Old forum notes on Miscellaneous Engine Topics

These are old Delphi forum notes extracted by Fastjeff on Miscellaneous Engines topics ....


The target, WOT (wide open throttle) rpm for Chryslers is 4,000 to 4,400 rpm. If, you reach a higher rpm the propeller pitch should be increased 1" per 200 rpm; if this target is not achieved decrease pitch at the same ratio. FIRST, make sure that your tachometers are calibrated properly and the throttle linkages are connected properly (making the full range of motion) and the engines are operating properly prior to making propeller adjustments. Failure to reach proper wide-open throttle rpm is caused by things other than propellers more often than you might think.


The WOT rpm test is the only way to determine if a boat has the correct propellers. Even though few boaters actually ever run at WOT, even briefly, running propellers with too much pitch can overload the engines throughout the entire RPM range. This can result in damage to the engines (tuliped valves and burnt pistons if the motor is lugging) and excessive fuel consumption (if the motor is over-reving).  I realize this is probably a long-winded answer to a relatively simple question but after many years sizing propellers at Michigan Wheel Corp I know that there is a lot of mis- information out there regarding engine operation and propeller size.


On the effect of horsepower and torque curves on boat performance, engines have a torque curve that peak at some relatively low rpm, (a sign of maximum engine breathing). The horsepower curve, by contrast, peaks near the top of the rpm scale. The horsepower curve tends to confuse people, for it's the product of torque times the number of firing strokes: At high rpm the torque curve is falling rapidly, but the rpm (and, hence, the firing strokes per minute) are still rising, so horsepower goes up. Eventually--and this is a critical point--the torque value drops so low that horsepower also falls, even though the rpm number continues to climb. This is the engine's peak horsepower value. Now, if one has a RACE boat, you'd want the prop it to allow that hot rod motor to reach and slightly exceed the peak horsepower curve point for maximum speed, for that's what you're after. (This is similar to putting low gears in your drag car so that the motor peaks in high gear at the end of the quarter. Great for acceleration, and rotten for highway driving and fuel mileage.)  In a cruiser, the idea of propping the boat as if it were a race boat is nuts, yet this is what you constantly hear. A far better plan is to set the cruising rpms/ throttle opening/ and prop selection at a point where the engine develops maximum torque (the torque curve peak). This is commonly referred to as the 'sweet spot'. With Chrysler motors, this rpm is between 2,800 and 3,200 rpms. That's where you want everything to harmonize--and even if your present combination (weight, bottom condition, fuel load, etc.) does not allow the motors to rev to 4,000-4,000 rpms at WOT.




I have found the 360 motors in my 32 footer are hardly working at cruising speed, and the little beauty planes off effortlessly.  With the original props (16 x 14s) the boat would plane on one engine while dragging the other dead prop along. (I'm still stunned by that!)  Accordingly, I put on the recommended (for 360s) 16 x 16s, which increased the boat's cruising speed, reduced engine wear from excessive rpms, and provided better fuel mileage. This worked so well that I’m planning on adding four blade props with even more pitch.


Reading (the above) post made me wonder what you were trying to improve. If you want to improve the speed, you need to increase your prop pitch, but if you want to improve your power you decrease your prop pitch. A motor will last longer at lower rpms and you will get better fuel economy. Prop pitch was explained to me years ago as the amount of distance the propeller would move forward in on revolution. The higher the pitch, the further it will go in one revolution so (without any slip) a 10 pitch will go 10 inches forward in one revolution, and a 20 pitch prop 20 inches at the same engine revolution. Higher pitched props will take a bit longer to plane out and reach their top speed.


