trim

[DaveA]

I have recently bought a new narrow boat and am concerned that it is not trimmed correctly. The bows seem high out of the water even with water tanks full. Is there a recommended "angle" for the boat when lying at rest. The high bow doesnt' look right and presumably causes the stern to drag along the canal bed??

[Gary Peacock]

Hi Dave

 

We would normally trim a boat to sit as near to level as possible with full tanks but trimming narrow boats ca be a bit of a juggling act.

 

Gary

[John Orentas]

Dave.

 

It is as much about appearance as anything else, the flat area above the propellor (the counter) should be a couple of inches into the water to stop cavitation and being below water will tend to prevent floating debris fouling the prop.

 

Generally speaking the deeper the boat is in the water the better it will handle. The hull will need to be ballasted horizontal in the water but in practise they look better slightly 'bow up'. As Gary indicated, full or empty tanks will make a big difference. You say you have just bought the new boat, is it fully fitted yet, generall fittings will be significant, even the personal bits and pieces which will live on the boat.

 

Ballasting and final trimming is usually done by adding and taking away concrete flagstones from under the floor, usually one complete layer of 1.5 / 2 inch concrete is about right, but there are lots of variables.

 

John Squeers

[David Schweizer]

There really is no "correct" trim for a narrowboat, although there is a more recent trend for boats to lie almost horizontally in the water. Boats built some while ago tended to have more of a "bow up" trim, similar to that of an empty working boat.

 

My boat, which was built 24 years ago, has a difference in draft of forteen inches between the bow and stern, it used to be more but I trimmed the front down by a combination of cast iron ballast plates under the galley units towards the front of the boat, and cast iron 56lb weights in the front well. I would like to bring it down a couple more inches but my front locker scuppers are now almost at water level so without some major engineering work it won't happen.

 

One of the advantages of a "bow up" trim is that you can nearly always get the front end in close to the bank even on a shallow canal when mooring. One of the disadvantage is that when you come across a shallow section in the canal channel, the bow tends to ride up unto it and leave you high and dry. A more level trimmed boat tends to bump into the shallow section and stop, making it much easier to reverse off.

 

The shape of the bow does have a significant impact on the visual effect of high bow. It is only my opinion, but if the bow is long with with fine lines then you can get away with it being higher up, but a bluff bow does need to be ballasted down to minimise the visual impact. At the end of the day it is a matter of personal preference, some of my friends refer to struggling up hill when walking from the back of my boat to the front, but I can live with that.

[GeorgeL]

Those who prefer a "bow-up" look would be better off getting the boat built with higher topsides at the bow. For best fuel efficiency the bottom of the boat should be horizontal.

Edited November 19, 2004 by GeorgeL

[DaveA]

Those who prefer a "bow-up" look would be better off getting the boat built with higher topsides at the bow.  For best fuel efficiency the bottom of the boat should be horizontal.

<{POST_SNAPBACK}>

[DaveA]

Thankyou to all of you replyed - info was very useful and i am already scrounging ballast!! One further question you may be able to help me with. When weight is added only at the bow will the boat pivot about some mid point with the stern rising by an equal amount?? Perhaps only Archimedes knows!

[John Orentas]

George.

 

In practise a horizontal trim can't be achieved for all circumstances. When a boat is underway the stern goes down a few inches depending on the speed through the water, it's distance from the bottom and the shape of the swim. Boats with very long swims have proportionally less bouyancy at the stern and the effect will be more pronounced. My boat at half power on a river goes down a full 12 inches.

 

If you trim your boat to be horizontal when underway, it will look terrible when stationary.

 

Dave A

 

Yes the stern will come up a bit, as the hull shape is not symetrical it's hard to predict how much. If you assume it will pivot about 2/3 of the hull length back you won't be far out, don't aim to get it horizontal, just get it looking right with the rear swim 1 or 2 inches below the surface. Nothing looks worse than a boat that is 'Bow Down'.

 

 

John Squeers

Edited November 20, 2004 by John Orentas

[GeorgeL]

George.

 

In practise a horizontal trim can't be achieved for all circumstances.  When a boat is underway the stern goes down a few inches depending on the speed through the water, it's distance from the bottom and the shape of the swim.  Boats with very long swims have proportionally less bouyancy at the stern and the effect will be more pronounced.  My boat at half power on a river goes down a full 12 inches.

 

If you trim your boat to be horizontal when underway, it will look terrible when stationary.

 

John Squeers

<{POST_SNAPBACK}>

 

That is why I said the topsides should be built up a bit. Those who cruise a lot will be concerned with minimising fuel consumption, which means keeping the bottom of the boat parallel to the bottom of the canal.

