WHAT IS CRUSING SPEED??

[coromar]

I have just bought a new (to me) boat. A Corvette 320 with 2 x 250hp Hino turbo intercooled diesels.

As I am new to planing boats, I have two questions :-

1. How do I know what the crusing speed should be?

2. What is the most economical speed to cruise at?

Any help would be most helpful.

 

[longjohnsilver]

Cruising speed is whatever you want it to be, most economical depends on how fast you want to go, displacement speed of about 7kts will be most economical but assuming you want to plane then about 75 - 90% of max revs will probably be best.

Corvettes have a good reputation for seakeeping and have a huge amount of space for their size. Have a good season.

 

[Nick_H]

Hi, and welcome to the forum. As a general rule maximum cruising speed should be at about 90% of maximum RPM, though many people cruise well below this to get better fuel efficiency. Most economical speed for a planing boat is just on the plane, often in the 15-20 knot range. Usually its a trade off between economy and speed, so a 20 - 25 knots cruising speed may be optimum, but depends on the boat

 

[powerskipper]

I find the easiest way to decide is first put her up on the plain and then easy to top speed,note speed then drop her back to about 2/3 or 3/4 of that and she should cruise happily at that, most economical is normally just on the plain as said already.

PS welcome to the forum.

 

[duncan]

Welcome - and great choice of boat.

Chucking in a 2p - different boats have different hull and different engines; each of these combinations has a set of characteristics. Some result in a very pronounced 'optimum speed for comfort, economy and efficiency, others have a wide range delivering parity on all fronts.
I believe the Corvette is primarily a semidisplacement hull and as such doesn't have 2 distinctive modes. However it still has to connform to the laws of physics and as such is either operating in displacement mode (hull moves through water displacing it) or planning (hull creates lift and raises boat above water). The former, as already stated, will be up to around 8 knots (Cruising speed 1)for your length hull and, below that speed everything will behave in a pretty linear way (hull speed/engine revs/fuel consumption and consistent handling). Above that you are starting to move into an in-efficient window where the engines are having to use significant power (fuel) to push against the water and starting to lift the boat - however at around 12 knots the hydrodynamic lift generated by the hull takes over and the engines are once again using their power to drive the boat forwards (Cruising speed 2). From there you will be able to increase the engine revs and gain additional hull speed in a linear way again until either the drag of the stern gear or the power delivery of the engines becomes a factor (Cruising speed 3 being at the onset of this). For most boat/engine combination this will be around 85% Max revs as outlined above. Additionally there may be a mechanical reason why the engines should not be run continuosly beyond a certain level - again this is commonly 90% max revs for pleasure craft/engine.
Finally there will be a relationship between the hull speed and passenger comfort - not to be ignored!
It follows that most planning craft have 2 cruising speeds - 1 (optimum displacement speed) and a range between CS 2 and 3. The more extreme (and or underpowered) the craft the more smaller this range is likely to be.
Hope this helps - but just like a car you really can feel when the ship is 'happy' and working well!

 

[Wiggo]

1. It's always about 2 knots slower than everyone else you're out with.

2. Suck it and see, cos it does depend on engines, hull etc. Note your mpg figures at different revs (it will take a while to accumulate this data), but you will probably find that your economy drops off a cliff above about 80% max revs. The last three or four knots really cost!

Nice boat, BTW. I think they're very pretty and confidence inspiring boats. Enjoy the season.

 

[hlb]

Years ago I was told that cruising speed is full power less 200rpm. Course this would be a bit rubish if full revs were 200rpm!!

 

[Wiggo]

Ah but that was in the old days when 200rpm was worth something... I 'spect you still remember 78rpm's don't you, Haydn?

 

[hlb]

Course I do. My sister had a wind up record player with all the pop records at 78rpm. Some times she let me listen to them with a new needle!! /forums/images/graemlins/tongue.gif

 

[coromar]

Thank-you for all your advice, i'll see how it goes once i'm back in the water.

ROLL ON SUMMER!!!

 

[Gludy]

"Most economical speed for a planing boat is just on the plane,"

I think this really depends on the boat - many seem to have about the same fuel consumption once onto the plane right up to say 90% max revs or cruising speed.

I do not think, the most economic is just onto the plane.

 

[BrendanS]

It really depends on the hull. Mine for instance is on the plane at 15knts, just, in a flat calm. Most economical speed is about 28knts, and that economy drops off steeply at a few knots above or below that speed.

 

[Gludy]

Exactly ... it varies with the boat and often with a planing boat, your get better economy at the cruising speed than just onto the plane speed.

So, it would be wrong to assume that the best economy is achieved at the lower planing speeds.

 

[ArthurWood]

I agree, Paul - for what it's worth /forums/images/graemlins/smile.gif

 

[gjgm]

think you ll find its the mood that takes you. Sometimes you feel like having a spin, others pottering along more sedately. But as in another post, you ll probably find comfort -including noise levels-will dictate(especially the crews' unless you want to be alone for ever more). Head sea/ following sea.. you ll just find what makes for a comfortable,enjoyable voyage on the day.
As for economy.. hmm. might be you're in the wrong pastime !
Enjoy your new boat !

 

[miket]

MBM/ MB&Y have certainly boat tested this boat (different engines, never come across Hino other than in older larger Bayliners) and will probably have included a graph plotting speed and fuel consumption against engine revs.

Cruising speed for me is what is most comfortable for me and the crew, the boat and given sea state at the time.

 

[Nick_H]

For my boat (46' planing flybridge), the most fuel efficient is just on the plane, and from memory most others where I've seen the stats is the same. Figures below, lifted from the MBY report, show best MPG at 16.7 knots (just on the plane), but only a slight penalty in efficiency to get an extra 4 knots cruising speed, well worth it I reckon. The efficiency does get noticably worse above 80% of max revs

RPM 1,800 2,000 2,200 2,400 2,600 (*2,600)
Speed 16.7 20.5 24.0 27.0 29.6 (*31.0)
Trim 5.0° 5.0° 5.0° 5.0° 5.0° (*5.0°)
GPH 14.6 18.4 24.0 31.7 41.6 (*41.6)
MPG 1.15 1.11 1.00 0.85 0.71 (*0.75)
Range 324 315 283 241 201 (*210)

 

[Gludy]

I accept your figres but it does vary for the boat - on my Squadie there is little difference after on the plane until I am at above 90% revs.

 

[Medskipper]

If you have an "ear" for engines sounds you will quickly learn to hear that sound when an engine is running most sweetly! that will be your cruising speed! Too little revs and the engines sound heavy too many revs and the engines sound strained, just adjust the throttles until she sound like shes purring sweetly! people who don't have an ear or a thought for their engines just rev the bo**ocks off them!

Have a good season

Barry

 

[BrendanS]

Exactly. Don't need speedo or leg trim guage at all these days. Throttle, trim leg, throttle, trim leg. all seems happy, leave it be for a while. Conditions change, throttle, trim leg etc.

It's all instinctive after a while, but not sure how'd you'd teach that to someone. When strangers helm my boat, I take leg trim out of the equation, they have enough difficulty adjusting throttle to conditions, and I guess would need a few months out on water before they could do all that by ear.