nose diving riveira

I am sure Ike has got it but we have seen no photos of the installation.. In the Wake boats I am familiar with the V8 sits at the rear of the boat and drives down through a gearbox to the prop shaft which passes under the motor .. This brings the weight to the rear ..clearly this is not possible in this design but illustrates the problem ...photos of the set up in question would be helpfull .....where is the fuel tank ???

 

There have been dozens, if not hundreds of this design built and it does have concave water lines forward and other speed limiting hull shape attributes, but does not have a reputation for this behavior. I have seen this precise engine and drive setup done both ways, that is the trans driven off the crank snout or the flywheel end. It is more common to see a velvet drive off the flywheel.

 

tunnels,
I haven't changed any dimensions or position of any major component.I followed the plans to the letter. The plans showed a dimension of 1200mm from the transom to the shaft hole along the keel. when I drilled this and setup the shaft, the prop ended up being 310mm foward of the rudder. this seemed excesive to me, but if it was wrong, I had no idea where it should be. I beleive that the motor is mounted to far forward, but i don't know how to test for that or calculate where it should move to.
I have a photo of the boat in the water, but I don't know how to attach it. it shows the boat sitting bow down, in the length of the boat, its 60 - 80mm.

Regards brian

 

Email me the photo: sideslip AT tpg dot com DOT au
I'll post it up.

Regards, Andrew.

 

Some photos of Aussiebrian's boat:



Looking at the photo of the boat in the water, I can imagine, with two adults on the forward seats, she'd be nose heavy.
It's also got a whole lotta prop for a fairly small boat.....would definitely contribute to a nose down force.
Brian, what rpms does the boat turn flat out, or can't you get there?

Regards, Andrew.

 

Quick question what is the weight of the motor and gearbox? ??need pictures of the motor and gear box and the shaft where it goes through the bottom . Also a nice side shot of the rudder , prop and shaft , including the bottom of the boat in that area . Just email them to me - stuartwrcom@gmail.com and lets have a look see at what you have done .

 

andrew,
we haven't pushed the speed yet untill the peformance issue is solved. whe have tryed different props in three blade and four blade, with not improvement.

 

the motor/gearbox weighs 400kgs. i'll have to get some more photos of the motor installation the boat is stored elsewhere will send couple of days.

 

Video:

I've only ever seen an outboard do this, I think weight too far forward and the planing surface is coming aft of the CG.
My thought is to try some "bow lifting" prop cuts, if available for inboards. They're available for outboards. The props you are using could also be generating stern lift too.

Regards, Andrew.

 

I have seen similar behaviour, it was trigged by large amounts of air following the flat bottom and ending up in the propeller disc. There it created an asymmetric thrust, like what we have in a surface piercing prop. The event is fairly sudden; either you have it or it is absent. It seems some of these flat bottoms have a "double hump", depending on CG placement, which might explain why your rig is suffering, and not others.

Please give us info on rpms, speed when the dip occurs, gear ratio and prop dimensions. I think Andrew is on the right track; the remedy is a combination of medicines.

Btw, the USCG experienced a similar behaviour on a prototype to a beach rescue craft; a sudden dive, followed by a dangerous broaching. I'll see if I can excavate the report from the office sediments somewhere....

 

dont give up on us !! some one will have the answers !!!

http://www.glen-l.com/weblettr/webletters-1/webletter12.html#layout

Find this site and open / go to the third page and just below the picture click on plate 37 and it should pop up and give you a drawing you need to look at .
I have taken the time to drawn out full size what i think you have and come up with the possability that you could have close to a 16 degree shaft angle and thats pretty high . The space between you rudder and prop (310mm ) is way to much and could be closed up to the 100 mm as you will see in the drawings on the Glen L web letter . Also by shifting the rudder shaft to 75 mm from the transom (outside ) you could crib a little more and suddenly the prop shaft has dropped from the 15 degrees to close to 10.5 to 11 degrees and thats getting much more realistic .
From the transom (inside )to the aft end of the shaft coupling could measure close to 1.600 with the motor and gearbox forward of that .I reckon on the centre point of balance of your motor and gear box being close to 2.400 from the transom which places the motor and gear box a little far foward .
I have drawn all this on my shed floor and it looks in proportion but dont know what you have so its all guess work .
With the steep angle of the shaft and wanting to push up and lift the back of the hull and it all of a sudden nose diving as mentioned by some one already Air is coming in under the hull from the transom . i would sure as hell being loading a stack of sand bags right in the back . If you already have 200kgs then another 200kgs should get you on the plane . Not fixing the problem but stopping the transom from lifting and getting airated .

 

While it is true that decreasing the shaft angle can only be beneficial, adding more ballast at the transom is not. It will result in a heavy boat (a killer for planing characteristics) and, besides, are we sure that the structure was designed to safely support 400 additional kgs of weight all concentrated at the transom? We are talking about increasing boat's dry weight by 30%! A too big variation for a planing hull, imho.
The only definite solution, imho, is to move the existing equipment (not to overload the boat with new stuff) in such a way to make the CG move backwards. A lots of work, but still better than having a crippled boat.

 

Aussiebrian,

I have read with interest this thread, you still claim that you have not altered anything from the original plan, is this statement still current after having read others comments and referrences?

 

Somthing has been changed But need to find what .I feel its something simple but not obvious at this point in time without some good pictures and some measurments . :idea:
I even drawn it out as close as possible to real size on my shed floor and the only things i can see is first shaft angle which seens quite high ! and second the motor and gear box could be a fraction to far forward . Its a heavy motor and gearbox for that size of boat .
Also fuel tanks and the battery where are they mounted ?
Last is the possabilities of hook in the bottom and if it has hook how much and where !
Iv spent hours look at questions and answer sites for ChrisCraft and Century boats of that type . The dead rise on those boats is very shallow and they plane really easy and get up and go very quickly .The guy has made a nice job of building the boat by what can be seen from the pictures and not concerned with the buiding its the mechanics thats the probelms i think !!

 

The shaft angle on all of these is pretty severe, but within the usual for this style of boat. The engine and trans is not overly heavy and this exact combination is very common. We will never know unless he takes a second look at the measurements, compares them to the plans and catches the mistake. The prop being further from the rudder is an indication, but nothing definitive until this is checked against the plans. This distance should be quickly scaled off the plans to see if it is off. If prop is falling over a foot forward of the rudder, placing it possibly several inches forward of where it is supposed to live. Assuming the shaft length and angle are as designed, this is a very real possibility for the problem, but it is still a guess without measurements and double checking the build.