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Has anyone tried moving the speedo to centerline

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jracoffman View Drop Down
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    Posted: 03 July 2017 at 16:32
All,

I am in the process of doing a significant electronics upgrade, dumping the old SIMRAD instruments on my 2010 H375 and going with new B&G instruments. Sad how outdated and obselete the old electronics are just 7 years later.

One issue I have with the Hanse is that the current position of the speedo/depth transducer is off centerline so you can often get materially different speed readings from one tack to another. Wondering if any one has tried a installing a new speedo at centerline. I am trying to do this, and wondering to what extent I can fit a smaller fitting under the floorboard in the forward cabin which I know doesn't have much clearance, and may require raising the floor an inch or two. How far forward of the keel is that? Other option is to put it in the small compartment under the Vberth just forward of the water tank, but that is awfully close to the bow and likely to come out of the water in heavy seas or chop.

Any advice or previous experience others have had with this is greatly appreciated.
Regards,
James
Gemini (USA 60468)
James C
Gemini, Hanse 375 #644
USA 60468
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Brufan View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Brufan Quote  Post ReplyReply Direct Link To This Post Posted: 03 July 2017 at 19:45
Leave it where it is.
It is the usual place for speed sensor (as far It factory fitted just at the aft end of V berth)
A small offset from centerline is not a problem
Bruno

hanse 355 - 57

S/Y Spicy Ginger

White hull, 2 cabins, Volvo D1-30, Selden rig, removable mainsheet track system (modifications done now), Simrad-Jefa autopilot.
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Johan Hackman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johan Hackman Quote  Post ReplyReply Direct Link To This Post Posted: 05 July 2017 at 18:32
Originally posted by jracoffman jracoffman wrote:

Wondering if any one has tried a installing a new speedo at centerline.


It's been irritating me for years that the the speed sensor is mounted on the starboard side of the keel on my 342. When heeling, the speed readings vary between the two tacks a lot. This does not only affect the speed information, it also affects the wind information. True Wind Direction varies 10 to 20 degrees between tacks and it doesn't help that the cups on the wind sensor move slower when the boat heels.

I have been contemplating either moving the speed sensor the way you descibe or replace it with an ultrasonic sensor. The ultrasonic sensor measures at what speed particles in the water moves at a distance from the hull, unlike the paddle wheel that measures the water movement at the surface. If you think about it, the hull is designed (correct me if I am wrong) to give different flows on each side when heeled. I mean, the wing profile theory applies even here, not only when it comes to the sails, the keel and the rudder. But then again, I am not an engineer so feel free to disagree.

I haven't been on many 370s but I recall being on one and that I found the speed sensor under the floor board in the fore cabin. Someone perhaps could fill me in on this?

Johan
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iemand View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iemand Quote  Post ReplyReply Direct Link To This Post Posted: 05 July 2017 at 19:24
On 312/341 they used to put the sensor next to the keel because of the missing hight under the floor boards. On the 370 the sensor is in front of the keel slightly of center (maybe 15cm). In my opinion that doesn't make a difference and is a completely different story to your issue!

BR THomas
Hanse 312 MJ 2004 - Hanse 370e MJ 2007
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Peter Russell View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Peter Russell Quote  Post ReplyReply Direct Link To This Post Posted: 05 July 2017 at 19:27
On the 370 it is indeed in the forecabin nearly on the centreline (probably 300mm offset to port).  I've not noticed a problem - often because it is so fouled it rarely works!
Peter Russell

Hanse 370 hull 499 "Outnumbered"



http://outnumbered.the-russells.net
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StavrosNZ View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote StavrosNZ Quote  Post ReplyReply Direct Link To This Post Posted: 05 July 2017 at 23:49
Unless you are racing why not put the blanking plug in the through hull and use SOG on the plotter for speed.

Eliminates this problem and the need to clean the paddle wheel at intervals.

For cruising its a trouble free solution.
Stephen
2010 H400, Auckland, New Zealand
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Johan Hackman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johan Hackman Quote  Post ReplyReply Direct Link To This Post Posted: 06 July 2017 at 09:56
SOG is a good substitute for boat speed in water without currents. It will however not be a good substitute when it comes to calculating true wind.

Johan
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iemand View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iemand Quote  Post ReplyReply Direct Link To This Post Posted: 06 July 2017 at 10:03
that is indeed the problem with SOG.

Interesting point: Why are all systems calculating the TWS based on speed in the water and not SOG? That would make much more sence to me!
Hanse 312 MJ 2004 - Hanse 370e MJ 2007
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Johan Hackman View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Johan Hackman Quote  Post ReplyReply Direct Link To This Post Posted: 06 July 2017 at 10:44
I shall attempt to answer that question.

First of all, it is mathematics.

Velocity is a so called vector, i.e. you don't only tell how fast your going but also in what direction. Look at the wind. You talk about the wind speed. But not only that, you also talk about its direction. For example, you are not just interested in that the wind tomorrow is going to be 10 knots, you will also want to know if its going to be blowing from North or South. The velocity of the wind is therefore a vector.

Now, in a car the direction is implied. The direction is always forward (never sideways) and in relation to the road. Therefore many people think that velocity is just a number and not a vector but it is a vector anyway.

Then you have the speed of your boat. That is also a vector but the first thing you will ask yourself is what is the velocity related to? Is it in relation to the water or is it in relation to the bottom of the sea, i.e. the ground? Boat speed as measured by the paddle wheel is in relation to the water whereas SOG is related to the ground.

When you measure the boat speed the direction is implied. The direction is where the bow points. The wind that your boat creates by moving forward is something I call speed wind for want of a better expression.

Your wind sensor gives you a vector - apparent wind speed (AWS) and apparent wind direction, or angle (AWA). This vector consists of two vectors - the true wind speed (TWS) and true wind angle (TWA), which is the wind blowing across the water - and the other vector that is your boat speed and the implied direction of that vector, i.e. straight forward.

In order to calculate the True Wind (you can't measure it when the boat is moving) you will have to withdraw the Speed Wind from the Apparent wind. This is a complicated mathematical operation done in the wind instruments.

SOG is consequently also a vector. It has a speed, but the direction is not implied to be straight ahead in the direction of th bow. You can actually be sailing six knots sideways if you are in a current.

So SOG cannot be used in that mathematical operation. It can only be assumed to equal to Boat Speed both when it comes to the speed and direction. As soon as the direction is not in the direction of the bow (i.e. when COG is not equal to your heading) the vectors summation will not work.

That's my answer why you don't use SOG to calculate the True Wind.

Johan


Edited by Johan Hackman - 06 July 2017 at 10:49
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iemand View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote iemand Quote  Post ReplyReply Direct Link To This Post Posted: 06 July 2017 at 12:36
Your boat is on a river where the wind is blowing with 20kn in the direction of the river. The tide is running with 2kn in the wind direction. You are doing 8kn thrue the water with the wind and 10kn SOG. Your apparent wind speed will measure 10kn and your system will show 18kn true. If you go back with the engine with 6kn thru the water you will go 4kn SOG. You will measure 24kn AWS and your TWS will show 18kn true.

This simple example shows that the calculation is wrong.

Hanse 312 MJ 2004 - Hanse 370e MJ 2007
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