Newboy Posted December 7, 2005 Report Share Posted December 7, 2005 What is cruising speed? Is it the speed the boat start to plane? Or the speed when the engine is most comfortable? Then how do we know if it is at its most comfortable? Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted December 7, 2005 Report Share Posted December 7, 2005 Kam Cruising speed is the speed your boat and engine runs most economically/comfortable at, although I would say cruising speed is the speed you are most comfortable at. To fast and you will get beat up to slow and you will be bored to tears. Modern outboard engines are comfortable cruising right through the rev range. I tend to cruise with the throttle at 2/3rds with the revs at about 4000-rpm. We are then trucking along between 18-24 knots dependant on weather. This works for me but other boats/engines will be different. Martin Quote Link to comment Share on other sites More sharing options...
Afishionado Posted December 7, 2005 Report Share Posted December 7, 2005 What is cruising speed? Is it the speed the boat start to plane? Or the speed when the engine is most comfortable? Then how do we know if it is at its most comfortable? It depends entirely on the boat in the terms of it's water footprint and whether it is a displacement, semi displacement or planing hull. One also has to factor in waterline length and beam, together with engine delivered HP at the propeller and to some extent engine weight. Cruising speed is dependant on all these things, not to mention the will of the helmsman and what he or she decides is a comfortable speed. Therefore you will see that there is no one speed that can be described as 'cruising speed'. One boat and it's crew may feel that 18kts is 'cruising', this may be the absolute top speed of a similar boat with a smaller engine. This second boat and it's crew may well sugest that 6kts is 'cruising speed'. It would be very foolhardy for any one to say that any particular 'speed' was a 'cruising speed' as there are so many variables that it would be impossible to give an answer for all boats on all occasions. Mad Mike Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted December 7, 2005 Report Share Posted December 7, 2005 Good answer Mike Quote Link to comment Share on other sites More sharing options...
that's_a_biggin Posted December 7, 2005 Report Share Posted December 7, 2005 another words the speed that you and the boat are happy with.. Quote Link to comment Share on other sites More sharing options...
TomBettle Posted December 8, 2005 Report Share Posted December 8, 2005 another words the speed that you and the boat are happy with.. Mike and Martin have given pretty well text book answers and you have summed it up very well. Your own cruising speed will change from day to day and be dependent on the weather, your mood and how much fuel you want to burn. Volvo tend to use about 10% of maximum revs when quoting approximate fuel consumption figures. In my mind 10% would be at the top end of an engines cruising revs, but as said, it is down to your own choice. Duncan has got some great technical blurb on fuel economy and some people relate cruising speed to the boats most economical speed, again as mentioned it is as much to do with comfort onboard as economy. My own boat, "Quest II", is no sports boat, shaft driven and semi displacement. Her rpm tops out at about 3500 with a clean hull and light that produces about 23knots, but guzzles fuel. Knock it back to 3000 RPM and light she will do a respectable 17 or 18 knots and burn almost half the fuel. You only have to come back to 2800 RPM and she is still planing at about 15 or 16 knots and again drops fuel consumption by a third again. This trend continues pretty much right through the Rev range and tickling along at displacement speeds almost all boats sip fuel. Add a full compliment of fuel, crew, fishing tackle, dirty hull from a season in the water and the general crap we all take on a boat and Quest II drops her performance by about 2 knots per 200 RPM, she won't quite pull maximum revs and uses a tad more fuel than she would normally. I am using Quest II as an example as it is my own boat and I am most familiar with her, however most boats will have a similar pattern. Outboard and outdrive boats can improve economy and comfort (and to an extent any boat with trim tabs) by correctly trimming the boat for the prevailing conditions. That is a trial and error thing and a bit of practise or tuition should allow you to get the best from your's. We are not normally in a hurry on Quest II and crusing speed is usually between 2800 and 3000RPM depending on our plans. The further we are going the lower the RPM to maintain the best economy. Why is it everyone else can do short, simple, concise replies and mine are always epics? Quote Link to comment Share on other sites More sharing options...
