[dennismc]

They were std equipment on Some Hatts to help reduce prop rumble, they are connected to the transom also.

[Capt J]

Originally Posted by dennismc

They were std equipment on Some Hatts to help reduce prop rumble, they are connected to the transom also.

That makes sense, and I've seen similar things from Hatteras. It's also possible the boat has had an extension put on and needed them for lift or something like that.......

[Capt Bill11]

What is the square partially filled opening on the right side of the picture all about? The original attempt to inject air?

[SHAZAM]

My old 65 SF had the same setup. I always thought it was to aerate the props.

[dan1000]

Originally Posted by Capt. Bill11

What is the square partially filled opening on the right side of the picture all about? The original attempt to inject air?

Half way up the right hand side of the picture is the attachment point of the strut to the hull. I'm not quite sure what to call this strut, but it's the strut that holds the bearing inside which the prop shaft turns.

In this picture, in case it isn't clear, the bow of the boat is off to the right, the stern to the left, starboard at the top, and port at the bottom. Between the blades of the prop, the port rudder post can be seen, but the port prop is not visible. The port strut (ahead of the port rudder post) is partially visible.

The similar set of holes above the port prop is not visible, because the keel between the props blocks them from view, although this is not terribly obvious from the picture.

Dan

[K1W1]

Originally Posted by dan1000

Half way up the right hand side of the picture is the attachment point of the strut to the hull. I'm not quite sure what to call this strut, but it's the strut that holds the bearing inside which the prop shaft turns.

Hi,

The name is a "P Bracket" as far as I know, it looks like there has been some fairing around the aft edge fall out in the first photo you posted.

[Capt Bill11]

Originally Posted by K1W1

Hi,

The name is a "P Bracket" as far as I know, it looks like there has been some fairing around the aft edge fall out in the first photo you posted.

That is sort of what it looked like to me. But I can't understand why they didn't fill it in before they painted the bottom.

[Innomare]

Originally Posted by YachtForums

I don't know how I missed this thread. Maybe 'cause there's so much Cat talk in tech and I'm a dog with diesels.

All of you have the right answer. Like K1W1 original suggested and Loren confirmed, this is a ventilation system. While it could certainly be a vacuum actuated system to drain bait tanks, only one hole would be required with a deflector. Besides, this was a motoryacht according to Dan, not a fish boat.

Like CatTech stated, venting the props allows slip, which can help swing a bigger wheel, which can help with acceleration and RPM's throughout the operational spectrum. But, it's not just a performance issue. While it may sound theoretical, venting creates an aerated boundary layer above the prop that helps insulate noise. The placement of holes, as well as the deflector to create a negative pressure zone over them looks just about right in my book, although it’s a pretty crude.

Great info! I had never seen this before. If I understand correctly, it's a patch to deal with insufficient blade tip clearance. Or - if you want to look at it in a positive light - a technical innovation allowing you to use a bigger propeller or smaller shaft angle with the same noise levels. Pressure pulses on the hull from the propellers are a major source of onboard noise, and I can see how a layer of air can counteract this.

A few more questions:
1. are the holes connected to a pipe above the main deck (atmospheric pressure), to a compressor or to the exhaust system?
Edit: just read in one of the posts above that they are connected to the transom, which seems logical.


2. aren't there any negative side-effects from the ventilation? We usually try to keep the underwater exhaust far away from stabilizers, props and rudders, because they loose efficiency when ventilated (except for surface-piercing propellers, which are specifically designed for this).

3. does this really help at all - given the more recent models don't seem to have it?

Bruno

[Capt J]

No, consider it more of a patch to fix a screw up.

[YachtForums]

Originally Posted by Capt J

No, consider it more of a patch to fix a screw up.

I meant to answer Brunos' question, but I've been playing catch up all week. Took off yesterday for the funeral of Clarence Clemens, a long time friend of the family. He's on his way to see the 'real Boss', bringing heaven a good dose of soul.

CaptJ, how do you surmise this system is a patch?

[Capt J]

Originally Posted by YachtForums

I meant to answer Brunos' question, but I've been playing catch up all week. Took off yesterday for the funeral of Clarence Clemens, a long time friend of the family. He's on his way to see the 'real Boss', bringing heaven a good dose of soul.

CaptJ, how do you surmise this system is a patch?

If the hull was designed with more space above the props, I don't think it would be added. The few existing thru-hulls that are filled in, indicate that it was more of a trial and error process that was incorporated after the boat was launched. Sort of like the first 1/2 dozen 63 MY's had to have that 1" lip added into the tunnel so they'd get more stern lift, then after that it was incorporated into the mold. Same with the 60' SF that had a lip or hump on the bottom at the transom, that they ground down on the first few because it ran too flat, then incorporated the change into the mold.