lakeman2

2003 Laveycraft 27' sabre cat.
I purchased this boat new in 2003 with a Mercury 575 sci Mercury xr drive 1:50 ratio,The boat handled great ! The only time it would porpoise was in smooth water at 60 mph to 72 mph. The boat loves a chop and runs on rails at 90 mph.

Last year it was time to sell or remodel the complete boat.I went the refurbish route.Boostpower built a 1050 hp motor and i went with a Inco scx upper and lower 1:35 gear ratio.Everything else was redone on this boat the same time.

First time out with the scx lower the boat would start the hop around 58 mph and you could not drive through it. The faster i went the worse the hop and handling was really.so bad i stood the boat up on the port rear a few times.I tried 4 blades 5 blades all different pitches and nothing changed.

Imco traded the scx lower for a standard size sc lower and i went out to test again.The boat handled the same way with the hop coming on the same speed .

I talked with a ton of people in the industry and from what i was told i found a deal on a Imco 2" shorty..The hop comes on the same speed but if there is a good size chop it will go away.but after many prop changes i am at 21% slip and the boat just does not feel right.I know the boat well.

After reading tons of threads and talking i think it is not the lower but the upper that is causing all my issues.The cav plate on the scx is big !!! I has noticed when i looked back at the drive spray there was a smaller spray that went off to one side.We mounted a gopro on and at 70 mph the cav plate is hitting the water and making a smaller roost.I read a thread from 2010 here when Kap was doing testing with the scx .They had a lot of hop in the boat so they cut the cav plate and it went away.When talking with Fred jr. at Imco he will not say anything about the cav plate being to big (i get it).He keeps telling me to try props.I have tried all kinds of props and nothing changes.

Has anyone here had the same problems and if so what was the fix.I am ready to get my angle grinder out and start trimming the cav plate.I am pretty sure that is where my problem lies.
HELP !!!!!

Tinkerer

What is the prop height in relation to the lowest point on the bottom of the hull?
Unless your drive is mounted TOO low I can't see the upper causing handling problems.
Your roost will always be more on the side that the prop blades are rising.
You have something way wrong. I am running about 4% at 112 MPH under ideal conditions and 8% at 107 MPH any time.

Tinkerer

It sounds to me like the drive is way too deep and you are trimming up to get the rpm's up.
At speed what is your trim setting? It should be only at most 1 number up from neutral.

JaayTeee

Do you still have the XR that was on the boat ?....if so, for testing purposes, put the lower off of that on the SCX and see how it behaves.... I also don't think the upper would be the issue

lakeman2

Thanks for the feedback.With the 2" shorty i am even with the outside sponson,I have tried a standard also,no change.I have tried all trim settings.I keep it mostly from 0 to 1 1/2.I know this boat well.The drive height with the xr was perfect.Now the scx sits back 3/4" more then a xr and is around 48lbs heaver.I have gopro film of the boat running at 80mph in simi smooth water with the cav plate making a small roost and on the water.This small roost is in addition to the normal roost.There have been a few guys that install the scx and the boat is very ill handing.
This is a good read and makes me think that the cav plate might be my problem.

http://www.offshoreonly.com/forums/d...o-drive-7.html

Please read page 6 about how it cured the hopping/porpoise..

offshorexcursion

iTrader: (5)

I would get the SCX lower back, the strength seems more important especially since the lower hasn't seemed to affected handling, since you are forced to spend so much dialing in at least it will be worth it.

Try more changes before grinding the cavitation plate.

What else has been changed on the boat, you said you went through everything. Did the COG change? Is the new engine heavier? Interior weight?

Maybe raise the drive, extension box, more prop testing, COG, etc. Cutting Edge offers a bravo style cleaver. WCM blueprint the lower. There are still options to help dial her in.

Maybe try running a bravo upper and lower again to see if that also handles different then before.

Tinkerer

If your prop shaft is even with the lowest point on the hull there is no way that the cavitation plate is hitting clean water. (Aireated water hitting the cav plate won't have much effect) It is at least 6 inches up. I can see the larger bullet on the SCX lower causing severe transom lift.
I am running at 2 inches up from the lowest point on my 26 Daytona with a neutral box and I CAN"T run a stock Maximus 5 blade. The boat handles very badly ( severe transom lift)
I have read many times that the SCX upper causes speed loss due to the larger gears but not handling isues.
Where was the prop shaft in relation to the hull before all of the changes?
Borrow a -3 lower and see where that gets you.
You maybe right at the point where you either need to go higher or go lower to stop the transom lift.
Is there any chance there is a problem with the hull?
It just sounds to me like you are having transom lift problems.
Find someone that will let you use a -3 shortie and stay out of the throttle. ( don't break their drive ) and see if the ill handling goes away.
Try some bow lifting props.
Wish you were closer I would love to take a close look at your boat.
Steve

lakeman2

Thanks for all the input !!!!! I have tried 4 blade and 5 blade props from 28 to 32 pitch.They were all Mercury stock props.The new motor is lighter and the interior is also a little lighter.How far out of the water should the cav plate be at 80mph ? In light chop water when it is porpoising the gopro shows the cav plate on top of the water making a small sideways roost.

Tinkerer

I have been told that the 26 / 27 Daytona center of gravity is too far forward. The same may go for your boat and you made it worse with the lighter engine and interior.
My new engine is heavier than the old one and I put the drive on a box. So I moved my CG back.

Tinkerer

Show us the video.

At 80 + MPH the water only hits the bottom of the bullet. I had to move my water pick up down for better water pressure. It was in the nose of the bullet.