Prop size (again)

[johnneale]

Can anyone advise me the correct prop to fit on my 24' (loa 22') tubby little motor sailer (4 tons) with a 1.5 litre BMC Captain engine + hydraulic gearbox ?

I've been told my existing prop is too small, though it can push me along at 6.2k with lots of black smoke and drinks fuel at 10 mpg (regardless of the speed- ie its the same at 5 knots as 6knots)

The engine also gets rather hot when running over 5.8 knots, but thats another problem.

The old prop is getting a bit battered now and I dont have a clew what size it is ?

Thanks in anticipation

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[duncan]

are you expecting more speed or the same for lower revs?

what revs is the engine pulling?

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[willtaylor]

I recently had a new prop supplied by BT Marine Propellers of Teignmouth. I found very good technical advice and service there at reasonable prices. The very helpful young man used a sophisticated computer programme and also asked intelligent questions in dertemining the size of prop. My previous prop had also been diagnosed as too small - correctly. I now believe - and was deteriorating.

Previously we achieved 6.5 knots at 2000 and 2250 revs, now

we we get 6.8 at 1500 and peaked at 8.2 at 2250. The higher speed is the theoretical maximum on our 28ft6in waterline. I asked the propeller man to limit the engine to about 2500 rpm max. In practice it will not exceed 2400 under load. Out of gear it runs up to 3000 and beyond.

Mine is a 1.5 Thorneycroft T90 in a 4.8 ton Moody 333 that I would be reluctant to describe as a 'tubby motor sailer', but there are grounds for comparison.

Therw ill be prop men near you who have all the prgrammes andf may also be intelligent and helpful. If not, try BT.

Regards,

Will

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[stamfordian]

Got a program on pc from Castle marine,downloaded it if you give max revs ,hp.,gearbox ratio,i,ll bonk it in to give you an idea.

<hr width=100% size=1><A target="_blank" HREF=http://www.arweb.co.uk/argallery/stamfordian>http://www.arweb.co.uk/argallery/stamfordian</A>

 

[willtaylor]

My own prop solutions

I recently had a new prop supplied by BT Marine Propellers of Teignmouth. I found very good technical advice and service there at reasonable prices. A very helpful young man used a sophisticated computer programme and also asked intelligent questions in dertemining the size of prop. My previous prop had also been diagnosed as too small - correctly. I now believe - and was deteriorating.

Previously we achieved 6.5 knots at 2000 and 2250 revs, now we get 6.8 at 1500 and peaked at 8.2 at 2250. The higher speed is the theoretical maximum on our 28ft6in waterline. I asked the propeller man to limit the engine to about 2500 rpm max. In practice it will not exceed 2400 under load. Out of gear it runs up to 3000 and beyond.

Mine is a 1.5 Thornycroft T90 in a 4.8 ton Moody 333 that I would be reluctant to describe as a 'tubby motor sailer', but there are grounds for comparison.

There may be prop men near you who have all the programmes and may also be intelligent and helpful. If not, try BT.

Regards,

Will

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[vyv_cox]

Download the Castle marine prop calc program <A target="_blank" HREF=http://www.castlemarine.co.uk/pitch.htm>here</A> I found its predictions to be in agreement with those of the prop suppliers.

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[johnneale]

Thanks fellas .... I knew someone would come up with the goods


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