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As we,instead of our normal 5knts, motored home today,the old motor coughed and spluttered,next minute we were doing 6.5knt so buggered if i know.3/4 throttle may of been something in the air filter?? .

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Harry, I used to change air filters at the beginning of each season regardless of if it really needed it or not. Another thing is people tend to under estimate the importance of good airflow to the air filter, too often engines are hidden away  in a box with no thoughts given to having an adequate supply of clean air to the engine. Another possibility in your case might be fuel, check filters and maybe consider having the injectors looked at but good to here the speed is back up to a more realistic  figure. it would be interesting to test the boat out in 25 knots with a lumpy sea to see how this performance increase helps in a choppy sea.

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Harry, I used to change air filters at the beginning of each season regardless of if it really needed it or not. Another thing is people tend to under estimate the importance of good airflow to the air filter, too often engines are hidden away  in a box with no thoughts given to having an adequate supply of clean air to the engine. Another possibility in your case might be fuel, check filters and maybe consider having the injectors looked at but good to here the speed is back up to a more realistic  figure. it would be interesting to test the boat out in 25 knots with a lumpy sea to see how this performance increase helps in a choppy sea.

changed oil+filter and fuel filter,had injectors looked at,no problem there,motor maybe getting tired and noticed a club member with same problem has taken kiwi prop off and gone to 2 blade folding,but we will just have to put up with what we have for the time being.motor sounds better and appears at this stage to be pushing 6.5,little less throttle but needs a scrub again.

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changed oil+filter and fuel filter,had injectors looked at,no problem there,motor maybe getting tired and noticed a club member with same problem has taken kiwi prop off and gone to 2 blade folding,but we will just have to put up with what we have for the time being.motor sounds better and appears at this stage to be pushing 6.5,little less throttle but needs a scrub again.

One thing I have noticed is that marine growth on the prop will kill performance faster than growth on the hull. My dads launch taught me this at a young age. After a long period of inactivity the boat was quite heavily fouled with slime and weed and  speed was down to 4.6 knots from the usual 7 knot cruising speed with the engine showing signs of overload. The boat was about to be bought home and I didn't envy the trip around to Halfmoon  Bay at that speed so decided to clean the prop and see if it made a difference. Speed was increased to 6.7 knots which was a little more acceptable and no overloading.

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I just thought I'd say I have a kiwi prop. It works very well, but MUST be correctly pitched for your boat, and the hp it produces. It's also essential to grease the 5 lubrication points every time the boat is out of the water, and to make sure the spring is good and the feathering action is smooth all the way through the movement.

I prefer the kiwiprop over my (way more expensive) spare Volvo three blade folder.

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IT your comment about matching propeller size pitch, diameter and blade area to engine load is very relevant with all propellers and even in today's age of computers it is not unusual for the experts to get it wrong. Usually after launch optimizing propeller efficiency is just a work in progress. Sometimes the cheaper option is to collect data of several second hand props ensuring tapers and attachment are the same or in the case of folders use blades from the same manufacturer of various sizes. The more data collected the more chance of getting the final prop selection right.

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Did you fit the new prop Chrisc, how did it work out?

Got the new prop on and can get 3200rpm at WOT, which should be 3600 rpm so I guess i need to get a bit more shaved off.

Interestingly, there has been a big speed improvement with the prop it its present configuration that what i got from it before it was reduced the first time.

Originally 4.5 knots at 2000 rpm, Now 5.4knots at 2000 rpm. I get max hull speed at 2600rpm, engine happy, no smoke.

Now my worry is the gearbox because I dont understand what all the numbers mean. The supplier said it was the right box for my engine, which produces 30hp at the flywheel. But does this include the fact that the engine is driving an alternator, a water pump and occasionally a refrigeration compressor?

The reason for my gearbox concern is that its maximum input is 32HP, but it also states 0.74hp per 100 rpm. This would imply that at 3600 engine rpm the box can only accept around 26.5HP, so who knows?

If there are HP deductions to be made for the ancilliary equipment that the engine is driving then I should be OK, if not then I may be in trouble.

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You should be fine. Firstly, you don't run the Boat at 3600RPM all the time, or you shouldn't. That is max intermittent rating. 3200 is not too bad. I would not be worrying about any more shaved off. 3240 is the 10% mark. And yes, the ALT will take power. 80A is about 5Hp. So a 40A is going to be ruffly 2.5Hp. That could easily be your 40RPM missing right there. I think you are spot on.

