Perkins 4-108, Max Revs and KiwiProp

[MidLandOne]

Hi Mike

If you've done the fuel side already I would suggest talking to KiwiProp in Auckland before actually going to the cost of replacing the exhaust on a "just in case" basis" (unless it looks like it perhaps should be replaced?).

I would tend to suggest to them that the only thing left apart from the prop you can think of is the exhaust but you don't want to go to the expense of replacing all that without having spoken to them first to get their thoughts on the prop sizing. Might give you an opening to tell them the whole story bypassing the dealer - they may come straight out with wih their own view of the prop size, whether right or wrong. As I said, my understanding from some others is that they are responsive to sorting problems.

[MikeNZ]

Here's an update of what's happened since my last posts.

Basically nothing....

I have passed all the info about the sea trails to the prop supplier who is going to contact Kiwi Prop to check that they still recommend the size supplied (it was Kiwi Prop who sized it for them originally). They have also agreed to check the engine for IP timing, compression and that the injectors are all firing well, so that these causes are all ruled out. The guy who is coordinating all this is away for a week so all this will be reported when he returns.

There is one puzzling thing I can't figure out in all this. Given that everyone agrees its most likely over-propped, why doesn't my engine black smoke when it loads up? When I open the throttle to full it tends to be more whitish in colour than black. I think I can see unburnt fuel in the mist coming out the exhaust. Could this suggest IP timing is wrong? Any ideas?

[CSY Man]

How did you calculate that displacement? I'm wondering what these 33's weigh typically full. And isn't the waterline 25ft?

Displacement as per travellift strain gauges, and also seems about right for all the gear, parts and tools I have added to the boat...

Factory water line is 25 feet, but the boat sits 6 inches deeper (each inch is about 1000 lbs) waterline increases when ya sit deeper, hence 27 is about right when loaded.

[MidLandOne]

Originally Posted by MikeNZ

There is one puzzling thing I can't figure out in all this. Given that everyone agrees its most likely over-propped, why doesn't my engine black smoke when it loads up? When I open the throttle to full it tends to be more whitish in colour than black.

Hi Mike

That is one reason why I have said that I would not rule out the engine being the problem.

Since my last post I have done a static bollard pull test in the marina with our own boat as an example in case it is of some assistance - 50HP, fixed 18 inch 3 blade sail boat optimised prop (Henley, small aspect ratio and blade area ratio). While is a different engine, etc it may give some definite idea of the type of result that one can expect with respect to load and smoke under static pull and motoring conditions.

Doing a static pull against the marina with a clean prop (it was PropSpeed coated in May) the exhaust cooling water turns grey with soot at 2,300 rpm and black smoke appears a little above that - max revs when motoring unrestrained is around 2,800 - 2,900 rpm (have forgotten the exact amount) which is as manufacturer's (Volvo Penta) recommendation and will black smoke at that if prop is fouled but not otherwise - so on static bollard pull against the marina that means it starts putting out soot at around 80% of manufacturer's max motoring revs.

We normally cruise at 2,100-2,300 so one can see that with a static bollard pull against the marina for our setup the load on the engine is much higher than for unrestrained motoring.

I have not looked at the plate on our IP but I think the no load revs for the engine (ie out of gear) are 3,200rpm so manufacturer's max motoring revs are around 10% less than this.

This is all as it was set up by the boat builder and the engine representative and we have no evidence that the boat is overpropped (we can maintain cruising speed into gale force winds and associated regular period seas with no problem - boat is 12 T in cruise with full tanks so we are not overpowered either).

Regarding the injection pump timing question, I am no diesel mechanic but as long as fuel system from tank to engine and exhaust system is ok I would definitely suspect timing if looking to the engine. That is not only the IP but the crankshaft to cam shaft timing (ie valve timing) to IP timing as well as measuring the IP plunger lift.

John

[eyschulman]

I don't understand why anybody is doing static pull tests except to overload your engines. The method of proping a boat is straight forward. #1 you use a photo timer to establish the accuracy of your tach or make a correction card-you do this at the dock in neutral at 1000,2000,and3000 rpm etc.#2 with motor warm in neutral at dock you open throttle all the way and record top rpm( wide open throttle no load WOT) - you match this reading to manufacturers stated fast idle(wot no load) specification. If your motor cannot attain that rpm you stop proceedure and find out why and correct( trottle linkage,fuel restriction, timming etc)If and when your motor is cpable of wot no load spec. rpm you go to the next step #3 the sea trail WOT full load.. Your boat bottom and prop must be clean-the water and fuel tanks full and the boat weighted down with typical gear and personel(no point of doing test on empty boat meant to carry 50 people + gear etc.) YOU take your boat out on the water on relatively quite day and warm up motor then run in four directions WOT and record the top rpm= wot full load value. you then match this value against the manufactures specs. rated rpm at full hp- If you do not get that rpm or higher your boat is over proped - no black magic no guessing no static pull nonsense. Because durring a season of use your bottom and prop may get dirty you should give your motor the benifit of the doubt and exceed the rated rpm by about 150+rpm. So if your motor is rated at say 50hp at 3300rpm you should get 3450rpm. if not you must adjust or replace your prop.Forget what the last owner,builder, your buddy with the same boat did or says your boat and motor will determine what prop to use by applying manufacturers specifications and the above sea trail method. Ed.

[MidLandOne]

Originally Posted by eyschulman

I don't understand why anybody is doing static pull tests except to overload your engines.

If you refer to the original and some subsequent posts you will see why the static test has been referred to. While I agree with you that a static pull will overload a correctly propped engine if that engine is not overpowered for (and consequently likely underpropped) the sail boat you will see that there has been some discussion to the contrary. Like you I consider it is not a relevant method of checking the prop selection unless one plans on being a tug required to do static bollard pulls.

