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Engine Overheating

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Jun 7, 2004
Hunter 310 Herrington Harbour South, MD
I have a Hunter 310 with a Yanmar 2 cylinder diesel. It runs fine all day under 3K rpm but when I push it over 3K it overheats in 5 minutes. When I back it down to a lower rpm the buzzer shuts off in 5 minutes. This situation has been repeated a number of times with the same engine behavior. I have checked the impeller and it was fine. The water strainer is clean. I used to be able to run this engine WOT with no problems in previous seasons. Has anyone experienced the same problem, and I would appreciate any suggestions.

s/v Calaloo
Oct 11, 2009
Lazyjack Schooner Fairhope, AL
I had a similar problem with my Perkins 4.108 - fresh water cooled - the fresh water side of the heat exchanger was partially plugged. Enough water could circulate to keep the engine cool at lower RPMs but when I cranked it up, it would overheat. The raw water side was fine, exhaust water volume was the same as always. If your engine is fresh water cooled you might want to check the heat exchanger.
Jan 4, 2006
Hunter H-310 West Vancouver, B.C.
Once You've Checked All the Easy Stuff .............

............ the last thing to check is the mixing elbow.

Not too difficult to replace but oh the pain in the wallet. Last year it was $325.00 for a new elbow.
Dec 2, 1999
Hunter Vision-36 Rio Vista, CA.
Assuming that you bottom/prop are clean, I would r/r the mixing elbow. This sounds like the problem, but you will not know until you inspect it.
Sep 24, 1997
Hunter 31_83-87 Middle River, Md
Check to ensure the belt is in good condition - it could be slightly worn and is slipping at the higher RPMs.
Sep 29, 2008
Hunter 33.5 Carlyle Lake in Central Illinois
When my 3GM30F was over heating at high RPM it turned out to be a bad seal on my strainer basket lid. I made a new one and tightnen the thumb nuts with a wrench and it solved my problem.
Aug 16, 2009
Hunter 1986 H31 California Yacht Marina, Chula Vista, CA
I'm going with Steve and Ralph on this and betting you will find the mixing elbow to be 70% carbonized. You're just not getting enough salt water flow through the heat exchanger at higher rpms. My quibble with Ralph is the "not too difficult". I found replacing the one on my 31 to be somewhere between extremely annoying and a major pain. To seperate the elbow from the exhaust outlet flange [terminology?] know someone with a very good vise and a very large monkey wrench unless you are going to just renew the whole assembly.
Oct 6, 2007
Catalina 387 Panama City, FL
When I had a H31 1987 with a Yanmar 2GM, I replaced the mixing elbow when it was overheating. Later when it started to over heat again I removed the heat exchanger water inlet hose and fitting (which was a 90 deg elbow), inserted a flat screw driver into the mixing elbow, broke up the carbon build up as much a possible. I then replaced the water inlet right angle fitting with a straight fitting. I had to move the anti-sphion to the other side of the engine compartment to make the hose go straight into the fitting. I could then hear that the water coming out with the exhaust had increased. Over heating stopped. Traded the boat 5 years later still not overhearting.

Feb 14, 2005
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Calaloo -
Proper Heat transfer depends on several things:
1. mass flow of water in the Hx --- the rate of cooling water
2. The temp of the inlet cooling water and the fouling characteristics of the Hx

This year again on the Ches. barnacles have become a problem in most areas, sometimes affixing and growing on and IN the inlet through-hulls especially (despite the use of appropriate bottom paint IN the entrance of the through-hulls); thus restricting flow

The FIRST assay of the problem with Hx is always to check the water flow. Go to your engine shop manual (or contact Bayshore Marine in Annap.) and look for the "specification water flow vs. 'target rpm'". It will be somewhere near 3.0 gallon per minute at 2400 rpm but verify to make sure for your particular engine model. Take a bucket and stopwatch and run the engine at the spec. rpm and measure/catch the flow as its coming out the exhaust through-hull - do this 3 times, and average the results.
1. If the output flow is less than spec. (±2%), then check for water flow resistance/partial blockage, etc. Start with the inlet through-hull and assay each and every component (in sequence) through to the injection elbow. You can 'momentarily' disconnect each component in the series to assay with the bucket/stopwatch.
2. If the output flow is within spec. (±2%) then consider that fouling is a probable cause - and then consider to 'de-scale' the engine and exhaust manifold, etc. with an appropriate boiler descaling compound ... (Rydlyme, etc.).

