rslick

iTrader: (1)

20' 1995 Cigarette 454 efi with pad bottom. The Bravo 3 that came on this boat worked OK but was a pain around the docks and I thought I could gain some speed with bravo 1. With the bravo 1 the prop shaft is 5-3/4 below pad bottom. I have tried a bravo 1 4 blade and mirage plus 3 blade. With both props if the drive is trimmed up the slightest bit from all the way down, the bow lifts and drops continuously and boat becomes unstable. The four blade worked slightly better. With the tabs down a bit the stability comes back and the drive can be trimmed a little. But who wants to drag tabs at WOT? What the heck is the problem?? The bravo 3 could be trimmed but not much before the bow came up and the boat felt loose. It never lifted and dropped continuously though. Does the bravo 1 need to be set back, raised or lowered? Do I need to put the bravo 3 back on? Any thoughts or suggestions will be greatly appreciated.

WETTE VETTE

There are a few things you can try, with some being more expensive than others. Have you tried going up on the trim some more to see if the prop bite frees up? That may help, but if it feels unsafe by all means don't push it. Another thing to try would be a cleaver style or 5 blade prop. Cleavers are tail lifting props and 5 blades buried on deep drives will also be tail lifting. It really sounds like you need a 2" or 3" shorty. With a shorty the prop will run a little looser and your bow lifting leverage will be reduced, which may solve the problem and give some MPH increases.

Good luck!

Craig

Outdrive1

Try a hydromotive prop for more stern lift, the shortie sounds like a good alternative as well. There has to be someone who knows that set-up and can give you advice on where your x-dimension should be. A 1995 Cig isn't that old of a boat, maybe the factory can give you some advice on what they've run on the boat.

dockrocker

Prop shaft 5 3/4 inches sure sounds awfully deep.

rslick

iTrader: (1)

Thanks for the input.

dockrocker

Where was the propshaft with the B3 on there?

AIR TIME

the b3 would be deeper and fater

Mr Gadgets

rslick.
One thought maybe the bottom of the boat. The 3 has lots of blade area and will hold the bow high and dry.. Put on the B1 and it doesnt do so good. Tries to hold the bow up but drops it... Are you getting clean water to the prop. Is there any air or bubles in the water that the prop sees. Is a drive shower letting air get to the prop.. You need clean water and a fairly flat bottom to pull up the bow and hold it there. Sometimes, I have no idea if Cig did this, the manufacture will build something into the hull to work with a special drive like the B3.. just some thoughts. Hope you can find it.
I would call Cig and ask them if this boat came with B1 and is there any difference if it came with the B3..

Good luck
Dick

Whitey

Sounds like the B1 can not lift the bow as well as the B3 did. The B3 has 6 blades so if you need lift it can give it, the b1 just can not do it with 3 or 4 blades. The x for either the B3 and B1 should be the same according to installation instructions. I wonder if the B3 was used to "cover" this strange ride. Measure what your X is and see if it is at the specs. Dwight

tcelano

You are experiencing porposing. Search for old threads on this topic, and you will find an exhaustive dissertation, including the paper I published a few years back on it.

Essentially, you have the CG too far aft for your setup. As you stated, you can stop the porpoising behavior by deflecting the tabs or keeping the drive in. The boat was balanced with the B3 to run right at the inception point, or the highest trim angle you can run and still exhibit stability for the combination of deadrise angle, loading and speed. By replacing the B3 with the B1, you likely moved you center of effort for thrust downward since you don't have that smaller prop pushing as well. Even if you only moved it down 1/4 or 1/2", that would be enough to upset the system.

The usual behavior for one of these rigs is to exhibit instability throughout the midrange, requiring tab deflection. When you get to WOT, you should be able to get the tabs basically all the way up. If you balance the boat with the CG further forward, it will run stable, but not as fast. Try shifting or adding weight if you have none to shift, forward in the boat. I currently have 25 lb bolted in the nose of my boat, and that allows me to get the tabs up and trim the drive out.

Your problem is not severe, and can be corrected by a simple CG adjustment.

Have fun.