What propeller

[cagey]

Advice please, it’s highly likely that I’m going to sell my Sigma and buy a heavy motor sailor and I’m entering an area I barely know. Starting from the bottom all the likely boats I’ve seen on the internet either have big heavy single props my lovely folding gori prop is so beautiful and they have descriptions I don’t know eg self feathering. Bearing in mind I can’t add anything else can someone give me a shove (with an explanation please ) as to what and why I should be looking for, Google this time isn’t much use, you have to at least know something to join in the conversation.
Thanks
Keith

 

[Laminar Flow]

We have one of those heavy motorsailers with a fixed 3-bladed prop.
We sail her hard and have, successfully, gone to considerable length to make her sail well. I have also done the math on whether I should change to a feathering/folding prop.
1) We don not have the room to swing a folder.
2) Next: does your gearbox allow you to let the prop mill. If so, there is already 40% less resistance than if the prop were fixed.

How much Speed will you lose? The loss of speed in relationship to overall resistance will be lower at higher speeds. At a relative average speed of 0.9 your speed will be diminished by:
- fixed 3-blade: 12%
- 3-blade left to free wheel 8%
- feathering prop 4%
- folder 1%

For us in numbers that means with a DWL of 28' and at a speed of 4.8kts (relative speed of 0.9) a loss of:

- Fixed 3-blade: 0.6kt
- 3-blade free wheeling: 0.4kt
- feathering prop: 0.2kt
- folder: 0.05 kt

at 7.2kts we lose 0.3kt.

For us this means, as we can let our prop mill without damaging the gear box, a gain of 0.2kts if we were to switch to a feathering prop.

You would essentially be better off to invest the money for a fancy prop in better sails, keeping her bottom clean or, if she's a Watson, to fair in the deadwood and profile the rudder.

 

[Tranona]

Folders are rare on more motor orientated sailing boats, usually because there is not room - many have props in apertures or in front of skegs. Feathering props are however common because they usually give as good or better performance under motor and reduced drag under sail. The benefit of this is felt more at low speeds, but it is questionable whether the cost and additional maintenance (on some designs) is justified on these grounds. For example if you are doing a lot of long distance sailing you can measure the gain in terms of daily runs but for typical coasting or cross channeling, when there is often a lot of motoring anyway the gain is difficult to evaluate.

The third type is the variable pitch or more likely self pitching type such as the Bruntons Autoprop which is popular with motor sailors, not from the lower drag point of view but because self pitching means that the pitch is adjusted to put optimum load on the engine based on speed and conditions. Fixed pitch props only fully load the engine at near maximum revs so at lower speeds the engine produces less power than it is capable of at the revs the prop demands. The effect is with a Bruntons the engine usually runs at lower and therefore quieter. This is particularly noticeable when motorsailing where with a fixed pitch prop the engine is lightly loaded at higher revs. Bruntons are particularly popular for those who passage make in the Med, for example where winds are often light or non existent and the lower engine revs leads to more relaxed motoring.

Like Laminar Flow, I would not add a fancy prop to a boat that is more dependent on its motor for passage making. Unlike your Sigma where the gain is useful, particularly if you are racing or like squeezing the last bit of sailing performance out of the boat, there are other things to spend money on that will provide better returns in sailing efficiency. On the other hand if the boat comes with a fancy prop, keep it as there is no real downside, except maybe the cost of anodes!

 

[Laminar Flow]

Re Bruntons.
At one point I too thought this prop would be ideal for a motorsailer. A Dutch friend installed one on his Southerly and was less than complimentary about it's performance, particularly when maneuvering and when motoring into any kind of a seaway. Subsequently, he switched to a Max of which he said it gave him much better performance.
I had offered to buy his Brunton off him, but he strongly dissuaded me and insisted that for friendship's sake he'd rather not.

 

[Kurrawong_Kid]

Re Bruntons.
At one point I too thought this prop would be ideal for a motorsailer. A Dutch friend installed one on his Southerly and was less than complimentary about it's performance, particularly when maneuvering and when motoring into any kind of a seaway. Subsequently, he switched to a Max of which he said it gave him much better performance.
I had offered to buy his Brunton off him, but he strongly dissuaded me and insisted that for friendship's sake he'd rather not.

Interesting! I have had 2 Autoprops but in each case the size was correctly calculated and the engine of sufficient power. I have been satisfied with their performance, as I was with the Gorilla in my previous boat.

 

[Tranona]

Re Bruntons.
At one point I too thought this prop would be ideal for a motorsailer. A Dutch friend installed one on his Southerly and was less than complimentary about it's performance, particularly when maneuvering and when motoring into any kind of a seaway. Subsequently, he switched to a Max of which he said it gave him much better performance.
I had offered to buy his Brunton off him, but he strongly dissuaded me and insisted that for friendship's sake he'd rather not.

They do tend to work better on some boats than others. Some find the low speed characteristics difficult to manage. I think most later Southerlys had Featherstreams. On the other hand I have met many who have been very pleased with the performance, particularly motorsailing.

I have had both feathering (a JF, earlier version of Featherstream) and fFexofold, but both bought primarily for reasons other than drag reduction under sail.