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[kiwimodeller]

First off can somebody please advise the conversion formula to convert Kg/cm2 to PSI. My Engine instuctions say the engine should be run on 1.5 to 2 Kg/cm2 and I would like to know what that is in real figures. Now to the real problem - I have a Saito V4 engine in a nice slim easily driven Windermere launch. I built it quite a few years ago and as I started with just the engine and then a hull I had to source bits from all over the place. I was told that the boiler I bought which came from Cheddar and is I think a Puffin horizontal single centre flue (will check the paperwork tomorrow) of about 6" length and 4" diameter would easily supply enough steam for the 9mm bore by 10mm stroke four cylinder single acting Saito (2.54 cc). Well it has always been marginal. It gets up to 40psi quite quickly but any sort of decent run has the pressure back to 20psi or less very quickly and then I have to let it just tick over for some time while pressure builds up again. The problem is made worse by the fact that the engine will not reverse or self start unless there is good pressure. Recently I gave the boat a rebuild and now the problem is worse! I rebushed the shaft, descaled the boiler, fixed a couple of minor steam leaks etc. I have rechecked prop shaft for binding, rechecked the engine valve timing is at factory specs etc but she will not run more than a few yards before the pressure drops off. I tried today something which I have been meaning to try for a while, some wire mesh rolled up and inserted in the funnel as I feel too much heat goes straight out the top instead of transferring to the water in the boiler. I think it made a small difference and shortened the time to get up pressure but it has not solved the problem. The Cheddar burner has a Number 10 jet in it which is quite large but it burns with a beautiful blue flame both outside and inside the flue. Any suggestions on how to get a bigger volume of steam and hold pressure are most welcome, I dont have any hair left to tear out! Thanks, Ian.

[oldiron]

First off can somebody please advise the conversion formula to convert Kg/cm2 to PSI. My Engine instuctions say the engine should be run on 1.5 to 2 Kg/cm2 and I would like to know what that is in real figures. Thanks, Ian.

Ian:

 Try this web site: http://www.engineeringtoolbox.com/pressure-units-converter-d_569.html
  or this one : http://mdmetric.com/tech/pressurecvt.htm

essentially, to convert to psi from kg/cm2, multiply kg/cm2 by 14.22

John

[derekwarner]

Ian .... ..something is amiss between your boiler steam production & your engine steam consumption

Try the MacSteam boiler/engine calculation page..........http://www.maccsteam.com/Info/How_big.html

I trialed the calculations with my older ACS boiler prior to purchasing the Saito Y2DR [9CC engine].......

I have attached my calcs [bases upon the McSteam  tables & also the 711 cubic inches of steam at 40psi that I should produce]   .... which suggest I will be OK...especially considering the low shaft speed I will be using

BTW....the Japanese nominated kg/cm2 as the base unit of pressure whereas the Europeans nominated the Bar......so very close to being near identical for our purposes..

Let us know how you get on with the calcs.......  Derek  

[flashtwo]

Hi Derek,

Just to clarify one point, do you have to subtract the piston thickness from the stroke to get the swept volume?

Ian.

[oldiron]

Derek:

  I'm wondering if Ian isn't on to something with slowing the flue gas flow to allow more heat absorption and keep his stack temperature lower. I agree, the boiler size and heating surface would indicate a suitable size boiler, but if the flue gases are passing to the stack too quickly the boiler size may be for naught. I'm not familiar with the Puffin design boiler internally, externally it looks very simple with a straight through flue. I've seen many full size boiler use a swirled steel strip through the flues to slow do the flue gases and allow more heat absorption. This may be an option here too.
My thoughts.

John

[gondolier88]

I always found the Puffin boiler needed the exhaust of the engine to steam properly- on natural draft it didn't steam as quick. I discarded the old 28mm copper funnel and substituted it for a twin walled copper funnel- the inner one of smaller diameter than the standard 28mm one, with a proper blast pipe arrangement in the smokebox, the blast pipe being 1/8" dia.

