ruddery sterny thingamy type question(s)

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monkey_trousers

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bf494.co.uk
Finally got Pansy out o the water at the weekend for a scrub and a once over. She is really in remarkable condition below the water line, nothing amiss apart from the odd inch or 2 of loose putty in a couple of seams.

one of the reasons for the haul out was also to have a look at how she's propped as we're pretty convinced its all just plain wrong

for what its worth, 120hp ford engine (rated 2700rpm) 1.5:1 box, 22" prop on a 2 " shaft, max revs we get is about 1600 and average 6 or 7 knots tops

now its possible that it might not just be the prop/box that is slowing her down and making her thirsty. I'll try and describe the set up, so forgive me if the terminology goes a bit wonky!

immediately astern of the prop is a large steel upright section between the keel and the outrigger over the prop cutout, the rudder isn't hung on this, its pinned at the keel and and on the outrigger above. Maybe it was put there as support for the outrigger??

I would of thought there would be nothing between the prop and the rudder, save perhaps a deadwood of some sort?

this steel upright is about 6" across, so I imagine it is going to deflect a large amount of the thrust from the prop sideways??

plus it explains why steering is a bit sluggish as the rudder has to move a good 15 to 20 degrees before its starts to clear the slipstream caused by this upright

should it actually be there??

the builder that looked at it reckon not, although he has plenty of experience with MFV's fishing boats of this type he's never worked on anything zulu or fifie ish, the nearest he gets is large Cobles

just wondering if anyone has any experience of this?? could it be removed perhaps? its certainly not original, or maybe replace it with something that is narrower, say the same width as the rudder

My limited knowledge suggest that if it were gone, or slimmed down considerably, the thrust from the prop would increas a great deal, helping with top speed and fuel economy and steering would be a lot better as well, or am I barking up the wrong tree!

will try find a picture to illustrate what I'm probably not describing very well!
 
I think you are correct. I used to own a 45ft Fifer and that nothing between the propellor and the rudder. I agree that if it was needed for strengthening then it should be streamlined as much as possible.
Perhaps someone on here can tell us more.
 
It certainly sounds as though the steel upright shouldn't be there. However, I am tempted to ask why was it put there? Was there some untoward movement of the deadwood which it was intended to correct? If the 6" dimension is transverse to the centreline, this really does sound dead wrong, and should be corrected. There should ideally be nothing between the prop and the leading edge of the rudder. Often non-engineering folk use steel in sizes that "look strong enough", although they are usually oversized by a factor of 10. It does sound like a bodge job. As usual, a picture is worth a thousand words.
Peter.
 
I forgot to take my camera down with us, so a friend took a load for me, waiting for him to email them

I did think that it should possibly not be there, the outrigger/stern post is rock solid. In fact we we were utterly delighted at her condition below the water line, not a nail out of place, every plank is rock solid and straight as a die.

Ideally we'd just pull this contraption off and see what happens, but the paranoia of 'why' it is there in the first place might make us fabricate something in its place that is at least a lot narrower - the width of the rudder say at most

releived that folks seem to agree that it sounds wrong though, It must be stealing a lot of our power and thrust from the prop hence the slow speeds and high fuel consumption

will try get the pics today
 
and finally!.....

rudder.jpg


that prop does look weeny down there
 
I think you should leave it there for at least 4 reasons;

1. even if it does disturb the prop outflow, that has nothing to do with the ability of the engine to develop max RPM. That is limited by the propeller pitch. Only once you get your engine developing its RPM can you start to worry about whether your hull is draggy. With your reduction gearbox, the propeller pitch should be small. With a slower prop of around 700 rpm (normal for this size of vessel) the pitch would be around the same as the diameter. Incidentally, I think the prop. dia is a bit small for this size and weight of boat.

2. You should think of this "vertical" support as akin to the skeg you see in front of the rudder on modern fin keeled sailing yachts. That skeg lines up the flow so that it goes over the unbalanced sailing rudder well. On a modern MFV the rudder is "balanced" (has part of itself forward of the rudder shaft). That forward part will create drag on a sailing boat which would like to sail with a small rudder angle. So if you ever want to sail this boat - the arrangement you have is a bonus, and one that others would pay to reinstate if it was not present.

3. The "vertical" post you describe is not needed to support the stern of the boat. However, I am sure it does a fairly good job of supporting the end of your keel (the skeg that supports the rudder). If you take away the "vertical" post, the only thing that braces the "keel extension" is the rudder ! And the rudder is the very thing that generates big side loads which will act on the keel extension/skeg.

4. the arrangement you describe was pretty normal for fifie motor boats, built as new - or converted later to motor. I think I can see it in an early colour picture of PANSY on slip somewhere in NE Scotland.

