Perkins 4-108, Max Revs and KiwiProp

[MikeNZ]

Hi everyone

A short while back I rebuilt my 1985 4-108. Long storey but the PO had failed to check the exhaust elbow which had rotted on the inside and let salt water backup into the cylinder head rotting out the No4 exhaust valve (this came to light 1 month after I took ownership- GRRRR).

Aside from the rebuild we had another major problem as the yacht was way over propped. I have just replaced cutlass bearing, shaft and coupling and fitted a PSS Shaft Seal and a brand new 16.5" X 23" KiwiProp. I left the sizing to the local supplier who also did all the shaft work, etc. BTW my prop calculation programs using my displacement (20,000lbs) my waterline (40ft) and my 1.91:1 reduction, etc suggested a 15" X 10" 3-blade. I told him this but he said KiwiProps used different sizing formulas. My last prop was a Flexofold 17X12 3-Blade which could only pull 1,700rpm at the dock.

When I re-launched the boat on Friday I motored back to my marina and tied her up and started my tests. Max revs I can get are just over 2,000. There is no black smoke but there seems to be a thin haze of diesel fuel with little exhaust steam. Temp is normal (85 Degrees C). This is way short of the 3,400 - 3,500 I had expected. My 4-108 is the "New" model rated at 52HP at 4,000rpm.

When I called the supplier back he tried to tell me that no boat can reach anything like operating revs while tied to the dock. This doesn't make sense to me. Anyone know for sure?

When out of gear my engine will rev to 3,950 rpm.

Prop supplier suggests we look at blocked exhaust and lack of fuel supply as he thinks his prop sizing is right (I think he is basing it on similar 4-108 installations they have done - see www.kiwiprop.co.nz if your are interested).

Anyone got any advice for me?

Thanks

Mike

[DeepFrz]

I would say you are way over propped. You should be able to reach close to your max rpm (-200 rpm perhaps) in gear and tied to the dock.

[MidLandOne]

As a generalisation for sail boats with a 3 bladed fixed prop, including high aspect ratio sail boat optimised ones, the engine should overload well before max revs when tied to the dock if the boat is correctly propped. For KiwiProp there may be subtleties in this that I do not know of though, however, the supplier has given the same advice and I suspect he is correct. The only way to test the prop properly is at sea and in a variety of conditions from smooth water to head seas/wind.

I am not particularly familiar with KiwiProp but as I understand it you cannot compare their pitches (in your case 23") with other props so no point making such comparisons with your previous prop.

The prop builder is the one best able to size the prop both from the point of view of experience and also the designed performance of the prop. Use of generalised prop selection formulae is fraught with danger in that there is much variety in prop design even in fixed props let alone applying the formula to specialist props such as the KiwiProp.

Sorry, nothing out of that that solves the problem, if indeed there is one. I would suspect that if the prop supplier was given the correct boat, engine, gearbox, etc details he would have selected a prop very close to the mark and if after sea trials you still have a problem I would keep an open mind as to its cause, either the engine or the prop could be implicated.

[eyschulman]

First things first- If you have not already checked your tach with photo timing do so - many units are way off. check at 1000,2000,and 3000 rpm no load(in neutral at dock) record deviations- next rev motor in neutral to max rpm and check against manual #s if not up to spec. correct(throttle ,filters, poor fuel sup. etc.) next step take boat out full load water fuel gear etc-warm motor go to WOT under load(wide open throttle) match the rpm against the rated Hp rpm in manual to be safe you must reach this or even better exceed by 150rpm(in the real world dirty bottom etc) An over load of a 100rpm can significantly shorten life of motor esp. newer terbo units. You should cruise at 300to 500rpm below rated Hp rpm for best life of motor proping a boat right is a critical proceedure often overlooked.

[over40pirate]

Just curious. Does the motor get the boat up to hull speed?

My 4-107 drives my boat at hull speed at 1600 rpm (so the tach says)
Seat of pants tells me the engine is running at the right speed.

[Randyonr3]

I've got one of the new 4-108s and when setting up the pitch on the "Maxi Prop" was told to set the prop to choke the motor at 200 to 300 less than full throtle... full throtle on my motor was 3600 so I set the prop to max the motor around 3400...
Even though I rarely take the motor over 1800........

[easterly]

I have a 199? 4 108 and a paragon gear trans about 2:1. I can't read the ratio, the tag is under the cooler. It has a 13X8 3 blade prop and will turn 3500 rpm @ 8 knots. The prop is correctly sized to the boat. I have seen too many boats over propped. The Perkins will heat up at 3500 rpm if run for a long time. I know alot of new boats came over propped. Full throttle in neutral on my engine is 4000+ rpm.

[Boracay]

When I upgraded the exhaust on Boracay from 2" to 4" the engine speed only went up 50-100rpm.

I have about 2" too much pitch. (I get 2150 of a max 2500rpm).

[CSY Man]

When tied to the dock and at max throttle in forward gear, ya should get 90 % of max RPM.

Running free yoa should get close to 100%.

Out of gear you should get 100% (4000 RPM) governed.

[eyschulman]

By the way one of the results of prop over load is excessive cylinder head and exhaust heat often blowing the last exhaust valve may also contribute to exhaust elbow problem. This is not a seat of the pants procedure you have to prop by the numbers- the motor specs and the info in my first things first message is the gospel. Many sailboats are badly over propped mainly a mater of ignorance. Big powerful modern diesel motor boats are less likely to be over propped and certainly when they blow there first motor at 500hrs and sixty grand later they won't do that again.

