Cruising speed

[DAKA]

The last three boats of mine have all being well capable of cruising in excess of 25kts but I can almost count on one hand how many times I have managed to average 25knts over 100 nm or so.

Sea state has almost always been the determining factor not engine power.

Is it because my crew are wimps or do I compromise too much Hull design for space ?

Who actually manages to cruise consistently above 25knts and what type of hull allows this ?




( I am asking because I still can't make my mind up weather I should add an extra inch of pitch to the props that should give a theoretical cruise speed of 27 knts, but swmbo/seastate will probably hold me back anyway )

 

[hlb]

Think 18-20 knots is quite fast enough in most seas.

Doubt if any boats going to be comfortable above that for any length of time.

 

[DAKA]

That agrees with my sensible side that says 'if it ain't broke don't fix it' /forums/images/graemlins/smile.gif

On the other hand speed is vital for photo shoots and dodging thunder clouds on the Radar /forums/images/graemlins/confused.gif

 

[DAVIDO]

Tend to cruise at 20-22knots, boat and engines seem comfortable at that speed and its rare you have to slow down.
Its plenty fast enough anyway its more about the journey than getting there IMHO.

 

[Channel Ribs]

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Think 18-20 knots is quite fast enough in most seas.

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I agree wholeheartely, you would need a fairly special/thirsty boat to remain steady at the kind of speeds you are after.

 

[hlb]

I agree wholeheartely, you would need a fairly special/thirsty boat to remain steady at the kind of speeds you are after.

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[DAKA]

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you would need a fairly special/thirsty boat to remain steady at the kind of speeds you are after.

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I am trying to do it with my current boat without going to too much expense ,(Prop re-pitch £300 ish.)

But I have found for speeds above 22 -24 knts the sea has to be flat calm and I only manage to find days like this once a season.

Some boats claim cruising speeds of 30 knts and I am just wandering how often anyone actually manages that for any duration.

edit

When we came back from CI this year we were pursued all the way back with cracks of thunder/lightning and constant VHF warnings of 50 knot squalls in our area.
We were in calm sunny waters and I felt the need to waste a little fuel in order to keep ahead of the lightning.
But was that a one off situation to be ignored ?

 

[D3B]

with calmish sea's I can do 28-30 knots and very comfortably /forums/images/graemlins/cool.gif /forums/images/graemlins/cool.gif...doesnt happen very often as watching the fuel guages is scary /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif

Over to Belgium this year was 22 kts average. but we stopped once for a comfort break for SWMBO, once for lunch and again because some bl@@dy great ship that was in the shipping lane we wanted to cross.
without the stops would have been much quicker /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

funny when i was ordering the boat i got flamed on here becasue i said i couldnt see the point of the extra cost of engines in a boat to make it go over 35 kts. i said you didnt get the weather frequently enough to justify the extra cost. /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

 

[ians]

Opinion from the sports boat side similar to Dougie realy
We generally cruise at 25/26 knots, nice quiet comfy speed with reasonable economy. engines purring at 3200rpm

ian

 

[A_8]

This summer have been pretty good in this respect, we've been able to cruise comfortable at 26-27 knots, 3400 rpm on several occations. Mostly its around 22-24 knots and when the weather gets bad(ish) I go down to 18 knots for added comfort.

 

[gjgm]

from the petrol (eek) sportside, I d say 3-3500 revs is about optimum between fun and money, which is about 30knts. A quick glance at the state of wifey usually indicates if thats a bit too bouncy, but its got to be getting a bit crappy for everyone before below 25.

 

[duncan]

as many water skiiers will know there is a speed at which water seems to go rather 'solid' - around 21 knots.

below this speed the water gives to the hull - much above and the hull moves instead (well has big forces played on it!).

if it's flat calm it doesn't matter, after that fine entry points, soft riding hulls etc etc etc all try and minimise the impacts but at the end of the day sub 22 knots will be more comfortable.......so, as people are creatures of habit, most plan around the likely and set the throttles for 'normal cruise' at around 18-20; unless they have bigger boats that can ignore these inconsequential forces!

