prop change

[mewewe]

i am running twin 1973 18 horse evenrudes on 16ft starcraft holiday 9x10and would like to increase top speed any ideas thanks

 

[Bondo]

Re: prop change

Ayuh,........

You've got to know the RPMs at WOT that your motors are rated for,+ What they're Doing Now at WOT.........

Other than that,...... The basic Rule still apply........

If your motors can Exceed the rated rpms at WOT,..... Increase your Pitch.......
If the motors can Not meet the rated rpms at WOT,........ Decrease your Pitch........

 

[Texasmark]

Re: prop change

Where in the water pickup on the '73 OMC engines? If lower than the engines of the '50's you might check where your anti-vent plates are with respect to the boat's bottom. Those older engines had to run lower in the water but newer ones may be able to run with it even or slightly higher. If you can raise them it will make a profound effect on performance.

Mark

 

[Frank Acampora]

Re: prop change

A single 35 will give you more speed than the twin 18s. --true-- LOL

But assuming you want to keep twin engines for whatever reason,

1. Since each engine is carrying a little less load: On twin engine installations typically each engine takes a prop with 1-2 inches more pitch than a single engine would. So- see if you can get two higher pitched props.

2. On anything except a dead flat bottom boat, as you move out from the center of the transom, transom height decreases so twin engines need to be mounted a little higher on the transom.

3. For reasons I don't fully understand, twin engines need to be trimmed with a little "toe-in" or "toe-out" --what-ever! When straight ahead, the prop washes should converge about 100-200 feet back from the transom. In your case, with such small engines, mounted close together, I would try the lesser amount first.

 

[Texasmark]

Re: prop change

Frank, maybe like land vehicular front tires. There is usually slop in the steering system. If both were centered there would be no lateral load on the steering and the tolerances would/could allow for oscillations. By putting some "toe" on them, you force them against one side essentially eliminating the slop and oscillations.

Just the other day I had a tractor going down the road and the front end was shimmying like crazy. The faster I went the worse it got.

Brought it home, put some toe on it (in...front of tires closer than rear) and solved the problem.

HTH

Mark