35-55 hp upgrade?

R

rsharer

Cadet
Joined
Sep 20, 2003
Messages
26
i have 1971 35 hp.its on a 15ft fiberglass trihull.it runs along ok.with 2 people i can just break a plane.i'm concidering a 55 hp ,reasonable priced,.how much of a power diference will i notice.is it worth the change over?i cant give you specs on my motors running state.i dont have a working speedomoter yet nor can i get my tach to work.but it seems to run better with the tilt allthe way down.(tight to the transom).i also cant find any numbers on the prop.but as a general responce will a 55 make that much of a diference.
 
eurolarva

eurolarva

Rear Admiral
Joined
Jun 24, 2003
Messages
4,182
Re: 35-55 hp upgrade?

You might want to verify the transom rating on your boat. A 55HP may be to large for a 15 ft boat. You should expect power to be about 150 percent more then you are experiencing with the 35hp. You might be pushing that boat about 15 to 17 mph with the 35HP. You will probably get around 27 to 30 with the 55HP. Iboats sells a GPS for around 90 bucks. It is a good way to find out how fast you are going and it is a cool toy to have around.
 
S

scotiany

Senior Chief Petty Officer
Joined
Aug 3, 2002
Messages
749
Re: 35-55 hp upgrade?

What brand of engine do you have on your boat now? The data plate on your boat should tell you the max authorized hp. For ease of replacement, I'd stay with the same make motor.
 
R

rsharer

Cadet
Joined
Sep 20, 2003
Messages
26
Re: 35-55 hp upgrade?

i have a chrysler 35 right now.the boat came with a 80 hp mercury.but the motor has lots of problems.the data plate is rated for a 90 hp.due to the age of boat(1969) i,d rather not push it to the limit.i figure 55-75 would be perfect for what i want.the speeds mentioned are probably right.with one adult and one child in boat i can pull another child on a wakeboard but not very fast.a tube really kills it.<br /><br /> the only concern i have is the chrysler 35-55 hp's all use the same gears and lower units.the only diference would be raw horsepower.Eurolarva said i should see a 150% increase.just20 hp will do this?
 
eurolarva

eurolarva

Rear Admiral
Joined
Jun 24, 2003
Messages
4,182
Re: 35-55 hp upgrade?

Going from 35 to 55 is an increase of 20 hp. Just by doing the math 20/35 = 57 percent more power. My math could be wrong here but a 55 should push that boat close to 30 mph. I am basing the 30 mph on a 16 foot aluminum boat. I dont know how much more yours weighs. The lower unit should be interchangable on the 35 and the 55 and a lot of the parts in the powerhead as well
 
S

scotiany

Senior Chief Petty Officer
Joined
Aug 3, 2002
Messages
749
Re: 35-55 hp upgrade?

With a 55 hp Chrysler motor on your boat, you should see 28-30 with ease. Figure on a 6 gallon tank being good for about 2 hours at cruise speed. One other thing, if your new engine hasn't been used for a year or more, mix 24-1 for the first 3 hrs.
 
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