The right size prop?

External Since: Sep 13, 2004 Posts: 1

Hi

I have a 25ft fin keel Marcon Tomahawk. I have a Stuart turner 10hp two
stroke engine. When I first bought the boat I had a new prop fitted
(Surveyor said it had leached or something). I let the yard I bought the
boat from do it. Didn't know enough about anything at the time. (still
don't!)

I have never had good performance from the engine. Despite a complete strip
down and re-build with new bearings seals etc. etc.and spot on timing under
load - set by timong gun. I make max 2 to 3 Knots and it always sounds quite
laboured and flat when under load, although it is perky and responsive out
of gear.

I was reading the engine spec the other day. It rates the engine as 10hp at
1650 rpm. I put a timing gun on it the other day. At full throttle under
load I never get above 800 rpm.

So is it possible that the yard put the wrong size prop on? How do I find
out what size prop I have at the moment. I have a bit of software to
recomend the correct size.

comments and speculation most welcome

Simon

External Since: Aug 19, 2004 Posts: 11

Hi Simon

Assuming that the engine is OK then either the prop is the wrong size or the
gearbox is the wrong ratio.

First check the gearbox ratio. (is there a gearbox?)

Some gearboxes have very different ratios in forward and reverse, does the
gear lever still work in the same direction as before , if not the new
propeller may be wrong handed.

To check the size of the prop you must have the boat out of the water, most
props are marked with the size on the boss (you may have to remove the nut).
Where is the old prop?

Assuming all above is OK then it sounds like the prop is too large. A
specialist may be able to modify it for you. They will also be able to
measure it for you.

If the old prop leached this means it dezincified, check your anode system
or it will happen again.

Alec

"Simon Neal" <sneal DeleteThis @REMOVE_pugilist.demon.co.uk> wrote in message
news:ci4q4o$3mm$1$8300dec7@news.demon.co.uk...
> Hi
>
> I have a 25ft fin keel Marcon Tomahawk. I have a Stuart turner 10hp two
> stroke engine. When I first bought the boat I had a new prop fitted
> (Surveyor said it had leached or something). I let the yard I bought the
> boat from do it. Didn't know enough about anything at the time. (still
> don't!)
>
> I have never had good performance from the engine. Despite a complete
strip
> down and re-build with new bearings seals etc. etc.and spot on timing
under
> load - set by timong gun. I make max 2 to 3 Knots and it always sounds
quite
> laboured and flat when under load, although it is perky and responsive out
> of gear.
>
> I was reading the engine spec the other day. It rates the engine as 10hp
at
> 1650 rpm. I put a timing gun on it the other day. At full throttle under
> load I never get above 800 rpm.
>
> So is it possible that the yard put the wrong size prop on? How do I find
> out what size prop I have at the moment. I have a bit of software to
> recomend the correct size.
>
> comments and speculation most welcome
>
> Simon
>
>

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