What makes a boat porpose?
 

I love my boat , 89 251 Checkmate Conviencer. Did all kinds of work to get it where it is now. But the boat just porposes so easliy. Its very fustrating. It stops at 4,000rpm and up but anything below that it sucks. I tried the drive down not much help. Kinda just makes it come up and down harder. If I give it more throttle it helps but then here I am at of 4,000 rpm again. I tried to zig zag alittle not much help. I just put Eddie Marine 23 inch Billet tabs on the boat But they do not seam to help the boat either. And wouldn't they be the same as drive trim anyway? I dont have Gauges on the tabs and that makes them hard to use. I hate to put any more money in the boat if they wont do the trick either. So what does make a boat porpose more so then others? Its a great boat, fast haul for a 89 and built very well. The boat runs and looks awsome! I hate to give up on it and sell it. But I refuse to bonce up and down the river all summer long!

Thanks
Russ

Re: What makes a boat porpose?
 

sounds like you need to blueprint the bottom, I bet there is a hook in it that needs to be taken out

Re: What makes a boat porpose?
 

My boat, 1989 24' Chaparral Villian III, has a similar problem but not as bad as you've said. With my drive trimmed all the way in it runs pretty stable. My best compromise between stability and efficient cruising is to have the trims down a little with the drive up a little.

I've heard of adding weight to the nose helps certain models. I tried a 50lb bag of sand in the anchor locker in my cuddy and it made a noticable difference. I'd like to try some more weight to see how it affects the porpose. I'm sure it's costing me some top end, but have it cruise smoothly is well worth it!

Search the General Q&A under the Tech section for "porpose" and you'll find some threads. I got the sandbag idea from there.

Re: What makes a boat porpose?
 

Being off balance will cause that. I know my boat isnt balanced very well but i just run some tab and get over it. Balance or a hook would likely be the cause.

Re: What makes a boat porpose?
 

I do know that Donzi 22 Classics (and Cig 20's....same basic hull), are very sensitive to what type of prop is being used. Seems there was a thread on this before and someone switched from a 3-blade Mirage to a 4-blade
hydromotive and it did wonders for his.
I'd try hitting the prop guru's and technicians and see if this rings true.
Good luck :cool:

Re: What makes a boat porpose?
 

Sounds like you have a major ballance issue. Let's start with a different prop and see if that helps things. From there we can look at drive depth, balance, hull design etc.

Re: What makes a boat porpose?
 

I think magnum 27 is half right; you need to blueprint the bottom, but the opposite reason. I think you have rocker and thats why the boat is falling off so often. A long straight edge will solve the question very quickly and then go from there. Of course, we both could be correct.

Re: What makes a boat porpose?
 

Agreed. What is the current prop?

Re: What makes a boat porpose?
 

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.

Try more weight up front? You could probably use a balast tank.

Re: What makes a boat porpose?
 

A very good friend of mine had the same boat, with an HP500 pumped up to 565HP. It would run 80mph all day, straight, true, and solid, and no porpoise. He had a Hydro prop and a B1 prop that he switch back and forth. I would try a few props, and if that does not help, have the bottom blue-printed over the winter.

Re: What makes a boat porpose?
 

Friend had a late 80's 25 Convincer with BB/Alpha. Same problem. He tried everything...props, tabs, Stern Jack. Nothing helped. He sold it before finding a solution. I think it was a hull issue.

Re: What makes a boat porpose?
 

I have a 88 28 checkmate. Mine will porpose at 3800 with no tab help. At 4200, it clears out if I dont use the tabs. I run a cleaver prop and have 550 hp. I can run it at 3300 rpms with the tabs down about 2 secs and the drive 1/4 the way up at about 40 mph. If you call checkmate, they will help you alot. They are very helpful. Good luck

Re: What makes a boat porpose?
 

Like we said above, change your prop! Your boat should not have a cleaver on it! Start with the Mirage+. If you can get on plane, then that will proably be your prop. If you can't plane with the Mirage, then try the Bravo 1. If the Bravo planes good, but doens't clean up the porpoise, then try the Hydromotive. My money is on the Mirage+ for you.

Re: What makes a boat porpose?
 

You might have got me confused with the person that started this thread, but I have to ask, why shouldnt my boat have a cleaver prop. I bought it like that and I have allways read that the cleaver is good for an older V-bottom boat. I am allways interested in learning. Thanks

Re: What makes a boat porpose?
 