Greg

[derekwarner]

flashtwo ....yes you are naturally correct that the double acting swept area of the piston on the rod side is minus the surface diameter of the piston rod .... however I suspect the MacSteam example is a ready reckoner & by disregarding this smaller surface area on the rod side we actually consume less steam......

oldiron ....I believe the subject of the older Cheddar boilers & premature heat loss via the stack [in a vertical boiler] was a subject on Paddleducks some years ago..........I will check later this evening.......Derek

[kiwimodeller]

Thanks for all the input so far, I must admit that it has me whacked. Using the 14.22 conversion factor quoted by John indicates that the Saito factory is saying the engine should run on only 21 to 28 PSI and it has run on fairly low pressure in the past but was prone to not wanting to restart easily when stopped or reversed. I checked the literature and the boiler is a Cheddar Puffin horizontal version 2. It measures up at 6" x 3 & 1/2" and I would have thought that the surface area was ample to run an engine as small in capacity (2.54cc single acting) as this one is. I would have thought the Cheddar engines for which the boiler was designed would be much larger capacity. I will do the Maccsteam calculation in the next day or two when work time allows. I was unaware of discussion about the Cheddar flues and funnels but it just felt like the exhaust temperature at the top of the funnel was much higher than on the other boilers I have and that this indicated poor transfer of heat to the water. I will try and get hold of a laser temp gauge to do some proper testing. I do not feel that there is much of a problem with draft through the flue Greg, the burner uses a Number 10 jet which is large compared to other burners of this size but it still seems to get sufficient air in to make a really strong blue flame and it does seem to make high temperatures judging by the exhaust temp. Good to thrash it out and I will keep on playing and reporting back. Cheers, Ian

[kiwimodeller]

Derek, if my conversions from metric to imperial are correct then the little Saito at 2.54cc is only .155 cu in. My boiler is the same pressure vessel size as the Maccsteam (3 & 1/2" by 6") which produces 711 cu in of steam at 40PSI. This then equates to being able to rev the engine to 4587revs which is well within factory limits for the engine and far more than it is doing in the boat under load. I can therefore only presume that the Cheddar boiler is not producing anywhere near as much steam as the Macc is rated at. My gut feeling is that the burner is producing ample heat but that the boiler is not getting that heat in to the water (hence the high exhaust temperature). It is interesting to read that the Macc boiler has a 35mm flue and a 28mm funnel. Malcolm Beak has in the past cautioned me against downsizing funnel or flue too much in case that leads to blowback of the burner but I am at a loss as to what else to try. The only other possibility I can think of is that there is some sort of internal fault in the engine that is allowing most of the pressure to go straight out the exhaust or to bypass the pistons and I will be trying to devise some way of checking for that sort of thing in the near future. Cheers, Ian.

[derekwarner]

Evening Ian....

1. print out & confirm your calculations for the Saito V4 .......from the MacSteam ...ready recknower & post the results here
2. from memory ...... ...someone in 2008 asked a similar question..........on PD's......... Derek 

[Circlip]

Seems strange that we haven't had any comments recently from Bunkerbarge on the subject? Possibly pressure of work.

  Regards  Ian.

[oldiron]

 For info, here's a shot of some of those twisted steel swirls inside boiler tubes to delay the gas flow for better heat absorption.

John

[ooyah/2]

Hi Kiwi,
To date you have had only 1- constructive idea about your pressure drop problem and there is no doubt that using the exhaust up the chimney as a blast pipe will improve the performance, keep the pipe about 1" from the top otherwise condensate gets into the firing tube.
Most of the replies are very much to do with full size not the toy engines that we use and all the calcs in the world will not relate to the small pipes and firing methods that we use.
You quote.
It gets up to 40psi quite quickly but any sort of decent run has the pressure back to 20psi or less very quickly and then I have to let it just tick over for some time while pressure builds up again.

Your boiler if it's the center flue type will have internal cross tubes so that rules out anything stuffed inside the firing tube.
Many years ago before I started making my own boilers and engines I purchased a Pintail plant from Chedder and installed it in a CLYDE Puffer and experienced the problems that you have.
I cured my problem by making new cylinders from 5/16" bore to 7/16" bore and scrapped the ceramic burner and fitted a plumbers blow lamp head and fired it into the boiler, it was the new burner that did the trick with a No 8 jet.

If when your boiler is being fired and you can take a lighted flame to the top of the chimney stack and get a flame , your jet is too large and the gases are not burning properly, do you get headaches in an enclosed space ( your garage ) , another sign of poor combustion.

Are you experiencing the age old problem of your gas tank cooling and creating a pressure drop?  I have seen a German design of tank where the exhaust is piped through a copper pipe silver soldered into and through the tank which will heat the L.P.G. and keep up the pressure.
In my Puffer I had a copper fuel tank that cooled down and after fitting the camping gas tank which had some residue heat coming back from the burner I had no problems.

