Cruisers Forum
 


Reply
  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
 
Thread Tools Search this Thread Rate Thread Display Modes
Old 13-02-2022, 10:07   #1
Registered User

Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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?
Bobby Bluegum is offline   Reply With Quote
Old 13-02-2022, 10:34   #2
Registered User
 
Bill O's Avatar

Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
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.
__________________
Bill O.
KB3YMH
https://phoenixketch.blogspot.com/
Bill O is offline   Reply With Quote
Old 13-02-2022, 10:40   #3
Registered User

Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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.
Bobby Bluegum is offline   Reply With Quote
Old 13-02-2022, 10:44   #4
Registered User
 
Cheechako's Avatar

Join Date: Aug 2006
Location: Skagit City, WA
Posts: 25,482
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)
Attached Thumbnails
Click image for larger version

Name:	4 108 PropCurves.jpg
Views:	99
Size:	352.4 KB
ID:	252887  
__________________
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard











Cheechako is online now   Reply With Quote
Old 13-02-2022, 10:53   #5
Registered User

Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
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.
kmacdonald is offline   Reply With Quote
Old 13-02-2022, 11:01   #6
Registered User

Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
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.
kmacdonald is offline   Reply With Quote
Old 13-02-2022, 11:03   #7
Registered User
 
Bill O's Avatar

Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
Re: over-size propellers

__________________
Bill O.
KB3YMH
https://phoenixketch.blogspot.com/
Bill O is offline   Reply With Quote
Old 13-02-2022, 11:10   #8
Registered User

Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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.
Bobby Bluegum is offline   Reply With Quote
Old 13-02-2022, 11:11   #9
Registered User
 
Bill O's Avatar

Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
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.
__________________
Bill O.
KB3YMH
https://phoenixketch.blogspot.com/
Bill O is offline   Reply With Quote
Old 13-02-2022, 11:13   #10
Registered User

Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
Re: over-size propellers

21.5 X 13.5 three blade with 3.5:1 gear. Best choice.
kmacdonald is offline   Reply With Quote
Old 13-02-2022, 11:29   #11
Registered User

Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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.
Bobby Bluegum is offline   Reply With Quote
Old 13-02-2022, 11:34   #12
Marine Service Provider

Join Date: Jun 2015
Location: Croatia
Boat: Elan 45 impression
Posts: 1,161
Re: over-size propellers

Quote:
Originally Posted by Bobby Bluegum View Post
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?
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

0
more is online now   Reply With Quote
Old 13-02-2022, 12:04   #13
Registered User

Join Date: Jan 2021
Location: Australia
Boat: Formosa 41
Posts: 20
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.
Bobby Bluegum is offline   Reply With Quote
Old 13-02-2022, 12:32   #14
Registered User
 
Bill O's Avatar

Join Date: Jul 2015
Boat: Bruce Bingham Christina 49
Posts: 3,328
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.
__________________
Bill O.
KB3YMH
https://phoenixketch.blogspot.com/
Bill O is offline   Reply With Quote
Old 13-02-2022, 12:42   #15
Registered User

Join Date: Jun 2010
Location: Easton, MD
Boat: 15' Catboat, Bristol 35.5
Posts: 3,510
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.
kmacdonald is offline   Reply With Quote
Reply

Tags
propeller, rope, size

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Gelcoat over aluminum over fiberglass in head - how? rls8r Construction, Maintenance & Refit 28 01-06-2019 22:24
Send Over-Pressure Shore Water Over the Side with Pressure Valve ? sdowney717 Plumbing Systems and Fixtures 7 28-06-2018 17:45
Miles over ground or over water? homerobarros Navigation 36 13-01-2015 15:58
Varnish over Polyurethane? And PU over Varnish? MarkJ Construction, Maintenance & Refit 8 06-08-2013 15:31
It's Never Over Until You Say It's Over maxingout General Sailing Forum 12 16-03-2009 16:32

Advertise Here


All times are GMT -7. The time now is 11:23.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.