Big engine in small sailboat but not much forward motion

[opusnz]

Is the gearbox installed and the shift cables linked in such a way that it is being run in reverse to get forward motion? Many transmissions have a different gear ration in forward to reverse. I have seen it before where someone put on the wrong direction prop to compensate for an incorrect installation. Or sometimes they have the wrong prop and then change the direction on the gearbox. It is dumb but I know one guy who was operating with the wrong prop in reverse and he could only make 2.5 knots going forward. Before you spend money and change the prop, check it out.

[Fore and Aft]

Hugo B Tophats are a fairly slippery design, and you should get a lot more speed than that. Dirty fuel filters would be my first check.
Then when motoring do you reach 3600rpm with the throttle wide open? If you are getting that and only 3.5 knots, then you are well under pitched. If you can't get to 3600rpm then you are over pitched. It's not unusual to have a mismatched prop and motor. I have been on plenty of boats where the budget never extended to a new prop, so the boat way underperforms.
Cheers

[Lee Jerry]

Originally Posted by capt jgw

Formula I've always used is 1.34 x square root of waterline length, for all non planing hulls. The physics don't change if you have masts and sails. The physics do change for long skinny hulls like a catamaran, they can go faster.

The equation just gives the "hull speed," the theoretical speed where the wave length of the bow wave equals the waterline length. It's an indicator of performance (drag), but not any kind of limit, even for a displacement hull. The physics do not change with multihulls, only the details.

Originally Posted by capt jgw

Most diesels hit max HP around 80-90% of max rpm, that's where you should hit max boat speed. You do that by having the correct propeller pitch.

This is still wrong. Where max HP occurs is largely irrelevant. You can look here for a brief description of the prop matching process (4th paragraph). In short, if you match at 80-90% RPM (in calm water, clean hull), then performance will suffer greatly at higher demand curves (e.g. in a seaway, with dirty bottom, etc.), when you really need the performance, because prop speed will be further reduced. For a given prop (fixed pitch), the largest factor in generating thrust is prop speed. In addition, matching at lower prop speed (like 80-90%) means the prop pitch will be higher (then if matched at 100% RPM), and then loading will be higher and cavitation will occur sooner at those same higher demand curves, further reducing performance.

[BayouSailing]

Remarkable that only in one post (#42, also a SE Louisiana guy apparently) did anyone mention keystock.

I've seen sheared keys before. You can get engine revs, you can get transmission revs, and it all dies right at the end. But that's the equation, as stated by many others:
- Max Revs in Neutral. It's either in spec, or not.
- Max Revs in Gear. It's either in spec, or not.
- Output shaft/Prop Shaft Revs from Transmission. Yes, they do slip. It's either in spec, or not.
- and finally, that darn key. Actually, one at both ends of shaft probably. Wouldn't it be a dream if it was the transmission coupler slipping and you don't even have to get wet? But check it out, point by point, and you'll find the culprit in pretty short order.

[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

[JimsCAL]

First, the first number is the diameter and the second the pitch. So 16 x10 is 16 inch diameter and 10 inch pitch. If you are reaching 3500 RPM in gear, it sounds like you are a bit under pitched. I say probably, because it is possible the tach is not reading correctly. Use of a handheld optical tach to check it would be a good idea. Cheap ones available on Amazon. If that checks out, a properly sized prop will help. In general that means the largest diameter you can fit properly and at the correct pitch to reach rated RPM at full throttle.

[wyb2]

Originally Posted by Hugo B

…
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. …

Maybe this has been addressed but I don’t think so: what is the “indicator”? If you are only going off a little paddle wheel transducer, could it be as simple as it reading low? They will do that if gummed up with a little slime. 3.5 knots should feel slow, but I could see the difference between 4.5 and 5.5 not being obvious from the cockpit. Double checking with GPS may be in order.

[TrentePieds]

Hugo:

In post #19 I gave you the prop dimensions that a very common "propeller calculator" yields for your engine in your boat. Here it is:

https://vicprop.com/displacement_size_new.php

Plug you parameters into it, and see for yourself :-)!

