Estimated top speed?

[aborgman]

So after my saga attempting to get my motor running: http://forums.iboats.com/showthread.php?t=191183&highlight=aborgman

I finally got out and ran the thing in some real water. It ran well - started reasonably easy, didn't run hot, much quieter than the old motor, no problems... but I expected more of a performance jump with this motor vs. the old motor.

Boat - 14' x 42" beam jonboat, weight ~500lbs loaded

old motor - Sears Gamefisher 1.2HP aircooled

new motor - 1952-54 Johnson JW-10 3HP watercooled

With the old motor the boat would run about 4.5mph at full throttle according to my GPS. I didn't have my GPS last time out, but the boat didn't feel like it was much faster than the 4.5mph with the old motor.

So at a guess, how much of a speed difference should I be seeing?


--
aborgman

 

[JB]

Re: Estimated top speed?

You won't see much speed difference, aborgman. You are still way down in displacement speeds. Maybe 6-8mph.

 

[aborgman]

Re: Estimated top speed?

You won't see much speed difference, aborgman. You are still way down in displacement speeds. Maybe 6-8mph.

Sounds about like what I guessed - I figured the ~100% increase in horsepower would lead to about a 50% increase in speed due to that whole nasty drag increasing a square of velocity deal...

Which mean this boat with the biggest motor you can throw on it (5hp) would probably be running in the 8-10mph range.

--
aborgman

 

[Kevin W]

Re: Estimated top speed?

Well you went from a 1.2hp air cooled (lighter?) engine to a 50 year old 3hp.
i would guess it weighs more. not a huge amount but some.
it was rated at 3hp in 1950s...most likely engine HP not prop HP
The smaller air cooled may be rated prop HP and the engine may be closer to 2 HP.
Also the 3hp is 50years old and most likely not as powerful as it was new.

I would stick with the 3hp though... i have a early 60's evinrude that is similar and they are much better than the loud air cooled engines(had one sold it).

 

[aborgman]

Re: Estimated top speed?

Well you went from a 1.2hp air cooled (lighter?) engine to a 50 year old 3hp.
i would guess it weighs more. not a huge amount but some.

It does weigh more, but as part of the total weight I'm thinking it's pretty immaterial - of a total weight in the 550lb range we're talking about a difference of about 14lbs in the motors.

it was rated at 3hp in 1950s...most likely engine HP not prop HP
The smaller air cooled may be rated prop HP and the engine may be closer to 2 HP.

Another possibility, although the air cooled motor is from the late '70s.

Also the 3hp is 50years old and most likely not as powerful as it was new.

Actually the thing looks brand new internally, and has great compression - the cylinder walls actually look significantly better than they do on that late '70s aircooled beast.

I would stick with the 3hp though... i have a early 60's evinrude that is similar and they are much better than the loud air cooled engines(had one sold it).

Yep... Even with no performance improvement I'll stick with it - I can actually use the boat without earplugs.

--
aborgman

 

[Kevin W]

Re: Estimated top speed?

The air cooled i had was an old Sears (clinton) from the 60's.
it was 3.5hp and it was most definately more powerful than my 3hp lightwin but the embarresment of the noise echoing off the trees was more than i could take.
plus some day i will need parts and the parts for the Sears where tough at best to get.
the lightwins are easy to find parts.

 

[kenmyfam]

Re: Estimated top speed?

I think you have to put enouigh power on it to "plane it" just a guess but around the 10hp mark will maybe get you there.

 

[wilde1j]

Re: Estimated top speed?

You cannot go faster than "hull speed", which varies slightly according to the specific boat, which JB nailed as ~ 6 to 8 mph, unless the boat can be caused to plane. I have a boat with T 175's, won't plane on one, and gets around 6.5 knots with one motor at WOT. Doubling the HP in the range you're talking about doesn't make a difference at all.

 

[steelespike]

Re: Estimated top speed?

You may be able to throttle back and still make your original speed.
Jon boat isn't exactly the best displacement speed design.
If your talking 500 lbs boat motor and people.You may get planing speed
with a 5 hp.We did easily.12 ft flat bottomed steel boat and a 5 hp Gale.
140 lb adult,2 50 lb kids motor oars misc stuff.
Also popped up easily with a tiller extension and a 180 lb adult on the middle seat.

 

[Scaaty]

Re: Estimated top speed?

3HP "weedless" drives are terrific motors (down to 2 myself..sold 2), but if some big guy rowing passes ya up and smiles, just give him the finger back. He will be long dead and gone, and the Johnny will still be tooting along. Trust me, ya aint even going to pull a water skiing squirell behind it anyway, but ya aint rowing..(run 24-1 oil if it wasn't mentioned..)

 

[Scaaty]

Re: Estimated top speed?

Deleted post (answered..just caught it.. )