I want to improve my cruising speed and fuel economy, which go hand in hand. (If you could double the speed at the same throttle setting, the mileage will also double.)  My 32 footer has plenty of power and will easily plane off at far less than ½ throttle.  I feel it needs more pitch than the recommended 16 x 16s, so I added 17 x 15 four bladders (which are equal to 16 x 17 three bladders, I’m told). This has improved both fuel mileage and made the motors much quieter, for running a boat in 'second gear' wastes gas and wears the motors out sooner. Conversely, it would be a very bad idea, and destructive, to run an over-loaded boat in this fashion, but mine is not. We have a ‘hot rod’ Marinette that lacks a gen-set and even a flybridge.  For those who still consider this pure heresy, consider these facts:

1. Both motors tach out at 3,900 rpms WOT, just as I want them to.

2. It effortlessly planes off at 1/4 throttle (and you’d better be holding on!)

3. It used to plane off on one engine with the old 3 bladers (that tached out at 5,000 rpms.)

4. At 2,500 rpms (about 22 mph) you cannot slip a pencil between the open butterflies and the carb wall--it takes that little throttle to move the boat, so obviously the motors aren’t "straining".

5. Cruising speed rpms are down, reducing both engine howl and wear.




No flow meters? I suggest you try this: Run along at your favorite cruising speed and mark where the throttle are with duct tape and a marker, Then, when you've stopped for the day, place the throttles in that position, pull those beautiful flame arrestors of yours off, and see how much the butterflies are open. If you're at minimal throttle, as mine is, then don't sweat it. But if you're butterflies are WAY open, you might want to take a bit of pitch off your props.  For example: I was running the other day with about 3/4 tank of gas. That would be 165 gal of fuel and about a 1/2 tank of water; three people on the boat, and my normal load out of extra junk I like to haul around. One thing I didn’t think about which will make a difference: Since last year I added a davit and dingy. Lets say 100 lbs for the davit, another 100 for the boat, 125 for the motor, and 6 gal of gas sitting on the swim platform.




Do you go to your boat, that's been sitting for a week, and have a hard time getting the engines started?  Once, without starting the engine, I pulled the carb and removed the top (for some work I was doing on it). To my surprise I found that there was only a 1/4 “ of gas in the bottom--that's all. (Normally there would have been well over an inch.) Where did it go? Evaporation. To get the engines started they have to be cranked and cranked until the fuel pumps finally refill the carbs (unless you are blessed with electric fuel pumps--and then I won't talk to you!) This takes at least 5 to 10 seconds. The accelerator pump will then be able to spray in some go-juice after a few pumps of the throttle. The choke will allow more gas to be sucked in, and away you go. But it's not gonna happen on the first few cranks. Heck, it's good to get the oil pressure at least started before she lights off anyway, but all that cranking every time we get back to the boat after a week or so is not doing the starters any good.





I began checking what it would cost to replace the exhaust elbows and risers assuming the exhaust manifolds are okay. (No reason why they should last any longer unless they are fresh water cooled, however.) The parts alone for a twin would exceed 1,500 dollars! Add new exhaust manifolds, and you’re looking at another 800 bucks!  I know of no surveyor who inspects these items, so if you are considering rebuilding your existing engines, then expect to have to replace these items as well if the boat is over 10 years old (in salt water/ 20 to 25 years in fresh).  My exhausts are original, 20 years old and have only seen fresh water, but I wonder what shape they are in. The number one reason for engine failure is water intrusion through the exhaust system from corroded elbows and manifolds.  Another reason why a complete engine replacement may be more cost effective than one would originally expect. Oh and do not forget those darn transmission coolers! In salt water they can fail in a couple of years. Ouch!

From what I've read, non-salt water use manifolds (not risers) last virtually forever (though I had one rust through at the bottom on me). Risers--where water and exhaust meet--are shorter lived, however.  Mine are at least 20 years old and are still in good shape. 

One way you can tell if water's leaking into the motor via the exhaust ports is this: When you cold restart the motor the starter sounds like RRRR--hestitate-RRRRR-hesitate-RRRR.  (It should be a smooth RRRRRRRRR.) I had this problem with mine turned out to be the swivel O-rings and stainless tube were shot. I repaired the riser, but a year later the problem returned.  This time is was a corroded exhaust manifold that leaked water down into the exhaust ports overnight.