I am not a fan of Ceramic burners but I do like the silence that is there when they are working and I have still to see one that the ceramic material glows with the heat but then when the plumbers blow lamp type is used you always know that the burner is on.
Here are some pics of the installation that I had.

I do hope that my experiences are of help to you,  as to the Saito Engine it looks a very nice piece of kit but with the piston valves do you get a large column of steam from the exhaust?
You don't say if the engine is an old or new one, could the bores be worn ?












George.

P.S. also not a fan of the Walt Disney type funny faces and coloured words.

[gondolier88]

Have you got any pic's of the full setup that we could look at Ian?

Greg

[flashtwo]

Hi,

May I suggest that the twisted swirls (excuse the tautology) have more to do with creating turbulence, therefore breaking up gaseous boundary layers, that would inhibit heat transfer, rather than slowing the gas flow; cross-tubes would cause a similar effect, if they were in sufficient numbers. If there are no cross-tubes, then a small length of stainless mesh could be inserted to cause some turbulence.

Perhaps George's blow lamp type burner produces a more turbulent flame than the ceramic type and assists the heat transfer. I've found with my blow lamp burners, that they sound a little bit load up close, but, once out on the water, they are not unduly noisy.

Refering to the old LPG cooling problem, I've been steaming with the (disposable) gas cylinder upside down to supply liquid fuel, running the 1/8th inch fuel pipe a couple of turns around the burner and then to another gas control valve before, finally connecting to the burner nozzle. With this arrangement, I ran the steam plant in the midst of the cold December weather and measured the gas supply temperature at 35degC with none of the evaporation taking place in the gas cylinder, which maintained a pressure of about 30psi - shame the pond was frozen!

Whilst on the subject of steam production, does anyone know how much superheat they get with their boilers fitted with the additional combustion chamber steam coil ? Looking at the thermal conductivity value for steam, it is nearly forty times less than for water, which seems to suggest that very little heat pickup would occur.

And, finally, are we sure that the piston thickness is not subtracted from the stroke to obtain the swept volume?

Ian

[gondolier88]

Ian,

The correct term for any model 'superheater' coil is a steam dryer as that is all it's good for, though no less important at that as I'm sure you're well aware. It doesn't help that in 99% of models the volume of steam passing through 6" 'superheater' is the same as a 6" length of pipe, thus the fluid velocity is unchanged, whereas in well designed ones it passes through slowly and the engine/turbine whatever is supplied from the superheater as a seperate pressure vessel to the boiler. An old book (1914) I'm reading at the moment has a very interesting boiler design in I hadn't seen before- called a Western River boiler, it used the flue uptake as a combined steam dome and superheater- the steam been taken from the barrel where the flue exits and a seperate steam chamber encases the flue for as long as possible and the steam taken off at the top. it struck me that this is in an ideal world probably the best setup for use in models such as tugs etc where a long flue is available- weight would be a consideration but it would be an interesting exercise in model sizes.

The thickness of the piston is always subtracted for swept volume, and as Derek correctly says the piston rod volume is also taken into account on engines where it matters.

Greg

[ooyah/2]

Hi Ian,
I am sure that Kiwi Ian's boiler will have cross tube as I don't think Cheddar made center flue boilers without them and as he is running a slim launch I don't think he could invert the gas tank and run a coil around his burner..
I have made in the past a gas tank with 2- take offs, 1- with the tube going down to the bottom of the tank to burn liquid and the other with a short tube  to take of gas with a tee connection to allow start up with gas then close the gas valve and open the liquid gas valve.
Even with a coil around the burner there was still pressure loss but your idea of inverting the tank doesn't rely on the gas pressure so much but more to the gravity side, hence you can burn in any weather and use up ALL of the gas in the tank.

In my early experiments with my Flash Steamer I wanted to fire with gas and made a large burner with a 1.25" dia St/St flame tube machined to .032 " thick with a St/St coil around it and a .025" jet using a copper gas tank but that was also effected with pressure drop until I hooked it up to a 4.5 kg butane tank which just would not fit into OOYAH, so it was on to paraffin pressurised tanks.
Here is a pic of the burner fitted to a RAYMAN ,Scott boiler and it was VERY noisy.



