Boat Speed.. silly questions

[Nostrodamus]

Yes, I am sure someone is going to give me a right answer but what is the point in setting boat speed by an impella? Mine is set to the speed over the ground from the chart plotter which I find more useful. Is there a a reason to set it to something else or am I just having a senior moment?

How do you work out what your boats theoretical top speed should be and how do you work out what it will be under engine. This is obviously in perfect conditions with a perfect clean hull and matched prop. What in the real world will be the difference between the two?

Knowing your boats top speed under engine what speed would you normally cruise at?

 

[rwoofer]

You kind of answered your own question. Through the water speed allows you to tell how the boat is actually performing.

If under engine it becomes obvious when your prop is fouled or you are not getting the right speed for the revs cause the engine isn't well. Under sail it helps you identify if you've trimmed your sails badly or caught some weed etc. etc.

 

[RupertW]

The other key factor is the tide. If you are doing 4 knots and the tide is doing 3 against then that 10 mile sail will take a long time. Knowing your speed through the water helps judge what your best tactics should be.

 

[Twister_Ken]

Theoretical top speed of non-planing hull in knots is 1.4x the square root of the waterline length in feet.

Also having STW available allows you to compare with SOG and work out tide effect - very useful on cross tide legs.

 

[awol]

No questions are silly - just the answers.

Speed through the water was all that was available before GPS and was modified by tide and leeway to give estimated positions. I use the difference between the log and GPS to measure tide - both strength and direction.

For boat "top" speed, it depends on the hull design. For a displacement yacht, hull speed is somewhere about 1.4*sqrt of waterline length in feet = speed in knots. Totally different for planing hulls, catamarans and hydrofoils. If there is enough power from sails or engine this speed can be exceeded.

Cruising speed under engine can depend on your tolerance for noise and depth of your wallet for fuel consumption. Probably somewhere around 80% of hull speed for a displacement yacht - could be more or less depending on how powerful or underpowered the engine is.

 

[Elessar]

Yes, I am sure someone is going to give me a right answer but what is the point in setting boat speed by an impella? Mine is set to the speed over the ground from the chart plotter which I find more useful. Is there a a reason to set it to something else or am I just having a senior moment?

How do you work out what your boats theoretical top speed should be and how do you work out what it will be under engine. This is obviously in perfect conditions with a perfect clean hull and matched prop. What in the real world will be the difference between the two?

Knowing your boats top speed under engine what speed would you normally cruise at?

that way you can tweak your sails, or in my case tabs and legs, for best speed and not be influenced by tidal stream.
You can also tell if the boat is off form, again without being confused by tide.
With both side by side you can instantly tell if you have found a favourable tide or not.
And you need the distance from the impeller if you need to work out an EP. Don't have to do that often these days with GPS but when you do.....

 

[oldharry]

For general navigation purposes , like you I use SOG derived from GPS. Thats around 90% of the time. But I still look for STW when trimming the boat in any way, or for squeezing the very best out of her.

As others have said, in the old days impellor derived speeds were all we had, pre GPS, and impecunity in the earliest days of sailing for me meant that even having a log was a luxury! Everything was done by eye - speed calculated by timing over the boats length: I used to keep a ready reckoner table set to the length of the boat. Depth was calculated by peering over the side: if you couldnt see the bottom, out came the lead line. Ah the good old days - wouldnt go back to that for all the tea in China - but somehow there isnt that same thrill of acheivment when the right bit of land, or nav buoy appears at the right time in the right place ahead! The number of times in those days when we would have given our back teeth to be able to reduce the growing 'triangle of uncertainty!' as the passage lengthened that was at the core of old-style navigation!

 

[Resolution]

impellors

In addition to the properly technical answers above, having a thru-hull paddle wheel type impellor also does two things:
1. Provides yet another source of intermittent leaks; and
2. Provides small crustaceans with a slightly exciting home.

 

[Stemar]

In addition to the properly technical answers above, having a thru-hull paddle wheel type impellor also does two things:
1. Provides yet another source of intermittent leaks; and
2. Provides small crustaceans with a slightly exciting home.

