Does a Bowthruster create too much drag

[CAPTAIN FANTASTIC]

Naturally, a bowthruster is a nice thing to have to get in and out tight spots. However, do they create too much drag?

 

[johnalison]

It depends on what you mean by too much. A friend fitted a bow thruster to his Hanse 370 a few years ago, for sensible health reasons. He reckoned that it took the edge off its performance noticeably. He later did reasonably well in the Round the Island race, but mainly because it was a drifting match and most of the boats retired.

 

[srm]

Naturally, a bowthruster is a nice thing to have to get in and out tight spots. However, do they create too much drag?

Two big holes instead of a smooth hull form. It all depends on your priorities.

 

[TernVI]

I wonder what hull shapes we'd have if the bow thruster was in the design from the off, rather than slapped in a hull that's been designed without one?
It would be interesting to see some actual numbers for the drag and speed lost?

 

[mjcoon]

I wonder how long you have to wait before fouling fills up the tunnel and fairs it off for you...

 

[dunedin]

That’s why some makes fit retractable bow thrusters - double benefit, almost zero drag when lifted, and better “grip” in water as lower down (and can also be further forward) when in use.
Not cheap, but ideal.

PS Also seen in the Baltic externally fitted bow thrusters, which give the better depth without cost of retractable but similar drag to the tunnels.

 

[Catalina36]

No. I've had one for 13 years, with a proper eyebrow to streamline it. I often sail under genoa alone because of three canine crew and not infrequently outpace similar sized boats under full sail.

 

[pvb]

I reckon that if you really need a bowthruster, you'll be prepared to accept any small performance penalty. Thrusters are great things!

 

[capnsensible]

I wonder how long you have to wait before fouling fills up the tunnel and fairs it off for you...

Forty eight years.

 

[Rappey]

It's a good question I've often wondered.
A non rotating 3 bladed prop supposedly creates drag equal to towing a bucket?
Amel have a hull section that lowers to use the bow thruster.

 

[DJE]

Naturally, a bowthruster is a nice thing to have to get in and out tight spots. However, do they create too much drag?

Frontal area of the tube holes on my boat looks like a couple of percent of the total hull. Mine's well faired in but say the drag coefficient is two or three times that for the rest of the boat, then the increase in friction drag will be a few percent. But as the speed approaches hull speed friction drag becomes less significant. My guess is the effect will be similar to that of a fixed-blade prop and will be more significant at low speeds.

 

[Laminar Flow]

As DJE points out these things play a greater role at slow speeds up to about a relative speed factor of 1 (Square root DWL x factor).

One designer I read, suggested there should be no through hulls in the forward section of a boat, period. An eight inch hole right through the bow would appear to be counter intuitive to that statement.

The increase in resistance is due to turbulence and the waste of energy used to create it.

The brow added to the tunnel to lessen the frontal impact of the flow probably doesn't help a lot when the boat is making lee way in the order of 4-6 degr. and in the case of a long keel boat, which depends on it's deep forefoot for windward performance, the pressure drop and turbulence would be doubly impactful. I mean how many 747s have you seen with a hole through the front of their wing?

It is for this reason only that I would not consider installing one in my (long keel) boat.

I have , recently, spent quite a bit of time trying to quantify, in terms of resistance, the changes I have made to my boat and as DJE alluded, form or wave-making resistance is directly proportional to speed.
The critical point comes at a relative speed of 0.9 when the curves for frictional resistance and form resistance diverge sharply as form resistance increases exponentially.
Interestingly, a relative speed of 0.9 is also the average sailing speed of most boats.

For my boat the total resistance at 4.8kt (0.9 relative speed) is 58kg.

Increase in resistance due to various brand typical features and consequential impact on speed:

Blunt stem head 4.5% or 0.2kt
6" wide deadwood 8.5% or 0.4kt
Flat plate rudder 4% or 0.1kt (things get a lot worse at stall)
3 blade spinning prop 8% or 0.4kt

Deadwood and rudder have been "fixed"

The impact of a bow thruster in head-on flow which, in the case of a sailboat only happens when going downwind or under power, would likely, and only when the tunnel has been carefully faired, be in the neighbourhood of 0.15 - 0.1 kts

 

[NotBirdseye]

I think the keyphrase here is... 'too much' drag. Obviously with a bow thruster you're not 'intending' or rather 'going out of your way' to win races otherwise you wouldn't bother. Others have pointed out alternative solutions to minimize any drag but the common consensus is, "Nope". Enjoy your new bow thruster and turning on a penny at push of a button.

 

[DJE]

In my case the thruster came with the boat so I didn't have to make the decision. She also came with a shallow keel and masthead rig - options chosen by the original owner - which both reduce peak sailing performance. But compared to the previous boat I have gone from typical sailing speed of 5.5 knots to 6.7 knots. So I can live with the small loss from the thruster. It is hopefully offset by the feathering prop that we fitted. And the peace of mind benefits when manouvering at slow speed are well worth it.

Other factors to consider are weight in the bow, loss of a bit of buoyancy in the bow and loss of stowage space.

 

[anoccasionalyachtsman]

On the plus side, any cross currents will go straight through the hole.

 

[olam]

Like the thruster ,Hate the drag.
Always wondered if you could have some kind of outer door like torpedo tubes .Would be fun giving the order.
Or a cover that pivots up into place with the force of forward water flow but then drops clear of the tunnel at low speed.
How about a lightly sprung Venetian blind set up which has just enough force to open when no forward flow but shuts under way.

 

[TernVI]

On the plus side, any cross currents will go straight through the hole.

No more lee bow effect?

 

[anoccasionalyachtsman]

No more lee bow effect?

A small price to pay.

 

[dunedin]

Like the thruster ,Hate the drag.
Always wondered if you could have some kind of outer door like torpedo tubes .Would be fun giving the order.
Or a cover that pivots up into place with the force of forward water flow but then drops clear of the tunnel at low speed.
How about a lightly sprung Venetian blind set up which has just enough force to open when no forward flow but shuts under way.

Or buy a retractable bow thruster in the first place

 

[LONG_KEELER]

Or buy a retractable bow thruster in the first place

"Quality remembered , long after price is forgotten"

Quote from : "ecky thump yorkshire"