 

[Nautical]

An interesting point indeed. For ages now I have been saying to those powers that be quoting cruising speed based on engine performance is pretty much a waste of time, we can all make a boat that will do 40 knts just make the bottom like a tea tray and bobs your uncle looks great on the sales blurb but pretty useless info. What counts is a boat that can maintain a steady 22 - 24 knts in a moderate sea, to be honest I would be happy if top end was no more than 30 knts but comfortable cruising at 24 knts was a achievable in most sea states up to f4/5 excluding of course those boats that are more performance orientated and peeps accept the accomodation limitations that brings.

We just introduced a triple engined 56 boat that does 40knts but in all honesty it is not something we see as a popular seller not because of the boat itself it is a fantastic piece of engineering but the real world of family cruising doesn't require a 40 knt boat hence all the 56's for production next year are the twin version which still does 34 knts tops but cruises beautifully at around 24 knts and very efficiently too with only twin 435hp engines, while the triple is a fantastic tool to demonstrate what we can achieve and really attracts attention (and scares the bejesus out of a few high performance boats) it is not intended as a volume seller because the real world of cruising dictates that 40 knts is really a non starter, except perhaps in something like the Sogica 55 or similar but they are mega expensive bits of kit for their relative size and need a minimum of twin 1000 hp engines so at 24 knts they are hardly working.

 

[KevB]

Personally, I think if you can cruise happily at 25 knots that's more than enough. We have a top speed of around 38 knots but rarely cruise anywhere at more than 25. Having a blast of 30 odd knots every now and again is fun but a bit too hectic for general cruising.

About three years ago we did non stop from St Helier, Jersey to Chi marina in 4.45 mins, 4 hours to the bar with an average speed of just over 30 knots. It was early morning in August with a slight mist, the water was like glass. Lovely.

 

[bradtarga34]

Whenever we can we tend to cruise around the 25-28 knot mark, which is a nice comfy and economical cruise speed on our boat. We would probably run a wee bit faster but as we normally cruise in company we normally hang back as they aren't as comfortable at the faster speed, especially if it's a bit choppy. Makes the boring run from our base through the Thames and out to Ramsgate go a lot quicker so we can get over to the continent that bit earlier.

 

[DAKA]

You really have hit the nail on the head with your post Kev in what I would like to achieve.

In reverse the opportunity to get to St Helier before the crew get fidgety and we end up in Cherbourg or Alderney !

I am unfortunately aware these 'perfect' days are few and far between and are usually accompanied by threat of Fog or Thunder storms but the point is there are days when 25-30 knots cruise should be available.

I am surprised with the overwhelming opinion ( including expert thanks Nautical ) that 24 knts is max. in practical terms but agree this has also been my experience to date.

 

[DAKA]

I hadn't previously thought of water going solid but it makes sense, do you think the skiers 21knts in increased to 24 knts for the heavier boat and then it is like trying to break the sound barrier ?
It could also explain why the antifoul starts to wear off at speed.

 

[Channel Ribs]

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What?

 

[duncan]

antifoul first - that's the easy one 'cos it's wear will be a direct function of drag which has about a cube relation to speed........ie as you go a little faster the drag goes up a hell of a lot!

re 'hard water' it's going to be related to the properties of liguids and from there their interaction with solids - overall the footprint presented to the water of a monoski is not hugely disimilar to a fast moving 25ft boat; the ski is smaller but concave. Only a few ribs I know have such fine entry points and steep deadrises that they can 'cut' water at high speed.

bigger simply enables the craft to absorb the same forces with less effect

 

[LittleShip]

"Think 18-20 knots is quite fast enough in most seas."

Tut Tut.......you youngsters, all you want is speed (thats the go fast kind!)

7knts is quite fast enough! 10 at the most. /forums/images/graemlins/grin.gif /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif /forums/images/graemlins/tongue.gif

Tom