Power Cat - Same rule as V hull?

[Jim Hawkins]

I have an 18' Seagull cat and am working at getting an F70 Yamaha motor propped right. Using an old prop, strictly for a base line to determine what prop to use I found the boat could only get to 4400 rpm's top. When talking to the "Prop Doctor" in Key West Dan's first comment was, "sounds like the motor is too low". I said no the motor is fine, the plate was exactly in line with the cat's center keel. But after second guessing myself I took the boat out and the pic is what I saw behind the transom.

Now I suppose this is a hydrological mess but I wonder, do cats act differently in a way that affects motor height? Mine has this small, keel like form directly in front of the motor but it is higher than the two side hulls and only about as wide as the motor shaft.. If you look at the picture closely you will see two grey lines in the whitewash. That is actually corners of the shape of the water leaving under the hull. Any thoughts on motor height for a power cat?

 

[jimmbo]

From the one pic you have it does indeed look too deep in the water. A couple of pictures of the engine/hull from the side and rear, taken level with hull bottom would shed a lot of light on what you are dealing with.
There was a PowerCat boat at our lake 40 yrs ago, it wasn't a Catamaran(twin hulls) but it was 3 hull design, and one 85 hp outboard on it.
Catamarans usually have to use twin outboards

 

[Faztbullet]

First its under powered and it needs to come up at least 1 hole......

 

[Jim Hawkins]

The fact that the motor is at the low end of the boats spec-ed HP range and that the motor is too low I took as givens.

My question was meant more to discuss any differences in the way a center mounted outboard is mounted on a cat as compared to a v-hull. My impression is that due to the water being funneled between the hulls that the small center keel in front of the outboard is not a reliable reference for setting the ventilation plate height, at least on this hull.

Just wondering what other cat owners have found.

 

[Sea Rider]

So this is a standard powered cat boat with 2 side pontoons, with nothing in middle bow to transom, right ?

What's the round portion seen protruding left of the sviwel bracket trim holes ? Is It the upper non splash plate ?

If so, Yam OB sits too high, at least 1.0" high, if impossible to lower it down any more for water flow to skim right under upper splash plate cat has issue due to a lower leg/ transom height mismatch.

When determining if OB sits at the sweet transom height must be done in flat calm no wind water cond while looking back what's going on between back of transom and front lower leg.

Happy Boating

 

[ahicks]

With no direct cat experience to base anything on, my bet is it's going to set up more like a high performance pontoon boat than any other type. Those transoms/motor pods are always higher than the bottom of the 'toons. For that reason there's usually some messing around/trial and error involved finding the ideal engine height there.

Here, my WAG would be to place the cavitation plate maybe half way between the top of the tunnel, and the bottom of the sponsons and see what happens for a hole shot, hard turns and fully trimmed out high speed runs.

Regarding the "keel" this boat has in front of the engine, the bottom of that is where I would have set the cavitation plate for a first try.

 

[Jim Hawkins]

More pictures,
Second pic is a drawing showing the hull as viewed from behind the boat. I don't know what you call the small shape between the hulls so for lack of a better term I have referred to it as the keel. The ventilation plate was set in line with the bottom of the keel. (Red arrow) The keel protrusion is smallish, about 9" wide and runs forward on a taper about 3 feet.

First picture is of course the motor and comparing it to the motor under power it looks like 1 hole is the very least the motor needs to come up. Is this the way of cats or only this hull or this particular situation?

 

[jimmbo]

Any chance of a pic of the boat on a trailer? A picture of an engine tilted doesn't show anything, except the prop, which out of Curiosity is what pitch?

 

[Sea Rider]

There's no such rule or rule of thumb about sitting any OB at the sweet transom height spot. In real boating world need to match OB's front lower leg against the incoming hardened water flow exiting right in middle of rear keel ,doesn't matter if being V, round or flat shaped ones.

Seeing your cat and OB configurations, to me OB sits way high. Check Pics for clarification and plain tech understanding...



Upper slash plate must skim right under hardened water flow at speed, right now sits way up, would dare say 1.5 to 2" with respect to water flow.

Ideal proven OB height configuration :

Need to lower OB downward if possible till that condition is fully met. If not, will need to chop transom down accordingly. OB must ride trimmed to 90º with deck load evenly distributed for top prop thrust while riding parallel to water level.



Once OB sings nicely sitting at the sweet transom height spot can go straight for a prop maximization dialing a less pitch prop for OB to rev to its full wot rpm range factory stated. Combo will perform much better when getting out of the hole at just fast displacement speed.

Side Note : Can't possibly fully trim out a cat powered by just a 70 HP-OB and pretend high speed runs, that simply won't happen, it's just a fast displacement speed cat which its pontoons rides parallel to water level, not a speed boat LOL!!

If any point is not clear, simply ask...

