Speed while motoring

[Supercat568]

Hello,
I've got a 1985 Pearson 27. The 84 and 85 were very low production numbers and it seems I'm on my own as far as boat specific issues.
Previous owner swapped out the original 1GM? for a 2GM20F, the heat exchanger style cooled engine. The boat gets to 4mph at mid throttle, adding more throttle doesn't add any speed, just turbulence. Plus, I get a vibration at a specific rpm. Not sure what rpm that is because the tach doesn't work, yet. It's got a two blade, probably the original prop but I'm not sure. When adding throttle past mid throttle, it feels like it's cavitating, but I can't tell.
This engine runs exceptionally well so power isn't the problem. I can't imagine that the orignal prop would have a problem getting the boat to 6 or 7mph at medium or higher throttle.
I haven't had a chance to check the prop shaft to engine setup but the mechanic told me that if there's vibration at all power settings, that would lead one to believe that the engine to prop shaft alignment is out of tolerance. He also says that the 2GM20 engines typically have a spot where you get a little vibration.
The vibration isn't a big deal, but why won't my boat go faster than 4mph?
Any help greatly appreciated
Thank you

[Nekton73]

First thought, how clean is your bottom? I know that's kind of a personal question for some, but it certainly could dramatically affect your speed if you have a lot of fouling or hairy growth.


Assuming a clean bottom, next I'd look at the propeller. You mention you think it is a 2 blade original, but are not sure... without knowing if the prop is correctly matched it's a bit of a guessing game.


Those are the first two places I would look for answers.



Just for clarity, do you mean Nautical miles per hour - KTS, or Statute miles per hour - MPH?


Your boat should have a theoretical hull speed of approximately

1.34 x sqrt of LWL = 1.34 x sqrt of 22.5' = 6.35 KTS

[Orion Jim]

You should have no problem attaining hull speed with an 18hp diesel. Dirty bottom or the wrong pitch prop.

[hugosalt]

As you stated, (if bottom and prop clean,) power should not be a problem. The 2GM has been installed in bigger/heavier boats such as Bene 311, S2 9.2 (30'}, Hunter 33 and even Hanse 34.
all capable of motoring at hull speed in benign conditions.
So next stop, as stated by Nekton, would be to check prop. Seems illogical for the previous owner to have upgraded the motor to a 2GM without upgrading the original prop but stranger things have happened.
Have to research not only prop size but pitch as well. Short of an alignment issue I'd focus on checking prop pitch.
Also if prop upgraded, cavitation could be caused by larger prop being too close to hull.

[Supercat568]

My bottom is clean and smooth. Boat bottom at least. What change in prop would come with increased hp? Wider?
I only have my phone for now so the speed is in mph as determined by Google maps.
Thank you for the replies

[Bill O]

Use a prop calculator online to determine the size and pitch prop for your engine/boat. You will also need your gear reduction info off the transmission plate to do the calculation.
If the PO used the original prop, then it probably is too small/not pitched correctly and why you "top out" w/your speed.
A two blade will be good as a sail prop, while a 3 blade will give you better motoring but more drag under sail.
Would be good to have a tach to check prop performance to engine rpm. Appears the tach sender on the 2gm is on the backside of the motor. When you do get the tach working, the best rpm for fuel efficiency will be at the torque peak (~2600 rpm).
On a quick search found and engine repair manual for your motor. https://www.slideshare.net/yjksmemd/...anual-80154113

[TrentePieds]

Google maps give you speed OVER THE GROUND (SOG). What you need to know is SPEED THROUGH THE WATER (STW).

There will be landmarks along you shore that are a known distance apart. Do a proper "speed trial" measuring the "time to run" between marks. Do it first one way, then the other way, and then do it again both ways, measuring the TIME TO RUN (TTR) with a stopwatch. Your i-phone should give you one, I would think. Then do the TDS (Time, Distance, Speed) calculation. Now you will know your speed through the water, which is what you want.

The 6.35 knots given by Nekton (above) is as fast as you will ever go THROUGH THE WATER, ON FLAT WATER, with that engine fitted with the optimal prop. Speed over the ground as measured having reference to satellite provided data is irrelevant.

Here is a calculator to determine what your prop specification SHOULD be.

https://vicprop.com/displacement_size_new.php

What they actually are, you determine by cleaning the hub and reading what is imprinted on it, e.g. 12D10P, meaning 12" diameter, 10" pitch. Even that may not be true because the pitch COULD have been "adjusted" but the new spec. not noted on the hub.

You may want to get this article under your belt. It doesn't relate to your boat specifically, but the principles are universal:

https://www.finesse-owners-associati...peller-sizing/

Play with the input values in the calculator to derive a "feel" for how different inputs affect the propeller requirements.

Have fun :-)!

TrentePieds

[desodave]

What TP said.


Do you know what the current is where you are motoring? If you're motoring at 6 kn STW into a 2 kn current, you're doing 4 kn SOG ... it's like trying to motor up a river.

[Supercat568]

Problem description: boat is new to me. the boat will accelerate to about 3mph and adding throttle after that doesn't increase speed. Tach doesn't work and all I have is my GPS for speed. I've motored enough to know that current and seaweed aren't the problem. I watched the engine and it's not bouncing around beyond normal, motor mounts are good, and it runs very well. It just feels like there's seaweed on the prop, exactly like it. The original engine was a 1GM10 and it was replaced with a 2GM20F. All I know about the prop is that it's a two blade and the service guy told me it looked good...they set the boat in the water before I arrived, I've never seen it out of the water.

