Why is my SIB so slow?

[Merrie Thames]

Morning all

Took the boat up to Chelsea and back from Richmond yesterday (4.2m Tohatsu with Tohatsu 20hp four stroke) and did some timings for a DST calculation when I got back. What I thought was a good speed turned out to be just 13kts. I'm sure this can't be the best it will do as my old 18hp motor gave 19kts.

I tried having the motor in a couple of different trim positions and moved the weight (2 up) around and it did handle differently but not faster. I did notice that using any more than 1/2 rpm caused the boat to be sucked back down and come off the plane.

Any suggestions welcome!

[chris123]

That does seem rather slow. With a 15HP outboard I can easily get 16 knots from my 3.5metre SIB which probably weighs quite a lot more than your 4.2metre SIB. I get 13 knots with a 9.9HP.

I'm not sure what you mean by:

I did notice that using any more than 1/2 rpm caused the boat to be sucked back down and come off the plane.

Does this mean you are getting 13 knots at half throttle? Assuming everything is inflated to the correct pressure, the outboard has the right shaft length for your boat, and the prop is a reasonable diameter / pitch (should be stamped on it, worth posting details here) I am wondering if:

1. You are getting a problem with fuel starvation.

2. The prop hub is slipping.

3. The prop pitch is much too low (not sure about this one)

A rev counter would help with the diagnosis, but assuming you do not have one, when you increase above 1/2 throttle does it sound like:

1. The outboard bogs down / does not increase revs as much as you would expect?

2. The revs seem to rise rapidly with no increase in speed?

The best way I know to check for fuel starvation is to run at full throttle for as long as possible, then pull the killcord without closing the throttle. You can then pop the spark plugs out and check the colour. The tricky bit is finding somewhere you can do this safely - it is a lot easier with a motorcycle!

Hopefully someone can suggest a simpler / safer approach.

Cheers

Chris

[Poly]

Originally Posted by chris123

The best way I know to check for fuel starvation is to run at full throttle for as long as possible, then pull the killcord without closing the throttle. You can then pop the spark plugs out and check the colour. The tricky bit is finding somewhere you can do this safely - it is a lot easier with a motorcycle!

Hopefully someone can suggest a simpler / safer approach.

if the problem is fuel starvation then it may go away temporarily if (when you have the symptoms) you blip the choke, or squeeze the fuel primer bulb - depending exactly what the problem is.

[m chappelow]

How about the propeller pitch ,or that a plug is faulty ,,when over a certain amount of revs ,,i had that with a near brand new engine last year , it was one plug that had a ceramic tip cracked on the electrode that vibrated up and down ,plug looked ok and would spark up to half revs then cut out on one cylinder ,when at low revs it would spark again ,,,,could you have left any transom wheels in the down position at all ,

[Merrie Thames]

Thanks for the advice everyone. The followign thoughts come to mind:

I really don't think it is fuel related as she will rev to full revs without any coughing, spluttering etc.

The prop could be the wrong pitch for the boat but it is the standard one that came with the engine so I have not had a look at that yet.

What I meant by: I did notice that using any more than 1/2 rpm caused the boat to be sucked back down and come off the plane. was that using any more than 1/2 revs made the boat come off the plane and slow down- a lot.

No wheels fitted either, or anchor left down (this time!!!).

So next time out, I will try adjusting the outboard's trim position and see if that helps. If not then it is time to start looking at props.

Thanks for the advice all.

[jyasaki]

Originally Posted by Merrie Thames

What I meant by: I did notice that using any more than 1/2 rpm caused the boat to be sucked back down and come off the plane. was that using any more than 1/2 revs made the boat come off the plane and slow down- a lot.

That's normal. A boat at rest takes a bunch of power to start moving, climb out of the hole (i.e. to begin moving enough to get the hull to displace the water from underneath and start planing on top), but much less power to tay there. A normal takeoff involves wrapping the throttle until the boat is up then back off the throttle to attain your desired speed.

Slowing down, there's a point where the throttle setting is no longer generating enough speed to keep the hull up on top, and the boat settles back into the hole. At that point, if you leave the throttle where it was, you'll just plow through the water at a greatly reduced speed; it takes a bunch more power to get it back up on plane again.

jky

[chris123]

Originally Posted by Merrie Thames

I really don't think it is fuel related as she will rev to full revs without any coughing, spluttering etc.

If the revs are going up but the speed is going down it definitely sounds like a prop issue to me.

Maybe try marking the prop relative to the hub nut and check that they still line up after a run?

Cheers

Chris

[BumbleAbout]

Originally Posted by Merrie Thames

my old 18hp motor gave 19kts.

That wouldn't have been a much lighter 2 stroke outboard would it ?

[prairie tuber]

Originally Posted by chris123

If the revs are going up but the speed is going down it definitely sounds like a prop issue to me.

Maybe try marking the prop relative to the hub nut and check that they still line up after a run?

Cheers

Chris

That is most likely what the issue is. Especially if the same thing is happening with other trim positions and motor heights, it is probably not a ventilation or cavitation issue. One other thing to check, is to see if their is a loose flap of fabric on the bottom of the hull particularly near the transom - that would also cause cavitation.

[C-NUMB]

Originally Posted by prairie tuber

. One other thing to check, is to see if their is a loose flap of fabric on the bottom of the hull particularly near the transom - that would also cause cavitation.

That is correct, that happends also to the aerotec in case air flor is not properly inflated, fine until 12-13 knots but then terrible cavitation due to a small "pocket" just in front of transom. Increased preasure corrects the issue. If engine trimmed out too much, similar symptons but then also the sound of the engine is louder.