 One day I pulled the 3/4" exhaust manifold drain plugs, that are in different places throughout the exhaust systems, out to feel inside Yuck!  I made up a little scraper type tool to clean what you can reach. You’d be surprised how much rust & gunk that’s on the walls. Be prepared to use some brawn, for it took a 18" breaker bar and a 2 foot pipe to break a couple open.  After cleaning let someone start the engines and watch the rust chunks come out of the stern exhaust pipes.



I don't pretend to really know if it is okay to run synthetic in an OLD engine and I've experienced that if the question is raised in a group of motorheads a heated exchange will commence. I have heard on the mechanical "ask the experts" shows, and have seen printed articles that concur, that you should not run synthetic in very old engines unless it has been run in them from when they were young.  Something about there is too much wear already.

When I bought my boat, the previous owner was running 10W30 full synthetic.  Both engines had low-ish oil pressure.  Port engine had significant metals in the oil per analysis and did need to be rebuilt almost immediately (glad I had the oil analysis and dropped the price accordingly).  Even before the rebuild, I changed the oil in both engines to a straight 30 weight (as recommended by Crusader) and the oil pressure went up to spec levels.  The guy who rebuilt the engine said that I could run synthetic in the rebuilt engine if I wished, but that I should find a straight 30 weight rather than multi-weight (some on the forum will disagree with that).  He said the straight 30 was needed because the raw water cooled engines don't get hot enough to trigger the switch in multi-grade oils. (Over 250 ‘ F at cruising speed isn’t hot enough?)  Anyway, if I can find single-grade synthetic (no luck so far) I will probably start using it in the rebuilt engine but I plan to use only single-grade regular oil in the other one (until it's rebuilt too)

.A few years ago I tried Mobil 1 full synthetic in my 1978- 318's. I experienced an oil pressure drop and also developed oil leaks, especially around the rear main seals. The following year I went to Valvoline SAE 40 non-foaming racing oil. The oil pressure went back to normal range and oil leaks stopped. I wish I could explain why…


I switched to Mobil 1 at the end of the first season with my boat. It has 360's that the previous owner claimed to have approx 500 hrs, but the boat didn't have hour meters. I run around 60 psi oil pressure at cruise, nearly 80 after first starting, and around 40 at idle. (Note: Running such high oil pressure is rough on the camshaft/ oil pump gears.)





The raw water pump housing bolts to the bracket in one of two ways.  The outlets--top and bottom--are similar and LOOK interchangeable, but the pump shaft MUST turn only one way for each orientation--for the port engine the housing is oriented one way, and for the starboard engine it’s the opposite way. That's why one needs to put "UP" and arrows all over the fool things BEFORE removing them.  I did not and nearly burnt out an impeller before realizing my (dumb) error.

(Check out this brilliant idea:) I worked out a way to replace the impeller without confusion and errors by removing the pump body and hoses at the same time. Detach the hoses, take out the whole pump and work with it on a bench. This prevents me from putting it back on backwards. Also leaving them till last will make it easier to get to the belt-tensioner bolts.  I am really unhappy with the engineering of the raw water pump.  I have seen other designs where you can change the impeller by simply removing a plate.  Some even use thumb bolts.

On impeller lifetimes, I was kinda thinking that they "should" be changed every 1-2 years, to be safe. From your comment it seems that every 3-4 is reasonably safe?

On the pump orientation issue, if I understood the explanation, you are talking about the orientation when bolting the pump to the bracket on the engine.  Getting it wrong (so that the pump rotated backwards relative to it's original rotation) would require a 180 degree rotation about the Y or Z axis (assume shaft is X axis). I am still confused about how this could even be done but it does seem that this could be the issue Joel had (as opposed to switching the In/Out hoses as I thought was the only way to do that).  Maybe it is a function of Chryslers, but I'm still baffled.  A friend of mine has Chryslers and maybe he'll let me take a look.

