Some help pitching a prop please.

[Opie91]

Did the current set up ever work? Here are a few things to check before you go after the prop:

- Fuel system (as mentioned above)
- Exhaust system. Make sure there is not too much back pressure.
- Transmission. Make sure it is going all the way in gear.

Good Luck...

[BriaF]

I'm on my forth diecon engine.
If you have engine issues or need some thing
Explained ?
Talk to them, they have been, helpful diligent & honest
In my dealings with them.
I highly recommend them to all !!!!

[transmitterdan]

Originally Posted by GILow

Thank you Transmitterdan, those are interesting numbers. Your prop size comes out a fraction smaller than what I devived using vicprop and one other bit of software (can't remember the name right now). They came up with a three blade 15.8 x 7.6

Would you mind telling me what figures you used to get the prop size you are recommending? I might be missing something.

Matt

I doubt if you are missing anything. I guessed at weight (30,000 lbs.) and rounded the prop numbers. Your numbers are close enough to experiment with.

[Cheechako]

Generally larger diameter props are more efficient. But your transmission ratio may be pushing you away from the general rule.

[GILow]

Originally Posted by RaymondR

I upgraded my engine installation from a Yanmar 3GM30, 27HP to a Kubota V2203, 50hp. It revs to 2,800 rpm.

The Yanmar had a box with a 2.36:1 ratio and was spinning a 15 1/2" x 11" prop. It would rev out when the prop was clean but stall down when the prop became fouled. The Yanmar revs to 3400.

I did not want to have to change the prop so put a box with a ratio of 1.52:1 box on and the V2203 revs out and in fact I now suspect it may be slightly under-propped which is OK as it provides a fouling allowance.

Whilst the prop you are spinning is quiet a lot bigger in diameter it appears to have a lot less pitch and you have quiet a bit more HP.

The point of this is that those calculations tend to look a bit iffy to me in comparison to what I am observing on the installation I have..

One of the give-aways on an over-propped engine is that they will blow lots of black smoke if you try to rev out the engine.

Diecon have been around for a while and I have not heard any complaints regarding their proficiency in marine matters. What do they say regarding the situation?

Has the boat been re-engined and what was in it before?

Raymond,

You make some very good points... but I am not sure I am reading this correctly. In order...

So your current engine actually revs at a very similar speed to mine, and the HP is lower, though not hugely so, and you are using much the same gearbox as I am. And you feel it may be under-propped with a 15.5 x 11". Hmmm... aside from any difference in the boats themselves (and I suspect a Roberts 39 is not all that different to a Swanson 42) that does suggest the calculated 15.8 x 7.6 would be very much too small, yes... a worry.

The current prop is bigger, but the 12" pitch is also bigger (steeper) than what you are using. Unless I am reading pitch back to front, but I think I have it right.

No black smoke at full throttle that I have observed, but I will double check. What currently happens is that the revs increase steadily to 2000 then stop increasing, no matter what I do with the throttle, but the engine develops a deeper "throaty" sound if I try to push past 2000. Not sure what that might imply.

Yes, Diecon know (knew) their stuff (gone now, bought by a QLD aircon company, but they still have guys around who are very helpful), and the guy who fitted out the boat really knew his stuff too, so I am worried about changing anything without a really good reason, but something is not right. S42s should do another 1.5 knots with just 75HP.

The boat was repowered back in 1986, and that included engine, gearbox and prop. The old engine was a Lysterfield (Is that the right spelling?) 50HP and it was just not enough.

Matt

[GILow]

Originally Posted by scoobert

unless i am mistaken, and i may be, they can change the pitch of a prop you already own.

Yes, you are right, you can have props repitched, and the price is reasonable too.

I have to concerns with it though. When pushed, they will admit that you don't end up with an optimal prop shape and secondly, the difference between what I have now and what the calculations say I should have suggests somewhat drastic changes would be required (think angle grinder) and I think that would exacerbate the problem of final shape, if it were possible at all with such a size difference.

Matt

[GILow]

I have just come up with a brilliant new business idea.

"Hire a prop". For people wanting to optimise their setup, somewhere you could go and hire half a dozen props around the right size and try them out before they buy.

Anyone want to setup the business? I'll be your first customer.

Matt

[Lancerbye]

I read this thread with some interest as I plan to change my props to a maxi-props in the future to get better sailing performance. I have a motorsailer that weighs about the same as yours maybe a little heavier. It has a 13 ft. 9in. beam and a modified shoal draft fin keel. It has twin engines but I only cruise on one at a time. My engines are derated to 72hp for marine use at 2500 rpm. It has 2:1 gear ratio and the props are 22x14. I have no trouble reaching 2250 rpm which is the max cruising rpm with one engine. That pushes me along at 7.5 to 8 knots. I normally cruise at 1500 rpm which gives 6.5 knots. these are 3 blade props. So when I read the numbers you posted I have a hard time understanding them. My neighbor with the same engine and gear ratio on a bigger and heavier boat had a 24 x 12 prop and changed to a new high efficiency prop which is 20 x 15. He reaches max rpm and does 6.5 knots at 1800rpm. With the size of the prop you posted even with the different gear ratio you should be easily able to get max rpm. Have you checked that the shaft turns easily? No trap wrapped around the shaft etc.

