Less speed but better handling

tj309 · 02-23-2005, 07:28 PM

Hi all - new to this site - I hope you can help.

I just bought a 24" Active Thunder Cat with twin 1999 300 hp Mercs. Motors have low water pickups, counter-rotate, and are mounted on 12" setback brackets in addition to the setback provided by the lift jacks. Props are 14.5 x 32 racing cleavers.

1st problem - during initial acceleration to planeing (or flying?) speed of 30 mph the bow goes up close to 40 degrees severely limiting forward visibility and scaring the hell out of my wife. Jacks are full down and motors trimmed fully in. I assume a trim tab in the center would fix this and allow for cruising between 8 and 30 mph? (which I cannot do now) Outside tabs would not work as they would impede water flow to the props, which leads to....

2nd problem - Same initial acceleration scenario - I have to hit 4500 rpm or so at which times the props severely ventilate until they grab some clean water. Then the bow slowly comes down and the boat accelerates like a rocket. The water flow to the props is partially restricted by the hull but these props should handle that? Or should I perhaps step down a couple of inches in pitch or go with a thru the hub exhaust? I could also lower the motors on the jacks a couple of inches relatively easily.

Bottom line - the boat (Iam told and the math backs this up) will go 110. I am willing to sacrifice as much top end as necessary to tame the lower speed characteristics of this boat which is the only way I will ever get my wife back in it.

Thanks for your help

TJ

mcollinstn · 02-23-2005, 07:44 PM

Cleavers are NOT designed to come out of the hole cleanly. They require lots of revs and some even require accelerating to the point of extreme cavitation, and then letting off the sticks so the wave hits you in the @ss from behind, giving you a shove where you hit the sticks again.

Sounds like you could do with a set of 5 blade thru hub props for wife approval. Five bladers with lots of blade area will come out of the hole like a ski boat.

Nose high?
Tunnel tab will do three things:
1) It will keep the bow down on takeoff.
2) It will let you trim the cat "lope" out of the boat at mid speeds (45-65ish).
3) It will stuff your face into the water if you try to use it at higher speeds in rough water (as the hull packs air, the tab can cause a high pressure area at the stern that, when you launch the boat, can let the bow drop unexpectedly and with unpleasant results).

That's why tunnel tabs are not usually factory installed. For the huge amount they help, they can also kill people if used improperly in a fast boat in big water..

But a tab will do what you desire. And it makes a nice step for swimming off the back of the boat.

Sean H · 02-23-2005, 08:23 PM

try some props first before even thinking about the tab....

tj309 · 02-23-2005, 08:35 PM

Quote:

Originally Posted by mcollinstn

Cleavers are NOT designed to come out of the hole cleanly. They require lots of revs and some even require accelerating to the point of extreme cavitation, and then letting off the sticks so the wave hits you in the @ss from behind, giving you a shove where you hit the sticks again.

Sounds like you could do with a set of 5 blade thru hub props for wife approval. Five bladers with lots of blade area will come out of the hole like a ski boat.

Nose high?
Tunnel tab will do three things:
1) It will keep the bow down on takeoff.
2) It will let you trim the cat "lope" out of the boat at mid speeds (45-65ish).
3) It will stuff your face into the water if you try to use it at higher speeds in rough water (as the hull packs air, the tab can cause a high pressure area at the stern that, when you launch the boat, can let the bow drop unexpectedly and with unpleasant results).

That's why tunnel tabs are not usually factory installed. For the huge amount they help, they can also kill people if used improperly in a fast boat in big water..

But a tab will do what you desire. And it makes a nice step for swimming off the back of the boat.

Thanks for the info. 5 bladers are hard to come by - I was hoping to trade the cleavers to somebody. Would 4 or even 3 work? What pitch?

BadDog · 02-23-2005, 08:54 PM

This begs the obvious, have you talked to AT about your boat? They should know. Every boat has some evil trait that with some seat time you learn to mitigate.

tj309 · 02-23-2005, 09:25 PM

who is AT?

mcollinstn · 02-23-2005, 09:29 PM

Sorry, 5 bladers are NOT hard to come by.

They just cost about double. Any good prop shop will get you close on first try, and will usually work with you to let you try a few and return the ones you don't like (in perfect condition of course).

3 blade round ear?
4 blade?
ANY prop will come out of the hole better than an over hub cleaver.

mmwalters · 02-23-2005, 10:52 PM

I would try a pair of hydromotive 4 blades They are tru hub props. Check with Julie or Matt at Throttle Up propellers as they are dealers for hydromotive and great people to deal with. They may even have set for you to try. You will most likely need to go down in pitch to plane out with thru hub props, and you can always drill the hubs to ventilate to maximize your setup. Where are you located and do you have small propshafts ot fat propshafts?
I think you will still have the bow rise as this a function of balance but it will last a much shorter time. The tab may still be a good idea. Does your hull have a center pod? That is a lot of setback for a small hull. My 30' motion with the same motors uses only 5 1/2'' setback. Many 28' skaters use no set back. So a motor bracket change is also a consideration. That would help minimize bow rise also.

tj309 · 02-23-2005, 11:04 PM

I am located in southern AL. I dont know if I have fat or regular prop shafts but I do know I am running a sportmaster gearcase (CLE I think). From what Ive seen so far from the members of this forum I need to trade my props for something less aggressive. I think 4 blade thru hub exhaust of 28 to 30" pitch would work? Any takers for 14.5 x 32 racing cleavers in excellent condition for a trade?

tj309 · 02-23-2005, 11:08 PM

Quote:

Originally Posted by mmwalters

I would try a pair of hydromotive 4 blades They are tru hub props. Check with Julie or Matt at Throttle Up propellers as they are dealers for hydromotive and great people to deal with. They may even have set for you to try. You will most likely need to go down in pitch to plane out with thru hub props, and you can always drill the hubs to ventilate to maximize your setup. Where are you located and do you have small propshafts ot fat propshafts?
I think you will still have the bow rise as this a function of balance but it will last a much shorter time. The tab may still be a good idea. Does your hull have a center pod? That is a lot of setback for a small hull. My 30' motion with the same motors uses only 5 1/2'' setback. Many 28' skaters use no set back. So a motor bracket change is also a consideration. That would help minimize bow rise also.

No center pod. I was also thinking so much setback may not be a good idea - perhaps I could remove the setback brackets and just mount the jacks on the transom? Shouldnt be too hard.

Thanks, TJ