Feathering prop problem

[KLBH]

I am on board a friends boat, a Oceanis 46.1 that has a Bruntons feathering prop, a Autoprop H5. The boat has a 50+ hp engine and when motoring into 20knts of wind at 2800 rpm we get about 2.5knts. If we turn across wind we get 6.5 - 7knts.
Anyone had a problem like this before r could shed some light on what might be wrong?

 

[eddystone]

Rather a lot of possible factors to consider but the implication that it’s the feathering prop malfunctioning would not be at the top of my list.
I would be looking at sea state rather than wind. If motoring into short steep waves your propeller may be out of the water at times so there’s a continuous process of the boat stalling followed by the propeller trying to get a grip again that will cut your speed down considerably especially if the prop is badly fouled. You would probably achieve better VMG tacking across the waves and maybe sailing.

 

[Sandy]

When was the hull last cleaned?

 

[srm]

Does the owner have previous experience of motoring into similar wind and sea states? If so what water speed did he get? Come to think of it are you measuring water speed or ground speed from GPS? If the latter you have to account for tide and the surface current due to the wind waves.
As Eddystone mentioned a number of factors but remember high volume shallow flattish bottomed hulls are easily slowed or stopped by wind and waves. but will blow along quite nicely on a reach. Motor sailing or even sailing to windward (gosh do modern sailors still do that?) may well give a better performance.

 

[Arcady]

I had experience of Brunton’s autoprop on two boats, and have experienced similar issues. Bear in mind that the pitch adopted by the autoprop depends on rpm and - critically - forward speed through the water. I found that when heading into a steep sea and headwind, boat speed would decrease, causing the autoprop to adopt a finer pitch, thus reducing forward drive and reducing speed still further, which in turn causes a still finer pitch to be adopted, and so on. The solution is a lot more throttle in order to build up boat speed once again. Bearing off, or running downwind coarsens the pitch hugely and consequently the difference in performance depending on relative heading can be very considerable.

 

[Daydream believer]

I am on board a friends boat, a Oceanis 46.1 that has a Bruntons feathering prop, a Autoprop H5. The boat has a 50+ hp engine and when motoring into 20knts of wind at 2800 rpm we get about 2.5knts. If we turn across wind we get 6.5 - 7knts.
Anyone had a problem like this before r could shed some light on what might be wrong?

yes I know exactly what you mean. My boat is much smaller at 31 feet . the boat will do 7.2 kts flat out engine revs in the dutch canals. I will normally cruise at 6kts in a long swell or flattish water. However, in a 1-1.5 metre short chop the boat hits the wave & stops. the prop feathers, then it gradually pitches up to get the boat driving again. The boat starts to move forward, but before momentum is gained the boat hits the next wave & stops. The prop once again goes into the feathered state with zero drive & so the cycle repeats.
If one looks over the stern one will see the water "pulsing" ie water will be seen jetting out the back as the prop gives drive then nothing as the prop looses pitch then it shoves water out again.
I do not know how the owner of the boat in question bought & specified the prop, but the pitch may be wrong. The prop is self pitching but it is designed to pitch to an optimum pitch for the boat. You cannot take a prop off one boat & expect to put it on a different boat & expect it to work. Brunton can alter this pitch by chamfering the trailing edge of the propellor blades. It is only a very few millimetres but makes a big difference
So i suggest that the owner speaks to Brunton, tells them the problem - they know about it- Ask them to do calcs for the correct prop pitch & then to check the pitch that the prop is set at. Mine was originally started out with far too fine pitch.They apologised & re set it. It has been increased but is still carp.
Your owner will find that if he motor sails with, say, the jib, the speed will increase dramatically, as the prop is brilliant as a motor sailing prop. Trouble is that one often gets this when the wind is dead on the nose. My solution is to rev the engine as hard as possible or try & tack upwind using the jib. to avoid excessive heal. Makes helming hard work though as concentration factor is high.
I would close by saying that weed on the boat is a killer & a clean prop is important. Weed seems to cause a lot of prop wash, regardless of advertisers claims. Watch the anodes. Mine wear out super quick!!!

 

[eddystone]

Interesting - is this generic to feathering props or specific to Autoprop - I have Featherstream?

 

[duncan99210]

I’ve got an Autoprop and find that you need to make sure you’re using enough throttle for the situation. I normally have the throttle set to produce 2000 rpm to get about 6 knots. Into a head wind or short chop, then I’ll need to open the throttle until im getting perhaps 2600/2700 rpm to maintain say 5.6/5.7 knots. Bearing away from the wind and swell will increase boat speed and enable me to back off on the throttle.
I‘ve found the boat doesn’t perform much differently when the 2 blade Volvo prop is fitted: more throttle required into wind, lower boat speed and so on.

 

[Daydream believer]

Interesting - is this generic to feathering props or specific to Autoprop - I have Featherstream?

I believe ( Correct me if I am wrong) the feathersteam is not variable pitch. It is just fully open, or feathered, so does not change pitch with speed, in the same way as a Brunton
But if you have one you will surely know if you get the effect described

 

[rgarside]

Probably more relevant to Autoprop, as it is self-pitching rather than pitching to a fixed stop. I have not experienced the same problem with my Autoprop, but can recommend consulting Brunton's. My engine would not reach full rpm, and had much black smoke at WOT. I took a set of speed and rpm readings and with the aid of that information Brunton's were able to adjust the prop so I can now get to full rpm. I understand they have records of measurements of their props when they leave the factory, these help them with any future adjustments.

 

[eddystone]

I believe ( Correct me if I am wrong) the feathersteam is not variable pitch. It is just fully open, or feathered, so does not change pitch with speed, in the same way as a Brunton
But if you have one you will surely know if you get the effect described

No you are correct - Featherstream is fixed pitch so it’s either open or feathered although the pitch can be changed by Darglow
However I did experience something similar to OP last Autumn plugging into wind, waves and tide trying to get back from Salcombe to Plymouth. Not helped by finding prop heavily encrusted with barnacles on lift out (and my reluctance to use high revs).

 

[KLBH]

Thanks for all the prompt replies, Daydream believer described our symptoms perfectly so it looks like my friend needs to speak to Bruntons.
In answer to some of your other suggestions which may not of applied in our situation could in the future.
The boat has just been launched after anti fouling, prop cleaning and greasing. We are delivering it back to its home berth in Sicily.
The chop was only 0.5 - 0.8m with virtually no tide to speak of here other than the wind current.
Thanks again for your feedback.