chine walk

Hello everyone ,

I am a new member. I live in the Caribbean and most time of the year the sea is rough. I have bought a Mainship Trawler 34 from 1979 repowered with a 250 cummins 6BTA. The shaft is 1 1/5 and the prop is 22x18 (three blades) . They have put fins under the boat almost but not on the egdes at the back. The boat leans on the left side if you're standing from behind of it. I was thinking that maybe because the former owner have removed the generator and that's why it leans on one side. I have put weight on the other side so it will stay straight and other parts of the boat. I 've put more weight, because I thought that it would help prevent the 'chine walk' .When I go with the waves I have to deal with the 'chine walk', although this is what I think it is what it's doing and it can be really scary. This happens only when I go with the waves and it doesn't happen when I go against the waves.

I'm not sure maybe I have to much weight and my prop is to small?
Should I remove all the extra weight? Should I change my prop to the original size?

Please help

Greetings,
Welcome aboard. Sorry, Mr. lj. Can't help you out. There are better minds than mine reading this...

Sorry, what actually happens to the boat when you are in a running sea, ie going in the same direction as the waves?

It will turn first to the right and than go left. It seems like the wave does what it wants with the boat.

This is an example of "chine walking" that I'm used to:

...They have put fins under the boat almost but not on the edges at the back. ...

WHY did they do that? Was it to correct this condition or did this condition occur after the fins were mounted?

Ok so what I am dealing with is something else, my mistake sorry. I thought it was chine walk. I bought the boat with the fins. The problem is that the former owner is sick and do not remember anything. I think that they wanted the boat to go faster and I think also that this is why they have changed the prop to 3 blades . These Mainships come originally with a 4 blade prop 20x20

If you are traveling slower than the seas (the waves are passing under you from the stern), what you are experiencing is what happens to a square stern boat presenting a flat surface to the wave front. It doesn't put equal force across the stern so the boat tends to want to go right or left and sometimes both on the same wave. A full displacement boat with a full keel will do this a little less than a semidisplacement hull. Trim tabs if they are down make it a lot worse. I would say the fins are probably not helping any as it gives more surface area for the forces of the wave to act on just like trim tabs. The first time you go at hull speed with a 5 foot following sea in a 22 foot C-dory and forget to bring the trim tabs up, you'll know what I am talking about. Running faster generally makes things a lot better.

Tom

Littlejack, I think what you are describing isn't chine walk, but more the typical action that happens when running in following seas. The wave picks up the boat's stern, the boat picks up speed as this happens, and you start to lose steering. When this happens to an extreme the boat broaches and if the following wave is large enough it will roll the boat over.

I think the solution to this is to maintain your boat speed so you can ride on the back of the wave and not let a wave push on the stern, lift it and cause you to lose steering.

If you're always climbing the back of the wave and traveling at about the same speed of that wave it won't be pushing on your stern.

Oops, tpbrady and I had the same thoughts, he just typed a lot faster than I did.

The fins might to reduce the tendency of the stern to get deeper in the water as the boat tries to climb the bow wave after hull speed is exceeded. The same thing you would do with trim tabs in a planing hull. They might also have some positive effect on rolling in the beam sea.

Tom

I want to go faster and I think that my prop is getting worse because when I bought the boat this prop was in really bad shape and it had to be cut. And he guy told me that it was cut from 19 to 18 but this is not correct. It is like 17.8. My engine is not reaching his rpm. I can hear the engine working but the prop cannot handle it. I think my problem is the prop.

Littlejack,

There are a lot of things in play here. Things you need to know are:

a. What is the wide open throttle (WOT) RPM for your engine? You need to be able to reach this RPM when your boat has its typical load on board. ( I do fully loaded with fuel, water, food, and a month's supply of wine, whiskey, and beer).
b. What is the maximum continuous RPM for the engine (Typically a couple hundred RPM below WOT)

Once you have determined what prop meets the criteria of "a" then "b" tells you how fast fast is. Trim tabs and things that can change the trim of the boat can affect "fast" a little but not a lot for a given prop. What they might allow you to do is reach the proper WOT with slightly higher pitch prop so you should be going a little faster when you drop back to cruise RPM.

Tom

Got any pictures of these "fins"?

Also, speed will greatly be effected by marine growth on the bottom and on the running gear (prop, shaft, and rudder).

Ted

Was the previous owner's name "Monty"?(Monty Lewis I think)
I owned a model 1 for 14 years and repowered it. I also installed trim tabs.

