Hull weight displacement

A general, non-powerplant specific, discussion on airboat technology, ie., hulls, rigging, polymer, etc..
jayhoff
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Hull weight displacement

Postby jayhoff » Sat Dec 16, 2017 8:08 pm

Years ago my uncle ran a 16' combee hull, 1/4 poly, 454 BBC DD, 72" whisper tip prop. I can remember he could run dry and dry load. He has passed and we have decided to bring his old rig back to life. Opted to switch to a 13' laser hull with quick slick bottom. The new rigging is identical to his original set up, double front double rear (rear driver) emt seat stand and cage, steel engine stand. Same 454 and whisper tip same pitch at rpms. Can't get this new build to run a lick of dry. Question is with the new hull being smaller would this displace the weight in less surface area causing it to not be able to break free? The hull has been refurbished can't find a hook in the bottom, also have experimented with the engine angle (level and 1/4 bubble down) but no progress. Any and all advice would be highly appreciated.
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mojoe
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Re: Hull weight displacement

Postby mojoe » Sun Dec 17, 2017 7:29 am

There are A LOT of variables that affect a boats ability to run dry. Power, WEIGHT, contour of the hull, rigidity, DRIVER and angle of thrust just to name a few.

I can’t tell you which changes you made that are hurting you the most, but don’t rule anything out....
Airboating is like marriage: There are unlimited ways a person can be right, and somehow still be wrong at the same time!!

jayhoff
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Re: Hull weight displacement

Postby jayhoff » Sun Dec 17, 2017 8:55 am

Thanks for the advice mojoe, we are still early in this marriage: probably somethings right, but a handful of things wrong. Gonna have to finesse her til I get her to love me.
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kwanjangnihm
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Re: Hull weight displacement

Postby kwanjangnihm » Sun Dec 17, 2017 9:29 am

are you running a standard or narrow whisper tip setup? what is your wot?
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gatorfan331
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Re: Hull weight displacement

Postby gatorfan331 » Sun Dec 17, 2017 9:59 am

What do you have on the bottom? Polymer or some type of slick bottom? Is it different than the other boat?

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Deano
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Re: Hull weight displacement

Postby Deano » Sun Dec 17, 2017 11:11 am

As usual, MoJoe is on right sheet of music. This subject matter is nearly as broad as is this board's reach.

When you are starting out with an unknown (or many as in this case), the best process to follow is one of methodical elimination.
Because everything effects everything else, you can't really fine tune any one thing and then move on to the next variable. What you can do though, is approach it that way knowing that you will need to then start over making the same tweaks, until you either arrive at the desired end or exhaust the possibility.

Breaking down and/or grouping variables may simplify the process.
At the most basic level you have engine variables and other (set up) variables. After many rounds of playing this came over the years, it is my preference to start with the engine, insomuch as, if you are lacking the power you need, time spent anywhere else will not be productive and will only increase your frustration level. Because I (and most other people) don't have an engine dyno and/or a thrust tester, this necessitates running the boat to determine and gauge said performance. This, in and of itself creates an exception to 'starting with the engine' because if the thrust angle is not where it should be (or real close), it may not be possible to attain the performance you are attempting to gauge.

If your angle of thrust is currently down in the back, and the bottom is in fact straight, the implication would be that it should perform better raised up to level, or most likely a bit more than that. I would guess, based on what you're working with, that you will end up two (give or take one) washers higher in the back. This is presuming the engine AND the stringers are parallel when level *which needs to be confirmed*. In an effort to avoid beating a dead horse, check this link for the effects of engine angle and what you want to shoot for.
https://southernairboat.com/phpBB3/viewtopic.php?f=85&t=28600
jayhoff wrote:Same 454 and whisper tip same pitch at rpms.
This statement is a bit ambiguous. Do you mean that the degrees of pitch is the same (independently of the engine), or do you mean the pitch is such that the engine turns the same RPMS? In the later case, your power could be well down and therein lies your entire problem.

Back to the engine.
Confirmed the newly installed throttle cable is in fact opening the throttle plates all the way?
Do plugs all read like they are firing as they should? Points clean & gapped, if not HEI? Cap good?
Confirmed the timing is optimal? What is your WOT timing?
Was the carb rebuilt? Does it need to be?
Fuel filters all clean? Don't forget a Holley will have one in fuel inlet.

If you confirm/correct the above variables, and then give us an update of where you are then currently at, we will all be in a much better position to help you discern your problem. There are many other variables beyond all this, but it's never productive to put the cart before the horse. I should also point out while going through this exercise it is always advisable to (1) only change one thing at a time and (2) record/track what effect(s) that change did or did not make. Taking such a methodical approach will ALWAYS be more productive and time effective than just making random changes and crossing your fingers. If you've ever been involved with setting up a race car, you've already seen and know this. It's also the best way to avoid an unwanted and unneeded heartburn.

There is a lot of experience here available to you for the asking. Be aware that the more information you share, the less abstract your situation is to those trying to help. Because we can't see what you're working with, the quality and quantity of the information you glean will be contingent on the quality and quantity of the information you share with us to bridge that gap.

ps Putting your location in your profile may help your cause. You might be around the corner from someone that may be able to help.
"The suppression of uncomfortable ideas may be common in religion and politics,
but it is not the path to knowledge; it has no place in the endeavor of science."
- Carl Sagan

Aeon
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Re: Hull weight displacement

Postby Aeon » Mon Dec 18, 2017 10:13 pm

was the cage on the old boat the same as the new boat? same spacing on the wire?
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