Help with boat/motor problem

[Toller]

I have built a wakeless launch for my son's rowing club. It is essentially an old 17' hobiecat with a deck made out of square aluminum tubes and plywood. I have attached photos of the boat.

I water tested it Saturday with a 9.9hp shortshaft motor. It weighed about 1100 pounds. In actual use it will have 2 people rather than the 3 tested, but will have seats and a console; so I figure the actual weight will be 1000 pounds.

At half throttle it did fine and got up to 10mph. With increased throttle it started throwing water up from the motor and at its full speed of 13mph it was throwing up a pretty big fountain.

The top of the motor mount is about 11" above the water line. I "think" it was pretty level during the whole run. We could shift the balance a few inches by moving the passenger up and back a few feet, but it was reasonably stable. In an earlier test we had the mount 6" further down, but the mount caught in the water and didn't work out, so it was raised.

So:
1) What is causing the fountain and how do we get rid of it. One person thought perhaps the prop wasn't deep enough, though with the mount 11" above the water is "should" have been deep enough, shouldn't it? Would going to a long shaft help?
2) The crew shells that the launch has to keep up with go 12mph; so it is barely adequate with a 9.9hp, but I am thinking that it isn't a good idea to run it at full throttle. What short of performance might we get from a 15hp? Or maybe eliminating the fountain would give us better speed?

Any help would be appreciated.

 

[Bondo]

Re: Help with boat/motor problem

I have built a wakeless launch for my son's rowing club. It is essentially an old 17' hobiecat with a deck made out of square aluminum tubes and plywood. I have attached photos of the boat.

I water tested it Saturday with a 9.9hp shortshaft motor. It weighed about 1100 pounds. In actual use it will have 2 people rather than the 3 tested, but will have seats and a console; so I figure the actual weight will be 1000 pounds.

At half throttle it did fine and got up to 10mph. With increased throttle it started throwing water up from the motor and at its full speed of 13mph it was throwing up a pretty big fountain.

The top of the motor mount is about 11" above the water line. I "think" it was pretty level during the whole run. We could shift the balance a few inches by moving the passenger up and back a few feet, but it was reasonably stable. In an earlier test we had the mount 6" further down, but the mount caught in the water and didn't work out, so it was raised.

So:
1) What is causing the fountain and how do we get rid of it. One person thought perhaps the prop wasn't deep enough, though with the mount 11" above the water is "should" have been deep enough, shouldn't it? Would going to a long shaft help?
2) The crew shells that the launch has to keep up with go 12mph; so it is barely adequate with a 9.9hp, but I am thinking that it isn't a good idea to run it at full throttle. What short of performance might we get from a 15hp? Or maybe eliminating the fountain would give us better speed?

Any help would be appreciated.

Ayuh,.... Welcome Aboard,.... Did ya try changin' the trim angle of the motor,..??

11" from the water line oughta be Ok,... I imagine the motor's built for a 15" transom...

Ya got any pictures of the motor on the craft,..??

 

[Bob_VT]

Re: Help with boat/motor problem

Welcome to iboats.

1000 lbs is quite heavy for a 9.9 and that hobi hull and if you are counting the seats and console with that weight........

I would almost opt for a 15 or larger.

The fountain effect is normally from having a motor mounted low but, that is with a hull/transom directly in front of the motor..... you are running that motor is clear water and as with a jack plate on a motor you can usually mount it higher. Just watrch your water pick-up so you do not overheat.

 

[Ned L]

Re: Help with boat/motor problem

Do you think the "fountain" is water that is actually being thrown up by the prop, or could it be being thrown up by the front face of the lower unit of the outboard sort of 'plowing' through the water? An otboard at 'higher speeds' is supposed to have only from just above the caviation plate and below in the water. If you are trying to push a good bit of lower unit leg through the water it can end up causing a good bit of 'splashing'.

 

[Toller]

Re: Help with boat/motor problem

Do you think the "fountain" is water that is actually being thrown up by the prop, or could it be being thrown up by the front face of the lower unit of the outboard sort of 'plowing' through the water? An otboard at 'higher speeds' is supposed to have only from just above the caviation plate and below in the water. If you are trying to push a good bit of lower unit leg through the water it can end up causing a good bit of 'splashing'.

I don't know what the fountain is. I think the motor is low enough that it shouldn't be the prop.
You are suggesting that the motor is too low and the actual lower part is throwing the water up? I suppose that kinda makes sense; on a normal boat the lower part is shielded by the boat, but on this there is nothing.

So if I have a 15" short shaft and it is 11" from the top off the motor mount to the water line, then I should try raising it up 4". Or I could try it with only two people and the lighter weight should raise it up a couple inches.

 

[JB]

Re: Help with boat/motor problem

Try to set it so that the antiventilation plate is right at water level when under weigh. That will put enough of the foot in the water for good "traction" but not so much that it makes drag and huge spray.

 

[Toller]

Re: Help with boat/motor problem

Try to set it so that the antiventilation plate is right at water level when under weigh. That will put enough of the foot in the water for good "traction" but not so much that it makes drag and huge spray.

I will test it again with just two people (that should raise it an inch or two) and put an inch shim on the motor mount.
Hopefully that will help.

I emailed someone who had built a wakeless launch entire from scratch. He told me the commercial ones use deflectors in front of the motor to keep water away from the shaft. That would seem to confirm the problem.