Recent work:
Hello, thank you so for the replies. The boat was salt water so getting the square set screws out of the coupler wasn't easy. I thought for sure they were going to break, but they didn't, phew. The boat is in the water till tomorrow so I couldn't take much more apart without worrying about sinking the boat. I've done the engine to prop shaft alignment procedure on an inboard ski boat I owned so I understand what it's doing and how to do it. The coupler wouldn't slide back on the shaft, so I couldn't really separate the two couplers very much, and even when I had them apart, the trans coupler still rotated with the shaft coupler, so I don't think I'm looking at the true alignment of the two....or am I? With them separated slightly, will a misalignment show up or is there something else holding them in place? On my ski boat there wasn't. But like I said, even with them separated by maybe 1/4" they still spun together, and I couldn't get both arms in there to spin one and hold the other. So I don't know yet what's going on. I'm unfamiliar with the dripless seal system and didn't want to sink my boat, or even have an emergency with water rushing in. If anyone watches the video please excuse the noises, I was jammed into the quarterberth and the phone was right in front of my mouth, my apologies.
What would a list of possibilities look like, everything that could be wrong and causing this issue?
Bent prop, loose prop, broken prop, really bad alignment, ......????
Thank you


https://youtu.be/WUj6r8aAEKU

[Jammer]

Probably the wrong pitch or diameter prop. Possibly an engine with a case of the wimpies.

You really need to know the RPM to figure this out. There are various cheap RPM measuring devices including an app for your phone that uses the camera and light as a strobe. You also need to know the gear ratio of the transmission either from the tag or from painstaking measurement. You also need to know the diameter and pitch of your prop, which are probably stamped on it, and can be measured with a tape measure or ruler.

[Don C L]

Where are you? In my area there is a prop shop and the guy is very knowledgeable about sailboat props and matching horsepower, hull and prop size and pitch. You may have a good prop shop near you. If it were me I'd jump in and remove the prop, or get a diver to do it, as a first step.

As far as speed over the ground (SOG,) there are navigation apps, like Navionics, that will spit out your SOG readily. I use iSail on my iPhone since it displays on a chart which I like. If you are in calm water and you know there are no currents, SOG is workable. But really getting the prop, gear ratio and engine hp all straightened out is the thing to do, regardless of what an app says.

[SailingHarmonie]

Somehow much of this thread has gotten stuck on the possibility of poor alignment of the propshaft and engine as a potential cause for the boat's poor performance under power.

Am I the only one who thinks this is nuts? Of course proper alignment is vital to the mechanical integrity of the boat's drive system. A badly aligned shaft will wear and damage many parts of the boat prematurely, and cause excess vibration and noise. But slowing the boat down? Huh?

If the boat has a clean bottom and prop, there are two things that matter. How fast the prop turns, and how much water it moves. In the simplest analysis, the first of these issues involves the engine, and the second the propeller.

Without an accurate engine RPM and corresponding propshaft RPM all the rest of this is just hand waving. This whole thing could be as simple as a cable linkage not fully advancing the throttle. A simple laser RPM tool is less than $20. Without that data speculation is worth than worthless because it will cause you to spend time, effort and money on things that might have nothing to do with the issue.

To the OP: You will not get useful advice until you provide the following data:

1. No-load engine RPM at full throttle, and idle.
2. Engine RPM at full throttle with the boat moving, and a contemporaneous recording of the boat speed. It would be nice to see a couple intermediate points as well, RPM vs Speed at lower throttle settings.
3. Either the transmission ratio OR a measure of the engine speed and the shaft speed at the same throttle setting.

Then the people offering advice can actually have a clue if the problem is in the engine room or the running gear.

The suggestion that the FIRST thing to do is pull the prop is wrong. The FIRST thing is actually measure the RPM and see where the problem is.

[Supercat568]

Got the boat on land today. The prop has 12R10 370318 stamped into the main body section. It's a two blade and looks in very good condition. Both blades are symmetrical whole looking from side and back. Leading edges look very good for what might be a 38 year old piece.
The prop shaft support bearing piece that supports it outside the hull fits very nice, tiny amount of movement, almost zero in and out movement, and it rotates nicely.
This was the first time I was hoping to find something wrong!
Does anyone know if this is the correct prop for the boat?
I went on a prop calculator site but some info they're asking for I couldn't provide.
Thank you

[Jammer]

12" diameter
R right hand
10" pitch

Did you get engine RPM at wide open?
Did you get the gear ratio of the transmission?


I experimented with a propeller calculator and it looks like your prop is probably wrong for the boat. I used 12 hp, 2700 rpm, and a 2.2:1 gear ratio. This gives a theoretical 15.3" diameter prop with 10" pitch and 2 blades, so the prop you have would have the correct pitch but be too small.



With a 1.4:1 gear ratio I can get the diameter of the prop to match what you have but the pitch has to be 6.4".



Either way, or in between, you have the wrong prop, and will not get the speed you want unless you change it. What you should change it to will depend on the max RPM of your engine and the gear ratio of your transmission, which you will have to find out to get this right.

[Supercat568]

It's got a kanzaki km3a gearbox with a reported ratio of 2.64/1. I'll attach a picture that I took a screenshot of from a video so it's hard to read.
I really hope you're right because if not then I'm in the dark as far as finding the problem.
Thank you