19' Sunbird hard to get on plane

[mlrman]

Hi,

I recently purchased a '91 Sunbird Corsair 198 19' cuddy cabin, pictured in avatar. It has a 4.3 V6 with OMC outdrive. The prop is currently a 14.5 x 19 3 blade aluminum with a couple small dings.

WOT is 4500 - 4600 RPM @45 MPH (speedo) if I bring the trim up on smooth water.....with only me in the boat, and it planes out quickly.

Here's the problem....with three or four adults sitting in the boat, the boat just will not get on plane. I have to have someone move near the front or even get inside the cuddy for it to plane, then we are fine.

I believe the engine is running well, so before I mess with a hydrofoil or trim tabs, do you think my prop setup is correct?

Thanks for any replies.

 

[steelespike]

Re: 19' Sunbird hard to get on plane

I'm not a real expert but will try to get you started. It seems to me that a 19"
prop should do better; both you alone and a few passengers. its not way off but I think your max rpm should be maxed out with a 19,with a little more speed.
A 18"4 blade prop would help you get on plane and generally improve your low and
mid range performance. An alternative would be a 19".But might lower your max rpm.
I feel a 17" would be extreme.Check that the throttle plates are opening all the way
and that the motor is in excellent tune.Bottom clean and smooth.

 

[Josh W]

Re: 19' Sunbird hard to get on plane

My brother recently bought a Sunbird as well, his is a 1989 Sunbird 198 Sport Cuddy. He is also having the same problem. When there are more than a couple people in the boat it takes forever to plane. We all need to get up to the front and/or in the cabin. He just bought a new prop and is planning to put that on before the weekend. I'll post back here and let you know if that did the trick.

I have a question too, we need to do a little bit of rewiring for some of the lights. Do you have any idea where we can get some kind of maual or wiring chart or something similar online? I haven't been able to find anything about this boat. I figured with it beingover 20 yrs old, there may not be a lot of stuff out there.

 

[usskydvr]

Re: 19' Sunbird hard to get on plane

Sounds like a design issue to me, but that aside you should be able ot get it on plane by putting your trim tabs DOWN. Once you are on plane raise them up to the point you just start coming off plane. (are you sure your hull hasn't taken on any water. I once ran into a situation where the transom leaked and allowed water to get into the the hull between the bottom and the floor saturating the foam filling and adding hundreds of pounds to the back end of the boat. needless to say floor had to be ripped out, transom fixed...etc..etc)

 

[steelespike]

Re: 19' Sunbird hard to get on plane

Welcome Josh,what prop did your brother have and what did he buy? is either one a 4 blade?
What was the max speed and rpm w the old prop.

 

[junior1113]

Re: 19' Sunbird hard to get on plane

id go with a 4 blade 19 pitch. 4 blades have better low end torque. might lose a couple rpms.

 

[mlrman]

Re: 19' Sunbird hard to get on plane

Good info, thanks!

Steelspike, the 14.5 diameter prop on my boat struck me as being narrow, too. If I went with a larger diameter prop say a 18" or 19", what pitch should I go with? Never thought to check throttle plates, will do. Hull is clean and smooth.

Josh W, yeah please keep us posted on the results of your bro's prop change results. As for your re wiring, I have my owner's manual with wiring diagram from 1991. I will scan email to you, send me PM with email info.

usskydvr, don't have trim tabs yet, but will invest in them if that makes my boat plane quicker, and run smoother. I just want to eliminate all other possible problems before buying trim tabs. The boat dosn't take on much water at all. Drain plug hole only puts out a gallon or so after a day on the lake.

 

[usskydvr]

Re: 19' Sunbird hard to get on plane

In your first post you said...
Quote " WOT is 4500 - 4600 RPM @45 MPH (speedo) if I bring the trim up on smooth water.....with only me in the boat, and it planes out quickly."

That led me to believe you had trim tabs. What then does 'trim up' mean?

also, no way can you put an 18" or 19" diameter prop on your outdrive.

 

[45Auto]

Re: 19' Sunbird hard to get on plane

What then does 'trim up' mean?

The outdrive has hydraulic cylinders on it that raise and lower it. Usual technique is to trim all the way in to take off, then "trim up" (raise the outdrive) until the boat starts porpoising for best speed once you get on plane.

with three or four adults sitting in the boat, the boat just will not get on plane. I have to have someone move near the front or even get inside the cuddy for it to plane, then we are fine.

A compression check on the motor to make sure it's in top shape would be a good idea before you mess with anything else. A weak cylinder or two could be the cause of your problems.

How big are the "adults"? 700 or 800 pounds in the back of the boat could possibly be enough to keep it from planing ......

Your RPM and speed are about right for the boat, although going down to a 17" pitch should help you get it on plane easier while still staying within the engine's operating range.