Sea trial

Capt.G

Capt.G

Member
Apr 14, 2018
43
Boat Info
2005 Sundancer 300
Engines
5.0 mercs bravo 3
Hey club members
I just purchased well in the middle of purchasing
A 2005 300 with twin 5.0 and brvo 3 hrs ate 280
My question is if a sea trail is performed and the motors perform good no skips smooth under load tests why would one get a compression test after that? Is it really needed ?
My thoughts are if a cyl is down on compression
It will have a choppy shake to the motor..
There for an inspection of electrical fuel and mechanical would then be needed. Thoughts ?
 
While you might notice low compression, you might also not. There's a lot happening in an engine room while both motors are running at 4500+ RPM that a low cylinder could be hidden.

I like data points, so having the actual numbers was important. I also take compression tests every 150-200 hours to see how things are going internally as well as oil tests.
 
Might as well find out now before signing on the dotted line, whether good bad or ugly. A good survey is relatively cheap too, a couple hundred buck that might save you thousands.
 
Strecker25 said:
While you might notice low compression, you might also not. There's a lot happening in an engine room while both motors are running at 4500+ RPM that a low cylinder could be hidden.

I like data points, so having the actual numbers was important. I also take compression tests every 150-200 hours to see how things are going internally as well as oil tests.
Click to expand...

Agree with all of this, but I would change "might" to "probably wont" notice low compression. Low compression from major damage you might notice, but a V-8 engine that is low in a couple cylinders due to ring issues, valves etc won't shake or vibrate so you would notice that something was wrong. At speed, boats vibrate, shake and make noises due to wave action that will mask what a compression test can tell you easily. Plus there is no "judgment" with compression test results. It is what it is. With a sea trial you are thinking "does that sound/feel right, maybe its oK?".

And lastly, you want a mechanical survey for the following reason (sorry for being a bit vulgar to those sensitive types):

A boat buyer on a sea trial (even an experienced one) is like a 14 year old kid with a boner. Everything looks good, and you swear your are in love, and this time its real.
 
make sure to run for 20-30 min up on plane.. my trial was short runs intracoastal limited.. which let the engine cool.. i had a piece of impellar in system and would overheat after running for 20 min.. never said anything about my low water pressure either...
 
When purchasing my first boat, I took it out on a sea trial with the seller and it ran great. I didn't notice any issues and so (being very foolish) I skipped the mechanical and just had the survey done. Fast forward 3 months and the boat can't get on plane with more than 3 people aboard. Turns out that there was no compression in two cylinders, and I ended up having to have the entire upper unit of the engine rebuilt. Cost about $5000 for this on a Mercruiser 5.0. Moral of the story is that this was likely a problem that had been festering for some time and only got worse during those first 3 months of ownership. Had I gotten a compression test and a proper mechanical, the problem likely (definitely) would have been detected saving me a very big headache. Always get a thorough mechanical inspection with a compression test. I had to learn that the hard way. Never again.
 
Creekwood said:
Agree with all of this, but I would change "might" to "probably wont" notice low compression. Low compression from major damage you might notice, but a V-8 engine that is low in a couple cylinders due to ring issues, valves etc won't shake or vibrate so you would notice that something was wrong. At speed, boats vibrate, shake and make noises due to wave action that will mask what a compression test can tell you easily. Plus there is no "judgment" with compression test results. It is what it is. With a sea trial you are thinking "does that sound/feel right, maybe its oK?".

And lastly, you want a mechanical survey for the following reason (sorry for being a bit vulgar to those sensitive types):

A boat buyer on a sea trial (even an experienced one) is like a 14 year old kid with a boner. Everything looks good, and you swear your are in love, and this time its real.
Click to expand...
That's why on a sea trial I'm just an observer. I rely on my surveyor to be conducting the sea trial with the company representative.
 
Most surveys are hull and systems only - they do not look deeply at the engines. An engine survey will remove any doubts of pre-existing and evident issues. At minimum I would look for a compression test and read the computer for recent faults and engine times. The mechanic doing the survey may offer just basic services or have packages that include fluid analysis. If problems are found it may save you more now than the cost of the engine survey. You might want to make sure they take a good look at the Bravo 3s because while they are great units repairs and replacements can be quite expensive.

-Kevin
 
wow awesome. great input for all . I"m sold. anyone know or recommend a engine surveyor in Rhode Island boat is in Narragansett area.
 
I did not get a survey because I knew next to nothing about boats. We did sea trial it and it ran great. I lucked out.. Bought it June 2015 and have not had to put anything into it that was a necessity. I have done tune ups, oil changes, new canvas, etc. But so far I have been very lucky. That being said, my next boat will absolutely have a full hull and mechanical survey. Won't buy another one without.
 
Compression testing will show if the engine will develop compression and that each cylinder is within range of each other however that is about it.
A more comprehensive and accurate test is a Leak-Down test and what the surveyors should be doing rather than the compression test. A Leak-Down test reveals leakage in valves, leakage past piston rings, head gasket leaks, and can determine severity of a cylinder not within range of the others. Plus a Leak-Down test does not kill the batteries with continuous cranking.
A leak-down test should be requested with the engine surveyor.
 
ttmott, for those of us who are not mekanikully inclined, can you explain what a leak-down test is and how it is done?

Curious GFC
 
A leakdown basically fills the cylinder with the valves closed and see how much pressure leaks out. Haven't done one in a long time but IIRC it measures a %. These tests are down with the engine off, key off etc. Just have to make sure both valves are closed and pull the plug, fill with air. If it is leaking you can get an idea of what is leaking. If it blows out the carb or exhaust then it's valves. If it blows out the PCV or breathers then it is rings.
 
boatman37 said:
A leakdown basically fills the cylinder with the valves closed and see how much pressure leaks out. Haven't done one in a long time but IIRC it measures a %. These tests are down with the engine off, key off etc. Just have to make sure both valves are closed and pull the plug, fill with air. If it is leaking you can get an idea of what is leaking. If it blows out the carb or exhaust then it's valves. If it blows out the PCV or breathers then it is rings.
Click to expand...
Yes, a calibrated air flow rate is fed into the cylinder through the spark plug hole and, simultaneously, pressure which is proportional to flow rate in the cylinder is monitored. The differential is leakage. As boatman37 stated leakage source then can be determined which is a much better "gauge" as to a problem or not and the severity of a problem. Here is a video that describes a bit better -
 
There is a lot of excitement, noise, drama, and anticipation in a Sea Trial. Hopefully you are not Sea Trialing on De-Nile either. If you are able to detect, by ear, a single cylinder with low compression under these circumstances, good on ya.

But we mere mortals should have the compression tested.
 
I found a surveyor that does compression checks also records data from motors of hrs at rpm. Ranges
Any codes Also records temps rpm etc. To compare with factory gauges and infraed temp. For manifold and risers and more. I am pumped hopefully it all looks good. This process is alot more then buying a car lol but I know it's needed
 
Capt.G said:
I found a surveyor that does compression checks also records data from motors of hrs at rpm. Ranges
Any codes Also records temps rpm etc. To compare with factory gauges and infraed temp. For manifold and risers and more. I am pumped hopefully it all looks good. This process is alot more then buying a car lol but I know it's needed
Click to expand...
It’s worth it. Make sure he sends out the oil samples for analysis too.
 
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