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13-02-2022, 10:07
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#1
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Registered User
Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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over-size propellers
I had an over-sized prop fitted to my Formosa 41 a couple years ago. The size and pitch was reccomended by a marine mechanic and accepted by the prop maker (after I'd supplied him the boats details, Perkins 4108 etc). It's a 21"x 14 pitch 3 blader. Gives 7.5 knots in calmish water at around 1800 rpm. However, in rough water, hard to get the engine above 1500rpm and speed drops back to 3knots. But then, that was also the case with the smaller prop that was on there (15"" diameter) - would get 2-3 knots at around 2300-2400rpm - Any comments? Should I maybe get it re-pitched downwards?
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13-02-2022, 10:34
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#2
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Registered User
Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
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Re: over-size propellers
21" is pretty large for that motor. We run a 21" maxprop (we can adjust the pitch) on a 4.236.
Would do the calculation to determine the correct size prop for your motor and trans. Tod do the calculation, you will need to look at your trans to find the gear ratios for fwd and rev.
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13-02-2022, 10:40
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#3
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Registered User
Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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Re: over-size propellers
Hi Bill. The specs I gave to the prop manufacturer were as follows:
LWL - 9.75m
Beam - 3.71m
Draft - 1.88m (long keel)
Weight - 13 tonne
Engine - Perkins 4108 rated 50hp, but realistically, at the cruising revs I use, (1400 -1850 revs) it would be more like 30hp in practice I would think.
The gearbox is a Hurth and the prop shaft turns 1 revolution to every 2.25 engine revolutions as measured by myself recently by hand-turning the flywheel one revolution.
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13-02-2022, 10:44
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#4
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Registered User
Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,482
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Re: over-size propellers
Hmmmm. It seems if the engine wont rev higher it is over pitched or too big. Take your pick.
I understand if the boat moves slow sometimes, but I dont understand why the engine stalls rpm wise... especially if it did with the smaller prop.
One wonders if under stress your tranny is slipping or something..? What tranny does it have?
However, the 4-108 isn't making a lot of HP at 1800 rpm either. It's only a 51 hp engine at what 3600 rpm, cant remember...? I liked to run mine at about a 2200-2400 rpm sweet spot in my 44 footer (avatar) I think I had an 18" prop. 8-8.5 knots in flat water.
If you conclude your tranny is ok, I would repitch or reprop to use some HP. At least for maybe 2400 rpm. Do you have the power curves for the 4-108?
Your boat is likely a bit heavier than mine was, and with more windage.
Actually, it looks worse than I thought; 22 hp or so at 3000 rpm! (PROP HP)
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard
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13-02-2022, 10:53
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#5
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Registered User
Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
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Re: over-size propellers
This is what I come up with: 50 HP, 3600 rpm, 2.5:1 gear.
17.5 X 9.5 three blade.
19.5 X 11.5 three blade with 3:1 gear. This gear would be you best choice.
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13-02-2022, 11:01
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#6
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Registered User
Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
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Re: over-size propellers
OK, with a 2.25: 1 gear
16.5 X 8.5 three blade. Poor gear ration selection. You need a new gear and prop.
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13-02-2022, 11:03
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#7
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Registered User
Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
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Re: over-size propellers
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13-02-2022, 11:10
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#8
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Registered User
Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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Re: over-size propellers
Hmmm, I've just discovered a site where you can plug in your boats details and it will calculate a size for your prop. Definitively the diameter looks that it shouldn't be much more than 20" and as you guys are saying, pitch should be lowered too. I'll call the prop manufacturer today and see what specs he actually made the prop to. After all, I gave him the mechanic's suggestion of 21"x14, but after he went through the boat specs I gave him, he may have modified what the mechanic had suggested. Thinking about the whole situation now, I realise that when the new prop was fitted, the boat had also just had a hull clean and new anti-foul and prop speed, and since then, has had an annual lift-out and ditto: clean and anti-foul/prop speed. However, with Covid etc, it's now been 18 months since last haul-out, so lack of use and dirty bum/prop would be aggravating the situation. Haul-out has been booked, so will shortly have a clean hull, but will also enquire about having a re-pitch and go from there.
