prop backwards or bent?

[Borealis]

I have a 10hp Saab propulsion setup in my 10 ton sailboat. An auxiliary power unit to say the least. My speed is pretty slow. Last summer I had a diesel mechanic glance at my prop and say the blades are backwards. Its a 2 blade feathering Saab prop. Now I have two situations:1. the prop blades were installed backwards (the number didn't line up), 2. the blades have a 10 degree bend (not pitch) to them, not like a traumatic damage type of bend but rather a deliberate manufacturing type of bend. if you imagine an animated cartoon of a boat traveling super duper fast and the blades are bending backwards from the extreme speed of the boat then that is what mine remind me of (I actually wondered for a moment if that happened). I have looked at 4 other Saab feathering props in the yard and they are all straight.

So, to set things right I switched the blades around to line up properly with the numbers but then the bend in the blades are facing forward. I shrugged my shoulders and put the boat in the water hoping that maybe i remedied a problem the previous owner was unaware of. However, the boat is agonizingly slow and I have no knowledge of its performance before I changed the prop.

Anybody have any thoughts about the bend, performance, cause, solution?

[David M]

Weird to say the least. Have you contacted Saab?

[Alan Wheeler]

A common cause of bending prop blades is to ground in mud. The effort exerted on the blades will bend them like Bananas instantly. It is possible the boat has been aground and the prop has been in something harder to move than water.
At a ratio of 1Hp/T don't expect wonders in acceleration either. That is a very small engine for 10T of boat.

[4900bruce]

Hello again Borealis, I cant comment on your prop shape , but ten horse power diesel in in that boat is way to small eg my boat same design 48 hp with a fixed 3 blade prop.feathering 2 blades wiil not be very efiecent.I used to have a rorc 32 ,7 ton and that had 12 hp motor which was embarrasing when motoring.

[4900bruce]

just remembered ,there are hull speed calculators and prop sizing calculators on the internet which will tell what speed you should be travelling at and what size prop and pitch you should have for that motor and boat.

[Ex-Calif]

Send photos...

I was given a prop and had no idea what is was other than it was a feathering prop. We had the original Volvo saildrive folding prop.

After I posted a few photos the folks here identified the prop within seconds (maxprop duo). I downloaded the manual from the internet which explained how to set the prop up. I expected a bit of "trial and error" but we seem to have nailed the adjustment on the first try.

With the same 10hp engine we now get 6.5 kts vs. 3.5 before.

I would guess if the blades were damaged you would see evidence on the blades of striking something - mud might be a different story - but also I would expect you to get vibration at any significant engine speeds.

photos would be nice...

[svHyLyte]

The flares at the tips serves two functions. Firstly they provide a little drag to ensure that the blades fold when not powered. Secondly, they redirect the tip-vorticies that form as the blade spins, which would otherwise swirl under the back sides of the blades killing some of the lift provided by the blades. A good example of this is the up-turned tips at the outboard ends of some of the higher performance jet powered private aircraft.

s/v HyLyte

Originally Posted by Alan Wheeler

A common cause of bending prop blades is to ground in mud. The effort exerted on the blades will bend them like Bananas instantly. It is possible the boat has been aground and the prop has been in something harder to move than water.

That certainly might be true if the prop is buried in mud and turned, but given the bottom configurations of most sailboats that seems a unlikely scenario.

If you are lucky enough to live in an area that has a propeller specialist you might bring some photos to their shop.

[Borealis]

Originally Posted by 4900bruce

just remembered ,there are hull speed calculators and prop sizing calculators on the internet which will tell what speed you should be travelling at and what size prop and pitch you should have for that motor and boat.

hey Bruce, send me an email, I'll send you pics of my C-Shell / Bird.

[Borealis]

Originally Posted by 4900bruce

just remembered ,there are hull speed calculators and prop sizing calculators on the internet which will tell what speed you should be travelling at and what size prop and pitch you should have for that motor and boat.

hey Bruce, send me an email, I'll send you pics of my C-Shell / Bird.
usnaquaman@hotmail.com

[Borealis]

photo, good idea, i may even have one from my last haulout...thanks.