Cat strut height (again)

[Simon.O.]

Yes I know there is a bit of info here and believe me I have read most of it, in fact I have written some of it.
Given that there are lot more folk now running 30"+ cats I figure there is some new info.
This is mine
http://forums.offshoreelectrics.com/...ad.php?t=10589

I have had it out again recently a with the new 5S to 9XL power setup and it is fun.
The 637 is a hoot and the 640 is insane.
I raised the strut this morning and it all got better.
I now have the prop/shaft centre line 12mm or near 1/2" above the sponson line.
I have attached a photo of the transom profile so you can see what I am talking about.

It appeared that the tail dropped a bit and the nose came up and it fair flew.
I have a few theories as to why this happened but I am keen to try for more strut height.

Is it out of order to have the prop/shaft centreline higher than this, as in 1/2" ?

[Fluid]

For maximum speed, keep raising the strut until the cat slows down or blows off. The exact strut height depends on the sponson deadrise, hull aoa and CG, prop design and model speed. Virtually all props develop some lift and will help keep the transom up - by raising the prop you allow the transom to settle and the bow comes up - sometimes. Changing the angle of the strut effects the bow and aoa too.

Running on the last few inches of the sponsons is fastest on most cats. The less hull in the water the faster the boat and the lower the amp draw, but the more unstable the cat is in wind or waves.


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[Simon.O.]

Jay, thanks for your reply. As usual i wait for your input before I do too much.

With the 637 it ran very well with nose just begining to lift as the tail settled in as you point out. There was a very fine mist above the main rooster tail and to me that tells me that the prop is maxed out. My Canard does this, any higher and the prop is too high.

The 640 sent it nose up and with a little tail lift (from th prop) but still riding the rear like a OPC tunnel. Yes very fast but as we know quite prone to blow off. This was on glass smooth water and looked great.

I figure that if the 637 was "perfect" (to me) then I need 1.5mm more up on the strut to accomodate the 640. This will take the strut to 13-14mm centre line above the sponson bottom and is why I asked the original question.

Your answer gives me the confidence to raise it a little more and run again.
The pond is like a mirror this morning but I am on duty so will have to wait a few days now.

Thanks again.

[Simon.O.]

Yes it has taken a while for me to get on the water again.
Between the weather, two jobs and summer chores my boats nearly got dry.

I tried the strut a bit higher today, well quite a bit higher and it went very well.
I now have the upper limit set and need to bring it down a little. About 1mm should do it.

I ran the 637 for kickoff and it was not enough blade so on went a 640 and that was enough to set the Cat up on the rear and haul well.
There was a lot of the fine spray above the roost that tells me the prop is now just that little bit too high.
Motor, esc and packs were fine for temps when I bought it in after about 6 passes.

I will wait now for some more clear weather and a day off and run it with the strut where I think it should be.
Now it is Autum so the boating weather gets better, my best runs have been on the very calm cold winter days. No ice here

Working on it !!!!
Thinking about a winter build too.