Hi guys

In order to get more speed at lets say 2000rpm, would I need to increase the propellors diameter or increase the pitch of the blades?

Pitch will do it Dex but there's concerns of over pitching and over working the engine and not being able to get to operating revs

Hi Vernon, I strongly suspect that the formula that Flexofold uses to recommend propeller sizes is over-conservative. Mine is the same, I keep thinking it needs to be a size up or more pitch.

If you give the guys at DH Porter a call and give them your engine and gearbox details, they'll tell you what prop size you need.

My impression is that yacht props consistently seem to be slightly underpitched.

It would be interesting to tab down some examples
mine is 16x12 Flex Fold 2 blade. With D1-30 on a sail drive. 37 foot Bavaria 6.9 tonne advertised displacement. It feels under propped, the engine runs out easily to max governed speed, and never feels like it is hunkering down! I suspect I could go up
a pitch, larger diameter I think equals more potential thrust.
I guess the guidelines are conservative, and heavy weather and waves would obviously add more load than flat water.
cheers Richard

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Dexport said..
Hi guys

In order to get more speed at lets say 2000rpm, would I need to increase the propellors diameter or increase the pitch of the blades?

Increasing the dia of the prop by 1 inch relates to an increase of pitch by 1 1/2 to 2 inches
The prop works more efficiently with an increase of dia rather than an increase of pitch.

You must have room to swing it along with the correct clearances

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2bish said..
Hi Vernon, I strongly suspect that the formula that Flexofold uses to recommend propeller sizes is over-conservative. Mine is the same, I keep thinking it needs to be a size up or more pitch.

Yep for sure. Mine can rev up to Max very easily and is 1kt slower at 2500rpm than my older prop.

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Jolene said..

Dexport said..
Hi guys

In order to get more speed at lets say 2000rpm, would I need to increase the propellors diameter or increase the pitch of the blades?

Increasing the dia of the prop by 1 inch relates to an increase of pitch by 1 1/2 to 2 inches
The prop works more efficiently with an increase of dia rather than an increase of pitch.

You must have room to swing it along with the correct clearances

Interesting! when I asked the salesman from Flexofold he said I could add more pitch but it would put the engine under more load. He said nothing about increasing diameter. I was also more thinking more diameter.

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Stockie said..
It would be interesting to tab down some examples
mine is 16x12 Flex Fold 2 blade. With D1-30 on a sail drive. 37 foot Bavaria 6.9 tonne advertised displacement. It feels under propped, the engine runs out easily to max governed speed, and never feels like it is hunkering down! I suspect I could go up
a pitch, larger diameter I think equals more potential thrust.
I guess the guidelines are conservative, and heavy weather and waves would obviously add more load than flat water.
cheers Richard

Mine is also the Flexofold 16x12, 2 blade on the composite hub. 37ft Sayer, 4.8ton at launch.
I can easily get to Max rev's. My old prop unfortunately fell off somewhere in Moreton bay but I got about 1kt more speed at 2500rpm.

If you think your prop is under-pitched it is probably when you are going fast with not much to battle against. A fine pitch will help you when the wind and sea are on the nose, and boat speed drops as you punch through. Life is a compromise.

rule of thumb for repitching props. Put engine in neutral push throttle to max revs take note this is flight revs
put engine in gear push throttle to max revs it should be 10 % under flight
for small boats an increase of 1 inch pitch is equal to a reduction of about 100 rpm

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Yara said..
If you think your prop is under-pitched it is probably when you are going fast with not much to battle against. A fine pitch will help you when the wind and sea are on the nose, and boat speed drops as you punch through. Life is a compromise.

Yeah I understand that but in general when its windy and rough I'm sailing. Its when there's no wind and its calm that I need to motor so I would rather have the speed/lower revs in this situation.

There is pitch and diameter and there is also blade area which is what gives you "grip" in the water.

The previous owner of my yacht had a larger diameter and larger blade area prop fitted to it which I am sure was a definite improvement.

A 26.25 ft LWL hull that achieves 6.5 knots in smooth water and 5.5 knots in a seaway is quite impressive I feel.

Yes it is 3 blade fixed but those props (I forget the brand) that have free swivelling blades that increase their pitch as you gain speed have to be the duck's nuts if you can afford one.

They give you the best fuel efficiency.

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cisco said..
There is pitch and diameter and there is also blade area which is what gives you "grip" in the water.

The previous owner of my yacht had a larger diameter and larger blade area prop fitted to it which I am sure was a definite improvement.

A 26.25 ft LWL hull that achieves 6.5 knots in smooth water and 5.5 knots in a seaway is quite impressive I feel.

Yes it is 3 blade fixed but those props (I forget the brand) that have free swivelling blades that increase their pitch as you gain speed have to be the duck's nuts if you can afford one.

They give you the best fuel efficiency.

max prop do one as do auto prop the auto prop has a much higher drag profile while feathered. i was keen on the max prop that increases pitch under load but was talked out of it by the us supplier as he said they were a bit unreliable in the small sizes (18 inch) so went with the conventional 3 blade feathering which i have been very happy with

Those auto props almost sound a bit of a gimmick. The design creates a phenomena where as the shaft speed decreases the angle of attack is corrected. It really has nothing to do with load and shouldn't be confused with a constant speed prop that sets the angle of attack or pitch according to engine load.
Most feathering props will increase the angle of attack as they slow down, this is of course how they feather, but they won't be providing thrust as they feather,
The difference is with other feathering props like the auto stream for example, is that for the prop to start to feather the driven speed of the prop shaft needs to be less than the windmilling speed of the prop. This is where the auto prop is a little unique as it's not the windmilling that feathers it.

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Dexport said..

2bish said..
Hi Vernon, I strongly suspect that the formula that Flexofold uses to recommend propeller sizes is over-conservative. Mine is the same, I keep thinking it needs to be a size up or more pitch.

Yep for sure. Mine can rev up to Max very easily and is 1kt slower at 2500rpm than my older prop.

Yeah, pretty much identical drop in performance to mine (3 blade folding bronze), 39ft boat with D1-30 saildrive.

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2bish said..

Dexport said..

2bish said..
Hi Vernon, I strongly suspect that the formula that Flexofold uses to recommend propeller sizes is over-conservative. Mine is the same, I keep thinking it needs to be a size up or more pitch.

Yep for sure. Mine can rev up to Max very easily and is 1kt slower at 2500rpm than my older prop.

Yeah, pretty much identical drop in performance to mine (3 blade folding bronze), 39ft boat with D1-30 saildrive.

Yeah so what happened is when I hauled out the other day I noticed that there was quite some play in the hub, so I made a video and sent it to Flexofold and they said I'd maybe mounted the prop wrong but Id also made a video of that 3 years ago so they ended up saying I should send the hub back to them. Its a composite one so I'm guessing there has been some excess wear. I've only done 800 engine hours since it was new so its not good. They will send me back a new hub and new blades with more pitch. I'll let you know how it goes in May when I return to the boat and get her in the water.