My BMC 1.5 get's hot

[nb.Stealaway]

My BMC is fine on the cut under normal cruising conditions (generally between 750 - 1000rpm) but will only go significantly beyond this for about 30mins before the temp gets too high (90ºC+). I would like to go on the Thames (Limehouse to Brentford) at some point but am concerned that at the moment the engine may not have much in reserve in case of emergency. Basically I think I'm probably over propped, and with too small a skin tank.

 

If I reduce the prop size, will this help at all, or would I find when going against a current that I would have to rev proportionately higher & that the over-heating issue would remain unchanged?

 

The stats; 45ft boat, BMC 1.5 with ZF 2:1 gearbox. Revs out of gear to about 2500 (from memory). In gear only revs to 1400 (Achieved while tied up - can't see if it'll go higher on the cut as by that time the boat is really shifting so I have to throttle back.) Prop is 17x16.5. Skin tank 5.5 square feet.

 

What do people think?

[alan_fincher]

The stats; 45ft boat, BMC 1.5 with ZF 2:1 gearbox. Revs out of gear to about 2500 (from memory). In gear only revs to 1400 (Achieved while tied up - can't see if it'll go higher on the cut as by that time the boat is really shifting so I have to throttle back.) Prop is 17x16.5. Skin tank 5.5 square feet.

 

What do people think?

Almost certainly your skin tank is on the margin for any sustained use......

 

The recommended area for a BMC is generally closer to twice what you have.

 

I'm no expert, but the 16.5" pitch of your prop also sounds a lot for a 1.5 on a 2:1 reduction, but others will know better than me.

 

However, I don't think that is the primary cause for your overheating - I think the skin tank is probably too small for serious sustained hard work, whatever prop you had.

 

Out of interest who built your shell ?

[nb.Stealaway]

Thanks Alan, that seems to be my thinking too. Would really prefer not to have to have a second tank fitted & the associated costs/complications and was wondering what effect changing the prop would have? It's nice to have the low revs (noise wise) on the cut, but I would like to get out on the river.

 

It a John Shotbolt Hull, no. 29, so he should have hopefully got things sussed.

[alan_fincher]

It a John Shotbolt Hull, no. 29, so he should have hopefully got things sussed.

If the skin tank to cool a BMC 1.5 is genuinely only presenting 5.5 square feet of cooling surface, then I'm afraid he hasn't.

 

Sadly this is not an uncommon problem. Our boat was Evans and Son, effectively the legendary Mike Heywood, after he had built huge numbers of shells. It had a skin tank barely more than 4 square feet, and I have shared stories with others with a shell of the same origins.

 

Even Liverpool Boats, (almost certainly the biggest volume builder in recent years), sometimes seem to skimp on skin tank area.

 

You might well get away with Limehouse to Brentford, because you really don't need to use any more power running in on the tide than you would elsewhere, but for serious work against a current, the skin tank sounds too small.

 

I can't see that reducing prop size will help much. In fact it could even allow the engine to run up to higher horsepowers, and hence need greater cooling than it currently does, (I think! - I'm sure someone will point out a flaw in that suggestion, if there is one! ).

 

Your basic problem is that if you need to generate a certain horsepower in the engine, there is a set amount of heat it will need to dump. If the skin tank can't handle that, problems will always be likely, I would say.

 

We had a single larger (but slender!) skin tank put on the outside of the swim - very much less disruption than making changes inside the boat. It doesn't affect handling, and the cooling water temperature now never rises much above that at which the thermostat is open - that was a result - but not that cheap to do.

[nb.Stealaway]

I will measure the tank accurately tomorrow but it is definitely around that size or slightly smaller Your logic sounds pretty reasonable to me, as I say, I was just hoping to find a more straight forward fix/improvement other than engine out/welder in as well as plumbing and expansion issues.

 

I have given alternate cooling some thought over the last year or so;

 

-Second skin tank locations (plumbing & expansion issues)

 

-External tank/tubes (don't like tubes and not sure how much bigger an external tank could be - I have polystyrene insulation so it can't extend much past where it is now)

 

-Even pumped raw water cooling for the engine side of the skin tank - a tank on a tank! Wonder if anyone's done this?

 

Don't know how much heat could be dumped through the domestic rad via the calorifier & circ pump? Probably not much & also not good on a hot sunny day .

