Cooling Configuration on 4-108 Perkins

[jimisbell]

To start, nothing I say below is to, in any way, belittle the offerings of suggestions from anyone on this thread. I am greatful for all the help. I just want to make a clean report on what did and did not help and where I am in the investigation. I appreciate ALL suggestions and even if I have tried them already I may try them again. So dont anyone be offended if I report a suggestion did not help. It was appreciated anyway and I tried it. I am the one that should be offended...by my own stupidity in not having solved this in almost 6 months...#8-)

To update those who missed the first post, the following are what I had done prior to the comencement of this thread:

I had replaced the raw water impeller though it was in good condition.
I had boiled out the heat exchanger tubes, minor improvement
I had checked the belt tension.
The flow out the exhaust pipe looks suffient. Yes that is a subjective observation as I really dont know how much there should be, but its more than the 6-354 put out and that engine stayed cool.
I had checked the thermostat and it works at the right temperatures
I had checked the water temp guage against a laser thermometer
I had cleaned the raw water strainer..even though flow seemed suffient. But there was a leaking gasket that was letting air into the system, minor improvement.

Since this thread started I have done the following:

I have now REPLACED the thermostat even though the old one checked good.
I have now removed a suspicious hose that was bypassing the Heat Exchanger, minor improvement was noted.

As you can see there were three things that resulted in minor improvement in the situation. In each case I was able to move to a higher speed before overhearing began to show. Eash time there was about a third of a knot improvement. Originaly it was 4.5 knots, I am now up to 5.5 knots.

Allan a report on your latest post:

Originally Posted by Alan Wheeler

Your circulating pump doesn't have any wearing surfaces, so clearances will not be a problem. You do need to check the front bearing for wear. The belt pulley should not seem wobbly.
Check the Salt water pump impellor. Ensure all blades are in good condition. If any are craked, replace it. If a blade is missing, ensure you find the missing piece. If the impellor is brand new looking, that too could be a suspect. It maybe possible that the previouse owner has replaced it and a broken blade has been traped further down stream.
I imagine this is the Engine fitted pump, so lets assume it is the right sizeing required. But if it is not the engine fitted one, have you got the right size pump?. If the pump is belt driven, ensure the belt is of correct tension and correct gearing to ensure proper speed. Ensure you have a large diameter intake hose. No "kinked" bends in it. No hose fittings other than the through hull and valve and ensure they are large enough. Impellor pumps do not like pulling water. They must have no restriction. Ensure the hose from pump to exchanger is free from kinks. And most importantly, ensure you have a clear water intake. Especially if there is a screen or filter fitted. Ensure no weed growth is inside the intake.
OK, with cooling aside now, what about RPM. Does the engine rev to full or close to full RPM when in gear. To check this, rev the engine to max out of gear and take note of the RPM. Now place it in gear and rev to max again. You should be able to get within 200RPM of max. If you can't, then your boat maybe over proped and over working the engine which will cause it to over heat.
There's a start, report after the checks and we will take it to the next step.

I like you I doubt the freshwater pump is the problem since it is virtually bulletproof. Impellers have been known to come off, but it is rare. But I am down to just this and the tranny cooler and the bottom of the boat.

The raw water pump was checked in May and again in Sept when the impeller was replaced.."just for grins"...it was in fine shape, but at that point I was running out of options so I replaced it ANYWAY.

The pump is the one that came with the engine and the part numbers match. The intake hose is 1.5" and goes thru a raw water strainer that has been cleaned and refurbished.

The bottom was checked by a diver in May and the problem existed at that time. The diver said everything was clean as a whistle.

The engine was thourally gone over and new belts put on and one new pulley in May by a professional mechanic. Belt tension was checked and found correct again in September.

As to RPM, I havent yet tested it as you detail, but the overheating begins long before RPM limits.

I will be pulling the boat for a bottom job on the 15th so I can check everything below such as prop dimentions, water intake, and whether I am towing a bucket on the keel...#8-)

[Alan Wheeler]

Thats great detail and no you will not offend anyone of us I am sure. It's good to get that level of info, it makes it clear and concise for us all to suggest.
OK, there are three things I can think of. One or two you may have checked, but here goes.
Check the oil cooler for blockages. I have come across one once before that had a block and upset the water flow/temp.
Have you verified that the main engine heat exchanger is of the recomended size for this engine? Eveything else you seemed to have checked very well and ruled out, so on the cooling side, that is my only last thought.
OK, the over prop could also be the other issue. If it is overloading the engine, it will indeed over heat the engine well before you reach max RPM. So a simple test is to warm up the engine, then open it up to max for just a very short interval and take note of the RPM it reaches. Now find a quite spot were you can sit and open the engine up to max RPM out of gear and take note of RPM. Now why I say to do it this way, is that the engine may be limited by throttle control as to what it can actually fully rev to. So going by the book can be a little inaccurate. The difference between min a max for the wee perkins should be ruffly 200RPM. If anything greater than that, your engine is most likely over loaded. If it is greatly different, then it will indeed cause over heating at much lower RPM than it should normaly operate to. You may also notice black smoke if you open the throttle right up when it is in gear, which is a sign of overload. but it's a bit harder for me to describe if what you see in smoke is bad or not, so RPM is a much more accurate analysis.

