Glen-L.com

Hi All.

My 18' Monaco (straight-shaft inboard) performs very well, except when I go for the top end. I have porpising at over approx 3500 RPM.

I am not a "speed freak", and I live on a small lake, so this is more of a "mental challenge" than a barrier to enjoyment of my boat.

I don't feel that visible trim tabs off the transom are an appropriate "look" on a '50s style Woody.


More facts.

- I shortened the boat by 10% from the design.
- My engine should not be run at more than 4000 RPM. I comfortably cruise at 2600.
- The bottom of the hull is straight and true
- The boat has a large rear-mounted tank of fuel that is full right now. (Moving the fuel tank (or other weight) forward is not an attractive option)


Question

In researching options, I found this post that describes 6" x 6" x 1/2" wedges that are placed under the hull, 14" either side of the rudder.

http://www.chris-craft.org/discussion/viewtopic.php?t=1018&highlight=porpoise

Does anyone have any experience with these? Other than having to plug some screw holes if they don't work, do you see any down-side to giving them a try?

Thanks.

_________________
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

A very common practice for well boats and open tiller handle skiffs is to install wedges across the transom. The key is to figure out how much you need with inboard motor boats. With outboards you can trim the motor up a bit to offset if you add too much. I would start with 3/4" by 12 inches across by 4 to six inches wide. YOu can even fasten from the inside too if you can get to the region and use a good bedding compound. If you glassed your bottom you also can just use some epoxy and place blocks to hold until dry and then tab them in place when you get them "tuned", ground down if you have to.

Thanks, Oyster.

I guess that makes sense... let's see if I understand... Would this work?

1) roughly fabricate the tabs (wedges) with more thickness than I think I need

2) in the area of the tabs, sand off the boat's bottom paint down to the fiberglass

3) epoxy the tabs to the bottom (no fasteners).

4) Shape/Feather the front edge.

5) test the boat's performance - if necessary, sand/grind down the wedges until I am happy with them (that may be back to flat)

6) repaint bottom.

One question...

In your recommendation, I assume you recommend dimensions for each tab of 12" athwartship, and 6" longitudinal. This is about twice the "convention" that the Chris Craft folk use. I realize that I can always remove it if is too much.

But how will I know if I have "too much" tab?

Also... I spend the 95% of my time cruising at 2600 - 3000 rpm. I am currently very happy with the trim and performance of the boat at those speeds... is there any risk that adding these tabs would screw that up? Are there any other consequences that I should expect?

B.

_________________
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

The wider the tab the more gradual the angle which also requires less thickness even though the angle carries further foward. Personally I rough cut my tabs on a bandsaw and then finish with a RO. YOu can also let the smoothe side be out and backfill the rough cut with the glues.


But with glass bottoms we also also create shims using 403 west system fibers too and let cure to a green stage and then skin the surface with 406 cabosil to fill the roughness that 403 creates. then use those for testing and shaping. These wedges may also improve your mindrange when loaded and when you get on plane you can also cut back on the throttle for efficency and the same trim as you have noted now.

You may get some bow walking in the beginning if you have too much but thats all that we have experienced.

Thanks, Oyster.

I've been doing some more research with the Chris-Craft sites.

Here's a picture of the fix that worked for someone who dropped a 283 in an older style hull, and found that he was porpising due to the extra power.




I'll keep "looking and learning"

_________________
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

pic didn't come through Bruce

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

Funny.

It came thru on preview, and looks fine on mine.

I downloaded the photo... hopefully, this works...

_________________
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

Got it that time....looks pretty extreme.

I,ve never seen wedges that abrupt and deep

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

I agree.

I think I would have them less thick, and more "tapered". The chap who did that had very severe porpising at much lower RPMs than me.

Another "old timer's" trick that I read about was to simply screw some left-over 4" long strips of half-oval rubrail trim to the back of the bottom. (equal sizes, equal distance from the middle)

that sounds like the "ten minute" solution. Maybe I'll try that first, and see if it makes a diff. I'm up at the lake right now, but my leftover rubrail is in the garage at home, so I can't go out and do it this afternoon.

_________________
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

How about a pack of those wedges that they sell for shimming door frames?

You could get some of that 5 minute epoxy to stick a bunch of them next to each other to the hull without drilling holes to see how that works.

Available at a hardware or box store,and you can add more or take off if they don't work with no holes breaching the bottom..

Get a trial,then make permanent ones later

_________________
Boat building can best be defined as an endless series of
tragedies obscured by the occasional miracle, followed by a good bottle of beer.

Don't Dream Your Life, Live Your Dream

Well in my usual suttle and not so pointed point of view regarding that abortion called a wedge, I would never put a basketball on the bottom of anything I have ever own, hidden or not. The same applies in those dolfins too, imo. At least when we do the ones I described it takes someone to point them out to the average man on the street to notice them. In most cases too they worked. Maybe try the rails. But the problem with any exposed fasteners long term water will almost always find its way into the wood from the threads.

Here is the Chris-craft service bulletin on the topic...

Obviously, the wedges should be "wedge-shaped"... and properly feathered at the front...

_________________
Bruce.

~~ Do what you love, and love what you do. ~~
~~ To me - only my boat is not yet perfect. Everybody else's is to be admired for I know the path they have walked (Dave Lott, 2010) ~~
Dow's Monaco Project

Hi Bruce,

I have some experience in this area with my Chris-Craft Riviera that we completed about two years ago. It has way more horsepower than the original boat and at wider throttle openings it would begin to porpoise. At normal speeds it was fine.

I wanted to be able to open the throttle right up if I wanted to so I mucked about with the wedges. I also saw the material you described earlier but thought they looked pretty severe.

I made mine out of plywood and covered them with epoxy and eventually painted them to make them invisible (nearly anyway).

They were intitially 200mm X 200mm and have a wedge from zero at the front to 20mm at the rear. I stuck them on with double sided sticky tape and gave them a quick test.

They made quite a difference. I then mucked around with the size some and ended up adding an extra 10mm at the rear. They are now about 220 X 200 with a wedge from zero to 30mm. They are epoxied and painted the same colour at the bottom paint. They are next to invisible.

I can now open her up and hold WOT with no problems. I can get 80kph (seen on a GPS) out of the boat whereas before I could only hold about 60kph. The attitude of the boat at slower speeds is also slightly better and I feel that it comes on plane easier as well.

Well worth a try.

_________________
Gordon
Tasmania, Australia

Since 1968 the date on the bulletin we have come a long ways in the way we treat our bottoms.

Bruce:
Do you have spray rails on the chine? These make a big difference also, mine are installed to do as trim tabs. Our Double Eagles needs them for a good plane without listing. Mine are 1" wide at transom and run a full 1" for the next 8' forward. that is 96" and the sq root of 96 is 9.6" so I figure I got about a 9"x9" trim tab on each side of the boat.
Not really trim tabs but just spray rails.

Butch Barto added them to his also, and says he runs better and picked up 200rpm.

_________________
God Bless our Troops; especially our Snipers