# Critical speed for powerboats

Discussion in 'Boat Design' started by meren, Apr 13, 2014.

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### merenJunior Member

Maximum propulsion power rating for RIBs and powerboats under 8 m (in EU) is determinated by an "avoidance line test", where a test driver has to make three successful 90 degree high speed turns within required maximum radius to both directions without loosing control and difficulties to keep at helm.

What are highest transversal g-forces that can be expected to be tolerated by a test driver without considerable risk of capsizing a boat or ejecting from boat under tests made according to standard?

In addition to there are dynamic instabilities which set limits for maximum speed, but now it is question of critical speed which could be calculated for each boat (LH) setting a g-force maximum.

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### TANSLSenior Member

There are formulas that quantify the heeling moment due to quick turn of the boat. Under one of them
with :
M heeling moment Tm x m
V speedd, m/s
D displacement Tm.
KG height C. of G. above base line, m
d draft at Lbp/2, m
L water line lenght, m
I do not think this formula will serve you as it is, but surely point you in where you should look.
This heeling moment should be compensated for the righting moment generated by the shapes of the boat .
As the value of g: calculates the centrifugal force of the rotating object at a certain angular velocity.
"g-forces that can be expected to be tolerated by a test driver" : depends to a large extent on the physical formation of the test driver. It would be nice to take seat belt and automatic disconnect of the engine by the effect of "man overboard".

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### Mr EfficiencySenior Member

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### AlikSenior Member

Tolerable accelerations of the driver much depend on circumstances such as seat type and construction, passenger/crew tolerance level, etc. As a rough guideline, we use safety assessment from HSC Code (Annex 3, Table 1) where they specify acceptable levels of side, fwd/aft and vertical accelerations for normal and extreme cases.

To be more precise, one needs to consider bio mechanics say of neck section of spine and torso, also type of support provided by chair. Also important to know what activity should be carried during turn; say only driving and holding oneself at seated position or other functions such as operation of gun, etc. I think here no calculation is possible and formulas and tables from HSC Code are very rough guidelines; testing is better.

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### Mr EfficiencySenior Member

So, the "nut behind the wheel" is the most important part ? This thread causes me to think bureaucracy in the EU is over-reaching itself.

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### PARYacht Designer/Builder

I have a lot of CE regulations "issues" many like this, with arbitrary goals and figures, needing to be met. I've done 2 CE compliant designs and I'll try not to do another, if I can avoid it. For example, if you attempt to design an "in the tradition of" sharpie, you'll never get it to pass.

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### TANSLSenior Member

That's the bad, and the good, with the rules and regulations. You have to meet them, and you can not do things anyway. you have to do what the regulation calls. But most designers, I think, are accustomed to working in compliance with the standards.
The CE rules are not easy to understand but they do things much easier for manufacturers of pleasure boats.

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### IkeSenior Member

Back in the late 80's I was the project officer on a US Coast Guard research project to determine lateral accelerations of outboard powered boats in a high speed turn. We were hoping to get some consistent data, from which we could derive a formula for horsepower ratings based on lateral acceleration. It didn't work out that way. To make a very long story short there is no correlation. We instrumented the boats with accelerometers to measure accelerations in three dimensions and dumped the data to a computer. The data was all over the the place, what is known in statistics as scatter. There are far too many variables based on the hull shape.

I have participated in doing the collision avoidance tests that the EU requires and they are really no more accurate than the ABYC quick turn test or the USCG Formula for horsepower. To put it concisely, it's simply another method to level the playing field. Everyone has to use it so everyone has the same handicap. It is a subjective test based on the experience of the person operating the boat. A very experienced operator can get the boat through the test at a much higher speed than an inexperienced operator. Our test driver was a highly experienced Coast Guard boat coxswain who had operated everything from dinghies to rescue vessels, including racing boats, and frankly some of the boats scared him badly at the horsepower for which they were rated. He nearly got tossed out of the boat on several occasions. The test is too subjective and the amount of acceleration that can toss the person out of the boat is dependent on the variables of the hull shape, especially deadrise and angle of chines, weight of the boat in relation to it's size, where the CG is when on a plane and so on.

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### PARYacht Designer/Builder

There you go Peter, you had to bring up the USCG HP formula thing. This is another one of my pet peeves. I have a few designs that would become completely unstable with the HP figures this formula suggests as a practical limit.

I do understand the need and use of these regulations, but the reality is the designer better have a clue and not just simply comply with these guidelines, formulas and recommendations.

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### IkeSenior Member

PAR, need I remind you it is maximum allowable. You can put any lower amount the builder or designer believes is more appropriate. If the formula says 100 and you feel it is not safe over 50 then 50 it is. Once you put the label on it is as good as cast in stone because now most states have passed laws making it illegal for an owner/operator to exceed the values on the label.

I agree with you that the designer should know what they are doing.

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### SukiSoloSenior Member

A couple of years back we had a 'DellQuay Dory' type with 25 HP Tohatsu on the tail, complete with CE certification. It was a 'Slapper' because it was most likely designed to take a lighter 2 stroke on the back. If you leant forward with your head about half a meter over the bow the 'slapping' stopped. This was in smooth freshwater and the porpoising/slapping could become pretty extreme, almost close to dangerous. The hull moulds were obviously not quite fair either. Still got (CE) passed though, we got rid of it after less than a year, and went back to another second hand beast the right shape!.

I've had lots of other non marine products passed for CE and generally the 'rules' are there correctly. I would quible with the trailer board lighting stuff from 2013 and a few other things but mostly things are reasonable. Not sure about why the EU (via RYA) wish to license amateur builders though? Just another layer of bureaucracy to get a CE mark on a racing boat that is quasi exempt anyway.

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