Stinger Placement in a Mono

[Fluid]

Offset can work in shallower vees or stepped hulls, but IME deeper vee hulls will only chine walk worse with much strut offset. It's all about the physics of course, with offset prop lift upsetting the delicate dynamic balance on the keel of a vee hull....

BTW your English is just fine Mark!



.

[properchopper]

Mark,

Good to hear your take from the "physics" perspective, but since I get intrigued by such discussions I'll ramble a bit here in that vein.

We're talking about vector analysis here. When an object propelled on a given path is subject to a secondary force not aligned with the initial (directional) force, there will be a new path (direction) which is a function of both forces combined relative to the strength ratio of both forces where Force [F] equals Mass [M] times [x] Acceleration [A] or simply F = M x A .

In a not very accurate analogy (but useful to explain what I'm talking about ) consider a pool ball being propelled straight ahead. It'll likely go straight across the table. Add some side force at, let's say, 90 degrees, from a (I can't come up with a more sensible example but WTF) hair blowdryer. It will go in a direction angled away from it's original intended straight path. Kind'a like what you characterize as propwalk I believe. So to make the pool ball go straight while enduring the side force -effect of the blowdryer one must propel the pool ball with a compensating angled initial force. Kind'a like offsetting the strut, Eh ?

NOW (dont'cha love the ominous tone of that?)

Consider a Slot Car going straight ahead. Hit it at 90 degrees with the blowdryer. Unless the label on the blowdryer says Pratt and Whitney, the Slot Car will still be heading straight (I'd imagine).

See where I'm heading ? A mono with a sharp keel and big 'ole rudder more closely resembles the Slot Car example. Of course there will be a sideways force-vector from the prop but it hasn't contributed to any troubling propwalk in any real life way in my experience [bragging/immodesty alert : - lost count after building well over 50 boats and have 14 National records under my belt]. (Crap, did I just say that)

My best overall conclusion to this debate can be derived from a term used over and over by my professors whilst an Engineering student (which didn't last all that long): "FOR ALL PRACTICAL PURPOSES"

While your reliance on pure physics does have some basis, I'm sticking with the "No Offset" camp 'cause FOR ALL PRACTICAL PURPOSES it's not necessary for the type of hull under discussion.

Cheeseburger time (Oh wait, car's in the shop AGAIN)

Best,

Tony

[Fluid]

...Oh wait, car's in the shop AGAIN...

That's okay Tony, nothing a bit of wet sanding and rubbing compound can't fix....

bullsborowreck.jpg

[properchopper]

That's okay Tony, nothing a bit of wet sanding and rubbing compound can't fix....

[ATTACH=CONFIG]108558[/ATTACH]

Thanks Jay. Damn, I'm running out of boats to sell to pay for the frigging car fixes. At least my mechanic gave my health a vote of confidence when replacing the fuel pump and recommended a new battery with a SIX year warranty (at which time, If I make it, I'll be 75 )

[Darin Jordan]

I place my stinger on the transom at the BACK of the boat...

No one had mentioned this, so I figured I'd make sure that base got covered...

Hahaha... Sorry... I couldn't resist...

[rickwess]

I place my stinger on the transom at the BACK of the boat...

No one had mentioned this, so I figured I'd make sure that base got covered...

Hahaha... Sorry... I couldn't resist...

Thanks!!!! I was wondering how to attach it to the pointy end.

[iop65]

" a mone with a big old rudder...."

a rudder that needs to eliminate the prop walk+ steering the boat needs to be bigger then a rudder that has just got to steer the offset boat that has eliminted the prop walk by move the driveline off center

bigger rudders causes more drag ...

of coures that in not going to make a world off difference ,but the effect will be there

and now i'm going to stop ,i think everbody has made his point and has his ways of building his boats....i gues going on would anoy some people

kind regards

[rickwess]

A side question if I may.

It looks to me that marking the dead centre of a mono is inexact at best. I think I can get a line perpendicular to the keel using a square, but bang on centre where a 1mm offset might make a difference, I have my doubts. There are just no square edges on the side to take an accurate measurement from. What's your technique?

[ray schrauwen]

Good point Rick.

[Chilli]

You can draw a pencil line across the transom where the sides of the hull meet the bottom. But in all honesty anything close to perpendicular is going to be close enough for a stuffing tube hole because the hole is located very close to the keel.

[mattmak]

Would this work?: use a compass - Two arcs with centers at each side will intersect along the perpendicular to the main center. As long as your two arcs' centers are equidistant to the keel and have the same radius. You can keep adjusting the radius of the arcs to get multiple points to form a line perpendicular to the keel..

You can use a digital or dial caliper that goes to 0.05mm (or better) to position and measure. Measure multiple times from all sorts of reference points to convince yourself it is centered enough.

I tried this on my full size boat and found out that the hull is not symmetrical. Keel to one side is 1/2" shorter than to the other side. That was disappointing..

[properchopper]

" a mone with a big old rudder...."

a rudder that needs to eliminate the prop walk+ steering the boat needs to be bigger then a rudder that has just got to steer the offset boat that has eliminted the prop walk by move the driveline off center

bigger rudders causes more drag ...

of coures that in not going to make a world off difference ,but the effect will be there

and now i'm going to stop ,i think everbody has made his point and has his ways of building his boats....i gues going on would anoy some people

kind regards

Really ? I'd like to know if you've tested this concept by doing in-the-water trials with different length rudders to see what length extension compensates for propwalk in addition to measuring the effect of the longer rudder's extra drag on forward velocity. It would certainly add credibility to what you're saying. And just for the record, I've never had to make my rudders longer to compensate for propwalk - I must be lucky. In fact, unless you use inline rudders, the effect of the commonly used offset rudders has as much as, if not more of a perpendicular force vector exerted on the back of the hull than propwalk while underway.

[rickwess]

Thanks Mike, I'm starting to realise that "close enough" is what I need to shoot for.

I apologise for starting a bit of a war about offset. My experience is with the Revolt which has a massive (in comparison to what is being discussed here) offset. In my ignorance, I thought finding dead centre would be easy peasy. I just spent 30mins starting at the transom and trying different ways to ensure:

1. I can make a perpendicular line to the keel
2. I can find dead centre

My conclusion........there is simply no place to use as an accurate reference point accomplish this. I will shoot for 0mm offset and 9mm up from the keel (I find metric so much easier to work with), but acknowledge that in the end it's just an approximation.

Thank you all for your help. This has been very educational.

[rickwess]

Would this work?: use a compass - Two arcs with centers at each side will intersect along the perpendicular to the main center. As long as your two arcs' centers are equidistant to the keel and have the same radius. You can keep adjusting the radius of the arcs to get multiple points to form a line perpendicular to the keel..

You can use a digital or dial caliper that goes to 0.05mm (or better) to position and measure. Measure multiple times from all sorts of reference points to convince yourself it is centered enough.

I tried this on my full size boat and found out that the hull is not symmetrical. Keel to one side is 1/2" shorter than to the other side. That was disappointing..

I started down that path while I was staring at the transom. This method relies on two accurate reference points. I can't even find 1. The sides and top of the transom do not have square edges.

[properchopper]

A whole lot of crazymaking going on here Rick - center the shaft line over the keel. The hull has a natural tendency to go straight as directed by the keel line. You want the prop thrust (as dictated by the stinger location) to be in agreement with this. As has been found, many hulls aren't symmetrical so use the keel line as the center of the hull regardless of what the ruler tells you.