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Chris The 30 horse power from Yanmar may be a bit misleading for instance the 1GM 10 is only 9hp and the twin 2ym15 is only 14hp the 3 cylinder  30 (forget the number is actually 29hp This horsepower rating will include the water pump and alternator but not the fridge commpressor. A typical fridge compressor can asily soak up 5 to 10 hp so best to have an electro magnetic clutch fitted to turn it off once the fridge has reached the required temp.

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Chrisc no 3600 rpm is a guide my new engine with.a 16x9 three blades Volvo prop only gets 3400 and that's close , it's handy to be a little over pitched on MY VIEW for it's a little quicker off the mark , more power responsive manuvering in marinas etc if you ever get to full rpm your putting everything under strain , I can do 8 kts flat out , 6.5 all day at around 2600 rpm into Most wind and fuel is nothing , with a beta 20 three cylinder ,I don't know if this is the same with a launch etc

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1GM 10 is only 9hp and the twin 2ym15 is only 14hp the 3 cylinder  30 (forget the number is actually 29hp

There are several different ways to rate HP, hence why the different figures. No modern day engine manufacturer uses the old real world specs, simply because everyone wants you to believe their engine is better than their oppositions. To name a few different tests, there is BHP, SHP, SAE and DIN. Then there is Max Intermittent and Continuous Rated. In the old days, you used to get all those test specs. Today you tend to get one or maybe two. For instance, my Perkins 6.354 has test specs in which the lowest number is just a mere 95Hp. That's 95Hp from a 6 cylinder 354 cubic in(7ltr) engine. The highest number is 130Hp. If it were sold today using what most makers use, it would be sold as a 130Hp engine.

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gearbox hp rating is similarly complex

 

my hurth ZF10 M manual listed 4 quite different hp rating depending on duty cycle and gearbox ratios 

 

ranging from

 

C - duty - continuous at fixed rpm, (commercial trawler etc) max input  20-31hp

 

P - duty - light (private planing hull) 30-51hp 

 

as it was my mitsubishi twin based engine was only 16hp so it didn't matter

 

but if someone had only looked at the headline specs of "up to 51hp"

 

and mated the gearbox to a 40hp engine and used it continuously at a fixed rpm they would eventually come to grief and the warranty would probably not be honored

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gearbox hp rating is similarly complex

 

my wurth ZF10 M manual listed 4 quite different hp rating depending on duty cycle and gearbox ratios 

 

ranging from

 

C - duty - continuous at fixed rpm, (commercial trawler etc) max input  20-31hp

 

P - duty - light (private planing hull) 30-51hp 

 

as it was my mitsubishi twin based engine was only 16hp so it didn't matter

 

but if someone had only looked at the headline specs of "up to 51hp"

 

and mated the gearbox to a 40hp engine and used it continuously at a fixed rpm they would eventually come to grief and the warranty would probably not be honored

Exactly right!

And this is why I wonder if I have been supplied with the wrong gearbox.

My engine is rated at 30hp @ 3600rpm. The gearbox has a maximum input of 32hp so on the face of it, all OK.

But hidden away in the manual are maximum HP ratings per 100rpm. For a pleasure boat, 1.12hp per 100rpm and for a light commercial 0.74hp per 100rpm. On the face of it still OK excerpt that displacement pleasure boats are to use the light commercial rating. So 3600rpm x 0.74 equals 26.6hp which would imply that the box is a shade too small, or perhaps not if I should make a HP deduction for the engine turning a raw water pump and 55amp alternator.

What I need is to find the power curve for the Perkins Parama M30 so I can see exactly where I'm at over my normal cruising rpm, but an internet search reveals nothing - I guess it must be a secret.

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PRM 80.

Got a workshop manual for it, and also for the Perkins engine, but engine HP output data is not in it and is notoriously hard to find - I don't know why. I have heard the Perkins M30 described variously as a 27 HP through to 30 HP engine.

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don't really want to 2nd guess professionals

 

but 30hp seems enough to push along an h28 at hull speed without needing to be red lined

 

and if the engines not needing to output its max hp the gearbox should be ok

 

if you were concerned you could look at adding an oil cooler

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True - I get hull speed at 2500rpm.

From the gearbox data, 2500 x 0.74 = 18.5hp, the gearbox's maximum input HP for those engine revolutions. The trouble being in the absence of any engine power curves I don't know the engine output at those revs. By simple extrapolation it should be 20.8hp but I don't think it works like that. Without the curves it all just supposition.

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