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Old 08 September 2010, 00:20   #1
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Evinrude 70/Avon 465 top end problem

I have a '92 Avon 465 with a '92 Evinrude 70 that I just bought. Took it out on the water for the first time yesterday after doing some work on the boat & engine. Things were running pretty well for the most part, but while the acceleration felt really good, top end felt pretty slow.
I didn't have a way to measure, but WOT felt about 25 knots or so... it also didn't seem like the engine was revved as high as it should at WOT.
The odd thing was that when I came off a wave & got a bit of air under the boat, the revs came up and I felt a big burst of acceleration until the boat was settled back down in the water.

I'm thinking it might just be the Stingray foil that came on the boat & I'm planning to pull that off the next time I take it out. Any other ideas? It really felt like there was a big drag on the boat that would release now and then and the boat would take off..

The prop is a Piranha 14x13. I also have a set of 13.75x15 blades I can try.
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Old 08 September 2010, 00:40   #2
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Country: UK - Wales
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Im not an expert but to me those props are both too small a pitch. Another member on here from same area as me has a humber 5m assault and an ex rnli 70hp evinrude and he runs a 17" pitch stainless thats double cupped, he gets 36kts at WOT.

The local sailing club also has an avon 465 with a 40hp honda 4 stroke and i have had that upto 29kts with a bit of tidal assist.

Try taking the fins off and the larger prop.

I think a lot of the guys on here will ask if you have a tacho to know what revs you are actually getting aswell.
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Old 08 September 2010, 11:40   #3
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How did you have the trim set?

Sounds like you might have been trimmed too far in at speed; motor was trying to keep the bow down with the result of ploughing through the water. When you came off a wave, the drag from the bow lessened and the thrust-to-drag ratio became more efficient.

Next time, once up on plane, trim the motor up until it ventilates (assuming you power tilt/trim), then back it down a tad til it stops slipping. If you don't have PTT, try setting the trim further out until you start breaking loose in tight turns, or have trouble getting up on plane.

The fin will cause some extra drag, but *may* also help to lower planing speed.

What part of California are you in (assuming you are)?


jky
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Old 08 September 2010, 13:01   #4
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Thanks for the replies... Ed, I agree that the prop pitch is on the low side for top end, but on the other hand, it should be over-revving at WOT if the pitch is too low right?

I did try messing with the trim a bit and in some flat water I trimmed it until the bow came up, it did go a bit faster, but not much.
When it comes off a wave, the acceleration is much more than just from the hull coming out I think. It's not just a gentle surge, but more of a kick, almost scary.

I'm in the Santa Cruz area.

Anyway, I was just wondering if anyone had had the same experience & maybe save myself some troubleshooting time...

Any chance it could be engine/carb related? Maybe while freefalling off the back of a wave the carbs get a shot of fuel?
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Old 08 September 2010, 18:13   #5
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yes id agree that the engine should be screaming if the prop was too small a pitch. there isnt anything on your hull is there to add extra drag? weed or anything?
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Old 08 September 2010, 18:52   #6
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Quote:
Originally Posted by ed_dash View Post
yes id agree that the engine should be screaming if the prop was too small a pitch. there isnt anything on your hull is there to add extra drag? weed or anything?
No, no growth or anythng..
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Old 08 September 2010, 21:54   #7
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where is the cavitation plate in relation to the bottom of your hull? is it lower down, inline or raised up above it? In my limited knowledge thats all i can think of, other than a problem with the engine. In which case get someone to have a look at it.
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Old 08 September 2010, 22:19   #8
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where is the cavitation plate in relation to the bottom of your hull? is it lower down, inline or raised up above it? In my limited knowledge thats all i can think of, other than a problem with the engine. In which case get someone to have a look at it.
It's about inline with the bottom
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Old 09 September 2010, 03:34   #9
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ok, that sounds about right although i know a lot of people raise it up a few inches.

I was thinking along the lines of drag, boat in water = slow, boat out of water = fast. Either weed or having the engine too low would have explained the extra drag and slower speed. Ohwell, hopefully someone else has better ideas than me.

Good luck and hope it gets sorted.
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Old 09 September 2010, 11:07   #10
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[QUOTE=Calidiver;368987]I did try messing with the trim a bit and in some flat water I trimmed it until the bow came up, it did go a bit faster, but not much.
When it comes off a wave, the acceleration is much more than just from the hull coming out I think. It's not just a gentle surge, but more of a kick, almost scary. [quote]

Still stand by what I said. The amount of drag can make a huge difference in accelleration.


Quote:
I'm in the Santa Cruz area.
I get down to Monterey usually a couple of times a month (and some of my dive buddies have been talking about diving around SC anyway.) Maybe we can set up a meet-and-demo sometime (though I'm heading north this weekend, so that's out...) Might be able to come up with some ideas.

jky
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