Trundlebug
Well-Known Member
I've just sent in my props for assessment, adjustment, rebalancing and polishing.
It's the first time I've done it since owning the boat - 8 years.
I've just had a call to say the props are 18x12, but their software advises the optimum for my boat (for top speed) is actually 18x10.
Well after due consideration I've decided to ask them to adjust to 18x11.
Yes, that will still leave the boat slightly over- propped.
But importantly it's a full displacement boat, and apparently quite a lot of commercial displacement boats and trawlers specify over pitched props to improve economy, if at the expense of all out maximum speed (which, let's face it displacement boats rarely achieve or travel at).
Are they right? It would surprise me if they weren't.
But I keep reading on here that everything must be matched exactly right. I can certainly understand it for planing and SD hulls, but in a generously powered displacement hull the emphasis is on comfortable and economical cruising ( at the lowest revs for noise etc).
So is it different in a displacement hull? Are there benefits to slight over-propping?
For information, engine spec max rpm is 3500, max I've ever achieved is 2900, but max hull speed and economic cruising achieved at around 1900.
I do have dripless shaft seals, one a Volvo rubber boot type and the other an HMI oil cooled type - which IIRC specs max shaft rpm at 1500.
This equates to 2895rpm, very close to my 2900rpm achieved and making me think that was the reason why it was set up the way it was.
I would appreciate expert input and thoughts. Have I done the right thing?
It's the first time I've done it since owning the boat - 8 years.
I've just had a call to say the props are 18x12, but their software advises the optimum for my boat (for top speed) is actually 18x10.
Well after due consideration I've decided to ask them to adjust to 18x11.
Yes, that will still leave the boat slightly over- propped.
But importantly it's a full displacement boat, and apparently quite a lot of commercial displacement boats and trawlers specify over pitched props to improve economy, if at the expense of all out maximum speed (which, let's face it displacement boats rarely achieve or travel at).
Are they right? It would surprise me if they weren't.
But I keep reading on here that everything must be matched exactly right. I can certainly understand it for planing and SD hulls, but in a generously powered displacement hull the emphasis is on comfortable and economical cruising ( at the lowest revs for noise etc).
So is it different in a displacement hull? Are there benefits to slight over-propping?
For information, engine spec max rpm is 3500, max I've ever achieved is 2900, but max hull speed and economic cruising achieved at around 1900.
I do have dripless shaft seals, one a Volvo rubber boot type and the other an HMI oil cooled type - which IIRC specs max shaft rpm at 1500.
This equates to 2895rpm, very close to my 2900rpm achieved and making me think that was the reason why it was set up the way it was.
I would appreciate expert input and thoughts. Have I done the right thing?
