Dumas Payn Pak '20 kit.......

[Mike Larson]

Hey guys! Have a buddy has the '20 kit built/finished, per plans, and the thing will not get on plane.... The sponsons are true and nice, ( the actual builder did a nice true job) 2.5 dihedral on each surface, with 4 degrees actual AOA overall....... ( kit form) Installed CMB 21 in hull and there isnt enough power anywhere to lift the rear of this boat...... With the hull at a simulated running attitude ( rear of hull level) you need a strut about 3" to get to the table! ( overestimated, but, there. ) As I'm looking for solutions for a freind I need some setup/hull mod measurements to solve the dilemma..... transom to table, sponson height at start of belly pan( slope) sponson height at bow, ( sorta like tunnel depth) Overall AOA of sponsons...., dihedral, and any other thing we can measure to get this thing motoring...... Granted most are running serious modifications to this hull, too, Since we plan to perform surgery on the hull anyway, any input would be welcome...... C/G, may be of concern? We have the air dam recommended around on it, too..... wont go fast enough to worry about blowin off...... thanks men.... suggestions/input appreciated..... mike.....

 

[namba276]

Rear radio box? You may just have too much weight in the rear of the boat. The thing on my boat that had significant positive results was to taper the bottom from the transom forward about 6-8 inches. when I built the first of my latest version, the area I discribed was pretty flat. since it's an epoxy boat, I had to sand the plug from 1/8" to nothing starting at the transom and working forward.this allows the tail to drop a little and break the boat loose. worked real well for me. check this on your boat. your pipe may be too long to allow the engine to devlop enough horsepower to raise the tail. or you may have too big of a prop. balance is a very important part of the set-up that is sometimes overlooked. Mine is not a dumas , but Ive built a ton of 20 sport boats in the last ten years. Won a few championships as well.

Bob Tuttle

 

[Hydro Junkie]

I have five questions for you:

1) Did he use only the wood supplied in the kit?

2) Did he cut lightning holes in the internal framing?

3) Did he use fiberglass or anything else to strengthen the hull?

4) Where is radio box and fuel tank?

5) Did he use the Dumas hardware kit

I built one of those back in 1982. The boat was so heavy that it wouldn't get going fast enough to plane out, even though I followed the directions to the letter. If your boat was built without lightening holes and used the Dumas supplied garbage wood for the internal framing, forget it and start from scratch. Did he use the Dumas hardware kit? That stuff is also garbage and could also be part of your problem. Have him answer the five questions and I think we can narrow down the problem.

 

[Mike Larson]

Thanks for the replies Bob and Hydro(?) I know youre the spt 20 king... and I'm interested in your latest '20 build for awhile now... I currently have a scaled down Dave Frank spt 40 I run, so I'm familiar with 20 class stuff.... The radio box is in the rear, but, as far forward as it could be planted, motor, too..... we tested an old school cmb valvola ( old model) with different pipes/lengths... and, I have a myriad of 21 props for all my riggers, tunnels, and my sport............. went from 437 to 442 with 440 most commonly various dia etc....... This is my freind/mentor's boat.... I measured angles, and depths, etc... The build is good........ I think the depth of the tunnel in the boat needs to be lowered to allow the boat a flatter running surface......... ie: cut the sponsons height down to at least get the hull into the thrust portion of the prop and strut ... ( at running attitude, rear of hull level, 2 inches plus to the table!) Changing the rear of the hull is possible but not probable, my pal has custom paint ($) and a minor sponson change touch up could be accomplished......We accumulated data from other boats, and some have 0 degree dihedral turn side, and 1 1/2 degree outboard side? Recommend a good stable dihedral? if the sponson change would help at all. As it sits both sides are at 2.5 even both sides... per build. Hydro, All wood came from what the kit supplied.......No lightening holes were cut for sure, and cannot be done now = heavy boat.... RTR weight 6 lbs 10 ounces less fuel........ My Frank hull is around this too.... 53 with a CMB green......... No extra stuff installed, all quality speedmaster hardware, sweet boat.... Radio box is mid hull, forward as possible, tank in nose... ( custom) Do you guys have a "rule of thumb" for the actual balance point of the smaller hydros? Percentage formula? Hit me that it could be drastically off, but, everything is as far forward as it could possibly be installed...... The boats worthless as it sits on a counter, we are looking to ressurect it, if possible with little cosmetic damage as possible... Thanks for the help, I'll be relaying info as it comes.... Mike

 

[Hydro Junkie]

At 6-10, your boat is way too heavy. I was informed years ago that a 36" Pak I had started from scaled down Newton plans was too heavy at just over 5 pounds without deck and cowling but with engine and running gear. Mine is all 1/16th with 1/4" transoms. Here are some things you might look at:

1) Change the bottom profile. The sloped area in the front of the tunnel provides more drag than lift. Replace the bottom, resetting the front section to somewhere in the 4 to 5 degree range and extend it further back. You will get more lift and less drag. As it is now, you are trying to push a barge. You could also cut litening holes at the same time.

2) Check the afterplane length. If it's over 17", you will need to extend the sponsons aft

3) Check your boat's balance point. It should be approximately 1" behind the sponsons

4) How much weight is on the prop? If it's over a pound, you're not going to get it to lift.

5) Have you tried a 1400 series prop? You are going to need a lifting prop on this boat, so your 400 series probably aren't going to cut it.