[btrayfors]

Jim,

Sorry for all your troubles, and I feel your pain having gone thru a similar situation with my 4-108 over several years.

One more thing to check: The fresh water circulation pump fits to the engine block with a thin steel plate in between. There is a gasket on either side of this steel plate. The two gaskets LOOK alike, but they are NOT alike. Some "shade-tree mechanic" in the islands had installed a new water pump on my engine, using identical gaskets. That blocked a critical port and was one of the reasons my engine was overheating.

Wheels' suggestion re: the heat exchanger size is also relevant. We found that although my heat exchanger was the right one, and although it had been cleaned with acid, etc., the pressure needed to pass water through it was higher than with a new one.

In the end, I decided to fit a larger heat exchanger, and bought one from Foley Engines in MA. I think it's for the Perkins 4-236. Anyway, I got it to fit OK with just a little modification to hose runs, and have not had an overheating problem since.

Good luck on your haulout.

Bill

[jimisbell]

Wheels,
The reason I was concerned about offending someone was that I posted a similar list on another forum in Boat Design and one person flamed me for being a smart A** so I didnt want anyone to think I was trying to be smart with my replies. If I was smart, I wouldnt be asking for help....#8-)

I will check the oil cooler and tranny cooler this weekend and next week the boat will be hauled for a bottom job so I can then check the water intake and the prop size. The boat came with an 18x18 prop in the engine room and I have determined that it is , while not perfect, of a diameter and pitch that should be usable if the one on the boat now is something horrific. However I suspect the prop is not the problem as I drove it 1400 miles (from Tampa to Corpus Christi) in May at 5 knots using 0.671 GPH which sounds like the prop is fairly close to correct.

Bill,
I think you are the person that has been referanced here, and that I could not find the thread for, as having had similar problems. Glad to see you on board here in this thread. Your comments on the gasket sound like just the sort of thing I would expect on this engine. It has been owned by a couple of moliarks that dont know an engine from a motor. I found a couple of things that suggest no mechanical knowledge on the part of the PO.

The heat exchanger on this engine is the one with the rubber endcaps that allow you to remove a couple of hose clamps and then slide out the tube bundle. I think its the latest version used on the 4-108. This is a 1983 engine, the one they call the "New" 4-108M

What did you have to pay for the oversized heat exchanger?

Thanks to all

[btrayfors]

Jim,

I paid $258.50 in Feb 1993, including shipping from Foley to the DC area. No problems in past 13 years :-))

Good luck,

Bill

[SkiprJohn]

Aloha Jim,
I'm not certain of your transmission gearing but an 18x18 prop sounds like too much prop for a 4-108. Let's see how large your installed prop is?
Kind Regards,
JohnL

[Alan Wheeler]

Bill that is invaluable info. Thanks for your information. It's worth pure Gold for info like that. Sorry I am fresh out of Gold however, so you will just have to settle for the thanks instead :-)
The RPM is the only true way of determining if you have the correct prop or not. There are programs that cqan get you in a close ball park and many of the prop companies then have enough experiance to recomend an exact size. The exact sizing is also determined by gear ratio. Boat speed you are getting has very little bearing on the matter, due to the ease of getting a displacement hull to speed. The load the engine "see's" is due to the friction the prop produces in the water. The friction has to be balanced against movement. By the way, 18x18 is a very big prop. I would be expecting a very high gear ratio swinging a prop of that size for starters.

[SkiprJohn]

Jim, Bill, Wheels,
From experience the 4-108 used a 1:1 Velvet Drive in some applications and that definitely would mean an 18x18 would be too large. I swing an 18x13 two blade with a Velvet Drive 2.57:1 with an engine close to the same size. 10ton boat 34ft waterline. Wheels is right to say that it is always a crap shoot until you get the prop on, boat in the water and test it out.
Jim, if your prop is too large you'll never solve the overheating problem.
Kind Regards,
JohnL

[jimisbell]

In this case the drive is 2.57:1 and the 18x18 is close to correct according to the formulas I am using. The wright is 18,000 lbs and the wl is 34 and the beam is 12 feet.

