A poll about P boats behavior in some rough(ish) stuff

[MapisM]

Scenario: boat cruising on a/p, constant throttles, steady behavior in flat conditions: 21kts @ 1700rpm.
The video shows the cruising conditions which are the subject of this poll, with F5 stbd quartering sea off the stern.
Now, what sort of speed excursion do you think it's reasonable to expect from a 57' flybridge weighing almost 30T?

 

[petem]

P, what do you mean by speed excursion? Do you mean deviation?

 

[[2068]]

It’s going to vary quite a bit when that bow stuffs a little and throws up all the spray.
My vote was probably too low.

 

[MapisM]

P, what do you mean by speed excursion? Do you mean deviation?

Yup, I guess deviation is a more proper term. Anyway what I meant is how much do you thing it's reasonable to expect the boat to slow down when "uphill", and go faster "downhill", vs. the 100% steady 21kts in flat seas.
Without touching the throttles, obviously.

 

[petem]

Yup, I guess deviation is a more proper term. Anyway what I meant is how much do you thing it's reasonable to expect the boat to slow down when "uphill", and go faster "downhill", vs. the 100% steady 21kts in flat seas.
Without touching the throttles, obviously.

Deviation is the word you want. Excursions are days out that old people take in buses!

 

[MapisM]

LOL, I see, thanks.
Actually, I'm 90% sure to have seen the word used also in some technical writings, where they surely meant a deviation above and under an average value.
But coming to think of it, I believe it was a PBB article, hence American...

PS: regardless, now that your doubt is sorted, you don't have any excuse for not voting!

 

[john_morris_uk]

I don’t expect it to be plus much, but you might easily knock several knots off your speed when you plough into a wave and move aside the many tonnes of water as wave/spray.

I haven’t voted because I don’t think you’ll get plus 4 but you might easily get -4 or more.

 

[MapisM]

Good point.
Indeed, after 3+ hours crunched pretty much like the video shows, I can confirm that you are right in principle, and the deviation ain't exactly symmetrical.
I just didn't want to make the poll too complicated...
...and many times, I actually saw some "plus" which were almost as large as the "minus", anyway.

 

[CLB]

In a following sea my, much smaller and lighter, planing boat has been known to lose as much as 6 knots when climbing the back of a wave and gain as much as 4 knots when surfing down the front on one. I would have thought a larger, heavier and more powerful boat would be less affected.

 

[BruceK]

In a following sea my, much smaller and lighter, planing boat has been known to lose as much as 6 knots when climbing the back of a wave and gain as much as 4 knots when surfing down the front on one. I would have thought a larger, heavier and more powerful boat would be less affected.

Yes. I have even been knocked clean off the plane hitting the back of large swells in overfalls. 25knts to 14 in the space of a boat length. Pretty thrilling when the fridge pops out and you discover why there is a 1 inch gap between the windscreen and deck.

 

[derekbland]

So when do we get the correct answer? It seems that like you I have too much time on my hands and so am keen to know as now can't think of anything else

 

[kashurst]

There isn't a right answer as such. Depends on the boat, the engines and the waves. A small boat may be less affected as it will tend to ride over, a really big boat will more likely push through the waves. However, M your engines sound lovely, looked like a fun but I guess tiring run.

 

[Nigelpickin]

I can’t watch the video so apologies but - in 20kts of wind following with say 2m of swell and say .5m of sea I’d be on the throttles so the constant revs part of the scenario is not in accord with the way I would helm the boat. Let’s say there’s a 10 second swell period, running diagolally under the boat, well you’d be going faster than the swell so if you just point and shoot you’re going to come down hard on some troughs and into others.
So for me it’s autopilot and cruise control off, feather the throttle and steer down into the trough at an angle so that you can power through and ride up the other side and keep the wipers dry.

In the messy, shorter sea it’s much harder to predict but there’s no substitute for eye and hand in those conditions.

But in your scenario with constant revs, I’d guess you’d take a 5knot hit every now and then and need to power up to get back on the plane but it depends on how clean the sea is and I guess some IT boats wouldn’t even blink let alone stutter in those conditions

 

[MapisM]

So when do we get the correct answer?

There isn't a right answer as such. Depends on the boat, the engines and the waves.

Apologies for not having spilled the beans yet, been busy crunching some miles.
In fact, I'm now posting from this lovely place buried in the deep South of Italy, just for the records.