 

When the propeller is pushing the boat forward, it is sucking water from under the boat ahead of the prop, and this in turn sucks the boat down.

 

It would be very interesting to experiment with ducts with openings at the sides of the hull, funnelling water back to the prop. That way you would avoid pulling the stern down, but you might create other inefficiencies instead.

 

I really think that very little research has been done into this. Nowhere else do you have a situation where a boat is operating in such shallow water. Deap sea ship designs for improved efficiency may be largely or wholly irrelevant.

 

George

 

PS: My concern is prompted by the fact that I believe fuel prices will rise enormously in the next few years, provided China and India continue to grow at anything near their current rates.

[John Orentas]

George.

 

I'm not sure what the 'topsides' have got to do with it. Narrowboats despite their appearance above water are very streamlined craft, improvements in hull shape would be likely to increase efficiency by only a few percent. The propellor action is probably not the major influence, if you were to tow a narrowboat at say 8 mph it would doubless show the same charactaristics as if it was self-powered, it is the venturi effects below the hull that has the greatest influence.

 

Ships do experience these effects. The Herald of Free Enterprise disaster, it was stated in the enquiry that the ship went 3 metres lower in the water when passing over the sandbanks off Zebrugger.

 

If we are looking for greater fuel economy we would have to be looking at the profile of the hull, adding chines, reducing the displacement and therefore the weight.

 

John Squeers

[Richard Bustens]

This is getting very serious surley a couple of go faster stripes would help ?

[John Orentas]

Hi Richard.

 

Don't be silly, how could stripes make you go quicker. I shouldn't be saying this, I haven't patented it yet, but I am working on an aerofoil wing to fit on the back of the tiller arm, that should make a difference.

 

 

John Squeers

 

Ps. I tried that www number I gave you, not much there. I am sure if you ring them they will send you a catalogue and price list.

Edited November 20, 2004 by John Orentas

[Richard Bustens]

WOW John

I want one of them.

Can you make me one for when i get the light coz the light has a big surface area and will slow down the boat with all the drag it creates so the aerofoil will help replace equlibriam

[Paul Evans]

Take no notice of John's comment about go faster stripes. It's a well known fact that strips of chequered plastic make things go faster. I remember all those Mini Coopers - one stripe added at least 10mph to the perceived top speed. A friend of mine tried it on a Ford Prefect and, after he had thrown four passengers out, proved it - at least downhill! Did wonders for the trim as well.

 

Couldn't you get a pointy cone - bit like Concorde's nose - to strap over that big lamp you covet. That would save John fabricating an aerofoil for you. Or perhaps you could have both. But watch the weight - you have to keep your boat trimmed.

[Martin]

I adjusted what must amount to a couple of tons of ballast on Octarine - just to get her somewhere near the bank against which we moor.

I have found that removing bow end ballast doesn't have an appreciable effect on the stern depth - the boat doesn't really pivot in that way.

This may be due to her overall weight being around 28 tons, or it may be common to all boats.

 

Additionally - and this is a gripe I have which is similar to those who insist on the proper use of apostrophes (and I am one of those too!) - CAVITATION (referred to earlier in this thread).

 

The effect being referred to here is VENTILATION and not cavitation.

This occurs when the prop is not deep enough in the water and draws air down from the surface resulting in reduced effectiveness.

Cavitation is found more on speed boats and higher speed cruisers when the water practically boils on the leading edge of the prop. You then get a pocket of air ahead of the prop blade which again reduces efficiency.

 

Ventilation is cured by lowering the prop in the water (generally) and Cavitation by changing prop geometry or cleaning the leading edge of nicks and dents.

Of course, you can cause ventilation by using too many revs for your speed through the water proving again that less can indeed be more.

 

Sorry to be pedantic, but there you have it!

[Tony Ross]

Just a quick note of caution to those re-adjusting the balast in trad style craft with integrated handrails. Bear in mind that the scuppers in the hand rail would have been cut out by the boat builder at the lowest point on the roofline so as to let rain water drain off. If you adjust the balast, this can lead to water pooling on the roof either before or after the scuppers and if left over a period of time can lead to rust on the roof.

[John Orentas]

Martin.

 

I think the word cavitation has now been absorbed into the language to describe the phenomenon of air interfering with the efficient operation of a propellor, if I was to talk about 'ventilation' very few people including myself would know what I was talking about.

 

Yes cavitation can happen with little or no interaction with the surface of the water, commonly on fast boats a badly designed prop. which does not accelerate the water smoothly will actually 'pull' a vacuum.

 

I'm not too good with punctuation, but I do try.

 

John Squeers.

[Big COL]

Ok guys

 

whats the correct terminology when applied to pumps?

cavitation or ventilation

[Paul Evans]

Are they sucking or blowing? Or could it just be hot air?

