tunnel hulls is it normal for a slight porpoise

[ozzie-crawl]

i have been playing with my vs1 setup and had some help from forum members on tips and i thank them again. my question is do tunnel hulls have a slight porpoise as normal running, i can get mine to run with out any porpoise but it then runs slightly wet, with it only running on the last couple of inchs of sponsons it always has a slight porpoise, having looked at countless videos even the 2008 nitro championship it seems they all porpoise unless running wet, even 1/1 tunnels do this, so is it just the hull venting air and normal or not

thanks scott

[old guy]

My Bandit 3.5 never did. On a K&B, the bottom of the shaft housing was 1/8" above the bottom of the boat with -2 degrees angle so that would make the center of the prop shaft about 3/8" to 1/2" above the bottom of the boat. put your boat on some glass with the leg hanging over and then lift the front up to were it's running and see how far down your prop goes. hope it helps
Old Guy

[ozzie-crawl]

thanks old guy, i use a setup board to do adjustments,just had another run
and its near perfect, runs the last few inchs of the sponsons it still has a very slight porpoise till its at full speed but its barely there but it turns better than ever.
i was curious as i see a lot of videos were there is a fair amount of porpoise to a small amount and thought it maybe normal

[andym]

runs the last few inchs of the sponsons it still has a very slight porpoise till its at full speed

I can not see a problem, you should run at full speed, always. Go hard or go home

[ozzie-crawl]

andy it corners at wot but scrubs of a little speed but stays very planted in the corner,once its in a straight line again it porpoises for about 20 feet then its gone
this is on near mill pond, but i think it will be even better in a little chop

[ozzie-crawl]

when i say porpoise its hardly there it the sponsons move maybe 1/2 at the tips
maybe i am just being fussy

[Simon.O.]

If the porpoise effect is that slow up and down at the bow but it is still running on the last few inches then you have it 99% perfect.
I have watched a lot of full size or 1:1 tunnels and most have that rythmic up and down at the bow look going on. Unless you are on glass water this will happen.
My tunnel and one of my cats does it.
Any work done to smooth it out usually involes a barge impersonation by the boat or a blowover. I am happy with the slow oscilation, blowovers are no fun.

Keep at it and see if you can smooth it out.

[ozzie-crawl]

thanks simon, yes i see 1/1 tunnels do the same thing,it runs with a slight bow up attitude and the bow motion is very slight and its running on the last few inchs of the sponsons. i have seen videos of FE tunnels and some air rite out but have a fair amount of porpoise,i have had mine doing the same in testing different setups
but as it is now it turns better than all the other setups i have tried.
the reason i was trying to get it perfect was my original setup had no porpoise what so ever, but i think my setup was wet and the water i was running in had a fair amount of chop letting it air out a bit, i can run that setup again but were i run now is near mill pond most of the time so it just runs wet
i must just be getting fussy in my old age

[Jimboat]

porpoising

ozzie-crawl - Porpoising is pretty common...any tunnel hull or vee hull can be susceptible to porpoising, depending on design and setup. Flatter bottom surfaces are more prone to porpoising than steeper deadrise hulls, but there are several contributors to the occurance and any tunnel hull can find the problem caused by dynamic instability.

The "bouncing" or porpoising comes from a rapid change in the location of the center of Lift as the boat accelerates. The relocation of static weights is one way of dampening the rate of change of the CofL...so it's not always obvious whether to move weight fore or aft in order to cause the "dampening". The solution can be calculated, but we use boat performance software for that. It's not too difficult for you to find out through testing, whether moving weight fore or aft will help your particular problem.

The resolution to a porpoising problem with a hull design is most always addressed by causing the boat to run with less trim. There are many different ways of achieving this.

Porpoising is a function of the lift generated by your hull, the deadrise of your running surfaces, and the trim angle that is needed to get that lift. The onset of porpoising is, in part, influenced by the weight balance in the boat. Altering the deadweights in your boat (fuel, payloads, etc) can affect the speed and trim angle that porpoising will initiate. If a boat is porpoising at a given speed and load, lowering the trim angle will reduce or eliminate the porpoising. There are several ways to get there, but the bottom line is to reduce trim at the velocity of porpoising onset. (Higher deadrise hulls are less suseptible to porpoising.) Even if the hull design is operating in the "Porpoising Regime" through a full range of velocities, reducing trim in some way will improve or resolve the problem.

Porpoising can be predicted on any hull design. (the new version of the TBDP software now does this "porpoising analysis" for tunnel hulls, vee hulls and vee-pad hulls).

For your hull design and setup, the speed and trim angle at which you will see the onset of porpoising can be determined. Changing trim angle, changing motor height, changing propellers, moving weight foreward, will all help to reduce porpoising at a given velocity. Changing design variables such as running surfaces design, deadrise, engine setback, etc. can help maintain the hull in stable planing regime, avoiding porpoise regime.

Trim tabs will also improve a porpoising problem, but these will also affect overall performance of the hull. Adding transom wedges can often help too, since they will allow for more negative trim travel, if it's required. Raising the prop shaft higher will shift CofG foreward and reduce trim angle. More HP also reduces trim angle. Reducing your trim angle in some way will always help a porpoising problem.

Consideration of your design and setup arrangement can predict whether you will see porpoising and at what speed you will experience porpoising with the TBDP Porpoising Regime Analysis for tunnel hulls, Vee Hulls, Vee-Pad hulls, Power catamarans and Mod-VP type hulls.

Also, have a look at this article on "Hump Zone/Why does my Boat Porpoise?". It may be of some help to understand the problem and possible solutions.
[also see this article on Porpoising]