tunnel trouble

[BoatDoc]

i just set up my prather 3.5 tunnel hull. i'm using a stock K&B motor. in fact, it's the same motor that i've been running on my prather vee. the tunnel doesn't seem as fast as my vee. from what i can tell it's not getting up on plane quite all the way. the CG seems to be about right. another thing is the turning. it hooks real bad in turns. it's hard to make a smooth arc. i fiddled with my dual rate...but to get rid of the hook i didn't have enough rudder to get out of any emergencies. any ideas on how to fix either problem?

[Doubledog]

Hows the trim on the engine? Tucked, level, out? (if memory serves correct)My old Prather/stock 3.5 ran good with a hair of out trim & the water line touching the bottom of the drive shaft. In other words, it was jacked high. Prop was a P215. Speed was mid 30's on a fair day.

[CFRACR]

The dual is not the fix!

Its the hull trim most likely. Tunnels are kinda like riggers in some ways. They need to be trimmed out just right for ride attitude. Small changes make a big difference. The engine mount shoul be adjustable, Up/ down & in/out.
The Up/down works out the cavitation, & the in/out give control to how much the bow is out of the water. Minor changes 1/16 can have a pretty big effect, so go in increments.

Basically proper setup is like this. In smooth water & very light winds you can get the bow up more without flipping. Not sure of your hull size but lets say average riding on 4.5-5 inches of stern. (higher the bow=faster speeds)
But if the water is a little rough & there are some winds then you want the bow a little lower or you will flip in head wind. Having these basic two changes due to conditons is about it. The cavitation may need to be tweaked out a little too.

With the bow up you will get the wider turns without the hooks. (hooks can = flips) The bow should natually lay a little lower in the turns and rise back up in the straights.

Hope this helps..... if you want somehere on my puter I have a vid of a lapcat with a 3.5 on it.

[BoatDoc]

thanks guys. right now the engine is tilted forward but the prop looks a little deep. i'll raise it up and see what happens. it was a perfect day for running too. i had good 'rigger water but i didn't bring my rigger with me :( there was a slight ripple in the water...maybe 1/4" to 1/2" at the most. i don't think the wind even topped 5 mph. my prop is either an x437 or x438...can't remember at the moment. i would have tried my 215 but it got damaged due to an outdrive malfunction (it tried coming out) a while back.

[CFRACR]

The prop of choice for the stock 2.5's seems to be the 440.

As for the engine trim, loosen the bolts that allow you to move the engine in & out. Put it @ neutral flush/straight with the bracket. Good place to start!!

Then see how it planes, if to high & cavitates alot then bring it in a smidge(1/16), if to wet go the other way the same amount. You will see that those changes can make a big difference. Could mean for example going from a level/flat bow & plowing to bow tip 6 inches off the water.

Good luck & give a update.........

Going to run my 7.5 tunnel in about a hour or so :D

[Ron Olson]

CF, you're rubbing it in again! The only open water that we have here is the river and that's at almost flood stage. Oh well, gotta work today anyway (well, I don't gotta but the pay is great today with me moving up a spot). I heard that there is some stupid Football game on today that everyone is going to watch. I need toy money so I'm going in.
Where were we? Oh yeah, the boat. Is this one of the older models with the turn fin mouted on it?

[BoatDoc]

well...i went to go run it today (67degrees, e-se wind at 5mph :D ) and decided it would be fun to cut my finger on my newly resharpened and balanced prop. at that point i called it a day. i moved the engine up...but didn't open up the throttle due to a bleeding hand. there are holes for a turn fin in the tunnel but it's not installed at the moment. i'll run again next saturday hopefully and i'll get back to ya guys.

[BoatDoc]

btw...the engine was NOT running when i decided to hack up my hand!!! just thought i'd mention that before i get lectured on prop safety!

[Ron Olson]

No lectures from me, I've sliced up my hands pretty well just sharpening and balancing props.

[Doubledog]

Take some of that radio box tape to it and keep going. :D

[BoatDoc]

it was right on a knuckle so every time a bent my finger i soaked the bandage i made...paper towel and electrical tape :D oh well, i've got next week to go and test out the changes.

