Wave Crest Phenomenon

Biggles · 31 May 2007, 19:40

Whilst mucking around in the Solent today I became very aware of a phenomenon that I had come across before but not really thought about. I was following a large tug in to Southampton water and riding on the waves it was producing. Just practicing a little throttle control. However whilst doing so I became distinctly aware that after sitting on the crest for a while when I wanted to just accelerate away forwards and drop in to the trough my engine really struggled to make any increase in speed. However if I turned right or left then accelerated the engine would act normally.
Anyone any idea what causes this ?

This has happened to me on a couple of boats. Its like your boat gets stuck to the top of the wave.

NR.

Aidan · 31 May 2007, 23:59

I am only guessing but would it be because your prop was in the wave crest as in airated water?

I am sure one of the Royal College of Physics will be on so I will get out of the way>>>

Biggles · 01 June 2007, 00:01

Quote:

Originally Posted by Aidan

I am only guessing but would it be because your prop was in the wave crest as in airated water?

Aidan this is our initial thoughts but as you say lets see what the bods from the Royal College of physics come up with.

codprawn · 01 June 2007, 01:24

It could be down to the speed of the wave you are sitting on. You may think you are doing 20kts but the wave may well be doing more.

Biggles · 01 June 2007, 01:33

I don't think we were doing 20 knots. The tug was doing about 15 Knots. But try as I might I couldn't get the boat to accelerate off the top of the wave. It was almost as if I had a massive trim that had my boats nose pointing vertical. Full throttle only gave me about 4000 revs but no increase in speed.

jyasaki · 01 June 2007, 01:59

What I've noticed is that while sitting "on top" of a wave (not a tug's wake, but an actual wave), I'm actually slightly short of the crest. The boat is riding a bit nose up, and I'm playing the throttle to stay there. To accelerate over the top, you have a little bit of uphill to overcome before dropping off the leading edge.

Might be what you're seeing. Might not.

jky

pathalla · 01 June 2007, 02:49

Quote:

Originally Posted by codprawn

It could be down to the speed of the wave you are sitting on. You may think you are doing 20kts but the wave may well be doing more.

Our waves go a little faster over here Cod..so we don't have this problem...

BogMonster · 01 June 2007, 11:33

I've noticed this when following one of our company's launches and riding the wake, it will go over but there feels as if there is a lot of drag. I don't know what it is but if it was aeration the prop should cavitate?

Biggles · 01 June 2007, 11:44

I have had the same effect from riding on an actual wave, the tug was just an example from yesterdays outing. jyasaki seems to have a good idea of what is possibly happening. It is a big power killer though.
I don't think its cavitation just because it doesn't sound like cavitation.

Nos4r2 · 01 June 2007, 11:53

Does this occur when most of the boat is hanging in mid air? I'm wondering if it's not something to do with the engine having to hold the boat in that position?

Biggles · 01 June 2007, 12:44

Nos, Seems like it. I'll do a little more experimenting when I can get another boat out with me.

Carl · 01 June 2007, 12:59

simple answer...........................get a bigger engine

jwalker · 01 June 2007, 13:30

I think there are two factors at play; 1. the boat is climbing up hill and, 2. the particle movement inside the wave will load the engine and prevent it reaching maximum power.

Water particles move in circular paths inside a wave; the size of the paths is dependent upon the water depth and the wave period. As the particle paths near the seabed they flatten out to ellipses. The deeper the water, the less the particle movement with depth. As a wave reaches shallower water, the particle motion bottoms out and causes the wave height the rise. Eventually the particle motion becomes so compressed the top of the wave becomes unstable and begins to break and the particles fall over to become moving water.

When you finally get over a breaking crest with your boat, 1. you are travelling down hill, 2. the boat is in a forward current of water and, unless you are in very shallow water, the water is moving rearwards through your propeller and unloading the engine so it will rev freely.

I think.

Biggles · 01 June 2007, 13:52

Well that makes some sense. Try as I might I couldn't get the engine to Rev above 4000 once I was cruising on that wave.

It certainly gets you thinking when your in that position as you are always keeping an eye on the wave forming up behind you, which seems to be getting bigger and faster the slower you seem to be going. With no power to manouvere it wouldn't take much to get caught out by the following wave.

jwalker · 01 June 2007, 14:09

Quote:

Originally Posted by Biggles

It certainly gets you thinking when your in that position as you are always keeping an eye on the wave forming up behind you, which seems to be getting bigger and faster the slower you seem to be going. With no power to manouvere it wouldn't take much to get caught out by the following wave.

In your example, sitting on a boat's wave, I don't think the wave behind you would ever catch up. If you dropped back into the trough you'd unload the engine and get your performance back.

jwalker · 01 June 2007, 14:43

Something else has just occured to me which is likely to have an effect. When you are moving at displacement speed, the bow wave your boat is making doesn't just move out sideways from you it also radiates outward around and under your hull sorta like a half cone. I'd guess there will be some distortion of the shape because of the shape of your boat hull. However, since you were following a tug's wave your boat speed must have been fairly low and there may well have been interference between your underwater wave and the tug's wave. It may well be therefore, that you are also trying to climb your own bow wave.

Just guessing.

jyasaki · 01 June 2007, 15:18

Quote:

Originally Posted by Biggles

Well that makes some sense. Try as I might I couldn't get the engine to Rev above 4000 once I was cruising on that wave.

That brings up another point. With less of the hull riding on the surface (i.e. the nose hanging out in space), it's going to settle a bit deeper for any given speed, and take more power to get you up out of that hole. It's quite possible that the engine is bogging down until the hull starts planing better.

Lots of factors in play here, apparently.

jky

Actually, I think Mr Walker and I are talking about nearly the same thing. Just read his last reply. Sorry if I'm paraphrasing.

9D280 · 01 June 2007, 23:35

Could this be related to an effect I've seen while "chasing" the wake of a Ribcraft 5.8 in an SR4 at around 20ish Knots? (I say ish 'coz I was too busy driving to watch the numbers!

) - Similar experiences with regard to "overtaking" the wave, but I found whether I was on the port or starboard side of the wake, I had an immovable rock solid list to Starboard. It was Ok on the Port wave, as I was leaning "into" the hill, however on the starboard side, it got a bit unnerving!

PS - ran perfectly flat & level outside the "wake triangle" and in the "direct" line of prop wash......

Rogue Wave · 02 June 2007, 12:35

how close were you?

Alex Brown · 03 June 2007, 17:01

I get the same whilst coming in through St Anns in a 4 or 5 metre swell - the effect is more pronounced if the wave period is minimal. When you're just behind the top of the wave, if there's more than 2 people on board, the engine really struggles to pickup quickly. Let the boat back into the trough (without being swamped from behind), the revs pick straight up then you launch over the same wave you couldn't previously get up

Can't say I've ever found it on tugwash, despite the local tugs kicking up some good surf....

edit: I wonder if some of this is due to the lack of instant pickup from the suzi-4s engine, whereas a 2s 150 would probably jump up quite quickly? I know when we've come in on a large following sea on the charter boats with the yanmar 300's, we try and keep the turbo spun up all the time to get a quicker response, and also so you can keep the bow up.

-Alex