Noise-Vibration

[Knot Now]

I know this is going to be a hard one.
I have a noise / vibration that comes and goes under load. It seems to be related to running with the swell. Usually its there when first bring her up on plane and then goes away. Any ideas? Is this a prop starting to spin or the outdrive? Any suggests on props for DP's with a 41. I am running A5s, the boat came with a pair of used and new A3s as spares. The PO said something about having the prop resized and to have them checked every year.
Phil
Knot Now

[Wizard]

Is it cavitation? I get that sometimes when quartering into a larger mixed swell -- a noise that comes and goes with each swell but goes away if I turn more up or downswell. Prop acts like it isn't down far enough below the keel. Mike (NoSlack) even wondered if I might benefit from an outdrive extension.

Any other 26' Billfishers notice this?

[DOGHOUSE26]

If you're going straight into the swell at moderate speeds it's probably cavitation. When the bow ploughs down after cresting a large swell it goes deep and creates a void in the water column which is mixed up by the time it gets back to outdrive.

Phil - what's your top RPMs loaded with the A5s ? With the A3s ? Most of us are running A3s or 4s; I can get 4000 with my A4s; you want to be in the 3800 - 4000 range.

PS - You better get Charlie's slip ASAP if you want it, it won't last!

[NoSlack]

Take a look at your tach when you hear the noise. If it's cavitation the RPM will surge up a bit, then return back to normal when the props take a bite.

Volvo makes 1" and 4" spacers for the lower end. The 1" kit runs about $400 and could help keep the props in cleaner water.

Mike

[Knot Now]

I already moved in J-22 (Charlie's old slip). I have not really tested WOT loaded. With a light load of fuel I have had her to 3800, I think it took about 5 sec. I am still learning how to trim this boat. I will try the WOT again after I fill with fuel this week. The old owner suggested I cruise at 3200-3400 when the seas permit.
Phil
Knot Now

[DOGHOUSE26]

Phil,

Take the Knot Now over to the Ski Beach side of the bay by Sea World where you will have plenty of room. Start with tabs all the way down until about 3000-3200 then slowly raise each side up to about half way to get the bow back up and the spray from the boat is almost under your bridge seat. You should get up to 22-26 nts 3800-4000 RPMs.

The previous owner was right on; cruise 3200-3400 RPM (400 RPM below your max RPM per Don Blackman)

Was that 3800 with the A5s?

Mike - $400 an inch seems a little high, strap ons are cheaper!

[Wizard]

Mike - $400 an inch seems a little high, strap ons are cheaper!

:eek: :D

[NoSlack]

I priced the kit just in case I needed it when I went to the Duo-prop. If $400 keeps you from eating a set of props up do to cavitation and keeps you on plane, it would be money well spent IMO.

The kit is a $5 block of aluminum coated with $395 worth of Volvo paint.

Mike

[Knot Now]

Jeff,
I filled the Knot Now and got bait and went to LJ kelp bed fishing - didn't have time for a test drive also. ;-) It was very flat on the way out, so I tried the WOT. I was able to reach and hold 3800 into a very slight swell (A5 Props). I need to check out the function of my trim tabs as one makes a larger difference than the other??? I have never tried the A3 props. I didn't have the noise-vibration on this trip at all. We had a great time fishing. My tank only took 95 gals from 1/4 on the gauge - does that sound right?
Phil
Knot Now

[DOGHOUSE26]

I'd be happy with A5s and 3800; about as efficient as it gets! I'll have to check my fuel gauge notes; I think I take fifty at 4/4; 75 at 3/4;100 at 1/2 and I think 130 at 1/4. What was your speed at 3800?

[Knot Now]

I didn't have the GPS plugged in during the test. I will try WOT again and note the speed. I am happy with the A5's if they are not putting too much load on the outdrive. Have not taken careful enough notes to check my milage, but we went over 100 miles today for 30-40 gals.