 

Hence my attention turned to the suspect monster prop

 

I know every boat is different so it's impossible to say definitively but would changing the prop really not help at all? It does seem to have quite a lot of pitch?

Is it really essentially that my engine has only sufficient cooling to propel the boat at 4mph for any extended period of time whatever the propeller?

 

Charles.

[Nose Miner]

I will measure the tank accurately tomorrow but it is definitely around that size or slightly smaller Your logic sounds pretty reasonable to me, as I say, I was just hoping to find a more straight forward fix/improvement other than engine out/welder in as well as plumbing and expansion issues.

 

I have given alternate cooling some thought over the last year or so;

 

-Second skin tank locations (plumbing & expansion issues)

 

-External tank/tubes (don't like tubes and not sure how much bigger an external tank could be - I have polystyrene insulation so it can't extend much past where it is now)

 

-Even pumped raw water cooling for the engine side of the skin tank - a tank on a tank! Wonder if anyone's done this?

 

Don't know how much heat could be dumped through the domestic rad via the calorifier & circ pump? Probably not much & also not good on a hot sunny day .

 

Hence my attention turned to the suspect monster prop

 

I know every boat is different so it's impossible to say definitively but would changing the prop really not help at all? It does seem to have quite a lot of pitch?

Is it really essentially that my engine has only sufficient cooling to propel the boat at 4mph for any extended period of time whatever the propeller?

 

Charles.

 

Hi Charles,

 

Changing the prop size will only affect, as has been said, the maximum amount of power you can extract from the engine. The amount of waste heat is a direct function of power output, so a smaller prop may well mean higher maximum power output and more heat. A larger prop will mean the engine is fully loaded at a lower speed and possibly below it's maximum power output.

 

It is worth asking the question though, is your engine actually running that hot? 90 degrees is still some way below maximum and in any event many temperature gauges can be considerably out. I was worried that my engine was running hot at 100 degrees, until I put an accurate thermometer on it and found that it was really only about 75. Does your engine actually overheat? ie start venting steam from the pressure cap?

 

I find that maximum possible engine speed is a good indicator of whether or not a boat is over-propped. If you can't push it up to 2000 rpm, it probably is! I think for the BMC 1.5 max power is 40hp @ 4000 rpm, so you should be able to achieve that sort of speed on the right prop for your boat.

Edited June 23, 2011 by Nose Miner

[blodger]

To improve cooling (ona 3 pot listsr) I put a (roughly 2 ft square double) radiator in the engine room in series with the calorifier that I could switch in or out with valves.

 

Was quite effective but hardly needed on the canals now since (still there for the rivers) I fitted a much larger calorifier when the seam on the original went.

[RobinJ]

My BMC is fine on the cut under normal cruising conditions (generally between 750 - 1000rpm) but will only go significantly beyond this for about 30mins before the temp gets too high (90ºC+). ... Basically I think I'm probably over propped, and with too small a skin tank.

Prop is 17x16.5.

If I reduce the prop size, will this help at all, or would I find when going against a current that I would have to rev proportionately higher & that the over-heating issue would remain unchanged?

If indeed it is 17x16.5 then yes it probably is!

A 1.5 with 2:1 box will probably not carry much more than 17x13 without reducing top revs. and loading the engine - great for slow speed manouvreability on canals.

The stats; 45ft boat, BMC 1.5 with ZF 2:1 gearbox. Revs out of gear to about 2500 (from memory). In gear only revs to 1400 (Achieved while tied up - can't see if it'll go higher on the cut as by that time the boat is really shifting so I have to throttle back.)

Skin tank 5.5 square feet.

Confirms both overpropped and under cooled!

Full speed revs. on a 1.5 is around 3000-3500!

It is capable of developing up to 50hp (normally rated 35-40), you will need something of the order of 9 sq.ft.

 

This is the problem I had when I fitted a 17x13 wide blade on a 9 sq.ft. tank without baffles. 3 hours if you were lucky, open up and 'old faithful'.

 

A temporary solution is to assist it with raw water through the heat exchanger, but you probably need to change the prop and/or modify the skin tank!

