P bracket or Skeg

[KAM]

Installing a new engine layout and can't decide between P bracket and skeg. I fitted a skeg on the last boat and never had a problem it was just a 316 tube fitted with a cutlass fed by a small inlet hole forward, felt it was better from the rope tangling point of view. Looking round the yard P brackets seem to dominate. Not sure what the advantage of a P bracket is, is it just original manufacturing cost. Presumably the water flow is better over the prop but they look so fragile. Not much data on drag available but have read some comments on vibration prolems with 2 bladed props and skegs. Would be interested in comments.

 

[Tranona]

A P bracket is normally used when the designer wants a clean run aft - and often where the bottom of the hull rises quite steeply so that a skeg would end up large. It also allows a clean water flow to a spade rudder. The downside of P brackets is that structurally they can be weak - particularly the bonding into the hull, and difficulty in providing support for a long shaft.

There is only one major production yacht builder using P brackets now, although many semi custom designs still use them, but usually much more substantially engineered than production boats of 20-30 years ago. (Look at Oysters for example).

The choice of which to use will depend on the under water configuration. A skeg, if it fits in would be my choice, but from a rope catching point of view there is little difference. The prop is still in a stream of water that may contain rubbish, so fitting an effective rope cutter is still a good idea.

 

[Neil_Y]

As Tranoma sais from a rope catching perspective there is little difference. Rope catching seems to be effected more by hull shape. We see far more on Cats (fishinhg boats) as the shaft line is close to the side of the hull and the prop sucks water down.

Rope or debris are likley to move a P bracket and leave it out of ailignment at the very least and possibly damage the fixings, which is why it is best to have fixings in a box so that failure doesn't result in sinking.

DIY P bracket installation needs some care with regard to the bearing carrier alignment. Chock the shaft and check you can spin the new bearing in the carrier and on the shaft, assuming you go for a clearance fit bearing. Once aligned don't hammer anything use pullers or presses.

 

[john_morris_uk]

There is only one major production yacht builder using P brackets now, although many semi custom designs still use them, but usually much more substantially engineered than production boats of 20-30 years ago. (Look at Oysters for example).

The real reason that many mass production manufacturers don't use p brackets is that they now install sail-drives. No alignment problems and they are quicker and easier to fit in the mass manufacturing process.

Nothing to do with structural integrity and strength of the p-bracket.

Lots of yachts have p-brackets and if they are fitted well, then there's nothing wrong with them.

 

[Tranona]

The new boat market is indeed split roughly 50/50 shaft and saildrive. However, Beneteau use solid shaft logs and Jeanneau P brackets. So the comment still stands. P brackets are in a minority on sailboats.

P bracket failures come up frequently here because of either poor installation or damage. This does not mean that there are not well engineered installations, but many in the early days were not.

 

[KAM]

Thanks for the comments. Think I'll go for the skeg. It's easier to line up, cheaper and 1 less hole in the hull. I wonder how much water flow a cutlass needs?

 

[Neil_Y]

We have a calculator for cutless bearing clearances which also details number of channels and water flow needed. It's on our site on the left, might be useful.