Motor Hinge Point

[GTR]

Hi guys

This was sort of discussed previously but I am asking this from a slightly different perspective. This is not about how far back the prop is or the distance to the 'sweet spot' what this is about is -

Has anybody tried varying the position of the hinge for the motor, in relation to the back of the sponsons, & what effect did it have on the handling?

I know ChrisW said on a post somewhere here that you have to have the hinge level with the sponson backs - I'm interested in knowing why and the observed effect of having it inside the sponson backs or outside them. I noticed on full scale they are a considerable distance behind the sponson backs.

Jimboat if you are out there and reading I am sure you could add to this discussion.

GT :blink:

 

[ScottVanSlooten]

It has to do with the pivit point and how it turns to far forward and you fight the sponsons and to far back you fight them . But as you move motor back you open the triangle of back of sponsons and prop and this helps handling . I run an areomarine 3.5 tunnel and they say to put the motor way up between sponsons and I run it back almost even with back of them to clear tunned pipe and the boat runs better then with pivit point closer to the back of the boat. Scott

 

[MikeP]

GTR,

Good question, I hope to see more input. Mind if I throw one more question on this pivot pin location.

I was wondering about the pin angle or caster and what effects it might have on handling. On deep vee's it has been stated the the bow of the boat can be adjusted up or down while turning by changing the caster of the rudder hinge.

Mike

 

[Bob Kensill]

Boy,that opens a new can of worms! How about using the Octura motor mount on an OS21 outboard. You could have adjustable rear caster.

 

[Capt Vic]

Hey Guys;

I couldn't help myself. You are on the right track, with outboards adjustments in hieght and trim, what you are calling "caster angle" are crucial!! As litle as 1/16" makes a great differance in speed and handleing. For speed bring the hieght up to "unload" the prop. You can run a taller pitch this way. As you gain speed the model will want to lift the bow and run loose now make adjustments with the trim to keep it on the water. Different water takes different adjusments. You can run with more positive trim on calm water. Or lower engine hieght and trim down for the ruff stuff.

I make a cool mount with all of this in mind. I have them for 7.5 and 11cc now and making a version for 3.5 that will be ready in 2-3 weeks.

Check out the link and you'll see what I mean.

http://community.webshots.com/user/captvic

Victor Gorecki

 

[Bob Kensill]

I took a look at your mount,very nice! What I was talking about using the Octura mount with the OS outboard is, that you can change the angle of the hinge pin without changing the angle of the engine. This way you could make the engine angle vary as you turn, to either lift the nose of the boat or force it down.

Bob

 

[snowdog-2112]

krpnova,

I'm not understanding what you're saying here. How can the angle of the hinge pin change without the pitch of the prop changing? I know I haven't been in this game long but what you're describing seems to be impossible. Would you please explain how this works?

Snowdog

 

[MikeP]

If you had an adjustable motor mount you could shim behind the motor mount between the hull and the mount to get a little caster in the hinge. Then reset the thrust angle and height back to what ever you had it set to.

Don't forget GTR's original question on hinge location in relation to the back of the sponsons.

Mike

 

[Guest]

HE HE WELL IT LOOKS LIKE YOU HAVE FINNALY ASKED THE BIG ONE . WELL , BOB KINSEL AND I HAVE BOTH LOOKED VERY DEEP INTO THIS , AND WE BOTH HAVE AN IDEA ON THIS AND IT LOOKS LIKE IT HAS BEEN TAKEN CARE OF BY THE LAWLESS LOWER UNIT ?? THIS IS WHY I RUN THEM , TRY IT YOU WILL LIKE IT , USE THE OCTURA MOUNT .

 

[Guest]

:lo SORRY , I SHOULD HAVE SIGNED THE ABOVE

JAMES IRWIN

 

[snowdog-2112]

Gang,

I'm going to have to bow out here because I still have no idea what is being discussed in this topic. I tried...I really did but I cannot understand what this is about. Thanks for trying though.

Perhaps I'll go stick my head back in the sand!

Snowdog

 

[TimD]

Bryan,

Think of it this way. Adjust the mount on the transom to say 5%+ and adjust the motor side of the pin to 5%-. that way the pin is at a 5% angle but the motor is still neutral. If you turn the rudder the trim angle will change from neutral in line with the pin angle.

Hope that helps.

Tim.

P.S You can do the same sort of thing with the T.T motor with an angled transom mount. I did that for years until I switched engines.

 

[GTR]

hey Snowdog

Don't be too confused I'm not sure which question got answered - the hinge pin location or the hinge inclination.

You need a double adjustable mount to get variable castor effect - eg an Octura mount on an OS leg. This would be easier to understand with a drawing I'll think about doing one if nobody can post a picture if they've already done it.

I still want to know about the hinge pin location. The Ocutra mount/Lawless set-up makes me think you can mount the hing inside the sponson trailing edges but keep the prop out a decent distance from them - am I right or have I missed it.

GT :blink:

 

[TimD]

GT,

I did some experimenting with the pin location /spacing from transom thing a while ago on a hull with a 1 inch inset transom. Is that what you are looking for?

Tim.

 

[GTR]

Hi Tim

What I was asking about was observed changes in handling depending on whether the hing pin was inside or outside of the line of the rear sponsons. I was hoping Jimboat might chime in with some of the geometry of why it should be one place or the other. I didn't want to confuse this with the position of the prop relative to the sponsons, although I do believe there is a realtionship I think they are two different thing at play. I guess there is an interaction between them but I'd like to understand why.

GT

 

[TimD]

GT,

I think the main difference would be steering sensitivity if you took the sponson / drive dog length out of the equation. Rather hard to test back to back unless you had a K&B leg and a Lawless leg (difference in length) and a spacer the same length as the difference. Use the K&B leg + Spacer verses the lawless without the spacer. That would keep the prop to transom distance the same but move the pin (and therefore the steering angle).

Anyone tried testing like that?

 

[Jimboat]

GT - The location of the hinge pin will determine where the drive (motor lower unit) will generate it's force (thrust) from (fore/aft of sponsons). Since you will have the bullet of the drive located at some dimension above/below the planing surface of sponsons, this makes the location of the drive fore/aft of sponson trailing edge influential to total "moment" of forces that it will contribute.

There isn't any "rule of thumb" that helps determine the "best" for all hull setups. As with most design/setup issues on tunnel hulls, it is different for each hull, and can really make a difference to performance and stability.

In general, moving hinge pin further aft of sponsons will move dynamic center of gravity further fore, which can generate more overall lift. This can be helpful when hull is having trouble "flying" under some circumstances.

 

[lohring]

There is another issue with spacing the engine back from the transom. To maintain the same center of gravity you need to add sponson tip weight. Therefore you end up with a boat that has a lot of its weight at the ends rather than at the center. This makes the boat's pitching motion harder to damp, and destroys the rough water handling. Cats, with their engines at the center of gravity, are good rough water boats. I find that the less tip weight, the better the boat handles in rough water as long as everything else is working.

Lohring Miller

 

[TimD]

Lohring,

Cool post B) So would I be correct in assuming you prefer to add more weight to closer to the CG than less weight at the tips to balance the hull??

Also raises the question - is the improved stability due to less "pitching motion" or just the increased total weight of the boat?

Tim.

 

[Slideblues]

OK Hang on a minute here I think ya lost me somewhere, The drive dog measurement from the transom does not change and your moving only the hinge pin ?

Gene