[Tony Brooks]

As Nose Miner says the 1.5 should rev around 4000 rpm in neutral (I think it is 4400) so if its only revving to 2000 rpm something more then the throttle stop may be wrong, specifically the injection timing may be incorrect and that could cause excess heat to be produced as would anything on the drive-line that is stiff and loading the engine.

 

If this is an ex-hire boat and the throttle stop has never been adjusted since it left service my guess this would be the most likely explanation for the low neutral speed, but if the pump has been changed or if they set the stop back before selling it then it is not. On many 1.5 injector pumps the stop is under a long allow tube secured in place with wire and a lead seal. This should not be disturbed unless you have good reason to doubt it is wrong or you wish to limit the maximum speed. If you "open it up" to get 4000rpm and it was not the problem you stand a very good chance of overloading the engine and making lots of black smoke - actually the stop may have been set down to cover up smoke caused by an over sized prop making smoke.

 

The 1.5 (not the 1.8) drives its injector pump from the same equipment that drives the distributor on the petrol version of this engine but it requires extra lubrication. This is achieved by an oil strainer and jet that are under large hexagon heads beneath the back end of the exhaust manifold. When the strainer blocks or on very high hours engines the drive wears and retards the pump. Hence the special tool for setting the injection pointer. It would be worth checking the jet and strainer.

 

 

I think it would be a good idea to make sure the overheating is not caused by something else before spending money on extra tanks etc.

[alan_fincher]

I find that maximum possible engine speed is a good indicator of whether or not a boat is over-propped. If you can't push it up to 2000 rpm, it probably is! I think for the BMC 1.5 max power is 40hp @ 4000 rpm, so you should be able to achieve that sort of speed on the right prop for your boat.

Not sure about that.......

 

The BMC B series engines are usually fairly de-rated when marinised, and most of the published figures I have seen would not suggest much more than 30HP at 3000 rpm for a 1.5.

 

My 1.8 would probably struggle to get up to 3000 rpm off-load, as it has been set-up. Not sure if that is right or wrong, but it's certainly not a big issue in use. I don't think mine is particularly over-propped, (although I admit I don't know the pitch of the 17" prop fitted), but probably in the "mid 2000s" is the most it can realistically be taken up to in gear, even in deep water.

 

That said, 17" x 16.5" on a 2:1 reduction on a 1.5 does sound more than I would expect, but I'm not convinced it's actually a real "monster", in this application, as the OP describes it.

 

[EDITED TO ADD: OK - cross posted with Tony and Robin, who both seem to think a BMC should be capable of a lot more RPM in marinised form than our one apparently does. I'm sure they have more experience that I do, and I guess ours is just set up not to go that high ? It never causes a problem in practice, because by about 2000 rpm there really is no need to go higher - in our case.....]

 

But despite all that, I think we both agree that a smaller prop may well allow the engine to get hotter, as an oversize one is probably limiting it's available maximum power at the moment.

 

5.5 square feet is certainly well below any normally recommended skin tank for this engine.

 

One further point......

 

The comments about "is it really 90 degrees" are valid. We found one of these very helpful in diagnosing our overheating problems, and checking out the solution to them.

 

An infra-red thermometer can be pointed at the skin tank, a hose, or any part of the engine at close range, and give a fairly accurate idea of actual temperature. If everything is working well, then if directed at the top of your skin tank, or the hose feeding it, the temperature should be consistent with the thermostat fitted, (unlikely to be above 82 degrees for a BMC), whereas, even with the engine worked hard, the bottom of the tank, and the return hose should measure significantly lower. If there is not a decent temperature gradient as you move down the tank, (i.e. water is being returned to the engine nearly as hot as it left it), then I'd say that is fair proof that a skin tank is not sufficient for the task.

Edited June 23, 2011 by alan_fincher

[Tony Brooks]

Not sure about that.......

 

The BMC B series engines are usually fairly de-rated when marinised, and most of the published figures I have seen would not suggest much more than 30HP at 3000 rpm for a 1.5.

 

My 1.8 would probably struggle to get up to 3000 rpm off-load, as it has been set-up. Not sure if that is right or wrong, but it's certainly not a big issue in use. I don't think mine is particularly over-propped, (although I admit I don't know the pitch of the 17" prop fitted), but probably in the "mid 2000s" is the most it can realistically be taken up to in gear, even in deep water.