 

I like 'cavitation' because I know it's something to do with the prop.'Ventilation' is what you get when you open the window (or shot full of holes). But thank you Martin for your usual erudite explanation and for expanding my fund of knowledge.

 

I have the problem Tony mentions when I empty my water tank for the winter (or at least half empty it) and fill the diesel tank to stop/reduce condensation. The bow is at least 3" higher and the stern an 1" lower. Ah well! Such is life.

[GeorgeL]

George.

 

I'm not sure what the 'topsides' have got to do with it.

 

John Squeers

<{POST_SNAPBACK}>

 

 

I'm simply suggesting that rising topsides at the bow would enable the boat to look right whether going ahead or stationary (when the bow tends to be lower).

 

I don't know what the answer to all this is, but I get the impression that nobody else does. Do you have any hard evidence to assert that narrowboat hulls are already very efficient, or is it just conjecture? If they are so efficient, are Alvechurch lying when they say their bulb is more efficient?

 

All I know is that normal NB hulls seem to produce a lot of wash compared to good seaboats of the same length going at the same (low) speed. It might be purely subjective, it might be because of the shallow depth, it might be something else. Is there a book on NB hull design?

 

George

Edited November 22, 2004 by GeorgeL

[David Schweizer]

Unless you have seen a sea going boat travelling on a canal with a depth of 3ft, I don't know how you can make the comparison George. The only "Sea Boat" I have seen on the canals is my friends genuime vintage Dutch Barge and that makes a hell of a wash!! One thing I do know is that narrowboats make far less wash on a wide deep river than most of those boats that are built specifically as river craft.

 

I suspect the reason that any boat makes a wash on the canals has got as much to do with the space between the bottom of the hull and the bed of the canal, asd it has to do with the shape of the hull (although hulls with longer swims tend to create less wash) If the water cannot pass the hull under the boat or to the sides, it will pass the boat on the surface ie. create a wash. Anyway that's my suggestion, no doubt someone else will prove me wrong.

[John Orentas]

George.

 

I didn't intend to brush aside your comments re the cabin heights I know you were refering to the aesthetics of the thing. The point about hull shapes, a narrowboat looks pretty crude and ungainly when viewed from above, but the important elements under the water are generally are very streamlined, quite canoe shaped in fact, probably as 'fine' as the better seaboats.

 

The Alvechurch bulge was discussed earlier and most people I think considered that it failed in it's attempt to replicate the bow of a super efficient ship for various reasons, so the exercise was not valid, I don't believe the experiment was a success though the company must be applauded for having a go.

 

John Squeers

Edited November 23, 2004 by John Orentas

[Martin]

Mr O:

"I think the word cavitation has now been absorbed into the language to describe..."

Quite right!

This is because everyone has got it wrong!

This doesn't make it right!

 

This bugs me like people who call those little three-wheeler cars "Robin Reliants"

They are Reliant Robins!

Reliant is the company name; Robin is the model.

It's like calling a car a "Focus Ford" or "Astra Vauxhall"

 

I did apologise for being pedantic!

Please don't take offence!

It's just me being picky!

[John Orentas]

Hi Martin.

 

Did you see any of that TV series, 'Grumpy Old Men'. I found myself nodding in agreement constantly. A bit frightening really.

 

One memorable quote, "Spending to much time staring into a computer screen, turns your brain into a mush".

 

John Peel going on about car radios that interupt your listening, to tell you about traffic congestion in Chipping Sodbury just as a who-done-it play is coming to a conclusion.

 

John Squeers

Edited November 23, 2004 by John Orentas

[rustyduck]

OK, here goes with my two-pen'th.

 

Narrowboat hulls ARE very efficient, but they could be better. The hull speed for a displacement vessel is the maximum speed that the hull can go before the craft will sink into the trough left by it's own bow wave.

 

This speed (in knots) can be found as the 1.34 * square root of the waterline length in feet.

 

So a 70ft narrowboat has a theoretical maximum hull speed of 11.2 knots.

 

Width is limited in our case, but that's OK as long thin hulls have less drag than short fat ones.

 

The only real improvement in efficiency can come from reduced wave production (think of turbulence as just waves underwater). The smaller the waves produced, the better. Cross sectional shape will have little affect as the operational speed is so far below the maximum hull speed.

 

So now we're down to bow and stern shape. The swims in our case. Bow up or down attitude will effectively change the swim shape, altering the the size of waves produced.

 

The bulbous bow will produce a small wave that interferes with the one created by the bow, reducing the overall size of the wave created, so less energy is wasted in making waves.

 

Nearly off topic alert - A Kort nozzle would probably give the best improvements in efficiency for a narrowboat plus it would add some protection for the prop.