True Average Speed

Major_Clanger

Major_Clanger

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Hi all,

I'm trying to work out a realistic average speed for my displacement hulled boat. I'm ignoring theoretical hull speed and my tendency to think she's faster than she is... Boat is a clean Contessa 26 with no prop.

Can anyone lend an opinion please? I'm currently working out a passage plan using 4kts but not sure if this is realistic. Obviously there are many variables involved, but a rough indication of what I can expect would be useful.

Thanks,
 
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I don't have a great belief in rigid passage plans where it matters if your speed is 4 knots or 5 knots.
I tend to look at the 'what ifs' and have flexible plans where if I'm heading West I'll stop in Studland if it's a slow beat or get to Weymouth if it's a nice reach.
4 knots is reasonable for a plan. You might do 5 90% of the time, but does that help? Do you want to run out of time on 10% of passages?
You might do 6 on a beam reach 20% of the time. I would let a plan for 4 knots stop me enjoying that.

Also in a typical day sail, planning 32 miles at 4 knots, and thinking it'll take 8 hours, works fine if you do it at 5 knots, by the time you have got going and spent time parking at the other end.
 
Are we talking SOG or STW, with or without engine?

Being a bit geeky I have a spreadsheet with passage time and distance which builds to give me an average speed. Its that average I use in planing.
 
Sandy said:
Are we talking SOG or STW, with or without engine?

Being a bit geeky I have a spreadsheet with passage time and distance which builds to give me an average speed. Its that average I use in planing.
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Planing, I'd say 10 knots,







Sorry.!
 
Sandy said:
Are we talking SOG or STW, with or without engine?
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OP is talking STW, i.e. boatspeed. Anyone would. The process of passage-planning then takes care of the net tidal effect over the period of time. If he planned on a SOG there'd be nothing for the passage plan to calculate.

Similarly, he's planning a boatspeed using whatever blend of power source he expects to use. It's then up to him to meet that on the day, using sails, engine, oars or facing aft and blowing.

Sandy said:
Being a bit geeky I have a spreadsheet with passage time and distance which builds to give me an average speed. Its that average I use in planing.
Click to expand...

If your boat really does plane, I guess your average speed works out quite high.
 
Major_Clanger said:
Hi all,

I'm trying to work out a realistic average speed for my displacement hulled boat. I'm ignoring theoretical hull speed and my tendency to think she's faster than she is... Boat is a clean Contessa 26 with no prop.

Can anyone lend an opinion please? I'm currently working out a passage plan using 4kts but not sure if this is realistic. Obviously there are many variables involved, but a rough indication of what I can expect would be useful.

Thanks,
Click to expand...

When I plan a round the cans race out in a tidal area I work on 5.5 kn average for the NHC boats ( which includes 26 footers and some bilgies) and 7 for the IRC. Maybe a crewed racing boat racing for just a couple of hours gets pushed harder than you would cruising but |I would have thought you could plan on 5 rather than 4.
 
I see predictive text has struck again!

I'v got to admit I've yet to get the old girl planing... Surfing her can be fun in the right conditions.

Interesting to see that ever other poster looks at boat speed when planning while I use several years average SOG. I wonder if ultimately there is much difference in the figures? Do you take into account a clean hull, new sails, number of crew or amount if beer in the bilges? There are to many variables to get a true figure.
 
I echo much of what lw395 and Sandy both said. It depends on too many variables.

My 27ft long-keeler can, with a cleanish bottom and a decent F3-4 on or just forward of the beam easily rock along at a consistent 6kts. But, like your C26, we're long-keeled and can't maintain decent speeds (or angles) too close to the wind as modern fin-keelers can, nor can we overcome the wave period in a short sea as longer boats can. But, and despite the large wetted area, our narrow-beamed, folkboat-like hulls are no slouches either.

Your absence of a prop will reduce drag, but your lack of an engine will mean that you can't motor-sail to maintain a planned minimum speed - which I'd normally want to be say 5kts for a Channel crossing.

Someone I once knew, who also had a small long-keeler ,reckoned that all boats, whatever their size, average 5kts under sail given long enough at sea.
 
