What to do with the cat?

[Per]

Hi all!

I need help to decide what to do with the running surface on my tunnel hull.
In Sweden we have a racing class called "Classic offshore". The boat model must be at least 25 years old, you choose your own pace an try to come as close to it as possible and then the contestants vote about which boat is most authentic. This boat is restored for this class.

Here is the background:

the boat is 4.65m long and 2.05 wide, the tunnel is 1.2 wide, weight without crew (two persons) is ~325kg Speed is so far 46knots with a OMC 3-cyl 56ci 70hp but comparable boats run at least 50-60knots.
From the beginning there were no cutout in the aft portion of the "pontoon" and the lifting strake was the same size from bow to stern. A previous owner have made a step / cutout and reduced the size of the lifting strake the last meter of the running surface.
The running surface also have a very small transverse step some .3m from the cutout. This however is very small and I doubt that it can get any air from the sides. the shape of the step is also strange (to me at least) since the surface before and after the step lies in the same plane, the step it self is actually only a wedge and a cavity other than that the bottom is straight. The wedge is only about 5mm high but it appears to me that this would act as if the bottom had a severe hook pulling the bow down.
From what I have been told this boat model (make and size) is a poor performer in tail wind but works well with a little head wind. I have not driven any other boat of this type before so I don't have anything to compare to but I know that it is faster running into the wind, not to mention the ride is a lot smoother...
It is very hard to get on plane with full fuel tanks and my navigator have to climb past me up on the deck, this is not really allowed by the rules (as far as I recall). Once on plane the boat feels like it would need to have the weight even further back. not an ideal situation since there is nothing to move besides me and that would take some serious modification.

It would not be to hard to grind down the wedge and fill the remains of the step creating a straight running surface BUT There must be some reasons that the step is there in the first place. I've seen a lot of different ways to build the running surface on a cat but I have never seen this configuration before and maybe they just got it wrong.

Do you think the running surface look good as is or should I alter it? If it were your boat what would you try?

Please share your thoughts and ideas!

Video of the boat:
This is from when I was breaking in an old (rebuilt) engine so the speed is not very high but gives you a good idea what the boat looks like though.






Pictures of the running surfaces: https://plus.google.com/photos/11139...NOatISW9_7sngE

Just for fun!

Navigators view:
http://www.youtube.com/watch?v=FJnJ_XKzcms

Our Classic Offshore site: http://www.classicoffshore.se/

English is not my native language so please ask if something doesn't make sense to you!

[zul8tr]

Din engelska är bättre än min svenska, with help from Google

The tunnel steps that I have seen do not have a hook in them, there may be some with a hook that works for those hulls. They are straight with with the alignment of the strake and the short step is carved into the strake creating the void to feed air into it to get an air water mix to reduce water drag aft of the carved in step. I believe in your case there is a lack of power to get that hook to operate as it was designed except into the wing which helps get the hull to lift. Usually hooks are put in by the manufactor to bow down the hull based on their testing. I notice in the video the engine sounds like it is slipping due to prop cavitating. Send some pics of the engine prop shaft depth relative to the tunnel sponson bottom. Do you have electric engine kick out or fixed, where do you run the tilt relative to the fore aft sponson bottom?

My first thought is to remove the hook and get it straight and leave what ever small amount of step remains, if any.

What rpm are you running at full throttle?

What is the prop pitch and gear ratio of the gear case?

Also all the tunnels I have come across have a rough bottom running surface to break up the water off the running surface and get it air entrained for less water drag.

[Fastjeff57]

Wedges like that have been added to boats that porpoise like crazy, to stop that from happening. Could that be the reason?

Jeff

[zul8tr]

Wedges like that have been added to boats that porpoise like crazy, to stop that from happening. Could that be the reason?

Jeff

In video his hull rides to low with alot of it on the water (high drag) signs of bow down. If he is trimmed out and still bow down the hook is killing the lift. If the hook is removed and trimmed right with right prop it should ride more bow up and no porpoise. He could also have a hull and high weight that needs more power to get the lift?

[Fastjeff57]

Aren't those wedges contributing to the bow down condition?

Jeff

[FrenchPhil]

Actually those wedges are still common in the smaller circuit cats so I wouldn't touch those yet. 325kgs is heavy for a 15' but as mentionned give us some setup info 1st.

[Per]

Thanks for your interest in this matter!

Yes Google translate is a very good thing isn't it!

zul8tr don't pay to much attention to how the boat rides in the video, I think the speed is only somewhere around 25knots and I was slowing down when I passed the camera. I only included it so you could get a good view of the boat.
The Penta engine in the video is still in my workshop for surgery, it had a severe case of overheating, right now the boat is powered by OMC.


Lets answer some questions:

Setup:
Engine, OMC 56ci 70hp from -92 with trim and tilt, completely stock.
Gear ratio 2.42
WOT rpm between 6300 and 6600
Prop Unknown brand over the hub cleaver 14"x24"
Prop shaft height is 2"-2,5" above bottom. did not find any good picture, sorry!
Top speed is so far 47knots with very little positive trim, (trimmed out from prop shaft parallel to the bottom)

Video of the gear case:
It is quite hard to see the water surface since the lake was like a mirror that day but one can see a shadow just by the leading edge of the gear case.



Another view:

http://www.youtube.com/watch?v=27aB7lmphug

Lack of power and slightly overweight, sure just like me I was told that when the hull was built it only weighed 75kg, that was before any of the reinforcement was added (transom, knees, stringers and such) but it was built around -83 so I guess no one really remembers...
I think the hull it self only weighs around 150kg is that heavy?

FastJeff57, about the porpoise, yes, maby the wedges are there to control some longitudinal instability? (speculation from my side)

FrenchPhil In what kind of cats are the steps /wedges still being used? do you have any pictures??

The fact that someone removed the aft portion of the strakes feels like he (most likely a guy, right?) was trying to create more bow lift.

Got myself a new prop for the cat today, a Winrace 14x25" slightly modified by previous owner, lets hope I get to test it before the winter arrives!

[fs5]

hi mate,i had a bad purpoise problem on my old tunnel hull.it turned out to be that the bottom yoke was worn out allowing the motor to rock back and forth.might be worth checking out..
http://www.youtube.com/watch?v=89vcs4Nyrmk

[Per]

The boat does not have any bad porpoising behavior and the gearcase doesn't seem to move around alot in the video i posted in my last post but I will check it out for sure! I got a feeling that the engine have had alot of hours so there might be time for new bushings anyway

[FrenchPhil]

You previously mentioned 325 kgs without crew.
Hers a pic of a s850 circuit cat.
You should try a smaller cleaver like 13x20 for comparison.
But I ran the prop slip calc & the 2.42 gearcase has to go.
As far as the slow take off with the nav, I had the same issue when I
had 90hp on the Argo, with the 2.4 Merc not an issue anymore...