Reprop in med question ?

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mm1

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Hi all,
I hope to take my boat to the med at some point(to stay for quite some time) as i understand it because of the heat there is a loss of power output,so(a) will i notice this & (b) will i need to reprop,the boat is a 10mtr motor cruiser with twin 200 v p diesel's.
regards mm1.
ps. i should be on the boat alot so i should be able to keep her bottom clean, mm1.
 
Hiya
The engine temperature will stay pretty much the same, goverened by the thermostat so the power output will remain the same. Even so you wouldn't be able to compensate for loss of power by changing the prop.
I know my S37 performed just the same in the UK as it does in the Med.
Enjoy the Med, I'm off next week and looking forward to swimming off the boat again.
 
mm1 said:
Hi all,
I hope to take my boat to the med at some point(to stay for quite some time) as i understand it because of the heat there is a loss of power output,so(a) will i notice this & (b) will i need to reprop,the boat is a 10mtr motor cruiser with twin 200 v p diesel's.
regards mm1.
ps. i should be on the boat alot so i should be able to keep her bottom clean, mm1.
Click to expand...

The hot air is less dense so there is usually a power loss. But it's very slight and wont be measurable without instruments. Remebeer that the (say) 30deg med feels much hotter than say 20degrees uk, to a human, but in physics terms you're comparing 300kelvin with say 290Kelvin, about 3%

I've had 3 boats that I've used in the UK then taken to Med, and have not needed to reprop. Never heard of anyone doing it. If you buy a new FL/PRin/Sseker they wont alter the props according to whether you sail in Stockholm or Malta.
 
mm1 said:
Hi all,
I hope to take my boat to the med at some point(to stay for quite some time) as i understand it because of the heat there is a loss of power output,so(a) will i notice this & (b) will i need to reprop,the boat is a 10mtr motor cruiser with twin 200 v p diesel's.
regards mm1.
ps. i should be on the boat alot so i should be able to keep her bottom clean, mm1.
Click to expand...

I have heard of this before but was told that it wasn't just a heat factor - I heard that it was because the seawater in the med was slighty denser due to higher salinity from evaporation.
 
mm1 said:
Hi all,
I hope to take my boat to the med at some point(to stay for quite some time) as i understand it because of the heat there is a loss of power output,so(a) will i notice this & (b) will i need to reprop,the boat is a 10mtr motor cruiser with twin 200 v p diesel's.
regards mm1.
ps. i should be on the boat alot so i should be able to keep her bottom clean, mm1.
Click to expand...

diesel engines like cold combustion air
 
jfm said:
Actually David that's just the cooling water jacket, which has a pretty loose connection with what's actually going on inside the cylinders (at >>>1000 degrees centigrade)
Click to expand...
Good point and it's made me think more about it. Engine efficiency is proportional to the difference in temp between inlet and outlet ie the gas expansion that can be achieved. I guess the water cooling is really not too much to do with that, it just prevents engine overheating. As you have previously posted air inlet temperature will effect efficiency but in the case of the Med, a tiny bit.
 
some new UK builds the most of I know are Fairlines have had to reprop over here
you just have to go down there and see if your twin Aq41s will still deliver there promised maximum revs, you still might have a loss in speed due to the hotter water and more saltier med seas
 
mm1 said:
Hi all,
I hope to take my boat to the med at some point(to stay for quite some time) as i understand it because of the heat there is a loss of power output,so(a) will i notice this & (b) will i need to reprop,the boat is a 10mtr motor cruiser with twin 200 v p diesel's.
regards mm1.
ps. i should be on the boat alot so i should be able to keep her bottom clean, mm1.
Click to expand...

This is a good question and one I'm thinking about myself. I bought my boat in Holland and on the sea trial, the boat achieved it's max engine rpm easily. However after moving the boat to the Med, the engines have never quite achieved their max rpm, even with a clean hull, and the max speed is 2 - 2.5kts less. I don't know whether this is because of the fact that the boat is now loaded with cruising gear or whether there is a small power loss or what so I'm thinking about having the props re-pitched. The trouble is this is quite difficult to get done even in popular cruising areas. My props will have to be sent to Barcelona from Majorca
In your case, if your boat easily achieves it's max rpm with 50-100rpm in hand, then IMHO I wouldn't bother changing anything but, if it already struggles to achieve it's max rpm in UK waters, then it might be worth re propping in the UK where it can be done more easily and probably a lot cheaper. Btw, sterngear fouling (ie barnacles) is much more aggressive in the Med and you will have to be donning scuba gear and scraping your props, rudders and shafts on a monthly basis during the summer
 
reprop

Hi all & thanks for replies.
It will be interesting to see when we get out there, i forgot to say they are duoprops so it will be an easy problem to overcome,although somewhat expensive,once again thanks, can't wait to get there & try it out,regards mm1.
 
I had original Caterpillar test data on my boat in Poole, Sunseeker test data in Med showing top speed 2 kts less, then returned to Sunseeker when 2 years old and they got original top speed again in Poole.

Mirrored my experience - a loss of 2kts in Med. AFAIK due to air and water density. I didn't feel it was worth changing props.
 
thefatlady said:
I had original Caterpillar test data on my boat in Poole, Sunseeker test data in Med showing top speed 2 kts less, then returned to Sunseeker when 2 years old and they got original top speed again in Poole.

Mirrored my experience - a loss of 2kts in Med. AFAIK due to air and water density. I didn't feel it was worth changing props.
Click to expand...

Yes, but it's 2 kts loss when the boat is clean. With the aggressive barnacle fouling we get in Palma, I can easily lose 6 kts or more over just a few months so to have a few rpm in hand would be useful. I'm not talking about changing props just changing the pitch slightly. I have a spare set of props for my boat so I can mess around with them and go back to the originals if it all goes wrong
 
PowerYachtBlog said:
some new UK builds the most of I know are Fairlines have had to reprop over here
you just have to go down there and see if your twin Aq41s will still deliver there promised maximum revs, you still might have a loss in speed due to the hotter water and more saltier med seas
Click to expand...

The difference in density between med seawater and UK coast seawater is in the order of 0.5%. And that's not all speed reducing- it makes the boat float higher so less drag. The UK/Med difference in air density is more like 3%. Seawater becomes less viscous (but not appreciably) as temperature rises, which reduces drag and makes the boat go faster not slower. But overall, the dominant factor (and only about a 3% level) is ambient air temp
 
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