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?

[apex1]

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]

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]

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]

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]

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]

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]

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.

[mr faria]

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?

The hydraulic hoses get extremely hot so were thinking of putting an oil cooler. We don't have a tie bar. Problem is when I get up on a plane exceeding 29mph on up the boat runs on it's bow, even running with trimmed all the way up. Being a cat the bow shoots one way or the other so you have to constantly correct your steering. On turns boat leans the wrong way, away from turn rather than into it, giving the feeling of rolling.

[Frosty]

So its power steering, pump assisted.

You do have a tie bar but it is Hydraulic, integral in the system.

If you don't this could be your erratic steering probs. Imagine your car not having the front wheels connected to each other.

You should have balanced rams that displace fluid from the rear as it is actuated from the front, this will be connected to the other ram. That is a hydraulic tie bar, meaning the rams are hydraulically "tied"to each other.

If you can run with trim full up, there is something wrong. We need to get into shaft drive angles and prop rakes --oh dear!

Your rotation of the props will want to pull or push the drives apart or together depending on your rotation, your Hydraulic tie bar should always be in compression.

Your platform is so big you would not be able to see if the drives had come out of alignment. You should have a cock on the hydraulic tie bar line to adjust alignment.

You are certainly not under powered, it would seem to me that your probs are installation related.

You need to contact Rick at Arneson.

[mr faria]

Yes it's got a power steering pump with hydraulic tie bar, running full trim up. I believe the platform prevents the props from ventilating so boat cannot out of the hole, then after great difficulty, acts like a trim tab pushing the bow down after getting on a plane. Thanks again I will be getting in touch with Rick.

[Rik]

Cut a hole, large one near the propellers and put a grate in place so that one cannot fall through into dangers way.

This will enable the vacuum to be broken and your plaining will improve.

As for speed handling, your drives are not in sync.

Do you have any photos of the rear view showing the tunnel height?