3. Provides a significant increase in Skipper's blood pressure when it doesn't work due to 2.

 

[Blue5]

Yes, I am sure someone is going to give me a right answer but what is the point in setting boat speed by an impella? Mine is set to the speed over the ground from the chart plotter which I find more useful. Is there a a reason to set it to something else or am I just having a senior moment?

How do you work out what your boats theoretical top speed should be and how do you work out what it will be under engine. This is obviously in perfect conditions with a perfect clean hull and matched prop. What in the real world will be the difference between the two?

Knowing your boats top speed under engine what speed would you normally cruise at?

SOG is the meaningful value for timing on the passage but the paddlewheel log is great for helping you understand why you are not achieving those maximum values and a good indication of tidal strength and direction.

The formula to calculate hull speed has already been posted and regarding cruising under engine, I tend to stick to about 2500RPM rather than an actual speed but again that depends on many variables.

 

[vyv_cox]

3. Provides a significant increase in Skipper's blood pressure when it doesn't work due to 2.

4. Livens up a boring passage by removing the crustaceans of 2. It never ceases to amaze me how much light comes vertically upwards from the sea when doing this.

 

[Nostrodamus]

So really what you are saying is that if you have not pre planned a passage it may help a little with tides otherwise like an appendix it can cause you problems and now serves no real useful purpose.

 

[Lakesailor]

Except that you would find it useful to check the speed achieved when adjusting your variable prop, as in the other thread you have on PBO.

 

[oldharry]

the other disadvantage of paddlewheel technology - apart from marine life queing up for a ride on it - is that it is very rarely linear. My log is set to read correctly against the GPS at around crusing speed - i.e in my case in the 5 - 6kt range. At 2kts there can be as much as .2 knots error i.e 10% - easily enough to make a landfall in thick weather 'exciting' with several miles potential error in distance run after a days sail!

 

[Nostrodamus]

Except that you would find it useful to check the speed achieved when adjusting your variable prop, as in the other thread you have on PBO.

Good point Lake sailor. Unless that is I can get to the Med where there is no significant tides and the water is warm so I can dive and adjust it in the water.
What use is a impella log then. There must be something.

 

[awol]

Good point Lake sailor. Unless that is I can get to the Med where there is no significant tides and the water is warm so I can dive and adjust it in the water.
What use is a impella log then. There must be something.

I am reviewing my stance on stupid questions .....

 

[Nostrodamus]

I am reviewing my stance on stupid questions .....

Well it did seem supid when I originally posted it as it is difficult to find a boat without one and as I have mine set to SOG I was trying to understand what I was doing wrong. I can see the advantages in certain circumstances but on the whole SOG works for me. Besides that I cannot bear the thought of going a mile in one direction only to come back to where I started. That would bug me as I could be two further miles south.

 

[jimi]

Speed through the water tells you how well you are sailing, SOG indicates the quality of your passage planning ;-)

 

[Robin]

Well it did seem supid when I originally posted it as it is difficult to find a boat without one and as I have mine set to SOG I was trying to understand what I was doing wrong. I can see the advantages in certain circumstances but on the whole SOG works for me. Besides that I cannot bear the thought of going a mile in one direction only to come back to where I started. That would bug me as I could be two further miles south.

What exactly do you have set to SOG? You cannot set a paddlewheel log to read SOG, it will only read boat speed and distance traveled through the water. If you mean you have your GPS or plotter set to give a display of SOG that is all it can do on it's own, but if it is interfaced to the log then it can display both as well as other computed results like tide rate and direction.

Most importantly if you have no means of measuring boat speed and distance traveled though the water then you have no easy way of navigating should your GPS fail and you need to use dead reckoning.

 

[johnalison]

Question like this pop up from time to time. I can give you one practical reason (among several) why I value the information I get from my impeller. A regular trip for me involves sailing, but often motoring, 18-20 hours across the North Sea. My cruising speed under power is about 6.5 knots. Several times I have noticed the water speed drop to about 6.1 knots without change in revs, noise or vibration as I have saildrive. A brief stop and burst of reverse invariably seems to throw off some material or weed off the folding prop, which we often observe floating off. I then continue on my way having regained nearly half a knot and maybe saved a couple of hours. Tidal variations make it impossible to appreciate the small change in speed via SOG. Water speed is useful when sailing too, even to non-racers.