do i need a new prop?

[tomgina]

hi everybody. i'm pretty new to all this, and its my first post, so sorry in advance for my ignorance....

anyway, i have an aluminum starcraft ski boat with a '77 70hp johnson outboard. the boat is kind of "inherited", so i'm picking things up without really knowing much about the history of the boat. when i got it, it had no trim tabs (rusted off) and it had a little trouble getting up on plane, especially with 4 or more people on board, but once it got up, gps said it topped out at about 35mph.

to help it plane, i added a hydofoil. the boat planes super fast now, but i lost about 7mph at the top end. at WOT, it really feels like something is dragging, and i assume that the hydrofoil is causing a good bit of drag behind the boat.

looking for suggestions to get the top end close to where it should be. would a different prop help? i dont know the current pitch, but the family who owned it was into skiing, so i'm guessing it is a prop geared toward skiing.

also, the boat does not have power tilt. could adjusting the tilt help? ever since i added the hydrofoil, if i am loaded with passengers in the front, i cannot even get the bow out of the water to get up on plane.

laslty, is the hydrofoil just killing me? should i take it off and add some small trim tabs instead?

thanks in advance of the help!

 

[usskydvr]

Re: do i need a new prop?

ESSENTIAL DATA REQUIRED
When posting a question about propellers, you cannot get a reasonable answer unless you provide the following information in your post: Items 1-4 are essential for meaningful response to most questions.
1) Performance issue you are trying to correct.
2) Current prop manufacturer, model, aluminum or stainless and as a minimum.
3) Current prop diameter and pitch (required).
4) Wide open throttle RPM and speed with an average load (very helpful)
5) Engine/drive make, model, year, and HP
6) Boat make model, year, length and weight

 

[tomgina]

Re: do i need a new prop?

thanks for the suggestions.

1. decreased top speed due to new hydrofoil (included in post)
2. aluminum
3. as i mentioned, i dont know. most likely a ski prop
4. no clue about rpm, no tach. before hydrofoil-35, with hydrofoil-28 (included in post)
5. 1977 70hp Johnson (included in post)
6. 16' 1977 aluminum starcraft ski boat. dont know weight.

 

[usskydvr]

Re: do i need a new prop?

Ok, first take a close look at the prop for any numbers stamped on it. Turn it
one full rotation when looking. Without knowing what prop you have will make any recommendations only GUESSES. You can also take the prop off and take it to a prop shop where they can tell you the pitch. Make sure the anti-cavitation plate (the flat surface about 6-10 inches above the prop shaft)
is parallel to the bottom of the boat and at the same level. ( you can use a yardstick to check) Make any adjustments ( lower/raise the motor , change the tilt with the tilt pin). The hydrofoil will cost some top speed especially if it is running too low in the water (this is where the engine height comes into play). Personally I would remove the hydrofoil and then perhaps change the prop. Keep in mind that you can't get both good top speed AND good hole shot when fully loaded with the same prop (especially with only 70hp)