duncan Posted December 8, 2005 Report Share Posted December 8, 2005 epics are good too Tom......... I am going to introduce the concept of a theoretical crusing speed in order to differentiate the comfort/boredom etc factors that apply to man from the physical constraints of the machinery - hopefully to arrive back at a working definition of cruising speed (as opposed to an actual speed) for most boats. Recappingon the factors involved from posts above - 1. hull shape and it's relationship with the water at different speeds 2. engine and it's use of power in relationship with hull speed, and therefore fuel 3. other engine constraints Dealing with 3 first many manufacturers recomend a maximum sustained revs for their engines, and some a 1 hour figure too. In my case they are 3100 and 3300 although the engine is govened at 3650 and will normally manage 3450 even when fully loaded. This immediately indicates that any cruiseing speed that fits with 1 & 2 above will be limited to that which 3100 rpm will deliver. Most OBs no longer have any restriction on them and are happy to run at full revs all day long. Dealing next with No 1 above, and ignoring running at tickover in displacement mode, most planning and SD/SP hulls will run efficiently from about their resistance hump speed +15% upwards if on a drive or OB. Where a shaft installation is fitted then over about 18 knots the drag on the P bracket/rudders will start to increase (exponentially?) and is another factor to be considered (but falls into No 2 above primarily). Thus theoretically most hulls will be equally efficient from this speed up and some (deep vs as an example or those with a transom pad or stepped hulls.......) may even become more efficient as speed increases and the wetted area of the hull reduces thus decreasing drag associated with the hull being in contact with the water (drag actually increases horribly as speed increases but thats in 2 as well!) So we have a hull defined lower cruising speed of R + 15%, or 'just planning' and a potentially restricted upper one based on the engines rated cruise, or maximum otherwise. OK so far? Now for No2 and the spectre of fuel consumption. Within the above operational range a good hull/engine combination will have quite a range of economical operation - a bad one could have a very specific 'point' of economical operation. The latter would normally be considered that crafts 'cruising speed'. SD/SP shaft driven diesel powered boats will tend to have a very wide range of operation for similar 'MPG' as will planning craft with engines at the top of the HP range for the craft. Underpowered 2 stroke OB craft will be in the bad category here! Generally diesel and petrol (inc 4 stroke) exhibit different characteristics here with the former delivering similar fuel consumption in terms of grammes/HP/hour over a wide range of revs where the petrol engine requires an increasing amount of fuel to deliver each HP. Add back in the increasing drag factor on the hull refered to above as speeds through the water increase, which is why we need more HP to go faster, and you will generally see a graph of fuel consumption for a petrol engine as an upward curve and diesel as a lesser one (almost flat for a planning hull). This also explaines why a SD hull, which is always working in a high drag environment, will also have pronunced curve to it's fuel consumption as Tom set out above. Finally most petrol engines are a compromise between normal running and max available HP so the Twin Choke Carb (before FI) was developed. As FI is mapped in a similar way we will deal with the simpler concept. Here the engine runs on a single choke (of each carb) up untill a preset point at which a second one starts to open permitting a significanlty increased flow of air (and fuel) through the engine - and therefore more power. Whilst this gives a lovely feeling of acceleration on the boat the euphoria is short lived for those without money to burn.....Typically this happens over the last 15% of an engines rev range. The more econmoically the engine has been mapped to run at 85% the bigger the hit when the 15% are used. It follows for a petrol planning craft that whilst the MPG will be higher at 85% throttle (quite a bit higher) it is still reasonable to consider such a throttle setting a cruising one on many hull/engine combinations. For a diesel the MPG are similarliy close enough right through to it's limit as set out in No 1 although a SD/SP hull will be on curve but the planning diesel should be almost linear over the range - certainly up to 90% revs. As an example I use 0.95 litres/nm at 14knots (2250rpm)increasing to 1.15l/nm at 28kt (3100) and 1.4l/nm at 35knots(3650rpm). I consider my cruising speed to be between 14 and 28 knots. Conditions will of course have an effect on nearly all of the above - hence the use of the word 'theoretical'. Quote Link to comment Share on other sites More sharing options...
Coddy Posted December 8, 2005 Report Share Posted December 8, 2005 What an excellent piece of information If I put this theory to the test say my Warrior 165 does 25kts minus 15% gives me an ideal crusing speed of approx 21kts, the speed I usually travel at is about 18 to 20kts conditions permitting. Normaly start to plane at about 11 to 12kts Seems the theory works Great information, thanks duncan Coddy Quote Link to comment Share on other sites More sharing options...
