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Old 23 September 2003, 06:23   #1
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Country: UK - England
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Boat name: Spare Rib
Make: Zodiac/Bombard
Length: 6m +
Engine: 90hp Yamaha
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Under-revving Yamaha 90

We bought a new Yamaha 90 2-stroke earlier this year for a Bombard Explorer 6m RIB. Having completed the running-in, we're finding it's only revving to 4200 rpm which is way below the limit. We have had carbs checked for water contamination and have adjusted the auto-choke settings following advice from Yamaha but this has not made a difference. Boat weight is approx. 600kg.

The prop is a standard 3-blade aluminimum - can't remember size but it is as supplied with the engine.

Is this more likely to be an engine problem or do we need to drop prop size? Engine is under warranty but I'm interested to know if anybody recognises these symptoms on this engine.
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Old 23 September 2003, 06:35   #2
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Thats quite a big boat for 90 hp, think you need to be running a 16 or 17" prop max. Yamaha props are expensive new, but with a brass conversion ring you can run Merc props which are cheaper. Take the prop off and have a look at the size, it will have two numbers one for diameter and one for pitch. 13 x 17 for example. Yam Al props are 16 or 18s, S/S are 17 or 19 etc. Merc Als are 17 or 19, S/S well lots to choose from. On a new engine the supplier should be willing to lend a prop until you find the WOT limit with normal load. Ask him for a loan of a prop for testing afterall you have just bought a new engine from him.

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Old 23 September 2003, 13:23   #3
tue
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Don't worry, its not your engine. I agree with Pete on this. We have this problem on Quicksilver boats when we supply new Mariners to them. After the run in is done customers find the engine doesn't rev out to max. We cure this this with a pitch drop on the propellers. As a rule one pitch is equvilant to 500rpm and your dealer should loan you several props of different pitches to try.
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Old 23 September 2003, 13:48   #4
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Don't wish to Hijack this thread and probably a silly question but but based on the above answers.

If WOT ( throttle fully open) is less that the max revs of the engine what does that imply??.

Will it be significantly slower or what?

If going down a size in pitch will increase the revs would the thrust ( if thats the right word) remain the same?

i.e low revs and bigger pitch = higher revs and smaller pitch.

I surpose it can't be that simple as lower revs would have less wear on the engine.


I'm probably going to get slaughtered for this

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Old 23 September 2003, 14:04   #5
tue
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Ok here goes:

Pitch is rather like another set of gears. For a given engine that wants to run at a given RPM, the faster the boat can go, the higher pitch you need. If you select too low a poitch, the engine RPM will run too high (above manufacturers recommendations), putting undesirable higher stress on moving part. You may have great acceleration but your top speed will suffer and your prop efficiency will definitely suffer. If you select to high a pitch you will force your engine to lug at a low RPM (below manufacturers recommendations) which is generally at a higher torque level and can also be very damaging to your engine. Top speed may not suffer too much, acceleration will be seriously reduced.

Sorry for a long winded answer but its the best explanation for prop pitch I could come up with.
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Old 23 September 2003, 16:14   #6
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There is an excellent book published by Mariner and Merc dealers about props. Cost is quite cheap and it explains props, ventilation and cavitation etc. They will probably have to order it but well worth the read.

There is another hard back book with a big brass prop on the cover. Its heavy going, and gives lots of theory which you don't need.

Pete
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Old 24 September 2003, 07:24   #7
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90hp two-stroke bad experience

Hi Tim, Peter here, marketing manager of Falcon Inflatables in South Africa. Regarding your problem with the Yamaha 90 - this is actually dangerous, we have had the experience called 'detonation' - i don't know how mechanically minded you are, but due to the fact that the Yammy has a mechanical timing advance, this WILL eventually cause the piston to melt, as it is firing prematurely and the motor is overloaded. The maximum prop on your configuration should be 15-pitch, but even lower if necessary - you need to get at least 4500-5500 rpm on flat throttle otherwise you will be left with many problems later. This is based on actual experience, if you need more info email me at info@falconinflatables.co.za
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Old 24 September 2003, 08:30   #8
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Everything You Need To Know About Propellors

Guys,

I have a copy of the Quicksilver publication 'EVERYTHING YOU NEED TO KNOW ABOUT PROPELLORS'
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Old 24 September 2003, 08:35   #9
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Damned 'Enter' key.....

As I was saying......

I have a copy of the Quicksilver publication 'EVERYTHING YOU NEED TO KNOW ABOUT PROPELLORS' as a 'Zip' file (8.4mb).
When you unzip it is a series of pictures of the pages of the booklet. You can either read it as is onscreen, or you can build a printable version with a WP or Publishing program.

It makes very interesting reading......

Let me know if you want the zip.
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Old 24 September 2003, 13:46   #10
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Propeller 'Zip' file (8.4mb).

Hi Bill,

Please may I have a copy. Send to

kevin.luter@portsmouthcc.gov.uk


Many thanks
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