Max RPM at WOT Underway

[spartonboat1]

I have a 1972 43'FBMY with the J&T 6-71N's @310hp. I have now owned the boat for ten years and at WOT can never exceed 2450rpm. I ran at WOT into a slight head sea the other day with 2/3 fuel and full water tanks and 1/2 full holding tanks. On that day I reached 2405rpm observed with digital tachs at WOT.

Since these engines are rated for 2500rpm, how significant is this issue of not reaching more than 2400-2450? The right engine was rebuilt about 300hrs ago, while the left is orginal at 2300hrs. I mention this, in case the power output may be slightly down on the left engine as a contributing factor. Finally, I had all my four props digitally traced off the boat and they were close to perfect...very little deviation from book.

Finally, as long as I run at 2200 rpm or lower the right engine remains below 180 degrees, but if running at WOT she will begin to exceed 180. I had the heat exchanger R&R'd, with full cleanout of the exchanger last year, since she ran warm at the drop of a hat over 2150- since then she runs a little cooler at cruise and starts to run over 180 only at 2300-2400rpm. Drop back to normal 2150-60 rpm cruise and she cools down in a minute or so.

So,
a) how much of an issue is the max rpm at WOT (my principal concern) and
b) how much of an issue is the exceeding 180 at WOT?

Thanks...

[MikeP]

When you say digital tachs, are you referring to digital dash tachs or a photo tach that you shoot at the engine damper which actually counts the revs?

A "digital" dash tach is no more accurate than an analog tach - they both use a sending unit based on generating an elec current that gets converted to a reading. The digital tach even has an additional conversion to make.

Try it with a photo tach before making the determination that anything is wrong.

Re the temp - obviously a properly designed cooling system in good shape should be able to maintain an appropriate (safe) temp regardless of the throttle setting. When you say it begins to exceed 180, I wouldn't call that a problem. HOW MUCH does it exceed before stabilizing - or does it stabilize at all. Also, is this a mechanical gauge reading or an elec gauge? If it's elec, there's no telling what the actual temp is. I'd stick a mechanical gauge in there and go to WOT for 15 minutes OR UNTIL IT hit 190. If it stays in the 180's, I wouldn't worry about it.

[Jaxfishgyd]

I have the exact same motors in our boat.... Was told by DD and Hatteras I should get 2450 rpm at WOT.

Here in Fla, my temp will range at WOT from 175-192 depending on if it's winter or summertime. DD told me that in summertime, anything under 198 was safe.... (uh, rather it didn't make it that hot myself)

I really dont' care about wot as I never run it there...... 2100 rpm is what I'm "told" to run it... but I cant afford the fuel burn.

At 1100-1300 rpm I"m doing 9 kts, temp is 180 and fuel burn is good.

[Trojan]

J & T rates the 6-71N 310HP DD at no load to 2650 RPM. The WOT RPM is rated at 2450 RPM. This is how I have my 43 set up. I don't think you have any issues. The temp is the main concern. Mine never reaches 180degs. It stays between 170 and 180 underway at 1800 rpm and 10.5 mph.

BILL

[spartonboat1]

Thanks for all the replies on point.

To answer some questions, no not a phototach, but is the bridge dash digital readout from the analog/gear driven tailstock cables.

I run between 175-179 degrees at all speeds on the stbd engine, and since I rarely run at WOT, except for once or twice a season as a "test" for max rpm, I never exceed 180. When run for a period of time at WOT seems to go up to 183-185, so probably ok. Yes, I do notice a difference in engine temps depending on raw water temps, per time of year. I.e. spring when Great Lakes are chilly and Fall, when they are comfy warm, by Great Lakes standards, as high as 60 degrees.

When I run at the hull speed of 1200-1300-1400 rpm both engines run cool, i.e. stable at 175port, 178 stbd.

At some point in time, I intend to run the in-boat full cleaning regimen, using the phosphoric acid stuff mentioned in another post. Maybe that will clean the entire internal coolant system, vs. just the heat exchanger.

Thanks for all the responses, esp. regarding engine temps and the max rpm at WOT under load/underway.

FWIW, since I had the heat exchanger R&R'd, my very slight consumption of coolant has stopped! Coolant sits at a consistant level between runs now.

Finally, my next goal is to stop the lube oil leaking of these DD's! During a full cruise 4.5 hour run the other day, I consumed about a quart per engine, Texaco Ursa 40. I see most of it in the bunker, so assume leaking. Stdb. engine oil (rebuilt) stays mostly clear, while the port engine (orig 2200 hours), doesn't really turn black, but just brownish.

Thanks again.

[Trojan]

Where are your engine drains going to. Do you have a trash can on the engine that you are not cleaning or draining. I let mine go to long on one engine and it dumped some oil sludge in the bilge, I had just cleaned .


BILL

[MikeP]

"not a phototach, but is the bridge dash digital readout from the analog/gear driven tailstock cables."

So it is no more accurate than an analog-faced tach - don't trust it as being correct until you verify it's accuracy with a photo tach. The only thing a digital dash tach does is allow you to say, "The tach indicates 1823 RPM" instead of, "The tach indicates a little over 1800 RPM."

[newinlet]

J&T 310 671's Are rated 2650 RPM no load & 2500 Loaded that is what it says on the ID plate on mine . As far as engine temps We run at about 185 with water temps about 70 summertime @ 2250 RPM any lower rpms or water temp she runs a little cooler. I cleaned my H/E in pool acid and I had good results. My thermostats are new and 180 F .

[ADaily3224]

Do you have coolant, (Antifreeze) in your engines? If so the viscosity is important. Too much antifreeze inhibits the ability of the heat exchanger to do it's job. I had DDC671's I kept a summer and a winter mix and changed it spring and fall. The winter mix was 50/50 the summer 75 water, 25 DDC additive, I forget the name. Anyway someone else will respond with the correct name.

The antifreeze mixture is important on those engines.

Tony D

[Genesis]

If you're running a DD in non-freezing conditions the best option for "coolant" is distilled water with a proper inhibitor/conditioner package. NO GLYCOL AT ALL. This obviously does not work up north, but up north you don't have 85-90F raw water termperatures. Down here in the south WE DO.

Nearly all DDs have marginal cooling capacity. In warm water everything must be scrupulously clean or you will have problems.

The problem isn't the viscosity - it is specific heat capacity. Water has the most. Glycol has quite a bit less. Therefore, the more glycol in the coolant, the LESS COOLING you get.

"Never exceed" on a Detroit is 195F. "Don't run over" is 185F. The reason for the "don't run over" is that at 185F the thermostats (assuming you have 160s in the engine) are FULLY OPEN - that is, there is no more modulating capacity available in the cooling system - it is running flat-out.

You can go from 185 to well beyond 195F before you can spell "overheat", and overheats on Detroits crack cylinder heads with a high degree of reliability. The cylinder head on a Detroit is a very complex casting and as such cracks very easily when abused. Cylinder heads are expensive and cracked ones tend to leak coolant into the oil (and oil into the coolant), which does tremendous damage to main and cam bearings.