Did a dream got shattered?

bertku--hope your boat project is doing well...
may i ask why you want a ss rudder and not composite?
i may have missed something so please forgive my lack of reading all your posts...

 

Hi Tugboat,
No particular reason. I work well with SS and wood, composite is still a big learning curve for me. Maybe I should try that out.
Bert

 

Well, the dream is coming a reality. I visited Wageningen in the Netherlands and had some very solid answers on many questions I had. Also I paid a visit to Delft and had discussions there. But one question, nobody could answer accurately. They stated that there are some software programs which could predict the power needed at certain size waves i.e. 2 meter and wind speeds of i.e. 20 knots. However they are not trustworthy. It was guessed that it would take some 10 to 20 years before some 100% accurate software programs would be available for small crafts. For large tankers, the value is about 5% more, but for a small, 7,5 meter displacement hull, regretful, nobody was willing to commit to a fixed value. Rick Willoughby was so kind to work out that I needed 800 – 1 Kw for flat, smooth water surface.
But we concluded that it was indeed a good idée to have a small storm sail on board.

Thus let me ask you all again. What total electric power do I need to push a 7,5 meter displacement hull through off shore waves of 2 meter with some wind of 20 knots, against a current of 2 knot. I don’t mind that the overall speed is then only a surplus of 1 Knot, as long it is able to move forward.
Bert

 

When and if someone gives you that number,figure out how many tons of batteries you'll need to run for more than an hour.

Here's what I posted for another dreamer of electric drive:

"You need 10kw @ 12 volts which is 833 amps an hour.
12 volts=160 pounds:
http://www.lifelinebatteries.com/rvflyer.php?id=7
$600 each
http://www.invertersrus.com/gpl-8dl.html

For # of hours needed,@833 amps@12 volts to 50% of battery capacity:

6 hours=360 min=87 batteries=7 tons=$53,000
2 hours=120 min=29 batteries.=4700 lbs.=$17,600

 

Guys from both Wageningen and Delft institutes have (correctly) told you that you cannot have a reliable answer to that question.
So what on earth makes you think that you will get an answer at Boatdesign.net or on some other forum? You cannot, you can only have wild guesses. My wild guess is 10 HP, for what it's worth. 5 HP for the aerodynamic resistance plus the current, the rest for the aded resistance due to sea waves. If you ever get to build that boat and drive it into a storm, you'll know the correct answer...

However, your requirements are sensless, imho. How did you arrive at those numbers - wave height, current and winds speed, and to the requirement of 1 kt speed over ground? Do you have any idea about how many miles you will advance @ 1 knot before your batteries die, leaving you just enough energy for a last mayday call?

 

That was an interesting thread, Bertku you are a determined man nothing wrong with that and all the advice given is valid with your safety and well being in mind.

It is true that we dont know who your are capable off so keep that in mind when some one has a strict reaction because they dont know what a person on the other side of the world will do with the advice and as you know boats and boat building mishaps = lives lost this is no simple matter and thus reactions will be harsh and strict. (and rightly so).
We have different cultures, upbringing and moralities. My culture is so different that what is right in mine can be offensive to others, that applies to everyday interaction.

Many names in this post have helped me big time (Richard, daiquiri and dskira at the lead, check my posts if you like) and i have asked the silliest questions in this forum. Yet they took the time and helped me out BIG time.
I try not to be biased no matter how strongly i am convinced with my point of view (and fail some times) because i know i have years and years of experience to gain to be at the same intellectual level as some of the senior members here (if i ever reach that).

Dont abandon your dreams easily fight for it but keep in mind the limitations presented by todays technology and the expert advice FREELY given for your own safety. No one here is obliged to help you but they do it out of sheer willingness.

I wish you the best of luck and sorry if i an unable to help with the calculations.

 

This applies to marriage, friendship, men and women !!!

