Prop selection again

Hi guys and gals,
I know this question has been asked many a time but I need to ask it for myself this time, I have a 24ft stejcraft (australian fibreglass) brand boat its about 15years old, I have a new 350 chev marinised engine in it with a volvo penta 280 stern drive leg. Now the problem is that to get the boat onto the plane I need to rev it up to about 4500 rpm, and then to maintain it on the plane about 4000 rpm, at this rpm as a constant Ive destroyed some engine rocker arms. I assume that my prop is to small its currently a 3 blade unit 14 inches wide with a depth of about 3.5 inches I dont know what the pitch is sorry. Unfortunatley I cant go to much bigger because the prop clears my exhaust plate by about 1.5 inches. I have a 350 in my car and it sits at 2000 rpm and is feels just right is it possible to decrease my boat engine rpm and still maintain forward speed Any one got any recommendations on what I can do to help fix the problem eg reving so hard
Any help is appreciated
Thanks Simso

 

Sounds like it could be any number of problems. Assuming the motor is in good condition and producing resonable power and since 4000rpm is required to maintain a Plane, I would start with the prop. Not familar with your sterndrive, but if it has a single prop and NOT a Duo-prop, your prop should probably have about a 17 or 19 pitch. Check and see if there are any numers on the prop hub.

If you're throwing alot of water (a large rooster tail), the prop may be too small or trim is too far out. Does bow of the Boat come up high when your trying to get on plane? If so, trim is not all the way down as it should be to get on plane.

Also, make sure your prop Hub (rubber insert?)is in good shape and not slipping on the drive shaft.

Hope this helps, Jango

 

I have no idea how to work out the pitch but I can answer the other ones, yes I do have a decent size rooster tail out the back in the centre, to get onto the plane its a really steep climb you can feel the front of the boat lifting fairly high. How does one work out ones pitch by looking at it.
Thanks for the advice so far
Simso

 

I know of no way to determine pitch other than having a Prop repair/repitching facility check it for you. Although, It may not be your prop .

"to get onto the plane its a really steep climb you can feel the front of the boat lifting fairly high"

As I indicated in previous post, your Trim may be way TOO FAR OUT. Trim is usually controlled by a switch on or near the throttle. Out Drive should be "All the way in" or Down during acceleration. Then, after or near planing gradually "brought out" for max. performance.

I suspect this will solve your problem

 

Simso- while the boat is sitting at the dock, or on the trailer, take a look at the drive. On (or inside) the hub of the prop there will be a number like " 14 x 19 " or something. In this example the 14 is the diameter and the 19 is the pitch. Now look at the angle the prop's pointed at. The shaft should be parallel to the boat's bottom or angled down slightly; if it's pointing upwards at all and pressing the trim switch doesn't bring it down then you need a mechanic. The trim should be all the way down, right to the bottom, while you accelerate. Then trim up very gradually once you're on plane. In all probability the problem is trim. It is also possible that you need a slightly steeper pitch prop but check your current one first.

 

Thanks guys I found a number and it was 814612, after doing an internet search I found that it is a volvo prop 15 x 17. As far as trim Its an old boat there is no electric or mechanical trim for the boat hull, the boat hull has a couple pieces of marine ply "wedge "stuck to the rear on the left and right. Tha actual leg is electric controlled but is mechanically locked when fully motored down into place, with the boat on dock/trailer and using the centre of the V hull as a guide the stern leg when down and locked sits angled slightly to the front, when appliying pressure and pulling rearwards its almost perfect 90 with the hull. Like I said its got me stumped, Im assuming the prop just isnt coarse enough or something along those lines
Any more suggestions, these are making me more aware of my boat anyway, so keep em coming please
Thanks Simso