On 6/5/2012 6:10 PM, Oscar wrote:
On 6/5/2012 4:38 PM, Wayne.B wrote:
On Tue, 05 Jun 2012 10:28:52 -0400, wrote:

The facts:
9'X5' inflatable with inflatable keel and hard floor.
5hp 4 stroke engine.
7 1/2 X 7 3 blade prop.

Here's what's happening:
Engine runs smooth as silk.
At about 1/2 throttle GPS measures 4 1/2 MPH
At full throttle GPS measures a little over 5MPH
There is no noticeable change in pitch or DB between 1/2 and full
throttle.
Engine doesn't seem to be bogging down.
Speed increase between 1/2 and full throttle is gradual even though
throttle is advanced quickly.
Boat bottom is clean.
Prop is in excellent shape.

Questions:
Is this normal?
Should I try a lower pitched prop?
Do I need more Horsepower?
What do you think?

===

As others have mentioned, you are running up against the hull speed of
the dinghy and don't have enough power to get on plane. My
experience with inflatables is that you need at least 8 or 9 hp to get
on plane with 2 adults, and 15 hp is even better. A Doel-Fin on the
outboard is also a big help.

I assume you have a rigid transom? If so it should have a
manufacturers plate somewhere which states your max horsepower rating.
That's usually about the right number for everyday use.

http://www.basspro.com/Doel-fin-Hydrofoil-Stabilizer/product/27337/88162?hvarAID=shopping_googlebase&om_mmc=shopping_ googlebase


I doubt they make doel fins small enough.
I suspected HP was the problem and you guys confirmed it.
I suppose the best thing to do is use just enough throttle to drive it
to hull speed and be satisfied.
thanks for your input.

You are correct, and that is exactly what the boat is designed for... If
you want to go faster, get the right boat.

On 6/5/2012 6:39 PM, JustWait wrote:
On 6/5/2012 6:10 PM, Oscar wrote:
On 6/5/2012 4:38 PM, Wayne.B wrote:
On Tue, 05 Jun 2012 10:28:52 -0400, wrote:

The facts:
9'X5' inflatable with inflatable keel and hard floor.
5hp 4 stroke engine.
7 1/2 X 7 3 blade prop.

Here's what's happening:
Engine runs smooth as silk.
At about 1/2 throttle GPS measures 4 1/2 MPH
At full throttle GPS measures a little over 5MPH
There is no noticeable change in pitch or DB between 1/2 and full
throttle.
Engine doesn't seem to be bogging down.
Speed increase between 1/2 and full throttle is gradual even though
throttle is advanced quickly.
Boat bottom is clean.
Prop is in excellent shape.

Questions:
Is this normal?
Should I try a lower pitched prop?
Do I need more Horsepower?
What do you think?

===

As others have mentioned, you are running up against the hull speed of
the dinghy and don't have enough power to get on plane. My
experience with inflatables is that you need at least 8 or 9 hp to get
on plane with 2 adults, and 15 hp is even better. A Doel-Fin on the
outboard is also a big help.

I assume you have a rigid transom? If so it should have a
manufacturers plate somewhere which states your max horsepower rating.
That's usually about the right number for everyday use.

http://www.basspro.com/Doel-fin-Hydrofoil-Stabilizer/product/27337/88162?hvarAID=shopping_googlebase&om_mmc=shopping_ googlebase



I doubt they make doel fins small enough.
I suspected HP was the problem and you guys confirmed it.
I suppose the best thing to do is use just enough throttle to drive it
to hull speed and be satisfied.
thanks for your input.

You are correct, and that is exactly what the boat is designed for... If
you want to go faster, get the right boat.

Yes.

On Jun 5, 9:28*am, Oscar wrote:
The facts:
9'X5' inflatable with inflatable keel and hard floor.
5hp 4 stroke engine.
7 1/2 X 7 3 blade prop.

