mounting a 90 hp 2-stroke

help.........i just purchased a 2008 90 2-stroke yamaha. i have a 18' sea serpent boat. i put the motor on the boat and when i drives the boat it goes all headlong and i cant trim it up because it loses its water. whats up with that??????????

[jonp]

Hello,

Not sure what you mean by "headlong" But sounds like you may need a long shaft engine if you are running out of water trimmed up... Long shaft meaning 25 inches vs standard 20 inches. Best to measure your transom.

Good luck

I previously had a 40hp yamaha on my boat and i had no problems. Both engines were standard (20") shafts. My transom is about 20.5" high. When i say the boat is going headlong i mean its going down in the bow, its very hard to handle like this and i cant trim it up to rise the bow because then the motor races(propeller slips). Any suggestions?

[jonp]

Try switching props. Perhaps the the prop hub is slipping under a load. Trimming out puts a heavy load on the prop. Not sure if that would effect water cooling though....

its a new motor and new steel prop(13.5x16). if the motor is to high on the boat, would that cause it?

[jonp]

Yes, mounting it to high can do it. How high off the transom? Can you insert or attach a picture?

the bolts were in the #2 holes from the bottom, and the transom was roughly between 20.5 & 21" high. i cant attach a picture because i have the motor took off to cut down my transom. would Lowering down the motor 1 or 2 holes make much of a difference? Also would a hydrofoil helf? Thanks for the replys

[jonp]

Cant really say what the mounting holes mean. There are several options for mounting if I recall. I will check my yamie (also a 90) to see what holes are used. I am mounted flush on the transom. Wish I was higher... I run it on 17 ft whaler with a 19 inch prop. No problems... I know I can go higher.

I will look in the morning. Seems to me that cutting the transom is a bit drastic. If you have 20 inch transom and standard 20 in shaft something else is not right... Did a dealer install this?

Anyone else chime in??

[jb123]

SO.... what youre saying is that you doubled the size of motor... and it LIFTED the transom ?? this i would like to see....

ok, let me try to explain this. i have a 18' sea serpent bowrider, i previously had a 40 hp yamaha on it. the 40 dident have enough power so i bought a 90 hp yamaha. both motors have the same mounting brackets and are standard length shafts. there are 4 adjustment holes in the mounting bracket up top, the motor was in the second hole from the bottom. i think i need to lower my motor down but the transom is too high(bracket was touching the transom on the top) so i cut the transom down to 18 3/4" high, now i can lower the motor down 2 more holes in the bracket. with the motor in the second hole from the bottom it would push the bow down to much and when i tried to trim the motor up it would loose its water(motor would race{blade losing water}).each mounting hole moves the motor 3/4 of an inch, so if i lower the motor down 1 or 2 holes will it solve my problem(will i be able to get the bow up). i hope this explains it, (doubling my hp did not make my counter grow smartass)

[jb123]

so the 90 weighs less than the 40 ? doubling the weight of the motor SHOULD lift the bow not sink it.

[cyrillefrenchy]

for me the right engine height is : the cavitation plate must be aligned with the bottom of the boat when trimed down parallele to the bottom
cyrillefrenchy

i know the 90 weighs more, but you also have double the hp(alot more push). so i was thinking with more hp it pushes the boat headlong(bow down). i got the motor on the boat now, its 20 1/4 inches high,should help i guess, if not i got 1 extra hole i can drop it. that will give me 19 1/2 inches to the bottom of transom. any comments or suggestions?

[jb123]

only when a motor is fully trimmed down should it ever push the nose down but with any xtra weight it should always lift bow, trimmed down, up or even sitting still. you MAY have wanted to install bevelled mounting blocks first before cutting transom.

beveling blocks?do you mean shiming the motor out at the top of the mounting bracket, does that make much of a difference? i figured with power trim i wouldent need to do that. i was adjusting the motor and trying it all day, but i still cant get it right. when im going 1/4-1/2 throttle its pretty good but when i goes wot it still goes down in the bow(very dangerous), i got it so i can trim the motor up but it dont rise the bow a bit at wot. i am thinking that mayby the bottom of the boat is cupped up??????????????