On my Crusader (starboard; haven't changed the port yet) and the Westerbeke gen-set, I don't think the pump can be mounted backwards for the pulley would not line up with the fan belt.  When I had my pumps apart, it did seem that they were designed to rotate in either direction, and that the direction determined the direction of water flow and. Thus, the IN and Out ports will flip-flop when you reverse the rotation.  Since mine seem to only be mountable one way, I think the danger is switching the port that the sea strainer connects to.

Even with a single engine boat, one needs to be concerned with which port connects to the sea strainer. On my twin engines, if I do not diligently mark the port connected to the sea strainer I might get it wrong and reversed the water flow.  My plan is to change out both impellers at the same time. Typically, I would remove both pumps, take them to my work area, change impellers, and re-install.  So it is very important for me to mark the sea strainer port AND to mark which engine the pump goes on.

Pumps on Crusader and Westerbeke are similar in structure to each other and both are very different from Chryslers..  My "housing" is on the side with the pulley and shaft and mounting plate.  The other side is just a more or less flat cover with the graphite bearing.  Instead of IN on bottom and OUT on top, both ports on mine leave the pump in the same direction so as long as you've identified the ports rotating the housing 180 on X axis (if you could mount it that way) would not have any effect.  On mine, the problem you mention is not an issue but it is easy to swap the hoses if not marked to give the same result - I'd think swapping hoses would be difficult on yours.  On mine it is important to identify the correct port for the sea strainer hose and I think to identify which engine if they could ever be accidentally swapped.  On yours it seems that "up" and engine ID are important.

How long can the graphite bushing in a raw water pump last? Until it's shot!  (Sorry, but that's the true answer.) The ones I've seen last for decades, but…. we've also seen some (and on this site) go 20 years. I change my every few years, just in case. 

If I decide to change out the graphite bearing on my stbd pump then I'll try to post pictures so that you can see what I've been talking about.  Big difference between the pumps.


The useful life of raw water pump impellers depends on what they are pumping.  Mine were 23 years old originals with 1000 hours when I replaced them. The originals were still soft and supple so I kept them as emergency spares.  They only see fresh water from the Great Lakes. The process for changing them is relatively easy.  Depending on your hose setup, you might be able to leave the pumps in place and just remove the rear housing.  Make a note of the direction of rotation so you can put the new ones in correctly.  A squirt of liquid dish soap helps lubricate them for assembly and initial use.  When you get the old ones out check the brass wear plates for scoring and excessive wear.  They can be replaced as well.



You, kinda, screw the impeller into the housing and, when it's partly in, you cut the tie wrap off.  (And if it pops back out, you try again.)

Are  "Full Reversing" transmissions recommended, or is it better to stay with the R & L rotation engines?" I recently needed to rebuild an engine - the counter rotating one.  I was seriously considering getting a new 454 cid or higher so long as I could use my current transmission.  The idea was to keep the same trans and prop and buy a new engine for the other side in a few years. I found that at least for big block engines, they seem to be rapidly phasing out the counter rotating engines.  So, based on that experience, I'd have to recommend against buying a NEW reverse-rotating engine, for sometime in the future it may be difficult to replace.



Converting from RWC to FWC can be troublesome if the flaky rust inside the block circulates to the heat exchanger and blocks the tubes inside the heat exchanger.  If you make the switch, you might want to check and back flush the heat exchanger after about a month to see how many flakes are in the heat exchanger.



I suspect that your 37' aft cabin is about the same weight as my 39' sedan but I have a bit more planning surface.  You've got 250 hp and I've got 350 hp.  So I guess I'm not too surprised that there’s a fairly significant difference.  John's site (http://www.marinette.com/perf.htm) has performance data on several Marinette configurations but not on any 37's (aft cabin or sedan).  The closest to your boat they show is a 39 Sedan with the 250 hp engines and the max speed on it is 30 mph (26 knots) @ 4000 rpm so I'm still suspecting some type of drag problem with yours.  Do you suppose you might have a "hook" problem with your hull from storing "on the hard"?  I think that is the problem (as opposed to a "rocker") that makes the boat run in a bown down manner and could affect speed. They also don't show the 32 Sedan, but a 32 Express with dual 225 hp should get 25 mph @ 3000 rpm.  The site also does not have a 39 with my engines.  The closest there is to Miss Cleo is a 39 Aft Cabin with 330 hp Chryslers and it's speed is listed as 34 mph @ 4000 rpm so if anything it seems my performance is a bit low but certainly not out of the realm of the specified performance.





