Can I make my boat more efficient?

[nb celestine]

Hi, My boat is 57ft and I think it sits too high at the bow.

Taking a rough measurement against a Thames lock wall, the boat falls 7ins bow to stern. Coming down the Thames last week with a mate in a boat alongside, he was doing 1100revs and I was doing 1400revs for the same speed. We both have Beta 38s, PRM 150 gearbox, but his boat is 3ft shorter.

 

I know there is guite a few variables involved like shape of hull, shape of swim and props and other things I cant think of, but I,m just wondering if the fall is too much, ( I'm thinking more of 2ins front to back). Would levelling it cause me to have less drag and so be more efficient?

 

My prop is 18ins and his is 17 but he could have a more suitable pitch.

 

Maybe mine is sitting to deep at the stern. When we were in a lock and both put the boats in gear together his picks up a lot better than mine.

 

I havent measured how deep the cavitation plate sits in the water but I would guess about 3 or 4ins.

 

Should I leave well alone along the lines of,"If its not broke, dont fix it" Thanks.

[Chertsey]

I suspect it's the depth of the prop that matters, rather than the differential between front and back. I'm in trouble otherwise, drawing about three foot at the stern and next to nothing at the front! And as long as there's enough water, we seem to go along pretty efficiently.

[nb celestine]

I suspect it's the depth of the prop that matters, rather than the differential between front and back. I'm in trouble otherwise, drawing about three foot at the stern and next to nothing at the front! And as long as there's enough water, we seem to go along pretty efficiently.

 

I might measure how deep his prop is tomorrow. When in the lock and both putting our boats in gear, his churns out more water than mine. Is it the deeper the better? or maybe he just has a better pitch.

[alan_fincher]

Any number of extra things to consider here.....

 

The pitch of the prop can have as much effect as the diameter, so unless you know it for each, hard to say they are in any equivalent.

The PRM box comes with different options for reduction ratio, I think. Do you know they both have the same reduction ?

Are the hulls of a similar profile - some slip through the water far better than others. Who built each ?

 

But for all that, the bit I think that may give a clue is the uxter plate, (I assume that's what you mean by cavitation plate) being 3 or 4 inches underwater, if that's a "while static" measurement. In my view it is better to have a leisure narrowboat ballasted with it's uxter plate only just immersed when static. They dig in a few inches when fully underway anyway.

 

I do not think the "bow high" thing detrimental. Our more modern boat Chalice is very "bow high", but "takes off" and generally gets along a lot better than many that are ballasted more level.

 

In terms of "taking off", don't forget that if you over-ballast a boat, all that extra weight has to be got up to speed, (and you'll need the engine to work harder to bring it to a halt). I think any obsession with ballasting level is more about appearance than performance. Or that's how I see it!

[Chertsey]

I'm not an expert but I think there are lots of variables and it is probably something of a black art.

[bizzard]

I might measure how deep his prop is tomorrow. When in the lock and both putting our boats in gear, his churns out more water than mine. Is it the deeper the better? or maybe he just has a better pitch.

Deeper the better.Waters denser less unruffled and less likely to draw down air,and cavitate. more grip on the water.

[alan_fincher]

Deeper the better.Waters denser less unruffled and less likely to draw down air,and cavitate. more grip on the water.

But you have to propel all that extra weight along, and displace all that extra water.

 

If that theory held, a loaded narrowboat should get along better than an unloaded one.

 

They don't, (even on the Thames!).

[DickBrowne]

Our experience on Pinmill is that pitch is very important. We changed from a 15x8 egg whisk to a 15x12 and dropped our cruising revs from 2000 (oh yes!) to 12-1300, far more relaxed. The new prop has bigger bats also, which helps.

[nb celestine]

Deeper the better.Waters denser less unruffled and less likely to draw down air,and cavitate. more grip on the water.

There seems to be a difference of opinion here. One says I should remove ballast to lift the uxter plate, one says the deeper the better.

 

Its tricky , I suppose.

