Question about prop to transom distance

[kfxguy]

I've heard both ways....which way would a boat be more stable at high speed going straight....prop closer to the transom or prop farther from the transom? In other words, use a short strut or long strut.

[Heath M]

Depends on the the boat, but for a Cat my guess would be a short strut.
Found this in an artical,
Porpoising: Up and down movement, when running in a straight line. Could result from a prop too far from the transom, by a too positive strut angle or a too shallow prop. Possible corrections are:

Bring the prop near to the transom, 1/2 inch each time, testing after each adjustment;
Adjust the strut angle;
Lower the strut.

[dasboata]

The longer the loser in my book !! Especially on a mono

[kfxguy]

Depends on the the boat, but for a Cat my guess would be a short strut.
Found this in an artical,
Porpoising: Up and down movement, when running in a straight line. Could result from a prop too far from the transom, by a too positive strut angle or a too shallow prop. Possible corrections are:

Bring the prop near to the transom, 1/2 inch each time, testing after each adjustment;
Adjust the strut angle;
Lower the strut.


Good info. I'm a little nervous about changing things up. The last mono I built did 91 but it did porpoise a little on the top end past 90. See if you can tell in this video. Trying to eliminate it.


http://youtu.be/dg7CShGoRec

[modvp55]

I can tell you from a full size boat that it depends on how the boat lifts, if the boat has issue lifting the nose further back helps and then generally it faster so as dasboata says makes them looser. My goal was always to get the shaft to run parallel to the bottom at the attitude needed, I would thus move forward if too loose and back if clued in when balance is at the correct percentage.

[srislash]

It does make sense on a mono having a shorter strut. It should help keep at least some hull in the water to maintain a straight track. If it is just running on the strut barrel it can rock in any direction, as it may with a long one. Running this way is the theory behind MBP's flat bottom stingers. At this point the boat becomes all about aerodynamics would it not?

[kfxguy]

Looking at the video, can you tell what it's doing? The nose is raising and lowering. I need to figure out to stop that, I'm basically building the new boat almost identical because I know the first one worked well....just gotta refine it a touch more. The bracket I'm using is the 2" strut bracket. Should I move the prop a 1/2" closer or leave that part alone and move the batts up a little? This one will have a little more weight in the nose tho....

[srislash]

With a bit more weight in the nose perhaps it will steady it out more. This way it will use the push of the prop to hold the nose up. Have you played with fishing weights as CG adjustment Trav? I have been, this is what the gas guys will do to steady the nose in race conditions. I used them while sorting out my CG with the 1815's. Worked great when you run out of battery movement. In your case it may help as to where the weight is put

[kfxguy]

With a bit more weight in the nose perhaps it will steady it out more. This way it will use the push of the prop to hold the nose up. Have you played with fishing weights as CG adjustment Trav? I have been, this is what the gas guys will do to steady the nose in race conditions. I used them while sorting out my CG with the 1815's. Worked great when you run out of battery movement. In your case it may help as to where the weight is put


Not yet but I put epoxy in the nose and a bit more 2 part foam in it. I also had the batteries as far to the rear as they would go before. I'll have more adjustment in this build. I need to narrow the tray in the rear that my servo and stuff mounts to and it will allow the trays to go farther back. I'm using long trays so I'll have enough forward movement too.

[modvp55]

How sharp is the back edge of the boat? Cleaner the water breaks off the back the more consistent the prop/lift will be.

[boilo56]

From my point of view, i think that the air flow going under the hull is making it porpoise. The boat is almost running only on prop, so i say aerodynamic problem.