Sounds to me like you are describing "surfing" and the "fins" are trim tabs.
In a following sea the tabs need to be full UP with that boat.
These boats are set up to return fuel to the port tank. If that set up still remains, then that could explain the port list. The generator should have been directly centered.
These boats will indeed "chine walk" but only when they get to 16 knots or more. That is very predictable in my experience and in talking with many others who have repowered with more than about 220 hp.

I repowered with a 270 hp Cummins and a 1.5:1 tranny. With that set up I was able to keep the 20 x 21 4 blade prop and could make over the 2600 rpm at WOT.
I don't know how the 3 blade relates to a 4 blade, but it sounds like it is "over propped" assuming a clean bottom and assuming you have calibrated the boat's tachometers with a handheld tach.
With that power you should also fair the keel if it has not been done. That will get more clean water to the prop and increase your rpm, at least it did when I faired my keel. (I gained almost 250 rpm).


Oh yes almost forgot, you need trim tabs fully UP to do a valid WOT run or you will be plowing the bow and not be able to get to WOT.

Jleonard is spot on about the tabs. If you have them, trim them all the way up. That will sink the stern a little, and get the forefoot out of the water or at least with less resistance. What you are experiencing (I think) is bow steer from sheering off to one side. When you over correct it will sheer to the other direction. This is a common problem with boats with fairly flat runs aft as there is a lot of buoyancy to make the stern rise and bury the bow.

Unfortunately, on a boat like yours there is no way to eliminate the tendency altogether. You just have to mitigate it, and get the feel for how your boat handles.

What you are experiencing is "bow steering". Everyone above has pretty much explained it. You have a wide wquare flat transom that allows a lot of buoyancy to be produced and acted upon by a wave. The forward portion of your hull is very fine and sharp with not a lot of buoyancy. So the wave pushed on the stern and buries the bow until it has to come up. If it keeps sinking then the stern will come around and cause a "broach". And that is the scary part. Like has been mentioned, go the same speed of the wave train for safety purposes. If you want to go faster, then you just have to be quick on the wheel and make sure you are not falling off the front of any waves. You would have to pick your path thru the waves. Which requires experience and a lot of work on the wheel.

I believe the 34 Mainship was originally powered with 165 hp? Going up to 250 hp, I can see where it could get a little scary.

Larry M said:

I believe the 34 Mainship was originally powered with 165 hp? Going up to 250 hp, I can see where it could get a little scary.

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2 engine options. Both Perkins 6-354s....one is the 165hp you mentioned and the other the 200hp version of the same engine. Although a small difference, that little bit of extra HP is what got you over the hump and gave you a realistic chance of cruising at 12 or so knots.

Baker said:

2 engine options. Both Perkins 6-354s....one is the 165hp you mentioned and the other the 200hp version of the same engine. Although a small difference, that little bit of extra HP is what got you over the hump and gave you a realistic chance of cruising at 12 or so knots.

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I wasn't aware that it was an option in the early Mainships. We had an 1981 with the 165. A friend repowered with 200 hp and the boat got up and moved. Yahoo! After driving with the 165 the extra hp made a definite difference in handling.

This may have been mentioned before, but there is another way to correct bow steer. Crank the wheel over hard the opposite way, and apply power briefly. This will "kick" the stern to correct the situation. You will just have to practice it, but it works.

Baker said:

2 engine options. Both Perkins 6-354s....one is the 165hp you mentioned and the other the 200hp version of the same engine. Although a small difference, that little bit of extra HP is what got you over the hump and gave you a realistic chance of cruising at 12 or so knots.

Click to expand...

34s here seem to use a 380hp Yanmar. Too much power imo, unless you want to plane.

BruceK said:

34s here seem to use a 380hp Yanmar. Too much power imo, unless you want to plane.

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Those are the modern 34 Mainships. The 1979 era 34 was originally powered with a 160 hp Perkins. In 1980 they changed to the 165 hp range 4 Perkins. Then in 81 they started offering the 200 Perkins so it's kind of a mix after that.
Around 85 or 86 they started putting 220 Detroits (4 cycle) in them.
The preferred engine for repower was / is the Cummins 6 bt at 210/220. Some crazies like me put in more power. Some used higher hp Perkins and I've seen a few with Yanmars. The highest hp I have heard of was 315 Yanmar.
Some have used alley engines but I think that ruins the boat.
I know after I repowered it was a much better boat and much more efficient.