 

That said, 17" x 16.5" on a 2:1 reduction on a 1.5 does sound more than I would expect, but I'm not convinced it's actually a real "monster", in this application, as the OP describes it.

 

[EDITED TO ADD: OK - cross posted with Tony and Robin, who both seem to think a BMC should be capable of a lot more RPM in marinised form than our one apparently does. I'm sure they have more experience that I do, and I guess ours is just set up not to go that high ? It never causes a problem in practice, because by about 2000 rpm there really is no need to go higher - in our case.....]

 

But despite all that, I think we both agree that a smaller prop may well allow the engine to get hotter, as an oversize one is probably limiting it's available maximum power at the moment.

 

5.5 square feet is certainly well below any normally recommended skin tank for this engine.

 

One further point......

 

The comments about "is it really 90 degrees" are valid. We found one of these very helpful in diagnosing our overheating problems, and checking out the solution to them.

 

An infra-red thermometer can be pointed at the skin tank, a hose, or any part of the engine at close range, and give a fairly accurate idea of actual temperature. If everything is working well, then if directed at the top of your skin tank, or the hose feeding it, the temperature should be consistent with the thermostat fitted, (unlikely to be above 82 degrees for a BMC), whereas, even with the engine worked hard, the bottom of the tank, and the return hose should measure significantly lower. If there is not a decent temperature gradient as you move down the tank, (i.e. water is being returned to the engine nearly as hot as it left it), then I'd say that is fair proof that a skin tank is not sufficient for the task.

 

 

I think we are back to maximum and continuous power ratings. I suspect the marinisers of engine for leisure use at the maximum fuel delivery for maximum continuous power which may well be at the speed you say - saves warranty disputes caused by "Phylis activities" However 2000 is too low and I think the reason for that needs looking at before too much money is spent. I agree an infra red thermometer will tell the OP a lot - it may even tell him his gauge is telling lies.

[David Mack]

My BMC is fine on the cut under normal cruising conditions (generally between 750 - 1000rpm) but will only go significantly beyond this for about 30mins before the temp gets too high (90ºC+). I would like to go on the Thames (Limehouse to Brentford) at some point but am concerned that at the moment the engine may not have much in reserve in case of emergency. Basically I think I'm probably over propped, and with too small a skin tank.

 

If you mainly boat on the canals and only occasionally venture out onto rivers there is a temporary workaround which may help. This works if your skin tank is on the side of the hull (not the bottom), and if the bilge area below the skin tank is separated from your (oily) engine bilge/drip tray.

 

Fill the bilge with an inch or two of water and rig up an electric bilge pump to continuously pump water from the bilge onto the inside surface of the skin tank. A perforated hose fitted along the top of the tank would help to get a reasonably even flow over most of the tank surface. This will effectively double the cooling area of the skin tank, and the circulating bilge water will be cooled via the bottom of the boat. (With acknowledgements to Peter Scott who I recall has described the technique somewhere on this forum).

 

Alternatively, buy a car heater/fan unit from a scrapyard and fit that in the cooling circuit. When the engine overheats switch on the fan. Depending on the layout of your engine area you may have to duct the hot air away.

 

David

[nb.Stealaway]

Thanks to everyone for they're input so far.

 

I have accurately measured the tank & it is only 5 sq feet

I have re-checked and the engine in fact actually revs to 3800 out of gear (tacho works of the injectors so should be accuate) so hopefully the engine at least is ok Sorry for the mistake earler. Thanks for the pointers anyway Tony & Nose, glad I don't have to strip the engine!

 

I did buy an infrared thermometer specifically for the purpose of seeing how hot different parts of the tank/engine get. I also bought one of these http://www.quickerbuy.com/products/digital-blue-auto-thermometer-panel-meter-two-external-882453926 , although I got mine off fleabay. I have the probes clamped to the hoses in & out of the tank so I can monitor while cruising. Obviously though these will read slightly lower as they are not immersed in the coolant.

 

I have found that at canal speeds the flow sits steady at about 75ºC with the return water temp of 50-55ºC.

When I 'open up' the engine (albeit tied to the quayside - so not an ideal simulation I know) first the return temp rises. After a little while the flow will then begin to rise as well and after about 30mins the flow is up to 95ºCish but the return (cooling) water is 80-85ºCºC!

