one such measured mile between Anvil Point and S Albans Head on the South Coast. Otherwise check at slack water neaps in a very sheltered bay out of the tide and again take the average of two GPS SOG readings one into one against the tide.

Don't bother about measured miles. If you have GPS then read both SOG and log over a short stretch, then turn round and repeat in the opposite direction. Best done under power so that you can keep the speed constant That way you can cancel out any tidal current effect. I won't include the maths here but leave it an an exercise for the student.

 

[jonic]

You might find this site interesting, figures on over 1,00 boats, you can adjust any parameter you like as well.

Have fun...

http://www.image-ination.com/sailcalc.html

That's a useful tool. Gave my Moody 38 as 7.59 knots so in line with my own experience.

 

[Babylon]

Why 'before you need to take in your first reef'?

...quote your max speed before you need to take in your first reef.

My boat (Vancouver 27, heavy displacement cutter-rig) sails faster if more upright (obviously?) and her max recorded speed under sail of 7.1kts (averaging 6.5kts on a fine reach) was achieved on Monday with a clean bottom in winds of 22kts under double-reefed main and stays'l.

 

[bazobeleza]

Methinks Tranona places too much faith in sqrt wll x 1.4 for hull speed, it is used often a genenal rule but there are many more parameters that need o be taken into account.

My old Nauticat 44 has a wll of 39ft a 'theoretical' hull speed by the above of 8.6 knots.

We set off from Liverpool, motor sailing, and arrived in Lagos, Portugal 140 hours later, Average speed 9.5 knots, engine set at between 1300 & 1800 revs

My original Nauticat manual gives 12.5 knots as the speed to be expected at 2850 revs when motoring. (though it goes through 2.5 gph at that rate of revs) and it is a 6 cyl 120hp engine.and beleza is a 20 ton long keel boat

Yes we used the tides, no we didnt get much lift from the portugal current because we were well offshore, we had good weather, but 50/50 wiht wind direction.

So I suggest no one should cast the wl sqrt 1.4 formula in stone.

 

[Tranona]

Methinks Tranona places too much faith in sqrt wll x 1.4 for hull speed, it is used often a genenal rule but there are many more parameters that need o be taken into account.

Never did say that it was anything more than a general rule. It is a very complex subject and you can find out more if you Google hull speed.

If you look at most of the posts here for "normal" auxilliaries such as Jonics Moody 38, my Bavaria 37 and many others, it is pretty robust. For boats such as rbstretch RM and many other semi planing hull forms which are becoming more common, it is not a good guide. That is because the hull form is designed to "break" the barrier, provided enough power is applied. Similarly it is not a good indicator for grossly overpowered boats (said in the nicest possible way) such as yours. Your boat will easily reach its hull speed with less than 50hp. Have a look at the power curves of your engine and you will find that is roughly what it is producing at 1800 rpm. More than doubling it will allow you to overcome the resistance and achieve higher speeds. BTW using passage times as a measure of speed is not a good idea - as others have suggested timed runs against a measured distance or SOG in non-tidal waters are the only accurate measuures.

Remember the maximum displacement speed is the point at which the power required to exceed it rises rapidly. Yachts with planing or semi planing hull forms such as the RM will not achieve the same speed using their engine as they will under sail, simply because the power from sail is higher - look at the SA/Disp ratio for an indication of power. The hull form with wide flat sections aft, high form stability and low CG allow the boat to use the power. The shape is the very opposite of your boat and you only achieve the speed by more than doubling the horsepower actually required to drive the boat at its most efficient speed. It is the same with semi displacement motor boat hull forms. Look how much extra power is required on, say a 30 footer to achieve 12 knots compared with say a long slim displacement motor boat of the same LWL which will achieve approx 7knots with less than 25hp.

As you will discover when you read the more technical explanations, the point at which power required to increase speed rises dramatically varies according to hull form - but ignoring planing hulls which can lift above the water, not by very much. Empirically the 1.4 Sq root LWL in feet is pretty robust. Even if you use a factor of 1.5 the difference on my Bavaria is only 0.5 of a knot. As it happens its power is well matched to the boat and in flat water achieves 7.6 knots at maximum power - exactly what is predicted by the "formula".

 

[Mandarin331]

much slower then an Altanta Viking 28 then

So it would seem.

I'll re-calibrate the log now, watch my next post after we record over 9 knots close hauled in force 2

 

[sailorman]

So it would seem.

I'll re-calibrate the log now, watch my next post after we record over 9 knots close hauled in force 2

Fingers X

 

[rwoofer]

Never did say that it was anything more than a general rule. It is a very complex subject and you can find out more if you Google hull speed.....snip

.....Remember the maximum displacement speed is the point at which the power required to exceed it rises rapidly. Yachts with planing or semi planing hull forms such as the RM will not achieve the same speed using their engine as they will under sail, simply because the power from sail is higher - look at the SA/Disp ratio for an indication of power. The hull form with wide flat sections aft, high form stability and low CG allow the boat to use the power.....snip

Fully in agreement and glad you had the time to explain it!

As mentioned by another poster, Frank Braithwaites book really does go into (too much) detail around using hull design to overcome the bump towards planing. As with a 49er it means you can get to the point of fully planing whilst going upwind.

 

[Mandarin331]

Fingers X

If not I'll recalibrate the log again

 

[PeterR]

If you want to see how fast a wide range of cruiser racers can go see: http://www.blur.se/boats/

These are polar figures which are computed but pretty accurate. A fully crewed stripped out boat might just exceed them by a small percentage. A lightly crewed boat full of cruising kit would probably not achieve them. In 20 knots at a true wind angle of 120 degrees most boats of this type will achieve 1.5 x the dynamic waterline length without planing.

 

[geoffatstanpit]

RNLI Atlantic IRB

I went on a conducted tour of our local RNLI IRB station last evening and took a pic of this RPM/Speed/Fuel rate notice at the helm position.

This is a 20ft(ish) RIB with 2 x 115 hp motors on the back.

What amazed me is the figure for 1500 RPM, both engines, 5.5 knots and 0.8 litres/hour.
My 5hp Merc on a Parker 21 gives less that 5 knots and 1 litre/hour at sensible revs (just under the pain threshold for noise!)

Surely a big rib going at 5.5 knots can't be more efficient than a slippery hull going at hull speed?