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Old 21 August 2001, 08:42   #1
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Country: UK - England
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What Prop ?.

I have a Ribcraft 4.8 and Mariner 50/4 stroke outboard.
The alloy prop that came with the outboard only let me get 5000 rpm with one person and 4500 rpm with 3 persons. I am aiming for about 5500 rpm with one person.

Does anyone have any sugestions or experience for porps, probably stainless for this type of setup.
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Old 21 August 2001, 10:02   #2
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Gary, what pitch is your current prop? It sounds as though you are running a prop with too higher pitch which is not enabling your engine to achieve the Wide Open Throttle setting. This will cause the engine to labour and not run at its optimum.

You might want to drop a couple of inches in prop particularly if you are going for stainless. An alloy prop will flex so that in reality a 13" pitch prop maybe 12.5" under load. A Stainless prop is far stiffer and flexes less.

Ideally you want to aim for a prop that gives you the performance under the load conditions you usually use the boat. So if you are always out with 3 people and full fuel load, select a prop that suits those conditions.

Final thought, is that your engine is a 4 stroke which in my laymans knowledge of 2 vs 4 stroke engines probably doesn't rev as high as a 2 stroke? 5000rpm maybe WOT for your engine. Your manual will tell you.

HTH, Alan
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Old 21 August 2001, 17:17   #3
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The pop fitted at present is a 14 pitch by 11.25 dia, as fitted as standard by Ribcraft. The RPM range in the manual is 5500 to 6000 RPM so 5500 with 3 persons on looked good, hopefully not over revving above 6000 with one person on.
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Old 21 August 2001, 17:30   #4
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Gary

On smaller engines I have never found that the cost of a stainless prop justifies the outlay. You will get slightly better performance over standard Aluminium but nothing to write home about and certainly not worth three times the price.

Just to note, if you have a fairly newish Ribcraft and you want to exchange props (provided you havenít dinged it!) Ribcraft will exchange it for a different pitch to suit the usage FOC.
Also if itís a newer engine sometimes in the first 100 hrs they loosen up and gain a few more revs.

Cheers

J
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Old 25 August 2001, 04:12   #5
JCW
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Gary,

I also have a Ribcraft 4.8 but with Mariner 60HP 4 stroke.
The standard prop supplied was a 15" aluminium. This gave 5700 RPM with 3 persons and 5900 solo.

I switched to a 14" stainless steel prop from Solas. This gave me the same RPM figures but found much improved grip. It slightly improved acceleration and reduced fuel consumption too.

If for instance the aluminium prop started to cavitate I had to throttle right back to regain grip. The stainless steel regains grip without having to do this. Therfore much better in choppy conditions.

Rules of thumb for your setup.

If you switch from a standard aluminium prop to a cupped progressive pitched SS. It will probably reduce your RPM by about 200 -300.

Every 1" reduction in pitch will increase your RPM by about 200 -300.

Try and pitch your boat so at WOT it pulls to the MAX RPM specified by the manufacturer.

Also I noted From Rib Internationals 'Grey Seal' They found a 13" solas SS to give them the best performance. There set up was a Yam 50HP and Ribcraft 4.8. There figures suggest that they were pulling Max RPM.
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