Yep hook doesn't cause porpoising it actually helps get rid of it. My blackhawk had A LOT of rocker and it porpoised horribly at low speeds without the tabs.

And if I remember right Russ is running a Volvo DP so all the mentioned props won't work.

The tabs should help at cruise speeds. They will lift your stern enough and flatten the boat out and keep it on plane. You said you just got them so I'd keep playing with them.

Re: What makes a boat porpose?
 

No Boat is right I'm kinda stuck with props. I have a Volvo Dp drive. The boat had a OMC drive and it porposed with that set up also.

I posted this question on the Checkmate site but most of the guys get hurt feelings talking bad about a Checkmate. Some did try and help though. But it seems that most Checkmate convicors have this problem.

I dont have gauges on the tabs yet because I dont know if I'm going to sell the boat this winter. Is there a measurement to the tabs before I put the boat in the water again that might be close?

Your scareing me about hook and rocker!

I like the idea about adding weight to the bow. Would that make the boat un safe? Tell me more.

Like I said the boat is awsome at high speeds. Really fast and stable. No porpose and not chine walk at all. It makes me want to just add crazy HP and just cruize at 65 plus all the time. But that is kinda silly. But is the boat good at those speeds because of RPM's or the MPH?

Thanks
Russ

Re: What makes a boat porpose?
 

I've got an 87 251 Checkmate Convinsor. I had the same issue, very ssensitive to porpoising and Chine walking. I swapped out the Alpha for a Bravo ( left the same "X" ) but still had the same problems. Switched to a Hydromotive 4 Blade prop ( from a 3 blade Mirage ) and it drasticlly improved and became much easier to drive. I have an Imco 2" lower I want to try but have not gotten a chance yet.

Dave B

Re: What makes a boat porpose?
 

OK, not following you so help me by expaining a little more. Is the Mirage a bad prop for getting on plane? It's what I have, btw.

The Bravo 1 is different from the Mirage how? More stern lift? And how does the hydromotive rank?

For me, 2 factors are most critical - getting on plan with a minimum of nose lift and no proposing w/out having to stab down the tabs.

Re: What makes a boat porpose?
 

Same here. I have a 89 251, 502/Bravo setup. Did some prop testing and found that the Hydromotive 4 blades make a world of difference for porpoise compared to other brands 3 blades. I was told that the Hydromotive props give you a bit of stern lift and older Checkmates like that. I'm assuming stern lift helps keep the bow down at crusing speeds. Boat has run the best with a 24 Hydro so far. I have a 26p Hydro 4 blade that was too much pitch for my combo, but the boat handled just as good as the 24p. It's for sale if interested.

Re: What makes a boat porpose?
 

Was the X-dimension raised on the boat from the original dimension? This will cause the boat to porpoise.

Re: What makes a boat porpose?
 

RussG,
What motor came stock in that hull? I ask that because I owned an 88 and did some bottom blueprinting. I found that Checkmate had two molds both have hook in the bottom on the (pad) so to speak. If you ordered a 454 mag it would have less hook. It is in there for a reason and you can take it out, but you wont like the result.
Do a search for bottom blue printing or my user name, you should be able to find the thread that I told the story about my 251. It is one of the best hull's Checkmate made and it will do a lot of things right.
I always ran a 25" Mirage, not the Mirage Plus and would run it to 6300rpm. It would run 75mph in 3-5' water on Lake Michigan, lots of fun..
Hope that helps
Dick

Re: What makes a boat porpose?
 

The old cleavers are a horrible prop! They offer absolutly no bow lift, they have no cup to help grab the water, they are a straight pitch, there is no dffuser ring to help load the prop either! For these reasons the prop has absolutly no value! Try a more modern prop and you'll be happy.

Re: What makes a boat porpose?
 

With high x-dimensions (drive height) the Mirage will tend to blow out when trying to plane off. If you can get on plane with a Mirage, then your drive is set deep. The Mirage+ is a round ear 3 blade prop. The cup is carried around the ear which helps to give the prop great bow lift. The Bravo 1 is a 4 blade elongated ear prop. The blades are longer than they are wide. The extra diameter helps the prop grab the water and hook up. The cup is still carried around the nose for bow lift. The Hydromotive prop is almost a cleaver. There is a very slight rounded nose. The cup is no longer carried around the nose so planing with the Hydromotive can be very difficult. However, the Hydromotive prop is a exceptionally smooth prop. This smoothness is a term used to help stabilize a boat from chine walking or porpoising. A true cleaver has a very sharp point. The trailing edge is totally straight. There is virtually no cup. Obviously, this prop will provide no bow lift and will be a bear to get on plane with. This prop is best reserved for very high speed, high power boats, boats that don't need help carrying the bow and outboard boats. A cleaver is a 10 on the cool scale, but when installed on a the wrong boat it will net you a 0.

