Remove Exhaust Outlet??

[penbroke]

I have a '66 18 with a Volvo 200B outdrive and thru-transom exhaust. Any reason to keep the exhaust outlet that hangs down in behind the prop? Seems like it is just blocking thrust and allowing air to be drawn into the propwash thru the exhaust passages in the drive. I was thinking of replacing it with a round stainless or alum. plate with drain holes and a maybe a trim tab welded to it. Any thoughts??

Frank
Still aground in Chicago.

[BigGrizzly]

Remove it and put the stright fin in its place it is worth 2 mph ! did my 16 in 1966 and it is still that way

[MOP]

The fin does work really well, try to make it so you can adjust for steering torque.

[penbroke]

Just as I thought, and it shall be adjustable.

How big a tab are you guys using? I was thinking of starting with about a 3 inch square and cutting the front at an angle so the bottom edge is around 1 inch with a vertical trailing edge.

Frank
Still aground in Chicago, but I feel the tide rising...

[penbroke]

Like this?

http://www.donzi.net/photos/penbroke_exhaust_tab.jpg

Frank

[BillG]

I removed the tab on my '66 2+3 and replaced it with an aluminum disc and it did not make any difference as far as torque went. I think the fin is just drag. I think the prop you use has more to do with the torque problem than the fin.

Bill G

[MOP]

My thoughts on the tab, I have felt for many years that the tab afforded a little safety also by helping to unload steering torque. I believe like us most shops set the tab to neutralize the torque at cruise IE: you can take your hands off the wheel and still maintain a reasonably straight course. A little offset will steal a tad of speed but extend the life of the steering. 90% of the tabs I have set were on big outboards, honest I could not see any lose in top RPM afterwards.

[penbroke]

NOW you tell me I could have bought one???

Frank

[boldts]

I had a 18 Classic with a 250 Volvo. Traded the boat before swapping the exhaust tab for a sleeker version, but let me tell ya, on a boat with-out power steering, that tab makes a big difference in the steering torque depending on which way it is directed. I couldn't image running the boat with-out it and I was only turning a 14" by 23" prop. Maybe my application was not apples to apples with what you all are running.

[penbroke]

Bolts: Actually the application is almost identical. '66 18, Volvo 200B, Ford 289 Hipo, Prop is stamped 14-3/8 x 23. I got the boat late last fall (just in time to put away) so I don't have any good speed/RPM numbers. The boat did handle well with the old outlet the couple times I ran it. Hope to get more serious about it real soon.

Madpoodle: Thanks, but I think I'll stick with this one for now. I may have some more material tho... Watch your e-mail for a parts wish list.

Frank

[MOP]

If you change to the fin style you will get a little better preformance all around, the difference compare a lemon sized object to a knife like object going throught the water. The flapper outlet disturbs the props thrust.

[penbroke]

Finally had a chance to run the boat last weekend. The tab made a HUGE difference! With the "lemon" I was fighting the steering wheel at almost any speed but with the tab the feedback was greatly reduced. I don't have any good information on what it does for speed but the boat feels a lot better at all speeds and seems to track better at dead slow as well.

Frank
Time for another shakedown.

[MOP]

I was waiting to see how you made out, that looked like a hot ticket.

[TuxedoPk]

The point of a nosecone is to make sure all the flow over the gear case is laminar. There are two kinds of flow: Laminar and turbulent. Laminar is smooth flow of water around and object without eddying or separation. Turbulent flow occurs when the flow separates from the object. The amount of drag caused by turbulent flow is significantly greater then the drag from laminar flow.

So the point of a nose cone is to keep the flow laminar. How does it do this? Well as you increase the length of the gear case it decreases the aspect ratio which allows for the smoother flow of water. It follows the principal a long skinny ship goes through he water better then a short fat one.

However, while the object is to create laminar flow if your gear case is already experiencing totally laminar flow and you increase the length of the gear case all you are doing is adding more surface area, which in turn creates more drag. So the object is to create the shortest gear case with the least surface area that will stay in totally laminar flow. I have not finished the calculations yet but it appears the numbers correlate well with what I have heard. It seems to me, mathematically, that the speed ranges where nosecones become an advantage from a speed perspective are as follows:

· Alpha 74-76
· Bravo 80+
· Volvo/TRS 72-75

It may surprise you that the number for the Alpha is lower then the bravo because the alpha is smaller. However the logic behind this is that the alpha has a hydrodynamicly poor design compared to the bravo. While it is physically smaller its shape, blunt front, and short chord length all contribute to this.

That being said there are other advantages. When I added a nosecone to a Volvo 280 and my boat was running 60 I noticed a large decrease in steering effort and a slight increase in control. Where does this come from? Adding a nosecone increases the “rudder” length of the drive allowing more precise control.

Another uncommon fact about nosecones is that in most cases (parabolic nose cones excluded) they actually will create more stern lift. The increased length and surface area in the stern of the boat create more area for the flowing water to push up on and in turn create stern lift. This also is applicable to propeller hubs. The smaller diameter hub the less stern lift created.

** I didn't suddenly get knowlegeable, this is from CDMA

[MOP]

Penbroke how much offset did you try? I found it can be a little tricky with the Volvo, even from new they have a little extra looseness in the steering. If you get to close to neutral the steering get wobbly, a bit of tourque helps my favorite drive.

[vonkamp]

I recently bought a 18 with a 280 drive. Someone installed hydraulic steering and removed the "lemon". I was going to put a plate where it was. I've only had the boat out once as it needs some work, but when I back off the throttle, the boat tends to veer left pretty hard eek! . Would installing a straight fin help to correct this?

[Surfer]

Could someone clarify what would replace the rubber flap, or do you need anything? Who is a good vendor in south Florida that would have the fin in stock, and for that matter who is the best Volvo (280) guy in south Florida?

[penbroke]

M.O.P.,
I was more concerned about checking the dozens of other changes I made since last fall so I just set it straight ahead for now. Need to do some more running but it takes a LOT of strain off the steering system.

Frank

[vonkamp]

Surfer,
In north Florida, Joel Stuart (Stuart Marine, 904-269-2599) rebuilt my volvo 280. I've been told he is the Volvo guru. A little pricy but if you buy the parts it will save a few $$$ and he did a great job on mine.

[MOP]

Penbroke you can kick that tab over about 20 degrees, what you are looking for is light steady load at cruise. As speed increases it will load the steering and keep the slack out.

[penbroke]

Thanks M.O.P. I think I have most of the other bugs worked out so I hope to be able to play with the tab next time out. The first shakedown was cut short by a cooling hose leak. I need another day of tweaking to find out where the top end is. wink

Frank

[kjly63]

Just found this old post:) Where can I find a steering fin for my 270 lower???
I havn't had any luck in finding one without the exhaust outlet.

Kenny