To Modify Skeg — Or Not ... That is the Question ... installing Autoprop

[randygva]

I am the proud owner of a Bruntons autoprop propeller. To be fitted on our 1981 Bristol 41.1 sloop sailboat.

But, it doesn’t fit due to the protective skeg forward of the prop that is the lower 2/3’s of the cutlass bearing housing.

the curvature of the lower part of the skeg ( see the attached picture) was simply overlooked. The blades will hit the skeg when they are unloading and rotating from forward to reverse thrust and back again.

I bought the unit at the fall boat show">annapolis boat show, but the boat was in the water and I just got around to the test fitting ... and quite the surprise.

To Brunton’s credit, they realized the mistake and have offered to provide a full refund. We likely will go that route...unless:

We can modify the skeg in one of two ways:

1. Remove an arc from the lower skeg 3/4 of an inch from where it would hit. This cut would preserve the protective function of the lower skeg. The skeg fitting seems pretty substantial in build and heft. Not sure its material construction.

Making this cut retains the ability to do the more drastic cut described in #2, below.

I think it might work.

Or

2. Cut the skeg virtually off buy removing all the material below the cutlass bearing. This arrangement seems common on many boats. But we lose the protection from the crescent forward of the prop.

Lengthening the shaft is not an option as we already have a dampener between the shaft and the transmission, and I don’t want to spend any additional money or time to but remove, install, align, etc.

The boat is a 1981 Bristol 41.1 with a 50 hp Yanmar. We are replacing a fairly aged PE Luke feathering prop. See second pic.

Big selling point for the autoprop is it’s matching thrust to prop angle throughout the power range.

We are also expecting some additional speed as compared with the PE Luke propeller

Welcome others thoughts and opinions. Regards from Maryland.

[Jim Cate]

Randy, one way to think of it is: I've never seen another boat with the "protective" addition to the strut. I think you would not be introducing all that much hazard by removing it. I suppose that in areas with large numbers of lobster or crap pots it could help avoid entanglement... not sure about that one!

And yep, I'd expect some improvement in propeller efficiency with the Brunton, for that old prop really looks primitive in terms of blade shape, even when compared to other feathering props, all of which have flat blades with no twist. When you add in the self pitching feature, it should be a significant improvement indeed.

Jim

[Uncle Bob]

Originally Posted by Jim Cate

Randy, one way to think of it is: I've never seen another boat with the "protective" addition to the strut. I think you would not be introducing all that much hazard by removing it. I suppose that in areas with large numbers of lobster or crap pots it could help avoid entanglement... not sure about that one!

And yep, I'd expect some improvement in propeller efficiency with the Brunton, for that old prop really looks primitive in terms of blade shape, even when compared to other feathering props, all of which have flat blades with no twist. When you add in the self pitching feature, it should be a significant improvement indeed.

Jim

+1 I would cut it to be the same as almost every other skeg. On another note, you are aware of the need to have a shaft brake with the autoprop I presume ?

[randygva]

Originally Posted by Uncle Bob

+1 I would cut it to be the same as almost every other skeg. On another note, you are aware of the need to have a shaft brake with the autoprop I presume ?

Jim, Uncle Bob

Appreciate the thoughts. The boat was built in the 80s in Bristol Rhode Island. Back in the day a LOT of lobster pots in Narraganset bay and offshore towards block island

No. I have not heard/seen discussion on a shaft brake ... descriptions suggest it will feather when no under power.

What / where how to apply a shaft brake. ?

[Uncle Bob]

Originally Posted by randygva

Jim, Uncle Bob

Appreciate the thoughts. The boat was built in the 80s in Bristol Rhode Island. Back in the day a LOT of lobster pots in Narraganset bay and offshore towards block island

No. I have not heard/seen discussion on a shaft brake ... descriptions suggest it will feather when no under power.

What / where how to apply a shaft brake. ?

Apparently if you have a hydraulic gearbox you require a shaft brake to prevent the prop auto rotating when sailing, the blades turn enough to present the width to the drag.(been described as towing a bucket) but a mechanical gearbox will lock the shaft if placed in gear. (if this is OK with the gearbox manufacturer)
A friend fitted the prop and was amazed at the difference it made.

[a64pilot]

I don’t have a hydraulic transmission, but my Autoprop doesn’t rotate when my transmission is in neutral, the blades feather.
Centripetal Force unfeathers the thing and induced flow feathers it

[Uncle Bob]

Originally Posted by a64pilot

I don’t have a hydraulic transmission, but my Autoprop doesn’t rotate when my transmission is in neutral, the blades feather.
Centripetal Force unfeathers the thing and induced flow feathers it

Curious, do you put the transmission in gear after stopping the engine?

