If you would be prepared to fit a transom transducer then a lowrance fishfinder might work for you.
The X51 (only because this is the one I have) comes with a standard depth transducer that mounts on the transom. An addition to this there is an optional transom log paddle which turns the fishfinder into a dual log / sounder.
Four screws, if you use short screws and silkaflex they should not even penetrate the hull, but you would have wires on the outside of the boat.
Not a very clear image of a transom mount speed sensor>
May I ask what you mean by 'not penetrate the hull'?
If you are worried about the possibility of leaks, I am not sure that I follow your logic. Nearly every boat that has a log fitted has it through a skin fitting and I can't remember if I have ever heard of a boat sinking through failure of the log fitting.
If you don't like plastic log hull fittings, then you can get bronze ones.
The sonic onces that work wihout a paddle whel are excellent, expensive, and still need to be fitted through the hull.
And in response to the suggestion of using a handheld GPS, Speed Over Ground (SOG) isn't the same as log.
We have a through hull log. Some wooden bungs in case of emergency, and a trailing 'Walker' log if all else fails.
It is NOT bad practice with GPS ... it is DIFFERENT practice with the GPS ... not everyone feels they need to know speed through the water, and infact using the GPS heading and speed it is quicker to plot on the chart where your actually going and current location, without the requirement for tidal calculations and EP/CTS ... an knowledge of the water flow is still very important though!
You can give the "what if GPS goes down" argument if you like, but I can counter that with the "what if your log gets clogged up" and "what if your log display breaks down" ... don't tell me they don't - cos they do.
But having both allows you to measure the current. If your GPS is showing 2kts and the log 7kts it shows that you are nudging 5kts of current. This is important at other times, you can see when the tide has changed and enter a race or know entry to a tricky entrance.
Both are valuable, the most valuable in MHO is the log.
Sailing on the east coast I find that my log is clogged up much of the time & that it is not all that accurate,& needs calibrating every few weeks.If I was buying a new one now I would go for an inside hull jobbie though they are rather expensive.The Blackwater mud has alot to answer for!It does depend where you sail of course & if you are on a drying mooring/marina.
I don't disagree ... but my nav software does show the current as well! /forums/images/graemlins/smile.gif
That said - we still have and will maintain the log as I like to think we can navigate without the use of GPS .... now where did I put that pencil and rubber??
I have my sounder transducer mounted on the end of a 3" wide pine board, with the cable running up a covered groove, everything faired to recuce drag. The board is attached to the pulpit and dips into the water over the transom. I like this arrangement becos there is no hole thru the hull, and the whole thing can be used in the tender for scouting away from the yacht, or indeed in any other boat, with a 22ah 12v bty weighing less than a kg. My next project is to mount the paddle of an electronic log the same way. I like being able to get at all the cable and the senders with ease.
thanks all..... I currently have a gps and find it very usefull... the need for no hull penetration is because I have just launched and I do not want to have to crane out again.... and dont have faith enough in my working speed to dry out and attempt to drill the hull!!! all very useful replies.