Babylon said:
Someone I once knew, who also had a small long-keeler ,reckoned that all boats, whatever their size, average 5kts under sail given long enough at sea.
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That is a bit of an old wives tale from the days of short waterline lengths. It is true that daily runs used to be in the region of 100-120 miles (4-5 knots) average, but even an old style 35' was constrained by its waterline length, of maybe 26-29' compared with today's typical 33' or even more. So daily runs of 120-150 are much nearer the benchmark now.

Of course in our tidal coastal cruising waters with shorter passages and arguably more windward work, averages both through the water and over the ground are much more variable and often particularly through the water, slower.
 
Sandy said:
Interesting to see that ever other poster looks at boat speed when planning while I use several years average SOG. I wonder if ultimately there is much difference in the figures? Do you take into account a clean hull, new sails, number of crew or amount if beer in the bilges? There are to many variables to get a true figure.
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Depends on the state of the tide at the time you plan to take the voyage. Your average for the same trip, let's say 7 hours, will doubtless end up accurate - but unhelpful on any given day as most people would like to know whether it'll be a 5 hour or a 9 hour trip, which depends on whether you're running with the tide or against it.

Also, unless you're running right with or against the tide, the phase of the tide during the passage time also affects where you'll end up on a given CTS, or to view it the other way, what course to steer in order to end up where you want. So crossing, say, from Poole to Alderney, the same plan at one point in the tidal cycle will push you towards Cherbourg or suck you down the Alderney Race, and at another point will swing you round to the West of the islands.

The point of passage planning is to know these things. Second-guessing what your speed will be over the ground defeats that objective.

As for the factors you list that affect boatspeed, yes, if you think they make a difference, certainly take them into account. Clean hull might make 0.5 - 1 knot difference compared with a season's growth; new sails - compared with shagged ones - might do at least the same, depending on wind conditions and point of sail; your crew and victualling weight probably less so, but depends on the size of the boat. Only you can make a sensible guess.
 
Babylon said:
Someone I once knew, who also had a small long-keeler ,reckoned that all boats, whatever their size, average 5kts under sail given long enough at sea.
Click to expand...

Or a little less on ocean crossings, interesting how close 100Nm days average out to reality over long crossings with a some good days then maybe a day or 2 becalmed. Not just me either, have a few beers in the major cruising crossroads bars and you'll hear similar averages, but not from those with monster fuel tanks and in a mad rush ;)
 
BelleSerene said:
As for the factors you list that affect boatspeed, yes, if you think they make a difference, certainly take them into account. Clean hull might make 0.5 - 1 knot difference compared with a season's growth; new sails - compared with shagged ones - might do at least the same, depending on wind conditions and point of sail; your crew and victualling weight probably less so, but depends on the size of the boat. Only you can make a sensible guess.
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I does often amuse me that the often quoted speed will be 4kts, then people say subtract up to 1kt for growth, sails, gear, prop.

So on that basis a boat in the water all season, with ancient sails, well laiden and a non folding prop will go nowhere!
 
ShinyShoe said:
I does often amuse me that the often quoted speed will be 4kts, then people say subtract up to 1kt for growth, sails, gear, prop.

So on that basis a boat in the water all season, with ancient sails, well laiden and a non folding prop will go nowhere!
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Come October it can very often feel like that. It can be disheartening.
 
Uricanejack said:
I use 5 knots.
Why? its easy.
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Reminds me of a chum I used to sail with. He used to plan for 4 knots (IIRC), and measure the course in a straight line, departure to destination.

It was true that the boat would do 4 knots, even a bit more, but it was a heavy old girl, a bit under-canvassed, and this speed would only be achieved in F4 upwards, and we didn't go out if there was F6 or more in the forecast. It would also only make this speed with the wind between a close reach and a broad reach, and if there wasn't much of a sea running.

We did quite a bit of motoring to stick to our schedule!:D
 
I can always start the motor and use a rumb line . If I have to. I generally try and avoid having to.
I tend to go with the flow and get there when I get there.
If it's not working I sometimes change there to somewhere else.
A typical passage plan
Set of after lunch
Hope to arrive somewhere in time for last orders.
Last passage wasn't working according to plan
Phoned our order in just before last orders.
Bacon cheese burger and an amber ale

It always good to have a contingency
 
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