Performance question on unusual C-Dory

strike3

I purchased a 1989 22’ classic that was customized in the following ways. An aluminum cockpit was added. It had an armstrong bracket installed. It came with 115 hp Mercury Command Thrust 4 stroke with 25” shaft. I felt it was just struggling to come up on plane. So I added a 13 3/4” X 15” prop. Permatrim tab. Lenco electric trim tabs. And a Jack plate to raise up the 25” shaft so that the anti cavitation plate was closer to the bottom of the hull.
The boat still feels sluggish. Her are my rpms and corresponding speed on flat run. They were my best speed while adjusting trim and tabs.
1000 4.0 mph
2000 6.2
3000 8.5
4000 13.2
5000 16.0
5800 wot 26.0

And I average 2.2 to 2.5 mpg
I know the boat is not a standard set up. But
Do these numbers sound right? Any suggestions on how to improve the power curve? Thanks for your expertise.

ssobol · Posted: Mon Oct 07, 2024 11:04 am Post subject:

If you are getting the WOT rpm, then the prop should be correct. I would check that the cavitation plate is just at the surface of the water, not necessarily aligned with the hull when the boat is planing. Seems like a big jump in speed from 5000 to 5800 rpm. Kinda suggests that the needs a higher speed to fully plane than a stock boat. Any idea how much your boat weighs?

My 22 gets 18-19 mph at 4K rpm. Around 30mph at WOT (~5700). Depends on the load.

thataway · Posted: Mon Oct 07, 2024 11:22 am Post subject:

There is something very wrong--and it may be the modification of the boat. It could be heavier, and it ir very possible that the engine is way too deep. You want to raise it an inch (above where it would be at the transom) for every 12" of engine bracket.

At this point, we need photos to look at the configuration.

Weigh the boat in cruising trim.

I hope that you bought a remotely controllable jack plate so you can experiment easily and maybe even move the engine up and down to find the sweet spot.

As Ssobol, you should be running in the 4000 RPM range at cruise. On a plane I ran my 22's from 3500 to 5000 depending on load, and sea conditions. The boats normally plane at low speeds--such as 12 to 15 mph. Your fuel consumption is more like a C Dory 25..(Or worse).

So, Ask Moderator (see member list--DaNag or TyBoo) and start posting some photos: Bracket from both sides, jack plate and engine mounting, and top of bracket--bracket and hull interface. Also the "alumium cockpit". \

Then weigh the boat on the trailer, no truck attatched--any major truck stop will have CAT scales. (or similar). Pick a non busy time. leave the trailer in place, jack up off the hitch--be sure the truck is on the "cab" part of scale, the boat is on trailer part of scale. Is there any other extra gear on board?

strike3 · Posted: Mon Oct 07, 2024 12:36 pm Post subject:

Thanks for the replies. Unfortunately my boat lives in the water 3 hours away. My jack plate is set up 3 holes up and is manual, but I will try and see if I can raise it. I do suspect that the boat is heavy but won’t be able to weigh it until I pull it out of the water in December.

thataway · Posted: Mon Oct 07, 2024 12:47 pm Post subject:

Do you have any photos of the boat? I will send you my email in a private post, and I will put them on this forum. You want the antiventillation plate just at the surface of the water when on a good plane.

When bringing the boat up to a plane, I put motor trim and trim tabs all of the way down. Then as the boat comes up to a plane, then bring trim tabs to neutral, and motor slowly uup, until the boat is running the fastest for that throttle setting. You may want to bring the throttle down as the boat picks up speed.