I found that you need a 14D x 8P if a 2-blader, a 13D x 8P if a 3-blader and a 12D x 8P if a four-blader.

A propeller's ability to absorb however much of the engine's output power remains by the time if gets as far as the prop is a function of the proportion of the "swept area" that is actually taken up by the projected area of the blades. That is why there is the "step down" in diameter depending on whether the prop is 2-, 3- or 4- bladed.

The DISTANCE the propeller will "screw itself" into the water with each revolution is determined by the pitch, so the distance the boat will move through the water on each turn of the "screw" depends on RPM. Not quite as simple as that for a whole lot of highly scientific reasons, so don't worry about that. Now, at least, you know why we are interested in the pitch.

For your present purposes you are wasting your time trying measure boat speed with ANY electronic gizmo or even with a chip log.

What you need is FLAT WATER and a MEASURED MILE. If you haven't an ACTUAL measured mile handy where you are, then ANY known distance will do. You'll just have to do a bit of arithmetic.

Using a Measured Mile goes like this: on FLAT WATER you pass the mark at one end of the mile. When it is abeam, you start your stopwatch. There is one on your i-phone. Then you run the mile, and when the marker at THAT end is abeam you stop your stopwatch. You note down the "Time (it took) To Run".

You do that four times, going first one way along the mile, then the other.

For each "run" you calculate your speed via the famous "STD formula" Speed = Distance/Time. e.g 4 = 2/.5

For example: 2 miles in 30 minutes = 4 knots; 1 mile in 15 minutes = 4 knots (1/.25); 2 miles in 20 minutes = 6 knots (2/.333).

You can do the math now. From what you said in post #50, your boat is probably just fine, but do clean the hub of the prop and see what it sez on it. Then get back to us.

If I understand your googling as you explained it in post #50, it won't do the job for you. You HAVE to go through the propeller calculator! And you HAVE to do the "speed trials".

Cheers :-)

TrentePieds

[Sailor_Jeff]

Ok, hope this helps. .

1. First it sounds like either the woodruff key may be broken or out. If you can safely do this, perhaps dive down in a safe place or haul it out and check. Both the one on the prop and check the (possible) two in the engine compartment. Again, be qualified to dive and please be safe when doing this, never dive or swim in a marina with electric power or by even another boat with electric power.

2. Second, provided that key is good, I'd look to see if it has the correct prop pitch and size. Prop pitch matters most, not so much how many blades. Make certain the pitch is not set up for a smaller engine. Because a fixed pitch prop kind of has a hull speed and seems like a bigger engine won't usually make it go much faster. The prop pitch must be set up for the HP and model of the Yanmar engine.

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.

You can Google to learn how to measure a prop to determine pitch.

2. If the existing prop is a folding or self feathering prop it could be stuck at a partial setting.

3. Does it work better or not at all in reverse?

4. if it's self adjusting has it stuck on a low pitch?

Unlikely what you describe is growth or trash.

Originally Posted by Hugo B

Hi

I recently bought a sailboat, which I am very happy with except one thing. When I use the engine there is very little forward motion, never more than 3.5 knots. This is a problem when I have to deal with currents, as she sometimes hardly makes headway in them.

The engine is a Yanmar 2GM20 and the propeller is a 3 blade propeller. The (monohull) boat is only 27 feet long and weights about 4 metric tonnes. I used to have a boat of similar size and weight with a 2YM15 Yanmar and she did 6 knots! Everyone keeps telling me in my new boat it must be the size of the propeller, but until I lift the boat I can not really measure it and look it up in relation to the engine size.

In any case, she is a full-keeler with attached rudder with the prop sitting in the cut-out space between the keel and the rudder. At the top, that space is the hull itself so there is no room to cut a bigger hole to fit a larger propeller.

I was thinking of perhaps fitting a 4 blade propeller which seems to give better performance at lower speeds, but not as good at high speeds. I am wondering, say at 2000 - 2500 RPM whether this would help or perhaps even make it worse. The other possible solution is to change the shape of the propeller to give better pitch, but the one that is there seems quite standard to me in shape when I look at other props on the web.

