The process of spinning out


After considerable thought I decided to share my experiences and knowledge about what is going on when a boat spins out. Some of this information came from my years of model boat racing, some of this comes from my experience driving and racing Mod VP boats and finally I would give a nod to Jim Merten Jr. whose dad: Jim Merten Sr. was somewhat famous in the sixties and seventies for racing the Parker Enduro, and Havasu. It goes against my better judgment to teach my competitors how to get through a corner in a Mod VP boat. That said, I am still compelled to share this by the thought that if I didn’t do so I may have no one to compete against, or worse yet see someone who is well meaning, yet unknowledgeable and inexperienced drive a COR boat into another crash. Hopefully, we will have a class to race in and should that happen; I have set myself to the task of educating those who do not have the same experience and knowledge as I do. I am doing this for a selfish reason, that being: I would love to continue racing in APBA and as a COR driver. I would also like to race against all of you who have the desire to do this in a safe and responsible manner. Perhaps we can, as a group, foster this attitude amongst ourselves and new members of the COR Family.

Types of spin outs
Gearcase blowout
For those of you around higher performance outboards for some time, most have experienced or read about this problem. The problem is this: the propeller is turning (most often right) creating forward thrust, but also a right or starboard force. This force must be counteracted by a left hand steering force to drive the boat in a straight line. The skeg is always “crabbing” to the left when the boat is going straight. This causes a bubble to form on the starboard side of the gearcase. This bubble doesn’t usually cause problems at slow speeds, but at high speeds (app. 75 mph) the bubble gets long enough to trail back and ventilate the propeller, when this happens the propeller loses lift, and thrust causing the bow to drop, this in turn with the rudder (skeg) angle can cause the boat to spin out…..bad things follow. This problem is usually fixed with the addition of a blowout ring, and a longer gearcase such as a Sportmaster or CLE case.

Sweeping turn
This is the spinout that seems to have plagued the COR class of late. In driving around a sweeping corner, drivers will want to maintain more speed than the boat will achieve with the bow trimmed down. This means the rudder (skeg) is turned, and the boat is not responding since the bow is “flying” and the boat is “pushing” to the outside of the course. For those experienced with this “problem” alarm bells will/should be going off between the ears. Even with the engine trimmed down to the stop, the boat will continue to push to the outside of the turn with enough throttle applied. In calm water, a shallow arc can be achieved without difficulty, but the boat is in a precarious state, the rudder is turned too sharply for the direction of travel. Now here is where we get in to difficulty, the driver might be tempted to let up on the throttle, but forget to straighten the wheel; if the throttle is slowly released as the boat slows the bow will finally drop. When the bow gets wet enough, the length of the turning moment increases to suddenly make the rudder effective and the boat will turn sharply or spinout. Unfortunately, it will not be predictable as to when it happens and may be unforeseen by an inexperienced driver.

Now imagine this same scenario, except the water is rough. The boat is “flying” not responding to the steering input, the boat is back to that precarious state. Now if the bow finds the right wave to “turn on” it will hook, only this time the boat could be carrying more speed and the spinout even more violent, furthermore the operator “did nothing” to initiate the turn, other than overdrive it into the corner.

Transom lift induced spinout
All of these spinouts have common factors that play into them occurring. This kind of spinout is induced by transom lift. Transom lift can be caused by such things as stern lifting propellers, gear case type, trim angle, depth of lower unit/skeg, setback and throttle input. The scenario for this type of spinout is as follows: trim down or up, full left rudder, boat is turning, speed is low, now if the throttle is applied rapidly, the propeller generates transom lift which reduces effective skeg area as well as increases a sideward force to the right rear, think of your propeller as a paddle wheel pulling the rear to the right…… boat hooks left. Of course, other things can make this more likely to happen such as water conditions, but at the heart of this is the point at which the throttle is applied. If the driver had waited until the boat had completed more of the turn, or had straightened out the rudder more, the spinout wouldn’t have happened. So, at first, this spinout is caused by inexperience, after it has happened several times to the driver, if it continues, it would be considered bad judgment, and for those experienced drivers still spinning out, I would characterize it as a lack of “driver patience.” That’s a whole “nother” topic.

Centrifugal Force Spinout
This is the least likely spin out to occur on a mod VP boat, but is important to understand from a CG standpoint. To illustrate this, imagine we could put a fin into the water on the port side sponson and as the boat corners; the fin would oppose the centrifugal force pushing the boat to the outside. The smart set-up guy would move this fin fore and aft until the center of the blade area was in line with the fore and aft Center of Gravity. Now the two forces would be inline and oppose one another, and not provide a turning moment to the boat, if the fin was moved forward the centrifugal force would push rearward more than forward and the boat would spinout to the left. Conversely, if the fin was moved to the rear, the boat will “push” or not turn until the speed, and thus the centrifugal force, is reduced. Now, before you set about mounting a fin on your boat, consider this: in a mod vp boat your “Fin” is the sponsons and your trim button can control the “placement” of the fin. As you trim up the “fin” moves back, and when you trim down the “fin” moves forward. The boat attitude will dictate the effective “blade area” and effective “placement” of the “fin.” If we, as drivers and set up men, can and do control both the CG and the trim angle and thus the attitude of the boat in the turn, we can safely drive the boat through the corner. The setup (CG) and the trim angle thus affect the maximum speed achieved around the corner; the downside is these two also affect straight line speed. So the clever racer will select and test different CG’s and trim angles to balance speed achieved in both the straight and the corner. There are those who would believe that the mod VP boat is inherently unstable, to those I would counter, how is it that some can and have for years of Mod VP racing history driven safely and fast through the corners. The simple answer is knowledge to set the boat up, judgment to drive it correctly and experience to know the pitfalls, this post is dedicated to the knowledge aspect of that equation, the other aspects will require some kind of a driver training experience.

Factors that affect spinout

Steering input—Less is better

Throttle input---Less is better

Boat speed--- Less is better

Boat attitude/trim---This is tied to propeller rake, trim angle, and boat balance, in general a boat that has “taken a set” is trimmed down and not “flying” will be less likely to spin out than the converse.

Propeller Rake More is better with positive trim, more is worse with negative trim

Propeller Diameter---- Smaller props are less likely to spin out than larger diameter props if the rake is the same.
Propeller Pitch---lower pitch will spin out less than higher pitch
Skeg effective area---Bigger skegs are less likely to spin out than smaller skegs

Water conditions---calm is better

Recommendations for discussion

No trim angle less than zero

No prop diameter greater than 14.5

Sportmaster size skegs

No CG (trailing edge of bottom pad to CG) less than 17% of total hull length

Gearcase blowout ring installed

I won’t claim to be any kind of “expert” on this subject. I will admit there are smarter and more qualified people to discuss the process of spinning out than me. If your experience is different than mine I would be happy to discuss it over the phone, even if you want to call me on the phone and tell me I’m full of it, I would like to talk to anyone who can carry on a thoughtful conversation on this topic.

DaBull's picture

While opinions may vary at least you are taking steps to make COR safer. Good for you.

Good article Kevin!

I Learned a lot here. Good article.