[GILow]

Originally Posted by Lancerbye

I read this thread with some interest as I plan to change my props to a maxi-props in the future to get better sailing performance. I have a motorsailer that weighs about the same as yours maybe a little heavier. It has a 13 ft. 9in. beam and a modified shoal draft fin keel. It has twin engines but I only cruise on one at a time. My engines are derated to 72hp for marine use at 2500 rpm. It has 2:1 gear ratio and the props are 22x14. I have no trouble reaching 2250 rpm which is the max cruising rpm with one engine. That pushes me along at 7.5 to 8 knots. I normally cruise at 1500 rpm which gives 6.5 knots. these are 3 blade props. So when I read the numbers you posted I have a hard time understanding them. My neighbor with the same engine and gear ratio on a bigger and heavier boat had a 24 x 12 prop and changed to a new high efficiency prop which is 20 x 15. He reaches max rpm and does 6.5 knots at 1800rpm. With the size of the prop you posted even with the different gear ratio you should be easily able to get max rpm. Have you checked that the shaft turns easily? No trap wrapped around the shaft etc.

Yes, those figures you give tend to suggest there is some other problem, but the prop spins freely enough, actually starts to freewheel at about 2.5 knots boat speed, which is really annoying as the Velvet Drive box does not allow you to lock the prop.

Had the boat out of the water a few weeks back, and there was not time to clean the prop, but I did think the fouling was only very light. Might be that is having more effect than I expected. (In which case I am going to look like a right idiot if cleaning the prop solves this.)

The two problems I have with this are:

a) When the prop was known to be spotless, I don't remember getting the engine over 2000 rpm, but then I never tried, so I don't know. (I didn't try as I had concerns about the cooling system at the time.)

b) Every prop calculator I try, and the two propellor suppliers I phoned here in Oz, all say the prop I have is way, way too big.

Finally, a possible difference between your current setup and mine might be the torque available. Apparently the Nissan ED 33 was reasonable for torque, but do you have a comparable torque curve for your engines? Could be that at 2000 rpm your engines could be putting out a lot more power than mine. I noticed the power curve for my engine is much flatter than I expected, so at 2000 rpm I am only getting something like 55HP. Your engine might be putting out much closer to 70HP at the same revs.

That would account for a lot.

Finally, in all of this, I do notice that my engine is maxing out at peak torque, which is a strange coincidence and makes me wonder if this was by design. Strange design if so, effectively limiting a 96HP engine to 55HP...

Matt

[GILow]

Originally Posted by Lancerbye

... I have a motorsailer that weighs about the same as yours maybe a little heavier. .

The only figure I could find for the Lancer 44 was 23,000 lbs or about 12 tons. We weigh between 15 - 17 tons.

Or is that figure for the Lancer wrong?

Matt

[GILow]

Originally Posted by Opie91

Did the current set up ever work? Here are a few things to check before you go after the prop:

- Fuel system (as mentioned above)
- Exhaust system. Make sure there is not too much back pressure.
- Transmission. Make sure it is going all the way in gear.

Good Luck...

Good check list there, but all are good, fuel system had better be right, I just completely replaced it. ALL of it. $$$ Ouch!

Trying to find out if she ever went over 2000rpm, but orignal owner has gone walkabout. (Fine Ausse tradition). I've sent him an email, he may shed some light on the matter when he reappears.

[GILow]

Originally Posted by Lancerbye

My engines are derated to 72hp for marine use at 2500 rpm. It has 2:1 gear ratio and the props are 22x14. .

I plugged the Lancer numbers in to Vicprop and got a recommendation of 20.5 x 17 for your setup, which is not far off what you have. (I guessed 10 knots desired maximum speed, that might be a bit conservative and two feet of moulded hull depth, may also be conservative.)

The point is, based on Vicprop coming up with something close to what you know works ok on your boat, it is a worry that it recommends something a LOT smaller for mine.


Matt

[Lancerbye]

There is a big difference between the manufacturers displacement numbers and the actual weigh of my boat. It sits 6 in. lower in the water than the original markings. The government registration ratings was 18 tonnes gross weigh, I think the displacement of 23000 that I have also seen, was for a pretty bare boat, with empty tanks. Maybe I have too much stuff in mine. There have also been a number of upgrades done to this one over the years which have probably resulted in some added weight. I got a quote from the guys who sell the Maxi prop and there was no change from there calculations. I would have expected that a heavier boat should require a bigger prop not smaller. The neighbor went with a reduced size and pitch calculations from a reputable prop dealer and had a hard time getting half of the hull speed. He brought it back and they increased the pitch from 10 to 15 and now it works great. From that little experiment I had to deduct that maybe prop calculations don't always work.

[JPA Cate]

Matt,

That big old wheel you're towing around really will slow you down. Have you considered a 3 blade folder? Jim is quite pleased with our Flex-O-Fold. Ordered it out of Massachusetts in the States.

Ann

[GILow]

Originally Posted by Lancerbye

There is a big difference between the manufacturers displacement numbers and the actual weigh of my boat. It sits 6 in. lower in the water than the original markings. The government registration ratings was 18 tonnes gross weigh, I think the displacement of 23000 that I have also seen, was for a pretty bare boat, with empty tanks. Maybe I have too much stuff in mine. .

Yes, 6 inches lower seems suggest a fair bit of "stuff". Actually, a quick crude calculation suggests that your are therefore displacing another 6 Tons! Yikes, that's a LOT of "stuff". What the heck did you add to your boat?

Originally Posted by Lancerbye

The neighbor went with a reduced size and pitch calculations from a reputable prop dealer and had a hard time getting half of the hull speed. He brought it back and they increased the pitch from 10 to 15 and now it works great. From that little experiment I had to deduct that maybe prop calculations don't always work.

Yes, well, plugging your slightly heavier numbers into vicprop came up with 20 x 14 which is a bit smaller than what you have found to work, so that supports the theory that the prop calculator might err on the size of too small.

Matt