30hp, sib380, 13 pitch not right?

[snakeeater]

After two days on the water with my new sib I think I adjusted everything right, but at top RPM the prop seems to loose grip.

When I quickly go to full throttle the prop quickly goes to full RPM with little Thrust. I do not get any bow rise
So I accelerate slowly. With two on board I can reach about 38kph like this, at full throttle. But the engine seems to be 1000rpm under max.

Now if I jump a small wave or take a corner too hard, so the prop catches a bit of air, it sometimes happens tat rpm goes up about 1000rpm to full rpm, but speed stays the same. (does this condition harm the engine?) Then I have to reduce throttle and accelerate again to have grip.

I have a 30hp selva 2stroke. There is a number 13 on the prop. Boat a 3.8m SIB with 150KG load.
With such a well powered engine on such a small boat, does it sound right to you to go for a 11 pitch or should the problem be somewhere else?

Boat is well balanced, Engine timed to the second hole (third rooster tails). Increasing engine height 25mm reduced splash and drag. I had trim tabs installed, but found with a well balanced boat they just cause drag, so I took them off for now.

[jyasaki]

Sounds like the prop is aerating. Pulling air into the prop when running high power. Try trimming the motor down a hole and see if things improve.

Usually, manual trim setups are a compromise; you're not going to get ideal performance over the entire range of use.

jky

[snakeeater]

It is rather like this: when I run high power my wake is clean, no splash and no air sucking, but the engine revs stay a good deal below max rpm (from my hearing). At speed the motor tilt is already slightly downwards. But of cause I will try one more hole down next time.

Now if the prop comes our of the water from a small wake, it sucks a little bit air. Then the revs jump up to max. Then the air stays around the prop and rpm stay max even though the boat is at same speed and stable water flow

The problem is that the prop does not get close to max rpm on WOT.

Also when opening throttle fast the prop seems to slip, rpm goes to max and i do not have acceleration.

From what I see i either have a lot of drag somewhere or the pitch of the prop is too steep for the boat

[jyasaki]

If you're not hitting max rpm WOT, then yes, you've got too much pitch. You need a tach to tell you that, though. I know very few people who can accurately judge rpm by the sound of the engine.

A shorter pitch *may* help with other problems as well.

For the most part, the ventilation goes back to what I said above, assuming the prop hub isn't slipping.

Drag would be easy to spot on a small boat: growth on the bottom of the boat, loose material billowing out underwater, or growth on the motor's lower unit.

jky

[snakeeater]

Thanks jky, will get a tach and a propeller and try more.
Just seemed strange that standard torque is not enough on this boat.

[jyasaki]

Motors are sold with a prop (sometimes.) Mfgr has no idea what it's going on, or how what it's going on is going to be used.

jky

[Tom465]

I have a 3.5 with a 30hp and I have the same problem as you.

Will be interesting to see what it is...

[snakeeater]

I would just have expected that the standard prop would have plenty of torque on an overpowered setup.

Well, prop is on the way, lets see what happens