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So, looking at the engine power curves and the gearbox data, my bum's out the window, as it were.

I like to motor at 2000 rpm. From the power curves at 2000rpm the engine is putting out around 17hp. From the gearbox manual, the box can accept 2000rpm x 0.74hp per 100rpm = 14.8hp.

The gearbox is overloaded.

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Alright. Totally confused now.

According to my post above based on the engine power curve and the gearbox data, the gearbox is overloaded.

I now have a Perkins Parama M30 spec sheet courtesy of Erice, and from it I see that the engine was originally supplied as a package complete with gearbox, the gearbox being a Hurth HBW 50. From the internet,this Hurth box has a maximum input of 20hp and can accept 13hp at 2000rpm - somewhat less than my PRM box.

What the heck am I missing here?

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on the face if it


it is concerning


my 2013? 16hp twin came standard with the ZF10 M rated from 33 - 51hp


so the gearbox was 2x over-spec for the engine's power rating


your, almost double, 29hp engine has a gearbox rated half mine


as the HBW50 became the ZF 5 M, which was replaced by the ZF 6 M


which is only rated at 23 - 27hp


seemingly under-spec...




so you'd have to think why they went double-spec on later engines?


perhaps because the earlier trans were not up to scratch?


so maybe


- keep revs well below red line, no problem  as you hit hull speed well below red line


- be meticulous in your annual trans oil changes for quality, quantity and timing


- look at adding an oil cooler to reduce oil heat stress


(if you did go the cooler route make sure to use the dipstick when changing oil and not the quantity for the gearbox before the cooler was added)


but this is just theoretical ponderings


have never needed to dig into ANY kind of transmission,

with the exception of mr. bojangles, the bucket of bolts in a concrete mixer dry clutch of my 1991 ducati superlight  (#536 of a limited run of 500.....f#ck!ng italians!)





my aging honda auto at 250,000km might need to be the next time i dig into a transmission 

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Don't panic Chris. First of all, it is all in the numbers.

None of those specs are the numbers you are using. As I said earlier, manufacturers use different specs to rate motors and you have to be careful as to what is being used. So the graph is showing with line No1, the Gross Power Output of the engine alone, no gear box. That is the Max intermittent power output possible this engine can ever possibly produce and that is worked out on a test bench with good cooling and for an intermittent time period because the engine cannot produce that power for long.
Line No2, is the Shaft output power measured at the shaft of the Gearbox itself. Once again, measured on a test bench in a way that could never be duplicated in your boat.
No3, is showing what Hp that particular propeller noted should be requiring from the engine and measured as SHP, which is different to BHP.
What is seen as a load at the output shaft is proportional to the load at the input. So just because the engine may produce say 20Hp at x RPM, the gearbox is not working hard if say you had no propeller fitted. If the propeller only sucks up 15Hp the the load the gearbox is sucking of the engine is 15Hp....kind of....because Torque increases as gear ratio increases, but ratio also increases mechanical energy loss and Hp decreases. In otherwords, Hp is lost to friction and thus heat as the gear ratio increases, but Torque goes up.

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Thanks Wheels, but I think we are a little at cross purposes.

I am altogether happy with the engine. I'm going to take a whisker more off the prop so that I get the recommended tip clearance, which in turn will enable me to get to within a few revs of WOT- probably around the 3500rpm mark.

It is purely the size of the gearbox in relation the engine output HP that concerns me. I destroyed a gearbox last season and didn't enjoy the experience very much.

The PRM80 gearbox can accept an engine imput max of 32hp, a max of 5000rpm, or a total of 0.74hp per 100 rpm across the operating range. So, at 1000rpm you can poke 7.4hp into the box, at 2000rpm, 14.8hp and so on. Clearly, the engine is delivering more HP than this at these revs.

I'm really hoping that there's another way of looking at these figures, but I don't know what it would be.

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Correct me here if I'm wrong, but the engine Hp is only part of the equation. To do 'work' across the gb, the propeller needs to be putting out the power also. The gb can't experience the power produced by the engine if the prop isn't applying it to the water.