Some 1987 Marinettes have the older log style manifolds and some have center dumps. My guess is they made the switch somewhere within that production year.  The rub rail says 1986 the hull plate in the engine room could say 1985 like mine does since it was made early in the year.  Also the engines from my 1986 have casting dates for 1984 so you can assume the hardware used was probably collecting dust in a warehouse for a year or more before the boat was outfitted. To change the oil, that garden hose thread at the end of the dipstick pipe is a rather unique way of doing it.  Did it come from the Manufacture that way, or was that an after market option?  (No. Later design feature.)


If you are using a fuel additive like Sta-Bil or Star-Tron, you don't have to worry about the gas gumming up the carb. Just make sure you add the proper amount to the gas tanks and allow the stuff to mix with the gas. Run the engines until the stabilized gas is running through the carb. Fogging can be done by spraying fogging oil into the engine (after pulling the coil wire and rotating the engine).  Pouring motor oil down the intake until the engine stalls is another way to do it (but DON’T do this without the motor running--you might ‘hydraulic lock’ the motor). Expect a little smoke in the spring as the oil burns off. I've always done it this way and the engines fire up just fine in the spring.  (An alternate method is to pull the coil wire and spray fogging oil into the intake as you crank the motor over.)


When I re-powered, the engine manufacturer wanted the boat propped to deliver redline rpm on engines. First problem we ran into is 14 x 10's couldn't hold back the engines. I should have gone to reduction transmissions but that’s water under the bridge. We changed to 14 x 12 with a #8 (heavy) cup. Boat ran fine with no strain on engines. Only went to 3700 rpm (31 knots). One issue that engine builders seem to leave out of the equation is that when propeller tip speed exceeds a certain speed it will cavitate. The effective surface area of the blade is reduced at higher and higher rpm's. If you have a 1:1 transmission like me part of your props are cavitating at max rpm. Went to 4 bladed props. Much smoother and hold the speed better in turns.


Over heating? Check your water flows going to manifolds. Pull the inlet hoses off one at a time and compare water flows. Thermostats will not affect the flow to manifolds regardless of engine temperatures. I cannot picture a muffler going bad unless it got too hot but there would have been other problems anyway. My mufflers are plastic. The water in exhaust does stop some noise. What kind of water pump do you have on your engine? Some boats have a pump on front of engine with an impeller pump hanging on engine (like an alternator).  Do you have any water intake screens? If so check. If you pull pump apart, mark which the housing before removing it (since they go on two ways but work properly only one way). Parts are readily available for these pumps were used on a wide variety of boats and engines. I just did a master rebuild I have a D65 Sherwood pump: the D stands for double impeller I replaced rear bearing (graphite bushing) front ball bearing and seals and both impellers.  I was told that when an engine is hard on belts, look at rear bushing/ bearing.  They were right it was bad, so I decided to do the whole works. I always carry extra parts.


I had the port water pump changed last fall. The water flow into the starboard engine looks pretty good based on the flow through the strainer that has a clear cover (appears to be similar to the port engine) and the strainer is clean. This engine does not overheat, but I will definitely check the water flow to the manifolds. I also have plastic mufflers so the engine noise is probably not a muffler problem. Maybe better flow to the manifolds will help. I will also check the water pump and see if I need to replace the impeller. The belts look pretty good. I guess I need to run the engines this summer and keep an eye on temperature, etc. and see if there is a problem before I do much else.