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Ajax_MD 12-21-2009 08:55 PM

Knotmeter- Clarification needed
The patient: '69 Coronado 25.

Ok guys, help me out here.

For the first time, I opened up the storage locker under the quarter-berth. Inside, I found what looks like a red, thru-hull plug with a funny little egg-beater thing on the end, and a small cable poking out of the top. I had an odd feeling, and climbed up into the cockpit and watched the knotmeter dial while I spun the egg-beater. Yup, the knotmeter needle moved. WHERE the heck is this thing supposed to plug in??

I have a sinking feeling about this: Under the galley sink, there is a thru-hull for the sink/icebox drain. Right next to that, is a red cap plugging off another unused thru-hull that looks just like the red knotmeter cap except that it has no provision for a wire, and instead has a metal grab-ring. It's just a plug/cap.

Does the knotmeter egg-beater plug in where this red cap is under the sink? How the heck am I supposed to swap them out without flooding the boat?!
It seems that I have a perfectly working knot-meter and no way to deploy the flow sensor!

GaryHLucas 12-21-2009 09:08 PM

Yep, the red sensor goes where the red plug is! It is somewhat comforting to remember that the geyser of water that occurs when you pull the plug can not actually reach any higher than the waterline. So pictures in your mind of it hitting the ceiling are just your imagination at work. However it is a good idea to locate the key on the sensor and orient it properly before pulling the plug all the way out. That way you can pop the sensor in quicker. The sensor should have an o-ring seal in a groove. It is a real good idea to lube it generously with silicone grease, otherwise it can easily be sheared on the way in. It wouldn't be a bad idea to replace it with a new one before you install it. O-rings are cheap and it pays to have a spare or two.

Gary H. Lucas

Sabreman 12-21-2009 09:16 PM

Don't ya love it when a mystery is solved?

There is a simple solution to inserting the knotmeter impeller while the boat is in the water. We do it all the time. Have a large sponge handy, the red impeller within hand's reach, and remove the retainer pin on the red plug. Wiggle out the plug until it's loose but not out of the through hull fitting. It may leak a little at this point, but not as much at it's gonna! :)

My only worry at this point is if the plug is welded in the through hull by a million years of bottom paint. If it is, then you'll probably have to give it a good yank. I find that twisting it right and left while pulling works to free it.

When you pull the plug, you'll have a 1 1/2" hole in your hull gushing really cold Chesapeake water (if the boat's in the water). Immediately place the palm of your free hand over the hole and grab the impeller. In one quick motion, remove your palm and insert the impeller. Don't put your face directly over the hole unless you want to get wet. A fair amount of water will gush into the boat, but in reality, it will only be about a quart of water. Mop up the excess with the sponge.

The first few times that you do this, it can be disconcerting. But with practice, it gets easy and it can be a game to see how little water you let in the boat. If you are uneasy doing this in the winter, wait until you haul in the spring.

Tip#1: Lubricate the plug, the impeller, and impeller shaft with Vaseline. We even goop Vaseline on the impeller (the egg beater) to keep off marine growth. Works like a champ! I only had to clean the impeller once last season! We used to pull the impeller each weekend.

Tip #2: Replace the o rings that should be on the plug and impeller. If this is the first time that you're seeing them, I'm guessing that they are quite old.

Good Luck and Happy Holidays!

Sabreman 12-21-2009 09:18 PM

Gary wrote his reply while I was typing my long winded one. I agree with everything that he says here.

JohnRPollard 12-21-2009 09:31 PM


Good advice above.

I'll just add that there's no need to swap out the "blank" and insert the knotmeter impeller just now. Leave the blank in place at least until spring. This will protect it from any possible ice damage, etc.

Next year, you may find as we do that the impeller gets fouled fairly quickly by marine growth. Which means you'll have to clean it often for it to operate properly. You can clean it by diving, or by pulling it out (swapping with the blank). Sometimes, depending where it's located on the hull, it can be reached with a long-handled brush.

FYI, we don't even bother putting ours in any more. It gets fouled very quickly, and we have boat speed (over the ground, anyway) via gps.

jackdale 12-21-2009 10:31 PM


Originally Posted by JohnRPollard (Post 553151)

FYI, we don't even bother putting ours in any more. It gets fouled very quickly, and we have boat speed (over the ground, anyway) via gps.

If you are using an integrated system, you may want to leave it in. The knotmeter, in conjunction with the GPS, can make set and and drift estimates. It can also be used with the wind instruments to make calculations for true wind angles and true wind speeds.

JohnRPollard 12-22-2009 08:04 AM


Originally Posted by jackdale (Post 553171)
If you are using an integrated system, you may want to leave it in. The knotmeter, in conjunction with the GPS, can make set and and drift estimates. It can also be used with the wind instruments to make calculations for true wind angles and true wind speeds.

Yes, true. However, we don't have an integrated system. Even if we did, I'd still leave it out because around here the impeller would be completely fouled in about 3 days of non-use. Ours is centerline on the bottom of the hull -- it's either a dive down to clean it or swapping out the blank with the attendant geyser, every time we use the boat. Just too much hassle for a very marginal increase in data.

Remember, while bottom growth is about as fast as anywhere here on the Chesapeake, tidal currents are thankfully negligible in most places. Yes, there's a correlation.;)

Sabreman 12-22-2009 09:01 AM

John - I had the same feeling until we raced Gov Cup two years ago. The impeller hadn't been in for 2 years. When I installed it for the race, I saw that it wasn't working, so we used a GPS for the race. GPS SOG lags actual boat speed it really isn't very good at telling what the boat is doing now. On top of that, we used a handheld and at night had to keep turning on the backlight to see what was displayed. This year, we used the knotmeter in conjunction with the GPS and it really helped us to know whether we were he aided or hindered by the current. At times, the current was ~1kt.

I highly recommend slathering Vaseline on the impeller. I read about it here on Sailnet and decided to give it a try this year. We're based on the Yeocomico off the lower Potomac, and marine growth is insane during the hot months. I only had a small bit of slime on the impeller in August after 2 months in the water. After that, there was nothing on it until we hauled in November.

Ajax_MD 12-22-2009 09:09 AM

Yeah.......with this new information, I won't even be attempting this until it's good and hot outside.

I have minimal instrumentation and I'm something of a restoral nut so I'd like to have it working.

I think what I'll do is dive on the thru-hull, and lightly insert a shallow, rubber plug with a string attached to it. That way if I encounter any problems removing the red plug (like damaging the interior fitting or something) the exterior plug will already be in place, preventing or slowing water from getting in.

If I encounter no problems, then I can just gently pop the rubber plug out with the egg beater and recover it with the string.

Anyway, that's all months into the future. Thanks for the clarification.

Ajax_MD 12-22-2009 09:11 AM

Silicone for the O-ring. That'll provide lubrication and prevent deterioration. Petroleum jelly might eat the rubber.

Petroleum jelly on the impeller itself to keep growth at bay sounds like a great ideal.

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