TomBettle Posted December 8, 2005 Report Share Posted December 8, 2005 Told you Duncan had some good blurb on this!!!! Quote Link to comment Share on other sites More sharing options...
duncan Posted December 8, 2005 Report Share Posted December 8, 2005 Coddy ------- Your ideal theoretical cruising speed would probably be 14 knots (12 + 15%) but up to 21 knots would normally be an acceptable cruising speed from a theoretical economical view. Thus you could consider 14 - 21knots as suitable cruising speeds depending on your priorities at any time - 14 will deliver better mpg but 21 gives you more fishing time! (23 would giev you even more fishing time but no money for bait!!!) In practice this of course translates quickly to 21 knots being your cruising speed if conditions allow................ Quote Link to comment Share on other sites More sharing options...
that's_a_biggin Posted December 8, 2005 Report Share Posted December 8, 2005 no one has taken water temperature into consideration.. so can we go all through that again... Quote Link to comment Share on other sites More sharing options...
blueboatdriver Posted December 8, 2005 Report Share Posted December 8, 2005 crikey Quote Link to comment Share on other sites More sharing options...
Afishionado Posted December 8, 2005 Report Share Posted December 8, 2005 Coddy ------- Your ideal theoretical cruising speed would probably be 14 knots (12 + 15%) but up to 21 knots would normally be an acceptable cruising speed from a theoretical economical view. Thus you could consider 14 - 21knots as suitable cruising speeds depending on your priorities at any time - 14 will deliver better mpg but 21 gives you more fishing time! (23 would giev you even more fishing time but no money for bait!!!) In practice this of course translates quickly to 21 knots being your cruising speed if conditions allow................ Absolute ruggish! Are you sugesting that 14, 21 or even 23 kts are possible with say a displacement hull with say an 18hp 2 cyl diesel? Of course not. There is no such figure as a crusing speed that is applicable to all boats. Cruising speed is and can ONLY be what a helmsman decides on the day, to suit the conditions AND TAKING INTO ACCOUNT WHAT THE BOAT IS CAPABLE OF. The next thing will be someone asking what a comfortable cruising speed is Vs maximum cruising speed Vs top speed. Mad Mike Quote Link to comment Share on other sites More sharing options...
that's_a_biggin Posted December 8, 2005 Report Share Posted December 8, 2005 if your reading this danny a cruising speed for snowflake would be all you got and then a bit more... Quote Link to comment Share on other sites More sharing options...
TomBettle Posted December 9, 2005 Report Share Posted December 9, 2005 Coddy ------- Your ideal theoretical cruising speed would probably be 14 knots (12 + 15%) but up to 21 knots would normally be an acceptable cruising speed from a theoretical economical view. Thus you could consider 14 - 21knots as suitable cruising speeds depending on your priorities at any time - 14 will deliver better mpg but 21 gives you more fishing time! (23 would giev you even more fishing time but no money for bait!!!) In practice this of course translates quickly to 21 knots being your cruising speed if conditions allow................ Absolute ruggish! Are you sugesting that 14, 21 or even 23 kts are possible with say a displacement hull with say an 18hp 2 cyl diesel? Of course not. There is no such figure as a crusing speed that is applicable to all boats. Cruising speed is and can ONLY be what a helmsman decides on the day, to suit the conditions AND TAKING INTO ACCOUNT WHAT THE BOAT IS CAPABLE OF. The next thing will be someone asking what a comfortable cruising speed is Vs maximum cruising speed Vs top speed. Mad Mike Mike The quote you have taken from Duncan refers directly to Coddy's own boat on the figures that Coddy himself povided. Cruising speed is the boats most comfortable, fuel efficient speed that still provides reasonable progress through the water for the particular hull type. It IS down to the boat driver to a very great extent, but if you own a plaining hull, but decide to potter around at 6 knots that would not be regarded as cruising speed as it is not seen to be making reasonable progress for that particular hull. 6 knots is regarded as displacement speed. In addition pottering around at 6 knots in a planning hull is often not very comfortable as a lot (not all) of plaining boats "wander" or "swan neck" at displacement speed and in open water can sometimes be a handful at slow speeds. Tom Quote Link to comment Share on other sites More sharing options...
