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Old 02-12-2010, 19:46   #16
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I've just read an article on another subject talking about over sized props.
One of the responses stated that an over size prop could cause over heating .

This has got me thinking. I've never had these V8's over 3000RPM, and as I've stated that once I pour on the power the temp starts going up.

At a boat show I was talking to a Rep from a prop co.

He said my 255HP Mercruisers should be pulling 4200RPM with a 1.9:1 gearbox with a 18" x 18" 3B super cup and it should pull 32 knots.

Now I have no idea what the gearbox ratio is other to say that it is original so it is as it came out of the factory and I've try to see what size the props are but I can't see any markings.

I'm lead to believe these are originals too.

I've had plenty of boats in the past , but this is the first cruiser with inboards , so therefore I don't have anything to compare it with.

Thanks for you thoughts.
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Old 02-12-2010, 21:32   #17
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What make & length of boat is this?

Different hulls react to how much weight that one brings on board.

Trim tabs can be an important feature. Once the boat is up to a certain speed then one has to lower the tabs to bring the bow down so the boat will plane on top of the water rather then pushing the water with the bottom of the hull (high nosed). Fastest way to stop a boat- raise the tabs.
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Old 02-12-2010, 21:50   #18
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The hull is a 31' Mariner Aust made by Bill Barry-Cotter.

The weight is suspose to be approx 7 tons.

That thread I was talking about is in Engines and Hydrids.

Thanks
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Old 02-12-2010, 22:31   #19
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Well, it appears to be a hard chine planing hull. So I guess the next question would be is the boat leveling out at full speed?
In reality once the boat is up on a plane there is less stress on the motors. If the boat is level then I would suspect a lack of cooling water, which brings me back to the thruhull fittings either facing the wrong way or their placement is off enough to allow air to get into the fittings.

Do you have any pictures of the bottom out of the water?
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Old 03-12-2010, 01:04   #20
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The thru hull vents are pointed the right way.

I'm a bit confused. You said if the boat was running level it wouldn't be picking up enough water.

Your last note was about trim tabs and how they should be down so as the the front of the boat was level.

As I mentioned before I've never pulled 4200 RPM the max revs I've pulled is 3000 RPM, which based on the other thread I starting to think the props might be an issue.
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Old 03-12-2010, 10:07   #21
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Quote:
Originally Posted by Flybridge 31 View Post
I'm a bit confused. You said if the boat was running level it wouldn't be picking up enough water.

No! If the boat IS running level it should NOT pick up air.

Your last note was about trim tabs and how they should be down so as the the front of the boat was level.
You need to level out the boat once the bow starts to come up.
See the videos below.



To eliminate drag are the key words!!! Hope this helps!

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Old 03-12-2010, 10:17   #22
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[QUOTE=Flybridge 31;570833]The hull is a 31' Mariner Aust made by Bill Barry-Cotter.

The weight is suspose to be approx 7 tons.

That thread I was talking about is in Engines and Hydrids.

Thanks[/QUOTE
Are they straight inboards with transmissions or inboard outboards?
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Old 03-12-2010, 23:29   #23
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Are they straight inboards with transmissions or inboard outboards?
Quote:
Originally Posted by Flybridge 31
It's a closed system and they are shaft inboards.
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Old 04-12-2010, 01:34   #24
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Quote:
Originally Posted by delmarrey View Post
Check your thruhulls! They could be sucking air.
This way or that? On planing hulls the screen should be facing forward. On Displacement hulls facing aft.

.
Another similar problem would be the raw water strainer, if you have one it would be located between the thru hull and water pump, most are easily cleaned..my engine was recently running hot for the simple reason there wasn't enough coolant in the reservoir
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Old 07-12-2010, 14:31   #25
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It's been checked!

If I had a dollar for every time that was said!

The max RPM is a big clue, dirty props or wrong props.

The high tech prop scans are a good idea, they can also advise on correct pitch for your boat and repair as needed.
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Old 07-12-2010, 20:13   #26
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Highlander, you're right.

It is now clear that the over heating problem is related to the props.

I was under the impression that theseprops were originals, but it is now apparent that they have been changed at sometime. Cause they's no way it would have left the factory with these props.

I've spoken to a prop manufacture who says if my diameter is too big he can resize my props, but I have to give him my gearbox ratios first. Which is fair enough.

He has said that the problem would be solved with the right size props.
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