[MikeNZ]

Thanks for the great responses.

Eyschulman - You are dead right about the over load contributing to my rebuild! Things with diesels ar so opten a combination of factors. I also agree there are no magic numbers and that sizing the prop is a usualy a multi-step operation. I just feel I'm not close enough on the first shot and I should be getting at least 3,000 rpm at the dock.

CSY Man - That's what I wanted to hear. So I should be able to get say 90% of 90% of my unloaded max rpm . This would be 90% of 3,600 which is 3,240, or therabouts?

Bora Cay - That's good to know. I remember a much older post by one of our members who said he had talked to Perkins who said the P-108s should have 3" exhausts to get up to spec horsepower, and they said that the 52HP quoted for the 4-108 was unachievable in a marine application. I guess I accept all that but what I'm dealing with is not a horsepower problem per se (heck I would like more but I get to hull speed now in favourabel conditions). My issue is all about extracting the most useable horsepower available and getting maximum longeveity from my rebuild. I know my engine is not in its power band at 2,000 rprm. I also want to prop it so I can get into the power band when I'm pushing into a headwind like when you round Cape Jackson wind against tide in a stiff blow (South Island Kiwis like Wheels and my Mana mates will no what I mean).

Easterly - I presume your 3-blade is a fixed prop rather than the feathering props I'm talking about? However, you are much closer to what I calculate I need. Glad to hear you can get to 3,500 at 8knots. That's pretty much what I'm after (8.5 knots in my case).

Randyonr3 - Your MaxProp gives you more adjustment than I think is available in the KiwiProp, but that's what I'm after.

Deepfrz, eyschulman and Over40pirate - yep, the boat gets to hull speed and I have checked the tach with a laser guage and a reflective strip on the crank pulley. I will go and do some sea trails when I get back from the UK (I'm on business now earning the money to pay for the new prop, etc!!).

Midlandone - Thanks. I understand what you are saying and I'm keeping my fingers crossed that sea trials prove all is well. But....with my last prop the difference between what I got at the dock and what I got at sea was very slight. Maybe the KiwiProp will be different but I have to say I'd be surprised.

Keep the comments coming. This site is such a good one for all issues like this. Its great to have such a wealth of experience to draw from. Thanks.

[CSY Man]

CSY Man - That's what I wanted to hear. So I should be able to get say 90% of 90% of my unloaded max rpm . This would be 90% of 3,600 which is 3,240, or therabouts?

Well, my 4-108 is rated @ 51 HP at 4,000 RPM and I thought all recreational 4-108s was rated the same..?
If so, 90% of max should be around 3,600 RPM and that is about what I get when tied to the dock.

The book also states that max-continous power is 3,000 RPM, hence anything over 3,000 is just for emergencies.
I usually cruise at 70% of max continuos, which turns out to be around 2,100 RPM and that seems to be the sweet spot with an hourly fuel consumtion of 0.64 US. Gallons, or about 2.422 liter per hour. In still water the speed @ that RPM is about 5.3 Knots

I turn a wide 3 bladed fixed pitch bronze prop pitched at about 10.5" and the size is about 16.5".
My boat is a fat and heavy little cruiser with a 27' waterline and a displacement of about 23,000 lbs "as is".

[MidLandOne]

I'm on the coast too Mike - a bit over a km along the road north of the roundabout at the P/Beach boat club. Boat is in Wellington.

I've had a look through the KiwiProp selection data, etc and seems to me that the prop you have is about what they recommend for your engine and reduction. Also, given the number of examples they give for the various 4-108's and their gearboxes they appear to have had plenty of practice.

With a correctly sized conventional prop on a sail boat one should not expect to get 90% of max engine revs when doing a static bollard pull on a dock. After all, the usual recommendation is if correctly propped one will then only get 90% max engine revs in calm conditions and considerably less again into seas and heavy wind.

This effect may be much less obvious on a boat that is high powered for its size and underpropped (which may be intentionally done to keep revs up at displacement speed) - but I suspect that your boat does not fall into that class - or if it is just underpropped.

We are 12 tonnes everything full, 40 foot, 50 hp, 3 blade fixed pitch sail boat optimised prop and we are not over powered and have yet to meet conditions around here that defeat us under power (Karori, Jackson inner pass, French Pass against the tide, and with rip seas/winds). But we definitely overload well under max engine revs if we try to pull the dock around, however I cannot recall at what rpm because I don't make a habit of doing it. The general recommendation is if running the engine routinely against the dock "for exercise" that revs should be kept down to avoid overloading the engine.

How great this affect is with a KiwiProp I could not comment but on the basis of their selection data I would think there is a good chance the prop is sized correctly and I would not discard the possibility that there may be something else wrong if you find it does not perform when doing trials away from the dock.

John

[MikeNZ]

CSY Man - What gearbox ratio do you run? I'm running a 1.91:1 which gives me a pretty high shaft speed. I'm guessing you are around 3.0:1?

[MikeNZ]

G'day John

Thanks for the follow-up. My main aim is to get revs into the power band under general operating and to be able to get to 90% of max revs when I really need it.
I kept running out of power with my last prop (max of 1,700 - 1,800).

I'm down at Mana. The Night Train is on D27. I'm down often in the weekends, so you would be welcome to drop down for a beer.

Mike