Propellor to rudder to hull position.

I have a 15 foot speed boat. It is powered by a 300hp 6cyl engine and is a midmount. The boat is lightweight and has gps verified speed of 57 mp/h. I have the boat upside down at the moment for re-finishing and now looking to replace the skeg, prop shaft and cav plate. I am looking into adjustable cav plate and reducing the prop shaft angle from 14 to 12 degrees or maybe a bit less. I want to make the cav plate bigger to extend the length of the boat and would like to know if I should bring the rudder and pintle back aswell? Would I need to extend the prop shaft to get the prop closer to the rudder? It seems the further under the boat the prop is, the more the boat seems to lift at speed so im hesatant to extend it, but worry about lack of steering if rudder is to far away. Is there any literature on this topic or formulas to use?

 

Go check out the Glen L marine inboard section and find there install ation pages . They are really helpfull for all the questions you have there . ! Just google it !!

 

One of the important things you need to understand is that the plate is an anti-ventilation and not an anti-cavitation plate. The main function of it is to prevent air being sucked by the propeller. It can also keep air from being sucked around the rudder to prevent the loss of steering.

 

post pictures !!

Pictures !! i love pictures !! they tell a long story of what you have and can see what you are trying to do !!

 

Gonzo is spot on for terminology and misconceptions.

Ventilation is also known, correctly, as aeration.

 

What do you hope to gain ??

what do you exspect to achieve by what you are trying to do ?? If it running ok like it is why change it !!

 

Just trying to increase the performance. High 50's (mp/h) is ok for this type of hull, but high 60's is not impossible as others have done it. (even without th hp we actually have)

I want to install adjustable cav plate, as in t-bar shift from driver seat. Trying to lengthen the boat a tad for safety mainly, If I'm going to sit the boat on its tail at speed, I want lots of boat length ideally.

These are the challenges of trying to make something do what it wasn't really designed for...

 

Let me know what it is an I might know something about it

Cav plate in OZ/US is the common term but its essentially does the trim tab function for a shaft drive boat

 

Now this is a plate !!

Is this what you had in mind ??

 

Sorry, Let you know what what is?

 

http://m1067.photobucket.com/albumview/albums/bluejinx202/image.jpg.html?o=0

 

should have made it go all the way across !!!

 

Seacraft?
You need to convert to a Vee drive to get that to go
Driver and that much engine forward it is too heavy upfront
Check out some 80's ski race boats in OZ

 

I'm just leaving it midmount and doing the best I can. Conventional set up has the engine 100mm forward than mine and 12 degree prop shaft angle where mine is about 14 degrees. As I said, while its upside down I thought I might sort some things out. Just doing the reasearch now because I don't want to ruin it either. There are a couple of midmount sixes in the seacraft club that are in the mid sixes for mph top speed, and v8 midmounts in the mid seventies. My boat does drop its nose down at 57mph and the drag increases dramatically.

That picture is not my boat as I couldn't find a picture of the cav on mine, but mine is the same hull. That one in the picture is one of those race boats from the mid eighties, and it's cav size is quite a bit bigger than my standard one, it has three stays and it's offset I think for prop torque?

I measured a friend of mines boat that is going well but is a larger v8 hull, he has a transom width of 1500 and a cav of 750, my boat measures 1360 so I'm going to make a 680 wide cav to start with and go from there.

I'm considering making engine mounts to move the engine forward and therefore lessening my shaft angle, but bolting the mounts further back and maybe pinning the weight back aswell?

Sounds like the prop should stay where it is as that keeps the weight relationship between the two as weight closer, and therefore back as in like the vee drive setups mentioned above.

 

I am building a crackerbox it's just under 16 foot and I'm running it with a 5.7 v8 with a 1-1 gearbox, keeping the engine as close as possible to the center of gravity and using a 66" shaft I was able to get it to 12 degrees and still have a 2" gap from the 13" prop to the hull and 4" from the end of the shaft to the rudder giving enough room to remove the prop if needed,the rudder has a plate on it to stop it from sucking in air.



To help work everything out I built a mock up of the motor stringers and the hull.