Last year when your gb failed, you were running the over pitched prop?

That would have been laying massive amounts of 'work' / power, back up the whole drive train. The engine sounds strong enough to meet the work produced by the prop, but not the gb.


If you have a smaller / flatter pitched prop, you aren't going to be able to generate the same forces (torque) in the gb as last year. So you should not get the same mode of failure in th gb, at least not nearly as quickly as last year.


That's what I think anyway.

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Last year's GB failure was indeed due to being hugely over propped. I didn't know then about all this WOT under load business, and could only manage 2400rpm instead of 3600rpm at WOT. Since I never went there, and the engine was very happy at 2000rpm which was what I cruised at I assumed all was OK.

Now that I'm older, a lot poorer after buying a new box, and a bit wiser, I am very keen on avoiding another gearbox catastrophe which is why I have been labouring the GB input HP point. Sorry to be harping on about it.

Personally, I agree with you Fish that the gearbox shouldn't get loaded up with the right sized prop - but the whole business of input rpm and HP and propeller loading and engine output HP just makes my head hurt.

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Well, you've de pitched your prop now, and you say your going to take the tips off a bit to get closer to WOT. What else can you do?

Maybe fit a gb cooler, if it has the ports etc. being a small box it may not.

The only other thing to do is, as you've sorted the prop, use it. If it blows again, then get a bigger one. You've already paid for this one and it's on the boat. Either that or go back to you old prop and tear around at 3.5 knots...

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First point. Don't cut the prop. Unless you have a hammering problem on the Hull. The gap is usually 10% of the Diameter. I know Muzza said 12%, but I was taught 10% and there is stuff all between the two anyway. The reason for the gap is simply because the water coming off the tip of the blade ends up being compressed between the tip and the Hull. Because water will not compress, the result is the Tips pound the Hull and it is purely a noise thing, nothing else. It has to be much closer before it becomes a mechanical issue.
By cutting down the Diameter, the efficiency of the prop decreases dramatically. You will have far more slip, which results in a lot less acceleration.
   Second Point.
The perfect Loaded RPM of your engine is 10% less than max. That is 3250, not 3500. At 3500, you will losing a lot of efficiency. 3250 is perfect and as you are darn close to that, I would not be worrying in the slightest of achieving more. You may have an error of 50RPM with you tacho for a start, so Unless you are measuring via an electronic measuring device all the way through the range, then you could well be trying to correct something that isn't wrong.
Third Point.
You are using the wrong specs.
Your box will handle 47Hp at full RPM, not 32Hp. 32 is for light commercial. You easily fit into the Pleasure range. Which is simply that you don't sit at max RPM for extended times or more than 5% of the time. Normal continuous rating should be at no more than 90% of Max RPM. Frankly, I doubt you will be at 90% even. The engine will have a rating of about 75% for continuous use. That is 2700RPM, still way above the 2000 you sit at.
Oh and a max of 500Hrs per year of use.
So the figure you use is 0.98 BHP/min.
Although that is a slight red hearing, because it is not something a Sailboat is concerned with.  Because normally, the slower the engine RPM, the less strain and thus heat the Box is producing. The less efficient the prop and less thrust, so less power required from the engine. Just because the engine can produce x power at x RPM, does not mean the prop will require that. The time this would be a factor to consider for a Work boat or launch that has a large multi blade prop and you were trawling a net or something and the Engine would be working hard at the lower RPM.


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I am fractionally less than the 10% tip clearance. I thought to take it down to the 10% mark because I can hear it hammering under the hull, and also with the previous small prop I could stand a glass of water on the engine at 2000rpm, but now there is a degree of vibration. Shaft tube bearings replaced last year.

Concerning the gearbox ratings, the manual is quite succinct in stating that vessels with a displacement or semi displacement hull form when used for pleasure boating should use the 'light commercial' ratings. Unfortunately.

