affect of headwind?

[skyhawk]

I was just watching a youtube video, Mermaid Monster. They were moored on a ball in a stiff wind.
Set me to thinking of a question....
obviously current will have a pretty direct affect on speed over ground
but what about wind in a slow boat such as a full displacement trawler
I don't know exact numbers
but just say a boat like that might have a typical wide open throttle speed of say 8 kts.
heading into an 8kt current would give a speed of pretty much zero, right?

now assume zero current
and ignore wave state for purposes of discussion
at around about what wind speed on the nose would stop the boat?...a boat such as their Nordhavn for example.

 

[psneeld]

My trawler would lose about 0.5 knots in 25-30 knot headwinds.

6.3 knots down to 5.8 knots.

 

[mvweebles]

Hmmm....wind and no waves. As Psneeld says, probably lose 10% in strong headwinds. But sustained 25 kts with any amount of fetch will build 4-foor chop pretty quickly, and likely get to 6-foor over a few hours. Most boars would do well to slow a bit to make for a more comfortable ride.

Keep in mind the N55 presents a lot of windage. Not exactly a svelte design even for a Nordhavn. The Dashew FPB is designed to slice through headseas so would have minimal loss.

Peter

 

[skyhawk]

drag varies by the square of velocity...so it's harder to extrapolate....but I'd recon you'd have no problem making very good headway then into the 50 kt wind they were seeing...

 

[mvweebles]

drag varies by the square of velocity...so it's harder to extrapolate....but I'd recon you'd have no problem making very good headway then into the 50 kt wind they were seeing...

50 kts of headwind? Would stop my boat cold. I can't imagine having no waves, but I'd have difficulty standing station in that type of headwind. At anchor, you'd likely be pressed into running the motor to relieve tension on ground tackle.

Peter

 

[rslifkin]

50 kts of headwind? Would stop my boat cold. I can't imagine having no waves, but I'd have difficulty standing station in that type of headwind. At anchor, you'd likely be pressed into running the motor to relieve tension on ground tackle.

Peter

In a winding river valley or something you can get high winds with minimal waves. Pretty much anywhere else, it won't happen.

 

[AlanT]

I was just watching a youtube video, Mermaid Monster. They were moored on a ball in a stiff wind.
Set me to thinking of a question....
obviously current will have a pretty direct affect on speed over ground
but what about wind in a slow boat such as a full displacement trawler
I don't know exact numbers
but just say a boat like that might have a typical wide open throttle speed of say 8 kts.
heading into an 8kt current would give a speed of pretty much zero, right?

now assume zero current
and ignore wave state for purposes of discussion
at around about what wind speed on the nose would stop the boat?...a boat such as their Nordhavn for example.

You can't ignore waves even for the purposes of discussion. It's like saying ignore gravity. Waves will have a considerably greater impact on speed (whether the boat's ability to generate velocity, or the captain's comfort with the battering) than wind.

Running for shelter we headed into 55kts in a 49' heavy sailboat (Hylas) which has a hull speed of about 8.5kts. When we made that turn into the wind we made between 0 & 1kt. Waves on our nose were short (interval) and ugly and around 8'.
~A

 

[guy with a boat]

We hit 50kt winds right on the nose on our way out of Tracy Arm in SE Alaska. The wind wave height was insignificant (2-3’ chop) and our biggest concern was the possibility that something on the deck might tear off and blow away. Everything stayed attached and the wind speed declined quite a bit when the fjord made its next big turn.

Our speed over ground didn’t change enough to notice. Especially for heavier boats, it won’t change much from wind speed alone. We have done lots of miles in strong winds and strong currents. Currents mean a lot and wind speed means very little for trip planning and speeds. The biggest factor related to wind (as already noted) is that in open water it will generate big wind waves if the duration and fetch is enough.

 

[markpierce]

Tracy Arm is one of my favorite destinations. Never experienced high winds there, however, but once dense fog preventing entry.

 

[markpierce]

Find steep waves more slowing than headwinds, needing to slow to reduce slamming/bucking.

 

[OldDan1943]

That all depend on the "picture" you present to the wind.
A boat with lots of canvas and or a fly bridge presents a larger "picture" to the wind.

 

[skyhawk]

We hit 50kt winds right on the nose on our way out of Tracy Arm in SE Alaska. The wind wave height was insignificant (2-3’ chop) and our biggest concern was the possibility that something on the deck might tear off and blow away. Everything stayed attached and the wind speed declined quite a bit when the fjord made its next big turn.

Our speed over ground didn’t change enough to notice. Especially for heavier boats, it won’t change much from wind speed alone. We have done lots of miles in strong winds and strong currents. Currents mean a lot and wind speed means very little for trip planning and speeds. The biggest factor related to wind (as already noted) is that in open water it will generate big wind waves if the duration and fetch is enough.

well that pretty much answers it then... in a Nordhavn no less, and larger but similar frontal profile.

 

[rgano]

Get a general estimate of your subject vessel's frontal area using beam and superstructure height. Lop off a few percent for bow streamlining and reduction in width as you go higher on the boat.

