Big engine in small sailboat but not much forward motion

[GrowleyMonster]

Originally Posted by Hugo B

Hi

Sailor Jeff wrote:

From yanmar book:

For the 2GM20

If the transmission is 2.21:1, then the prop should be 14 X 8
If the transmission is 2.62:1, then the prop should be 16 X 10
If the transmission is 3.22:1, , then the prop should be 18 X 11

The above prop sizes are for 2 or 3 bladed props. First number is diameter second number is pitch.

So if I am underpropped, for example the transmission is 2.62:1 but I only have a 14X8 3-blade prop (because the space between the keel and the rudder does not allow for anything bigger) then it would make sense to get a 4 blade prop.

This is because when you increase the number of blades, the prop diameter can be smaller to get the same power output. So if I do not have the space but fit a 14X8 4-blade prop I should get more power compared to having a 14X8 3-blade prop, because the power of the 14X8 3-blade prop is equivalent to a smaller 4-blade prop, i.e. something like a 12X8 4-blade prop. True?

I wouldn't think that the original prop was four bladed, and so unless something else was changed, a four bladed prop should not be needed. Also the four blader will give you a bit more drag, though it could be mitigated slightly by lining up two blades right in the cutout. Anyway I wouldn't start prop shopping yet, because you don't know your reduction ratio or crusing speed prop shaft RPM or what prop you have already installed.

[GrowleyMonster]

Another thing. When you remove your prop, it might be a good idea to measure your aperture, too, just to see what your blade tip clearance is.

[JimsCAL]

Stop getting hung up on needing a four-blade prop. The most important thing is use the largest diameter prop that will fit in your restricted opening (with required clearance) AND THE PROPER PITCH. The correct pitch for your transmission ratio is far more important than the number of blades on the prop.

[rslifkin]

Originally Posted by JimsCAL

Stop getting hung up on needing a four-blade prop. The most important thing is use the largest diameter prop that will fit in your restricted opening (with required clearance) AND THE PROPER PITCH. The correct pitch for your transmission ratio is far more important than the number of blades on the prop.

Number of blades (and blade area) will determine optimum diameter. So if a 3 blade of an ideal diameter and pitch is too big to fit in the aperture with adequate clearance, then going to a higher blade area design or a 4 blade will allow a reduced diameter. You can just reduce the diameter and add pitch, but that will typically give worse low speed thrust compared to having the extra blade area.

[JimsCAL]

Originally Posted by rslifkin

Number of blades (and blade area) will determine optimum diameter. So if a 3 blade of an ideal diameter and pitch is too big to fit in the aperture with adequate clearance, then going to a higher blade area design or a 4 blade will allow a reduced diameter. You can just reduce the diameter and add pitch, but that will typically give worse low speed thrust compared to having the extra blade area.

I guess I disagree with a lot of this. The optimum diameter is driven pimarily by the prop RPM, i.e. the engine RPM a rated power and the transmission ratio, not the number of blades. Since many installations may not permit the optimum diameter being used, it becomes a compromise. The key thing is to try to maximize is efficiency - how much of the engine power is converted into thrust. With a relatively low HP engine like the 2GM, your really don't need a lot of blade area. That's why going to 4 blades is probably a waste of time. Prop pitch is key.

[rslifkin]

Originally Posted by JimsCAL

I guess I disagree with a lot of this. The optimum diameter is driven pimarily by the prop RPM, i.e. the engine RPM a rated power and the transmission ratio, not the number of blades. Since many installations may not permit the optimum diameter being used, it becomes a compromise. The key thing is to try to maximize is efficiency - how much of the engine power is converted into thrust. With a relatively low HP engine like the 2GM, your really don't need a lot of blade area. That's why going to 4 blades is probably a waste of time. Prop pitch is key.

It's driven by both. Try out a prop calculator, and generally as you increase blade count (with the engine and trans details kept the same), the recommended diameter gets smaller. If you can fit the optimum diameter for a 2 or 3 blade with fairly long, skinny blades, that's more efficient. But if you can't, you end up going to either fatter blades or more blades (or both) to keep the blade loading down and avoid cavitation (and poor low speed thrust).



I do agree that this application probably won't need a 4 blade thought. I doubt it's diameter restricted enough to need it.

[Lee Jerry]

Originally Posted by Hugo B

Hi

First of all, thanks very much for all the great and interesting advice. I am learning lots of bits and pieces from all the replies!

After diving down and knowing a fouled prop is most likely not the reason I took up the most common advice here and did some engine RPM checks.

In the marina, in neutral (prop not engaged), I could go up to 4000 RPM with WOT. So it seems there is no basic throttle problem and overall I think the engine by itself is OK.

Yesterday I went out and checked how things behave under load, just motoring (no sail up of course). Unfortunately there was a lot of wind and a 2.5 m swell. I made sure the wind was coming from the beam (which I assume is the most neutral position), but I could not avoid the swell of course.

At WOT I could go up to about 3500 max. Regarding speed, I would say probably 4.5 knots on average, the indicator was reaching 5 here and there but that could be because the boat was surfing down a wave. Things were a little wild out there so it was difficult to get a good reading. I must try again when the sea is flat with light winds.

Seeing I can get to about 3500 RPM it looks like I am not overpropped, but the max speed of 4.5 knots means I am likely still somewhat underpropped.

When a I google prop dimensions for a Yanmar 2GM20 I get 16 X 10. Can someone perhaps explain to me what that is? Is the first number diameter and the second pitch somehow? I cannot fit a bigger prop in the space between the keel and rudder, so it either has to be a prop with a better pitch or still perhaps a 4 blad prop (although most of you say that it only improves speed in the mid-range).