[jimisbell]

Originally Posted by btrayfors

Jim,

In the end, I decided to fit a larger heat exchanger, and bought one from Foley Engines in MA. I think it's for the Perkins 4-236. Anyway, I got it to fit OK with just a little modification to hose runs, and have not had an overheating problem since.

Good luck on your haulout.

Bill

OK, that sounds good, but just for grins, consider this...you could cool it with evaporating freon and run ice water in the passages, BUT...wouldnt that hurt the engine if you were taking 150 HP off the shaft? IOW, putting in a larger cooler will bring the temperature down, but is that the solution or are you just allowing the engine to be overstressed without becoming overheated??? I am just asking, I dont know the answer.

[Alan Wheeler]

OK, I really think your prop is way over pitched. Diameter does not matter (to a point), but pitch is the most important aspect. Diameter will determin your slip % and the amount of resistance it takes to trun the thing in the water, but pitch determines the speed foward per rev.
OK, firstly a 34ft hull will have a speed of 5 -5.5kts. Lets work on 5.5kts. The engine must be able to swing the prop to a max RPM and I am presuming this is 3000RPM. I am asuming you have a gear ratio of 2.57:1.
Prop slip is one of those hairy ones. Usually at slow RPM, a prop tends to be less efficient and a slip of 50% is not uncommon. But as RPM increases, so does the efficiency of the prop and a slip of about 10% is usually a common figure. At a 50% slip, your prop should have a pitch of only 11.5" and at 10% it would be closer to 6.4".
If you have cupped blades, you take another inch of pitch off.
Does that help at all??
As for the cooling and going to a bigger size exchanger, you can never have too much cooling, and Diesel engines like being worked hard, but the over working of the engine is really shown in the amount of black smoke you produce. I imagine getting a prop pitched would be cheaper and easier than plumbing in a bigger exchanger and finding the engine is still over working. Diesel consumption is determined by RPM, not load, give or take about 20%, so the fuel usage could be between 0.6 and 0.7 around that figure of 0.67, depending on how hard it is working.

[jimisbell]

So, just what do you think the pitch should be?

I dont know yet what the pitch is on the installed prop that I will find out on Monday when she is hauled out.

The "spare" that was in the engine room was a 18x18. I doubt that it could be repitched as low as 18x10 as I think you are suggesting. I had it rebuilt so its like new so I would probably be better off to sell it and find a new one if thats what I need.

[btrayfors]

Jim,

The engine temperature will be regulated by the thermostat. Diesels like to run at 180-190 degs F. The larger capacity heat exchanger will just keep the regulation closer to the design temperature, providing more cooling capacity in the event of partially clogged or corroded water jackets, occluded ports, etc. It also has more capacity to overcome the degrading of the heat exchanger itself (corroded or clogged tubes)....a common source of overheating.

Wheels,

A 34' waterline boat has a hull speed closer to 8 knots than 5.5 (1.3 or 1.4 x SqRt of 34).

Jim didn't say whether the 18x18 was a two-blade or three-blade prop. It makes a difference.

I tend to agree, though, that the prop may be a source of trouble. Jim: what's the maximum RPM you can achieve underway? If you can't get 3,000 RPM or more, then you probably have too much prop (diameter, pitch, and/or blades).

A good cruising speed for a 4-108 is somewhere between 2400 and 2800 RPM. I run mine at 2300-2500 RPM with a 17" 3-blade MaxProp. My son, with approximately the same size and type hull, runs his closer to 2800 RPM with a fixed 3-blade prop 17" in diameter. We both have 2.5:1 reduction gears.

The 4-108, as Wheels said, is a SMALL engine. It's typically way overrated at the quoted power of 50HP or 47HP or 42HP. It's really more like 25HP.

Bill

[SkiprJohn]

Aloha again Jim,
I'm pretty certain I know the source of your overheating problem. We'll know better when you haul out and check the size of your installed propeller. No prop manual I've ever read recommends an 18x18 for a 4-108. Maybe you have a different reference?
John

[jimisbell]

I am not sure what the RPM is as the Tach has not been running properly, off and on. Tomorrow I am going to fix that so I can get accurate information on the RPM.

But based on the discussion as it has evolved recently I am going to stop messing with the engine, except for getting the tach going right, until Monday when we haul her and I can see what the prop is. That seems to be the more likely culprit at this time.

I have been using the Following numbers in a program called propcalc.xls:

Displacement =20,000 lb (loaded weight)
WL =32 (actually somewhat longer at above weight)
Max speed =7kt.
Engine HP = 35 at 2500 RPM
HP usable 34 at prop rotation of 973
Torque = 183 FT LB
and a Speed to length ratio of 1.237

This tells me I need a 16" pitch which you guys seem to think is too much.

This is why I have been saying 18" was close, but if the program is giving me incorrect numbers and it should be 10 to 11" then that could be my problem.