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Old 09 July 2010, 11:20   #1
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Gear Ratio, New Honda BF90 D?

Out for the first time yesterday evening with my new Honda 90. Wow! Managed to get enough time on it to be able to give her a momentary burst of full throttle. Smooth, quiet and responsive... BUT:

Running at 3800 rpm, the old Mercury 90fs gave 23 knots. Honda, 19knots.

At full throttle Merc 5500 revs. Honda (brief burst) wound up in the blink of an eye and was passing 6000 rpm before I could pull back on the throttle . Bit much for a brand new engine I thought. No sign of the acceleration slowing down only 300 rpm less than the maximum recommended.

Conclusion, the boat's under propped. The Merc prop was 13 dia x 17 pitch. Just looked at the Honda - 13.25 x 17 pitch. !!

Same horsepower (in theory), same pitch, yet 4 knots slower at cruising revs.

So, anyone know the gear ratio of the Honda? Am I right in thinking I need a coarser pitch prop?
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Old 13 July 2010, 13:40   #2
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the merc is geared higher than a honda. Cant rember what it is. But the Honda is at 2.33:1.
Give it some time to run in before giving it the beans, Propped right it should be pulling beween 5500 -6000 depending on usage type.
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Old 14 July 2010, 19:35   #3
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the merc is geared higher than a honda. Cant rember what it is. But the Honda is at 2.33:1.
Give it some time to run in before giving it the beans, Propped right it should be pulling beween 5500 -6000 depending on usage type.
Yes, I thought so, thank you. Merc is 2.07:1. That must explain the speed difference.
Been out for the last three days, following the recommended break in procedure. Had a couple of minutes full throttle - just over 6000 revs. with more weight aboard than usual. I'm going to try the next prop up, but may wait a bit until the engine's had a bit more running. I'd have thought an 18" pitch would be right, but dealer offering a 19". He's the expert, but I feel that may be a bit much.

So far I'm very impressed with the engine and it's in a different league to the Mercury 90 4str - responsive, quiet and very fuel efficient. At the speeds I've been running, fuel consumption seems to have reduced by about 30% compared to the Merc. I'm not interested in 'hole shot' (yeach!), but, to me, the acceleration seems stunning with no hesitation from the engine. It's also much lighter than the Merc.

Only gripe is that the engine doesn't tilt up as far as the Merc and so about half the gearbox is in the water when moored, so will need antifouling.
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Old 16 July 2010, 18:04   #4
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Yes, I thought so, thank you. Merc is 2.07:1. That must explain the speed difference.
Been out for the last three days, following the recommended break in procedure. Had a couple of minutes full throttle - just over 6000 revs. with more weight aboard than usual. I'm going to try the next prop up, but may wait a bit until the engine's had a bit more running. I'd have thought an 18" pitch would be right, but dealer offering a 19". He's the expert, but I feel that may be a bit much.

So far I'm very impressed with the engine and it's in a different league to the Mercury 90 4str - responsive, quiet and very fuel efficient. At the speeds I've been running, fuel consumption seems to have reduced by about 30% compared to the Merc. I'm not interested in 'hole shot' (yeach!), but, to me, the acceleration seems stunning with no hesitation from the engine. It's also much lighter than the Merc.

Only gripe is that the engine doesn't tilt up as far as the Merc and so about half the gearbox is in the water when moored, so will need antifouling.
Hi Tony I am just back from 2 weeks hols!! Glad you are fixed up and on the water again. I an heading up to tay on sat for 2 weeks so hope to see you.
Cheers:
Billy
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Old 17 July 2010, 12:25   #5
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Hi Tony I am just back from 2 weeks hols!! Glad you are fixed up and on the water again. I an heading up to tay on sat for 2 weeks so hope to see you.
Cheers:
Billy
Hope the weather settles down a bit for you Billy. There is just a chance that I may be in L Sween over the weekend 24/25th. giving support to some open water swimmers. Not sure whether that'll go ahead yet. All weather permitting of course, but could be out and about the following week too.
Cheers Tony
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Old 17 July 2010, 16:31   #6
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Only gripe is that the engine doesn't tilt up as far as the Merc and so about half the gearbox is in the water when moored, so will need antifouling.
Think you just need to adjust the tilt limit switch on the tilt tube to allow it to raise a bit further.
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Old 18 July 2010, 06:23   #7
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Think you just need to adjust the tilt limit switch on the tilt tube to allow it to raise a bit further.
Ah! I didn't know there was one. Mind you, the engine tilts until the pump alters tone, indicating (I deduce) that the ram has reached maximum travel.

The instructions say to then lower the engine onto the tilt lock and keep pressing the down button. It makes a lot of noise, but this eventually retracts the trim rams into their cylinders - important as they are under water and will suffer fouling if left exposed.

I think the angle of transom is the problem, so antifouling seems the only answer.
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Old 18 July 2010, 16:47   #8
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the acceleration seems stunning with no hesitation from the engine.
It would when under-propped

Given the difference in gear ratio between the Merc and Honda, if the Merc was right with a 17" pitch, then the Honda should be right with a 19"
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Old 19 July 2010, 07:45   #9
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It would when under-propped

Given the difference in gear ratio between the Merc and Honda, if the Merc was right with a 17" pitch, then the Honda should be right with a 19"
Thanks DD. Yep! The Merc was exactly right, in the middle of the recommended rev range at WOT.

Now got the Honda to the point where I can give it some welly for a couple of minutes. Straight up to 6300, which is the maximum revs. and that with a still tight engine.

Weather's been so grotty here the last few days, I haven't changed the prop yet. Hopefull soon.
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Old 22 July 2010, 18:45   #10
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Thanks DD. Yep! The Merc was exactly right, in the middle of the recommended rev range at WOT.

Now got the Honda to the point where I can give it some welly for a couple of minutes. Straight up to 6300, which is the maximum revs. and that with a still tight engine.

Weather's been so grotty here the last few days, I haven't changed the prop yet. Hopefull soon.
Phew! Will I ever get it right? 19" prop on today. Still does 6000 revs WOT (and the engine's only done 8 hours so it'll still be tight)

4000 revs with the old Merc 90 17" prop - 26.6 knots
4000 revs with new Honda 90 19" prop - 23 knots.

21" next ???
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