Regarding your question on Super heaters in a boiler I have a Scotch boiler  4.5" dia x 5" between the end plates 2- coils of 1/4" copper tube in the back fire box fitted to my steam tug, the bottom of the coils are  directly in line with the burner  being fired through a 1.25" firing tube..
The coil is fed by 5/32" copper pipe coming from the wet header and into the 1/4" steam dryer and exiting back into 5/32" pipe to the engine.
Just taking the O/D of the pipes the 1/4" is 2.5 times the area of the 5/32" pipe so there is plenty of room to dry the wet steam and let it expand.,
I have never tried to get a temp reading but the pipe coming out of the steam dryer goes blue with the heat and on winding cotton string as an insulator around the pipe to the engine it eventually burns and falls off, so there must be a difference in temp from the wet steam to the dried steam.

This bluing effect happens on all of the boilers that I have made with the steam driers fitted, here is some pics of a vertical multi- tube boiler and a  Scotch multi- tube marine boiler similar to my tug boiler.








  Scotch marine multi tube boiler similar to my tug boiler.






Finally the boiler in my tug which I had out to fit the new D10








Sorry that I can't give you definite answers but maybe my experiments will help.

George.

[AlexC]

Hi Guy's,

Interesting topic, however, I must disagree with Greg on one point: -

The thickness of the piston is always subtracted for swept volume, and as Derek correctly says the piston rod volume is also taken into account on engines where it matters.

The piston thickness is NOT subtracted for swept volume.

The overall cylinder length takes the piston thickness in to account as follows: -

Cyl length = Stroke + piston thickness + top steam clearance + bottom steam clearance.(see note below)

Therefore swept volume is: -
         = Stroke x Piston area in one direction... and
         = Stroke x (Piston area - Piston rod cross section area) in the other direction (taking account of the piston rod area).

Total swept volume = sum of both the above.
 
Note... in practice the O/A cylinder length would also take account of the length of any location stubs on the inside faces of the cylinder covers, however these would not have any bearing on the Steam Swept Volume.

In most small model steam engines the area of the piston rod is generally ignored since the small resulting error is usually outweighed by other losses, however it must certainly be considered if it is a significant value, (eg. in a trunk piston design).

On the subject of boilers: -

The Cheddar boiler has a flu dia of 28mm OD (26mm ID) and uses a 22mm dia burner.
It has 6 x 1/4" dia cross tubes.

Rough calculations give Flu Heated surface area of 19.294 sq ins.

With an additional heated surface area for the 6 tubes = to 4.823 sq ins.


The Maccsteam boiler has a flue dia of 35mm ID and uses a 28mm OD burner
 and 6 x 5/16" dia cross tubes.

Rough calcs give Flu Heated surface of 24.489 sq ins (which is approx 27% more than the Cheddar).

With an additional 7.6525 sq ins heated surface area for the cross tubes (which is approx 59% more than the Cheddar)

I think you will agree with me that this large difference in available heated surface area coupled with the much larger burner results in the MaccSteam boiler being an altogether different animal.

My ACS 3 1/2" horizontal has 5 rows of 2 x 5/16" OD cross tubes ( 10 tubes total) in a 35mm flu and also uses a 28mm burner.

I think the answer is HEATED SURFACE AREA... no point firing a lot of heat into the flu if the available surface area can not transfer it to the water, the extra heat just passes straight through and does not do any work and is just wasted ... From Ian's description of his set up... Funnel exhaust is very hot... it would seem that this is the case.

Quite what Ian can do to improve his set up is not a simple question.

Slowing the combustion gas flow (up to a point) can help but taken too far and he will end up with a flu full of unburnt gasses (with no oxygen available) which would reduce the efficiency of, or even extinguish his burner.

I do not think MORE heat is the answer.

So... how does he increase the available heated surface? is really the question.

Hope this is of some use.

Best regards.

AlexC.

[kiwimodeller]

A great deal of info and I will have to read through it all several times to take it all in but a few things I can say are :- George, I have a plumbers torch burner set up with dual gas cannisters (for increased surface area which helps slow the cooling effect of draw off) and I will try that burner in this boiler at the weekend. I do my testing in the studio inside the house so i have to do it when my partner is out and get rid of the smell before she comes home. I am fairly sure that the gas/air mixture of the original burner is fairly close to ideal. I have tried both larger and smaller jets but returned to the one that Cheddar supplied. I use a large disposable gas cannister which does not get too cold and even when it is cold I still get a bright blue flame which can be made to roar if I turn up the gas and burns gently blue if I turn the gas down so mixture seems to work well. I want to test both burner temperature and temperature at the top of the funnel and am working on getting a Laser Thermometer. I am still inclined towards the theory that the heat is passing through the flue too quickly and not transferring to the water so I will continue to experiment with slowing the flow through the flue. Also the engine has a separate exhaust for each bank and another one that comes out of the crankcase. When I run it next I am going to separate the exhaust tubes from the manifold and that should give me an indication if there is much blowby past the pistons and in to the crankcase (remembering this is a single acting engine). Unlike Maccsteam Cheddar did not specify the number or size of the crosstubes but i know there are some in the horizontal part of the flue but none in the vertical part. Derek I have rechecked my calculations and I am sure that this size of boiler, if it matched the Maccsteam similar boiler for output, would be ample for such a small capacity engine. Will keep experimenting a following suggestions and posting results. Cheers, Ian.