If you really want to reduce drag at the aft end of the boat, and you have sufficient space aft of the propeller, a faired wooden piece bolted onto the forward face of the offending post would not do any harm - and might do some good

Edited to add - this post was written before I saw the picture just posted - but conclusions remain the same.

The pic does show how small the prop is. If you could fit a 28 inch diameter propeller with at least 3" clearance above the top blade, that would give you a lot more prop area to get power into the water.
 
22" prop is too big for 120hp at 1.5 reduction.

with 1.5 reduction the engine should acheive 2450rpm with a shaft speed of 1800rpm.

That would require a prop around 18.5x11 and should give a hull speed around 9kts.

Figures calculated by Castle marine 'Propcalc' programme which I have found reasonably accurate, although I guessed at some of the figures. Propcalc suggests the gearbox ratio is not ideal, and the setup would be more effient with a higher ratio: try putting different ratios in to get a good combination. 1800 rpm is too high a shaft speed for a heavy dispalcement hull, and I would think you would do better with a 2:1 box when a 22x15 prop would be ideal at 1450rpm shaft speed. There is room to swing an even bigger prop, so you might consider 2.5 or even 3:1 gearbox with a correspondingly bigger and therefore more efficient prop for pushing a heavy hull. The lower the prop speed the less slip occurs.

Try it yourself with the actual dimensions of your boat at: http://www.castlemarine.co.uk/pitch.htm

I dont think your hull/keel configuration would make too much difference, and certainly would not cause the engine to slow down.
 
I think you are spot on Oldharry.

The reduction ratio is very small, which makes the prop speed very high. Far too high for a heavy boat like this.

It looks that someone has been forced to fit a small diameter propeller because they found out that the prop RPM was too high for a proper size. In fact it seems that the diameter and pitch are still too much for the engine.

If the diameter and pitch of the existing prop were reduced further, the engine might achieve higher RPM - but the problem then would be that the prop would probably be too small to develop the horsepower available from the engine.

A boat of this size should normally have a prop diameter of around 26 or 28 inches. That would require a slow turning prop (slower is always more efficient in any case). A 3:1 gearbox giving around 700 or 800 rpm at the prop would make sense.

Not least because I think she was originally built with a Kelvin K series engine - they had no reduction box, and a max RPM at prop and engine of around 750 RPM.

But the bottom line is - cannot fit a bigger prop unless the RPM is reduced.
 
we'd pretty much concluded a while back that the box is just plain wrong, I suspect the previous owner got a 'deal' on the engine and box together and tried to match a prop to suit, this engine/box would no doubt be great on a modern plastic boat, but not for us. On paper, 22" prop for that box/shaft speed is there or thereabouts, but being the big old tub she is, it's just not 'right'

plan is to find a 2.5 or 3:1 as suggested here (and by a couple of 'prop shops' I've talked to) and then up the prop size accordingly

If you look on the post, there are already a couple of bits of angle iron that have been welded on to give some fairing and deflection to the thrust, but that whole upright post thing is over 6" wide, so my uneducated conclusion was that it must be causing at least a certain amount of 'blockage' I had though that the in-ability to achieve rated rpm was that we might be over-propped, but maybe its down to the layout

Personally I'm a bit twitchy about the idea of removing it altogether, but might have a look at re-fabricating a replacement that is a lot slimmer possibly

thanks for the replies though it does confirm a lot of what I'd 'guessed'!

I suspect the course of action will be to get the box swapped first, then play around with props, then if we still feel the need, re-work the post

was offered a 28" prop while she was out but it was a right hander unfortunately. We did also dump about 4 tons of ballast at the weekend as well, which has improved things already, not so much speed, but certainly fuel economy. 'Proppy' issues aside she is in superb condition for her age though, even the builder that will be working on her with us was impressed!

We might just end up experimenting for a bit with props, trying a few to see what gives us the best overall balance of economy and speed.... any one got any 2" shaft left handers they want to lend us!
 
Yes, I was looking at the angle iron `fairing´ But there is room for a wood/fibre/epoxy nicely shaped foil to get the flow sorted for the rudder. As said, bigger prop, lower rpm. ( still concerned as to low engine revs on no load) Keep us informed.
Andrew
 
I thought that a picture of the stern arrangement of my boat may be helpful. This drives Kala Sona along at over 7 knots with a 16HP Albin AD21 engine. She is 27ft 5ins long and weighs in at around 5 tons. [image]http://s139.photobucket.com/albums/q319/KevinCroll/[/image]
 
Thank you. That is Sikkens Cetol. A rub down with wet and dry and two maintenance coats every year although I think that it may go to two years. The transom has always looked very good. I have done the toe rails with the same stuff and they still look good after three years.
 
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