At this point I normally bottle it as I don't want to knacker my engine

No steam has ever come out though - am I ok too really push it that far?

 

I already kind of knew my tank is undersized but have only had the gauges to monitor what's happening for the last couple of cruises & wondered what part my prop had to play in all this.

From what I can tell so far, it'll have little effect other than noisier cruising on the cut!?

 

Unfortunately it looks like I'm back to thinking of cooling ideas. The car heater sounds interesting but the boat is a trad stern so I'm not sure where I could mount it?

 

Charles

[Neil2]

On the bmc forum I raised the issue of skin tanks as my 1.8 only has 5.5 square feet as well. However, I haven't experienced any overheating problems and I assume my engine develops more heat than yours. I do have a 70 litre (estimated) calorifier though, but I assume once the stored water gets hot it isn't helping cool the engine coolant.

 

The general advice on the forum was that on canals the skin tank should be adequate. It seems to be.

 

I think I would be concerned about rivers, but having said that we did a few hours on the Weaver at 4.5 - 5 mph and it seemed fine. Not much current on the Weaver of course.

 

In due course I'm thinking of adding a radiator or car heater in line with the calorifier, but I love the idea of recirculating bilge water, real lateral thinking that.

[Iain_S]

If you mainly boat on the canals and only occasionally venture out onto rivers there is a temporary workaround which may help. This works if your skin tank is on the side of the hull (not the bottom), and if the bilge area below the skin tank is separated from your (oily) engine bilge/drip tray.

 

Fill the bilge with an inch or two of water and rig up an electric bilge pump to continuously pump water from the bilge onto the inside surface of the skin tank. A perforated hose fitted along the top of the tank would help to get a reasonably even flow over most of the tank surface. This will effectively double the cooling area of the skin tank, and the circulating bilge water will be cooled via the bottom of the boat. (With acknowledgements to Peter Scott who I recall has described the technique somewhere on this forum).

 

Alternatively, buy a car heater/fan unit from a scrapyard and fit that in the cooling circuit. When the engine overheats switch on the fan. Depending on the layout of your engine area you may have to duct the hot air away.

 

David

 

I can vouch for this. When we took Copperkins across the Ribble link, we had overheating problems. On the way out, buckets of water were poured over the inside of the skin tank. On the way back, I rigged the hose to an internal tap and water was sprayed onto the skin tank. This kept the temperature within the normal range, and didn't use a lot of water. (I didn't measure it, but didn't have to use the bilge pump ) Peter later rigged up the bilge pump, to make the system a bit more self contained. Not needed now, however, since baffles were fitted to the skin tank!

 

Iain

[Nose Miner]

Thanks to everyone for they're input so far.

 

I have accurately measured the tank & it is only 5 sq feet

I have re-checked and the engine in fact actually revs to 3800 out of gear (tacho works of the injectors so should be accuate) so hopefully the engine at least is ok Sorry for the mistake earler. Thanks for the pointers anyway Tony & Nose, glad I don't have to strip the engine!

 

I did buy an infrared thermometer specifically for the purpose of seeing how hot different parts of the tank/engine get. I also bought one of these http://www.quickerbuy.com/products/digital-blue-auto-thermometer-panel-meter-two-external-882453926 , although I got mine off fleabay. I have the probes clamped to the hoses in & out of the tank so I can monitor while cruising. Obviously though these will read slightly lower as they are not immersed in the coolant.

 

I have found that at canal speeds the flow sits steady at about 75ºC with the return water temp of 50-55ºC.

When I 'open up' the engine (albeit tied to the quayside - so not an ideal simulation I know) first the return temp rises. After a little while the flow will then begin to rise as well and after about 30mins the flow is up to 95ºCish but the return (cooling) water is 80-85ºCºC!

At this point I normally bottle it as I don't want to knacker my engine

No steam has ever come out though - am I ok too really push it that far?

 

I already kind of knew my tank is undersized but have only had the gauges to monitor what's happening for the last couple of cruises & wondered what part my prop had to play in all this.

From what I can tell so far, it'll have little effect other than noisier cruising on the cut!?

 

Unfortunately it looks like I'm back to thinking of cooling ideas. The car heater sounds interesting but the boat is a trad stern so I'm not sure where I could mount it?

