yamaha 20hp to 25hp

[gogetter]

hi was wondering if anyone knows anything about beefing up my 2 stroke 20hp 95 model motor to a 25 or bigger. it has great compresson but lacks the get up andgo when i have 2 people in it. its on a 3.7m tinny.:(

[marty+jojo]

I would be sussing out a different prop, a lot less hassle. I used to have a 3.7m tinnie with a 20hp yammie and it flew 2 up.
Marty.

[cobiaman]

i had a 3.7 with a 10 on the back and used to do about 15-20 knots 2 up, check the trim also

[blaze]

check height of outboard on transom= cavitation plate about 1/2 to 1 inch above bottom of boat.
Check WOT with the boat loaded for a normal trip- if you dont know the WOT then you wont know if you have the right prop.
If all these are within spec, buy a bigger outboard.
cheers
blaze

[gogetter]

i had a 3.7 with a 10 on the back and used to do about 15-20 knots 2 up, check the trim also

yer mate done the trim thing also added a foil as well and no dif. think i will look at the prop... is there a bigger jet for the carbies maybe? everyone says it should fly but it don't:(

[gogetter]

check height of outboard on transom= cavitation plate about 1/2 to 1 inch above bottom of boat.
Check WOT with the boat loaded for a normal trip- if you dont know the WOT then you wont know if you have the right prop.
If all these are within spec, buy a bigger outboard.
cheers
blaze

what does WOT stand for?

the motor is a short leg and is riht for the boat. :-/

[Noelm]

WOT is "wide open throttle" or the Max RPM your Motor can do with a light Load, you will need some sort of Tacho to find this, if you are not getting full RPM for your Motor, then get a prop that will let you get it, the 20 to 25 conversion, is a lot more work than a jet, been spoken about countless times for almost every Motor at some time, some can be done for a fair cost, most are prohibitive and doomed to failure because of a lot of internal casting/block/piston differences.

[Noelm]

OH, and the Motor may well be the correct length for the Boat, but it may still need to be raised up a couple on inches, if it is too low, getting it right can have a big impact on top speed, a Fin type of thing does not help speed in any way, it should be above the Water when on the Plane anyway, I also have a 3.8 tinny with a 20 Merc, and except for when we have 4 People (too crowded anyway) it performs great, 2 people it fair hoots, three it is more than just OK, but 4 just pushes it over the edge a bit and it struggles.

[gogetter]

WOT is "wide open throttle" or the Max RPM your Motor can do with a light Load, you will need some sort of Tacho to find this, if you are not getting full RPM for your Motor, then get a prop that will let you get it, the 20 to 25 conversion, is a lot more work than a jet, been spoken about countless times for almost every Motor at some time, some can be done for a fair cost, most are prohibitive and doomed to failure because of a lot of internal casting/block/piston differences.

well i know straight away that i'm not getting rev out as the motor struggles as it sounds loaded up not reving. as the motor revs harder with just one in it.

[gogetter]

OH, and the Motor may well be the correct length for the Boat, but it may still need to be raised up a couple on inches, if it is too low, getting it right can have a big impact on top speed, a Fin type of thing does not help speed in any way, it should be above the Water when on the Plane anyway, I also have a 3.8 tinny with a 20 Merc, and except for when we have 4 People (too crowded anyway) it performs great, 2 people it fair hoots, three it is more than just OK, but 4 just pushes it over the edge a bit and it struggles.


i will check the height.

[gogetter]

check height of outboard on transom= cavitation plate about 1/2 to 1 inch above bottom of boat.
Check WOT with the boat loaded for a normal trip- if you dont know the WOT then you wont know if you have the right prop.
If all these are within spec, buy a bigger outboard.
cheers
blaze

so the cavitation plate is where i fitted the foil?

[Noelm]

yes, that would be it, most tinnies the plate needs to be a tad above the bottom of the hull, but it takes a bit of trial and error, just lift the motor say about 1" first, then another untill it cavitates too much, then lower to the height before it was not so good, you can just use a small bit of Timber on top of the Transom where the Motor sits to hold it at the correct height while you do up the clamps, also if you have a foil, and it is too low, you will be making the power problem worse, as the foil is also be dragged through the Water, make sure the trim bar is set for optium performance as well, also trial and error, this is where a tacho comes in handy, because you are not just guessing, you can see a difference straight away.

[Harro75]

was looking at buying a new yamaha outboard in the 20 to 30 hp range,last year. dealer said only difference between the 20 and 25 is the siz.e of exhuast port

[litenup]

Mate, there is one model of 20 that can be modified. requires a bafle to be removed from exaust apparently. A mate did it and made his 3.8m go a lot better. Trouble is I can't remember which model. My brother got it done to his from new. I'm sure if you contact a Yammie dealer they will help. Cheers Pete.

[captain rednut]

hi gogetter i sent you a pm