Sea trial

[eyschulman]

I have noticed in talking to owners of power boats that I evaluate during a CGAUX safety check ( Not a safety issue )that not too many are familiar with the process of a formal sea trial related to the propping and loading of their engines. I would think the posters and readers on TF would probably score higher in this area of boat management. I am curious to know how many have done this with their boats and do follow up with regular WOT checks for overloading. Are their posters and readers who have no idea what this is about as are many of the boaters I have come across? Does this site have a detailed description of this process which is a easy DIY? If not would that be a valuable addition to the site?

 

[caltexflanc]

I am going to surmise most do not, at least in comparison to those who go planing speed on a regular basis. I had something of a prop fetish, plus having 2 stroke Detroits that I ran at low speed, I was used to running the engines way up for a few minutes on a regular basis, so might as well test WOT while I was at it.

 

[River Cruiser]

I have EH-700 naturally aspirated Hinos 175 hp, about every trip out I'll run up to wot for a minute or two just to make sure no problems have jumped up.

 

[refugio]

I play with props on my dinghy. My trawler has a 40x23 prop on a 3" shaft - it doesn't matter if that's the right prop because I cannot afford to buy a new one. ?

 

[psneeld]

It's nice to pass along a "hard" statement on a specific issue...but there is some disagreement about cruising props and their viability.

 

[FlyWright]

About a week ago I would have said my engines run at their WOT of 2800. The stbd tach always read steady at all speeds and was rock solid on 2800 at WOT (my rated WOT). The port lagged behind 300 RPM, even with the engines synced. I chalked it up to a bad feed from the port alternator since the port tach often acted a little jumpy ...ever since I bought the boat 7 years ago.

Then I replaced my stbd alternator last week. Now my tachs match perfectly and are rock solid at all speeds and, at WOT, my tachs are reading about 2500 RPM. No more jumpiness on the port tach. I'm now cruising at 1800 RPM (indicated) instead of my former 2000 RPM to achieve the same sound, throttle position, stern wave, and speed.

It's said that a man with a watch always knows what time it is, but a man with two watches is never quite sure.

For 7 years, my stbd tach has been my steady tach and the one I relied upon for setting engine RPM. Now it matches the port tach perfectly when synced. I'm not sure I can trust them to give me accurate data.

So the $64,000 question is.....

What's the best way to confirm actual engine RPM on a diesel?

 

[refugio]

What's the best way to confirm actual engine RPM on a diesel?

Put a reflective mark on something that rotates and use an optical gun on it. I forget the tool name - it's hot here and I had a liquid lunch. Light tach? Something like that.

But the better gauge is temperature - either cylinder head or exhaust gas. I had a pyrometer on my Romsdal years ago - with a CPP I just loaded it to the optimal temp.

 

[BobH]

Optical tachometer or more accurately "Digital Photo Sensor Tachometer".

Bob

 

[refugio]

optical tachometer or more accurately "digital photo sensor tachometer".

??

 

[caltexflanc]

Then what you do is a chart showing tach gauge reading vs digital. Kind of like a compass deviation card. In some cases you can adjust the tach to match, more or less.

 

[BandB]

We don't just check at WOT. We have charts and records at all rpm's and speeds including fuel consumption. So when we open both engines to a particular rpm we know what speed to expect. Now we also do open to WOT at least once most days we're out. The exception is when conditions are too rough. We learned long ago on much smaller, faster boats to keep an eye on max rpm's and look for problems or changes. It's not the issue of going fastest but simply that WOT gives you the quickest indication of what is going on. It's an easy objective measurement. Honestly, we haven't had a major problem with tachometers.

 

[markpierce]

While capable of WOT at 2400, my favorite RPM is:

 

[BandB]

While capable of WOT at 2400, my favorite RPM is:

Ours is typically 1800 to 2000 as well so very different boats and speeds but similar rpm at cruising. Now we also go much slower if conditions require.

Someone mentioned tender and it is also important although we seldom run ours at WOT of 8000 rpm, but normally between 3000 and 5000 and often at 1000 to 2000.

 

[markpierce]

Ours is typically 1800 to 2000 as well so very different boats and speeds but similar rpm at cruising. Now we also go much slower if conditions require.

1800 gets me 6.3 knots. 2200 (full but not flank speed) gets me to 7.3 knots (hull speed), used when bucking a strong tide or in a losing attempt to keep up with friends who have twin engines and semi-planing hulls. (Pineapple Girl will soon pass while FlyWright will restrain itself and Mahalo Moi takes a photo.)

 

[BandB]

1800 gets me 6.3 knots.

A little faster for us, but the point of knowing and checking is the same.

