Overheating

The water here is soft so no furring in kettles. but I don't know what its like in Oban. I believe its good for Whiskey though. Although I was supposed to have had diesel filters changed I may indeed look at them as well now. I think he may have poked out the crud at the end of the heat exchanger tubes as the ends looked nice and clean but they were blocked in the middle which I couldn't see till I took it out. His tool wasn't long enough. Sorry ladies.... She is coming out of the water next week and the prop calculator says she should have an 8" pitch. The exhaust comes out under water and I couldn't see any black bits as the water is full of sediment. All will be revealed shortly....
 
SteveGorst said:
The water here is soft so no furring in kettles. but I don't know what its like in Oban. I believe its good for Whiskey though. Although I was supposed to have had diesel filters changed I may indeed look at them as well now. I think he may have poked out the crud at the end of the heat exchanger tubes as the ends looked nice and clean but they were blocked in the middle which I couldn't see till I took it out. His tool wasn't long enough. Sorry ladies.... She is coming out of the water next week and the prop calculator says she should have an 8" pitch. The exhaust comes out under water and I couldn't see any black bits as the water is full of sediment. All will be revealed shortly....
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I meant calcium etc in the seawater! Or are you sailing on fresh?
What is the gearbox ratio to give an 8" pitch?
 
SteveGorst said:
2:1 gearbox
3600 rpm
7 knots aim
7 tonne boat
WL 28'5"
engine 35hp
3 blade prop
I think that's all it asked for
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Your displacement is way out. The boat is relatively light displacement for its size (one of the shock horror things about it when it was introduced!). Plus a low (for the times) ballast ratio of 36%. It has more in common with modern AWBs than its contemporaries.

Design displacement is 4.8 tonnes which is the figure the prop calculator is looking for.
 
Tranona said:
Your displacement is way out. The boat is relatively light displacement for its size (one of the shock horror things about it when it was introduced!). Plus a low (for the times) ballast ratio of 36%. It has more in common with modern AWBs than its contemporaries.

Design displacement is 4.8 tonnes which is the figure the prop calculator is looking for.
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Whose calculator?
I suggest putting in the lower displacement and seeing what difference it makes.
It's the real, loaded displacement that dictates the wave drag.
Also play with the values.
Put in your estimate of what the prop might be, and the rpm and speed values you've observed.
Then see if the power required is anything like what your engine should deliver at those rpm.
 
lw395 said:
Whose calculator?
I suggest putting in the lower displacement and seeing what difference it makes.
It's the real, loaded displacement that dictates the wave drag.
Also play with the values.
Put in your estimate of what the prop might be, and the rpm and speed values you've observed.
Then see if the power required is anything like what your engine should deliver at those rpm.
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Propcalc. Even fully loaded will not get to 7 tonnes. Using design displacement has always worked for me. obviously you can "play" with different values, but using one that does not reflect reality is perhaps not a good idea.

Think you are making it all too complicated. The boat is overpropped if it will only get to 2600rpm and we know it is governed to run to 3600. So instead of producing 35hp it is limited to around 30hp. Better to start from scratch to get the right size prop as the engine has more than enough power to achieve hull speed. As I suggested earlier, Beta will almost certainly know the the size that works.
 
I'm going by the actual weight not the design weight. I cant see why the calculations should be based on something that is not true.
 
SteveGorst said:
I'm going by the actual weight not the design weight. I cant see why the calculations should be based on something that is not true.
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How have you determined the "actual" weight? if it is from a crane then it is unlikely to be anywhere near accurate. Difficult to see how you can add over 2 tonnes to the design weight (which would be pretty accurate). So neither is "true" and I have used design weights many times and got the right size (or the calculator did). However, always good to get a second opinion hence the suggestion to ask Beta.
 
We don't know the existing prop's size or condition.
You are making the assumption that the prop size is the problem.
I am open to other possibilities.
Given that the cooling system has been poorly maintained, causing the headline problem, I'd be wanting to know that the fuel system wasn't in a similar state.
Then there is the possibility that the prop is fouled.
 
lw395 said:
We don't know the existing prop's size or condition.
You are making the assumption that the prop size is the problem.
I am open to other possibilities.
Given that the cooling system has been poorly maintained, causing the headline problem, I'd be wanting to know that the fuel system wasn't in a similar state.
Then there is the possibility that the prop is fouled.
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Go back to the original post. This is not new and the engine won't go over 2300 under load. So the prop may be fouled, but there is no mention of this changing over time, which is what you would get if it is fouling - slow change if fouled blades, sudden change if rope etc round prop.

So best approach is to get a reliable recommendation for what the prop should be then haul the boat to compare with what is. Cleaning the HE may well stop the overheating but won't do anything for the restricted revs.
 
A friend of mine has an identical boat and he was craned out last year and as you say the weight of 7 tonnes was from the crane. I've tried knocking a tonne off the weight(in theory) and using the calculator and it does make a difference of an inch. What is included in the design weight? Anchors, chain, fuel, water, generator, tins of food, bottles of wine, tools, beer, outboard, dinghy, extra sails, BBQ etc I can see all these adding up to 2 tonnes easily. When racing I sometimes go to the effort of taking everything off and you can see the boat come out of the water by an inch or so, that was my old boat. I guess I could work out how much an inch of water over that area weighs. Archimedes.

Yes I have asked Beta and they referred me on to someone else, I'll phone them shortly but as I am not going to be buying anything off them I was reluctant to waster their time. Despite all theoretical mathematics however I think that just gives you a starting point. Nothing beats actual sea trials and adjustment. As has been said with a clean hull and prop and new fuel filters. New Fuel? Underlying all this though imho she is just going too fast at low revs, hence my over-propping thoughts. The lack of black smoke is an issue though so I will investigate other issues as suggested.

I will dry her out against the harbour wall this weekend clean her, change filters, and test.
 
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