blocked pitot tube -- cutaway diagram of F40 lower unit??

[Mike Topaz]

Hi, My pitot tube is blocked on a tiller-steer 2016 F40. I am aware of the fixes posted on this forum using small hand-held drill bits, compressed air, and irrigation with mild detergent in a syringe. However, I would like to understand the problem by visualising it through a diagram BEFORE I go poking around in with a (hard) drill bit in (soft) alloy. I can see the inlet on the leading edge of the lower leg is a narrow ring of stainless steel (SS) set in the alloy -- but does this SS tube extend all the way up the lower unit to join the rubber messenger tube - or does the pitot tube enter an unshielded gallery cast in the alloy of the lower leg? I have surfed the internet and cannot find a diagram or picture.
(NOTE: I had to buy a whole new lower leg for my first 40hp yamaha outboard (1994 2 stroke, 2nd hand) when corrosion in the pitot tube assembly caused calving of the alloy each side of the lower leg unit. I did not even know pitot tubes existed as nobody I know has a speedo fitted to tiller-steer outboards on "tinnies". Very wary now of flushing pitot tube now, though!!)

Thank You, Mike

[TownsendsFJR1300]

Can't answer your question but can provide an answer for preventive maintenance..

After flushing the engine, I take an old washing machine hose (3/8 rubber line with a garden hose end) with a "V" cut in the end.

I'll put the "V" over the pitot tube opening, using a trigger handle, force fresh water into the hole. My speedometer (if key on), will show about 17 MPH and the fresh water
will shoot back out.

To finish (not absolutely necessary, but I have Schrader valve "T"ed into the tube up in the console. I then hook up a very small bicycle air compressor into the valve. It'll then blow EVERY DROP of water out of the lines..

I don't believe my '06 has a SS tube inside however the factory speedometer since this mod, (12 years old), still works great (down to 4 MPH-lowest speed it will register).


Now blowing with some air to the top of the tube (just as it exits the engine-should be an in-line connector), may help get rid of you blockage.
*Pulling the LU will allow you to put HIGH pressure AIR from the top of the LU and hopefully loosen your blockage.


I'm not sure if they make a pointy, rubber tipped grease gun fitting, but if so, jambing that into the top end of the LU (no hoses) and pressurizing / pushing grease thru the orifice may force your clog out. Two folks will likely be needed-one to pump, one to hold the tip tight to the LU..

Good luck

[99yam40]

I am not sure how you would get all of the grease back out of there if you did try shoving grease into that tube.

I would think there are very few is any people that would know how all of that system is made up in the lower unit since it is not something you would take apart to even see it normally

[TownsendsFJR1300]

I am not sure how you would get all of the grease back out of there if you did try that

Air and brake cleaner. Even by hand, with a small drill bit could pull a good part of the grease out.. That hole is very small, wouldn't take much..



I'd use high pressure air and tapping the LU (near the blockage) before I started drilling ANYTHING..

[99yam40]

but pushing grease into the tube from the top would fill the whole tube up not just the intake orifice.
would take a lot of flushing to desolve all the grease and wash it out.
Just my thoughts , I would not use grease, but then he said he does not use it for a speedo, just does not want to corrode away the lower unit.

My 1999 C4 has not corroded away or been flushed at all.
never been used either, so I have no idea why his other one got eaten up like he says it did.

[boscoe99]

Could be as simple as drilling a vertical hole straight down from the top of the lower unit. Then drilling another horizontal hole from the leading edge of the lower unit so as to insect the bottom of the hole that was just drilled from the top down.

Some may have a stain less steel insert added. A visual inspection should indicate if both the horizontal hole and the vertical hole have an insert.

[Mike Topaz]

Thanks. Actaully, I found this site by reading Townsend's post and he regsitered.
No, I have no need or want for a speedo.
Yes, I will definitely be flushing that NASTY pitot tube when I eventually get it unblocked. I have flushed it since new by removing the blind plug from the hose up near the tiller arm where it comes out of the loom of wires and applying a water hose onto it. Water used to come out the pitot tube on the leg when I did that. Now it is jammed solid. I tried to suck back through the tube, and then poked thick mono fishing line into the tube and it entered about 12mm (1/2 inch) and no more.
Once I bought a new leg for my old outboard, I actually took off the blind plug from the pitot tube, lengthened the tube hose, and had the water running through the hose and over the stern full-time. I was that paranoid about a blockage. Now I have another one in my new motor!
I hate the darn design of these pitot tubes!!!
Maybe we are a small market here in Australia, but we love our tiller-steered "tinnies" and use them in the worst places possible (shallow muddy estuaries with hidden sandbars in the murk)).
So "pitot tubes" here are just a recipe for a disaster in my humble opinion.
Most folk don't even know there is one on the motor, and flushing them is not in the run-down when you buy a motor. Nobody I know has a speedometer on a tiller steered outboard but we do use our sounder-GPS combos to tell speed over ground. Pitot tubes just give you an inaccurate water-pressure proxy.
Surely Yamaha-san has a picture or drawing of how they make the lower leg unit?
There is much talk of the "lost foam" process so even a picture of the foam before it is lost would scratch my itch for understanding.