Anyway, kashurst is spot on, there's no such thing as a right or wrong answer, I reckon.
My reason for posting the poll was simply that I have been used for many years to a boat whose cruising speed was 8 kts, and anything the sea could throw at her was barely sufficient to slow her down by half a knot (if that), and also the speed increase downhill was barely noticeable.
So, I was somewhat surprised to see the new boat often slowing down from 21 to 18 kts, and accelerating up to 24 knots just a few seconds afterwards. In this sense, the "correct" answer would be +/-3, but as others already explained, the pattern was nowhere near as regular as I'm now describing it.
In fact, in a few (very few, actually) occasions, I've seen the SOG going down to 16kts or so, and in a couple of occasions when the boat was "in sync" with the following wave, I've seen her reaching up to 25kts - though these were pretty much rare exceptions, TBH.

Oh, and in reply to Nigelpickin: I see your point in principle, and I agree that theoretically it could make sense to control both the steering wheel and the throttles manually, but with my boat and in those conditions, I never even considered that option.
In fact, the a/p kept her on a reasonably straight course (as can be seen also in the video), and the engines "recovered" almost immediately, after ploughing into some waves - though never as badly as BruceK reported, thanks God!
So, I had no reason to think that I could have done much better by helming myself - not to mention do that for 3+ hours... :ambivalence:

 

[MapisM]

I can’t watch the video

Do you mean that you have some constraints like low bandwidth or whatever?
I'm asking because in my delivery trip thread scubaman reported that another YT video was not viewable at his end, so I'm wondering if I'm the one who is doing something wrong...! :nonchalance:

 

[Nigelpickin]

Do you mean that you have some constraints like low bandwidth or whatever?
I'm asking because in my delivery trip thread scubaman reported that another YT video was not viewable at his end, so I'm wondering if I'm the one who is doing something wrong...! :nonchalance:

The video clip is fine I’m struggling with my connection as I’ve just cancelled everything in Portugal prior to setting of for Lanzarote on Monday.
I’ve watched it now - I’d be very interested to hear, should you find yourself in those kind of conditions again and you have the inclination, your appraisal of how your boat handles with you reading and riding the waves versus AP reaction - for me three hours in those conditions would pass far more quickly with a bit of kinetic interaction with the prevailing conditions...
Looks like a great trip, I’ll be grateful for 20kts following for my passage next week

 

[MapisM]

I’d be very interested to hear, should you find yourself in those kind of conditions again and you have the inclination, your appraisal of how your boat handles with you reading and riding the waves versus AP reaction - for me three hours in those conditions would pass far more quickly with a bit of kinetic interaction with the prevailing conditions...

I'll be glad to give it a try and report back, but don't hold your breath ref. converting me to such kinetic interaction...
Aside from the fact that steering and throttling constantly ain't exactly my idea of boating for pleasure, I'm almost sure that I couldn't have done much better than the a/p did - in fact, at 20+ kts, it doesn't take many degrees of rudder to keep the boat on track, even with quartering seas off the stern. I had the Multi instrument set on Pilot during the crossing, to see when and how much the a/p was working, and I can't recall to have ever seen more than 4 deg either side. And most of the time, the corrections were even less that that, like 1 or 2 deg.

Of course, in more extreme conditions potentially leading to broaching, I agree that no a/p can replace a decent helmsman.
But I'd rather do my best to avoid finding myself in such conditions!

PS: fair winds for your passage!

 

[Firefly625]

I cant see video either..

But I think link is a bit odd , maybe just on my PC...

if posted like this



although it doesn't play on this website any more, the link does get me straight to the video on you tube, instead of what I have at the moment with is a big black box which does not a lot..

 

[Deleted User YDKXO]

Yup, I guess deviation is a more proper term. Anyway what I meant is how much do you thing it's reasonable to expect the boat to slow down when "uphill", and go faster "downhill", vs. the 100% steady 21kts in flat seas.
Without touching the throttles, obviously.

Obviously it depends on the height and steepness of the waves. F5 really doesnt tell us much. To answer your question, you would be surprised just how much the speed can vary but in my experience its not a =/- single figure. On recent boats I've owned which are quite heavy like yours, in a biggish following sea, the boat can easily slow by more than 5kts when it hits the back of the wave but might only gain 2-3 knots when surfing down the wave. It might be different for a lighter boat. My point is that overall in any kind of choppy sea conditions, the boat will achieve a lower average speed compared to flat water and use more fuel too, which is perhaps a far more important consideration

 

[MapisM]

Yup, all agreed, M.
As I told to JM_uk who raised this point before in his post #7, I just wanted to keep the poll as simple as possible...