Find out the pounds force of the propeller(s) and compare to the calculated force of the wind using F=A x P where A is area you calculated and P = .00256 x V (wind velocity in MPH) using various wind velocities until you exceed prop power.

This is purely a math exercise because of all the other factors mentioned above.

 

[Nomad Willy]

Peter wrote;
“Hmmm....wind and no waves. As Psneeld says, probably lose 10% in strong headwinds. But sustained 25 kts with any amount of fetch will build 4-foor chop pretty quickly, and likely get to 6-foor over a few hours. Most boars would do well to slow a bit to make for a more comfortable ride.”

That’s been my experience. Never ever have I increased rpm to buck head winds. That would include 40 knot winds and 5’ seas. Quite a few times I’ve gone on for hours like that and probably bigger seas at times. When I’d change throttle it was always less .. not more.

 

[Shrew]

Tides and sea conditions both change and influence so much, I've pretty much stopped paying attention....to a degree. Yes, it's nice if you have have the tide at your back. For us, that means cruising at 10 kts instead of 8. But then again, fighting the tide means you're at 6 kts. (+/- .5-1 kt).

If I can plan around it, GREAT!! IF not, Oh well. More often the trip is planned to wind, sea state and time. This usually means it's more like "Hey, we're getting 10.5kts NICE!!"

Since high winds almost always lead to bigger seas in open water, we tend to avoid travelling during those times and choose to pick our days instead.

 

[FF]

"Find out the pounds force of the propeller(s) and compare to the calculated force of the wind"

Many boats will do better but 20lbs of thrust per 1 HP is a good working approximation.


IT gets interesting when noodeling backwards using fuel burn to estimate HP underway.

 

[Gdavid]

Peter wrote;
“Hmmm....wind and no waves. As Psneeld says, probably lose 10% in strong headwinds. But sustained 25 kts with any amount of fetch will build 4-foor chop pretty quickly, and likely get to 6-foor over a few hours. Most boars would do well to slow a bit to make for a more comfortable ride.”

That’s been my experience. Never ever have I increased rpm to buck head winds. That would include 40 knot winds and 5’ seas. Quite a few times I’ve gone on for hours like that and probably bigger seas at times. When I’d change throttle it was always less .. not more.

My experience is the same, I have never had to increase throttle for head winds, I have reduced throttle to decrease the frequency at which the hull is meeting the waves. It is very rare to be in strong winds with negligable fetch to ready know. The first few minutes of a sudden onset storm would give you this scenario but only briefly.

That said, on diesel boats, our throttles are just setting the engine speed and the governor is adjusting the fueling to maintain that engine speed. Our boats may be impacted more than we realize. Up until the point that prop slip is significantly increasing, we won't see any change unless we are monitoring fuel consumption real time or exhaust gas temps.

 

[rslifkin]

I think the only time I've intentionally sped up in a head sea is when it's a strong wind, but short fetch due to nearby land. That can build a very close, steep chop, but at only a foot or so tall. If I'm not up on plane, pushing up a little closer to hull speed can avoid the boat trying to pitch a bit in response to the waves and let it just punch through them, reducing the motion to just a series of gentle thumps (as the waves are close enough together to be on several at a time).

 

[rgano]

We just did some ROUGH calculations based on 20 pounds thrust prop for every HP at the prop. For a twin prop 120-HP engine boat at normal cruise speed, you could probably stop it with head winds at 70-80 MPH.

 

[AlanT]

We just did some ROUGH calculations based on 20 pounds thrust prop for every HP at the prop. For a twin prop 120-HP engine boat at normal cruise speed, you could probably stop it with head winds at 70-80 MPH.

That tracks my experience pretty well (49' heavy sail boat under power into 55kts True Windspeed, in protected waters but still 6-8'waves) 75HP 0-1kt entering Vela Luka on Korcula Island.
""It was a hair-raising, exhilarating, downwind sail, perfect for excitement and thrills, but with the knowledge that a single mistake would spell disaster - an adrenalin rush for sure. It lasted 3 hours but the wheels came off when we rounded the point at the end of the island and had to motor 4 miles in the opposite direction into Vela Luka bay against 55 knots and large close waves. For a while we made only 0.5 knots forwards while leaping up and down about 10-20 feet every few seconds. Wind-driven spray made visibility practically zero. At this time we saw a French yacht BALOU in distress, he was tacking to and fro in the entrance to the bay. He called in a mayday. He was unable to make any progress, had unidentified other problems and his engine had failed. We were powerless to help him, and could barely make progress ourselves for an hour or so, then we managed to get to the comparative shelter of the hills where the seas moderated allowing us to crawl into harbor, synapses snapping, adrenalin rushing and with just a little bit of fear ebbing away.""

Croatia

 

[rslifkin]

Thinking back, I think the worst speed loss from wind I've ever seen was about 0.5 kts. That was in 30 - 35 kt gusts right on the nose funneling through a river valley. Because it was a river, the worst sea state seen was less than 1 foot.

 

[diver dave]

We just did some ROUGH calculations based on 20 pounds thrust prop for every HP at the prop. For a twin prop 120-HP engine boat at normal cruise speed, you could probably stop it with head winds at 70-80 MPH.

Might be good to know if reducing anchor loads in vicious winds.