Evinrude 70/Avon 465 top end problem

Calidiver · 08 September 2010, 04:20

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.

ed_dash · 08 September 2010, 04:40

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.

jyasaki · 08 September 2010, 15:40

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

Calidiver · 08 September 2010, 17:01

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?

ed_dash · 08 September 2010, 22:13

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?

Calidiver · 08 September 2010, 22:52

Quote:

Originally Posted by ed_dash

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..

ed_dash · 09 September 2010, 01:54

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.

Calidiver · 09 September 2010, 02:19

Quote:

Originally Posted by ed_dash

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

ed_dash · 09 September 2010, 07:34

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.

jyasaki · 09 September 2010, 15:07

[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

Calidiver · 09 September 2010, 22:22

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

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 [/QUOTE]

jky,

It does feel like it's drag related, that's why I was thinking it could be from the foil. just looking at the foil, it looks badly designed from a hydrodynamic point of view.. and the feeling when the boat takes off is of coming "unstuck". I'm thinking that the water may be maintaining attached flow on the foil most of the time, but then aerating when I come off a wave, dropping a bunch of drag for a couple of seconds.

I probably won't have the boat back in the water for a few weeks as I'm out of town for a while. Are you tank diving or freediving? I'm a freediver myself.

-Jonah

bosun · 20 September 2010, 19:23

I'm the guy ed dash mentioned with the 70 evinrude and 5m humber,I've done loads of playing around with props on my boat and from what you say I'd guess you need to junk the fins and put a 17'' prop on it,then do the trimming out 'till it looses grip as suggested to work out your trim angle. My engine is set up with the cavitation plate an inch and a half above the bottom of the boat which is probably too much for most situations (only did it to try and outrun eds boat !) rule of thumb is level with the bottom of the hull I think.
I've got a tacho on mine and it tops out at just under 6000rpm but will happily rev to 6500 with a shorter prop (don't know how long it would last tho')
I think you need to aim for 5750rpm at WOT?