Re: What makes a boat porpose?
 

The shortie will make the problem worse. Having the higher x-dim means the prop will have a tougher time carrying the bow. You may no longer be able to use the Hydromotive as the prop will cavitate when planing off. If you can get on plane, you should see a speed increase, but be prepared for the porpoise to come back.

Re: What makes a boat porpose?
 

I'm not familiar with the drive or the proping. I know that many Bravo 3's come with uncupped props. Do your props have cup in them? If not, I'd take the top a good prop shop and let them beat on the props for a while.

Let me back up and explain why a boat porpoises. When the bow goes up and down the hull rises out of the water and falls back in. When the hull falls into the water it starts to build lift and tries to lift it's self out of the water. The problem is that the boat can't hold it's self up and it falls back into the water in a cyclaic motion. The prop is the easy variable as we can easily change the amount (or lack of) bow lift that the prop provides. A Mirage will provide more bow lift than a Bravo 1 which provides more bow lift than a Hydromotive. The correct prop for curing the porpoise will be the prop that is able to carry the bow, finely ballancing the hull lift with the prop lift so the boat can remain stable.

A uncupped prop will provide absolutly no bow lift. All the bow lift must now come from the hull design. That's where we get into rocker and hook. A rocker or a curve up in the hull bottom will help carry the bow. My cat has at least 4" of rocker in it. A hook is where the boat bottom drops down. Often a hook can be found in the last couple of feet. My old Concord had 3/8" hook in the last 3' of hull. This was done to make the boat plane off easily (like dropping the tab and leaving it down) at the cost of speed. Forcing the bow into the water also improves the ride and makes the boat more stable. If a mfr. comes out with a unstable boat they may add a hook from the factory to help stablize it. This is the lazy way of fixing a poor performing hull. Hooks can also form from a improperly loaded trailer or boat lift and from pulling the hull too quickly from the mold.

Re: What makes a boat porpose?
 

Mr Gadgets- it had a ford 460.

Cord- thanks for the info. The props do look like they have some cup to them. I dont know how much though. The porposing does start when I hit waves at a certain distance apart. Smooth water its fine. But I boat in the Delawere river so there is no chance for that. I dont think my boat has a problem holding the bow up. I have plenty of power. Its planes real fast. And at cruzing speeds looking at the side of the boat water starts to deflect at the last part of the rear of the boat. I have read that people with this boat has this same proplem. And they all have different drives. I quess with different X hight.

Clear cut- I dont know I didn't do the conversion.

How about adding weight to the bow of the boat!! Since prop. swapping is not going to be easy for me.

Thanks
Russ

Re: What makes a boat porpose?
 

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.

Re: What makes a boat porpose?
 

Well, that's a bit of the mystery around boats. Now to clarify, most boats that are prone to porpoising will actually do it in slick water conditions. Rough water causes the hull to react and it's unable to establish it rhythm.

You hit upon one other point that has not been covered: setback. The further the drive is set back, the more leverage that it has. Greater set back could help solve a porpoising condition.

As to determining the best setup combination, there is no substitute for testing. A race boat will go through dozens of prop, x-dim and setback combinations. It's pretty common on a civilian boat to go through 6, maybe even 12 different setups.

Remember, if you have an opportunity to try a prop, take advantage of it! You'll never know what the results are and they could pleasantly surprise you.

Re: What makes a boat porpose?
 

sell it :evilb: :evilb: :evilb:

Re: What makes a boat porpose?
 

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.

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).

Re: What makes a boat porpose?
 

Darn good summary!

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.

Re: What makes a boat porpose?
 

Yes, that is correct

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!

Re: What makes a boat porpose?
 

If the prop is unable to carry the bow it will porpose. The easiest way to correct the problem is CG and prop.

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???

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.

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

Re: What makes a boat porpose?
 

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!

Re: What makes a boat porpose?
 

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!!!

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!