 

His is a Liverpool shell, by the way. Mine a Price Fallows. Dont know what the gearbox reductions are on each boat.

 

Thanks for the replys

[bizzard]

But you have to propel all that extra weight along, and displace all that extra water.

 

If that theory held, a loaded narrowboat should get along better than an unloaded one.

 

They don't, (even on the Thames!).

I wasn't concerned about weights and loads,I was just stating that a propeller is at its best efficiency deep and so less disturbed by surface popple,wavelets ect.

What you mean is, the weight and extra shove through the water although the props deeper they will cancel each other out.

A cargo ship unladen and not in ballast usually have their prop tips threshing around out of the water.So if traveling empty have to take on water and or fuel ballast,obviously for stability purposes and secondly to get that prop down deep so that it grips.

[Iain_S]

When in the lock and both putting our boats in gear, his churns out more water than mine. Is it the deeper the better? or maybe he just has a better pitch.

 

Or is using a higher power setting. (Trying to avoid "throttle" ) Are both tachometers accurate (or inaccurate to the same extent)?

Iain

[alan_fincher]

There seems to be a difference of opinion here. One says I should remove ballast to lift the uxter plate, one says the deeper the better.

Well are you only going to use it on the Thames ?

 

Having the base of the boat maybe 4" closer to the bottom of a big river than it strictly needs to be may not make a big difference. But it will not half slow you down on some shallow canals.

 

How far into the water is your friends uxter plate, compared to yours ?

 

If you do try pushing it deeper in, make sure you have enough "side" on your weed-hatch before removing the cover!

[nb celestine]

I'm not an expert but I think there are lots of variables and it is probably something of a black art.

 

 

Brian Moore, the ex England hooker talked about the Black Art Club. I thought he was referring to the front row of a scrum.

 

Not so sure now.

[sebrof]

But you have to propel all that extra weight along, and displace all that extra water.

 

If that theory held, a loaded narrowboat should get along better than an unloaded one.

 

They don't, (even on the Thames!).

 

You are talking about different things.

 

All else being equal, a deeper prop will bite the water better.

 

If the price of a deeper prop is a heavier boat sitting lower in the water, then obviously the increased drag will counteract the more efficient propellor.

[bottle]

Hi Celestine

 

My friend and I have the same shell builder and length, same engine, different fit outs.

 

He travels at 1100 rpm and I at 1250 rpm, for the same speed over the water.

 

What we do not know is whether the props are different, if the boats weigh different and a lot of other things.

 

I will be contacting Crowthers with a mind to improving my 'bog standard' prop.

[Bob18]

One thing to remember is that most modern diesels are more efficient at a sweet spot in the rev range, for most this is in the region of 1200-1600 rpm. A better guide to engine efficiency would be the fuel burn not engine revs...

[The Pipe]

Deeper the better.Waters denser less unruffled and less likely to draw down air,and cavitate. more grip on the water.

You’re mistaking ventilation of a propeller for cavitation two totally different animals.

As for the rest.

Yes a fully loaded working boat will probably be still slower on the Thames than when unloaded this does not mean it is working more efficiently in that state just that it is underpowered for its displacement.

A 70 foot boat weighing 15 tons in simple terms has to shift 15 tons of water to someplace else to move 70 foot, put 25 ton of coal in it and you now have a 40 ton boat that has to shift 40 tons of water to move 70 foot. It would be interesting to see the comparison between a working boat at 15 tons and 40 tons by taking a relationship between fuel consumption/ speed and your displacement , you may find you are running allot more efficiently fully loaded than with your bows up in the air.

My own boat works better at 24 tons for a 63 foot boat if I could I would push the head down even more and I think she would be more efficient through the water.

[Chalky]

Rather than looking at engine RPM, drag, props etc why not look at what really matters - specific fuel consumption. (Just noted Bob18 says the same).

[The Pipe]

Rather than looking at engine RPM, drag, props etc why not look at what really matters - specific fuel consumption. (Just noted Bob18 says the same).

