Increased preformance

C

catalac08

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hi
you can upgrade MANY 9/10HP ENGINES TO 15HP AS THEY ARE ACTUALLY THE SAME ENGINE AND AL;L THAT IS OFTEN INVOLVED IS A BIGGER CARB JET SIZE. i KNOW THAT YAMAHA YOU CAN DO THIS AND i UPGRADED MY NEW TOHATSU 25 HP TO 30HP (DONE BY DEALER BUT ONLY INVOLVED REMOVAL OF A THROTTLE INTAKE RESTRICTOR)

What really concerns me about this practice is - is a 15 hp an uprated 9.9 or is a 9.9 a downrated 15. either one engine is too heavy for the Hp or the other it is over working and life may be affected

Good chance ask your dealer, also examination of the Tohatsu engine specs will show if thes eare the same engine i.e. bore/stroke ewtc
 

William_H

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If you want more performance then it is most likely that a better match of propeller pitch to hull would give you better performance.
Generally o/bs are prop pitched to suit a light dinghy which is capable of relatively high speed planing. So if you are driving a displacement hull at low speed. Firstly your top hull speed will in parctice be limited by the hull however if you cant' acheive that hull speed because of headwind then a finer pitch prop will get you nearer to hull speed.
The finer pitch will get the engine to rev at max power revs wheras a course pitch (probably as supplied will limit) engine revs so not allow it to generate full power or (possibly) will have so much cavitation that it won't push the boat well.
The down side of a fine pitch prop is that for gentle cruising the engine will rev higher so be noisier.
Any increase in carby or jet size will only give that extra bit at top throttle setting and then only if the engine is at correct top speed.
The hull max speed is a variable quantity but a speed where more horsepower gives progressively less improvement in speed. ie the hull speed can be exceeded a little but only by disproportionate increase in power. It is often described as the square root of the hull waterline length in feet. But in practice something like 25% more than that figure.ie a 25 ft boat is 5knots + 25% or about 6.25 knots. A figure which can be exceeded but not easily by much.

So do you really want more performance and if so why? olewill
 

achwilan

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Yes, as William_H told you, and assuming you are not a MOBO, you must replace your prop by one designed for displacement hulls (shorter pitch and wider blades; standard ones are designed for faster dinghies). The hypothetic additional cavalry you are longing for is a waste...
 

William_H

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High Phil No I am celebrating being "ole" . The best part is no one asks you to explain. ie you can get away with anything.
" silly ole bugger" dismisses any madness. May be I am cheating at 60yo. ole ole will
 

davidbains

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William, I'm shocked you've only just qualified for your bus pass.
I'm a year ahead of you and am firmly resisting ageism even when it's to my benefit!!
You're quite right about fine props, sometimes called work boat props up here.
I transfomed the motoring speed of my tri by fitting the finest prop available to a 25HP Yamaha.
The black box failed after two years though, didn't like getting wet! Herr Diesel rules now.
My brother has just moved from Fremantle to Keri Keri NZ, I was thinking contacting you on my next visit!
Little Dude hasn't told us what boat he's using yet.
 
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