arneson surface drive 12

[mr faria]

Cannot get out of the hole even with 2 ASD12's using 5 blade high speed props mounted on a Robrady Caribbean 52' catamaran and powered by 2 C18 Caterpillar 1015hp engines. The hull design was modified with a very low platform 8" above the props when they are fully trimmed up. Can the platform be the problem?

Some pictures? Data of the boat?

[Frosty]

Yes the platform can be a problem from the hole. The props need lots of air. I assume you are fully trimmed up when taking off?

Do you mean the engines wont increase RPM or that the bow comes up

[gonzo]

You need to make sure the props are ventilating. In some applications they use exhaust to get them started.

[mr faria]

Originally Posted by Frosty

Yes the platform can be a problem from the hole. The props need lots of air. I assume you are fully trimmed up when taking off?

Do you mean the engines wont increase RPM or that the bow comes up

Yes the props are fully trimmed up during take off and yes the engines won't increase RPM. To get out of the hole I have to turn and do a circle or move 5-10 people to the bow depending on the load.

[Frosty]

Ok so it does get up there eventually. More air, and yes exhaust are commonly directed just in front of the props, this also helps to reduce exhaust back pressure.

As this is a cat I am surprised, a cat is shallow draft and should not have this prob.

It could well be your platforms. Can you remove the platforms to try it without them?

[mr faria]

Originally Posted by Frosty

Ok so it does get up there eventually. More air, and yes exhaust are commonly directed just in front of the props, this also helps to reduce exhaust back pressure.

As this is a cat I am surprised, a cat is shallow draft and should not have this prob.

It could well be your platforms. Can you remove the platforms to try it without them?

I agree the props need more air, back pressure was thought to be the problem, we modified the exhaust by adding additional pipes after the riser up and out the back but this made no difference. Draft is 3.5'. Platform is a fixed fiberglass structure and the owner doesn't want to remove it (yet).

[Frosty]

You could move the exhausts to under the platform, yes a big job but it wont harm anything.

You could cut 4 inch holes in the platform to try to get air to the props.

What shape is the transom, how does it finish, it should be flat sharp finish to the water flow aft making the water break from the transom. A vacuum break is easy to make.

Do you think the Arnesons where fitted correctly and trim up is far enough.

Could do with some pics!!

I know you have Arnesons but read here what Seafury say about platforms and vacuum breaks

http://www.seafury.com/considerations.html

[mr faria]

Originally Posted by gonzo

You need to make sure the props are ventilating. In some applications they use exhaust to get them started.

Placing exhaust or air behind the props to ventilate should do the trick but platform is only 8" above the props when fully trimmed up during take off. Thanks, will have to find a way try to convince the owner to modify.

[Frosty]

That platform is sealing to the water. Cut some holes on the skirt.

[gonzo]

That's right. There has to be some way for the air to go in.

[mr faria]

Originally Posted by Frosty

You could move the exhausts to under the platform, yes a big job but it wont harm anything.

You could cut 4 inch holes in the platform to try to get air to the props.

What shape is the transom, how does it finish, it should be flat sharp finish to the water flow aft making the water break from the transom. A vacuum break is easy to make.

Do you think the Arnesons where fitted correctly and trim up is far enough.

Could do with some pics!!

I know you have Arnesons but read here what Seafury say about platforms and vacuum breaks

http://www.seafury.com/considerations.html

The platform is solid and does not have a skirt. The transom does have a flat sharp finish to break from the water and the Arnesons are fitted close to specification (the cat hulls were a little to narrow so the steering jacks are on the inside and a wee bit high angled), steering hydraulic hoses had to be increased from 2500 to 3500psi as the hoses would burst. This I believe is caused by the platform acting like a trim tab forcing the bow down the faster you go. Top speed attained is 52mph. Steering is a nightmare at that speed.

[Frosty]

The Hydraulic pressure will not be 2500, theres nothing to push but steer the drive leg.

Do you have a hydraulic or mechanical tie bar.

Why is steering a nightmare?

[mr faria]

Originally Posted by Frosty

That platform is sealing to the water. Cut some holes on the skirt.

Thanks a lot for your expert advice. Will be working hard to convince the owner the props need to be ventilated! One less headache goes along way.

[mr faria]

Originally Posted by gonzo

That's right. There has to be some way for the air to go in.

Thanks for your advice as well, will try and get the air down to the props, hard part is convincing the owner.