Mr Gadgets

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

Cord

iTrader: (2)

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.

Cord

iTrader: (2)

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.

Cord

iTrader: (2)

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.

Cord

iTrader: (2)

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.

Russ G.

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

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.

Cord

iTrader: (2)

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.

SLINGSHOT

sell it

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.

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).