over-size propellers

[Bill O]

Agree, you don't need to spend $2K for a prop when a reconditioned one will do the job. If you look around you can find a shop that has them.

(Btw: note to Bill O, I had a crane haul my Formosa out a few years ago onto the hard. She had about the normal amount of fuel and water aboard, plus all the usual crap I have in all the lockers, and the guy leaned out the window and yelled, "according to my weight monitor, she's exactly 13 tonnes.")

You do know the crane weigh monitors are not that accurate. Most of them do not work, let alone calibrated. Also call me cynical, but if someone tells you it's "exactly" 13 tonnes, I highly doubt it.

The data for a Formosa 41 has a dry wt. of 28,000 lb and can carry 60 gal. of fuel/120 gal. of water. With full tanks, you are at 29,380 lb or 13.11 tonnes (long). Add lots of stuff accumulated over the years, you'd be pushing at least 32,000 lb (14.28 tonnes).

Not trying to be rude here and tell you your boat needs to go on a diet, but have you needed to change/increase the depth of the waterline over the years? This is the tell.

It will be better to use a higher displacement wt. than a lower one for the prop calculations. This will help you dial it in better.

[Bobby Bluegum]

Ouch Bill, you got me there! Yes, have to admit that after finding marine growth starting to creep along the waterline (non anti-fouled), I did raise the anti-foul level a few years back, so good point you make. For the record, my bilge fuel tank takes a bit over 300ltrs, plus there's 80ltrs in each of the cockpit auxiliary tanks (port & Stbd) and my water tanks take a total of 140ltrs. So they've obviously been changed at some time before I bought her 16 yrs ago. I have now booked a haul-out in mid April for anti-foul and prop mods, will report back on any improvements. (I will also have a sniff around to see if there is a s/h replacement one that may do the job.)

[kmacdonald]

I used 30000 lbs in my calculations.

[kmacdonald]

Go to ebay and search sailboat prop. There are a lot there that will work.

[EWOL Props]

Definitely this is a too big prop.
We would recommend an 18"

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?

[Bobby Bluegum]

Thanks EWOL, and what pitch?

Bob~

[EWOL Props]

Sorry Bob, I did not indicate the pitch because we are used to supply props that can be optimized on installation at different pitches.

Anyway if you stick to a fixed blades propeller...your boat has a 13 Tons displacement, so I'd suggest a pitch of about 9"

[Bobby Bluegum]

Thanks Ewol, appreciate that. Will mention those figures when talking to the prop guys here in Sydney.
Best,
Bob~

[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.

[Sailorman97]

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?

This is a problem. You are killing that motor and using lots of excess fuel. The engine is overloaded. Eventually you will destroy it

In neutral with the engine warm push the throttle all the way forward. Do not worry you will not hurt the engine The rpm you see at this point is the rpm you must achieve underload. If it can not make it then your engine is overloaded and it is overloaded at all speeds.

I imagine if you open the throttle full and leave it the engine will over heat and blow black smoke

What info i could find says 3600 rpm. You should be able to make 3600 rpm in any condition, dead flat or 10 ft waves.

You should be running at 2160 as a minimim rpm or you wiĺ glaze the cylinders. You can run it all day at 3000 rpm and not hurt it and 3600 for short bursts say 1 hr.


In general if you take 1 inch off the diameter you will raise the rpm 150. You need to find 1800 rpm so you need to take 12 inches off the prop. Clearly that is not doable. So you need a new prop.

You can get a new prop now or a new engine and a new prop later

[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.

[Bobby Bluegum]

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.

Yes Bill, I now realise the boat is closer to 14 (metric) tons than 13 and will pass that on.

[billdomb]

Originally Posted by Bobby Bluegum

Hmmm, I've just discovered a site where you can plug in your boats details and it will calculate a size for your prop.

Interesting. What WAS that site?

[Bobby Bluegum]

Originally Posted by Sailorman97

This is a problem. You are killing that motor and using lots of excess fuel. The engine is overloaded. Eventually you will destroy it

In neutral with the engine warm push the throttle all the way forward. Do not worry you will not hurt the engine The rpm you see at this point is the rpm you must achieve underload. If it can not make it then your engine is overloaded and it is overloaded at all speeds.

I imagine if you open the throttle full and leave it the engine will over heat and blow black smoke

What info i could find says 3600 rpm. You should be able to make 3600 rpm in any condition, dead flat or 10 ft waves.

You should be running at 2160 as a minimim rpm or you wiĺ glaze the cylinders. You can run it all day at 3000 rpm and not hurt it and 3600 for short bursts say 1 hr.


In general if you take 1 inch off the diameter you will raise the rpm 150. You need to find 1800 rpm so you need to take 12 inches off the prop. Clearly that is not doable. So you need a new prop.

You can get a new prop now or a new engine and a new prop later

You could well be right with everything you say, however, I need to say this: I have now run the motor for the best part of 2500 miles with this prop running the motor at around 1400rpm either motor-sailing or cruising when the wind is too light and according to the mechanic that recently serviced it, it's running like a swiss watch. It's a yacht...not a stinkboat, I enjoy the slow pace and I enjoy the lack of a noisy motor which is why I went for a bigger prop in the first place. I know of another yachtie with a 4108 who's had it for some 16 years and says he rarely runs the motor above 1350-1400 and it still (like mine) starts instantly, has no black smoke and uses around 2.5ltres/hr. The mechanic says "you yachties, ya never run yr motors at correct revs!" and yet he can't explain why the motr still ticks over without any troubles. As I said originally, there is occassion when I need some extra grunt, but so far, it's been an occasional minor inconvenience....altho one I now want to address, which is why I've put up this thread...and why I'm taking the prop back to the prop maker to get it adjusted to the max possible. If that doesn't make much difference, then I'll get a new one made up.
Just as an aside, the (original) Perkins tacho has a green section which is presumably the low and high revs the motor should be run at, which starts at 1500rpm and goes to 2500rpm, although the dial continues up to 3500. So I presume from that, the 'red line' for that particular model (circa 1973) is 2500rpm. There is absolutely NO WAY I'm going to run a 50yo motor (despite that it was re-built in 2006) up to 3500, or attempt to cruise the boat at 3000rpm. Why on earth would I? I'm not trying to win races...those days are long gone (yep, I was once the Folkboat champion of my state) As I see it, as long as I'm running the motor at the low end of the green band on the tacho - I'm in the operating range of the motor...and especially so now that it has to work a little harder to swing the maxi-prop around. There is no black smoke at 1500, (but it does put out a dark grey smoke at 1600 and as I originally bsaid, it won't rev above 1650.) It runs for days at that level, requires only a tiny bit of oil top up from time to time, so even when I sort out this prop issue, I will still be running the motor at the low end of the green section (1500-1600rpm) unless I need extra grunt getting over a bar or against wind, tide etc. I don't get any indication that I'm wrecking the motor and using it the way I do, don't anticipate that I will be any time soon, especially after the prop guys have detuned the prop somewhat....but we'll see.

[Nicholson58]

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?

Replace it or have it re-pitched. A good prop repair guy can flatten this out to perhaps 12. It’s clear I think the pitch is too aggressive. On calm water a shallower pitch will run faster but you will be able to develop the motor full power in crappy conditions.

We have an infinitely variable pitch prop 24” x 3 blade. We vary pitch by sea state and flatten it out for harbor maneuvering. Motor sailing we stretch it out. Too great a pitch slows the engine and in the extreme can stall the motor.