[ooyah/2]

Hi Ian,
Firstly my thoughts go to your fellow Kiwis on the quake, I hope that you are not involved.

AlexC is quite correct in that a greater surface area would be advantageous and firing more heat will not cure the problem.

 As you already have the steam plant and can't increase the heating area I would try the plumbers blow lamp with a smaller jet and check out the condition of the bores and piston valves.

Another thought  what size of prop are you using ?
As this engine is a very high reving one I think you said 4,500 rpm if you are using a small prop the steam will be rushing through and your boiler may not be producing enough steam to maintain the high revs, and could be the cause of the non starting by not having enough boiler pressure.
 I would try as big a prop as will fit and let the engine work, it may make the boat too fast but you can always throttle back or turn the burner down and conserve your steam.
Personally I am a fan of bog props on steam engines, make the engine work.

Just my thoughts , hope it helps.

George.

[kiwimodeller]

Thanks for the good wishes George, we are fine as we live in the North Island about 800 miles away from the quake. Full time coverage on TV here of course but one feels a bit helpless as there is not much we can do except donate to the Red Cross fund. Reading back through all the correspondence it all seems pretty logical and given the surface area calculations AlexC has kindly done it seems likely that the Cheddar boiler is never going to work as well as the Macc one but given the very small capacity of the Saito I am surprised that the Cheddar boiler does not manage to do a better job than it does. Surely the Cheddar Puffin was more than 2.54cc  (.155cu in)? I cannot think of any way to increase the heating surface area without cutting the boiler apart and rebuilding it which is beyond my skills but I do think slowing the flow through the flu might help a bit provided I do not cause too much back pressure or mess up the mixture so I will continue to experiment with that. I will also do my best to check out the engine and will pull it apart as a last resort. As for prop size I am running a 4 blade prop which is as big as I can fit in and it really loads the engine down. The factory info says the engine is capable of a no load speed of about 6000rpm. The 4,500rpm is what the Maccsteam calculations suggested it would do with their boiler. I understand how using too many revs ups the steam consumption dramatically but I doubt it is doing more than 12 to 1500 revs with this prop. Certainly if I lift the prop out of the water the revs climb dramatically. I will do some more experimenting in the weekend including the Primus burner and let you know the result. Regards, Ian.

[AlexC]

Hi Guy's,

Ian,

Firstly I am very pleased to hear that you, and hopefully the rest of your family, are not caught up in the devastating aftermath of the earthquake in Christchurch and my thoughts and prayers go out to any who are involved.

You are quite correct in thinking that the Cheddar Puffin is a larger capacity than your Saito V4.
The Puffin is approx 4.5cc total swept volume versus your Saito at 2.5cc.

Having said that, the Puffin was generally used at a more sedate 700 - 800 rpm but driving a large dia high pitch prop… making full use of the TORQUE available.
However, even at this low rpm, the Cheddar boiler (under optimum operating conditions) was hard put to keep the steam pressure at around 30psi during a run. More often than not it was down around the 20 – 25psi mark.

With your Saito being a little over ˝ the total swept volume of the Puffin your estimate of between 12 – 1500 rpm under much the same operating conditions would seem about right for much the same pressure drop.

George’s suggestion of a large diameter high-pitched prop was certainly a good one… steam engines like to work hard and slow since this makes best use of the available TORQUE.

Now this is where the 2 engines differ greatly.

The Puffin is a 2 cylinder in line double acting engine with its cranks set at 90 deg. to each other … this means that at any point in the rotation cycle at least 2 cylinders are producing thrust.

At 30psi the Puffin can produce a potential 1.973 lb ins. of torque and there are no dead spots or drag created by non-producing cylinders and all the torque is available to drive the prop (other than from any bearing and prop shaft drag that may be involved).