[CFRACR]

Tisk, tisk! :o You should have known better Doc. Even us non-newbies make a boo-boo here and there. :D

How bout some pics???
The boat, not your finger. :D

[BoatDoc]

if i can get ahold of a camera i'll post a few.

[Fluid]

Hooking is common with many/most OPC models, the Prather is no exception. While some use a turn fin, and the relatively flat sponson bottoms on the Prathers may require it, adding tunnel ramps/stumble blocks will cure the hooking. What happens is that the as the boat slows in the turns it loses air pressure in the tunnel and settles, putting too much of the front of the sponson in the water; it grabs hard and the boat hooks around this fulcrum. Ramps mounted in the front of the tunnel - with their lowest point about 1/8" above the sponson bottom - keep the bow up in turns and prevent the inside sponson from grabbing too hard and spinning the boat.

The slower speed of the boat makes the hookng worse. Since the motor works well on another boat, the problem is likely that the tunnel is running too wet. As others have suggested, fly the bow higher and see how that heelps.

If the prop you used was a two-bladed one, then it's an x438....not quite enough prop for a stock motor, although it is useful for breaking in the motor. A better all-around choice is an x440 which loads the motor better and allows a slightly higher engine position.

[BoatDoc]

yup, it was the 438. i checked it when i resharpened it. i had a few dings to work out. i guess i'll need to try out a 440 one of these days. the LHS doesn't carry octura so i'll order one when i get the hardware for my FE mono. i'll see about the stumble blocks too. i need to get some foam to build up another project so i'll cut a set from there and glass 'em up. unless there is a better way to build them. will cutting the cav plate help? i've heard that it kind of drains performance in a surface piercing set up.

[Fluid]

The cavitation plate may not add anything, but stock OB tunnels run fine with them in place. I'd not cut it off until you get the model running sweet.

The blocks can be made from wood, foam, etc, whatever you're comfortable with using. Their ride surfaces should be smooth with sharp edges, most easily attained by gluing on a piece of 1/16" sheet styrene.

[BoatDoc]

i got my x440 in the other day and got it sharpened and balanced. the boat ran great. the motor is tipped slightly forward with the center of the prop about 1/16" below the sponson edge. at 7/8 throttle only the last 4" of the boat was touching. the lake was kind of busy so i didn't get to make any good speed passes and i left early because of a flip...which lead to the discovery of a leak in the radio box. i'll try to post pics if i can edit them to fit.

[BoatDoc]

with the camera we had it was kinda hard to get a good shot...but we tried. this one shows the roost. it's about 3-4 times the length of the hull. just trying to show the speed here.

[BoatDoc]

ok...just a little vanity! that's me :D gosh, i'm a looker ;)

[BoatDoc]

just getting on the throttle in this one. that x440 will throw some water!!! great throttle response too. she jumped right up on plane and shot off like a bat out of hades

[Doubledog]

COOOOL

(first time I've seen the man behind the screen name):D

[Hydro Junkie]

Ditto, but what's up with that skinhead haircut?(LOL)

[BoatDoc]

not only the haircut...but if you look closely at my right arm, you'll see my favorite beer tattooed on my wrist! i'm just a plain old guiness boy! :D

[Doubledog]

Noticed it but didn't want to sound flirty.:D You'd mentioned about getting new tats a few weeks ago & boat funds was low \/.

[Hydro Junkie]

Doc, haven't you learned yet that it's the boats that need the funds, not the tatoos?

[BoatDoc]

HJ, i know, i know. 21 tat's and counting! but if it will make you feel better i'll let you in on my recent shopping list:
rudder for 'rigger, 2 F-1 struts, 2 wire drives, LRP V7.1 ESC, RC-Hydros 6-14 ESC, OPS .21 SuperMarine w/pipe, comm lathe, 4 cell matched GP3300's, 6 cell matched GP3300's, several feet of 12AWG wire, deans connectors out the wazoo, 8x2 armature for my electric mono, D6 6 turn motor for the rigger, X440 prop for the tunnel hull, and 2 sets of plans. i think i may have forgotten a few things. all things total i spent around $800 to $900. my bank called me because they thought my card had been stolen! and by the way, the templates are ready for the atlas. it should start coming together here in the next couple of weeks.