Propeller Choices

boatmike

boatmike

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I have an Aquastar 33 Semi Displacement TSDY with TMD41a diesels (150HP @3800RPM) I have 20in x 15 4 blade props which is WAY over propped and I can only get about 2500 RPM , Engines are in good nick and gearboxes (2/1 reduction) are new. Michigan in Newton Abbot suggest resizing and re pitching to 18 X 14. Vicprop suggests 17.5 X 13 for a 4 blade. Norris suggest resizing to 18in and leaving pitch at 15. I am concerned that 18 X 14 might still be a bit OTT but a good move. I think 18 x 15 is still too much....... Boat is approx 28 ft OAL at WL and 11ft beam x 4ft draught and weighs in at about 6.5 tonnes (I think) My calcs say I should max out at about 16 knots. Anyone got any similar (ideally the same) boat who have any performance data and prop sizes that would help me make a choice?
 
I think you are correct at 16kts with two 150's in your vessel
Get in touch with Castle Marine in Caernarfon
They are brilliant at making/designing/altering and calculating props
They have a good 'online' calculator too
Used them lots of times and never dissapointed
 
boatmike said:
I have an Aquastar 33 Semi Displacement TSDY with TMD41a diesels (150HP @3800RPM) I have 20in x 15 4 blade props which is WAY over propped and I can only get about 2500 RPM , Engines are in good nick and gearboxes (2/1 reduction) are new. Michigan in Newton Abbot suggest resizing and re pitching to 18 X 14. Vicprop suggests 17.5 X 13 for a 4 blade. Norris suggest resizing to 18in and leaving pitch at 15. I am concerned that 18 X 14 might still be a bit OTT but a good move. I think 18 x 15 is still too much....... Boat is approx 28 ft OAL at WL and 11ft beam x 4ft draught and weighs in at about 6.5 tonnes (I think) My calcs say I should max out at about 16 knots. Anyone got any similar (ideally the same) boat who have any performance data and prop sizes that would help me make a choice?
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Were the old boxes 2:1
 
Yes they were. The reason we have a problem is the previous owner thought he had TAMD engines (200HP) and they are TMD (150HP) even then I think the prop choice was OTT.
 
I have a Broom Crown with twin TAMD40A’s (148hp). The Crown tops out at around 16.5knots, maybe 17.
She cruises nicely at 13 knots
 
There seems to be some confusion re terminology here re 'lightly loaded' (isn't the English language wonderful?)

My way of looking at it is -
If the engine can achieve it's maximum rev's easily, then it is (relatively) lightly loaded at anything less than maximum revs.
If the engine is struggling hard to achieve maximum rev's (and is still a few hundred rev's short), then it is more heavily powered - if the revs go down for the same amount of power delivered, then the torque goes up.
 
Dino said:
I have a Broom Crown with twin TAMD40A’s (148hp). The Crown tops out at around 16.5knots, maybe 17.
She cruises nicely at 13 knots
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Do you know the displacement of the broom? Do you know the spec of your props? Dia x pitch? 3 blade or 4?
 
Afraid I don’t have any light to shed regarding the correct prop size you need.

However Ben is right that a lightly loaded diesel is a happy diesel and you are wrong.

With the boat as badly overpropped as it currently, such that it will only achieve 65% of its recommended revs then if you run it at over displacement speed for any length of time the EGTs will go through the roof with the likely outcome being dropped exhaust valves and melted pistons. Glazed bores will be the least of your worries.
 
Scarron said:
Afraid I don’t have any light to shed regarding the correct prop size you need.

However Ben is right that a lightly loaded diesel is a happy diesel and you are wrong.

With the boat as badly overpropped as it currently, such that it will only achieve 65% of its recommended revs then if you run it at over displacement speed for any length of time the EGTs will go through the roof with the likely outcome being dropped exhaust valves and melted pistons. Glazed bores will be the least of your worries.
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That's totally irrelevant! The current situation is that the engines are OVERLOADED due to being overpropped. When I get the prop I need and achieve 3800 RPM at WOT I won't normally run at over 3400 when cruising. That means that the engines will be 90% of their normal RPM. That is NOT lightly loaded but the engines will happily run like that forever. If you swan about at 1500 RPM THAT would constitute lightly loaded and diesels definitely do not like that.
 
boatmike said:
That's totally irrelevant! The current situation is that the engines are OVERLOADED due to being overpropped. When I get the prop I need and achieve 3800 RPM at WOT I won't normally run at over 3400 when cruising. That means that the engines will be 90% of their normal RPM. That is NOT lightly loaded but the engines will happily run like that forever. If you swan about at 1500 RPM THAT would constitute lightly loaded and diesels definitely do not like that.
Click to expand...

Doh!
 
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