DommyBoy Posted December 9, 2005 Report Share Posted December 9, 2005 Chris, Mine and Danny's boat is a true vessel.........not a poxy snowflake! Gnasher is the fastest boat in southern waters!!!!!!!!!!! Ha ha. Dom Quote Link to comment Share on other sites More sharing options...
duncan Posted December 9, 2005 Report Share Posted December 9, 2005 Mike, Are you sugesting that 14, 21 or even 23 kts are possible with say a displacement hull with say an 18hp 2 cyl diesel? nope - haven't said that, or implied that, anywhere There is no such figure as a crusing speed that is applicable to all boats again I have never suggested that there was - my original post started ".....to arrive back at a working definition of cruising speed (as opposed to an actual speed) for most boats" not sure why you are looking for a fight with me - have I upset you in some way? Quote Link to comment Share on other sites More sharing options...
Newboy Posted December 9, 2005 Author Report Share Posted December 9, 2005 Mike, have you forgotten to take your medicine again...... Quote Link to comment Share on other sites More sharing options...
Afishionado Posted December 9, 2005 Report Share Posted December 9, 2005 Mike, Are you sugesting that 14, 21 or even 23 kts are possible with say a displacement hull with say an 18hp 2 cyl diesel? nope - haven't said that, or implied that, anywhere There is no such figure as a crusing speed that is applicable to all boats again I have never suggested that there was - my original post started ".....to arrive back at a working definition of cruising speed (as opposed to an actual speed) for most boats" not sure why you are looking for a fight with me - have I upset you in some way? Absolute ruggish! You will note the and the spelling of ruGGish both of which represent a degree of humour in my post. Looking for a fight? My goodness you are being touchy Why would I want to do that? However I do admit that I miss read your post and thought you were being specific about boats in general. I still think it is impossible to sugest a cruising speed for any boat as there are so many variables not the least of which is sea state. This can vary over a very short distance as in making a headland and becoming under the affect of a weather shore, or the change from flood to ebb. Mad Mike Quote Link to comment Share on other sites More sharing options...
Adam F Posted December 9, 2005 Report Share Posted December 9, 2005 another words the speed that you and the boat are happy with.. I think 'thats a biggun' has the right idea! Quote Link to comment Share on other sites More sharing options...
duncan Posted December 9, 2005 Report Share Posted December 9, 2005 Mike, I absolutely agree that even quite small changes in the sea conditions quickly turn the science into a complex art form. Large changes can make it almost completely irrelevant! It's only on those rare occassions that we are blessed with a mirror calm sea that there is ever any question of 'what speed should I cruise at?' those oh so rare opportunities.................... Quote Link to comment Share on other sites More sharing options...
Afishionado Posted December 9, 2005 Report Share Posted December 9, 2005 Mike, I absolutely agree that even quite small changes in the sea conditions quickly turn the science into a complex art form. Large changes can make it almost completely irrelevant! It's only on those rare occassions that we are blessed with a mirror calm sea that there is ever any question of 'what speed should I cruise at?' those oh so rare opportunities.................... Oh they are so rare I give in to the 'little boy within' and can not resist going Harry Flatters just for the thrill of it. Only a dwindling fuel supply limits my extravagance. Mad Mike Quote Link to comment Share on other sites More sharing options...
Maverick Martin Posted December 9, 2005 Report Share Posted December 9, 2005 Oh they are so rare I give in to the 'little boy within' and can not resist going Harry Flatters just for the thrill of it. Only a dwindling fuel supply limits my extravagance. Mad Mike Blow the extravagance you only live once enjoy it Quote Link to comment Share on other sites More sharing options...
jack Posted December 10, 2005 Report Share Posted December 10, 2005 I am very worried about you chaps who dash about in boats,burning up the worlds rescources !!!!!!!....I DONT THINK SO.... . Come on you Lentil eating "save the planet " brigade..lets have it !!jack Quote Link to comment Share on other sites More sharing options...
that's_a_biggin Posted December 10, 2005 Report Share Posted December 10, 2005 lifes too short for cruising.. give it some welly... Quote Link to comment Share on other sites More sharing options...
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