 

[klee wyck]

The green monster's manual says WOT is 2800. I guess I have never tested that without the transmission engaged. While underway it will max between 2300-2400. Though I am 48 on deck I am only a tad over 40 at the water line due to the double ender effect so I think my theoretical hull speed should be around 8.1 knots. It takes about 1800-1850 to get that. The boat sounds and feels best around 1400-1500 which gets me 7.4-7.6 knots and where I run mostly these days. Going from 1500 up to 1800 just seems to make her squat a little more and gets me no more than half a knot.
Interesting other note is that my hydraulic get home that only generates 12 hp off the generator will get me close to 2 knots in calm seas with no current.

 

[healhustler]

\
Interesting other note is that my hydraulic get home that only generates 12 hp off the generator will get me close to 2 knots in calm seas with no current.

That is interesting, Bill. Any idea what the approx. weight of your vessel is? I've often contemplated putting a PTO on my genset for a get-home, and now that it's in the new hatch under the veranda, it's possible to do it with a mechanical set-up (shaft and prop) instead of hydraulic.

 

[klee wyck]

HH,
The build spec calls out a little over 48K but the last time I was in a sling they said it weighed 55K at that time. Somewhere in that range I guess. Lots of steel.
When I first engage the get home it takes a bit to get her moving but then does pretty well once it gets going. The hydraulic unit normally runs the bow thruster but changing a couple valves turns the main shaft instead.

 

[eyschulman]

I am probably going to get some flack here. It is my understanding that most engine manufacturers specify in there specs what RPM and what temperatures (exhaust )the motor is supposed to achieve to obtain the intended toque and power curve. There is also usually a RPM that is specified for no load. It is my understanding that the builder of the engines intends the motors to be set up that way irrespective of what RPM you actually run motor. The people who sold me my JD engines would not give me an extended warranty until they did exhaustive on boat on water testing so that these and other parameters were indeed at acceptable levels. One of the things they were very concerned about was that there should be no over propping resulting in overloading of the engines. I understand that there is a component of boating people who disagree with this most notable among sailboats and some slower FD motor boats usually with smaller motors. Many claim they do not run their motors near peak so no damage done and claiming better economy. I have also come across statements by acknowledged master mechanics that an overloaded motor is overloaded at any RPM even if no damage is caused. On the other hand one of the often quoted causes of shorter than expected engine life is overloading and evidently in my case JD supports that enough that they would not give a extended guarantee without proof of proper set up. I suppose if you only have a year or two warranty and you blow a engine they would be happy to sell you parts or a new one. The actual performance of the testing for loading and proper propping is a relatively simple DIY thing once there is an understanding of the process. The use of exhaust pyrometers and modern electronic load information is also quite helpful in monitoring what's going on. On boats with adjustable pitch the process is dynamic and can be changed as needed. With fixed props if your curve is off it takes a pulled prop and a prop shop and some times a different prop to get things straight maybe that's why there is so much resistance.

 

[jimbob]

not to far off topic i hope, is this an issue that an engine survey would look at?

 

[caltexflanc]

not to far off topic i hope, is this an issue that an engine survey would look at?

Absolutely! As noted above, both load (sea trial) and no load (at the dock). A good engine surveyor will bring their own gauges and have the boat run at various RPMS during the sea trial.

 

[klee wyck]

Doc,
All is not lost. I went back to look at the survey from 2012 where there was both a general marine surveyor and a specific engine surveyor on board during sea trial. They did, in fact, look at the parameters you suggest in regard to loading. The numbers were not exactly as I previously posted but close and I will try to attach that section. Bottom and prop were pretty fouled during this trial as boat had not been real active prior to purchase.

 

[ghost]

It was asked what proportion of boaters understand the ramifications of wot speeds. I've run experiments and calculated it at just slightly less than the proportion who understand the ramifications of the dot size of their infra red temp gun.

 

[eyschulman]

not to far off topic i hope, is this an issue that an engine survey would look at?

It should be a major issue in an engine survey(rarely covered in standard boat survey). your surveyor should photo time your motor at the dock and confirm accuracy of tachometers and make a deviation chart then sea trial first for WOT no load then out on the water WOT full load. If this is not done get a different surveyor. You would also be wise to get owners permission to open turbos for a look. Any large high output engine that has been run over propped or with signs of moisture in turbo becomes a much higher risk for future premature failure.

 

[Moonstruck]

Yes, a good engine survey should cover this. I just had the heat exchangers service with new exhaust elbows installed. I ran the boat with the mechanic in the engine room. We ran it up to high idle to check the governed top rpm. Then we took it out and after a warm up ran the engines at top speed. This while checking temperatures at critical points of water flow and exhaust. The engines turned their rated rpms at 34 knots. All points were checked for leaks and waterflow. All this should be done on an engine survey.

There is a lot of money in that engine compartment. It is good to know the engines are running in top form.

Edit: Forgot to add that the tachs were checked with an optical tach while doing the check. They were withing 50 rpms. With the synch on they were dead on it. She was smooth and ran like the clappers.

 

[jimbob]

thanks all, hope to need a good survey in the near future