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What makes a boat porpose?

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Old 08-09-2005, 06:00 PM
  #31
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Arrow Re: What makes a boat porpose?

Quote:
Originally Posted by ECeptor
Thanks Cord, this is turning into a very informative thread. Given the number of rigs I see porposing their way down the lake I'd say it's one of the most mis-understood, most common problem in boat set up.

So, here's a few more questions based what I've learned so far: there's a balancing act between the hull, prop, and drive (x dimension and setback) on the forces that hold the nose of the boat at a certain height for a given speed. Waves impacting the boat disrupt that balance and can excite the boats natural harmonic frequency and the hence it porposes.

So, 1 desired effect and 2 key forces involved.

Desired effect 1 - certain attitude of the boat (nose high or low)

Key force 1 - hulls natural running attitude (hook, rocker, straight, etc.)
Key force 2 - prop's bow lifting capability

Porposing could then be defined as the latent effect of having an instable balance between forces 1 and 2 creating an unstable system. The opposite would be a hull/prop combo that was naturally correcting. Imagine balancing a broom handle on your finger - inheriently unstable. Dangle the same broom stick downward and you have a self correcting, naturally stable system.

Damn, now I think I have myself confused even more!

Here's what I'm sure of so far:

hook = nose down
rocker = nose up
weight in bow = nose down
trim tabs down = nose down
drive trim down = nose down
drive trim up = nose up
prop cup (Mirage) = nose up
straight prop (Hydromotive) = nose down

What I'm unsure of is how to determine which combos of the above yield a self correcting system.
Darn good summary!
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Old 08-09-2005, 06:21 PM
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Default Re: What makes a boat porpose?

[QUOTE=Ron P]Checkmate runs the inner set of chines all the way to the transom. Everyone else stops about 4 or 5 feet short of the transom. This gives you tons of stern lift, maybe too much.

BINGO!!!!

THIS IS CORRECT. THE BOAT WAS DESIGNED FOR MORE TOP END SPEED THIS IS WHY IT IS HAPPENING.The boat is just not long enough for that design.The newer models have been corrected.Alot of there smaller boats have had that problem as well.
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Old 08-09-2005, 07:17 PM
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Default Re: What makes a boat porpose?

Quote:
Originally Posted by ECeptor
Cord, more questions.

I picture a boat like a teeter-totter with the last 1' or so of the bottom as the pivot. Looking at it that way tabs, hook, weight in the bow all help keep the nose down.
Yes, that is correct

Quote:
Originally Posted by ECeptor
Given that how does cup in a prop both help keep the nose high (presumably by the prop pushing down hence lifting the nose) and help the boat come on plane (I assume having the nose stay down help the boat get on plane).
The cup at the trailing edge of the prop provided the bite to propell the boat forward. The cup at the outter edge of the prop provides the bite to lift the bow. Think of the prop as your hand. If you curl your fingers you now have cup. It's much easier to do work, say a chin-up, with curled fingers than a open hand. That's how cup works.

Remember that in the above paragraph I mentioned the cup being in two different positions on the prop? As the prop rotates the trailing edge cuts through the water like a knife with the cup helping it to grab the water. You would assume the same would be true for the cup on the tip, but that is incorrect. The trailing edge is square to the prop shaft, but as the cup turns around the tip it become parallel to the prop shaft. As the prop swings around, the cup at the tip will dig into the water and try to pull the drive into the water. The boat will pivot on the transom as you mentioned above and the bow will lift.

Now your next question will be...how does a stern lifting prop lift the stern? Well a stern lifting prop doesn't have cup around it's tip. A Hydromotive or a cleaver would be an example of a stern lifting prop. With this class of props, there is nothing to pull the stern of the boat into the water, so the hydrodynamics of the hull will lift the stern out of the water. A "stern lifting" prop doesn't actually lift the stern, it simply doesn't have any bow lift. The hull is doing all the lifting.

One of the reasons why the Bravo 1 is better than the older Hydromotive's is because the Hydromotive's don't have a diffuser ring. If you look at the hub of the Hydromotive it will have a straight body. The Bravo 1 has a straight body with a slight flair right at the end. This flair is called a diffuser ring. Like cup it causes the water to be thrown out so it is held on the blade. When you are planing, the inner hub portion of the blade is what is doing the majority of the work. The diffuser ring helps create a high pressure area at the hub so the water isn't able to break down and cavitate. Hydromotive left the diffuser ring off their prop because it adds a couple of mph at the expense of poor planing.