Here's what's happening:
Engine runs smooth as silk.
At about 1/2 throttle GPS measures 4 1/2 MPH
At full throttle GPS measures a little over 5MPH
There is no noticeable change in pitch or DB between 1/2 and full throttle.
Engine doesn't seem to be bogging down.
Speed increase between 1/2 and full throttle is gradual even though
throttle is advanced quickly.
Boat bottom is clean.
Prop is in excellent shape.

Questions:
Is this normal?
Should I try a lower pitched prop?
Do I need more Horsepower?
What do you think?

BTW Oscar, thanks for the boating topic!

On 6/5/2012 6:30 PM, Tim wrote:
On Jun 5, 9:28 am, wrote:
The facts:
9'X5' inflatable with inflatable keel and hard floor.
5hp 4 stroke engine.
7 1/2 X 7 3 blade prop.

Here's what's happening:
Engine runs smooth as silk.
At about 1/2 throttle GPS measures 4 1/2 MPH
At full throttle GPS measures a little over 5MPH
There is no noticeable change in pitch or DB between 1/2 and full throttle.
Engine doesn't seem to be bogging down.
Speed increase between 1/2 and full throttle is gradual even though
throttle is advanced quickly.
Boat bottom is clean.
Prop is in excellent shape.

Questions:
Is this normal?
Should I try a lower pitched prop?
Do I need more Horsepower?
What do you think?

BTW Oscar, thanks for the boating topic!

My pleasure.

On 6/5/2012 7:12 PM, wrote:
On Tue, 05 Jun 2012 10:28:52 -0400, wrote:

The facts:
9'X5' inflatable with inflatable keel and hard floor.
5hp 4 stroke engine.
7 1/2 X 7 3 blade prop.

Here's what's happening:
Engine runs smooth as silk.
At about 1/2 throttle GPS measures 4 1/2 MPH
At full throttle GPS measures a little over 5MPH
There is no noticeable change in pitch or DB between 1/2 and full throttle.
Engine doesn't seem to be bogging down.
Speed increase between 1/2 and full throttle is gradual even though
throttle is advanced quickly.
Boat bottom is clean.
Prop is in excellent shape.

Questions:
Is this normal?
Should I try a lower pitched prop?
Do I need more Horsepower?
What do you think?

If you have any V in the floor, 5 HP should be enough to just get
about 600 pounds up on plane.

I would be interested in assessing the RPM at full throttle, before
making a judgment on HP considerations.

A tach wouldn't tell me anything more useful than my ears have already
determined. I'm positive that WOT RPM is way too low.

On Tue, 05 Jun 2012 21:25:10 -0400, Oscar wrote:

I'm positive that WOT RPM is way too low.

===

In that case a prop with lower pitch is the right way to go. At least
the motor will be operating within its designed range and you might be
able to get on plane if you're really lightly loaded.

On 6/5/2012 11:14 PM, Wayne.B wrote:
On Tue, 05 Jun 2012 21:25:10 -0400, wrote:

I'm positive that WOT RPM is way too low.

===

In that case a prop with lower pitch is the right way to go. At least
the motor will be operating within its designed range and you might be
able to get on plane if you're really lightly loaded.


You almost have me convinced to put a different prop on it. If I do, I
will Tach it first like someone else suggested.

On Wed, 06 Jun 2012 09:26:03 -0400, Oscar wrote:

On 6/5/2012 11:14 PM, Wayne.B wrote:
On Tue, 05 Jun 2012 21:25:10 -0400, wrote:

I'm positive that WOT RPM is way too low.

===

In that case a prop with lower pitch is the right way to go. At least
the motor will be operating within its designed range and you might be
able to get on plane if you're really lightly loaded.


You almost have me convinced to put a different prop on it. If I do, I
will Tach it first like someone else suggested.

===

I put a lower pitch prop and a Doel-fin on our Honda 4 stroke and it
made all the difference in the world. As a bonus you end up with a
spare prop. :-)