Happy Boating

 

[Jim Hawkins]

Thanks Sea Rider but the thing you showed as the upper splash plate is the thing I marked in orange. For reference, note the hole at the rear of the motor where the bolt to hold the zinc trim tab is.

I don't have a pic of the motor on the trailer.

 

[Faztbullet]

It needs to come up.. its too deep.

 

[Jim Hawkins]

and so it shall Faz, so it shall...

 

[Sea Rider]

Sorry first posted pics were confusing, thought that marked border in orange was the upper splash plate. Plainly in agreement, need to raise OB accordingly till water flow at speed skims right under the protruding lip of the upper splash plate as in second pic. That's the sweet lower leg/transom height for Tohatsu & Yams too.

Will need to raise transom at least 2" as a start and keep raising it up till that suggested position is reached while doing water test at wot. That's a alum or fiberglass cat ?

Happy Boating

 

[ahicks]

Agreed, pic looks like engine is too low. That splashing around should be much cleaner. With the ventilation/cavitation plate (whatever you want to call it) set even with the bottom of that "mini" keel (I would have set it there for first try too), surprised that keel isn't doing a better job of deflecting/smoothing the water flow to the lower unit. Pics look like it's acting like it's not even there!

Wondering if this pic may not have been taken at a speed below where the hull is really designed to be run. The bottom of the tunnel appears to still be in the water. Thinking the bottom of the tunnel should be funneling air under the boat helping to lift it? About how fast was the boat going? Water flow to/around the lower unit may change drastically with just a slight increase in speed (when the bottom of the tunnel is lifted out of the water). Kinda like the surge you get when a hydroplane gets "up on step".

 

[Jim Hawkins]

Wondering if this pic may not have been taken at a speed below where the hull is really designed to be run. The bottom of the tunnel appears to still be in the water. Thinking the bottom of the tunnel should be funneling air under the boat helping to lift it? About how fast was the boat going? Water flow to/around the lower unit may change drastically with just a slight increase in speed (when the bottom of the tunnel is lifted out of the water). Kinda like the surge you get when a hydroplane gets "up on step".

Ah, you bring up a good point. Should the tunnel be out of the water with air passing through? It would seem it should be from all I've read about a cats ride being cushioned by the compressed air passing between the hulls. If the air does not exit aft it could be a big part of my problem. The boat was doing about 20 -22 mph when that photo was taken.

As a side note, This boat has a lot of weight I can take off. Forward and rear decks can be removed, center console could be made lighter as well as the soggy deck (that is typical to some older cats) can be rebuilt lighter. I can easily take off 300 to 400 lbs. My 2 previous boats were projects where I gutted the boat, lightened it way up and ran small motors with good performance and I plan to do the same with this one.

There are so many issues with this boat right now I have to adjust each one in increments because they all affect each other.. Move the motor up 1 notch, take some weight off, then as speed picks up try a lower pitch prop. Tweaking. I WILL make this boat work like I want it, light and economical. I prefer a bare bones boat to one with all the bells and whistles.

Thanks for that last post ahicks, it is to the point of what this post was about - cat hulls

 

[ahicks]

Sounds like there's a LOT of room for improvement! I like the focus on light weight and minimum power for a project like this - just to see what can be done.

To prioritize a little maybe, not real sure I would mess with the engine height just yet. You need to get your hull speed and rpms up to see if the engine height issue doesn't take care of itself when the hull gets up a couple more mph. No point in raising it just to find out it needs to be lowered when the hull starts working right.

Putting a lower pitch prop on it might help get it "on top", but then there's the potential it may turn out the smaller pitch prop is too small once the hull gets "on top".

With a better understanding of what you have going on, my focus would be putting the boat on a diet! 1 hundred pounds is 1 mph at least! It may not take much.....

And what about your trim? Did you try raising the nose up? If that's possible, that might help pack/trap a little extra air in there....

 

[Faztbullet]

A cat and tunnel hull need to run in excess of 60mph before trapped air will start lifting hull. My 16ft Texas Tunnel with 572ci wouldn't start packing air till 80mph then you could feel hull lift and lock riding on the pump in excess of 120mph...

 

[ahicks]

Tunnel hull fishing boats are pretty popular in Florida, and I see those up on top at speeds a LOT lower than 60mph. Have never driven or ridden in one so no real experience here.

 

[Jim Hawkins]

Wow, this boat will never go 60 mph and if it does, I don't want to be in it!

ahicks, I did do a trial diet, with 2 less passengers, the boat stripped of gear, the cooler seat removed and the front deck removed (easily 500 lbs.) the problem was still there, little change from a full boat so up one notch goes the motor. Will post results.

 

[garbageguy]

Good project - got any pics of the whole setup so we can see the whole boat, hull, and motor trimmed down, at rest either on trailer or in the water ? - both are useful, I think you can get more help the more that can be seen, and its most helpful for those of us dealing with a similar thing. Thanks