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Message 12 of 12
Posted by member Andy Mcmullan on Friday 18 September 2009
I have a 15 pitch prop on the 80 Yam 4-stroke on my 98 Pilot. I replaced a 19 inch prop that would only let the motor rev to about 5400 rpm with my normal load. The new prop lets the motor rev to about 5800rpm and she sounds much better. It revs up quicker and feels far more responsive.
I think ideally I would like a 13 inch prop (13 X 13) as I would like to be able to max out at 6K revs which is the max for the motor.
If I recall correctly another Hardy Pilot owner on the forum also got excellent results with a 13X13 prop on the 70Hp 4-stroke Suzuki.
I've read numerous times that if the motor is unable to rev out to near it's maximum then its not 'geared' properly with too course a pitch and is effectively working too hard.
I don't mind losing top speed with the finer pitch as I feel over 20 Knots in the Pilot is a tad too fast. I'd prefer sharper acceleration.
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Message 11 of 12
Posted by member Dominic And Nicola Gribbin on Wednesday 2 September 2009
Hi James.
Yes, correct we are using a 17inch pitch prop on our 2006 series Honda 75hp.
Boat planes at 13 knots upwards. Comfortable cruising in calm conditions at 17 to 20 knots around 4000rpm.
My calculaltions give us an approx figure of 3 to 4 miles per gallon at such speeds. Clearly this improves traveling more slowly.
I do think our prop is probably not maximising the max rpm achievable but this prop optionwas always a compromise of torque over top end speed.
In conclusion large hp 4 strokes are rather thirsty when opened up.
Hope this helps.
Dominic.
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Message 10 of 12
Posted by member James Smith on Tuesday 1 September 2009
Dominic?
I see on another thread you mention you are using a 15" pitch prop and in this one a 17" pitch. Seeing as how you have probably traveled the most miles i was wondering which it is you are using?
What rpm does your honda cruise at and what speed? And if i may be so bold... what sort of economy are you getting from your 75 honda at cruise? I have the same engine with a 15" pitch prop and if i cruise at 4000 rpm it seems to drink fuel at an alarming rate.
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Message 9 of 12
Posted by member Martin Hanley on Friday 29 May 2009
Hi All
At last we have a solution! Our engine supplier came out for a test run.
He gave her a good trashing!!! and was happy the engine height was correct ( a little rooster tail at top speed, with the cavitation plate visible). The engine reved to 7500rpm!!!! The rev counter was set incorrectly and over reading by 1000 rpm so we have been running in well under the recommended revs, hence our lack of performance. A quick adjusment of the counter was needed.
Top speed was 22knots crusing at 15knots (3 people on board). He did feel a 19 inch prop will improve peformance, This will get the engine to work a little harder and drop the top end revs to 6000. This should give us more grunt and a little less acceleration. I tried a 19 inch prop a while ago with some improvement but not running at the correct revs!
So..what prop to buy?? 3 or 4 blade, go up in diameter, Stainless etc etc?? Will keep you posted.
Again thank you for your help.
Martin Hanley
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Message 8 of 12
Posted by member Dominic And Nicola Gribbin on Sunday 24 May 2009
Hi Martin.
Correction to my above figures.
I have just been working on our Pilot all day. When at minimum tilt the cavitation plate is 1/2 to 1/4 inch below the stub keel/transom line (not 1" above).Effctively almost level with it.
Hope this helps.
Dominic and Nicola.
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Message 7 of 12
Posted by member Martin Hanley on Friday 22 May 2009
Hi Thanks again for your comments.
The engine is 2Hours old! I am running it in i.e 10 mins @ 45oo and short bursts @6000 for no longer than 1 min.
Our engin supplier is coming for a sea trial next week so he should know how to sort it!! I'm thinking engine height up is the way to go.
Interesting to see 1 inch above works out!
Will keep you posted
Regards
Martin
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Message 6 of 12
Posted by member Martin Hanley on Thursday 21 May 2009
Hi
Thank you very much for your comments.
I'm still strugling with this "Black Art" of prop engine height!
I made a second sea trial with a 19 inch pitch prop in flat water and found a marginal improvement, 20Knts flat out 6500 RPM.
The cavitation plate is 1/2 inch below the transom.
Prop diameter is 13 3/4 inch and is brand new.
Max revs for the engine are 6000 but have been well in excess of 6600!
The prop calc programe from castle marine calculated a prop size of Dia 13 3/4 x Pitch 10.85 !?!?
Do I now try a 21 pitch and then see about lowering the engine?
Has any one have a 21 inch pitch prop for me to try?
Regards
Martin Hanley
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Message 5 of 12
Posted by member Steve Hook on Thursday 21 May 2009
Hi Martin......You dont mention if the Merc is brand new/second hand if new then the fitter if merc authorised should help with the trials & provide trial props....am a bit concerned with the high revs as over reving a new engine could cause serious damage check the running in details.......did the rev counter come as a matched unit for the engine (is it reading correct)If the engine is seconhand did you trial it on its boat to check performance ....keep at it Steve
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Message 4 of 12
Posted by member Dominic And Nicola Gribbin on Thursday 21 May 2009
Hi Martin.
Don't know if this is of any use but our Honda 75 is set up with a 17" pitch prop and the engine set 1 inch above (yes, above) the lower transom/stub keel line. Ours is a 1993 Pilot.
Given the rather transom heavy leanings of the Pilot this has proven a good compromise setting and we can comfortably achieve 20 knots flat out (unladen)
We suffer no cavitation issues either with this height setting.
Try a higher setting?
Best wishes
Dominic and Nicola.
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Message 3 of 12
Posted by member Andy Mcmullan on Saturday 16 May 2009
It sounds like your motor could be under propped. Most people struggle to get their motor to hit max revs but yours seems to be over reving maybe?
What is the max revs for your engine? Usually the figure quoted is 5-6K revs and being able to reach the max with a normal load is ideal.
What are the two sizes stamped on your prop, eg diameter x pitch?
I take it your prop isn't damaged?
As far as height goes, the cavitation plate of the motor should be about level with the bottom of the transom.
Cheers, Andy
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Message 2 of 12
Posted by member Steve Hook on Friday 15 May 2009
Hi Martin
Dont get to down hearted as your almost there. I have a Pilot 20 & fitted a new Yam 80 ( see Yam 80 tests) & first trials did not perform as expected....not sure what prop pitch you need on the Fisher but the engine hight can be crucial......a good starting point is for the cavitation plate to be in line with hull at the transom..you should see a diagrem in the handbook for the engine.....It should just be a lift up of the engine & re- bolt as the holes should line up......The tech guy at yamaha told me that the ideal cav position is 2in below the water when the boat is on the plane.......best of luck & hope that some of the Fisher owners help you out...Keep us posted...Steve
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Message 1 of 12
Posted by member Martin Hanley on Thursday 14 May 2009
We have just had a Mercury 80 EFI fitted to our 20 Fisher. Our first sea trial after a 6 mounth refit was dissapointing.
Our enging fitter said the engine was " A little bit low "
Prop pitch: 17 inch
Engine height: Will post later
4000 rpm: 10Knts
6000 rpm: 14Knts
17Knts (just!) @ 6500rpm
Flat sea
Feeling very down after all our hard work, will hassel our engine fitter etc.
Any help appreciated
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