[Suijin]

I would cut off the whole fin below the bearing and put a rope cutter on.

The skeg fin is not doing squat anyway and is potentially going to make line fouling matters worse. If the prop is engaged it’s going to suck in any line that contacts the fin and if the line wraps the fin then you’re looking at a bent skeg and possibly even shaft and a $5k repair.

I would cut it higher than the line in your photo, right up against the bearing surround and grind it fair. Just don’t get it too hot in the process.

[furisgrampa]

Autoprop recommends leaving the transmission in forward gear, and shutting the engine off!, also, make sure you service the prop every 1000 hours?

[bad.wabbit]

The skeg is likely made using a two part mold so when you cut make sure to fiberglass across the cut. Possibly easiest to cut close to the cutless bearing- then your glass work is around a nice radius.

[Bleemus]

Cut it short. If you go for the long option you will have a nasty cavitation.

[S/V Alchemy]

Originally Posted by Uncle Bob

Curious, do you put the transmission in gear after stopping the engine?

My buddy with an Autoprop on a 55 HP Volvo on his steel ketch shut off in forward gear, and then starts in neutral, goosing it into forward or reverse as needed.

Interestingly, I have a steel cutter with a four-blade Variprop and a hydraulic transmission and I do not need to do this. The prop feathers when the shaft stops and stays feathered irrespective of leaving it in gear or not while sailing. I've watched the shaft as this has been done and it stays stopped.

[Gaia]

We have had an Autoprop on our steel Gazelle ever since we left Canada 23 yrs ago. The earlier iteration was poorly designed and suffered ball bearing failure when we crossed the Atlantic. This happened without the engine being used! Brunton did not admit this but changed the design and sold us the new design at cost. The new design has been great although water ingress into the housing can be a problem. If this happens and is not corrected the bearings corrode. Regular maintenance is advised. We have also on occasion when immersed for long periods, have had to remove the prop underwater to correct a loose blade. Easy in the Bahamas, not so amongst the sharks in La Reunion! Nor in the murky waters north of South America. Best done in shallow and clear water. Carry spare bearings, easily cross matched. And carry what you may lose when removing the prop underwater.like the shaft key or prop nuts. Also write down the socket size required and the Allan key size for the grub screw. Apply grease liberally on assembly.

You will also find it worthwhile to apply paint to the zinc anode where there is little material where the bolts go through it.

We have for 23 years feathered the prop while sailing by engaging it in forward gear. We have a 3YM30 Yanmar. Allowing the prop to rotate while sailing, which happens for us over 5 kts, causes a large increase in weather helm. When this happens and we then set it in gear the boat speed increases by a knot or so.

One of the major advantages of this engineering marvel is much more thrust aft. Useful in an emergency and when setting the anchor. Also great while motorsailing at low rpm as of course the prop's major attraction is auto pitching. It makes the flat blades of the Max prop look obsolete ��

Expensive but worth keeping imho.

Jim

[yachtgemini]

Brunton have known about this problem for many years..

About 7 years ago I bought a Brunton in the USA and at instal time I said it was hitting the strut, they said it would not be a problem...

I sailed the boat back over the pond and after a haul out I had some tip damage to the prop... I had all the emails verifying the original advice and made a claim but Brunton only offered me a discount on the repair and told me to cut my strut as you seem to be contemplating.

Bottom line is I had the strut cut and what had appeared to be a solid strut was stainless steel and actually had a void in it. Then I had to get an expert welder to try and patch it. In spite of the expert and careful welding, the strut warped and has caused lots of problems. Next winter I have arranged for a new strut to be fitted and aligned...( did I mention the water tank and A/C unit that will need to be removed for access.)

I have had 2 autoprops and they are a good product. But their UK customer service was lacking and their advice to cut the strut was poor. I will not be buying a third prop.

My advice is not to cut the strut and to simply return the product.

PM me if you want more of the story.

Ross

[waterman46]

Originally Posted by Uncle Bob

+1 I would cut it to be the same as almost every other skeg. On another note, you are aware of the need to have a shaft brake with the autoprop I presume ?

I could be wrong but I thought many if not most transmissions will lock in reverse, ie. you just put the transmission in reverse after stopping the engine. That locks the shaft. If left in forward, it will freewheel, no good for prop or transmission. In reverse, shaft locks and feathering of blades will happen.
We have Autostream prop, and that's how it works for us.