 

Charles

Yup, assuming you have actually still got coolant and a pressure tight system, if the rad cap isn't blowing steam then it's not actually in an overheat situation. I think it would boil at about 105, depending on the pressure rating of the cap. Having said that, you don't want to be running a system which is just a degree or two away from blowing!

 

The temp figures you quote say it all, water to the skin tank at 95, returning from the skin tank at 80-85 isn't really sufficiently cooled. I'd be looking at a 25 or 30 degree difference between the two for adequate cooling, which would I think get your running temp down to about 80 and keep it there.

 

Thinking laterally, and assuming you don't have any space to fit another skin tank on the other side of the swim, how about fitting one horizontally on the base plate. You won't have ideal thermal circulation, but it would dump some extra heat!

[David Mack]

The temp figures you quote say it all, water to the skin tank at 95, returning from the skin tank at 80-85 isn't really sufficiently cooled. I'd be looking at a 25 or 30 degree difference between the two for adequate cooling, which would I think get your running temp down to about 80 and keep it there.

 

Not needed now, however, since baffles were fitted to the skin tank!

 

There's your answer. The likely reason your 5.5 sq ft is so much less effective than others skin tanks of similar size may be that yours isn't fitted with internal baffles, so the hot water coming in from the engine is taking the short route directly across the tank to the outlet, and not staying there long enough to transfer much heat to the water outside. The baffles should be horizontal and extending internally most of the way across the tank from alternate sides, so that the water is forced to take a zig-zag route from tank inlet to outlet.

 

Iain or Peter can tell us how baffles were retrofitted to Copperkins.

 

David

[n.b.Goldie]

Thanks to everyone for they're input so far.

 

I have accurately measured the tank & it is only 5 sq feet

I have re-checked and the engine in fact actually revs to 3800 out of gear (tacho works of the injectors so should be accuate) so hopefully the engine at least is ok Sorry for the mistake earler. Thanks for the pointers anyway Tony & Nose, glad I don't have to strip the engine!

 

I did buy an infrared thermometer specifically for the purpose of seeing how hot different parts of the tank/engine get. I also bought one of these http://www.quickerbu...ernal-882453926 , although I got mine off fleabay. I have the probes clamped to the hoses in & out of the tank so I can monitor while cruising. Obviously though these will read slightly lower as they are not immersed in the coolant.

 

I have found that at canal speeds the flow sits steady at about 75ºC with the return water temp of 50-55ºC.

When I 'open up' the engine (albeit tied to the quayside - so not an ideal simulation I know) first the return temp rises. After a little while the flow will then begin to rise as well and after about 30mins the flow is up to 95ºCish but the return (cooling) water is 80-85ºCºC!

At this point I normally bottle it as I don't want to knacker my engine

No steam has ever come out though - am I ok too really push it that far?

 

I already kind of knew my tank is undersized but have only had the gauges to monitor what's happening for the last couple of cruises & wondered what part my prop had to play in all this.

From what I can tell so far, it'll have little effect other than noisier cruising on the cut!?

 

Unfortunately it looks like I'm back to thinking of cooling ideas. The car heater sounds interesting but the boat is a trad stern so I'm not sure where I could mount it?

 

Charles

 

I too had concerns about the temperature of the water returning from the skin tank so I bought an inline temperature sender housing with matched sender and guage from Car Builder Solutions.

 

 

Monitoring the temperature convinced me to have an extra cooling tank fitted as the returning water was heating the PRM gearbox rather than cooling it!

 

Regards

Ditchdabbler

[nb.Stealaway]

I think the tank has vertical baffles, my reasoning being I can insert a rod down through the bleed vent, but when you try to rotate it, you can feel it hit a solid obstruction. Hope that makes sense. I'm not sure how else to tell, there's no discernible weld marks. Perhaps an ultrasonic tester would work, other wise I'd have to cut the tank open

Haven't seen those sender housings before, but my system already seems to indicate that coolest cooling water is hotter than desired. They're a lot prettier than my jubilee clips though.

 

Unfortunately my boat has concrete ballast so at the rear of the cabin bilge there are two vents/drains which allow the concrete to breathe & drain after any accidents. As a result though, I prefer to keep the bilge bone dry if I can help it.

 

The idea is very interesting though. I'd have though the water would evaporate, but perhaps not?

