prop questions

[moterboat]

have a 17ft tri hull, when I reach 42 mph @5400rpm (still climbing)the engine revs up and I back off,I marked the hub & prop and it did not slip,I noticed that the cavi
tation plate is 1' above the bottom of the boat, Im running a 13x19 S.S. cupped,looks like I have 1' left bettween the cavitation plate and the edge of the prop, reason Im posting these questions is that I read another post that says if you run a larger diameter prop,you can raise the motor up more,if I can find a prop thats (14)x19 would that solve my problem of the engine height? would the 1' make the difference I need?if you think this will work,would this change my rpm's or pitch in anyway?(since Im totaly satisfied with the wot rpm x speed.)
Thanks guys,I have been working on this boat on and off for 10 years,had it in the water twice,those were test runs...would love to get it ready for fishing.. there used to be a 78 rude 115 on it w/ a ?x17 pitch and it never did this.

 

[moterboat]

Re: prop questions

Thanks for the help guy's.
lowered motor and remeasured and the cavitation plate is 2.5 inchs above the bottom of the boat..going to cut the transom down 3.0 inches..

 

[steelespike]

Re: prop questions

Hold on,Are you sure you are measuring the right part.
It isn't likely you could run at all at 2.5" above the bottom.
You would normally have trouble pumping water and getting any bite at all.
Measure transom height at a 90 degree angle to the keel.Measure motor from where it rests on the transom to the anti vent plate (just above the prop.Demensions should roughly match.If transom and motor don't match difference is normally about 5 inches.

 

[moterboat]

Re: prop questions

I trimmed motor all the way down and measured from the bottom of the boat to the cavitation plate,is this right,I did not quite understand the way to measure you posted. I always thought that the cavitation plate(plate above the prop) (that the trim tab bolts to) (that a hydrofoil usually bolts to), is supposed to be even with the bottom of the boat,is this correct?? the cavitation plate is above the bilge drain hole in the back of my boat. am I correct in calling this the cavitation plate??

 

[moterboat]

Re: prop questions

I trimmed motor all the way down and measured from the bottom of the boat to the cavitation plate,is this right,I did not quite understand the way to measure you posted. I always thought that the cavitation plate(plate above the prop) (that the trim tab bolts to) (that a hydrofoil usually bolts to), is supposed to be even with the bottom of the boat,is this correct?? the cavitation plate is above the bilge drain hole in the back of my boat. am I correct in calling this the cavitation plate??

Im going to upload some pic's of it..

 

[moterboat]

Re: prop questions

here is photos of the plate & engine & bottom of boat,I may have been looking at the wrong plate...

http://s215.photobucket.com/albums/cc107/boatmoter/

click on pic to enlarge it..

 

[steelespike]

Re: prop questions

First what motor are you running?
Yes that is right but proper name is antiventilation plate.Even with the bottom is considered a starting point.Depending on the shape of the boat
and the design of the motor.Measuring points I suggested are to determine actual height of the transom. and length of the motor leg.You need the transom height measured at a right angle from the keel line to the top of the transom.If you measure down the actual transom it will be slightly longer than the real verticle height.Motor measurements are to determine the actual length of motor leg.(usually about 20 in. or 15in.)Just to be sure what we are dealing with.

 

[moterboat]

Re: prop questions

Running A 1979 Rude 140 On A Old 69 -17 Ft Tri Hull

 

[moterboat]

Re: prop questions

First what motor are you running?
Yes that is right but proper name is antiventilation plate.Even with the bottom is considered a starting point.Depending on the shape of the boat
and the design of the motor.Measuring points I suggested are to determine actual height of the transom. and length of the motor leg.You need the transom height measured at a right angle from the keel line to the top of the transom.If you measure down the actual transom it will be slightly longer than the real verticle height.Motor measurements are to determine the actual length of motor leg.(usually about 20 in. or 15in.)Just to be sure what we are dealing with.

I POSTED SOME PICS OF THE PLATE TO THE BOTTOM OF THE BOAT AND YOU CAN SEE THE BOTTOM OF THE BOAT AND SHAPE IN ONE OF THE PICS,
WOULD APPRICIATE IF WOULD LOOK AT THE PICS AND LET ME KNOW WHAT YOU THINK.. THANKS...

 

[steelespike]

Re: prop questions

I don't see any pics.Am I missing something?

 

[moterboat]

Re: prop questions

TO VIEW THE PIC'S,CLICK ON THIS LINK,TO ENLARGE THE PIC,CLICK ON THE PIC...

http://s215.photobucket.com/albums/cc107/boatmoter/

 

[MikDee]

Re: prop questions

The plate that's dead even with the bottom of the boat is your cavitation plate! It's where it should be. So what's the problem?

 

[moterboat]

Re: prop questions

Read my first post on this page..

 

[MikDee]

Re: prop questions

A 1" bigger diameter prop might help you out with prop blowout, and you will probably loose some rpm (maybe 200rpm), but it sounds to me like that's not the case. I'd say that your prop hub is slipping, if you are overrevving. Leave your motor height just where it is, it is perfect for the time being.

By the way, mark the prop nut, and the hub, clearly, and check again,,, (the prop nut is fastened to the shaft, it won't move from it's position, but the hub, and prop itself is seperated from the shaft by a rubber sleeve, so if it moves, it won't match up with the mark on the prop nut.

 

[moterboat]

Re: prop questions

Thanks,Im going to try marking it that way, repost with the findings as soon as I get a chance to take it back out....