seting up a small but "heavy" tunnel hull

[150aintenuff]

ALL you cat owners plz respond...


OK heress my question... we as a dealer have just taken on this boat line


aksano marine catamarans

and they are a 13' catamaran hull rated for 50 hp.. with a 50 merc 2 stk they run about 40 mph (owner of company claims 50 BUT he is running a 16P aluminum prop and the calculators get it to about 40)

here is my problem... I am the one that gets to clean up the factory rigging (everyboat comes prerigged)( soon to change) and the boat i tested today had the typical tunnel/CAT porpose problem on full throttle full trim.. i could feel the boat come up on to the tips of the sponsons and start working as designed then just as the boat hit full throttle it would do nothing but porpose. speed droped and then the prop vented. top speed was a meager 30+ mph ( 32 mph GPS ) with a yahama 50 EFI with a MEASLY 12P aluminum (6300RPM) I tried a 16 P (36 mph GPS)and it worked well with 1 person @ 5600 RPM but with 2,3 persons it wouldnt get to 5000....so had to use the 12 the factory sent with the boat. no other choices as the 16 was a loaner from another yahama dealership.

how to eliminate the porpose and what would be the best way to setup the hulls for even the biggest of morons to run them but not KILL the performance... If it were my hull I would have raised the engine about 3 " but the factory had used the 2nd hole on the bracket on the top and the single holes 1/2 way down the bracket on the engine rather than the slides so I couldnt adjust the height.. the cavitation plate was even with the center pod and the whole propshaft and LU is below the tunnel and sponsons..

what would be the best height and prop to get 40-50 MPH out of these with out making joe blow moron first time boater un able to drive these.

advertised weight is 600 lbs but I found they were more like 750 + engine... in my mind WAY TO HEAVY...

my old water logged seaswirl P14 will run CIRCLES around them on top end in holeshot with 45 CRANK HP and a 13P prop and that just doesnt seem right...

I wasnt allowed to tweek anything on this one but the next one is going to be totally done by me and it will be able to run more than a 12 P prop ( another yahama 50 EFI) or else I will personally sink the SOB... I was rather PISSED that it wouldnt turn the 16 P because if weight was accurate with myself and gear and motor would have been just under 1100 lbs and the prop selector showed a theoretical pitch of 16.8 so either the hull weighs ALOT more (1400+ loaded) or their design doesnt work like it should.


give me Ideas thanks in advance.

I am running the owners extreem model(shown on website as type R) tomarrow and he is going along and the good ol GPS and radar are comming out to show REAL speeds... ( marine police shop is at the boatramp so I will use their Lidar unit to check GPS)

the one thing they do do well is turn... ON A DIME!!! like a Cat should..

just lack luster performance... and I NEED MORE!!!! with out alot of seat time because the outfit i work for wont let me have the freedom I need to get the things right. ( need some WD40 on their wallet)

[150aintenuff]

made a few changes I found in my post to better describe the product.... sorry for confusion.

[jimmyz32]

get a water pressure guage and start bringing up the motor till it stops. just my 2 cents. don't let the water psi go under 6 psi. have fun.

[Hottrucks]

pretty kool looking but you need to get them moving better I agree with moving it up but I would also like to see a 70 hp on it!! When you get the next hull try to pick it up 3 guys can pick up about 650 lbs but not much more!

The reason it porposes is it on the bubble of taking flight and the prop cann't hold the bow up enough (lack of ponies and prop)if it gets much faster it will not be a moron proof boat as it will take air and although it may not Blow Over it will become less stable in the turns if you don't let up and set the hull back in th e water.
The big quetion is how much just for the hull and how well are they built??

[150aintenuff]

they are built like tanks... launched it off of a 4' high barge wave at WOT and it had a very solid thump and that is all no groans moans or creeks...

the hull only runs about 5 thousand bucks. and they are trying to get 70 hp ratings they are currently testing one with a 75 but dont have any numbers yet. new one is comming iin today and i may tweek it some to get it right provided it isnt drilled wrong like the last one was.

only complaint was lack of speed and the porpose... 70+ hp should solve both.

if interested feel free to PM me and i will get you the info packet on the boat...

[VectorPat]

One thing is you are pretty limited to prop selection. Those motors need to propped to right at 6000rpm, those things are really a dog if you dont. And make sure you give it some breakin time, dont keep banging the rev limiter on a new motor. Those speed numbers are impressive for a 50hp of any make for the weight of that hull.

[Jimboat]

npartin - 40 mph with a 16" prop @ 5500 rpm should be possible (9% slip), but that is a pretty heavy rig. If you send me more design details (dimensions) I could give a better prediction of performance). I suggest you raise the engine as high as possible, while still getting water pressure.