 

Hi WestvanHam, and others

First at all, the 25Kwh I have available is 36 Volt and is less than 500 Kg, included mounting. Which means, at calm sea some 25 hours, add to this the solar energy input and wind energy, I could do quite a distance up and down. But that is not the point. The point is, we will come accross some bad weather situation and then I do not wish to be stuck. Therefore I need to know upfront what to expect. I personnally quess some 2 x 3,5 Kw i.e running with 2 motors, about 10 Hp in total.
5 Hp is too little as I have confirmation that a 22 feeter had problems with his engine to battle against the wind at the Nordsea.
Thus my question is not unreasonable. What has been your people experience at the sea with running an small engine and battling against the odds. This will give me some solid inputs to make decisions.
Bert

 

Thanks daiquiri for your input. Please remember that in our discussions in Delft, it was agreed by all, that a small storm sail will be an essential item.

When we motored from Agina to Poreus (Greece) we were motoring with a 37 feeter straight into the wind with choppy waves straight onto the bow.
However this was 22 years ago and I failed to ask the question to Nick (he steered at that time) with his "Yellow bird", how big the engine was. Pity.
Bert

 

Some problems are not suitable for solving by putting all the variables in equations.

Air resistance; if you look for example at the drag coefficient of cars there is an enormous variation e.g. Mercedes E220 is .24, early Ford Mustang is .46.

Wave resistance; will the hull slam, wasting energy, or pass smoothly over.

Small errors in either of these factors, or wind speed, wave height etc. will turn your 1 knot forward speed into going backwards.

Initially the best you can do is to compare your design with a similar existing boat and find out what HP they need for the worst conditions you could be caught out in with a decent factor of safety and a decent speed over the ground.

You are pioneering in this design, at the end of the day you need to accept that sizing the motors is going to be a best guess, only sea trials will prove how accurate the guess was.

 

Things to consider,that you need to calculate:

-is 7kw really enough?..
-peukert's effect
-50 % discharge of battery for long life.

Plus the other factors that "late" mentioned above- you say you go 25 hours at slow speeds, I'd be surprised if you go much over an hour to 50% discharge.

Peukerts is in no way a linear drop.

http://solarjohn.blogspot.com/2006/11/what-is-peukerts-law-and-why-should-i.html

Tell me what and how many batts you have.

I never get myself into those situations.

 

If that helps........

we once have been trapped in the mouth of river Elbe by a NW force 6-7 wind and a tide coming in with the wind. Vessel 35 meter, 200 tonnes, 300 hp, beefy engine. We made almost zero speed over ground for the first two hrs of the incoming tide! Too high the waves, too shallow the water, insufficient power, no way to turn around!

All clear?

 

You are probably right. But I hate to gamble. I was rather hoping that some inputs from other similar size boats would come forward. Although the designer is dead, I cannot ask him. He stated 10 hp. But without any indication whether this is at calm sea or with currents, wind and choppy waves.

Sofar, I have the input of a 22 feeter which needed more than 5 Hp to cope.

The golden rule is: 1/3 one way 1/3 to come back and 1/3 for reserve. If the sea is flat and smooth, about 8 hours one way. But I hate think what will happen, if I need suddenly more, should the weather turns around. Although we have reasonable forecasts and are not on the Nordsea, where the weather can turn around within half an hour. Also it depends on the season.
Bert

 

Sorry Apex1, you knew that the tide is +/- 6 meter there by Hamburg. No designer can cope and create a design for that kind of situations. The skipper should have planned his trip better. But this kind of answer does not help me very much. What is your personal experience for a small < 10 meter vessel under reasonable conditions?
Bert

 

No, sorry, there was no wrong planning! And I was the master btw. Just weather can change, no matter if we like it.
We have quite many designs capable of such situation, and they serve well and safe! Just this ship was neither designed for the North Sea, nor for such condition. We have never been in any danger, but did not make headway, as mentioned.

I was never at sea in a boat below 10 meters, and I call everybody completely insane just only thinking about!

Conditions are not, and never, "reasonable" because YOU do not decide in which conditions you sail. You can decide under which to start a trip, but thats all. Many, many things can, and will happen, changing your nice plans to become a nightmare.

You are far too much focussed on a personal bias and the related technical questions. The sea not even laughs about that............the sea has no will to kill you, it just happens.

Before you ask about my experience with such sort of craft, there was a recent occasion, when I did a yacht transport. The craft is about 10m and we enjoyed a trip in the Aegean Sea in winter. The smallest boat I ever sailed.
look:
http://www.boatdesign.net/forums/wo...ywood-carvel-planking-34539-3.html#post398091
that was a NA commenting!

Regards
Richard