3 vs 4 blade -boat lists under full throttle only

bluesail

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Feb 18, 2013
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I have a 16.5 ft aluminum vee hull fishing boat with a Merc 40 tiller outboard. Currently has a 15 pitch 3 blade prop. RPMS were correct at WOT with this prop but at FULL THROTTLE it seems the outboard wants to make the boat list several inches to the port side. Trim tab is already adjusted to the max (which helped some but not enough). Someone suggested that a 4 blade prop would help. Good advice or just speculation? Runs good at 3/4 throttle or less.
 

RogersJetboat454

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Re: 3 vs 4 blade -boat lists under full throttle only

Welcome to Iboats.

I don't think a 4 blade prop is going to make much of a difference over a 3 blade.

What does make a difference to stopping list (propeller wise) is either twin counter rotating props (seperate engines), or Duo props (I/O stern drive). Neither is feasible for your situation.

Your list is from simple physics. The boat is slightly rotating on the axis of the prop shaft to the opposite direction of prop rotation.
Sounds odd? OK picture this... If you were to hold the prop shaft still, and allow the boat/motor to spin, it would be spinning to the left. Since the water isn't a solid object, it's not going to get enough grip to spin the boat (that would just be a silly unusable boat), but it does provide enough grip to allow your light weight aluminum boat to be tilted.

This isn't a phenomenon strictly encountered by guys with aluminum boats, and 40hp engines either. It's pretty common to some degree among all boats running a single prop.

You may want to look into upgrading your trim tabs if it really bothers you. A trim tab with a larger surface area would be better to try and stop extreme list.
 

bluesail

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Re: 3 vs 4 blade -boat lists under full throttle only

You hit it exactly - it definately seems as if the prop is trying to "spin" the boat. I've had this problem before with other boats but always was able to correct it by angling the trim tab. This boat is more severe. I suspect its due to the relatively light boat (under 500 lbs with motor) and its small hull area at the bottom of the stern. I didn't know they made different size trim tabs - are these available from Merc or are they after-market items?
Thanks
 

steelespike

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Re: 3 vs 4 blade -boat lists under full throttle only

Does the boat get up on the water like it should? The rear 1/4 to 2/3s in the water.
Or does it lay down flat like its glued to the water?
Have you tried different trim settings?
 

jestor68

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Re: 3 vs 4 blade -boat lists under full throttle only

I think by "trim tab", he's referring to the anode on the lower unit. You adjust that to counter steering torque.
 

jestor68

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Re: 3 vs 4 blade -boat lists under full throttle only

I have a 16.5 ft aluminum vee hull fishing boat with a Merc 40 tiller outboard. Currently has a 15 pitch 3 blade prop. RPMS were correct at WOT with this prop but at FULL THROTTLE it seems the outboard wants to make the boat list several inches to the port side. Trim tab is already adjusted to the max (which helped some but not enough). Someone suggested that a 4 blade prop would help. Good advice or just speculation? Runs good at 3/4 throttle or less.

I doubt that a 4 blade would help. Might actually make things worse.

00440 (1).jpg One of these would likely make it straighten up and fly right. :) It's called a Doel Fin.
 

bluesail

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Re: 3 vs 4 blade -boat lists under full throttle only

Does the boat get up on the water like it should? The rear 1/4 to 2/3s in the water.
Or does it lay down flat like its glued to the water?
Have you tried different trim settings?

Boat planes as it should. I have tried various trim positions with no affect on listing issue. There are really not too many trim options anyway. If I trim out very far the motor cavitates, too little trim and it starts to nose drive and "bow steer". The motor doesn't have power trim so you can't change the trim while under way.

Also, Jestor 68 is right - I am referring to the adjustable anode fin when I say trim tab.
 

emilsr

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Dec 16, 2010
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Re: 3 vs 4 blade -boat lists under full throttle only

Can you shift some weight around? Move the battery to starboard?
 

jestor68

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Re: 3 vs 4 blade -boat lists under full throttle only

I would be interested to know what WOT rpm you are getting with the 15" prop. You need to be turning right at 5500 rpm with just you in the boat.

I have run some preliminary numbers(guessing) through the Mercury prop selector and come up a 13 pitch. This prop would produce less torque at high rpm than the 15", lessening the torque effect on the hull. It almost sounds like you are forcing the hull past it's design max speed(yes, every hull design as one), causing an instability.

If you are not getting up to max rpm(5500), changing to a prop with less pitch may help.

An inspection of the bottom for any irregularities might also be in order.
 

bluesail

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Re: 3 vs 4 blade -boat lists under full throttle only

WOT was 5450 but that was using a Chinese knockoff "Tiny-Tach" that has since died. I would not testify to its accuracy! I recently bought a genuine "Tiny-Tach" that I'll recheck it with when things thaw out some.

I'm getting 32 mph (GPS verified)at full throttle (but with the severe list). There are no hull irregularities but I do have a paddlewheel and transducer puck on the starboard transom.

As far as weight distribution, my gas tank and small anchor are in the stern on the starboard side. I also sit on the starboard side of the motor. The trolling battery is near the bow on the centerline. There is no other weight in the boat.
 

mpsyamaha

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Re: 3 vs 4 blade -boat lists under full throttle only

Ive never used them before personally, but this sounds like it might be a good case to try smart tabs. I have noticed with smaller boats of various types with max hp on them, that listing to the port at full throttle is common. In my current boat (17ft v hull cc), the tendency to lean to port is noticeable, but i am able to negate it by shifting more gear to the starboard side. In your case it seems like you have already done all you can with shifting weight, and I have doubts switching props will make much difference.
 

Georgesalmon

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Apr 14, 2012
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Re: 3 vs 4 blade -boat lists under full throttle only

Take a good look at both sides of the bottom of your boat with a straight edge. I've seen aluminum boats that have a "rocker" (bulge) on one side but not the other. I've also seen them with a "hook" (indent) on one side. Either one of these could cause your boat to run with a list at high speeds. Over the years I have seen a lot of hooks caused by all roller trailers not set up properly. Both of these problems can be fixed by hammering down a hook from the inside or jacking the hull up from the outside to fix a rocker. This is delicate work and unless your capable abd carefull best left to a pro. It doesn't take much change in hull shape at the stern of the hull to make a huge difference.
 

emilsr

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Re: 3 vs 4 blade -boat lists under full throttle only

As far as weight distribution, my gas tank and small anchor are in the stern on the starboard side. I also sit on the starboard side of the motor. The trolling battery is near the bow on the centerline. There is no other weight in the boat.

In that case I probably wouldn't worry about it unless you run WOT for long periods of time. Hopefully not. If it's fine at 3/4 throttle, and that's where you're doing most of your running I'd leave it alone. If you set it up to run flat at WOT it may very well list the other way at 3/4 throttle....trim changes with speed on boats and airplanes. Racers set up for maximum speed, the rest of us set up for cruise speeds. You can drive yourself crazy trying to set it up to perform perfectly under all conditions.

It's worth trying a different prop if you can do it for little/no cash; a prop change may or may not work. Again, if it's running good (other than the list at WOT) I'd leave it alone and spend that money on gas and bait.
 

steelespike

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Re: 3 vs 4 blade -boat lists under full throttle only

What year is your motor and do you know the gear ratio?
 

bluesail

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Re: 3 vs 4 blade -boat lists under full throttle only

Motor is a 2005 - I believe gear ratio is 2:1
 
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