Seeing how everything now seems to point to the prop, any technical advice regarding props for this engine and boat (particularly specific details about prop size and pitch) would be most welcome!

Thanks

This is good, you're getting there. Definitely need to repeat with milder conditions though. And you will need to get better data when doing it. The results / conclusions are only as good as the data going into it.

These have all been mentioned, but I'll summarize:

Run in one direction, then turn around and run the reciprocal course (in the same or similar water). This minimizes the effects of wind, waves and current.

For speed, unless your knotmeter has been recently calibrated (and maybe even if it has been), it isn't good enough. The easier increased accuracy is GPS speed (averaged over the two directions). The best option is the recording the time over a measured distance as described by TrentePieds above. (It doesn't have to be a mile. Any known distance will work, but longer is better up to a point, to minimize error.) Pick two marks and use your GPS to get the distance by marking their location when you're as close to it as you feel comfortable. You can use two government marks or drop one or two of your own. This also makes the first item easier (no compass calculations to get the reciprocal course). You could also use this opportunity to calibrate your knotmeter at the same time.

Your tachometer needs to be checked too. This does not need to be super accurate and doesn't have to be done at the same time as the speed runs. Just put the optical tach on the engine and note the gauge reading and the optical reading at WOT and several lower throttle settings. Best case is they read the same, but you'll probably find a small offset (which may increase slightly with engine speed). You could also shoot the shaft speed if you need to confirm the reduction ratio. The engine speed could be checked in neutral at the dock.


It sounds like the issue is not quite as dramatic as the original post made it sound. If you're still not happy with the performance once it's been quantified better, you will then need to figure out definitively what prop diameter and pitch you currently have before making any changes. Then it would be appropriate to discuss what those possibilities might be.

[TrentePieds]

Bravo, Lee Jerry - excellent summary :-)!

TP

[George_SD]

1) how are you measuring the speed? Speed through water? Speed over ground?

2) I have 2GM20F on a 32F and it can get me up-to about 6-6.5 depending on the prop. Original 2 blade fixed prop (gave me 0.5kn more) was 16x13. Now I run a folding 16x12 for 6kn of speed @ 3200RPM.

[GrowleyMonster]

Originally Posted by Lee Jerry

This is good, you're getting there. Definitely need to repeat with milder conditions though. And you will need to get better data when doing it. The results / conclusions are only as good as the data going into it.

These have all been mentioned, but I'll summarize:

Run in one direction, then turn around and run the reciprocal course (in the same or similar water). This minimizes the effects of wind, waves and current.

For speed, unless your knotmeter has been recently calibrated (and maybe even if it has been), it isn't good enough. The easier increased accuracy is GPS speed (averaged over the two directions). The best option is the recording the time over a measured distance as described by TrentePieds above. (It doesn't have to be a mile. Any known distance will work, but longer is better up to a point, to minimize error.) Pick two marks and use your GPS to get the distance by marking their location when you're as close to it as you feel comfortable. You can use two government marks or drop one or two of your own. This also makes the first item easier (no compass calculations to get the reciprocal course). You could also use this opportunity to calibrate your knotmeter at the same time.

Your tachometer needs to be checked too. This does not need to be super accurate and doesn't have to be done at the same time as the speed runs. Just put the optical tach on the engine and note the gauge reading and the optical reading at WOT and several lower throttle settings. Best case is they read the same, but you'll probably find a small offset (which may increase slightly with engine speed). You could also shoot the shaft speed if you need to confirm the reduction ratio. The engine speed could be checked in neutral at the dock.


It sounds like the issue is not quite as dramatic as the original post made it sound. If you're still not happy with the performance once it's been quantified better, you will then need to figure out definitively what prop diameter and pitch you currently have before making any changes. Then it would be appropriate to discuss what those possibilities might be.

Indeed. The only indication that a problem exists in the first place, relies on the knotmeter, and assumes that it reads correctly.

Just about any GPS unit has an easy method for recording a waypoint, and a method for calculating distance between them without the user doing any math. So a simple but still accurate way to find speed at a given RPM is to start the speed run, go a minute or so to get the boat up to speed, then at the top of the minute, record a waypoint. Keep going for exactly 6 minutes, a tenth of an hour, and record a second waypoint. Set a route with the two waypoints and get the distance between them in nautical miles, and multiply by 10 for the speed in knots.

The instantaneous speed given by the GPS device is probably accurate enough to debunk or sorta confirm the accuracy or lack thereof, in the boat's speedometer, but doing a speed run over a measured or known distance, or a speed run lasting a given length of time and finding the distance, will be a much more positive and accurate determination of speed than an unverified knotmeter.

No GPS? That would probably be incorrect. You DO have a GPS, if you have an Android or other type of modern smart phone. All you need is a GPS navigation app that will record two sets of coordinates, and measure the distance between them in Nautical Miles. Certainly, OpenCPN is more than capable of doing this, but there are lots of simpler apps that will work.

[Mauruuru]

I'd check the prop for fouling. I had my boat scheduled for a haul out and was headed for the marina down the ICW. Boat seemed fine till I got out of my canal into the main ICW flow. I was headed into tidal flow and at full throttle couldn't make way. Thought something was broken or prop had fallen off. Got a tow to the marina and when they hauled the boat the 3 blade prop was totally encrusted in barnacles. My prop was an egg beater all it did was churn the water. This all happened in just a couple of weeks of sitting at the dock. start here.......