The Saito in comparison is a 90 deg. V4 arrangement with single acting cylinders and having its cranks set at 180 deg. to each other.
Each crank has 2 cylinders (pair bank) but they are 90 deg apart from each other.

This has the effect of producing the worst-case (starting) situation where only 1 of the 4 cylinders is actually producing any useful Torque which at 30psi would only be approx 0.582 lb ins.
This would rise to 1.164 lb ins. after a further 90 deg rotation of the crank when the second cylinder of the pair bank (pair attached to each crank) reached TDC.
Some of the torque produced by the single cylinder would be used up in overcoming the drag of the remaining 3 cylinders until the second cylinder of the pair was in torque production, but there would still remain the drag of the remaining 2 cylinders.

All this would mean considerably less torque was available to drive a prop and it is just possible that this is the cause of your poor starting, especially if you also include any bearing and prop shaft drag.

I suppose you could see if this is the case by trying, say a 2 bladed or 3 bladed prop, with a finer pitch.

This may help in the starting situation, but of course the average rpm will increase along with your steam consumption and through the water speed may be compromised a little.

I might suggest that a 3 bladed prop with the same pitch (or slightly less) as your 4 bladed one would possibly be the best compromise.
It would appear you have to achieve the best balance between STEAM USAGE and worst-case STARTING TORQUE.

Certainly a little more steam pressure would help, however, this is not going to be easy to achieve, especially if your rpm needs to increase due to a lower pitch prop requirement in order to achieve better starting.
A larger or more powerfull (HEAT WISE... NOT TEMPERATURE) burner may help a little; but only if you can keep the HEAT inside the flu long enough for the smaller surface area available to do it's job. The blowlamp style may be better in this respect, since it will also produce a more turbulant gas flow, thus stripping away any boundary layer which may be present with the softer gas flow of the ceramic version, however, they are much more prone to heat reflection back out of the flu causing blow out.

Some of the 'Hegner' boilers use blowlamp style burner heads and they are set up such that the flame end of the burner head is approx 1/2" - 3/4" away from the open end of the flu but centred on the flu... any closer and they blow out.
Set up correctly they do a very good job and are quite reliable... albeit a little noisy... but only close up.

Trying to compare the 2 engines (Saito vs Puffin) is of little value really in your quest, since they are completely different in their power/torque application regardless of steam pressure.

I hope this all makes sense and gives you some more food for thought and ideas for experimentation.

AlexC.

[kiwimodeller]

Alex, thanks very much for your work on my behalf and for explaining it so clearly. Reading all the replies had started me in the directions you are suggesting and your theories confirm what I had suspected. I will try a different type of burner, varying amounts of restriction in the flue and a couple of different props which I already have for other projects. Will report on all the above as I get to try them. Unfortunately I do not have a home test tank so I need to go down to the pond to try things out but will post the outcomes. Thanks again, Ian

[ooyah/2]

Hi Ian,
Alex has explained the engine situation much better than I could and I think that a Flame burner ( plumbers lamp ) will allow the heat to penetrate up into the flue but I would be careful about restricting the flow of gases through the flue.
How about this to give you more steam.

32 ft of  1/4" dia St/St tube in an insulated casing.


Three burners each with .025" jets


All as complete unit


Powered by a 1.125" dia  poppet valve uni flow  engine doing 15.000 rpm


I AM BEING FACETIOUS, but it sure would give you more steam.
George.

[kno3]

Alex, thanks very much for your work on my behalf and for explaining it so clearly. Reading all the replies had started me in the directions you are suggesting and your theories confirm what I had suspected. I will try a different type of burner, varying amounts of restriction in the flue and a couple of different props which I already have for other projects. Will report on all the above as I get to try them. Unfortunately I do not have a home test tank so I need to go down to the pond to try things out but will post the outcomes. Thanks again, Ian

Hi, if you have a bath tub, you can use it as a test tank. Tie a string to the stern of your boat and to a small dynamometer and this way you can compare the bollard pull with different burner or propellers.

Regarding the burner of your Cheddar boiler: is it ceramic?
If yes, I would advise to change it to a blow-lamp type, which gives a longer flame and could improve your boiler's steam production. In my experience and as advised by others more knowledgeable than me, ceramic burners aren't that good in narrow centre flues. They work best when they can have a large diameter because they radiate heat to a large surface which has to be reasonably close. This is not the case in your boiler.
For example, Clevedon Steam makes replacement burners for the Cheddar boilers, which use no ceramic but a metal perforated screen and produce a rather long flame. Let me know if you need the contact data.