Sometimes when there is a porpoising at a particular speed, you may have to setup the boat to be able to "drive through" this zone, in order to realize top performance.

It could be that the reason your tunnel experiences a "porpoise" at a particular speed is quite simple. The tunnel hull gains it's performance from a unique balance between aerodynamic lift generated by the aerofoil/tunnel configuration, and the hydrodynamic lift generated by the running pads (on the water). The "hump" or "transition zone" occurs at a different velocity with each tunnel boat and setup. The hump zone is is unique to tunnel hulls, and represents the speed at which the balance of lift becomes predominantly aerodynamic (air lift from tunnel and aerofoil) compared to hydrodynamic (water lift from sponsons).

At the speed that the transition occurs, the hull will always experience some longitudinal instabilty - like porpoising (but not always in that form). Your hull experiences a dynamic CofG shift through the "hump" zone. The transition velocity can be accurately determined for any given hull design and setup, and can even be altered by hull design, weight distribution, propeller selection and hull setup. Engine height adjustment can help find the best setup to "smooth out" the transition. Weight movement will also change the speed at which the "hump zone" occurs.

The best tunnel designs actually minimize the "hump zone" to one that is very mild and through a narrow velocity range. Once boat is designed/built, the best solution is to design your setup so that the hump zone occurs at a velocity that you do not need/intend to spend time in...in other words, you are just "passing through". Setup, trim and weight balance can help this alot.

The boat's reaction as it goes thru the hump zone (for a given setup) will always be the same, too. So driving/handling experience helps. The "effects" of the hump zone can be "softened" thru design and setup changes, but will always be there. First the power to attain top performance of the hull needs to be available, then the boat has to be setup to be able to use the full power through to top performance.

[150aintenuff]

I will try to get some specs to ya.. may go in on my own time and measure the hull and send pics via Email to ya... currently the HP rating is 50 hp and they are testing one with a 75 but i think 60 might be about max they are going to allow... the hull I feel needs about 3 " of motor setback to be effective and the Jack would let the engine get high enough as right now they are using the second hole on a 3 hole bracket with only about a 1/2-3/4" above the top of the hole from the top of the transom (transom is VERY short for standard engine brackets.... I personally would like to try a SS engine mounted LOW on the transom... (would drop engine 3 " and be ABOUT right in my mind.... as the LS are about 1.5" low in my opinion... (cav plate .5" lower than bottom of centerpod...) the porpose felt like what you are describing... would start to lift and run NICE and then either the prop would blow out OR the hull would fall both enducing that lovley porpose.... and it happened about 35 MPH... (5000 with 16P GPS) i know they NEED a stainless prop to run right and tell me about prop limitations.... right now I have access to about 3 I can try... 12P 16P and a bent 14P..... but that will change soon.... or else they arent going to sell any... because i wont build them unless I can do it right... and they know that... so they are giving me some freedom on these..

vector I know not to run new engines on limmiter very long... I tried to only do about 3-5 15 second runs at WOT and a few handling tests @ 5000 then i droped it to 3500 and slowly crused back in to dock, allowed it to idle for about 5 min and then ran a demo for the new owners and ran it to 5800 for 25 seconds and then gave it to the new owner to try and showed them the owners manual for propper breakin prociedure... and we followed that prociedure back to the dock and the rest of the day(owner was in commenting on the boat today)...

[150aintenuff]

got together with owner of company and found out the way they assemble them at the factory... and noticed a few screwups not only in their mold but in their rigging design... they rather than using an X pattern (as i was tought) to find exact center of hull and then using a standard pattern jig to drill the top two holes they measure from bracket to right side of mounting area only.. their claim was that all the boats they build are the same center to center on the sponsons so only need to measure 1 side.... that was proven wrong when on an X pattern their last boat we got is 1/2-3/4" to the right and canted 1/4" to the right as well. so in order to fix it I am going to recenter the engine using a jackplate and re drilling the mounting holes on the engine side to compensate.. I do have to use a 2 piece jack to fix their mistake but it looks like it will work ok as they want us as the dealer to pay to have it repaied if they reglass and rerig. but we did get them to agree in letting us pre rig the rest that come in. and once our red hull sells i will be spending more setup time getting it perfect snd it has a 3 blade ballistic on it now but that may change if i dont like it... but it should run better than the 32MPH the last one ran...

looking to see 40-45 and that will be good enough for those boats and still be fairly idiot proof... would like to see a 75 -90 hp on one though with a bobs hydrolic jack on it though... then 60+ would be easy to see and the boat would float just fine as with 3 people on the swimsteps they are bairly getting wet and there is like 8" of freeboard left with a 50 4 stroke yamaha.

any takers... will rig one up for ya if interested.