One more thing I can think of is that the engine is positioned "backwards" in the boat, with the fan belt aft and the gearbox in the front when you access the engine from under the stairs. Perhaps this changes the transfer of power to the shaft drive in some way?

Any ideas or suggestions how to improve speed would be most welcome.

Thanks

Hugo

[Ballsnall]

Get yourself a strobe type taco and confirm the reduction gearing and pull the prop and confirm the diameter and pitch.

Without this information it's all just guess work, if you want the correct solution you need the correct data.

[Sailor_Jeff]

Hi, Jeff here, I agree with what you said. He needs to confirm the ratio of the gearbox. Then check the prop.

Originally Posted by Ballsnall

Get yourself a strobe type taco and confirm the reduction gearing and pull the prop and confirm the diameter and pitch.

Without this information it's all just guess work, if you want the correct solution you need the correct data.

[GrowleyMonster]

Originally Posted by Ballsnall

Get yourself a strobe type taco and confirm the reduction gearing and pull the prop and confirm the diameter and pitch.

Without this information it's all just guess work, if you want the correct solution you need the correct data.

Absolutely correct.

@Hugo B, zap the prop shaft with a laser tach, divide the reading into engine RPM, to find gear ratio. Pull the prop. Knowing what google says it ought to be isn't the same as knowing what it actually is. You need the ACTUAL diameter and pitch, and you need to visually inspect it, up in the dry. I doubt if the key is sheared but it is possible. Pulling the prop will tell you if the prop key is in good shape or not. The prop needs to be examined for bends and dings, too. Obviously the key any keyed shaft couplings is a potential failure point, too. Just make a chalk mark on shaft and coupler, and if ever the marks no longer are in alignment then obviously you had slip and your key there has failed.

Essential data is being ignored instead of measured and considered. Calculations with the best intentions give random garbage out, when you feed them random garbage in. You don't know for sure where the problem is until you pull the prop and measure it, and find the RPM of the prop shaft.

On the other hand, it runs, and it pushes your boat. So it's not all bad. Can you live with it? But I would at least find out what prop you have, what its condition is, what your gear ratio is, and ensure that your key has not failed. And don't forget your zincs, unrelated to your problem but people go years without zincs and then they mysteriously snap their nice bronze shaft when they hit something with the prop, and find that their shaft is just metallurgical cheese from years or decades of electrolysis.

You could set up a temporary test tank to measure diesel fuel consumption. Extend your TWO fuel lines, the feed and the return. Connect them to a container with graduated markings. A piece of clear vinyl 1-1/2" hose with a plug in the bottom works. An acrylic tube of the same diameter works. Whatever. Mainly you just need to get the engine pulling its fuel from it, and delivering the return fuel back to it, and be able to easily measure small amounts so you can do a good accurate fuel consumption test at one RPM setting in minutes instead of days. You will of course have to bleed the engine's fuel system. The end of the fuel feed line must be covered by fuel. Once the engine is running on your mini test tank, you load the engine by putting the prop in gear, set a test RPM, say 1000 or 2000, whatever, fill the test tank up to a mark, and it is a simple matter of running the engine for a measured time, measuring the fuel level at the end of that time, and crunching the numbers to get gallons per hour at that RPM. You could do idle speed, WOT speed, and every 500 RPM increment between them, and graph your results. Near bumps in the curve you could later maybe go every 100 RPM to zero in on sweet or sour spots, and find your optimum fuel consumption vs distance run speed, if you really wanted to get detailed. At the very least, you want to measure fuel consumption at idle and your usual cruise speed. Compare with other owners with similar boats.

Speed, to me, really isn't an issue, as long as it is a usable and practical speed. Your range and fuel consumption under power, are.

https://www.amazon.com/AGPtek%C2%AE-...hometer&sr=8-4
is a cheap option for a tach.

[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?

[Searles]

Don’t understand how the distance between the keel ( deadwood ) and the rudder affects the ability to increase the pitch on the blades of the existing prop ,which would the verry first thing to try ,pictures would be good or accurate sketches .⚓️⛵️

[Searles]

P/S the Disc area ratio would be good to know .⛵️⚓️