 

It's looking more & more like the engine will have to come out if I want to fix this. At least I might finally stop that oil leak? Maybe not, it is a BMC after all

 

p.s. To fit horizontal baffles do you cut curved strips (matched to the curve of the swim) out of flat plate, or is there some 'boatbuilder magic' way of bending flat bar?

 

Charles

Edited June 23, 2011 by nb.Stealaway

[David Mack]

Late night thought: you have got it plumbed so that hot water from the top of the engine goes into the top of the skin tank, and cold(er) water goes back from the bottom of the skin tank to the engine, haven't you.

 

And with the engine running and up to temperature, is the whole of the inside surface of the skin tank pretty hot? If there are any areas which are colder than the tank outlet, then the cooling water circulation must be bypassing those parts of the tank.

 

David

[Iain_S]

(snip)

Iain or Peter can tell us how baffles were retrofitted to Copperkins.

 

The job was done by Ray at Streethay. I didn't see it in progress, but I assume a section was cut from the hull side and welded back up again once the baffles were fitted. We originally thought we were going to need a bigger swim tank, and there were some discussions about this, and ideas like piping canal water through the existing tank (which is a lot wider than most), but the discovery that there weren't any baffles simplified matters.

 

Iain

[RobinJ]

I have accurately measured the tank & it is only 5 sq feet

Definately a bit small!

I have re-checked and the engine in fact actually revs to 3800 out of gear.

Thats good!

I have found that at canal speeds the flow sits steady at about 75ºC with the return water temp of 50-55ºC.

When I 'open up' the engine (albeit tied to the quayside - so not an ideal simulation I know) first the return temp rises. After a little while the flow will then begin to rise as well and after about 30mins the flow is up to 95ºCish but the return (cooling) water is 80-85ºCºC!

At this point I normally bottle it as I don't want to knacker my engine

No steam has ever come out though - am I ok too really push it that far?

The important thing is the differential temperature, so initially we have (75-55) 20 degrees, and latterly we have (95-85) 10 degrees. That is quite small, when water returns from the skin tank if ought to be as close as possible to ambient (20-25?).

However the BMC will usually go up to above 96 before it begins to boil and if it hasn't erupted in steam then that shows it can handle a fair amount of stress. I suspect your saving grace is the fact that the tank has good baffles, but if they are vertical you may have trouble cleaning the crud out?

[nb.Stealaway]

The flow into the tank is at the top & the return to the engine is at the bottom. As the baffles are vertical I don't think this makes much difference with the vertical baffles. With horizontal ones, wouldn't the water be inclined to sink as it cools anyway?

The heat does seem to progress across the tank until is is all pretty hot. From memeory one corner was a liitle cooler, but not much.

 

I totally get that the temperature differential is too close - I believe it's an inverse square law as well, ie. the greater the difference the faster things cool or heat.

 

Hadn't thought about crud accumulating, maybe I should try & blast it though?

 

Charles

[stort_mark]

Thanks Alan, that seems to be my thinking too. Would really prefer not to have to have a second tank fitted & the associated costs/complications and was wondering what effect changing the prop would have? It's nice to have the low revs (noise wise) on the cut, but I would like to get out on the river.

 

It a John Shotbolt Hull, no. 29, so he should have hopefully got things sussed.

 

Must admit I was under the impression that our boat was Shotbolt #29. Got that from Shotbolt's a year ago or so. Maybe I'm mistaken cos I can't find the e-mail right now.

 

fwiw we had identical problem. 5.5 sq ft skin tank, BMC 1.5. We got Calcott to fit an external tank to extend it to 9.5. Works a dream.

 

- Mark

[nb.Stealaway]

Hi Mark,

 

Stealaway is a 45ft trad built in 1994. She has an oval brass plaque fixed to the front of the well deck stating 'Steel work by Shotbolt Eng Ltd, Ramsey, Cambs' with a central area stamped '029'. I assumed all Shotbolt boats would have one, originally at least. I've not seen any of his other boats in the 'flesh' though, so I suppose someone could have just stuck the plate on!

 

Does yours have a big prop as well?

 

Also, I take it your external tank must be on the other side of the swim? Did you have to fit an expansion tank due to increased coolant volume?

 

Regards,

 

Charles.