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What is legal in UIM Ski GP3, and what the rulebook does not say

GP3 is the strictest exhaust rule in the UIM Ski Division and one of the plainest closed lists in the sport. The harder half is what the class leaves unstated, including a power ceiling with no stated way to measure it.

What this rests on

Basis
From a rulebook
Source
UIM 2026 Aquabike rulebook, Ski Division GP3, article 501.02, read for this site's class and part pages and read again on 11 September 2026.
Difficulty
Intermediate

The exhaust must remain entirely of origin

Article 501.02.03.03 requires the system as delivered by the manufacturer. There is no stinger allowance, no insert, no waterbox substitution and no through-hull change; the flap may not even be removed. Our data records this as the strictest exhaust rule in the UIM Ski Division, and it is stricter than the equivalent stock class rule in the American rulebooks, which at least permit an insert.

Exhaust sections 1 to 3, from the engine A box at the left stands for the engine. Bolted to it, a short tapering casting numbered 1 leads into a narrow straight tube numbered 2, with a small arrow inside it pointing right. The tube then flares into a wide body that widens, runs straight and begins to narrow again, numbered 3. A dashed line at the right edge marks where the drawing continues in the next panel. engine 1 manifold 2 head pipe 3 expansion chamber Exhaust sections 4 to 6, out through the hull Carrying on from a dashed line at the left, the narrowing end of the wide body becomes a thin tube numbered 4, and a dashed water line rises into it from below. A shorter, slimmer hose joins it to a closed box numbered 5. From the far side of the box a tube runs right to an open end, an arrow carries on past it to the outside, and the number 6 sits under it. 4 stinger hose 5 waterbox out 6 outlet through the hull cooling water

In order from the engine: manifold, then head pipe, expansion chamber, stinger, a hose, waterbox, and the outlet through the hull.

Gas from the engine meets these sections one after another. A class document can open some of them and hold the rest to the original part, so each section carries its own name in the rules.

How we know: the order is the one IHRA's rule P.2.1 lists and US patent 6,113,446 describes in paragraphs 28 to 30; the stinger and its cooling line are IJSBA's Ski Lites rule SKL.3.6.

Not to scale.

Nothing inside the engine may be touched

Article 501.02.03.02 refuses internal modification of any kind and names the methods: grinding, surfacing, polishing, machining and shot peening. Full porting is therefore out, an aftermarket head has no allowance, and the intake manifold has no article at all. What the class does permit at the top end is narrow and specific, which is the pattern throughout.

The carburettor list is five items long

Article 501.02.04.01 permits replaceable type jets, needle valves and needle valve springs, removal of the choke, and an aftermarket primer. It then says no other carburettor modifications, which removes the usual argument about what silence implies. Fuel injection has no allowance in this class, so an injection conversion is not a grey area here.

The two things the rulebook leaves genuinely open

The class states a maximum original engine power of 85 horsepower and never says how that figure is measured or verified, which makes it a ceiling nobody can check at the ramp. Separately, no reed valve article exists for GP3 at all. Under the closed list that reads as aftermarket reeds being illegal here while they are expressly legal one class up, and the rulebook never states the refusal in terms. We record both as uncertainties rather than answering them, because answering them would mean inventing the sentence the document is missing.

Closed list

Ski Division GP3 (501.02) names the changes allowed; anything it never names stays as the manufacturer supplied it.

  • A change the document lists

    goes through

    • Legal
    • Legal, with conditions where the list sets a limit

  • A change it never lists

    stops

    • Not permitted, not listed in the rules

Set of limits

The class states what may not change, or how far it may go, and leaves the rest open.

  • A change inside every stated limit

    goes through

    • Legal, with conditions where a rule names it
    • No rule, this is an open class where none does

  • A change past a stated limit

    stops

    • Not legal

Other classes in the same rulebook written this way

  • Our reading: UIM Ski Division GP1 and Runabout GP1, which carry no stock clause and no closed list; the rulebook never calls them open (501.04, 502.04).

Ski Division GP3 is written as a closed list, so the first thing to ask of any change is whether the document lists it.

How we know: the UIM 2026 Ski Division GP3 document (501.02), as the class records hold it: the rule type each record carries, and the closed-list clause in 501.02. The endings are the answers the class pages print.

What we cannot tell you, and why we are saying so

This site has never checked a single product against a UIM class document. Every legality record here is against the American bodies, so we can tell you what the GP3 rulebook says and we cannot tell you whether anything in our catalogue passes it. That is why no parts are linked from this page. A shop that answered a UIM question with an American check would be giving you a confident answer to a different question, and the difference only shows up at inspection.

The numbers that decide eligibility before any part does

A two stroke atmospheric ceiling of 800cc on displacement, a minimum dry weight of 135 kilograms, a sound limit of 86 decibels taken at 24 metres, a minimum rider age of 15, and at least 75 degrees of handle pole movement with the pole mounted ahead of the engine compartment. Homologation runs on 50 specimens with a conformity certificate. No model list is published, so eligibility is argued from those figures rather than read off a roster.

Symptoms this addresses: Racing UIM rather than a domestic series · Not sure whether aftermarket reeds are legal in GP3 · Told the exhaust must be entirely original · Need to know what a closed list means for parts nobody mentions. All symptoms.

Cite this page

“What is legal in UIM Ski GP3, and what the rulebook does not say”, BaddJet archive, https://baddjetarchive.com/tech/class-rules/what-is-legal-in-uim-ski-gp3/

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Class rules · Intermediate

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