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Yes but once again, they are referring to a Launch rather than Sail boat. You have likely noticed that all Displacement and Semi Displacement Launch operators set their throttles using the principle of how high the Bow is pointing toward the sky. We used to use the rule of thumb of 5Hp per ton for a Displacement Launch and 3.5Hp per ton for a Sailboat.

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Without dancing all around the issue again, as all angles seem to have been discussed, if you can reach rated RPM ie 3600 with a clean bottom then the box will not be overloaded . And more important you will not be running in lug mode, thus  allowing full horsepower to be available if ever required and avoiding damage to the engine (premature carbonning of exhaust elbow etc etc.) And the prop will run quieter with more tip clearance  15% was always rule of thumb and what classification agencies use as absolute minimum, however on a lot of yachts we sneak it down to nearer 10% depending on hull shape.

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What I'm going to do is go up on the grid at Rocky Bay, whip the prop off and into Briski who will take just a few mil off the diameter to put me into the 10% clearance range.

This, I hope, will enable me to get to within a 100rpm or so of the magical 3600rpm WOT, quieten down the drumming noise and reduce the resultant vibration.


I know that I have been labouring the point relating to all this engine/gearbox business, but this has been a result of my ignorance of these issues, and it was this ignorance that caused me to fork out $1200 or a new box after wreaking the existing one.


But I'm pretty comfortable now with my course of action, and as always, a big thanks to all you fellers who took the trouble to guide me through.

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What I'm going to do is go up on the grid at Rocky Bay, whip the prop off and into Briski who will take just a few mil off the diameter to put me into the 10% clearance range.


Chris, Briski is now quite a journey, they are no longer in Howick but based just east of Maramarua about midway between the village and Smythes Quarry or just before the Heavens Road passing lanes when east bound. Henley's are  a lot closer to Auck's. Brent is the guy who runs Briski.

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Thanks for that Muzza.

Briski built the prop for me a year ago at their Maramarua premises. It's not too far from Half Moon Bay where we keep the car whilst sailing. Also, Brent offers free tweaking of props he makes until the customer is satisfied. A darned good service!

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I thought to resurect this post because I have just destroyed my third gearbox in 300 hours run time. I'm really getting quite good at it.

Obviously there is a lot more to this engine-gearbox-prop matching business than meets the eye.

To recap, my engine puts out 29hp max. The PRM80 gearboxes I've broken so far can accept up to 32hp. I can attain maximum engine revs (3600) with the current propeller. Alignment between gearbox and engine and gearbox and propeller is good. Engine mounts are new.The PRM80 is the box recommended by the supplier.

So all should be ok but obviously not.

I am convinced that the answer lies in the gearbox manual fine print which states that the gearbox can accept 0.74hp/100rpm. At my preferred cruise revs of 2000, the gearbox can take 14.8hp. From my engine power curve the engine puts out 16.8hp at 2000rpm at the flywheel. I presume that you can deduct a hp or so for the alternator. But it would appear that unless one knows the propeller power requirement across the engine hp range, it's all just guesswork as to whether a gearbox is suitable.

The other thing that is exercising my nasty suspicious mind is that the PRM80 gearbox has been discontinued in favour of the PRM90. Same dimensions but it can accept 0.83hp/100rpm or 16.6hp at 2000rpm. This puts it in line with all other mechanical gearboxes in its class. This is what I am installing now. It makes one wonder is the PRM80 was a bit of a dud..

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Chris, i'm worried about your bank account...the new gearbox is only about 11 or 12 % stronger than the old one which failed after only 300 hrs. Do you know what happened to the old one , as in what part of it failed? is it a similar reason to the previous ones? 

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I’d like to know as well, same as Jim. Seems to me something is fundamentally wrong, and which part failing could provide a hint.

Are you absolutely certain the shaft is straight? What’s the condition of the cutlass bearing? Is there an issue with balance? Are the engine bearers still in good order and properly attached to the boat? (Saw a boat recently with similar issues, caused by alignment, which itself was caused by an engine bearer coming away form the hull!!)

It might be time to get someone else to look at it - another pair of expert eyes may see something missed before?

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