I am not sure who the "anybody"s are that you refer to but I think it is only the original poster and myself who have done the test quoting results - I did the test to demonstrate the point above not as any demonstation that such a test is relevant to prop selection.

Everything else you have said has already been said.

[MikeNZ]

Originally Posted by MidLandOne

Hi Mike

That is one reason why I have said that I would not rule out the engine being the problem.

Doing a static pull against the marina with a clean prop (it was PropSpeed coated in May) the exhaust cooling water turns grey with soot at 2,300 rpm and black smoke appears a little above that - max revs when motoring unrestrained is around 2,800 - 2,900 rpm (have forgotten the exact amount) which is as manufacturer's (Volvo Penta) recommendation and will black smoke at that if prop is fouled but not otherwise - so on static bollard pull against the marina that means it starts putting out soot at around 80% of manufacturer's max motoring revs.

We normally cruise at 2,100-2,300 so one can see that with a static bollard pull against the marina for our setup the load on the engine is much higher than for unrestrained motoring.

John

Thanks for the detailed response, John.

I think you might be right that my lack of black smoking (either at sea or tied to the dock) is pointing at something mechanical and this may be the sole cause, or it may be a combination of something mechanical and over-propping. My frustration is just not knowing.

I hope my post and your reply hasn't raised any heckles - all the help and advice that comes from the forum is invaluable and I very much appreciate everyone's input.

I'll post an update when the guys get a chance to check over compression, etc.

Cheers

Mike

[Wainui]

Hi Everybody
I have a Cal 48 sailboat over here in the US which is 25,00lbs
Recon 4108 driving a 2-1 vdrive, to a VP max prop 3 blade 17''.
I get 3,900 in neutral
Tried 12 deg (6.8'') pitch which gave me 3,200 WOT about 7knts (2,500 cruise)
When motoring with the wind or waves, every now and then the RPM would increase by 200 momentarily as the boat picked up a little speed.
My mechanic (worked on Perkins for about 30 years) believes that I need to increase the pitch to stop this occuring.
I adjusted the pitch to 14 deg (8") and now get about 2,800 WOT. I have not been out enough to determine if the surge problem has been solved.
He states that the 4108 in sailboats will only get about 35 hp due to exhaust size and drivetrain conditions.

Mark in Ventura
Kiwifornian

[CSY Man]

don't understand why anybody is doing static pull tests except to overload your engines

Some of us do static tests to measure the pitch of the prop:

This was mentioned beofre on the thread, and I will repeat in case it did not sink in:

Tied to the dock with a prop sized correctly, one should get 90% of max RPM at full throttle.

If ya don't like it, don't do it...

[Alan Wheeler]

For a displacement hull, A static test should not return a big error. The prop is spinning through the water at a far far higher speed than it is moving forward. Once again, the more incorrectly over proped you are, the bigger that decrease in revs' become. Please remember HP is linked to RPM with a Diesel.
Doing a Static test is a common and valid means of testing many boats and I have seen this commonly performed on net towing fishing boats to work out the best fuel/power ratio's.

[Wainui]

Forward to this week
I found out that the surge in RPM that I was getting was due to a low level of transmission fluid in the Borg/Warner. I have changed the pitch back to 6.8" as I like the ability to rev up to at least 3,000 rpm. I now cruise at 2,400 rpm which seems to be the sweet spot for this engine
regards

Mark in Ventura
Kiwifornian

[CSY Man]

I now cruise at 2,400 rpm which seems to be the sweet spot for this engine

Any tendencies to overheat at that RPM..?

[Wainui]

No
Temp at 2,400 rpm is 180 deg
It goes up to 185/190 at 3,000 rpm

[MikeNZ]

An update (almost resolved I think.....).

I can't believe the length of time its taken to finally learn what's caused my confusing over-propping issues, but now I think I have it understood.

The most recent revelation is related to the issue that had me stumped....If I was over-propped (I was convinced I was) why didn't my newly rebuilt Perkins 4-108 black smoke when I used max throttle?

I have been isolating all the other possible mechanical causes suggested by my prop supplier, and you guys.

I have:
removed and tested injectors - all fine
replaced exhaust hose, in case it was blocked or collapsed - it was fine
tested compressions - all perfect

The last job was to remove and test the IP. You will recall I could achieve just under 4,000 rpm with no load. The company that tested the IP found it was set some 400rpm too low and re-calibrated it to the factory specs stamped on the pump body (4,350 I think). Pump was fine apart from that.

Went out for test run yesterday and the results were almost exactly the same as last time (but max revs now 2,400, up 50rpm) BUT it now black smokes and feels like its really trying to work harder. The propellor supplier came with me and has now (finally) agreed that we are overpropped!

I had previously also spoken to the Kiwi Prop people who said that (provided it wasn't a mechanical problem) a reduction in pitch of 1.5 degrees should give around 400rpm, and a reduction of 3 degrees will give me 800rpm. My prop was factory set at 16.5 X 22 degrees. If I de-pitch it (using the set screws on the blades) 3 degrees to 16.5 X 19 I think I will be about right. This equates to the equivalant of a 'normal' 16 X 11 3-blade, as far as I can ascertain.

To recap, all my prop calculations said 15 X 10 or 15 X 11. Funny that!

I'm away for a fortnight but have a quick slip booked when I get back to re-pitch the prop. I'll post results then. Fingers crossed.

[MidLandOne]

Looks like a good result is close .

I wonder if the previous owner had a "play" with the injection pump to limit revs so engine couldn't rev high enough to smoke on overload rather than replace the previous oversized prop?

John