[Mage Marine]

Well since it is a tunnel it has to perform above a certain speed to get the hull out of the water, meaning enough air under the hull to lift it. It sounds like to me you aren't even getting it close to getting it up on top, thats why the porpussing is happening, you don't have enough HP to get out on top, the porpussing, I like to spell it that way, happens right before a tunnel begins to get out on top, I own 5 different tunnel boats that do any where from 91 to 107mph and they all do that at diffenent speeds depending on the hull design. The taller the sponson height the faster the boat has to go to compress the air necessary to get out on top. You are trying to get a heavy ass as hell plane off the runway with a 2 HP engine! Won't work with any combination you do with that thing! The boat sounds to me to be designed wrong from the beginning, alot of designers do, It probably looks cool on the trailer though!

[Jimboat]

Often the reason a tunnel experiences a "porpoise" at a particular speed is quite simple. The tunnel hull gains it's performance from a unique balance between aerodynamic lift generated by the aerofoil/tunnel configuration, and the hydrodynamic lift generated by the running pads (on the water). The "hump" or "transition zone" occurs at a different velocity with each tunnel boat and setup. The hump zone is unique to tunnel hulls, and represents the speed at which the balance of lift becomes predominantly aerodynamic (air lift from tunnel and aerofoil) compared to hydrodynamic (water lift from sponsons).

At the speed that the transition occurs, the hull will always experience some longitudinal instabilty - like porpoising (but not always in that form). The hull experiences a dynamic CofG shift through the "hump" zone. The transition velocity can be accurately determined for any given hull design and setup, and can even be altered by hull design, weight distribution, propeller selection and hull setup. Engine height adjustment can help find the best setup to "smooth out" the transition. Weight movement will also change the speed at which the "hump zone" occurs.

The best tunnel designs actually minimize the "hump zone" to one that is very mild and through a narrow velocity range. Once boat is designed/built, the best solution is to design the setup so that the hump zone occurs at a velocity that the driver does not need/intend to spend time in...in other words, you are just "passing through". Setup, trim and weight balance can help this alot.

The boat's reaction as it goes thru the hump zone (for a given setup) will always be the same, too. So driving/handling experience helps. The "effects" of the hump zone can be "softened" thru design and setup changes, but will always be there. First the power to attain top performance of the hull needs to be available, then the boat has to be setup to be able to use the full power through to top performance.<!-- / message --><!-- sig -->

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[150aintenuff]

talked to shawn and oz (owners of the company) and they botched their plans to add more power ability to the rating... so I am stuck with 50 hp unless someone wants to buy a hull only and power it with more.. but I spoke with randy(GPI racing) and he aggrees with mage that it is a design problem as well as being under powered... and there isnt enough lift built in to the hull to get it to dry out the areas between the sponsons... typical really given their background from glacier bay... so I get to re think the rigging and make it work as well as I can given the options I got... (30-50 hp and a limmeted selection of stainless bow lifting props) so IMHO they are alot to be desired but do look good on a trailer... the hull really has potential IF there was enough power on it because they are billing them as a jetski alternative... but how many jet skis only run 32 MPH any more..... NONE if they could run 60+ then they would be cool.

[AlaskaStreamin]

I don't know if you caught my daughters thread (North to Alaska part III). Her 12' cat has fairly flat sponsons so it plains off easily. The stock 40 Merc props are 10" diameter and cavitated too much. I am now using a OMC prop which is 12 3/4" and it works pretty good.

[150aintenuff]

did not catch that post... but may look in to it...


also update fron shawn from aksano... there is a 60 hp in the works.... thanks to enough whining of unhappy 50hp four stroke demo's not running with their test hull with a 50 2 stroke... unfortunatly it is a 60 four stroke..... so i dont expect much.... I wish they would get with the Etec program and try a 70 Etec... would weigh less than a 60 yahama anyway...

for those willing to try we have 1 instock without power and it can be arranged so rigging off site could occur so the sky is the limit.... so long as it stays on the water at speed.....

[150aintenuff]

BACK TO TOP.... talked to OZ today and got an invite to do a factory tour and rig mock up using a 3 inch jack plate with a 60 merc 2 stroke later next week... ( an excuse to see my sister and spew my knowlege on the hulls.) GPI I know you sent them a prop could you give me the info again on it to add to my notes before I go up there.