SailNet Community banner
61 - 80 of 86 Posts

·
Registered
Joined
·
25,130 Posts
Here's another reason for WOT. One's boat may run nice and smoothly at 60-80% of WOT, but feel like it wants to rattle out of the boat at WOT, due to misaligned drive train, old soft engine mounts, bent shaft, etc, etc.
 

·
Registered
Joined
·
5,125 Posts
Most engine manufacturers state the WOT rpm, the amount of time the engine is allowed to operate there, and the recommended cruising operation (usually 70-80% of WOT). Engines are rated for how long they can operate at any given rpm. Recreational engines are not rated continuous duty, so they are derated for cruising rpm operation. Commercial engines are rated to operate continually at WOT. The difference is usually the cooling system and the governed speed.

Gensets, which use some of the same engines as for propulsion, are operated continually at WOT, and usually with larger loads than a propulsion engine sees.

There should be no fear of operation an engine at WOT for a significant time (ie, several minutes). I think most people fear this because they aren't used to hearing their engine at that speed and think it is tearing itself apart. I don't like to hear ours at WOT, but understand it isn't being damaged.

Mark
 

·
Dirt Free
Joined
·
3,043 Posts
I was unaware of the WOT "test". It hardly seems like a "little stress".... but I don't know much about engines. I do know that my MD17D which is quite old has a recommended OEM "RPM operating range.". As I noted in an earlier post that I never have had a need to even run at the high end of this range because I can achieve hull speed as lower RPMs.
Perhaps there are times when a burst of full throttle may be called for. I'd like to hear of some examples. But surely not for 15 minutes WOT.
My naive sense of this matter.... is that if the mfg gives a recommend max operating RPM, the user should observe it.

WOT seems to be excessive stress. Who knows, it may cause some damage.
MFG's don't just state their max, their diesels are goverened to it. Only way to go over is if you have an oil runaway.
 

·
Registered
Contest 36s
Joined
·
7,143 Posts
Thanks for the comments.... they are somewhat helpful.
I don't know what WOT is on my engine. I do know that 2100 RPM is what is needed to reach hull speed. I have pushed the RPMs to high 2000s at times for very brief periods... perhaps needing to give a burst of speed for some reason. The engine did not suffer. Actually I am not sure that the throttle can add more fuel... it seems to be limited by the morse cable installation travel. As I said mine may be limited to the high 2000's RPM.

What is WOT technically?
 

·
Dirt Free
Joined
·
3,043 Posts
Thanks for the comments.... they are somewhat helpful.
I don't know what WOT is on my engine. I do know that 2100 RPM is what is needed to reach hull speed. I have pushed the RPMs to high 2000s at times for very brief periods... perhaps needing to give a burst of speed for some reason. The engine did not suffer. Actually I am not sure that the throttle can add more fuel... it seems to be limited by the morse cable installation travel. As I said mine may be limited to the high 2000's RPM.

What is WOT technically?
Running the engine to the max rpm to which the engine is governed by the mfg.
 
  • Like
Reactions: SanderO

·
Registered
Joined
·
25,130 Posts
There is an operating manual for every engine and they state WOT rpm. When you push the throttle fully forward, it should reach nearly that exact RPM, maybe +/- 50rpm. Just running smoothly at WOT isn’t the point.
 

·
Registered
Contest 36s
Joined
·
7,143 Posts
Apparently WOT test means something.... but it's not clear to me. Apparently some buyers believe this test will tell them something. And for sure one would hope that the engine can run at high revs without "failing".

My research reveals 3 goals for a WOT test
1. confirm that the prop's pitch etc is matched to the engine
I am not sure how many minutes WOT is needed to check the prop. But a "bad prop" limits WOT... which it a prop test!​
2. confirm that the engine's cooling system works (at those RPMs) and doesn't overheat
This apparently can show a sub par cooling system even though at normal RPMs the engine does not over heat. Good to know I suppose​
3. determine at what RPM the engine's governor limit is.
Why this is important is not clear... I suppose to prevent the engine from being able to run too fast when bad stuff happens. This is not a time test. Push the throttle and note max RPM. done​
My sense is that a prop design is calculated. Whatever prop is fitted.... can't measurements determine if it correct? (in my case... I have the OEM prop fixed 2 blade. I presume it is proper and the buyer who doesn't want to believe me.... can call the factory and do his own prop research. If it is a proper design it will not inhibit a WOT RPM reduction.

So cooling is likely the key issue... and reduced cooling could have multiple causes... including blocked intakes, hoses, elbow, missing impeller blades and so forth.
 

·
One of None
Hunter 34
Joined
·
8,647 Posts
It was almost a comedy on the sea trial for my hunter 34. The owner, the broker, my surveyor, and me I just took a seat and watched there wasn't room for three full size men and me in the smallish cockpit
 

·
Registered
Contest 36s
Joined
·
7,143 Posts
As mentioned by someone earlier, running loaded at WOT can also show engine mount issues and cooling system leaks, among other things.

#3 isn't really a thing, since governed WOT is known for most engines.

Mark
I suppose #3 is to determine if the engine is properly governed.
Bad mounts would be revealed.
 

