question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

[billpa]

Hi,

First, what a great forum!

A friend just sold me a 1980 Crestliner Nordic 20, with a Mercruiser 140 I/O.

During the sea trial, I noticed that it was important to position people toward the bow of the boat, or the boat would trim up significantly and slow down dramatically, basically falling off plane. If any adult was at the stern of the boat, it would not plane well.

The motor's sterndrive was trimmed all the way down during this test. Trimming it up made the problem worse. There are no trim tabs on the boat.

I am considering a Doel-Fin or similar hydrofoil to address this issue. Any thoughts on whether this would help?

Bill

 

[IVAZ]

Re: question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

The foil will help but it has a lot of other issues that will be created cause of it. Although I have never run smart tabs, everyone that?s used them has had good results.
It?s a 20' boat with a little 4 cylinder, its not going to plane well at all. Do you know what the weight is? Your boat being underpowered is the biggest issue.

 

[180shabah]

Re: question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

If your boat will not maintain plane then you have more issues than a lack of stern lift. Unless you had it heavily loaded, even a 4cyl should be able to keep it on top of the water.

Did you visually confirm that the drive was trimmed all the way down/in?

Those fins are a hack job at best. Trim tabs are a better solution, and Smart tabs are the least expensive available. They are not helm adjustable, but they only cost about $100.

 

[IVAZ]

Re: question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

If your boat will not maintain plane then you have more issues than a lack of stern lift. Unless you had it heavily loaded, even a 4cyl should be able to keep it on top of the water.

Did you visually confirm that the drive was trimmed all the way down/in?

Those fins are a hack job at best. Trim tabs are a better solution, and Smart tabs are the least expensive available. They are not helm adjustable, but they only cost about $100.

It will take it a little while for it to get on a plane but your right, it should be able to maintain one.
I do wonder what the total weight was when the sea trial took place.

 

[APPALOOSA2]

Re: question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

What RPM are you turning when it's wide open on plane. What pitch prop are you running.
I suspect you have too much prop.

 

[ziggy]

Re: question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

i'm no fan of the whale tail either. i had one on my last boat and all it did for me was break my anti ventilation plate when i hit something on the river... on my present boat, i run nothing. just plane ol boat and drive...

 

[billpa]

Re: question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

Hi, and thanks for the great feedback!

The boat's weight is supposedly 1600#. To that, we had about 900# of people/gear, and 25 gal fuel.

When we had the large # of people, with everyone near the bow the boat maxed at about 20mph, at 3500 RPM WOT. The Mercruiser manual states an RPM range of around 4200-4600, so it is running too slowly. When one 150+ lb person moved to the stern, the boat had trouble staying on plane, and ran much more slowly. We may have just been pushing the boat beyond its comfort zone, but it's rated 3500lb max payload.

With just me (220#) on the boat, it had no trouble staying on plane, and ran at about 25MPH at about 3700 RPM. Still too low.

The issue may well be the prop. I suspect the original owner had the original prop on there, or a replacement of the correct pitch - the boat has been fairly well maintained.

But, the prop has some small nicks, and one of the blades is slightly curled near the tip of the blade. I would not be surprised if the prop damage is the culprit. I plan on replacing the prop the next time I get to the boat.

If anybody has any idea what prop pitch should be on a 1980 Crestliner Nordic 20, with the MCM 140, I'd love to hear it! I called Crestliner about this, and they called me back (wow!) but we haven't made contact yet.

Yes, I visually inspected the motor trim, and it was full down.

The smart-tab option is brilliant, thanks. I have ordered those. I worshipped the adjustable trim tabs on my former boat. And for the price, it's an experiment I'm willing to try. I got the "pro troller" series which solves the next problem I was facing - slow trolling without a trolling motor.

Bill

 

[cooter2506]

Re: question about doel-fin with Mercruiser MCM 140 I/O (1980 vintage)

when you got on plane did you trim out some to try to acheive more rpms and speed? If you did and that didnt help there is a sticky in the adults only section that will tell you how to test not acheiving WOT.