your boat, and how fast she goes

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benlui

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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.............
 
Atlanta Viking 28
beam reach 7.4knts (hull speed 7.5knts)
motoring 5knts+ 20hp bukh.
 
Mcgruer designed 8 metre CR - vastly overweight with steel hull and cruising gear. (30ft waterline - 42ft overall)

Sail, to windward calm sea + 1 reef down 6.9 knots.
broad reaching average of between 8 to 9 knots recorded over an hour.
Engine max a bit over 7 knots (never used flat out for any length of time)

>>>>
Trident Voyager 35. (only one seasons use by me so still gathering data)
Sail, reaching 7 and a bit knots.
Engine, probably 7 + knots but never run an engine flat out.

These speeds are sustainable - ie obtained from difference between log readings at hourly intervals, not instant speedo readings with the needle at maximum swing.
 
Jeanneau Sunmagic 44

Beam reach 9.5kts
fastest 13.9 kts on GPS 54 kts wind speed south of Amorgos
1st reef about 20 kts When kids were little first reef was at 16 kts
Moto Yanmar 55. 7.5kt at 2700 rpm 8.5kt at 3500rpm

No tide. no waves under 16 kts wind speed in med
 
Last edited:
benlui said:
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.............
Click to expand...

This is in many ways a non-question.

Maximum speed of a displacement monohull is governed primarily by its waterline length. Maximum "displacement" speed is the point at which the power required increases rapidly. The formula Speed = 1.4* sq root LWL is generally accepted as a good estimate.

So for your boat with a LWL of approx 22ft maximum speed is 6.56 knots. It may be possible to get near to 7 , but 8 knots is physically unobtainable unless you are surfing on a wave. You should be able to reach your maximum speed under both sail and power. However, it is surprising how little power is required to achieve that. Assuming you boat weighs about 3 tons (probably an overestimate) you will easily reach the speed with your 21 hp engine. However, you can also get almost that speed (6.3) with the next size down engine (14hp) - assuming you have the correct size prop.

The only way to go faster than 6.6 is to get a bigger boat. So my Bavaria 37 with a LWL of just over 29ft has a maximum of 7.59 which it easily achieves with (only) 29hp. Still way less than your estimate of 8 knots!

The formula for calculating LWL for a given speed is (speed/1.4)^2. so 8 knots requires a LWL of 32.65 or a LOA for cruising boats with "ends" of about38/40ft. jonic's Moody 38 is a good example.

Two other observations. The factor of 1.4 is an estimate, but is well supported empirically. Do not underestimate the increase in power required to get that last knot or so up to maximum speed. Under motor typically a 50% increase in power.

Suggest you google hull speed and get the longer technical explanations if you are interested.

BTW Maximum speed is not necessarily a good guide to passage times, although generally the larger the boat (and therefore greater theoretical speed) the shorter the passage times.
 
Tranona said:
This is in many ways a non-question.

Maximum speed of a displacement monohull is governed primarily by its waterline length. Maximum "displacement" speed is the point at which the power required increases rapidly. The formula Speed = 1.4* sq root LWL is generally accepted as a good estimate.
Click to expand...


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

Any ideas please.

Mike.
Click to expand...

Google catamaran hull speed and you will find more than enough to keep you out of the pub for the rest of the day!
 
Dehler 35CWS. Fast hull, but a bit overweight and with cruising sails.

Beam reach in best conditions = about 8kt (surfing = considerably more)

30hp engine, 3-blade folding prop, flat out = about 7 kt
 
Westerly Konsort-

Beam reach F4-5= 6-7.5 knots. Reefed main crossing Swansea bay in a gale 8, surfing we had 9.5 knots!!

Volvo Penta 25hp cruising speed around 6 knots. Pushed to around 3000RPM we have had 8 knots SOG.

Ian.
 
Tranona said:
This is in many ways a non-question.

Maximum speed of a displacement monohull is governed primarily by its waterline length. Maximum "displacement" speed is the point at which the power required increases rapidly. The formula Speed = 1.4* sq root LWL is generally accepted as a good estimate.
Click to expand...

This is all true but one thing that nobody seems to mention on this thread is how the LWL changes when a boat is underway. Mine is 28' LWL for example on a 34' LOA. The boat will easily exceed the formula speed on 28' under sail. This is because like many other yachts, particularly of her era, rule bending was the name of the game when it was designed. A photo of the boat sailing from the side will soon make it obvious, that as soon as the thing has got a bone in its teeth, the standing wave meets the bottom tip of the retrousse stern with a spooky precision adding about 3 feet to the LWL in my case.


I do however feel that the OPs log must be over reading as the figures seem rather high for a 28 footer. I would be happy with those speeds on a 34 footer.

Of course the other question here is how the speeds of all our boats' are effected by different sea conditions because in real terms thats going to be the crux of the argument. Racing or cruising.

especially with the summers we are having!
 
contessaman said:
This is all true but one thing that nobody seems to mention on this thread is how the LWL changes when a boat is underway. Mine is 28' LWL for example on a 34' LOA. The boat will easily exceed the formula speed on 28' under sail. This is because like many other yachts, particularly of her era, rule bending was the name of the game when it was designed. A photo of the boat sailing from the side will soon make it obvious, that as soon as the thing has got a bone in its teeth, the standing wave meets the bottom tip of the retrousse stern with a spooky precision adding about 3 feet to the LWL in my case.
Click to expand...

That is of course correct - however the difference between 28 and 31 ft LWL is less than .5 knot. On the other hand if the boat has a high SA/Disp ratio the extra power may have an impact.
 
Tranona said:
That is of course correct - however the difference between 28 and 31 ft LWL is less than .5 knot. On the other hand if the boat has a high SA/Disp ratio the extra power may have an impact.
Click to expand...

When i bought the Sunmagic. I comment to the previous owner who used to race her about the height of the mast compaired to a Benny44 mast . His answer was it not the size of the mast, It`s the size of the sails you can carry. This has proven true over the 15years of ownership
 
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