Correct but it does not mean that changing the trim of the boat by putting extra in ballast to pull the head down will use extra fuel, it may use less because it is more efficient in moving the water from front to back.

All you can do is try it and see.

[international_boatbuilder]

 

My prop is 18ins and his is 17 but he could have a more suitable pitch.

 

Thanks.

 

Not rocket science, your boat should be swinging a 17x10.

Don't believe me? ask Crowthers or Beta themselves.

[bizzard]

You’re mistaking ventilation of a propeller for cavitation two totally different animals.

As for the rest.

Yes a fully loaded working boat will probably be still slower on the Thames than when unloaded this does not mean it is working more efficiently in that state just that it is underpowered for its displacement.

A 70 foot boat weighing 15 tons in simple terms has to shift 15 tons of water to someplace else to move 70 foot, put 25 ton of coal in it and you now have a 40 ton boat that has to shift 40 tons of water to move 70 foot. It would be interesting to see the comparison between a working boat at 15 tons and 40 tons by taking a relationship between fuel consumption/ speed and your displacement , you may find you are running allot more efficiently fully loaded than with your bows up in the air.

My own boat works better at 24 tons for a 63 foot boat if I could I would push the head down even more and I think she would be more efficient through the water.

They both amount to the same effect on the prop,Ventilation by air drawn down or a propeller either too small and or revolving too fast for a heavy displacement and so its grip efficiency on the water breaks down,can't shift the water aft anymore to provide propulsion so just makes the water boil,nowt doing,cavitation. This situation puts huge strain and stress on a propeller,a high frequency vibration and has been known to shatter them. bizzard.

Edited October 9, 2011 by bizzard

[onionbargee]

When the flat bottomed narrowboat's bow is higher than the stern the bottom plate is like a ramp forcing through the water, when it's level there is no obstruction to the water flow under the hull. this shows up as extra wash, which is wasted energy. If you rip the arse out of an unloaded working boat, with some ballast up the back end the wash is outrageous.

 

that's my pet theory, if anyone can rip it apart then go ahead.

[bizzard]

When the flat bottomed narrowboat's bow is higher than the stern the bottom plate is like a ramp forcing through the water, when it's level there is no obstruction to the water flow under the hull. this shows up as extra wash, which is wasted energy. If you rip the arse out of an unloaded working boat, with some ballast up the back end the wash is outrageous.

 

that's my pet theory, if anyone can rip it apart then go ahead.

Quite correct its trying to climb over its own bow wave. You'll notice that the faster you try and go like that the bow wave moves further and further aft.

[alan_fincher]

When the flat bottomed narrowboat's bow is higher than the stern the bottom plate is like a ramp forcing through the water, when it's level there is no obstruction to the water flow under the hull. this shows up as extra wash, which is wasted energy. If you rip the arse out of an unloaded working boat, with some ballast up the back end the wash is outrageous.

 

that's my pet theory, if anyone can rip it apart then go ahead.

It's a theory!

 

An alternate one is that with the bow barely in the water, but the back end deep in, the whole of that very gently ramped bottom is pushing the water away in a way that distributes the amount being moved sideways along (nearly) the whole length of the boat. There is no point along it's length where masses of water all need pushing out the way in a short distance.

 

With the bows down three foot or more in the water loaded, the whole lot has to be pushed sideways in just the length of the bow and associated swim.

 

Hence a loaded boat can create a far bigger wash at the same speed than the bow in the air empty one.

 

Well that's another theory!

 

If I'm honest, I think both these extreme theories probably don't stand full scientific scrutiny, and the actual situation is a lot more complex than such simple explanations.

 

I think there may also be issues (in either example) about "refilling the hole" that the boat is moving forward out of, and this can sometimes cause greater difficulties than pushing the water out the way in the first place!

[bizzard]

A displacement boat given a huge amount of power and not able to climb completely over its bow wave and onto the plane will just bury itself in its own hole and sink,Known as maximum displacement hull speed.