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13-02-2022, 11:11
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#9
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Registered User
Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
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Re: over-size propellers
Bobby,
I think you need to determine how you want to use your motor?
With the over-sized prop you can get up to speed quickly, but can't increase your rpm to utilize your full hp from the motor (you've found this out already).
The normal sized prop (as kmacdonald has calculated) will allow you to have access to more rpm, but will not give you the relatively higher speeds at the low rpm you typically run.
When do you want to be?
Will agree the current 21" prop is heavily loading the motor, possibly in a negative way.
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13-02-2022, 11:13
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#10
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Registered User
Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
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Re: over-size propellers
21.5 X 13.5 three blade with 3.5:1 gear. Best choice.
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13-02-2022, 11:29
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#11
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Registered User
Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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Re: over-size propellers
Thanks for the info gents...very helpful. Bill, you've hit the nail: "How do I want to use the motor." Answer, as quietly as possible...and even then, it's too loud! In practice, I cruise with the revs at the start of the 'green' section on the tacho, with is 1500rpm (the green section goes up to 2500rpm) and in calm to moderate seas, (with a clean hull & prop) gives around 5knots...all good. But recently, for instance, heading out of Sydney Heads against the tide (prob 1.5 knots) and a 15k breaze against a 2m swell (6ft), I was down to 1knot at times...and revs wouldn't go up past 1600rpm! Scary stuff with North head half a mile to port. So yes, my speed in calmish/medium seas is great, but no grunt when I need it.
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13-02-2022, 11:34
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#12
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Marine Service Provider
Join Date: Jun 2015
Location: Croatia
Boat: Elan 45 impression
Posts: 1,161
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Re: over-size propellers
Quote:
Originally Posted by Bobby Bluegum
I had an over-sized prop fitted to my Formosa 41 a couple years ago. The size and pitch was reccomended by a marine mechanic and accepted by the prop maker (after I'd supplied him the boats details, Perkins 4108 etc). It's a 21"x 14 pitch 3 blader. Gives 7.5 knots in calmish water at around 1800 rpm. However, in rough water, hard to get the engine above 1500rpm and speed drops back to 3knots. But then, that was also the case with the smaller prop that was on there (15"" diameter) - would get 2-3 knots at around 2300-2400rpm - Any comments? Should I maybe get it re-pitched downwards?
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use this
https://www.vicprop.com/free-propell...ng-calculators
and also lower HP of your engine minimum 10-15 % because after leave the factory all engines start aging and losing power slowly.
o yes use power RPM curve of you engine.
like Cheechako sugest
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13-02-2022, 12:04
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#13
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Registered User
Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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Re: over-size propellers
Yes, thanks for that prop calculating site...have only just discovered it...and yup, I popped in HP @ 45hp to allow for deteriorating engine power (although I had it rebuild to factory specs 10 years ago - about 2500 hrs back.) Prop diameter is def too large. Once again, simply taking a professional's advice without doing my own research...although I did tell him I wanted to run motor as quietly as possible! Anyway, I will discuss with the prop maker about de-tuning the pro I have as a starting point.
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13-02-2022, 12:32
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#14
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Registered User
Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
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Re: over-size propellers
For the prop calculator may want to increase the 13T weight. That was the dry weight, then add for fuel, water and all the other "stuff" you have onboard.
I don't remember the amount of pitch change one can adjust a fixed prop, but only expect a few inches. This may only get you to 2K or so on your motor.
You may need to purchase a different prop (used/reconditioned?) to get the right size/desired performance.
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13-02-2022, 12:42
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#15
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Registered User
Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
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Re: over-size propellers
If the present gear ratio is really 2.25:1 you will need a new gear. The prop you have is light years off and won't be of any use unless you get 3.5:1 gear and that may be your best option.
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