Porposing

[ironlake2]

what causes a boat to do this. I have a mark 55 on a 12 foot crestliner jetstreak that does this around 28 to 30 mph but then I hit the throttle and the front end just lays down level and it goes like hell.
gps said 39.8 mph with a double cupped 12 inch pitch prop. this was all done for me by a prop guy in brainerd mn. he also did a varaible pitch on it for a super hole shop. boat total weight is 550 lbs with me and a full tank of gas.

[velox]

Almost any flat bottom boat will porpoise at some speed.
A boat needs a lift equal to the weight of the boat. A flat bottom boat has an excess amount of lift area when the bow is at high angle relative to the direction of travel.Once the bow gets too high it falls reducing the angle of the bottom of the boat below what is required. This is a dynamic instability.
When you go faster the angle of the bottom required becomes less and if you are lucky all of the forces even out and you stop porpoising. Even then in very calm water you may start porpoising. A Vee bottom boat rises out of the water by the amount required to plane the boat at any given speed and does not porpoise like a flat bottom boat does.
One of the most beautiful sights I have seen was back in the day, a group of E class Raveau flat bottom boats approaching the start line at full bore with their bows floating along like they were being held by a magic force.
They were usually in a slight chop, and at perfect dynamic balance.
In the early 60's I had a 13 foot Raveau with a stock Mark 55. It would do in the high 40's, so you should be a little faster than the high 30's.
I don't remember what the prop pitch was, but I am pretty sure it was over 13 inches. You may have to raise you engine a couple of inches on the transom to turn a bigger prop.

[ironlake2]

right now the anti ventilation plate is even with the bottom of the boat. I am leary about going higher as the water pump inlet on the mk 55 is right under the plate. does the prop push plenty of water in there so I can go higher.
the motor is pristine and do not want to damage it.
the 12 inch pitch with the double cup is supposed to load like a 13 according to the prop man.I do not have any trouble with cavatation since the cupped prop was used and I also have a kickout in the transom bracket so I can put a long straightedge on the bottom of the boat and have it line up exactly with the merc ventilation plate.

[transomstand]

This it?

http://i221.photobucket.com/albums/dd58/transomstand/feathercraft.jpg

[ironlake2]

There was a guy at Tomahawk a couple years ago with boat just like the one you have in the photo and he had mark 58 on a quicksilver tower and foot with open stacks and his top speed was 48 mph. Everyone thought that was so fast, but if you can run in that speed area with just a kg9 you are getting way more out of you 40 cu inches than he does out of the 44 cuber.

[Mark75H]

The KG9 lower unit is dramatically different from the Mk58.

The gear ratio spins the prop faster and the water pickup is low on the bullet ... it was the "XS" of its day

[ironlake2]

The KG9 lower unit is dramatically different from the Mk58.

The gear ratio spins the prop faster and the water pickup is low on the bullet ... it was the "XS" of its day
the mk 58 had a mk 55 quicksilver tower and foot on it with aq racing prop. that kg 9 must have been quite the motor to compete with a 44 cu inch 58 on a quicky with open stacks

[ironlake2]

Bare in mind, this is my opinion and it is worth about half as much as you are paying for it. The Creatliner can be made to run faster by stiffening the bottom. I doubt it will run as fast as a Raveau, but I have a 1957 12' Feather Craft Rocket with a KG9 that runs in the high 40's. The key is keeping the bottom straight. There is a photo of my Feather Craft somewhere on Scream and Fly. Just don't ask me how to find it.

do you mean straight as and arrow or straight as in no lows and high spots on the keel, and how do you make it stiffer.
Is you feather craft stock or did you straighten it out.

[ironlake2]

This it?

this is the first photo where I see the bottom of a feathercraft up on top of the water. the ones that run at tommahawk all plow through the water in the rear end and cause drag due to the narrow rear end. how does this motor combo get that rear end up like that.

http://i221.photobucket.com/albums/dd58/transomstand/feathercraft.jpg
this is the first feather craft I have seen with the rear up on top of the water. the ones that i see run at tomahawk plow along with that narrow rear end down in the water limiting speed
if you want to see my jetstreak at speed go to retrocrestliner.com and click on boats. then scroll to 12 foot jetstreak dave roxin and go down towards the bottom of the page for photo of my 1st jetstreak. there is an excellent photo how the boat perfroms at speed and you see only a small portion of the very rear is in the water.

[velox]

do you mean straight as and arrow or straight as in no lows and high spots on the keel, and how do you make it stiffer.
Is you feather craft stock or did you straighten it out.

All of the above! It appears the boat is aluminum.
You may be able to add some stiffiners to the rear 3 feet of the bottom to stiffen it. If the bottom flexes it will have a hook forcing the nose down causing more drag. It also adds another dynamic that could cause porposing.
Its been about a hundred years since I set up a boat like that, but I think we actually had the top inch or so of the prop higher than the bottom.
I do remember overheating my engine also, scuffing the pistons.
The good news is, Lil Abner rebuilt my engine for free and it ran better than ever! I won a lot of races with that engine on the Raveau.

[seeroy]

Bare in mind, this is my opinion and it is worth about half as much as you are paying for it. The Creatliner can be made to run faster by stiffening the bottom. I doubt it will run as fast as a Raveau, but I have a 1957 12' Feather Craft Rocket with a KG9 that runs in the high 40's. The key is keeping the bottom straight. There is a photo of my Feather Craft somewhere on Scream and Fly. Just don't ask me how to find it.

Olegator - I love the picture. Your Feather/KG-9 would look great next to Frank Vidal's Raveau at OFF2009. Serious Old School!!! - Steve