Thanks for your replies.

[boscoe99]

Thanks. Actaully, I found this site by reading Townsend's post and he regsitered.
No, I have no need or want for a speedo.
Yes, I will definitely be flushing that NASTY pitot tube when I eventually get it unblocked. I have flushed it since new by removing the blind plug from the hose up near the tiller arm where it comes out of the loom of wires and applying a water hose onto it. Water used to come out the pitot tube on the leg when I did that. Now it is jammed solid. I tried to suck back through the tube, and then poked thick mono fishing line into the tube and it entered about 12mm (1/2 inch) and no more.
Once I bought a new leg for my old outboard, I actually took off the blind plug from the pitot tube, lengthened the tube hose, and had the water running through the hose and over the stern full-time. I was that paranoid about a blockage. Now I have another one in my new motor!
I hate the darn design of these pitot tubes!!!
Maybe we are a small market here in Australia, but we love our tiller-steered "tinnies" and use them in the worst places possible (shallow muddy estuaries with hidden sandbars in the murk)).
So "pitot tubes" here are just a recipe for a disaster in my humble opinion.
Most folk don't even know there is one on the motor, and flushing them is not in the run-down when you buy a motor. Nobody I know has a speedometer on a tiller steered outboard but we do use our sounder-GPS combos to tell speed over ground. Pitot tubes just give you an inaccurate water-pressure proxy.
Surely Yamaha-san has a picture or drawing of how they make the lower leg unit?
There is much talk of the "lost foam" process so even a picture of the foam before it is lost would scratch my itch for understanding.

Thanks for your replies.

If you have no need nor want of a speedometer why the concern about a blocked pitot tube? Many just never connect them and use GPS for speed. As you note, with certain boats many have no need at all to be concerned with boat speed. Certainly that was the case of my tinnie. Which actually was made of aluminium.

Pitot tubes on outboard motors were implemented long before Yamaha came along. They appear to be pretty much the same design across all brands of outboard motors. Used on damn near every airplane since the beginning of aviation.

[99yam40]

from what he said, his old lower rotted away at the pitot tube area for some reason,
apparently he thinks washing the tube out with fresh water will help keep this one from doing the same.

seems I remember reading a thread showing a rotted out lower , but not sure if it was his since he is a new poster or at least a new screen name

[boscoe99]

If I were never going to never use the pitot tube, and was significantly concerned about corrosion in the system, I might fill the area with an epoxy compound of some kind to prevent water ingress.

One mans solution to one problem is another mans different problem it appears. 99yam you should be old enough to remember when motors did not have a pitot tube. One was purchased and added to the boat. Folks obviously wanted them integrated within the motor.

[boscoe99]

Yamaha Offshore V6 gear case. It appears that the pitot tube opening does have some sort of a pressed tube being installed. The vertical tube could well be cast into the part when the housing was manufactured. Looks to me like it might have been drilled given the taper at the bottom of the hole.

[boscoe99]

Yamaha SHO V6 gear case.

[99yam40]

it would make some sense to have a tube of different metal(harder) pressed in to keep corrosion down some.

I wonder since the aluminum lower outer surface is painted and the exposed tube is in contact with the water, could stray currents flow thru the tube to the aluminum it is pressed into and cause a faster rate of corrosion.

[TownsendsFJR1300]

but pushing grease into the tube from the top would fill the whole tube up not just the intake orifice.
would take a lot of flushing to desolve all the grease and wash it out.
Just my thoughts , I would not use grease, but then he said he does not use it for a speedo, just does not want to corrode away the lower unit.
.

My thoughts were directly to the LU, (which would have to be removed), NOT going thru any hoses. High pressure anything would likely blow the hose off.


Both my GPS and depth finder (Hummingbird "spinning wheel" unit near the transducer) also give me my speed.

The Yamaha system IS surprisingly close, but then again, your reading speed over water, NOT true ground speed (tides running in and out, river flow, etc)

[Mike Topaz]

Thanks boscoe99!
This is exactly the diagram I was looking for. Especially the view that the inner, vertical gallery going up the leg is of a larger diameter than the pitot hole -- and the possibility that it is drilled in later after the casting (and perhaps not painted).

Boscoe99 -- having a blocked pitot tube with salt water sitting in a void inside an outboard leg in a tropical climate IS a recipe for disaster.

My 1994 40 Hp leg was expensive evidence of this. Pieces of alloy leg calved off before I was even aware the pitot tube existed. I bought the motor 2 nd hand.

Once the fault was diagnosed, the corrosion inside the leg was so bad that when I pushed in a drill bit I "struck oil"!

Gearcase oil. I guess from the gearshift spindle tube, but who knows what had happened in there??

I needed a new lower leg, fully imported from Japan, which put me out of the water for 4 months waiting and saving money. The part alone cost over half what I eventually sold the whole boat, motor and trailer for.

So some folk might think nothing of having a blocked pitot tube -- but not me -- I'm paranoid about it and determined to keep flushing it as Townsend says.

Maybe I could even get a removable plug made to cap it when fishing the mud (but still flush it).

Sorry, no gory photos of the rotted out leg