FWIW - Im located on the upper-Ches. and have had barnacles periodically blocking (inside) my raw water inlet since July. Micron CSC, etc. is no longer adequately protecting hulls, etc. on the upper Ches. due to the heavy 'slime' deposits. Most there are changing to Micron-extra. These are 'barnacles' and not 'zebra muscles' (yet, although the are moving down the Susquehanna towards the Ches.). Up here, the barnacles are getting worse and worse every year ... probably because the 'slime' deposits are getting worse and worse due to 'stagnation' of the water and extreme high turbidity - the Ches. is not 'turning over' due to the prevailing westerly winds instead of prevailing southerlies and SWers ---- hence SLIME then barnacles. Ive had barnacles growing many 'feet' up & well into the inlet cooling lines of my engine, refrig., AC ... and Im having to periodically use Rydlyme in most of the rubber cooling inlet lines to keep everything 'clear' - the damn barnacles are 'way' INSIDE the inlet cooling lines and 'beyond' the stainers.
Oct 14, 2005
1983 Hunter H34 North East, MD

your barnacle comments has stirred my curiosity about their presence up on the North East. My boat was pulled two weeks ago and I haven't done my final winterization check yet so will be curious to shine a flashlight up "where the sun don't shine". I did have an overheating issue coming out of Georgetown in October, but after blowing out the intake I had no problem running at cruising revs for three hours so am attributing it to sucking in a baggie or some other temporary blockage.

On the other hand, slime has been a recurring problem for the past couple of years up here. Racers scrub bottoms before every race. I have been using CSC as had the previous owners. Slime built up along the waterline, down to about a foot below, and appeared to be no more than normal from previous years. Didn't take the boat out much this past summer and didn't see what the bottom looked like before they powerwashed it when it was pulled. Depending on what I decide, I may need to consider "Extra" for next season.

One thing to note was that the water remained relatively clear until the heat of mid summer. We had at least 3 feet of downward visibility until then. By Labor Day the water was quite turbid due to the lack of rainfall. On weekends we've had the prevailing SW's for races so haven't noticed a wind issue here.
Feb 14, 2005
Tayana 37 cutter; I20/M20 SCOWS Worton Creek, MD
Re: RichH...

Yup, the barnacles are mostly attached to about 2 ft. down, rarely/occasioinaly below that. Probably because the slime isnt much of a problem below 2 ft down. A 'trick' I learned on my last passage 'south' ..... Micron Extra 3 ft. down from the water line; then, any good bottom pain below that - makes sense because of the extortion prices for 'Extra'. I put on a quick slop-coat of M-extra (on top of CSC) to the entire hull in July; the boat was hauled a few days ago ---- Nada!, no slime and NO barnacles. Barnacles were extra-BAAAAD and extra-BIG on Worton this season. CSC just 'isnt working anymore' vs. 'barnacles' where I am. The boat slipped next to me bought a 'bargain brand' ablative from WM .... and had barnacles on top of barnacles on top of MONSTER barnacles for about 3 ft. down from the waterline.

At Worton Creek all summer the turbidity was like thickened-diarrhea-brown color all summer, then when the prevailing winds changed to SW'ers in Oct. the water returned to normal - just a crappy 2 ft. visibility. I got a severe head-to-toe skin viral rash from swimming in the bay in July -- took 3 weeks to recover.

Mar 2, 2008
Cal 25 mk II T-Bird Marina, West Vancouver
Don't forget that a very common reason for overheating is have the wrong propeller. ie.. being "over-proped".
1st - Ask yourself a couple of basic questions, such as:
- Is the boat new to you? is.. what is the drive train history?
- Is the motor capable of achiving full motor load rated RPM?
- Have you changed the propeller diameter and/or pitch recently?

2nd - bottom fouling.

3rd - check raw cooling water rate as described by others above.
- intake throughhull, strainer, pump, mixing elbow, etc.

4th - fresh cooling fluid circuit.
- fluid level, pump, heat exchanger, etc.
Jun 7, 2004
Hunter 310 Herrington Harbour South, MD
Thank you all for your very valuable opinion and recommendations. I should have mentioned that the exhaust elbow has been replaced a couple of years ago. I have always been using Micron Extra paint and haul out every other year except that this past year I did not so it's been three years now. I'm hauling next week and staying on the hard for the winter. Unfortunately my Hunter 310 has the exhaust dumping below water line so I can never check the water flow, at least in the water. I'm thinking of removing the heat exchanger. Is it as easy as removing the cover on one side and sliding the exchanger out, or do I need to remove the exhaust elbow from the other side as well? And once I remove the exchanger, how do I clean it? Thanks to all.
Dec 25, 2008
catalina 310 Elk River
I use a strip of carpet to give her a back scratch about 1/month, dove on the keel and rudder with the brush. When I pulled her last week she was in good shape, all except the bottom of the keel which was covered with barney. The inlets were sliming up, I will make a note to ream them more often next year.
Jun 7, 2004
Hunter 310 Herrington Harbour South, MD
Well, I hauled the boat yesterday and a rope was wrapped around the prop. I hope this was the cause of my overheating problem at high RPMs. Too bad I won't be able to confirm this until spring.
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