over-size propellers

[merrydolphin]

This would work wonderfully IF you had more like a 3;1 reduction, but you are probably like most aux sailboats around a 1:8 TO 2:2 at best. The reduction gear ratio is so overlooked in this equation. Especially on a heavier boat like this.

[Bobby Bluegum]

Thanks Nickolson...a very useful and encouraging reply!! I've spoken to the prop guy and he's confirmed that he'll be able to take "at least a couple of inches out of the pitch." He's also promised that if I give him a few days notice ahead of bringing it in, he'll have it ready for pick-up the next day...which will fit snugly into a 3 day haul-out for the annual anti-foul. If it still won't allow the motor to rev out to 2000+rpm, I'll get another one made up. Have checked out ebay for s/h ones, nothing smaller with a r/h swing and the correct ID (taper:length 38mm, shaft 32.9mm, 10mm key.) Will report back when all is done. Thanks everyone for invaluable advice and comments.
Bob~

[Mickeyrouse]

I’m dubious about the horsepower rating. We had a 4-107 on our boat for years. Which was rated variously ( depending on source) at 37 to 40 HP. The 4-108 as I understand it is almost an identical engine, with 108 CI displacement compared to 107. In some literature the engines are referred to interchangeably. If you’ve propped for 50 HP it sounds like the prop needs more power to drive it. The problem wouldn’t show up as much in calm water, but would really stick out powering into a chop.

[LittleWing77]

Originally Posted by Bill O

Will agree the current 21" prop is heavily loading the motor, possibly in a negative way.

Not possibly - absolutely.

VERY hard on an engine if it's over-propped. Like driving a stick-shift in 1st gear and still pushing the gas to make it go faster. Gak!

Please find a prop installation expert (NOT your prop's manufacturer, by the way). First, find out what Perkins' recommendations are for your 4-108 engine, then utilize a prop expert to fine-tune prop selection for your hull, transmission and typical use.

Here's an interview with an Australian prop expert in Hervey Bay. Not sure if that's close to/ useful for you, Bobby, but the interview is informative (you'll want to ensure your prop tech understands sailboats, versus just powerboats):

https://youtu.be/GOAJKQWYl3o

I can recommend an excellent prop installation company here in Canada (sorry, not much use to you in Aus, Bobby), for our Canadian CF Members:

Denis Fournier
Canada Propeller
1398 Star Top Road
Ottawa, ON K1B 4V7
1-866-621-7767
denis@canadapropeller.com

Unfortunately, Canada Prop does represent two particular brands, so their perspective may be a tad skewed (caveat emptor), but Denis sure does know props!

Sorry if I've come late to the party, I do see that you're going the route of re-pitching your existing prop. Hope that goes well.

Best of luck,
LittleWing77

[Bowdrie]

Originally Posted by LittleWing77

VERY hard on an engine if it's over-propped. Like driving a stick-shift in 1st gear and still pushing the gas to make it go faster.

Yes, a large "over prop" condition is hard on the engine.
But it's not like mashing the go-pedal in 1st. gear.
It's like trying to climb a hill in top gear, when even pedal-to-the-metal won't get the engine revs up.

[Uncle Bob]

Originally Posted by Bobby Bluegum

Thanks Nickolson...a very useful and encouraging reply!! I've spoken to the prop guy and he's confirmed that he'll be able to take "at least a couple of inches out of the pitch." He's also promised that if I give him a few days notice ahead of bringing it in, he'll have it ready for pick-up the next day...which will fit snugly into a 3 day haul-out for the annual anti-foul. If it still won't allow the motor to rev out to 2000+rpm, I'll get another one made up. Have checked out ebay for s/h ones, nothing smaller with a r/h swing and the correct ID (taper:length 38mm, shaft 32.9mm, 10mm key.) Will report back when all is done. Thanks everyone for invaluable advice and comments.
Bob~

Bob, could you tell me which prop guy you are using here in Sydney, I am needing some work done on mine and am looking for recommendations.
Thanks in advance, Bob.

[Bobby Bluegum]

Hi Uncle Bob, the guy you should talk to is Glenn at Porters Propellers (the company has been in biz for over 60 years) who are out Parramatta way. Glenn and his wife own the biz and he's a great guy. (My over-sized prop was what I ordered, having been advised by a marine engineer at Coffs Harbour - they were not to specs advised by Glenn btw.)
Porters are highly regarded, you'll find them via google.

[Uncle Bob]

Originally Posted by Bobby Bluegum

Hi Uncle Bob, the guy you should talk to is Glenn at Porters Propellers (the company has been in biz for over 60 years) who are out Parramatta way. Glenn and his wife own the biz and he's a great guy. (My over-sized prop was what I ordered, having been advised by a marine engineer at Coffs Harbour - they were not to specs advised by Glenn btw.)
Porters are highly regarded, you'll find them via google.

Thanks for that, I have spoken to Glenn and visited the workshop (definitely my kind of place) as well as another establishment. Glenn left me feeling confident, the other not so. I value someone else's view very much.

[valhalla360]

Originally Posted by Sailorman97

I see people talking about switching transmission or the gears. This iMHO is nuts. You are talking thousands. The current gear reduction is quite reasonable.

If the OP left everything stock...sure.

When massively increasing the prop, there are consequences in trying to make it work.

[Bobby Bluegum]

The problem originally came about because the original pro that was on the baot when I bought it turned out to have a crack along one of the blades where it joined the prop hub. This was discovered after a shipwright had burnished it back ready for a coat of prop-speed. He then said that it was way too small (it was around 14"-15" diameter, which explained why it never got above 4.5k and maybe 5k in a dead calm. That's when he chimed in with his instructions to order the over-size prop, knowing that I tended to motor sail in wind forward of the stays. I should have come on here before ordering it and got the wisdom of the old sea-dogs of the Cruiser's Forum...and I say that with great respect and affection! They're good!

[GordMay]

Originally Posted by Bill O

Bob, would still change the weight in your calculations to get the correct pitch.
Since 13 tons is the theoretical dy wt. listed in sailboat data. How many inches of waterline had you had to add? Don't have the exact numbers, but would add ~1 ton (to the 13) for every inch you have had to add to the waterline.

POUNDS PER INCH IMMERSION (PPI): The weight required to sink the yacht one inch.
It is calculated by multiplying the LWL area by 5.333 for sea water or 5.2 for fresh.
The PPI usually increases as the hull sinks into the water as the LWL area is also increasing due to the shape of the hull above water.

“Ted Brewer Presents A Primer on Yacht Design” ➥ https://www.tedbrewer.com/yachtdesign.html

See also “Pounds/Inch”, for your boat model, on “Carl's Sail Calculator” v3.55
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