The Bravo is also a better planing prop because it is a variable pitch prop. Look at the inner portion of the blade on a Bravo 1 and you'll notice that it has a twist to it. When planing, the inner portion of the blade does most of the work. All of the boat is in the water so there is a lot of drag and the engine is out of it's power band. Both of these factors indicate that less pitch is needed to get the boat on top of the water. I don't think the Hydromotive or the Mirage+ are variable pitch props. I could be wrong though. When you are running at speed, the base of the blade does less work and now the tip becomes more critical. This is why the Bravo 1 has more pitch in the tip of the blade. This is also why prop diameter is critical to boat performance. The factory cleaver is a very small diameter prop, 14 1/2" if I recall correctly. Outboard props for a bass boat are 14 1/2". The Bravo 1 is 15 1/4" and the Hydromotive is monster at 15 1/2". The Hydromotive is about the largest diameter prop that can be fit on the Bravo 1 drive. The feller above still has the original cleavers on his boat. These props have a very small diameter. There is very little blade area. They have no cup at all at the tip so there is no bow lift. There is no diffuser ring so the props won't plane very well and they are a straight pitch! Now you can see why these are such a horrible prop to have!
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Old 08-09-2005, 08:39 PM
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Default Re: What makes a boat porpose?

Quote:
Originally Posted by ECeptor
Here's what I'm sure of so far:

hook = nose down
rocker = nose up
weight in bow = nose down
trim tabs down = nose down
drive trim down = nose down
drive trim up = nose up
prop cup (Mirage) = nose up
straight prop (Hydromotive) = nose down

What I'm unsure of is how to determine which combos of the above yield a self correcting system.
If the prop is unable to carry the bow it will porpose. The easiest way to correct the problem is CG and prop.
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Old 08-09-2005, 09:19 PM
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Default Re: What makes a boat porpose?

Couple more questions about porposing, thanks for all the great info.

I have a 2001 496HO in a 27' Fountain that has this condition at WOT. I normally run a stock Bravo 1 prop & have tried a labbed prop Bravo 1 and gained speed but did nothing for porposing. Now I know I can add tab to smooth out the ride but lose top speed as the result. I also know that the X demisions are set high on the newer Fountains, would a drive spacer between the upper and lower help with my condition???? I would also guess that if I could get the porposing to stop and boat carrying the hull properly it should pick up some top speed???
Am I thinking this out correctly??? I know they set the X demensions high to help top end speed, but could it be too high cause porposing actually resulting in hurting your overall top end???
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Old 08-09-2005, 09:39 PM
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Default Re: What makes a boat porpose?

Russ, I think you need a little more time with the tabs. A 23" tab should control the porpoise at slower speeds. Some seat time will help.

Some great info on this thread! But no one has mentioned that props, trim, tabs etc really don't control the bow, they control the stern. A "bow lifting" prop doesn't lift the bow, it sucks the stern down which in turn lifts the bow. And a "stern lifting" prop doesn't suck the stern down as much allowing the stern to ride higher which in turn drops the bow. Same with trim and tabs, they actually lower or raise the stern which in turn controls the bow attitude.
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Old 08-10-2005, 09:31 AM
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Default Re: What makes a boat porpose?

Thanks for the info guys!

One more thing I forgot to mention. My props are a little large. I only turn 4,600rpm at wot. If I use smaller props could this help? I would like to turn 5,000 or so. When I'm not porposing at 4,000rpm is it because of the RPM's or the speed 58mph.

I will keep working with the tabs.

I would like to add some weight in the bow and give that a try also. How much should I add?

Russ
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Old 08-10-2005, 09:51 AM
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Default Re: What makes a boat porpose?

Quote:
Originally Posted by Russ G.
I would like to add some weight in the bow and give that a try also. How much should I add?

Russ
I went to Lowe's and grabbed a 50lb bag of sand and wrapped it up in about 3 trash bags to insure it wouldn't 'leak' into my cuddy. You could pick up 4 of those and put them in your back seat then play with moving them up to the far front of your cuddy. Picture the teeter-totter which tells you to move them as far forward of the stern as possible.

I know it's a kind of a red-neck solution but it does work, is easy to tune, and is damn cheap!
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Old 08-10-2005, 09:55 AM
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Default Re: What makes a boat porpose?

Quote:
Originally Posted by noboat
Some great info on this thread! But no one has mentioned that props, trim, tabs etc really don't control the bow, they control the stern. A "bow lifting" prop doesn't lift the bow, it sucks the stern down which in turn lifts the bow. And a "stern lifting" prop doesn't suck the stern down as much allowing the stern to ride higher which in turn drops the bow. Same with trim and tabs, they actually lower or raise the stern which in turn controls the bow attitude.
Exactly!!! I think I finally have a solid mental picture of how this works.

Cord - thank you for the explaination. I've asked many others, researched the web for hours and never had a solid explaination like that.

I was just thinking - worst possible combination is a prop that sucks down the stern (bow lifting) so much it causes the hull to be unstable (porpoises easily) that it has to be corrected by stern lifting trim tabs which induce drag and cut both efficiency and speed!

Maybe that is the second worse combo -worst being a setup that can't/barely will come on plane!!!
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Old 08-10-2005, 10:42 AM
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Default Re: What makes a boat porpose?

We shimmed or drives 1/2'' lower than stock on our non stepped 38' Kevlar Top Gun with 29 pitch Bravo I props..
Now if it,s choppy enough we don't require any tab at all and in the smooth we use way less than before ,and now we can also bring our drives out well below our previous 65mph or so, this boat porpoised real bad below 3800 rpms.
We also added a few hundred hp per side which I believe also may have helped. The drive depth has made a possitive difference on our rig. Good luck!
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