·
Registered
Joined
·
91 Posts
Our past 2 Volvo's state within 500rpm of WOT.

Mark
I have a 2012 Volvo D2-40. I pulled the following text from the manual.
Cruising Speed
Avoid operations at full throttle for best fuel economy. We recommend a cruising speed that is around 500-1000 rpm below the maximum rpm at top speed (full throttle).
Depending on the type of hull, choice of propeller, load and sea state etc., maximum revolutions at top speed may very, but should be within the full throttle range; refer to the Engines section.
The engines sections lists engine RPMs at 2800-3200 for D1-13, Dq-20, D1-30, and D2-40 engines. I typically cruise at 2200 rpm which puts me around 6.5kts in chop for a boat that has a theoretical hull speed of 7.6. I believe I have gotten a little over 7 kts at around 2500.
Buying a boat without a thorough test of its engine to WOT would be like buying a car without fully testing its ABS brakes. I know, not the same, but stay with me. just because a boat can motor through the harbor, doesn't mean it can operate within the manufacturer's design at WOT. Just because your car can stop in a parking lot doesn't mean the brake system will function to the designer's intention when you stomp on the brake peddle on the highway. I'm not saying you're going to measure all the engines tolerances to compare, but a surveyor with an average amount of knowledge should be able to determine if you need a proper mechanic to examine the engine further.
 

·
Registered
Contest 36s
Joined
·
7,143 Posts
WOT is whatever the engine is governed to. On the MD17D, it is 3000rpm.

Mark
The Volvo-Penta MD17D model was made from 1981 to 1984 and had a slight increase in performance. This motor achieves 36 horsepower at 3,000rpm and is capable of 3,350rpm. MD17D has a compression ratio of 17.5-to-1 and features a displacement of 102.51 cubic inches. The heavy duty MD17D heavy duty has a max of 2,900rpm.
 

·
Registered
Joined
·
5,125 Posts
I have a 2012 Volvo D2-40. I pulled the following text from the manual.

The engines sections lists engine RPMs at 2800-3200 for D1-13, Dq-20, D1-30, and D2-40 engines. I typically cruise at 2200 rpm which puts me around 6.5kts in chop for a boat that has a theoretical hull speed of 7.6. I believe I have gotten a little over 7 kts at around 2500.
Yeah, I got the 500 delta number from our previous MD2030, which I remembered off hand. Your D2-40 is 400, and our current D2-55 is 300. I think the difference is the difference in governed rpm - 3600 for the MD2030, 3200 for the D2-40, and 3000 for the D2-55.

My point was that Volvo gives an operating range for WOT of within 10-13% of governed max rpm, not an absolute number, or one to get very close to. This allows the boat to be propped with a bit of leeway.

They also give these three engines a Category 5 rating, which states that they can operate at max rpm for up to 1 hour in every 12hrs of operation period. Otherwise they should not be run above 90% of full load engine speed.

So Volvo is fine with running WOT for 1hr.

Mark
 

·
Registered
Joined
·
483 Posts
Discussion Starter · #78 ·
I am scheduling my engine mechanic to check out the engine and do annual maintenance sometime in the next few weeks. My Yanmar 3GM30 has 3400 stamped on the front label as its max RPM. Normally, I run around 2800 RPM to exercise the engine in open water. I had other things going on today, so tried running just at the dock. I ran it up to 3400 RPM today, out of gear at the dock, and had no issues, it ran fine; except it started sucking mud up from the creek bottom (the creek is only 4 feet deep so I have about only 2 ft under the hull - advantage of the swing keel design), so I backed down. I tried running WOT with the 3 blade Max Prop engaged at the dock and it would not reach 3400, only got to 3000 RPM, but it did expose a faulty dock line cleat which the screws bent and eventually pulled out of a rotted section of the pilling. Fortunately I had multiple lines on the boat. None of this exercise caused anything strange in engine motion, cooling or vibration other than more noise. the new serpentine belt system hummed along smoothly.
Motor vehicle Gas Rim Machine Automotive tire


I think I will be ok with a future Sea Trial test, but I will ask the mechanic to make sure.

thanks for all the discussion.
 

·
Registered
Joined
·
483 Posts
Discussion Starter · #79 ·
I should try this again on open water, I guess if I cannot reach the rated RPM it may mean that the MaxProp settings are too high, and I am over propped. If I can only get to 3000 out of 3400, should that be a big enough issue that I should adjust the MaxProp settings? I had the MaxProp rebuilt a few years and had a discussion with the engineer at PYINC and he recommended the current setting based on my boat specifications. I will wait until I actually try this and perhaps start a new discussion based on that question.
 

·
Registered
Joined
·
257 Posts
Yes, the surveyor will put it in gear, the engine needs to be under load. In my case we did it at dock, I didn’t have engine temperature gauges, so he used a laser thermometer to check engine temperatures and possibly a handheld tachometer. I was at the controls at all times, he was down below monitoring the engine.
In addition we later did a sea trial of sorts, we motored to the yard to pull the boat.
 
61 - 80 of 86 Posts
Top