Stern squatting under power

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catalac08

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Hi
My boat 8m sailing cat squats badly (I think) under power, approx 6 inches at cruising revs and more with more power. Engine is only a 30hp outboard with an ultra long leg which otherwise works very well. From what I have read an extra inch of leg immersion equates to a 100 rpm loss at the prop. I assume therefore that apart from the additional hull drag the extra leg immersion will be slowing things down/using more fuel than necessary.

It is not a big job to alter the transom to give a little more tilt to the outboard leg which in theory should help the propellor if not lift the stern not to drive it down (at max adjustment already) but my question is really whether squatting under power is inevitable or if this is fixable?
 
On a normal boat, squatting under power is usually a sign that you are trying to exceed hull speed - the bow is lifted on the bow wave & the stern tries to drop down into the trough.

30hp is a powerful o/b for an easily driven hull (you will have nice long narrow hulls) - what sort of speed is this happening at?
 
to start with, what sort of angle is the leg at when the boat is at rest ?

Have you got sloping transoms ?
 
SaraBande; a Catalc 8m looks like this;-

12006a.jpg


It is basically a displacment hull & a high revving 30hp o/b is probably about twice the size needed for hull speed in normal conditions.
 
Hi
Thanks for responses - the squatting is 6 inches at 5-6 knots only (my normal cruising speed under power) and well below hull speed but at max speed, about 8.4 knots this increases to about 8-9inches squat. The transom is vertical but the furthest forward hole adjustment brings the leg not quite vertical i.e. parallel with the transom. When sailing the boat is trimmed nicely fore & aft. I appreciate the motor may be considered too big for the boat and under light weather conditions many of these boat manage quite well with a 9.9 HP engine but with the wheelhouse there is a lot of windage and the bigger engine enables good progress to be made against wind and tides because these are not good boats with the wind on the nose, under sail only.

Posted on Mobo forum as guessing that is where the expertise is for trim experience and issues.
 
Hmmm, 8m = 25' ish, doesn't it? So your displacement hull speed is around 5-6kts IIRC. My 25' Westerly used to start to squat at about 6kts & max speed (with a 9.9 johnson - cue song!) was about 7kts at which point she was squatting noticeably.

I suspect that your "problem" is exacerbated by the fact that Cats have narrower hulls (plus enormous rocker in your case) so buoancy at the ends is quite low. This may allow the stern to "sink" as the bow lifts to the bow wave. This will be exaggerated as the bow wave gets further back towards the rather wider & deeper midships cross-sections.

I do understand the benefits of over sized o/bs, which is why I said "normal conditions"! It is also why I have a 30hp i/b engine in my 31'er.

I suspect most on here are more experienced with planing hulls & trim tabs about which I know less than beggar all!
 
tks for the pic SeaRush. With that info, I suspect that the "ultra-long" leg is creating a long turning moment below the centre of buoyancy, so lifting the bows.

Is there any opportunity to borrow a short leg o/b ?
 
Ok, so boat mass is 3 tons, and say Cof B is halfway, and 30 hp o/b propellor is ? 2ft below waterline, and the leg is raked aft (out), then it won't take much effort to lift the bows and depress the stern, especially looking at the rocker.

Next question (this is all lovely theory as my propellor is about 4ft below the w/l and horizontally aligned), does the propellor wash break the surface of the water with strong turbulence ? I'm looking for evidence of the thrust being directed upwards to the surface instead of parallel to it.

I have ferreted around the various Catalac sites, and seen pics of the boat squatting even at low speeds. One owner fitted sugar scoops to help solve the issue.
http://www.bobcatandcatalac.btinternet.co.uk/Idea.htm
The pic there, of the extremely pronounced rocker, shows how easy it would be for a mis-aligned and overly powerful prop to lift the bows and sink the stern.

And, a 30 hp o/b weighs around 70 - 80 kg, which is like having a human bean standing on the stern; that is not inconsequential to the trim.
 
Last edited:
Searush said:
? .. your displacement hull speed is around 5-6kts.

I suspect that your "problem" is exacerbated by the fact that Cats have narrower hulls (plus enormous rocker in your case) so buoyancy at the ends is quite low.
Click to expand...

I think you've hit the nail on the head!
 
rich said:
I know this will sound silly,but has anyone tried mounting an outboard on the front of a cat to pull it like sails would,
Click to expand...

What a nice idea, you win the De Bono (though back to front isn't really lateral) thinking prize for the evening, though the effect on steering might be an issue.
 
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